[Federal Register: December 14, 2010 (Volume 75, Number 239)]
[Proposed Rules]
[Page 78093-78146]
From the Federal Register Online via GPO Access [wais.access.gpo.gov]
[DOCID:fr14de10-20]
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Part V
Department of the Interior
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Fish and Wildlife Service
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50 CFR Part 17
Endangered and Threatened Wildlife and Plants; 12-Month Finding on a
Petition To List the Sonoran Population of the Desert Tortoise as
Endangered or Threatened; Proposed Rule
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DEPARTMENT OF THE INTERIOR
Fish and Wildlife Service
50 CFR Part 17
[Docket No. FWS-R2-ES-2009-0032; MO 92210-0-008]
Endangered and Threatened Wildlife and Plants; 12-Month Finding
on a Petition To List the Sonoran Population of the Desert Tortoise as
Endangered or Threatened
AGENCY: Fish and Wildlife Service, Interior.
ACTION: Notice of 12-month petition finding.
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SUMMARY: We, the U.S. Fish and Wildlife Service, announce a 12-month
finding on a petition to list the Sonoran population of the desert
tortoise (Gopherus agassizii) as endangered or threatened and to
designate critical habitat under the Endangered Species Act of 1973, as
amended (Act). After review of all available scientific and commercial
information, we find that listing the Sonoran population of the desert
tortoise is warranted. Currently, however, listing the Sonoran
population of the desert tortoise is precluded by higher priority
actions to amend the Lists of Endangered and Threatened Wildlife and
Plants. Upon publication of this 12-month petition finding, we will add
the Sonoran population of the desert tortoise to our candidate species
list. We will develop a proposed rule to list the Sonoran population of
the desert tortoise as our priorities allow. We will make any
determination on critical habitat during development of the proposed
listing rule. In any interim period we will address the status of the
candidate taxon through our annual Candidate Notice of Review (CNOR).
DATES: The finding announced in this document was made on December 14,
2010.
ADDRESSES: This finding is available on the Internet at http://
www.regulations.gov at Docket Number FWS-R2-ES-2009-0032. Supporting
documentation we used in preparing this finding is available for public
inspection, by appointment, during normal business hours at the U.S.
Fish and Wildlife Service, Arizona Ecological Services Office, 2321
West Royal Palm Road, Suite 103, Phoenix, Arizona 85021. Please submit
any new information, materials, comments, or questions concerning this
finding to the above address.
FOR FURTHER INFORMATION CONTACT: Steven L. Spangle, Field Supervisor
Arizona Ecological Services Office (see ADDRESSES); by telephone at
(602) 242-0210; or by facsimile at (602) 242-2513. If you use a
telecommunications device for the deaf (TDD), please call the Federal
Information Relay Service (FIRS) at 800-877-8339.
SUPPLEMENTARY INFORMATION:
Background
Section 4(b)(3)(B) of the Endangered Species Act of 1973, as
amended (Act) (16 U.S.C. 1531 et seq.), requires that, for any petition
to revise the Federal Lists of Endangered and Threatened Wildlife and
Plants that contains substantial scientific or commercial information
that listing the species may be warranted, we make a finding within 12
months of the date of receipt of the petition. In this finding, we
determine that the petitioned action is: (a) Not warranted, (b)
warranted, or (c) warranted, but the immediate proposal of a regulation
implementing the petitioned action is precluded by other pending
proposals to determine whether species are threatened or endangered,
and expeditious progress is being made to add or remove qualified
species from the Federal Lists of Endangered and Threatened Wildlife
and Plants. Section 4(b)(3)(C) of the Act requires that we treat a
petition for which the requested action is found to be warranted but
precluded as though resubmitted on the date of such finding, that is,
requiring a subsequent finding to be made within 12 months. We must
publish these 12-month findings in the Federal Register.
Previous Federal Actions
On October 15, 2008, we received a petition dated October 9, 2008,
from WildEarth Guardians and Western Watersheds Project (petitioners)
requesting that the Sonoran population of the desert tortoise be listed
under the Act as a distinct population segment (DPS), as threatened or
endangered rangewide (in the United States and Mexico), and critical
habitat be designated. The petition contained detailed information on
the natural history, biology, current status, and distribution of the
Sonoran population of the desert tortoise. It also contained
information on what the petitioners reported as potential threats to
the Sonoran population of the desert tortoise, such as livestock
grazing, urbanization and development, mining, international border
patrol activities, illegal collection, inadequacy of existing
regulations, altered fire regimes, off-highway vehicle use, drought,
and climate change. We acknowledged the receipt of the petition in a
letter to the WildEarth Guardians and Western Watersheds Project, dated
November 26, 2008. In that letter we also stated that we had reviewed
the information presented in the petition and determined that issuing
an emergency regulation temporarily listing the species as per section
4(b)(7) of the Act was not warranted. We also stated that we intended
to make our finding on whether the petition presented substantial
information that the requested action may be warranted, to the maximum
extent practicable within 90 days of receipt of the petition, according
to the provisions of section 4(b)(3) of the Act.
On August 28, 2009, we made our 90-day finding that the petition
presented substantial scientific information indicating that listing
the Sonoran population of the desert tortoise (Gopherus agassizii) may
be warranted. The finding and notice of our initiation of a status
review was published in the Federal Register on August 28, 2009 (74 FR
44335).
On April 10, 2010, a stipulated settlement agreement (WildEarth
Guardians and Western Watersheds Project v. Salazar, 10-cv-86-ACT-RHS
(D. NM)) was filed. In this agreement, we agreed to submit a 12-month
finding to the Federal Register on or before December 5, 2010. The
stipulated settlement agreement was signed and adopted by the District
Court of New Mexico on April 15, 2010.
This notice constitutes our 12-month finding for the petition to
list the Sonoran population of the desert tortoise as threatened or
endangered.
Other Federal Actions
Throughout this finding, we use ``Mojave'' to describe desert
tortoise populations north and west of the Colorado River, as well as
any reference to the biotic community known as the ``Mojave Desert'' or
``Mojave desertscrub.'' These uses are consistent with the previous and
current spelling of the common name in Federal actions that have
addressed this population. We use ``Mohave'' in the geographic context
to remain consistent with its reference by the U.S. Board of Geographic
Names (e.g., Mohave County). In addition, while the Sonoran population
of the desert tortoise is not currently formally recognized as a unique
taxonomic entity, for ease of reference, we refer to the Sonoran
population of the desert tortoise as the ``Sonoran desert tortoise'' in
this finding.
On December 30, 1982, we published a notice of review which
determined the desert tortoise throughout its range in the United
States and Mexico to be a Category 2 Candidate species (47 FR
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58454); this was reaffirmed on September 18, 1985 (50 FR 37958).
Category-2 status was granted to species for which information in our
possession indicated that a proposed listing as threatened or
endangered was possibly appropriate, but for which sufficient data were
not available to make a determination of listing status under the Act.
On April 2, 1990, we issued a final rule designating the Mojave
population of the desert tortoise (occurring north and west of the
Colorado River) as a threatened species under the Act (55 FR 12178; see
final rule for a summary of previous actions regarding the Mojave
population of the desert tortoise). Currently, the Mojave population of
the desert tortoise is recognized as a distinct population segment
under the Act. As part of that rulemaking, we designated any desert
tortoise from the Sonoran population as threatened when observed
outside of its known range, due to similarity of appearance under
section 4(a) of the Act.
On December 5, 1996, we published a rule that discontinued the
practice of keeping a list of Category 2 Candidate species (61 FR
64481). Since that time, the Sonoran desert tortoise observed inside
its known range has had no Federal Endangered Species Act status.
For a detailed account of previous Federal actions that pertained
to the desert tortoise in the United States, please review the
following Federal Register documents: ``Proposed Endangered Status and
Critical Habitat for the Beaver Dam Slope Population of the Desert
Tortoise'' (43 FR 37662, August 23, 1978); ``Requirement to withdraw or
supplement proposals to determine various U.S. taxa of plants and
wildlife as Endangered or Threatened or to determine Critical Habitat
for such species'' (44 FR 12382, March 6, 1979); ``Reproposal of
Critical Habitat for the Illinois mud turtle and Beaver Dam Slope
population of the desert tortoise'' (44 FR 70680, December 7, 1979);
``Listing as Threatened With Critical Habitat for the Beaver Dam Slope
Population of the Desert Tortoise in Utah'' (45 FR 55654, August 20,
1980); ``Review of Vertebrate Wildlife for Listing as Endangered or
Threatened Species'' (47 FR 58454, December 30, 1982); ``Notice of
Findings on Four Petitions, and Review of One Species'' (50 FR 13054,
April 2, 1985); ``Review of Vertebrate Wildlife'' (50 FR 37958,
September 15, 1985); ``Finding on Desert Tortoise Petition'' (50 FR
49868, December 5, 1985); ``Findings on Pending Petitions and
Description of Progress of Listing Actions'' (53 FR 25511, July 7,
1988); ``Findings on Pending Petitions and Description of Progress of
Listing Actions'' (53 FR 52746, December 29, 1988); ``Emergency
Determination of Endangered Status for the Mojave Population of the
Desert Tortoise'' (54 FR 32326, August 4, 1989); ``Desert Tortoise''
(54 FR 42270, October 13, 1989); ``Determination of Threatened Status
for the Mojave Population of the Desert Tortoise'' (55 FR 12178, April
2, 1990); ``Finding on a Petition to List the Sonoran Desert Tortoise
as Threatened or Endangered'' (56 FR 29453, June 27, 1991); ``Proposed
Determination of Critical Habitat for the Mojave Population of the
Desert Tortoise'' (58 FR 45748, August 30, 1993); ``Determination of
Critical Habitat for the Mojave Population of the Desert Tortoise'' (59
FR 5820, February 8, 1994); ``Determination of Critical Habitat for the
Mojave Population of the Desert Tortoise'' (59 FR 9032, February 24,
1994); ``Notice of Final Decision on Identification of Candidates for
Listing as Endangered or Threatened'' (61 FR 64481, December 5, 1996);
and ``90-Day Finding on a Petition To List the Sonoran Population of
the Desert Tortoise (Gopherus agassizii) as a Distinct Population
Segment (DPS) with Critical Habitat'' (74 FR 44335, August 28, 2009).
Species Information
Taxonomy
The desert tortoise is in the genus Gopherus (Rafinesque 1832), or
gopher tortoises, and is a member of the Testudinidae family, or
terrestrial tortoises. The North American tortoises formerly comprised
two genera, Gopherus and Xerobates, with the latter including X.
agassizii, the desert tortoise (Crumly 1994, pp. 7-8). Scientific
nomenclature assigned to the desert tortoise has undergone a series of
changes since its initial description by Cooper (1863) as X. agassizii
(Barrett and Johnson 1990, p. 5); the currently recognized scientific
name for the desert tortoise is Gopherus agassizii. Further information
is available in Barrett and Johnson (1990, p. 5) or in the detailed
account of desert tortoise phylogeny (evolutionary development) and
systematics (taxonomic classification) by Crumly (1994, pp. 7-32). The
desert tortoise is known in Mexico with the common names of ``tortuga
del monte,'' ``Gal[aacute]pago de desierto,'' or the
``xtam[oacute]osni'' (Rorabaugh 2008, p. 35).
Physical Description of Sonoran Desert Tortoises
Adult Sonoran desert tortoises range in total carapace (straight-
line top shell) length from 8 to 15 inches (in) (20 to 38 centimeters
(cm)), with a relatively high domed shell (AGFD 2001, p. 1; Brennan and
Holycross 2006, p. 54). The record length for a Sonoran desert tortoise
is 19.4 in (49 cm) total carapace length (Jackson and Wilkinson-Trotter
1980, p. 430). The carapace is usually brownish with a definite pattern
and prominent growth lines (AGFD 2001, p. 1). The plastron (bottom
shell) is yellowish and is not hinged (AGFD 2001, p. 1; Brennan and
Holycross 2006, p. 54). The hind limbs are very stocky and elephantine;
forelimbs are flattened for digging and covered with large conical
scales (AGFD 2001, p. 1; Brennan and Holycross 2006, p. 54). Male
Sonoran desert tortoises are differentiated from females by having
elongated gular (throat) shields, chin glands visible on each side of
the lower jaw (most evident during the breeding season), a concave
plastron, and larger overall size (AGFD 2001, p. 1).
Distribution
The desert tortoise includes portions of southern California,
southern Nevada, southwestern Utah, and the western, northwestern, and
southern portions of Arizona in the United States, and also includes
the Mexican State of Sonora into the northern portion of Sinaloa. One-
third of the geographic range of the desert tortoise occurs in
northwestern Mexico (Bury et al. 2002, p. 86). The specific
distribution of desert tortoise is influenced by habitat and climatic
characteristics (vegetation community for food), soil and substrate
characteristics (for shelter), and precipitation pattern (for water
availability) within the appropriate elevation range.
The distribution of the Sonoran desert tortoise in the United
States is considered to be entirely within Arizona and comprises
approximately 26.8 million acres (ac) (10.8 million hectares (ha));
east and south of the Colorado River (Barrett and Johnson 1990, pp. 4-
5; Lamb et al. 1989, p. 84). Sonoran desert tortoise distribution in
Arizona is limited to the northeast by the limits of the Sonoran
Desert. The Arizona portion of their range constitutes approximately 52
percent of their total distribution. In Arizona, the Sonoran desert
tortoise occurs primarily on Federal land but also occurs on a variety
of non-federal lands as well as on ten Native American reservations:
(1) Fort Mojave Indian Tribe; (2) Colorado River Indian Tribe; (3)
Hualapai Tribe; (4) Fort McDowell Yavapai Nation; (5) Salt River Pima-
Maricopa Indian Community; (6) Gila River Indian Community; (7) Ak
Chin; (8) Tohono O'odham Nation; (9) Pasqua Yaqui Tribe; and, (10) San
Carlos Apache Tribe (AIDTT 2000, p. 40).
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In Mexico, where 48 percent of their range occurs, the distribution
of the Sonoran desert tortoise extends from the international border of
Sonora and Arizona, south to the vicinity of Guaymas, and north of the
R[iacute]o Yaqui (the southern and southeastern-most border of their
distribution), in southern Sonora (Germano et al. 1994, p. 77; Fritts
and Jennings 1994, p. 51; Bury et al. 2002, p. 88; Van Devender 2002a,
p. 5; Edwards et al. 2009, pp. 7-8). This includes approximately the
western half of the State of Sonora from the Gulf of California coast
east roughly to the transition to unsuitable woodland and conifer
forest areas in the higher elevations of the Sierra Madre Occidental.
In 30 timed searches conducted August to September 1983, and beyond the
known distribution of Sonoran desert tortoises in Sonora, Mexico,
Fritts and Jennings (1994, p. 52) found several patterns in Sonoran
desert tortoise distribution. First, most Sonoran desert tortoises in
the eastern and northern extent of their distribution in Mexico occur
below the 2,600 foot (ft) (790 meters (m)) elevation contour (Fritts
and Jennings 1994, p. 52). Second, populations may be the densest and
the least patchy between elevations of 900 and 1,600 ft (270 and 490 m)
(Fritts and Jennings 1994, p. 52). They were also not found in habitat
in Mexico that received an average of 3.9 in (10 cm) or less of rain
annually (Fritts and Jennings 1994, p. 53).
One question about the distribution of the Sonoran desert tortoise
concerns the origin of a small number of tortoises that have been found
in far southeastern Cochise County, Arizona, an area generally
considered well east of the known distribution. There is some evidence
that these tortoises may represent a naturally occurring population
based on the presence of suitable habitat (Rorabaugh 2009, pers.
comm.), similar animal communities (Rosen 2009, pers. comm.), and
historic and current observations of tortoises in the area (Hulse and
Middendorf 1979, p. 546; Radke 2009, pers. comm.; Van Devender et al.
1976, pp. 300-303). However, these observations have traditionally been
discounted as released pets rather than a natural population (AIDTT
2000, p. 3; Germano et al. 1994, p. 81). Also, recent genetic analysis
of a Sonoran desert tortoise collected from this area in 2009 indicated
it was most closely related to tortoises in the Phoenix, Arizona, area
and is likely, therefore, a ``released or escaped captive'' tortoise
(Edwards 2010, pers. comm.). We recognize there is a fair amount of
uncertainty regarding the origin of this population. However, because
Sonoran desert tortoises are infrequently documented from this area and
recent genetic testing indicated that observations represent released
captives, we conclude that desert tortoises from this area do not
represent a naturally-occurring, disjunct population. Consequently, we
will not evaluate potential threats to the tortoises in this area of
Cochise County in this finding.
Habitat
Sonoran desert tortoises are most closely associated with the
Arizona Upland and Lower Colorado River subdivisions of Sonoran
desertscrub and Mojave desertscrub vegetation types. They occur most
commonly on rocky (predominantly granitic rock), steep slopes and
bajadas (lower mountain slopes often formed by the coalescing of
several alluvial fans (fan-shaped deposits at the ends of canyons
formed when fast flowing streams slow and widen)) and in paloverde-
mixed cacti associations (Ortenburger and Ortenburger 1927, p. 120;
Burge 1979, p. 49; 1980, p. 48). Sonoran desert tortoise density has
been observed to be higher in the Arizona Upland subdivision of the
Sonoran desertscrub than in the Lower Colorado subdivision of the
Sonoran desertscrub or in Mojave desertscrub (Berry 1984, p. 434; AIDTT
2000, p. 4; Boarman and Kristan 2008, p. 19). In addition to the use of
vegetation to meet energy and nutritional needs, the Sonoran desert
tortoise uses vegetation for predator avoidance, thermal protection,
and in social behaviors (Avery and Neibergs 1997, p. 13; Grandmaison et
al. in press, p. 3). An important attribute of Sonoran desert tortoise
habitat is the presence of cryptogamic crusts (soil crusts with unique,
microscopic association of flora and fauna) (Bowker et al. 2008, p.
2309). These occur on the surface of Sonoran Desert soils and assist
with nitrogen-fixing to enhance soil fertility, improve water
infiltration into soils, and prevent or lessen effects from wind and
water erosion, all of which help to sustain vegetation vital to the
Sonoran desert tortoise (DeFalco 1995, p. 22; DeFalco et al. 2001, pp.
1, 9).
Sonoran desert tortoises rarely occur in oak woodland habitat.
However, one such population occurs at approximately 5,000-ft (1,500-m)
elevation in Chiminea Canyon in the Rincon Mountains of Pima County,
Arizona (Van Devender 2002a, p. 23), and they are also known from
similar elevation in the Atascosa and Pajarito Mountains in south-
central Arizona. Zylstra and Steidl (2008, p. 747) found that habitat
selection by Sonoran desert tortoises was most closely associated with
topographic (degree of steepness of slope) and geomorphologic (rock
type and structure) influences rather than by vegetation type.
Specifically, Zylstra and Steidl (2008, p. 747) found that the
likelihood of observing Sonoran desert tortoises increased with
increasing slope, with a strong association to aspect (the direction to
which a slope faces), with east-facing slopes preferred over north-
facing slopes. However, the season of use may affect which slope-
aspects (the direction a particular slope faces) Sonoran desert
tortoises are likely to use based on their needs at that time (Zylstra
and Steidl 2008, p. 752). Specifically, Sonoran desert tortoises have
different thermoregulatory and physiological needs based upon their
seasonal behaviors, such as hibernation or seeking temporary shelter
during the tortoise's surface-active seasons.
In addition to steep, rocky slopes and bajadas, Sonoran desert
tortoises also use inter-mountain valleys as part of their home ranges
and for dispersal at all age classes (Averill-Murray and Averill-Murray
2002, p. 16). In the Ironwood National Forest, Averill-Murray and
Averill-Murray (2005, p. 65) found tortoises or their signs (such as
scat (droppings) and burrows) on 92 percent of transects in boulder
habitat, on 71 percent of transects that included incised washes (dry
stream beds that flow in response to precipitation), and on 25 percent
of transects that had neither boulder habitat nor incised washes.
Sonoran desert tortoises were found up to one mile (mi) (1.6 kilometers
(km)) away from the nearest slope, indicating that they occur in low
densities in inter-mountain valleys. Averill-Murray and Averill-Murray
(2005, p. 65) stated that maintaining these areas ``may be important
for long-term population viability.'' Washes might also be selectively
chosen by reproductive female Sonoran desert tortoises as all eggs and
hatchling desert tortoises observed by Barrett (1990, p. 205) occurred
there. Sonoran desert tortoises on the 40-square-mile (sq mi) (64-
square-kilometer (sq km)) Florence Military Reservation in Pinal
County, Arizona, primarily use xeroriparian habitat (a habitat
association with plant species tolerant to hyper-arid conditions) along
washes, with caliche caves (caves formed along steep banks of washes
within cemented, sedimentary rock formations of calcium carbonate)
within washes being an important component to occupied habitat (Lutz et
al. 2005, p. 22; Riedle et al. 2008, p. 418). Another frequently
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used habitat type on the Florence Military Reservation included gently
rolling alluvial fans dominated by creosote bush (Larrea tridentata)
and white bursage (Ambrosia dumosa) during all periods of the year;
somewhat atypical for Sonoran desert tortoises in other portions of its
range (Lutz et al. 2005; p. 22; Grandmaison et al. in press, p. 4). In
this habitat, Sonoran desert tortoises often used packrat middens
(organic debris piles constructed for nesting purposes which often are
comprised of wood material, cactus pads, etc.) as shelter sites,
especially those with suitable canopy cover, an absence of cattle
activity, and proximity to roads and washes (Lutz et al. 2005, p. 22;
Grandmaison et al. in press, p. 2).
Sonoran desert tortoises in Arizona generally occur within
elevations from 510 to 5,300 ft (155 to 1,615 m) (Barrett and Johnson
1990, p. 7; AGFD 2001, p. 4). According to the AGFD's Heritage Data
Management system, 95 percent of Sonoran desert tortoise observations
in Arizona have occurred at an elevation of 904 to 4,198 ft (275 to
1279 m) (Zylstra and Steidl 2009, p. 8). However, one example of an
extreme exception was a Sonoran desert tortoise observed at 7,808 ft
(2,379 m) in a ponderosa pine-dominated coniferous community in the
Rincon Mountain District of Saguaro National Park in Pima County,
Arizona (Aslan et al. 2003, p. 57). The nearest road was 8.6 mi (13.9
km) away by trail and nearly 2,000 ft (610 m) lower in elevation from
the observed location of the tortoise, which strongly dismisses any
notion that human activity was responsible for its location at such a
high elevation (Aslan et al. 2003, p. 57).
Sonoran desert tortoises in Mexico are generally found at lower
elevations, ranging from approximately 1,000 to 1,640 ft (305 to 500 m)
in elevation in rocky outcrops in desertscrub and foothills thornscrub
habitat (Bury et al. 2002, p. 89). As in Sonoran desertscrub habitat in
Arizona, Sonoran desert tortoises in Mexico often use shrubs as
temporary shelter sites, and species such as mesquite (Prosopis spp.)
and ironwood (Olneya tesota) may play important roles in the natural
history of Sonoran desert tortoises in Mexico (Bury et al. 2002, p.
100). Sonoran desert tortoises in Mexico have not been documented in
flatter areas between mountain ranges (Bury et al. 2002, p. 89),
although we presume they use these areas to some extent for dispersal
much like they do in similar inter-mountain basins of Arizona. With the
exception of the El Pinacate Desert Bioreserve in northwestern Sonora,
Sonoran desert tortoises have not been documented using the extremely
arid Lower Colorado subdivision of the Sonoran Desert in Mexico (Bury
et al. 2002, p. 89). However, based on their presence in El Pinacate
and the general lack of surveys in Mexico, the Sonoran desert tortoise
may potentially be found in this habitat in northwestern Sonora in low
densities. The extent of Sonoran desert tortoise distribution in
northeastern Sonora, an area characterized as a transitional zone of
foothills thornscrub, tropical deciduous forest, and Madrean oak
woodland, is poorly understood (Bury et al. 2002, p. 89).
Burrow Use
Adequate shelter, often in the form of constructed burrows, is one
of the most important habitat features for the Sonoran desert tortoise.
Burrows are constructed under rocks and boulders, beneath vegetation,
on semi-open slopes, within the sidewalls of washes, or by using rocky
crevices which may or may not be altered by the tortoise (Burge 1979,
p. 44; 1980, pp. 44-45; Barrett 1990, p. 205; Averill-Murray et al.
2002a, pp. 136-137, Grandmaison et al. in press, p. 14). Sonoran desert
tortoises construct burrows in a variety of soil types including silt,
silt with loose gravel, diatomite (a light-colored porous rock composed
of the shells of diatoms) and diatomaceous marl (a crumbly mixture of
clays, calcium and magnesium carbonates, with remnants of shells), and
well-lithified (process whereby loose particles are converted into
rock) volcanic ash, as observed in the lower San Pedro River Valley of
Arizona (Bailey et al. 1995, pp. 363-364). Burrows are used for
thermoregulation, nesting, and protection from predators, and the lack
of suitable conditions for constructing burrows may be a limiting
factor in Sonoran desert tortoise populations (Barrett and Humphrey
1986, p. 262; Bailey et al. 1995, p. 366; Zylstra and Steidl 2008, p.
752). In fact, Sonoran desert tortoise population densities appear to
be highly correlated with available burrows, or potential burrow sites
(Averill-Murray and Klug 2000, p. 69; Averill-Murray et al. 2002b, p.
126). Sonoran desert tortoises often use a group of relatively closely-
located burrows as focal areas of activity in their home range. In
doing so, they establish circular or slightly linear movement patterns,
and may temporarily move on to another such cluster of burrows within
the same active season (Bulova 1994, p. 140; Averill-Murray and Klug
2000, p. 62; Lutz et al. 2005, p. 21).
Burrows influence a variety of Sonoran desert tortoise behaviors
and physiological characteristics. During the winter dormancy period
(colder, winter months of inactivity), female Sonoran desert tortoises
typically use more shallow burrows that are more susceptible to
variation in ambient temperatures and consequently females emerge
earlier in the spring (as early as late February) than do males, who
often remain dormant until the commencement of the summer monsoon
(AIDTT 2000, p. 7; Ernst and Lovich 2009, p. 547). Averill-Murray and
Klug (2000, p. 66) and Bailey et al. (1995, p. 367) suggest that
shallow burrows may account for responsiveness of females to warming
periods in early spring for additional foraging opportunities to
increase energy reserves for egg development, as shallower burrows are
more reflective of ground-surface temperatures. Alternatively, cool,
less variable temperatures in deeper burrows selected by male Sonoran
desert tortoises may enhance sperm development and viability, as cooler
temperatures allow more sperm production (Bailey et al. 1995, p. 367).
The season may influence the locations and dimensions of burrows
used by Sonoran desert tortoises in order to meet their behavioral and
physiological needs (Barrett 1990, p. 205; Bailey et al. 1995, pp. 363,
366). Finally, particularly in hatchling and juvenile size classes, the
burrow microclimate can affect the rate of water loss in desert
tortoises, which results in behaviors (drinking pooled rain,
withdrawing into their shell, seeking long, deep burrows) to avoid
lethal dehydration in relatively hot, dry seasons (Wilson et al. 2001,
p. 158; Bulova 2002, pp. 184-186).
Other forms of shelter used by Sonoran desert tortoise include
packrat middens, which are often shared with other native reptiles,
including other tortoises (Averill-Murray et al. 2002a, pp. 136-137;
Lutz et al. 2005, p. 22; Grandmaison et al. in press, p. 2). These
shelter types provide less insulation than earthen burrows and are
therefore used for shorter duration, especially during the months with
extremely hot or cold temperatures. This was the most commonly used
shelter site at Florence Military Reservation.
Seasonal Behavior and Long-Distance Movements
The Sonoran desert tortoise is diurnal (active during daylight
hours) but sometimes emerge at night in response to rainfall (Ernst and
Lovich 2009, p. 544). Sonoran desert tortoises may be surface-active
every month of the year, but in the winter, surface activity is likely
a response to thermoregulatory needs or movements between burrows
(Averill-Murray and Klug 2000, p. 66).
[[Page 78098]]
Temperature and precipitation are important predictors of Sonoran
desert tortoise activity (Meyer et al. 2010, p. 11). Precipitation
amounts and timing vary among the populations of desert tortoise. The
lowest amount of rainfall (usually during the winter) occurs in the
northwestern-most portion of the species' range, and gradually
increases and becomes seasonally bimodal pattern (rains in winter and
summer) to the south into the southern-most extent of the species range
in northern Sinaloa, Mexico (Germano et al. 1994, p. 76). Sonoran
desert tortoise surface activity largely mimics the warm-season
precipitation pattern (Averill-Murray et al. 2002a, p. 139; Van
Devender 2002a, p. 7). Like the Arizona populations, Sonoran desert
tortoises in Mexico seem to be most active in late summer (Ernst and
Lovich 2009, p. 544). Sonoran desert tortoises are approximately half
as active during the spring as they are in the summer, with females
typically becoming surface active to forage in late March, while males
typically emerge (but are not necessarily active) in late April
(Averill-Murray et al. 2002a, p. 138).
The summer monsoon (occurring typically from late June through
September), characterized by both excessive heat and frequent
thunderstorms, is the peak activity season for the Sonoran desert
tortoise (Averill-Murray et al. 2002a, pp. 139-140). During this
period, new growth of perennial plants is initiated and annual plants
germinate, providing forage for tortoises (Averill-Murray et al. 2002a,
p. 140). The onset of the summer monsoon triggers Sonoran desert
tortoises to drink, flush their bladders, and rehydrate, establishing a
positive water and energy balance, and spurring reproductive behaviors
(AIDTT 2000, p. 7). Sonoran desert tortoises have been observed to seek
out rocks with surface depressions during summer months to drink
puddled water from monsoon storm events (Oftedal 2007, p. 23). Surface
activity in Sonoran desert tortoises begins to wane as early as late
September and ends by mid-December as they prepare for hibernation.
Temperature and photoperiod (the duration of daylight) are likely the
cues used by Sonoran desert tortoises to commence hibernation (Bailey
et al. 1995, p. 367; Averill-Murray et al. 2002a, p. 147). Periods of
hibernation (typically from mid-November through mid-February) in
Sonoran desert tortoises appear to vary greatly among populations and
among years but appear to correlate with seasonal temperatures (Bailey
et al. 1995, p. 367; Averill-Murray and Klug 2000, p. 66).
The behavior and ecology of hatchling Sonoran desert tortoises is
poorly understood because their small size makes them very difficult to
observe in the wild. Their scat is small, inconspicuous, and ephemeral,
and burrows used by individuals in this size class resemble those of
other terrestrial vertebrates in Sonoran desert tortoise habitat
(Germano et al. 2002, pp. 271-272). This size class is thought to be
the most vulnerable, experiencing the highest mortality rates (Morafka
1994, p. 161).
Home range sizes of Sonoran desert tortoises vary with
precipitation levels, contracting during wet years and expanding during
dry years in response to the availability of forage plants (Averill-
Murray and Klug 2000, p. 67). The home range of Sonoran desert
tortoises may be as small as 6.4 ac (2.6 ha) but can vary widely, with
males having larger home ranges than females (Barrett 1990, p. 203;
Averill-Murray and Klug 2000, pp. 55-61; Averill-Murray et al. 2002a,
pp. 150-151). In the lower San Pedro River Valley, Meyer (1993, p. 99)
found Sonoran desert tortoise home ranges varied between 45 and 640 ac
(18 and 258 ha) in size. Sonoran desert tortoises are known to exhibit
high fidelity to their home ranges, with exception to dispersal
movements when they move to new areas (Zylstra and Swann 2009, p. vi).
They likely habituate to specific attributes of their home range,
including the location of mates, water catchments, mineral licks, and
burrow sites (Berry 1986a, p. 113).
Sonoran desert tortoises are known to make long-distance movements
between populations in adjacent mountain ranges. In an extreme example,
Edwards et al. (2004, p. 494) tracked an adult female Sonoran desert
tortoise moving 20 mi (32 km) between the Rincon and Santa Rita
mountains of southern Arizona (also see Zylstra and Swann 2009, p. 10).
During this long-distance movement, this tortoise encountered several
barriers to movement that required human intervention to overcome such
as fence lines, railroad tracks, an interstate highway, and several
captures (including a temporary adoption) by humans (Edwards et al.
2004, p. 494). In another example, in the San Pedro Valley of southern
Arizona, a sub-adult Sonoran desert tortoise was captured and marked in
1992. It was recaptured in 2005 approximately 14 mi (23 km) from its
original point of capture (Meyer et al. 2010, p. 18). Dispersal
distances of hatchling Sonoran desert tortoises are not well
understood, but are likely shorter than those of adults because of the
complex habitat of boulders and vegetation (where they occur) may
inhibit long-distance movements (Van Devender 2002a, p. 14).
Gibbons (1986, p. 104) suspected that long-distance movements by
turtles can be explained by: (1) Nest site selection; (2) seasonal
migration; (3) departure from unfavorable habitat conditions; or (4)
movement by males in search of females. Averill-Murray and Klug (2000,
p. 68) suggested that long-distance movements may be interpreted as
random wanderings, infrequent travels to known sources of biological
needs, explorations, adaptations for genetic exchange, or for dispersal
to other suitable areas. Precipitation may influence the likelihood of
long-distance movements, especially in individuals approaching
reproductive age in populations that experience above-average
precipitation for a 2- to 3-year period (AIDTT 2000, p. 8). Averill-
Murray and Klug (2000, p. ii) stated, ``A large cohort of young
tortoises that experiences a relatively wet and productive environment,
with high survival, may provide the stock for dispersal between
populations as they approach sexual maturity, in addition to replacing
aging adults within the local population.'' Long-distance movements by
Sonoran desert tortoises observed by Averill-Murray and Klug (2000, p.
69) suggest the potential for metapopulation (interrelated population
dynamics between regionally proximal populations) relationships between
local populations inhabiting regional hillsides. Habitat features may
also influence the Sonoran desert tortoises' ability to make long-
distance movements. Dispersal of Sonoran desert tortoises between
populations might be less likely through sparse desertscrub in very
hot, dry river valleys in the Lower Colorado River subdivision of
Sonoran desertscrub. Van Devender (2002a, p. 16) suggested that
populations occurring in the Eagletail, Maricopa, Sand Tank, and
similarly situated mountain ranges might have existed in isolation for
decades, if not centuries.
There are no data to evaluate long-distance movements in
populations that occur in Mexico. Although Sonoran desert tortoises in
Mexico are known to occupy slopes, arroyos, and bajadas, they are
infrequently observed using valley bottoms (Fritts and Jennings 1994,
p. 52). Sonoran desert tortoise populations in Mexico have been poorly
studied, but we presume individuals make similar long-distance
movements between populations.
[[Page 78099]]
Longevity
Estimates of longevity in wild Sonoran desert tortoises vary
considerably from 30 years to over 100 years (Germano 1992, pp. 369-
370; 1994, p. 176; Zylstra and Swann 2009, p. vii). Using a growth
equation to extrapolate longevity in Sonoran desert tortoises, Germano
et al. (2002, p. 271) estimated that the average oldest ages attained
for Sonoran desert tortoises is 62.2 years in females and 64.4 years in
males; however, they admitted that correlating age with size is
problematic in turtles. Zylstra and Swann (2009, p. vii) suspected that
Sonoran desert tortoises may reach 80 to 100 years of age in the wild.
Sonoran desert tortoises have been shown to live longer in the wild
than those from the Mojave population.
Bladder Physiology
The bladder in the Sonoran desert tortoise is unique and serves an
important function in its survival. Sonoran desert tortoises are
capable of drinking large amounts of water when it is available, and
may even construct water catchments by digging earthen depressions,
likely as an adaptation to the infrequent and unpredictable nature of
rainfall events throughout their range (Ernst and Lovich 2009, p. 546).
The bladder of Sonoran desert tortoises is a large and bilobed (divided
into two lobes) organ critical for withstanding the effects of seasonal
and short-term drought because of its ability to store water, dilute
excess dietary salts and metabolic wastes, and reabsorb water into the
bloodstream (Averill-Murray et al. 2002a, p. 146; Ernst and Lovich
2009, p. 545). In seasonal or short-term drought conditions, the
concentration of urine in Sonoran desert tortoises allows them to
forage on dried vegetation by reducing the dehydration effects of such
forage types (Averill-Murray et al. 2002a, p. 146; Ernst and Lovich
2009, p. 545). Water serves an important role in flushing salts from
the body of Sonoran desert tortoises and resetting the electrolytic
balance, preparing the Sonoran desert tortoise for the next dry period
(Averill-Murray et al. 2002a, pp. 140, 146).
Diet, Foraging Behavior, and Potassium Excretion Potential
The Sonoran desert tortoise is an herbivore, and has been
documented to eat 199 different species of plants, including herbs
(55.3 percent), grasses (17.6 percent), woody plants (22.1 percent),
and succulents (5 percent) (Ogden 1993, pp. 1-8; Van Devender et al.
2002; pp. 175-176; Brennan and Holycross 2006, p. 54; Oftedal 2007, p.
21; Ernst and Lovich 2009, p. 562; Meyer et al. 2010, pp. 28-29, 44-
48). Of the numerous nonnative plant species that have become
established throughout the range of the Sonoran desert tortoise, only
red brome (Bromus rubens) and redstem filaree (Erodium cicutarium) are
frequently eaten and considered relatively important nonnative species
in the diets of Sonoran desert tortoises (Van Devender et al. 2002, p.
183). However, physical injury to Mojave desert tortoises resulting
from consuming nonnative grass species (i.e., red brome and cheatgrass
(Bromus tectorum)) has been documented, and sharp seeds have been found
lodged between the tortoises' upper and lower jaw. This injury may
adversely affect their foraging ability or become a source for
infection (Medica and Eckert 2007, p. 447). Though this study focused
on Mojave desert tortoises, this may affect all desert tortoises
wherever these plant species occur (i.e., within the Sonoran Desert in
Arizona).
Significant differences in the nutritional quality of native versus
nonnative forage for desert tortoises were not found by Hazard et al.
(2010, pp. 139-145). Nagy et al. (1998, pp. 260, 263) compared the
nutritional values of native and nonnative grasses (native: Indian
ricegrass (Achnatherum (Oryzopsis) hymenoides); nonnative: Mediterrean
grass (Schismus barbatus)) and forbs (native: desert dandilion
(Malacothrix glabrata); nonnative: redstem filaree), finding that the
two grasses possessed similar nutritional value. The dry matter and
energy digestibility of the two grasses were much lower than those of
the forbs, providing little nitrogen, and tortoises lost more water
than they gained while processing grasses. The native forb was more
readily digestible than the nonnative forb as dried mass, but the
inverse was true as fresh mass (Nagy et al. 1998, p. 263). However, the
native forbs provide significantly more nitrogen and water than the
nonnative forbs, which is important in maintaining a positive water
balance. Results of these feeding trials suggest that the proliferation
of nonnative grasses leading to the exclusion of forbs places desert
tortoises at a nutritional disadvantage. If, instead of eating to
obtain a given volume of food, tortoises consume just enough food to
satisfy their energy needs (as commonly noted in other vertebrate
groups), then the native forbs provide the best nutrition. Nagy et al.
(1998, p. 260) concluded that the life stage of the plant and the plant
type (forb or grass) were important predictors of nutritional quality
versus a plant being native or nonnative to a particular region. In
summary, research has shown that forbs are more valuable to Sonoran
desert tortoise nutrition than grasses, and that native forbs are more
valuable than nonnative forbs in a dried state, which may be important
in periods of drought.
Diets of Sonoran desert tortoises vary among populations in
response to seasonal availability of plant species and in response to
precipitation amounts (Martin and Van Devender 2002, p. 31). In years
of low winter rainfall, Sonoran desert tortoises are less selective in
plant species consumed because there are fewer options, but in years of
high winter rainfall, Sonoran desert tortoises have exhibited highly
selective foraging habits (Oftedal 2002, pp. 205-206). During years
when monsoon rains are light or irregular, Sonoran desert tortoises
consume dried plant material (Averill-Murray et al. 2002a, p. 140).
Within Saguaro National Park in southern Arizona, Sonoran desert
tortoises frequently ate annual legumes in the spring (high in water
content, low in potassium), and annual and perennial grasses
(supplemented by prickly pear fruit (Opuntia engelmannii)) during the
monsoon when ponding water can replenish water reserves (Oftedal 2007,
p. 17). In most years, Sonoran desert tortoises consume enough calories
during the summer monsoon to fuel growth and store fat for the next
year (Van Devender 2002a, p. 10).
Desert tortoises are uniquely vulnerable to changes in their
potassium levels (Oftedal 2002, p. 208). Because potassium cannot be
easily stored in the body, excess potassium must be excreted to avoid
toxicological effects (Oftedal 2002, p. 208). Therefore, Sonoran desert
tortoises that must forage on plants with high potassium content must
also flush their bladders more frequently and therefore risk a net loss
in metabolic water levels and subsequent dehydration (Oftedal 2002, p.
209).
The potassium excretion potential (PEP) is an index of water,
nitrogen, and potassium levels in a plant that relates to a desert
tortoise's ability to efficiently excrete potassium. PEP is a critical
consideration for determining the value or risk of particular forage
species during times of drought or major perturbations to habitat, and
for comparing potential effects of forage competition between tortoises
and livestock. A positive PEP value for a plant species (preferred by
tortoises) means there is more water and nitrogen
[[Page 78100]]
in the food than is needed to excrete potassium, and vice-versa for a
negative PEP value (Oftedal 2002, p. 215; Ernst and Lovich 2009, p.
545). Sonoran desert tortoises have been documented to selectively
forage on high PEP plant species to minimize water loss associated with
metabolizing potassium (Oftedal 2002, p. 214; Ernst and Lovich 2009, p.
545). High PEP values can be found in certain species of primroses,
filaree, legumes, mustards, and spurges (Ernst and Lovich 2009, p.
545). Sonoran desert tortoises have been found to be seasonally
selective for high PEP forage species, based on the abundance and
diversity of plants and precipitation (Oftedal 2002, p. 223; 2007, pp.
3, 22).
In addition to herbivory, Sonoran desert tortoises are also
geophagous; in other words, they consume bones, stones, and soil for
additional nutrient and mineral supplements, for mechanical assistance
in grinding plant matter in the stomach, or to expel parasites in the
intestinal tract (Sokol 1971, p. 70; Marlow and Tollestrup 1982, p.
475; Esque and Peters 1994, pp. 108-109; Stitt and Davis 2003, p. 57;
Walde et al. 2007b, p. 148). Sonoran desert tortoises are highly
attracted to sites with exposed calcium carbonate and have been
observed congregating at these sites year after year eating these soils
(Meyer et al. 2010, p. 11). Soil condition and quality are important to
the Sonoran desert tortoise, not only for nutrients derived from eating
soil, but also production and maintenance of vegetation that is
consumed by tortoises (Avery and Neibergs 1997, p. 13).
Desert tortoises have been observed eating scat from black-tailed
jack rabbits, wood rats, collared peccaries, and even desert tortoises.
This behavior could possibly aid in the transfer of gut microflora such
as bacteria or fungi or it could be used as a source of additional
nutrients (Walde et al. 2005, p. 77-78). Bostick (1990, p. 149)
asserted that desert tortoises feed ``primarily on dung'' although this
claim was refuted in the literature (Boarman 2002, pp. 27, 35, 38).
Infrequent observations of sand, bird feathers, arthropod parts, and
snake and lizard skins have also been made during fecal analyses of
desert tortoises (Ernst and Lovich 2009, p. 560).
Reproduction
The Sonoran desert tortoise breeding season begins with the summer
monsoon when male-male combat over receptive females can be observed,
such as at sites with exposed calcium carbonate soils, where tortoise
densities may be higher (discussed above) (Meyer et al. 2010, p. 11).
Sexual maturity and first reproduction in female Sonoran desert
tortoises occurs from 12 to 22 years of age, or at 8.7 in (22 cm) in
midline carapace length, and reproductive activity is highly influenced
by winter and spring precipitation (Averill-Murray and Klug 2000, p.
69; Averill-Murray et al. 2002b, p. 119; Bury et al. 2002, p. 100;
Germano et al. 2002, p. 265). Females may store sperm for up to two
years, meaning that one season's mating produces the following season's
clutch of eggs (Palmer et al. 1998, pp. 704-705; Averill-Murray et al.
2002a, p. 141). Female Sonoran desert tortoises may lay one clutch of
1-12 eggs per year, usually around the onset of the summer rainy
season, although they may not produce a clutch every year (Averill-
Murray 2002b, p. 295). Eggs hatch in September and October (Van
Devender 2002a, pp. 10-11; Averill-Murray 2002b, p. 295). The average
clutch size is 3.8 to 5.7 eggs, and in contrast to Mojave Desert
tortoises, clutch size is not positively correlated with female body
size (Mueller et al. 1998, p. 313; Averill-Murray 2002b, p. 299;
Averill-Murray et al. 2002b, p. 119). Late oviposition (deposition of
eggs) dates recorded on the Sugarloaf study site in central Arizona in
1998 and 1999 suggest that eggs and hatchlings may occasionally
overwinter in nests (Averill-Murray 2002b, p. 299). Female desert
tortoises have been known to urinate on their nest sites before and
after nesting; this may be to aid in digging the nest, and might make
it more difficult to dig up the nest after the soil dries, or possibly
to hydrate soils in contact with eggs as the rigid-shelled eggs of
desert tortoises have been shown to uptake moisture from the soil
faster than it evaporates from the shell exposed to air (Patterson
1971, p. 199; Spotila et al. 1994, p. 112). Female Sonoran desert
tortoises that survive to reproductive age are believed to produce as
many as 85 eggs over the course of their lives, with perhaps two or
three of those hatchlings surviving to reproductive age (Van Devender
2002a, p. 11).
Desert tortoises exhibit environmental sex determination, which
means that incubation temperatures during embryonic development
determine the sex of the tortoises. Higher incubation temperatures
produce more females and lower temperatures produce more males (Spotila
et al. 1994, pp. 109-111; Rostal et al. 2002, p. 313). Incubation
temperatures at or below 86.9 degrees Fahrenheit ([deg] F) (30.5
degrees Celsius ([deg] C)) result in the production of all male desert
tortoises, whereas temperatures of 90.5 [deg]F (32.5 [deg]C) result in
all females, and eggs incubated at the ``pivotal'' temperature of 88.3
[deg]F (31.3 [deg] C) develop a 1:1 sex ratio (Rostal et al. 2002, p.
313).
Predation
As adults, Sonoran desert tortoises are relatively protected from
natural predation because of their hard shells. Mountain lions (Felis
concolor) appear to be the only natural predator in the Sonoran Desert
with the jaw strength required to puncture or crack the shells of adult
Sonoran desert tortoises. However, mountain lion predation is not known
to contribute to elevated mortality rates within monitored Sonoran
desert tortoise populations (AIDTT 2000, p. 8; Meyer et al. 2010, p.
18; Riedle et al. 2010, p. 165). Dickenson et al. (2001, p. 254)
recorded 14 Sonoran desert tortoise mortalities in the Little Shipp
Wash and Harcuvar monitoring plots from 1990-1994, five of which were
attributed to mountain lion predation. Javelina (Tayassu tajacu)
predation on Sonoran desert tortoises was suspected in the San Pedro
Valley of southern Arizona (Meyer et al. 2010, p. 18). Other mammalian
predators may include badgers (Taxidea taxus), ring-tailed cats
(Bassiriscus astutus), bobcats (Felis rufus), skunks (Spilogale
gracilis, Mephitis mephitis, M. macroura, Conepatus mesoleucus), kit
foxes (Vulpes macrotis), gray foxes (Urocyon cinereoargenteus), coyotes
(Canis latrans), and domestic dogs (Canis familiaris) (Boarman 2002, p.
17; Ernst and Lovich 2009, p. 563).
Both golden eagles (Aquila chrysaetos) and common ravens (Corvus
corvax) have been documented to prey upon all size classes of Mojave
desert tortoises in California (Berry 1985, pp. 1, 6-10). Such
predation might also occur on Sonoran desert tortoises. The greater
roadrunner (Geococcyx californianus) is also a suspected predator on
juvenile Mojave desert tortoises, based upon one field observation of
roadrunner tracks next to a freshly killed individual (Berry 1985, p.
11); such predation might also occur on Sonoran desert tortoises.
However, because avian predators rely exclusively on their vision to
detect prey, we expect lower rates of avian predation on Sonoran desert
tortoises occupying Arizona upland Sonoran desertscrub because the
dense, complex habitat structure likely limits birds' ability to detect
tortoises. Habitat-associated protection from avian predation may be
less pronounced where Sonoran desert tortoises occur in the sparser
vegetation of the Lower Colorado River subdivision of Sonoran
desertscrub.
[[Page 78101]]
Sonoran desert tortoises are most vulnerable to predation while in
their eggs or as hatchlings and juveniles predominantly because of
their size and undeveloped, softened shells (which do not adequately
harden until approximately 7 years of age) which provide little
protection and are easily compromised. Higher mortality rates in the
hatchling and juvenile age classes may also be partially due to their
higher metabolic rates, which necessitates longer periods of surface
activity to obtain suitable amounts of forage. Longer surface activity
may cause greater risk of detection by predators (Morafka 1994, p.
163). Nest predation levels may be high in some populations. Seventy-
five percent of Sonoran desert tortoise nests suffered predation over a
two-year period at the Sugarloaf study plot in Maricopa County, Arizona
(Averill-Murray 2002b, p. 298). Gila monsters (Heloderma suspectum) are
a primary predator on tortoise eggs, and female Sonoran desert
tortoises in the process of oviposition will actively defend the burrow
and aggressively pursue Gila monsters in attempting to drive them away
(Barrett and Humphrey 1986, p. 262). Coachwhips (Coluber flagellum) and
gophersnakes (Pituophis catenifer) have been reported consuming
juvenile Sonoran desert tortoises (Amarello et al. 2004, p. 178; Ernst
and Lovich 2009, p. 563). Presumably, other snake species such as
common kingsnakes (Lampropeltis getula) with generalized prey
preferences consume eggs or hatchling Sonoran desert tortoises, but we
did not find other examples in the literature.
For more detailed information on all aspects of Sonoran desert
tortoise biology, see Barrett and Johnson (1990, pp. 1-95) and Bury and
Germano (1994, pp. 1-212).
Monitoring and Population Status
Monitoring and Statistical Analyses
We are unaware of any structured, long-term monitoring program for
Sonoran desert tortoises in Mexico; therefore, we are unable to assess
the current status or population trends in that part of the range.
Therefore, we discuss only Arizona studies in this section.
Twenty-eight individual Sonoran desert tortoise populations in
Arizona have been studied since the mid-1970s but few populations have
been studied for more than a few years (Averill-Murray 2000, p. 1;
Averill-Murray et al. 2002b, p. 109). Monitoring plots (also referred
to as ``plots'') have varied from 0.2 to 1.5 sq mi (0.3 to 2.4 sq km)
in size (Averill-Murray 2000, p. 4). Beginning in 1987, AGFD and the
U.S. Bureau of Land Management (BLM) have established and maintained 17
plots in Arizona as long-term monitoring plots and have surveyed them
in a somewhat irregular, but repeated fashion. Each plot has been
surveyed between two and nine times during this timeframe, with 11 to
86 person-days (cumulative days spent by researchers working on plots)
spent during each survey (AGFD 2010, p. 1). These long-term monitoring
plots are located in six counties within Arizona, and their locations
were chosen to represent Sonoran desert tortoise distribution in the
State.
General monitoring objectives for the 17 plots are to document
abundance, density, and changes of Sonoran desert tortoise populations
across the State using capture-recapture methods (Averill-Murray 2000,
p. 3). Records of demographic characteristics of each population,
including sex ratios and age/size structure as well as individual
health and signs of disease within each population were also recorded
during monitoring activities (Averill-Murray 2000, p. 3). Monitoring
protocols used from 1987 to 2000 are summarized in Averill-Murray
(2000, pp. 3-7).
The Sonoran desert tortoise is a difficult species to monitor in
the wild because of its slow movement and camouflaged appearance,
especially in the smaller hatchling and juvenile age classes. These
factors can significantly hamper a surveyor's ability to detect them in
the field (Zylstra et al. 2010, p. 1311). In addition, Arizona Upland
subdivision of Sonoran desertscrub (where Sonoran desert tortoise
population densities are the highest) is complex, often with many large
boulders, somewhat dense vegetation, and challenging topographic
relief. Drought and emigration also affect the reliability of data from
Sonoran desert tortoise population monitoring because the tortoises may
be inactive (in their burrows) or have left the population (dispersed).
In these cases the absence of observations might be mistaken as
mortality. Also, Sonoran desert tortoises can occur in low densities
with little surface activity both seasonally and daily (Zylstra et al.
2010, p. 1311). Alone or in combination, these factors, in addition to
a relatively short sampling period for such a long-lived species, make
subtle population trends difficult to distinguish and overall
population trend analysis problematic.
Low detectability may have been responsible for long periods
between recaptures of marked desert tortoises in an 18-year desert
tortoise study from 1980 to 1997 in the San Pedro Valley, Arizona. For
example, a sub-adult Sonoran desert tortoise was captured and marked in
1992, and was not encountered again until 2005, when it was
incidentally observed approximately 14 mi (22.5 km) from its original
point of capture, 8 years after the conclusion of the study (Meyer et
al. 2010, p. 18). Within the entire duration of this study,
approximately 30 percent of 577 marked Sonoran desert tortoises were
never recaptured, with only 15 total carcasses found, indicating
potential emigration, long-term burrow use, or difficulties in
detecting individuals in complex landscapes (Meyer et al. 2010, p. 20).
The amount of time between recaptures of Sonoran desert tortoises can
be significant; durations between recaptures of some individuals in the
San Pedro Valley study were as high as 18 years (Meyer et al. 2010, p.
20).
Several authors have investigated how detectability may bias
results of Mojave desert tortoise monitoring. For example, Anderson et
al. (2001, p. 583) studied the degree to which field observers can meet
the assumptions underlying line-transect sampling to monitor
populations of desert tortoises in Mojave desertscrub. They found that
when all Mojave desert tortoises are not detected along the centerline
of the transect route (which routinely occurs), biases in sampling data
result (Anderson et al. 2001, p. 583). Anderson et al. (2001, p. 593-
596) noted that surveyor numbers and level of experience contribute to
the reliability of line transect methods. Freilich and LaRue (1998, p.
594) experimentally tested the effect of personnel experience on Mojave
desert tortoise survey outcomes in Mojave desertscrub. They found that
observers consistently overestimated the number of desert tortoise
burrows (falsely assigning other animal burrows as those made by desert
tortoises), and found fewer desert tortoises and scat than were
actually placed on test plots. Their results indicated that experience
played a relatively small role in detecting Mojave desert tortoises
(Freilich and LaRue 1998, pp. 593-594). In an effort to increase
detections, some investigators have tested the use of tortoise
detection dogs in Mojave desert tortoise monitoring projects (Cablk and
Heaton 2006, p. 1926; Heaton et al. 2008, pp. 476-477; Nussear et al.
2008, pp. 109-111). Because Sonoran desertscrub is more dense and
complex than Mojave desertscrub, detection is even more difficult in
Sonoran desert tortoise monitoring. Zylstra and Steidl (2009, p. 16)
found that line transect methods are
[[Page 78102]]
not an efficient means with which to monitor Sonoran desert tortoises.
The seasonal timing of surveys and fluctuating influence of
precipitation on Sonoran desert tortoise surface activity also create
problems with monitoring populations and interpreting results. Sonoran
desert tortoises often become inactive, residing in their burrows,
during periods of seasonal or short-term drought. For example, in a
multi-year mark and recapture study of Mojave desert tortoises in
Joshua Tree National Park, Freilich et al. (2000, pp. 1487-1488) found
that in years of below-normal precipitation, desert tortoise home
ranges decreased, individual captures decreased, and the effort
required to find each tortoise nearly doubled; indicating the
significant influence of precipitation on the possible discrepancy
between the number of tortoises that can be observed versus the number
of tortoises that actually occur within a monitoring plot.
In an attempt to improve monitoring protocols to account for such
complicating factors described above, Averill-Murray (2000, pp. 7-13)
critiqued the original protocols used for long-term monitoring plots of
Sonoran desert tortoise populations in Arizona. This work became the
basis for several changes in monitoring protocols, beginning in 2000.
Although line transect methods have not been implemented on Arizona's
Sonoran desert tortoise long-term monitoring plots, the capture-
recapture methods currently used likely violate assumptions about equal
detection probability (all animals having the same probability of being
captured during every sampling occasion) (Zylstra and Steidl 2009, p.
9).
While monitoring of Sonoran desert tortoise populations in Arizona
has been ongoing for several decades, attempts to quantify temporal
trends in abundance have been hampered by the data limitations
discussed above (Zylstra and Steidl 2009, p. 5; Zylstra et al. 2010,
pp. 1311-1317). Effective monitoring is largely dictated by the
objective of the monitoring, whether that objective is to detect
changes in distribution, abundance, density, or survival. In addition,
using existing plot data to establish rangewide trends in Sonoran
desert tortoise populations is generally problematic because the
current set of monitoring plots does not represent a random sample from
the species' entire range in Arizona (Averill-Murray and Klug 2000, p.
25). Despite the history and effort dedicated to monitoring Sonoran
desert tortoise populations in Arizona since 1987, there are
limitations of these data with respect to interpreting rangewide trends
of the Sonoran desert tortoise. Averill-Murray (2000, pp. 12-13)
identified problems with extrapolating the results of the plot
monitoring data to making range-wide assessments outside of the plots.
We elaborate on these problems in our assessment of Boarman and Kristan
(2008) below.
Boarman and Kristan (2008, pp. 3-12) analyzed mark and recapture
data from the 17 Sonoran desert tortoise long-term monitoring plots
throughout Arizona that were surveyed on the average of once every 4
years from 1987 to 2006. Boarman and Kristan (2008, p. ii) concluded
that the Sonoran population of the desert tortoise in Arizona
experienced statistically significant declines, at an annual rate of
3.52 percent over the 20-year period; equating to a cumulative 51
percent decline in overall numbers during this timeframe.
We received several comments from the public in response to our 90-
day finding that addressed the Boarman and Kristan (2008) report (AGFD
2010, pp. 4-6; Carothers et al. 2010, pp. 5, 8-12; Ogden 2009, pp. 3-
12, Smith 2010, pp. 4-5). Commenters criticized the method and manner
with which Boarman and Kristan (2008) used statistical tests, as well
as the conclusions they made. Significant concerns were noted with
respect to the type of statistical tests used by Boarman and Kristan
(2008) because data were extrapolated beyond the statistical tests'
ability to avoid inherent biases (AGFD 2010, p. 4). Problems associated
with the statistical confidence intervals for monitoring plot data used
by Boarman and Kristan (2008) were also identified (Ogden 2009, pp. 2-
3). Also, monitoring plot data used in Boarman and Kristan (2008, p.
20) were not designed to compare population trends among individual
plots (Ogden 2009, p. 2). Carothers et al. (2010, pp. 8-12) identified
numerous additional problems with the statistical analysis provided by
Boarman and Kristan (2008). Collectively, based upon comments received
from the public as well as our internal review, the number and
magnitude of potential problems associated with Boarman and Kristan's
(2008) statistical analysis call into question the validity of their
conclusions. After careful review of the report and the questions
raised by reviewers of the report, we decided that the conclusions
pertaining to overall Sonoran desert tortoise population trends do not
represent the best available information and, therefore, we did not use
the report in this finding. However, other information in the Boarman
and Kristan (2008) report was used in our analysis of the status of and
threats to the Sonoran desert tortoise and is cited in this finding.
For a more detailed analysis of the Boarman and Kristan (2008) report,
see our ``Review of Boarman and Kristan (2008)'' provided at http://
www.regulations.gov (Docket Number FWS-R2-ES-2009-0032).
Survivorship and Population Densities in Arizona
Viable populations in turtles usually require that both juvenile
and adult size classes have high survivorship (Averill-Murray and Klug
2000, p. 70). Data on the recruitment of juveniles into Sonoran desert
tortoise populations, and their survivorship, are generally lacking due
to the difficulty detecting juveniles in the field (AGFD 2010, p. 3).
Data on juvenile and adult survivorship in Sonoran desert tortoises
require long-term, repeated population monitoring, which in turn,
requires long-term, reliable funding sources. Consequently, these data
are conspicuously rare or absent for most Sonoran desert tortoise
monitoring plots making population viability estimates for Sonoran
desert tortoise populations within Arizona problematic at best. As
expected for a long-lived species, survivorship in Sonoran desert
tortoises (using data generated from a few long-term monitoring plots
in Arizona) is generally high for adults but potentially lower for
juveniles and hatchlings (Zylstra and Steidl 2009, p. 7). Where enough
data from long-term monitoring plots or independent studies exist,
survivorship has been calculated for adults in the following plots or
study areas: Sugarloaf Mountain (96-98 percent), Florence Military
Reservation (88-97 percent), Little Shipp Wash (94-97 percent), Granite
Hills (94-97 percent), and Eagletail Mountains (94-97 percent) (AGFD
2010, p. 2; Riedle et al. 2010, p. 165).
Densities of Sonoran desert tortoises among populations vary
considerably. In 2000, the density of Sonoran desert tortoises, as
determined by surveys on long-term monitoring plots and other
monitoring plots during the 1990s, varied from 15 to 150 individuals
per square mile (2.6 sq km) (AIDTT 2000, pp. 5-6; Averill-Murray and
Klug 2000, p. i). In the San Pedro Valley of southern Arizona, the
average density of the Sonoran desert tortoise population was 38
individuals per square mile (Meyer et al. 2010, p. 17). Stager et al.
(2010, p. 37) suspect that Sonoran desert tortoise populations in
Mohave County, Arizona may be naturally lower due to limited burrowing
habitat available to them to survive cold winters and hot summers.
[[Page 78103]]
Periodic, Localized Declines in Arizona Populations
There are no records of actual extirpations of Sonoran desert
tortoises from any of the monitored populations. However, periodic,
localized, and sometimes substantial declines have been documented in
at least five of 17 monitored populations (Hart et al. 1992, p. 60;
Averill-Murray et al. 2002b, p. 124; AGFD 2010, p. 4). Because of their
life history, Sonoran desert tortoise populations may be slow to
rebound from declines (Howland and Rorabaugh 2002, p. 340). The AGFD
(2010, p. 4) suggested that observed declines in certain plots
demonstrate localized, stochastic events and are not indicative of
population trends as a whole across the distribution of the Sonoran
desert tortoise. Sonoran desert tortoise populations are particularly
vulnerable to elevated mortality of adults. Sustaining the adult,
reproductive age class within Sonoran desert tortoise populations is
important because mortality rates of juveniles are high and because it
takes a long time for a Sonoran desert tortoise to reach sexual
maturity (Howland and Rorabaugh 2002, p. 339). The relatively higher
visibility of adult Sonoran desert tortoises leaves them more
vulnerable to human impacts like collecting or shooting, and their
tendency to move longer distances make them more susceptible to road
mortality (Howland and Rorabaugh 2002, p. 340).
The largest population decline noted at any Sonoran desert tortoise
monitoring plot was observed on the Maricopa Mountains plot, where
substantially more tortoise carcasses were found than live tortoises in
successive years from 1987 through 1991 (Hart et al. 1992, p. 54;
Averill-Murray et al. 2002b, p. 124). Regional drought from 1984-1992
was a suspected cause of the die-off of Sonoran desert tortoises in the
Maricopa Mountains (Hart et al. 1992, p. 60; Averill-Murray et al.
2002b, p. 124). However, in 1987, the estimated density of Sonoran
desert tortoises on the Maricopa Mountains plot was
uncharacteristically high at 146 tortoises per square mile (2.6 sq km),
suggesting that the population may have been in the process of
naturally correcting to carrying capacity (the state at which a
population level is commensurate with available resources) (AGFD 2010,
p. 3). Since 1991, the Sonoran desert tortoise population on the
Maricopa Mountains plot has experienced relatively high survivorship
and shown evidence of reproduction. No additional carcasses have been
documented, indicating the population may be stable, if not returning
to the previous 1987 level (AGFD 2010, p. 3).
The AGFD (2010, p. 3) and Hart et al. (1992, p. 120) confirm
Sonoran desert tortoise populations declined from initial population
estimates (as demonstrated by density estimates and relative carcass
numbers) on three additional plots (Hualapai Foothills, San Pedro
Valley, and East Bajada), suspecting that drought conditions may have
played a role in the observed declines on these plots (Ogden 2009, pp.
12-13). An observed decline on the Tortilla Mountains plot in 2001 may
have been an artifact of low surface activity in response to below-
average precipitation, because an increase in carcasses was not
detected (AGFD 2010, p. 3).
For detailed information on monitoring and survey results from the
previous three decades for the Sonoran desert tortoise in Arizona, see
the following reports: Schneider (1981), Shields and Woodman (1987),
Wirt (1988), Woodman and Shields (1988), Holm (1989), Shields et al.
(1990), SWCA (1990a; 1990b; 1990c), Hart et al. (1992), Murray and
Schwalbe (1993; 1997), Woodman et al. (1993; 1994; 1995; 1996; 1998;
1999a; 1999b; 2000; 2001; 2002; 2003; 2004; 2005; 2006; 2007; 2008;
2009), AIDTT (2000, pp. 5-6), Averill-Murray (2000, pp. 3-7), Averill-
Murray and Klug (2000, pp. 3-25), Averill-Murray et al. (2002b, pp.
110-112), Walker and Wood (2002), Young et al. (2002), and Zylstra and
Swann (2009).
It should be noted that an average generation time for a Sonoran
desert tortoise is 12-15 years and that monitoring of Sonoran desert
tortoise populations has only occurred for about 30 years, representing
approximately two generations. Many threats described below have been
potentially acting on Sonoran desert tortoise populations for many
decades, longer than populations have been studied. Below, we discuss
the effects of various threats to individual Sonoran desert tortoises.
However, due to limitations in monitoring data, we are unable to
discern how Sonoran desert tortoise populations may have responded to
these threats over time, or identify any long-term, historical trends
in tortoise populations. We have not observed any extirpations among
monitored populations.
Distinct Population Segment
We consider a species for listing under the Act if available
information indicates such an action might be warranted. ``Species'' is
defined by the Act as including any subspecies of fish or wildlife or
plants, and any distinct population segment (DPS) of any species of
vertebrate fish or wildlife that interbreeds when mature (16 U.S.C.
1532(16)). We, along with the National Marine Fisheries Service (now
the National Oceanic and Atmospheric Administration--Fisheries),
developed the Policy Regarding the Recognition of Distinct Vertebrate
Population Segments (61 FR 4722; February 7, 1996), to help us in
determining what constitutes a DPS. The policy identifies three
elements that are to be considered regarding the status of a possible
DPS. These elements include: (1) The discreteness of the population
segment in relation to the remainder of the taxon (group of similar
biological organisms); (2) the significance of the population segment
to the taxon to which it belongs; and (3) the population segment's
conservation status in relation to the Act's standards for listing
(i.e., whether the population segment, when treated as if it were a
species, is endangered or threatened) (61 FR 4722, February 7, 1996).
The first two elements are used to determine if a population segment
constitutes a valid DPS. If it does, then the third element is used to
consider whether such DPS warrants listing. In this section, we will
consider the first two criteria (discreteness and significance) to
determine if the Sonoran desert tortoise is a valid DPS (i.e., a valid
listable entity). Our policy further recognizes it may be appropriate
to assign different classifications (i.e., threatened or endangered) to
different DPSs of the same vertebrate taxon (61 FR 4722).
Discreteness
Under the DPS policy, a population segment of a vertebrate species
may be considered discrete if it satisfies either one of the following
two conditions:
(1) It is markedly separated from other populations of the same
taxon as a consequence of physical, physiological, ecological, or
behavioral factors. Quantitative measures of genetic or morphological
discontinuity (separation based on genetic or morphological characters)
may provide evidence of this separation.
(2) It is delimited by international governmental boundaries within
which significant differences in control of exploitation, management of
habitat, conservation status, or regulatory mechanisms exist that are
significant in light of section 4(a)(1)(D) of the Act.
Based upon available information, the international boundary
between Mexico and the United States is not considered for delineation
of discreteness because
[[Page 78104]]
the edge of the DPS is not located at the International Border and,
therefore, will not be addressed further.
The Colorado River and R[iacute]o Yaqui are two perennial rivers
that form biogeographical barriers (a natural barrier that prevents the
migration of species) to movement of tortoises between the Mojave and
Sonoran desert tortoise populations, and between the Sonoran and
Sinaloan desert tortoise populations, respectively. The Colorado River,
separating California and Arizona, comprises the northern and western
boundaries of the Sonoran desert tortoise population as identified in
the April 2, 1990, final rule designating the Mojave population of the
desert tortoise (occurring north and west of the Colorado River) as a
threatened species under the Act (55 FR 12178; see final rule for a
summary of previous actions regarding the Mojave population of the
desert tortoise). The eastern boundary is the extent of the range of
the Sonoran desert tortoise where desert habitats end and grassland,
chaparral, and mountain habitats begin, which are areas that do not
contain desert tortoises. The southern boundary of the Sonoran desert
tortoise DPS, as considered in this finding, is the R[iacute]o Yaqui in
southern Sonora, Mexico; south and east of there, desert tortoises are
considered Sinaloan populations. Potential threats to the Sinoloan
desert tortoise are not evaluated as part of this finding.
In view of this biogeographical isolation, significant ecological
divergence has occurred between the Mojave and Sonoran populations of
desert tortoise, largely due to significant differences in geology,
vegetation types, and precipitation cycles where the populations are
distributed. Desert tortoises in the Mojave population are most dense
in the intermountain valleys that have soil types favorable to the
construction of large, deep burrows (Bury et al. 1994, pp. 66-70).
However, Sonoran desert tortoises reach maximum densities in the rocky
bajadas and hillsides of higher slopes, with reduced densities in the
intermountain valleys (Berry 1984, p. 434; AIDTT 2000; p. 4; Van
Devender 2002a, p. 7; Brennan and Holycross 2006, p. 54; Zylstra and
Steidl 2008, p. 747). At the southern end of the DPS, Edwards et al.
(2009, pp. 7-8) suggested that Sinaloan population of desert tortoise
uses Sinaloan thornscrub and tropical deciduous forest habitats. These
different habitat types with differing soils and vegetation communities
are created by higher precipitation levels. However, some level of
gradation may occur in the vegetative transition zone between Plains of
Sonora subdivision of Sonoran desertscrub and Sinaloan thornscrub
habitats of central Sonora such as in the vicinity of the R[iacute]o
Yaqui (Edwards et al. 2009, p. 8).
In addition to habitat differences, morphological differences have
also been documented among the three populations of desert tortoise.
Several morphological differences in carapace size and shape have been
documented between the Mojave, Sonoran, and Sinaloan populations of
desert tortoise: The carapace of the Mojave desert tortoise is the
widest and tallest of the three, the Sinaloan desert tortoise carapace
is the most narrow and least domed, and the carapace of the Sonoran
desert tortoise is intermediate between the two in those dimensions
(Germano 1993, pp. 324-325; AGFD 2001, p. 1). Using eight independent
shell measurements, Weinstein and Berry (1987, pp. 26-28) documented
three distinct phenotypes (physical appearances) in desert tortoise
populations within the United States based on morphometric (body
measurement) analyses: The ``California'' phenotype (Mojave
population), ``Beaver Dam Slope'' phenotype (Mojave form in Arizona),
and the ``Sonoran type'' (Sonoran population). Desert tortoises from
southern Sonora and northern Sinaloa in Mexico were not studied as part
of this effort.
Differences in reproduction strategies between the Sonoran and
Mojave populations of desert tortoises also occur. Mojave desert
tortoises lay up to three clutches of eggs per year with larger clutch
sizes (more eggs), earlier in the year (April to mid-July) (Wallis et
al. 1999, p. 405) while those in the Sonoran population lay one clutch
per year of smaller size, later in the year (June through August)
(Averill-Murray et al. 2002a, p. 141). These differences led Averill-
Murray (2002b, pp. 119-122) to the conclusion that Sonoran desert
tortoises invest all reproductive effort into a single clutch which
hatches at the peak of forage and water availability and abundance
owing to late-summer rainfall. Whereas desert tortoises in the Mojave
population (maturing at smaller body sizes) (Berry et al. 2002a, p.
259) have higher clutch numbers to offset higher mortality from greater
variability in environmental conditions.
The Mojave, Sonoran, and Sinaloan populations of the desert
tortoise have been found to have significantly differentiated genotypes
(genetic characteristics) (Lamb and McLuckie 2002, p. 74; Van Devender
2002a, p. 24). Genetic distances, expressed as percent sequence
divergence (an estimate of percent difference in the genetic code), are
substantial among the three populations of desert tortoise. Divergence
is 5.1-5.6 percent between the Sonoran and Mojave populations, 4.2
percent between the Sonoran and Sinaloan populations, and 5.1 percent
between the Sinaloan and Mojave populations (Lamb and McLuckie 2002,
pp. 74, 77). Considering geographic distribution, genealogical depth,
and a suite of other characteristics, the Mojave, Sonoran, and Sinaloan
populations of desert tortoise are considered to be ecologically
significant units (populations or groups of populations historically
isolated from one another, and thus representing deep phylogenetic
(evolutionary development of species over time) subdivisions within
species) (Lamb and McLuckie 2002, pp. 81-82). According to
mitochondrial DNA markers, the Sonoran and Mojave populations appear to
have diverged some 5 million years ago (Lamb et al. 1989, p. 83; Lamb
and McLuckie 2002, p. 76).
McCord (2002, p. 62) presented three possible causes of the
significant genetic differentiation between Sonoran and Mojave desert
tortoises. First, genetic differentiation between Sonoran and Mojave
desert tortoises may have been the result of differences in rainfall
patterns between the winter-dominated rainfall pattern of the Mojave
Desert and the summer-dominated rainfall pattern of the Sonoran desert.
Second, genetic differentiation between Sonoran and Mojave desert
tortoises may have occurred because the Sonoran desert tortoises may be
represented as a relict population (remnant survivor from the past) of
the tropical deciduous forest-evolved population of the Sinaloan
population (based upon their general absence in valley bottoms due to
heavy flooding during summer rains, a phenomenon generally absent in
the Mojave Desert). Last, genetic differences between Sonoran and
Mojave desert tortoises may have resulted from their mutual competition
with the Bolson tortoise (Gopherus flavomarginatus), another desert
tortoise species which was widely distributed throughout Arizona in the
Pleistocene, but which never occurred in California. The competing
Bolson tortoise population may have acted as a wedge between the
Sonoran and Mojave populations, driving them even farther apart, in a
process known as competitive displacement.
To explore the evolutionary track the three desert tortoise
populations may have taken and the extent of their current genetic
differentiation on the landscape, Edwards et al. (2009, p. 8) collected
genetic samples from desert
[[Page 78105]]
tortoises within three regions of Sonora, Mexico: Twenty-two samples
from near Alamos, Sonora (tropical deciduous forest in extreme southern
Sonora), 19 samples from near Ciudad Obreg[oacute]n (foothill
thornscrub in south-central Sonora, south of the R[iacute]o Yaqui), and
14 samples from two sites north of Hermosillo (Sonoran desertscrub in
central Sonora). When they compared genetic data with previously
collected samples from Arizona, they found a ``continuum of genetic
similarity'' in genetic samples taken from desert tortoises from the
Hermosillo area of Sonora, Mexico, 528 mi (850 km) northwest to the
Kingman, Arizona area when they compared genetic data with previously
collected samples from Arizona (Edwards et al. 2009, p. 8). This
confirms the similar genetic relationships of Sonoran desert tortoises
throughout the DPS. Genetic samples from the Ciudad Obreg[oacute]n
region, southward, showed clear genetic distinction and supported prior
evidence for a third distinct population of desert tortoise, referred
to as the Sinaloan population (Edwards et al. 2009, p. 8). The southern
limits of desert tortoise distribution in northern Sinaloa are likely
influenced by the growth of disease-causing bacteria and fungi present
in the soil of burrows, exacerbated by the hot, humid, and wet
conditions during tropical summer rainy seasons (Van Devender 2002b, p.
43).
Evaluation of Discreteness
Some biological similarities do exist among the three populations
of desert tortoise (Mojave, Sonoran, and Sinaloan). For example, some
overlap in habitat use occurs. It is well known that Sonoran desert
tortoises generally occur on steep, rocky slopes and bajadas in
contrast to the Mojave desert tortoise, which occurs primarily along
the valley bottoms. But to a lesser extent, Sonoran desert tortoises
also use valley bottoms and Mojave desert tortoises also use steep
slopes and mountain bajadas (Gardner and Brodie 2000, p. 51; Averill-
Murray and Averill-Murray 2002, p. 16; Lutz et al. 2005, p. 22;
Grandmaison et al. in press, p. 4; Riedle et al. 2008, p. 418).
However, there are many more numerous and convincing data in the
scientific literature to support the discreteness of the three
recognized populations of Gopherus agassizii, including differences in
their ecology, behavior, morphology, physiology, and genetics
(Weinstein and Berry 1987, pp. 26-28; Germano 1993, pp. 324-325;
Germano et al. 1994, p. 82; AGFD 2001, p. 1; Averill-Murray 2002b, pp.
299-300; Berry et al. 2002a, p. 259; Lamb and McLuckie 2002, pp. 74,
77; McCord 2002, p. 62; Van Devender 2002a, pp. 24-25; Van Devender
2002b, p. 45; Zylstra and Steidl 2008, p. 747; Edwards et al. 2009, p.
8).
We have reviewed the best available commercial and scientific
information and find that the Sonoran population of the desert tortoise
as it occurs east and south of the Colorado River, south to the
R[iacute]o Yaqui, in Sonora, Mexico, is discrete, under the Service's
DPS policy, from the Mojave and Sinaloan desert tortoise populations.
We base this conclusion on ecological (habitat use), physiological
(reproductive characteristics), morphological (shell dimensions), and
behavioral (seasonal activity patterns) differences that are further
supported by analysis of genetic differences that concluded significant
divergence has occurred among the three populations.
Significance
If a population segment is considered discrete under one or more of
the conditions described in the Service's DPS policy, its biological
and ecological significance will be considered in light of
Congressional guidance that the authority to list DPSs be used
``sparingly'' while encouraging the conservation of genetic diversity.
In making this determination, we consider available scientific evidence
of the discrete population segment's importance to the taxon to which
it belongs. Since precise circumstances are likely to vary considerably
from case to case, the DPS policy does not describe all the classes of
information that might be used in determining the biological and
ecological importance of a discrete population. However, the DPS policy
describes four possible classes of information that provide evidence of
a population segment's biological and ecological importance to the
taxon to which it belongs. As specified in the DPS policy (61 FR 4722),
this consideration of the population segment's significance may
include, but is not limited to, the following:
(1) Persistence of the discrete population segment in an ecological
setting unusual or unique to the taxon;
(2) Evidence that loss of the discrete population segment would
result in a significant gap in the range of a taxon;
(3) Evidence that the discrete population segment represents the
only surviving natural occurrence of a taxon that may be more abundant
elsewhere as an introduced population outside its historic range; or
(4) Evidence that the discrete population segment differs markedly
from other populations of the species in its genetic characteristics.
A population segment needs to satisfy only one of these conditions
to be considered significant. Furthermore, other information may be
used as appropriate to provide evidence for significance.
The current range of the Sonoran desert tortoise, as described in
the discussion above pertaining to discreteness, represents several
hundred miles of occupied habitat spanning across an international
border. This population segment is confined by two large perennial
rivers: The Colorado River in its northern periphery (separating the
Mojave and Sonoran populations), and the R[iacute]o Yaqui at its
southern periphery (separating the Sonoran and Sinaloan populations).
These two rivers represent significant biogeographical barriers to
genetic exchange between adjacent population segments and, therefore,
preclude recolonization of this expanse of habitat from adjacent
populations, should the Sonoran population of the desert tortoise
become extirpated. Thus, the loss of the Sonoran desert tortoise would
constitute a significant gap of several hundred miles in the range
between the Mojave and Sinaloan populations of desert tortoises, and
may constitute as much as 40 percent of the total range occupied by
desert tortoises as a whole, rangewide, which affirms its significance
to the entire species.
In addition, our evaluation of discreetness above found extensive
scientific support concluding that the Sonoran desert tortoise differs
significantly in its behavior (reproduction, seasonal activity),
ecology (habitat use and burrow construction), morphology (physical
characteristics), and genetics from either the Sinaloan or the Mojave
populations. Because of these distinctions, the loss of the Sonoran
desert tortoise population would result in the permanent loss of a
unique biological entity and would diminish the natural variation
within the species as a whole.
Evaluation of Significance
We have reviewed the best available commercial and scientific data,
and based on that review, we find that the Sonoran desert tortoise is
significant to the continued existence of the taxon. We base this
conclusion on: (1) The large geographic range of the Sonoran
population, which is significant (approximately 40 percent) to the
taxon as a whole; (2) a gap of several hundred miles that would result
from the loss of the Sonoran population, which would effectively bisect
the species' range; and
[[Page 78106]]
(3) the behavioral, ecological, physical, and genetic distinctions
among the three desert tortoise populations.
Determination of Distinct Population Segment
Based on our review of the best commercial and scientific
information available, the Sonoran population of desert tortoise is
discrete from the Mojave and Sinaloan populations and significant to
the species as a whole. As a result, we have determined that the
Sonoran population of desert tortoise qualifies as a DPS and a listable
entity under the Act.
In the August 23, 2009, 90-day finding (74 FR 44335), we discussed
a local population of Mojave-genotype (genotype: genetic code) desert
tortoises (that also share Mojave phenotype (the physically-expressed
genetic code) and habitat-use characteristics with the Mojave desert
tortoise population) occurring within the delineated Sonoran population
in the Black Mountains area of western Mohave County, Arizona. This
population is isolated from the threatened Mojave DPS that occurs north
and west of the Colorado River. The exact geographic extent of this
Mojave-genotype in Arizona is currently undefined and we expect there
is interbreeding between desert tortoises with the Mojave and Sonoran
genotype along the periphery of this population in the Black Mountains.
Therefore, we include this population of desert tortoises as part of
our status assessment for the Sonoran desert tortoise in this finding.
Distinct Population Segment Five-Factor Analysis
Section 4 of the Act (16 U.S.C. 1533) and implementing regulations
(50 CFR part 424) set forth procedures for adding species to, removing
species from, or reclassifying species on the Federal Lists of
Endangered and Threatened Wildlife and Plants. Under section 4(a)(1) of
the Act, a species may be determined to be endangered or threatened
based on any of the following five factors:
(A) The present or threatened destruction, modification, or
curtailment of its habitat or range;
(B) Overutilization for commercial, recreational, scientific, or
educational purposes;
(C) Disease or predation;
(D) The inadequacy of existing regulatory mechanisms; or
(E) Other natural or manmade factors affecting its continued
existence.
In making this finding, information pertaining to the Sonoran
desert tortoise in relation to the five factors provided in section
4(a)(1) of the Act is discussed below.
In considering what factors might constitute threats to a species,
we must look beyond the exposure of the species to a particular factor
to evaluate whether the species may respond to that factor in a way
that causes actual impacts to the species. If there is exposure to a
factor and the species responds negatively, the factor may be a threat
and, during the status review, we attempt to determine how significant
a threat it is. The threat is significant if it drives, or contributes
to, the risk of extinction of the species such that the species
warrants listing as endangered or threatened as those terms are defined
in the Act. However, the identification of factors that could impact a
species negatively may not be sufficient to compel a finding that the
species warrants listing. The information must include evidence
sufficient to suggest that these factors are operative threats that act
on the species to the point that the species may meet the definition of
endangered or threatened under the Act.
In our review of the best scientific and commercial data available,
we found numerous threats are impacting Sonoran desert tortoises or
their habitat throughout their range. Some of these threats occurred
historically, some are current, and some will continue into the
foreseeable future. As described in detail below, these threats include
nonnative plant species and altered fire regimes, urban and
agricultural development, barriers to dispersal and genetic exchange,
off-highway vehicles, roads and highways, ironwood and mesquite tree
harvest, improper livestock grazing, undocumented human immigration,
illegal collection, effects from field research and manipulation,
predation from feral dogs, human depredation and vandalism, drought,
and climate change. The effect of habitat disturbances on Sonoran
desert tortoises may differ among age classes, but may be most
significant to hatchlings or juveniles (Tracy et al. 2006b, pp. 271-
272).
Factor A. The Present or Threatened Destruction, Modification, or
Curtailment of Its Habitat or Range.
Nonnative Plant Species and Altered Fire Regimes
The most significant modification to Sonoran desert tortoise
habitat is associated with the ongoing invasion of nonnative plants in
Mojave and Sonoran desertscrub habitats, permanently altering these
ecosystems and causing a change in the frequency, duration, intensity,
and magnitude of wildfires in a region that largely evolved in the
absence of invasive nonnative plants. These ecosystem-level changes
cause both direct and indirect effects on the Sonoran desert tortoise
and its habitat.
Much of the available research on the effects of nonnative plant
species invasions and wildfire used in our analysis has focused on
Mojave desertscrub habitats, largely due to the presence of the Mojave
desert tortoise, which is already listed as endangered. However, Brooks
and Matchett (2006, p. 158) suggest that research from the Mojave
Desert is applicable to the Sonoran Desert when stating, ``Both (Mojave
and Sonoran deserts) occur at elevations above the hyperarid
shrublands, are often positioned on the lower slopes of mountain
ranges, and possess moderate woody plant cover.'' Therefore, we used
the information available from research on Mojave Desert habitats in
our assessment of the effects of nonnative plants in the Sonoran
Desert.
Nonnative perennial plants like buffelgrass, fountain grass, and
Lehmann lovegrass were historically introduced to the Sonoran Desert of
Arizona as livestock forage and to prevent soil erosion. For example,
buffelgrass was included in the nonnative plant species recommended for
release by the Tucson Plant Materials Center of the Soil Conservation
Service until at least 1987 (Bahr 1991, p. 156). These nonnative plant
species subsequently became common and widespread in Sonoran
desertscrub in Arizona (Brooks and Pyke 2001, p. 5). They have since
colonized new areas, often taking advantage of disturbed soils, such as
those resulting from construction associated with roadways, power
lines, and railroad tracks (Bahre 1991, p. 155; D'Antonio and Vitousek
1992, p. 65). Construction and maintenance of roads and highways can
also significantly enhance the likelihood of nonnative plant invasions
by increasing nitrogen deposition in the soil, the dispersal potential
of nonnative seeds, and adjacent soil moisture (Brooks 2007, pp. 153-
154). Roadside ditches along highways are particularly important
dispersal corridors for nonnative plant species such as red brome and
buffelgrass (Esque et al. 2002, p. 313).
Mechanisms that allow the spread of nonnative species generally
pertain to ground disturbance, but the plants may also be spread by
other mechanisms. For example, Smith et al. (2000, pp. 79-80), and
Brooks and Esque (2002, p. 337) both found that elevated atmospheric
carbon dioxide levels, predicted as a result of climate change
(discussed in
[[Page 78107]]
Factor E below), are likely to favor nonnative plant species, such as
red brome, over native species in desertscrub habitats. Increases in
atmospheric nitrogen deposition may also be advantageous to nonnative
plant species. Brooks (2003, pp. 344-345) suspected that increasing
human populations will lead to increased levels of atmospheric
pollution and nitrogen deposition and stated, ``Increased levels of
soil nitrogen caused by atmospheric nitrogen deposition may increase
the dominance of invasive alien plants and decrease the diversity of
(native) plant communities in desert regions, as it has in other
ecosystems.'' Sonoran desert tortoise habitat may be particularly
vulnerable to even minor increases in soil nitrogen levels, because the
ratio of increased nitrogen to plant biomass is higher compared with
that of most other ecosystems (Brooks 2003, p. 344). This suggests that
even small changes in nitrogen levels could result in substantial
changes in the plant community that supports Sonoran desert tortoise
habitat.
The prevalence of nonnative grasses in many areas of Sonoran
desertscrub habitats has resulted in high amounts of flammable fuels in
interspaces between native plants that would otherwise be free of
vegetation. This situation serves to promote the ignition and carrying
of wildfire (Brooks 1999, p. 13). In our review of the best scientific
data available, red brome, splitgrass (or Mediterranean grass, Schismus
spp.), and buffelgrass were considered the nonnative plant species that
pose the greatest concern to the Sonoran desert tortoise and its
habitat, because they are thoroughly integrated into some areas of the
desertscrub communities, and serve to promote and carry wildfire (Bahre
1991, p. 155; D'Antonio and Vitousek 1992, pp. 65, 75; Brooks 1999, p.
13; Brooks and Pyke 2001, p. 5; Brooks and Esque 2002, p. 337; Esque et
al. 2002, p. 313; Van Devender 2002a, p. 16; Brooks and Matchett 2006,
p. 148; DeFalco 2007a, p. 1; Zouhar et al. 2008, p. 157; Abella 2010,
p. 1249; AGFD 2010, p. 13). Red brome is known to carry wildfire in
Sonoran desertscrub habitat north of Tucson, natal grass is known to
carry wildfire in desert grassland habitat south of Tucson to Nogales,
Arizona, and buffelgrass is known to carry wildfire in Sonoran
desertscrub and foothills thornscrub south of the international border
to central Sonora (Esque et al. 2002, p. 316). Other nonnative plant
species identified in the literature as present in Sonoran and Mojave
desertscrub communities include Saharan (or Asian) mustard (Brassica
tournefortii), thistles (genera Centaurea and Cirsium), crimson
fountaingrass (Pennisetum setaceum), natal grass (Melinus repens), and
Lehmann lovegrass (Eragrostis lehmanniana) (Brooks 2001, p. 4; Brooks
and Pyke 2001, pp. 3, 5).
We are not aware of any good estimates of the number of acres of
desertscrub that have been invaded by nonnative plant species, but
Thomas and Guertin (2007, Appendices I and II) calculated the number of
records by county for many known invasive, nonnative plants in Arizona
that are harmful to Sonoran desert tortoise habitat. These data
illustrate general locations where certain nonnative species are most
common and describe which nonnative species are the most reported in
each area. Thomas and Guertin (2007, Appendices I and II) reported the
following for Arizona as of 2007 (relative number of reports of
densities being ``extremely high,'' ``high,'' ``moderate,'' and
``occurs,'' all within the distribution of the Sonoran desert
tortoise):
(1) Buffelgrass is the most-reported nonnative plant species in
Arizona, at 16.3 percent of total reports with 6,287 reports (p. 3); it
reaches extremely high densities in Maricopa and Pima Counties, with
high densities in Pinal and Yuma Counties and moderate densities in
Santa Cruz and La Paz Counties, but it also occurs in Yavapai, Gila,
and Cochise Counties (A-I, p. 60);
(2) Schismus spp. is one of the top 20 invasive plant species, at
2.4 percent of total reports, with 919 reports (p. 3); it reaches high
densities in Maricopa, Pinal, and Pima Counties, with moderate
densities in Mohave, Yavapai, Gila, La Paz, and Yuma Counties, but it
also occurs in Santa Cruz County (A-I, p. 69);
(3) Red brome is one of the top 20 invasive plant species, at 3
percent of total reports, with 1,152 reports (p. 3); it reaches high
densities in Yavapai, Gila, Pinal, and Pima Counties, with moderate
densities in Mohave and Maricopa Counties, but it also occurs in La Paz
and Yuma Counties (A-I, p. 24);
(4) Saharan mustard is one of the top 20 invasive plant species, at
3.3 percent of total reports, with 1,261 reports (p. 3); it reaches
high densities in Maricopa, Pinal, Pima, La Paz, and Yuma Counties,
with moderate densities in Mohave, Yavapai, and Gila Counties, but it
also occurs in Cochise County (A-I, p. 21);
(5) Centaurea spp. had a total of 3-318 reports (depending on
species) (p. 9) and reaches high densities in Pima County, with
moderate densities in Mohave, Yavapai, Gila, Pinal, and Cochise
Counties (A-I, pp. 15, 28-30);
(6) Bull thistle (Cirsium vulgare) is one of the top 20 invasive
plant species, at 3.1 percent of total reports, with 1,195 reports (p.
3); it reaches moderate densities in Yavapai and Gila Counties (A-I, p.
35);
(7) Crimson fountaingrass is one of the top 20 invasive plant
species, at 2.6 percent of total reports, with 999 reports (p. 3); it
reaches high densities in Pima County, with moderate densities in
Yavapai, Gila, La Paz, Santa Cruz, and Maricopa Counties (A-I, p. 61);
and
(8) Lehman lovegrass is one of the top 20 invasive plant species,
at 2.5 percent of total reports, with 980 reports (p. 3); it reaches
high densities in Pima and Cochise Counties, with moderate densities in
Yavapai, Gila, Santa Cruz, Maricopa, and Pinal Counties, but also
occurs in La Paz County (A-I, p. 45).
No spatial data were provided for natal grass, but there were 191
observations (Thomas and Guertin 2007, p. 10).
Buffelgrass has widely invaded Arizona and northern Mexico since
its introduction in 1939 (Stevens and Fehmi 2009, p. 379). While
buffelgrass invasions are occurring and are poised to seriously impact
the southwestern United States, the species has already exacted
significant tolls on Sonoran desertscrub communities in Sonora, Mexico,
because its expansion continues to be facilitated through intentional
plantings and cultivation. Consequently, the clearing of Sonoran
desertscrub and Sinaloan thornscrub in Sonora to plant pastures of
buffelgrass for livestock grazing creates a near monoculture (area
covered by a single plant species) that is highly prone to wildfires,
and therefore represents a substantial threat to the Sonoran desert
tortoise in Mexico (Bury et al. 2002, p. 104; Walker and Pavlakovich-
Kochi 2003, p. 14; Van Devender and Reina 2005, pp. 160-161; University
of Arizona 2010, p. 2). Buffelgrass has been planted in Sonora's
desertscrub lands since the 1950s and at least 5.5 million ac (2.2
million ha) of potential Sonoran desert tortoise habitat has already
been converted into a near monoculture of buffelgrass (Stoleson et al.
2005, p. 62). Buffelgrass has become established in both the lower
valley habitats and into the granite boulder-strewn areas of adjacent
foothills, and has altered historical fire regimes, regionally
converting large areas of Sonoran desertscrub into habitat resembling
the African savannah (Bury et al. 2002, p. 104).
In Arizona, the Southern Arizona Buffelgrass Coordination Center
[[Page 78108]]
(SABCC, a coalition of non-profit organizations, Federal, State, and
local governments, conservation organizations, private businesses, and
individual citizens) reports dense stands of buffelgrass on public
reserves, State and local lands, and private property, including
Saguaro National Park, Coronado National Forest, Bureau of Land
Management's (BLM) Ironwood Forest National Monument, neighborhoods of
Tucson, Sahuarita, Marana and Oro Valley, and along roadsides
throughout this region of Arizona (SABCC 2010, p. 1) These areas are
all within the distribution of the Sonoran desert tortoise in Arizona.
Brooks and Minnich (2006, p. 9) stated that southwestern desert
ecosystems likely evolved in a fire regime best described by ``low
intensity, patchy burns and long fire return intervals.'' Wildfire
capable of carrying itself in Sonoran desertscrub is a recent
phenomenon in evolutionary and geological contexts and only became
apparent recently in the Sonoran Desert (Brooks and Pyke 2001, p. 5;
Esque et al. 2002, p. 312; Zouhar et al. 2008, pp. 155, 160). From 1937
to 1986, only 1 percent of all lightning-caused fires in the Rincon
Mountains area of southern Arizona occurred in desertscrub habitat; 5.6
percent occurred in desert grassland habitat (Bahre 1991, p. 126).
While historical wildfires in desertscrub habitat were exceptionally
rare, after successive years of above-average levels of precipitation,
enough native fuels can develop to carry wildfire in desertscrub
communities, such as happened south of Florence, Arizona in 1979 (Bahre
1991, p. 141; Brooks and Esque 2002, p. 336; Brooks and Minnich 2006,
p. 9). While increased precipitation enhances plant growth and
subsequently increases the likelihood for wildfire starts in
desertscrub habitat, drought can have an inverse effect with respect to
certain nonnative plant species. Red brome, for example, is sensitive
to drought conditions and, therefore, might contribute to reduced fuel
loads and decreased fire frequency during long-term drought (Brooks and
Esque 2002, p. 337), which might help to minimize the likelihood of
wildfires in areas where red brome has formed a monoculture. Smith et
al. (2000, p. 79) noted, ``This shift in species composition in favor
of exotic annual grasses, driven by global [climate] change, has the
potential to accelerate the fire cycle, reduce biodiversity and alter
ecosystem function in the deserts of western North America.''
Wildfire ignitions in the Sonoran Desert region historically
resulted from lightning but ignitions are now more common from human
sources such as burning trash, parking vehicles over dry vegetation,
fireworks, discarded cigarettes, and accidental starts from backcountry
recreationists (Esque et al. 2002, p. 313). Human-caused wildfires in
desertscrub habitat are most common near urban developments, major
roadways, and in areas where off-highway vehicle use is unregulated,
while lightning-caused wildfire in desertscrub is typically located in
more remote wilderness areas (Brooks 1999, p. 13). In central Sonora,
ranchers intentionally set fires to maintain the vigor of buffelgrass
for livestock forage (Esque et al. 2002, p. 313).
Numerous wildfires, varying in size, have occurred in recent times
in many areas throughout the Sonoran Desert including the: (1) Pusch
Ridge Fire of 1987 on the southern slopes of the Santa Catalina
Mountains; (2) Skyline (1992) and Rock Peak (1993) fires in the San Tan
Mountains; (3) Mother's Day Fire of 1994 on the eastern slope of the
Rincon Mountains (Esque et al. 2002, p. 323; 2003, p. 104); and (4)
Cave Creek Complex fire of 2005 northeast of Cave Creek, Arizona, which
burned 248,310 ac (100,487 ha) of Sonoran desert tortoise habitat; the
largest wildfire ever recorded in Sonoran desertscrub in the United
States.
The BLM has kept records of wildfire in Sonoran desert tortoise
habitat. From 1990 to 2008, there have been 61 wildfires, affecting
21,977 ac (8,894 ha) in Category I Sonoran desert tortoise habitat; 285
wildfires, affecting 33,364 ac (13,502 ha) in Category II Sonoran
desert tortoise habitat; and 508 wildfires, affecting 109,460 ac (44,
297 ha) in Category III Sonoran desert tortoise habitat (USBLM 2010, p.
9). In total, during the 1990-2008 period, 164,801 ac (66,693 ha) of
categorized and uncategorized Sonoran desert tortoise habitat has
burned on BLM lands (USBLM 2010, p. 9). Combining the known area of
habitat affected by fire on both BLM and other lands, an estimated 1.5
percent of habitat in Arizona has been adversely affected due to
wildfire in recent years; rangewide this is estimated to be 0.8
percent, although total acreage data on wildfires in Mexico are unknown
and the total percentage of affected habitat is likely higher because
of the higher incidence of buffelgrass and lessened capacity to fight
wildfire in Sonora, Mexico. The total area reported as burned is a
relatively small proportion of BLM lands and has not likely been a
significant impact to most Sonoran desert tortoise populations in
Arizona so far. As the invasion of nonnative plants continues to
expand, the high number of fire starts has a greater potential of
creating larger and more destructive wildfires, especially where they
occur in remote, inaccessible areas as a result of lightning strikes.
Indirect effects of wildfires on Sonoran desert tortoises are
variable and can be significant, including habitat changes such as
altered nutrient availability and quality, loss of perennial plant
species that are important as temporary cover from predators, loss of
thermal refugia, altered tortoise behavior, shifts in biotic community,
pronounced desert tortoise emigration from burned habitat, and lower
growth and reproductive output (Esque et al. 2003, p. 107; DeFalco
2006, p. 5; McLuckie et al. 2007, p. 8). While a single fire in an area
may or may not produce long-term reductions in plant cover or biomass,
repeated wildfires in a given area are capable of ecosystem type-
conversion from native desertscrub to nonnative annual grassland, and
render the area unsuitable for desert tortoises (Brooks and Esque 2002,
p. 336). Increased frequency in wildfires caused by nonnative plant
species invasion increases light intensity at ground level and soil
nutrient availability, and reduces competition from native perennial
plants. These changes further promote dominance by nonnative plant
species (Brooks and D'Antonio 2003, p. 29). Wildfire in desertscrub
habitats can reduce native and nonnative seed banks (Brooks and Draper
2006, p. 2). In Mojave desertscrub, the effects of fire are most
pronounced under shrubs, where fire can kill seed banks and reduce
annual grass diversity, due to higher burn intensity (Brooks 2002a, p.
1; 2002b, p. 1088). Microhabitat associated with shrubs in Sonoran
desert tortoise habitat is an important source of temporary shelter and
provides foraging opportunities while tortoises are thermoregulating.
Fires associated with nonnative plant species have already affected
Sonoran desert tortoise populations in Arizona. The AGFD (2010, p. 13)
reported results from an unpublished study after the Edge Complex Fire
of 2005 in the Four Peaks area on the Tonto National Forest, which
indicated higher numbers of Sonoran desert tortoises (or their scat
were observed in unburned versus burned habitat), but they acknowledged
that the study was preliminary and very limited in scope (AGFD 2010, p.
13).
In Sonora, Mexico, 5.5 million ac (2.2 million ha), representing an
estimated 22 percent of Sonoran desert habitat in Mexico, or 11 percent
rangewide, has been planted to bufflegrass. This figure still does not
account for the land area
[[Page 78109]]
where buffelgrass has become naturally established or the 11.9 million
ac (4.8 million ha) (or one-third of the land area of the state of
Sonora) that are suitable for future natural establishment of
buffelgrass (Stoleson et al. 2005, p. 62). Combining the current and
predicted number of acres converted to buffelgrass in Mexico, 34
percent of the Sonoran desert tortoises' habitat is lost or at risk
across its range. In the area of El Batamote, 29 mi (47 km) north of
Hermosillo, Sonora, buffelgrass has invaded Sonoran desert tortoise
habitat in the adjacent foothills, which has led to wildfires that
burned so hot that the soil was scorched and the bedrock cracked (Esque
et al. 2002, p. 321).
In addition to impacts from fire, Franklin and Molina-Freaner (in
press, p. 1) found that these large-scale conversions from desertscrub
to grasslands in Sonora have reduced plant species richness by half,
and reduced tree and shrub cover by 78 percent, vastly affecting the
ability of Sonoran desert habitat to meet the species' thermoregulatory
needs (that is, using vegetation as cover to regulate body
temperature). These changes have resulted in substantial changes in
primary productivity (creation of organic nutrients and the lowest
level of the food chain, the plant community) and vegetation structure
(the physical structure of plant sizes and shapes as a mosaic on the
landscape) which can affect the forage base and habitat suitability for
Sonoran desert tortoises, as well as lessened the feasibility of
restoring native plant communities in Sonora without aggressive land
management (Franklin and Molina-Freaner, in press, p. 1). Dense stands
of buffelgrass have also been shown to physically disrupt tortoise
movements in the closely related Texas tortoises (Gopherus berlandieri)
(Fujii and Forstner 2010, p. 61), so this may also be true for Sonoran
desert tortoises. The grass can become so thick that the tortoises
cannot walk through it, and the grass may be too tall for the tortoises
to walk on top of it.
In addition to damaging Mojave and Sonoran desertscrub habitat,
wildfires can directly injure and kill Sonoran desert tortoises.
Wildfire may kill a desert tortoise by incineration, by elevating body
temperature, by poisoning from smoke inhalation, or by asphyxiation
(Brooks et al. 1999, p. 40; Brooks and Esque 2002, p. 335; McLuckie et
al. 2007, p. 7). Survival rates of Sonoran desert tortoises may be
contingent upon several factors, including soil type, substrate,
vegetation, tortoise activity during fire, whether tortoises are active
and above ground or in shelter during a fire, weather, fire behavior,
and shelter depth (McLuckie et al. 2007, p. 8). The desert tortoise is
most vulnerable to the direct effects of wildfire when they are surface
active and away from primary cover sites such as burrows, caliche
caves, and rock shelters, because these structures reduce direct
exposure to heat and smoke (Brooks and Esque 2002, p. 335). Gravid
(with fertilized eggs) female Sonoran desert tortoises may be more
likely to perish from wildfire than other tortoises because peak
wildfire season in Sonoran desertscrub occurs during the months of May
and June. This is when reproductive females are actively foraging on
spring growth to compensate for energy used in egg development; (Esque
et al. 2002, pp. 323-324; 2003, p. 106).
Sonoran desert tortoises that survive the wildfire itself may
struggle to survive in post-burned Sonoran desertscrub habitat due to:
(1) A reduction in forage and shade structure, such as packrat (Neotoma
sp.) middens and shrubs; and (2) increased visibility to predators
(which may be further increased in intermountain valleys where
temporary shade, predator avoidance, and available forage are
particularly important in long-distance movements in these dispersal
corridors) (Esque et al. 2002, pp. 325-326).
The effects on Sonoran desert tortoises of one particular fire were
studied in some detail. Within Saguaro National Park, the Mother's Day
Fire of 1994 burned 340 ac (138 ha) of Arizona Upland Sonoran
desertscrub habitat that was occupied by Sonoran desert tortoises,
killing an estimated 11 percent of the tortoise population (Esque et
al. 2003, p. 105). To assess how Sonoran desert tortoises used burned
versus unburned habitat following this fire, transmitters were attached
to 12 tortoises, 6 each in burned and unburned habitat within or
adjacent to the Mother's Day Fire footprint. Surprisingly, no
differences were observed in movement or activity patterns between
tortoises in burned and unburned areas, nor were long-term effects of
the fire on surviving tortoises noted over the 6-year study period
(Zylstra and Swann 2009, p. 7). These results indicate that different
tortoise populations may respond differently to wildfires and that
numerous variables and factors are at work.
One of the principal reasons that nonnative plants pose a
significant impact to Sonoran desert tortoise habitat is because few,
if any, reasonable methods currently exist to control the ongoing
invasion of these plants or to remediate areas where they have become
established. Mechanical removal is one option that has been implemented
on a small scale in some areas, but is extremely labor intensive and
not practical for treating large areas. Prescribed fire has been
proposed as an alternative means to control nonnative plant species
invasions, but also carries obvious inherent risks to habitat and to
Sonoran desert tortoises (Brooks 2006, p. 31).
It is also important to note the limitations of Sonoran desert
habitat with respect to post-disturbance (for example, after fires)
regeneration (ability for native vegetation to recover). Desertscrub
regions receive low annual precipitation totals, and the plant
communities have correspondingly low growth rates. Based on the type of
disturbance, recovery time estimates range from 40 years to centuries
(Abella 2010, pp. 1271, 1273). Combined, these factors result in slow,
post-disturbance recovery periods and it may take a long time before
any area becomes suitable for Sonoran desert tortoises to recolonize,
if at all. The presence of nonnative species such as buffelgrass,
cheatgrass, or red brome in disturbed Mojave or Sonoran desertscrub may
further limit post-disturbance recovery, delay recovery, or prevent
recovery altogether (Brown and Minnich 1986, p. 411; Brooks 1999, p.
18).
In our review of the best available information, we have documented
that nonnative plant species pose a significant threat to the Sonoran
desert tortoise and its habitat, both in Arizona and Sonora, by
promoting and carrying wildfire in an ecosystem that evolved in its
absence. Wildfires that are facilitated by nonnative plant species
invasions may have direct and indirect adverse effects on tortoises and
tortoise populations. The threat from nonnative plant species to the
Sonoran desert tortoise occurs throughout the species' range and is
expected to increase over time with the expansion of nonnative plants.
There is currently no viable solution to curbing this continued
expansion across the landscape. This threat also acts synergistically
with other threats discussed in this finding.
Urban Development and Agriculture
Human population growth results in the disturbance or loss of
Sonoran desertscrub or the conversion of land for urban and
agricultural development. Arizona increased its population by 394
percent from 1960 to 2000, and was second only to Nevada as the fastest
growing State during this timeframe (Social Science Data Analysis
Network (SSDAN) 2000, p. 1). Since 1990, Arizona's population has grown
by 44
[[Page 78110]]
percent. From 1960 to 2000, population growth rates in Arizona counties
where the Sonoran desert tortoise occurs have varied by county but are
no less remarkable, and all are increasing: Maricopa (463 percent);
Pima (318 percent); Pinal (54 percent); Santa Cruz (355 percent);
Cochise (214 percent); Yavapai (579 percent); Gila (199 percent);
Graham (238 percent); Yuma (346 percent); LaPaz (142 percent); and
Mohave (2,004 percent) (see SSDAN 2000). The population of Phoenix,
Arizona, grew 67 percent from 1980 to 2000 (Berry et al. 2006, p. 7).
Urban expansion and human population growth trends in Arizona are
expected to continue into the future. Maricopa-Pima-Pinal county areas
of Arizona are expected to grow by as much as 71 percent in the next 15
years, creating rural-urban edge effects across millions of acres of
public lands currently supporting Sonoran desert tortoise populations
(AIDTT 2000, p. 10; BLM files--Lands Livability Initiative). In another
projection, the population in Arizona is expected to more than double
within the next 20 years compared to the 2000 population estimate (U.S.
Census Bureau 2005, p. 1). Many cities and towns within the
distribution of the Sonoran desert tortoise have already experienced
substantial growth during the 8-year time span, 2000-2008: City of
Avondale (118.3 percent); City of Buckeye (392.5 percent); Bullhead
City (20.3 percent), Town of Carefree (30.5 percent); Casa Grande (56
percent); Town of Cave Creek (44.2 percent); City of Chandler (37.5
percent); City of Coolidge (24.9 percent); City of El Mirage (195.6
percent); City of Eloy (22.3 percent); City of Florence (20.3 percent);
Town of Fountain Hills (23.2 percent); City of Gilbert (84.5 percent);
City of Goodyear (203 percent); City of Kingman (32.2 percent); Lake
Havasu City (33.3 percent); City of Litchfield Park (34.2 percent);
City of Mammoth (45 percent); Town of Marana (139.9 percent); City of
Maricopa (2,508 percent); Town of Oro Valley (32.5 percent); Town of
Queen Creek (544.5 percent); Town of Saguarita (507.3 percent); City of
San Luis (58.5 percent); City of Somerton (63.2 percent); City of
Surprise (187.3 percent); City of Tolleson (43.2 percent); and, Town of
Youngtown (62.2 percent) (U.S. Census Bureau 2008, pp. 1-4).
This population growth has spurred a significant increase in
urbanization and development in these areas. Regional development is
predicted to be extreme in certain areas within the distribution of the
Sonoran desert tortoise in Arizona. In particular, a wide swath from
the international border in Nogales, through Tucson, Phoenix, and north
into Yavapai County (called the Sun Corridor ``Megapolitan'') is
predicted to have 8 million people by 2030, an 82.5 percent increase
from 2000 (Gammage et al. 2008, pp. 15, 22-23). If build-out occurs as
expected, it will encompass a significant proportion of the Sonoran
desert tortoise distribution in Arizona, and will in effect permanently
isolate Sonoran desert tortoise populations that occur on either side
of the Interstate 19, Interstate 10, and Interstate 17 corridors.
The land area permanently altered by human activities from urban
development and agriculture has grown to 13 percent of all land in the
western United States, Lue et al. (2008, p. 1130). Lue et al. (2008, p.
1133) concluded that in low-productivity habitat, such as desertscrub
habitats, slight human disturbances can have pronounced effects.
Significant urban development occurs within intermountain valleys,
within or adjacent to occupied Sonoran desert tortoise habitat, which
increases the likelihood of effects along the rural-urban interface,
and may also inhibit movement of individuals between populations on
nearby hillsides or mountain ranges. Disturbances to Sonoran desert
tortoise habitat on the landscape can take many forms and cover extreme
distances. Roads, canals, pipelines, and railroad tracks are examples
of linear habitat destruction. We discuss the potential effects of
linear disturbances below in the section titled, ``Development as a
Barrier.''
Development pressure across Arizona has slowed due to the recent
economic downturn and decline in the housing market. However,
development will likely continue in the future, although perhaps at a
slower pace than in the earlier part of this century. We also recognize
that economic trends are difficult to predict into the future. The most
recent draft Pinal County Comprehensive Plan (February 2009)
acknowledges that the county is in the middle of the Sun Corridor
Megapolitan and proposes four shorter-term growth areas in defining
where development will likely occur, or be encouraged to develop, over
the next decade, but does not discourage growth outside of these areas
(Pinal County Comprehensive Plan 2009, p. 109). These four growth areas
(Gateway/Superstition Vistas, West Pinal, Red Rock, and Tri-
Communities) fall completely within the range of the Sonoran desert
tortoise. The Gateway/Superstition Vistas growth area alone encompasses
176,000 ac (71,225 ha), or 275 sq mi (712 sq km), of State Trust land,
and it is anticipated that 800,000 to more than 1 million people will
one day live in this development (Pinal County Comprehensive Plan 2009,
p. 115). The loss of 176,000 ac (71,225 ha) constitutes a loss of 0.7
percent of Sonoran desert tortoise habitat in Arizona; rangewide, 0.34
percent. The Pinal County Comprehensive Plan (2009, p. 117) identifies
many miles of new freeways and principal arterials in the analysis area
at build-out, which the plan acknowledges may take over a half century
to realize (Pinal County Comprehensive Plan 2009, p. 115). The effect
of roads on Sonoran desert tortoises is discussed below.
Additionally, the Maricopa County Comprehensive Plan calls for
growth areas to the south and east of Chandler and Mesa, Arizona, which
are within the range of the Sonoran desert tortoise (Maricopa County
Comprehensive Plan 2002 (revised), p. 92). City comprehensive plans
within the range of the Sonoran desert tortoise also call for future
growth areas. For example, the City of Eloy has designated six such
areas encompassing 15,520 ac (6,281 ha), mostly along the Interstate 10
corridor (City of Eloy General Plan 2004, pp. 7-6 through 7-10). The
loss of 15,520 ac (6,281 ha) constitutes a loss of 0.06 percent of
their habitat in Arizona; rangewide, 0.03 percent. While much of this
area has already been impacted by development or irrigated agriculture,
any remaining dispersal habitat for the Sonoran desert tortoise will
likely be negatively affected as development and its associated
infrastructure progress into these areas.
Much of the past and projected development within the range of the
Sonoran desert tortoise in central and southwestern Arizona has
occurred and is expected to continue as a conversion from agricultural
uses to municipal uses. Land traditionally used for agriculture is not
occupied by Sonoran desert tortoises, but has a comparatively minor
effect on adjacent Sonoran desert tortoises. When these lands are
converted to municipal uses, the effect to adjacent Sonoran desert
tortoise populations increases human access, and use of adjacent
undeveloped land increases as a result of development of these former
agricultural areas.
The human population of Sonora, Mexico, doubled in size from 1970
(1.1 million) to 2000 (2.2 million) (Stoleson et al. 2005, p. 54). The
population of Sonora is expected to increase by 23 percent, to 2.7
million people, in 2020 (Stoleson et al. 2005, p. 54). In discussing
threats to Sonoran desert tortoise populations adjacent to, and
stemming from, urbanization in Sonora, Mexico, Fritts and Jennings
(1994, p. 53)
[[Page 78111]]
stated, ``Tortoise populations adjacent to large population centers
such as Hermosillo, Guaymas, and Caborca probably have experienced
long-term harm, including direct human exploitation, habitat
degradation, road kills, predation by domestic dogs, and use as pets.
However, we found evidence of tortoise populations on hillsides and
mountain slopes near each of these cities, which suggests that some
tortoise populations have survived despite perturbations by humans.''
Therefore, Sonoran desert tortoises may persist as depressed
populations adjacent to urban development, but without long-term
population trend data for these areas, we are unable to know for how
long.
Urban development has been identified as a concern for Sonoran
desert tortoise conservation in several areas within Arizona because of
the associated increase in human-based threats to populations in close
proximity. Averill-Murray and Swann (2002, p. 1) stated that urban
development adjacent to the Saguaro National Park in Pima County
threatens the Sonoran desert tortoise via several mechanisms including
harassment and predation by feral or off-leash domestic dogs, illegal
releases of captive Sonoran desert tortoises and exotic species that
may transmit diseases to wild Sonoran desert tortoises, elevated
mortality on roads, and illegal collection for pets. Averill-Murray and
Swann (2002, p. 7) stated that mid- to large-scale development projects
on the bajadas and foothills of the Rincon, Santa Rita, Santa Catalina,
Tortolita, and Tucson Mountains has likely led to area-wide decreases
in Sonoran desert tortoise populations. However, no population
estimates for Sonoran desert tortoises before development of these
areas exist, and, therefore, population responses to development of
these areas cannot be ascertained.
In addition to the Tucson metropolitan area, urban encroachment on
Sonoran desert tortoise habitat occurs adjacent to the greater Phoenix
metropolitan area, in the area around South Mountain and adjacent to
the Superstition Mountains (AGFD 2010, p. 7). Sonoran desert tortoises
are known or suspected to still occur in 12 of the 16 Maricopa County
and City of Phoenix urban mountain parks and reserves. The four parks
where no tortoise sign has been found in recent years are completely
surrounded by urban development (AGFD 2010, p. 7). Urban development
has occurred adjacent to five monitoring plots, but only the Hualapai
Foothills plot is completely surrounded by developed lands (AGFD 2010,
p. 7). A development consisting of 48,000 single family homes, south of
the Colorado River in western Mohave County, is also currently being
planned (THS 2009, p. 4; Mardian 2010, p. 1).
Because less area is being used currently for agriculture in the
United States, habitat loss due to agricultural development is more of
a historical issue. However, impacts to Sonoran desert tortoise
dispersal habitat within valley floors from historical agricultural use
and wood harvesting are still evident. The vegetation and soils of many
valleys in the Sonoran Desert were shaped by the periodic flooding of
dynamic wash systems, which partially recharged a shallow, fluctuating
groundwater table. Because of agricultural development, these valleys
no longer experience these defining processes and there has been a
permanent loss of meso- and xeroriparian habitat which are known to be
corridors for movement by Sonoran desert tortoises (Jackson and Comus
1999, pp. 233, 249; Lutz et al. 2005, p. 22; Riedle et al. 2008, p.
418).
Agriculture in Sonora, Mexico, has shifted from small-scale, local
markets toward large-scale agro-industry, with Sonora producing 40
percent of the country's total wheat crop (Stoleson et al. 2005, p.
59). While agriculture in Sonora is largely constrained to valleys
(along the Rio Sonora), many types of habitat used by Sonoran desert
tortoises have been cleared for agriculture, including Sonoran
desertscrub, thornscrub, and tropical deciduous forest (Stoleson et al.
2005, p. 60). In 1994, the total irrigated acreage in Sonora was
128,000 ac; in 2004 that figure rose to 530,509 ac (214,689 ha), an
increase of 314 percent (AQUASTAT 2007, p. 2). This constitutes an
estimated loss of 2 percent of Sonoran desert tortoise habitat in
Mexico; rangewide, 1 percent.
The projected growth of the human population in Arizona and
northern Mexico and subsequent urbanization discussed above is expected
to place onerous demands on lands where the Sonoran desert tortoise
occurs, increasing the need for infrastructure associated with
development, such as power lines, power plants, pipelines, landfills,
roads, sand and gravel mines, and removal of boulders for landscaping
(AIDTT 2000, p. 10). In addition, these growth projections will
increase human visitation to formerly remote Sonoran desert tortoise
habitat as urban-rural interface expands, whereby increasing human-
associated threats discussed in detail below (AIDTT 2000, p. 10). The
AGFD (2010, p. 7) concluded that ``* * * as urbanization continues to
expand, (Sonoran desert tortoise) habitat will continue to be lost.''
In a Global Information System exercise, we calculated that currently,
75 percent of potentially occupied Sonoran desert tortoise habitat
within Arizona occurs within 30 mi (48 km) (a reasonable distance a
person might travel to recreate outdoors on public land) or less of a
city or town with a population of 1,000 or more. As the human
population of Arizona grows and development expands as expected, we
assume that 100 percent of Sonoran desert tortoise populations will
occur within 30 mi (48 km) or less of a city or town with a population
of 1,000 or more, in the foreseeable future. Tortoise populations are
being increasingly exposed to humans and human activities, and
therefore to numerous threats that would otherwise be minimized or
nonexistent. We discuss these types of threats and how they affect
Sonoran desert tortoises and their habitat below in Factors B, C, D,
and E.
Some forms of development are likely to increase. The interest in
renewable energy projects is expected to increase significantly in the
future. Solar radiation levels in the southwestern United States,
including Arizona, are some of the highest in the world, and interest
in tapping into this source of potential energy is growing. Potentially
significant tracts of BLM lands in southwestern Arizona have been
identified for possible solar energy development, encompassing large
percentages of Arizona's valley bottomland in La Paz and Yuma Counties
and adjacent to or within the foothills of the Black Mountains of
western Mohave County, which could isolate Sonoran desert tortoise
populations and affect genetic exchange among regional populations in
those areas (USDOE 2009, p. 1). Since most solar projects are in the
early planning stages and have yet to be officially approved by the
BLM, we are unable to ascertain the amount of Sonoran desert tortoise
habitat likely to be impacted. However, we acknowledge that large areas
within the distribution of the Sonoran desert tortoise in Arizona are
being considered for solar projects.
In one example, 12,100-15,100 ac (4,897-6,110 ha) of BLM, State,
and private land containing Sonoran desert tortoise habitat along the
southern bajada of the Black Mountains in western Mohave County,
Arizona, has been identified for development of the Sterling Solar
Generating Facility within the next 4 to 6 years (Needle Mountain
Power, LLC 2010, pp. 4, 8, 11). At build-out, the Sterling Solar
Generating
[[Page 78112]]
Facility will consist of solar fields, power blocks, buildings,
retention ponds, rainwater catch basins, evaporation ponds, wastewater
and water treatment facilities, water storage tanks, on-site housing, a
substation, a visitors center, a substation and switching station
interconnection with the Western Area Power Administration power lines,
and septic tanks (Needle Mountain Power, LLC 2010, p. 11). We expect
the construction of this facility to render at least 13,100 ac (5,300
ha) of Sonoran desert tortoise habitat as unusable because this type of
construction requires the complete grading (removal of vegetation) of
the project footprint. It could, therefore, significantly affect the
Black Mountains desert tortoise population, especially in consideration
of other effects acting in combination with those poised from the
proposed housing development and highway construction in the immediate
area (THS 2009, p. 4; ADOT 2010, p. 3; Mardian 2010, p. 1). The
estimated loss of 13,100 ac (5,300 ha) constitutes an estimated loss of
0.05 percent of their habitat in Arizona; rangewide, 0.025 percent.
Other solar energy development and transmission corridors pose
similar threats to the Sonoran desert tortoise as development and
roadway projects (see discussion below). An average utility-scale solar
facility to generate 250 megawatts of electricity would occupy about
1,250 ac (500 ha) of land (BLM 2009a, p. 1), and would involve removal
of all vegetation within its footprint. Additionally, concentrating
solar power facilities requires liquids such as oils or molten salts to
create steam to power conventional turbines and generators, as well as
various industrial fluids, such as hydraulic fluids, coolants, and
lubricants, all of which may present a contaminant risk should these
fluids leak onto the ground (Scott 2009, p. 12). New transmission lines
would need to be built for these facilities, as well as roads to
maintain the facilities, posing additional threats to the Sonoran
desert tortoise through the destruction or contamination of remaining
habitat and increased potential for road-kill mortality.
In conclusion, the literature documents that urban development and
population growth in Arizona and Sonora has been remarkable, and no
information is available to suggest these trends will not continue into
the foreseeable future. Sonoran desert tortoise habitat is permanently
lost where urban development occurs. Sonoran desert tortoises and their
habitats that occur adjacent to developed areas are also threatened by
the increased incidence of an array of human activities or influences
such as off-highway vehicle use, facilitation of the spread of
nonnative plant species via soil disturbances, and increased wildfire
ignitions. These threats act in combination with other threats
discussed elsewhere in this finding, including ironwood and mesquite
tree harvest, livestock grazing, nonnative plants and altered fire
regimes, roads and highways, and undocumented human immigration and
interdiction.
Development as a Barrier
Urban development, canals, and transportation infrastructure, such
as roads and railroads, disrupt ecological processes, increase
mortality in animals, promote the degradation, loss, and isolation of
wildlife habitat, and cause fragmentation of populations (Spang et al.
1988, p. 9; Saunders et al. 1991, pp. 23-24; Averill-Murray and Klug
2000, p. 68; Seiler 2001, p. 3; Howland and Rorabaugh 2002, p. 335;
Edwards et al. 2004, p. 496). Sonoran desert tortoise populations are
island-like in their distribution, meaning they are generally
concentrated on the bajadas and hillsides of mountains, and less-
distributed within the valleys between these areas. As a result, they
may be particularly vulnerable to large-scale disturbances that affect
the suitability of intervening habitat (Spang et al. 1988, p. 9).
Factors that affect inter-population dynamics in Sonoran desert
tortoises include distance between populations, physical size of
habitat areas, sizes of source populations, and the ease of which
intervening areas can be crossed by dispersing individuals (Howland and
Rorabaugh 2002, p. 335).
The effect of potential barriers to inter-population movements of
Sonoran desert tortoises (discussed above in the Species Information
section) is not equal across their range. The ability for the Sonoran
desert tortoise to move among populations is also important for
allowing shifts in their range in response to climate change, and to
promote recolonization after fire or other regional disturbances (Beier
and Majka 2006, p. 2). Dispersal of Sonoran desert tortoises between
populations through sparse desertscrub is less likely in very hot, dry
valleys in the Lower Colorado subdivision of Sonoran desertscrub and
populations in mountain ranges, such as the Eagletails, Maricopas, and
Sand Tanks, have likely been existing in isolation for a long time (Van
Devender 2002a, p. 16).
Genetic analysis of blood samples collected from Sonoran desert
tortoises in Saguaro National Park in Pima County, Arizona, suggest
that intermediate gene flow still occurs, or occurred recently, among
isolated populations at the rate of at least 1 migrant per generation
(12-15 years) (Edwards et al. 2004, p. 485). However, thousands of
acres of tortoise habitat have been recently lost to large residential
developments in the foothills of the Santa Catalina, Tortolita, Rincon,
and Tucson Mountains in the greater Tucson metropolitan area (Edwards
et al. 2004, p. 485).
The importance of allowing movement of individual tortoises between
populations is observable by evaluating historical gene flow. Edwards
et al. (2004, p. 485) used seven microsatellite DNA markers to examine
the genetic relationships of tortoises in eight populations in southern
and central Arizona, in the vicinity of Tucson and Phoenix. They also
calculated migration rates among these populations to estimate
historical rates of gene flow, and, therefore, the importance of
individuals moving between populations (Edwards et al. 2004, p. 485).
Edwards et al. (2004, p. 496) found no evidence of recent loss of
genetic diversity that would indicate genetic bottlenecking that could
occur from lack of mixing among Sonoran desert tortoise populations in
southern Arizona. However, the authors acknowledged that a small sample
size and small number of genetic markers (alleles) used in their
analyses would likely not detect this genetic effect. Despite reduced
mixing among populations, Sonoran desert tortoises may be capable of
maintaining small effective population sizes (still viable populations,
despite small size), even with a low degree of genetic diversity
(Edwards et al. 2004, p. 496). However, Edwards et al. (2004, p. 496)
also stated, ``Because effective population sizes of Sonoran desert
tortoises are small, dispersal events probably play an important role
in the long-term maintenance of these populations.'' This suggests that
while dispersal and movement of tortoises may be rare, they may be
important events. Therefore, barriers that prevent this movement could
result in significant genetic impacts, by preventing mixing of
populations over the long term.
The effect of urban barriers limits inter-population movements of
Sonoran desert tortoises resulting in ``closed'' populations. Experts
believe that an isolated population of Sonoran desert tortoises that
experiences significant declines in population size could not overcome
losses simply through an increase in reproduction, based on evidence of
past gene flow (Edwards et al. 2004, p. 496). Therefore, if a
[[Page 78113]]
population were to experience a catastrophic decline as a result of a
stochastic event such as drought, the immigration of new tortoises from
adjacent populations would be necessary for population recovery
(Edwards et al. 2004, p. 496). Urban barriers effectively prevent this
immigration of new tortoises, resulting in closed, or isolated, Sonoran
desert tortoise populations, which are now evident within the
metropolitan areas of Phoenix and Tucson. Mountains and associated
foothills with Sonoran desertscrub habitat occur in these urban areas,
and although development within this habitat has been restricted by
zoning laws, development is still allowed to virtually surround the
bases of the mountains, isolating tortoise populations. Examples of
this development include the Union Hills, White Tank Mountains,
McDowell Mountains, Black Mountains, and South Mountain Park in the
Phoenix metropolitan area and Tumamoc Hill, Tucson Mountains, and
Saguaro National Park West in the Tucson metropolitan area (Edwards et
al. 2004, p. 496). Zylstra and Swann (2009, pp. 10-11) remarked that
the increasing negative effect of human-made barriers on Sonoran desert
tortoise movements between populations may require translocation
(moving animals out of harm's way into more secured areas of suitable
habitat), or occasional augmentation of populations with tortoises from
other populations, to remain viable.
Translocation has been considered an option, and implemented to
some degree for Mojave desert tortoise conservation and recovery. In
assessing the viability of translocation as a recovery and conservation
tool for the Mojave population, concern has been expressed for
potentially moving tortoises into areas where threats to desert
tortoise populations remain, which could negate any conservation value
associated with the action. Our (Mojave) Desert Tortoise Recovery
Office stresses that translocation of tortoises should not occur under
such circumstances, emphasizing the need to address threats which
impact all tortoises regardless of origin.
Translocation of desert tortoises has received mixed reviews in the
scientific literature and, as noted, may not be a viable option for the
Sonoran desert tortoise. There are several factors that must be
considered in deciding whether or not to translocate tortoises into new
areas, including temporary or longer-term holding conditions of
tortoises; the propensity for post-release, long-distance movements;
drought; the status of receiving population; and disease screening,
among other factors (Berry 1986a, p. 113; Field et al. 2007, pp. 232,
237, 240, 242; Martel et al. 2009, p. 218). Translocated Mojave desert
tortoises have been shown to settle at release sites, travel in
straight lines for substantial distances, or disperse up to
approximately 4 miles (6.4 km) (Berry 1986a, p. 113). Translocated
desert tortoises may disrupt social hierarchies in receiving
populations by displacing residents or they may be displaced themselves
(Berry 1986a, p. 113). Howland and Rorabaugh (2002, p. 341) suggest
that translocation of Sonoran desert tortoises may not be a viable tool
for conservation because most intact Sonoran desert tortoise
populations in Arizona are currently considered relatively healthy, and
likely occur at or near carrying capacity. Mullen and Ross (1997, pp.
145-146) found that translocated Mojave desert tortoises have a lower
survivorship than resident individuals (especially when moved during
the summer versus during the spring), but that negative effects
commonly associated with translocations are generally short-lived (1-2
years).
A 2004 population viability analysis for the Mojave desert tortoise
recommended that a minimum of 50,000 individuals are required for a 50
percent chance of persistence for 500 years, yet extrapolation of
Sonoran desert tortoise population data from southern Arizona suggest
that most populations number less than 20,000 individuals, with some as
low as several hundred (Edwards et al. 2004, p. 496). Because the
average generation time of a Sonoran desert tortoise is approximately
12-15 years and much of the urban development is relatively recent, the
full effect of developments as barriers to genetic exchange among
Sonoran desert tortoise populations cannot be fully assessed at this
time (Edwards et al. 2004, p. 486). Edwards et al. (2004, p. 495)
further cautioned that their estimates of gene flow are contingent on
what occurred pre-settlement, and should not be taken as evidence that
natural immigration or emigration still occurs.
In conclusion, the literature documents that urban development and
population growth, roads and highways, canals, railroad tracks, and
other types of development threaten the Sonoran desert tortoise by
creating barriers to movement in Arizona and, perhaps to a lesser
extent, in Sonora, Mexico. The creation of barriers affects the
tortoises' genetic exchange capacity within and between populations,
which in turn affects their ability to recolonize habitat in the event
of population declines or extirpations, and may lead to isolation and
eventual genetic bottlenecking. This threat acts synergistically with
other factors as discussed above.
Off-Highway Vehicles
Off-highway vehicle use may pose a variety of threats to the
suitability of habitat within the range of the Sonoran desert tortoise.
Off-highway vehicle use in Sonoran desert tortoise habitat can result
in damage to soil, riparian areas, wetlands, water quality, and air
quality. This damage occurs due to reduced vegetation cover and growth
rates, soil compaction, diminished water infiltration, diminished
presence and impaired function of soil stabilizers (biotic and abiotic
soil crusts), noise, wildlife habitat fragmentation, spread of invasive
plant species, and accelerated erosion rates (Boarman 2002, pp. 43-51;
Ouren et al. 2007, pp. 5, 11; USGAO 2009, pp. 10, 13; Vega 2010, p. 3).
Off-highway vehicle use in Sonoran desert tortoise habitat can also
potentially affect Sonoran desert tortoises directly by crushing
individuals or their burrows (Boarman 2002, pp. 43-51).
Off-highway vehicle use has grown considerably in Arizona. Between
21 and 56 percent of Arizona residents (depending on the county in
Arizona) consider themselves off-highway vehicle users as of 1999, and
projected increases in population growth are expected to increase
recreation on public lands, in particular off-highway vehicle use
(AIDTT 2000, p. 10). As of 2007, 385,000 off-highway vehicles were
registered in Arizona (a 350 percent increase since 1998), and 1.7
million people (29 percent of the Arizona's public) engaged in off-road
activity from 2005-2007 (Sacco, pers. comm., 2007). Over half of off-
highway vehicle users reported that merely driving off-road was their
primary activity, versus using the off-highway vehicle for the purpose
of hunting, fishing, or hiking (Sacco, pers. comm., 2007). The BLM
(USBLM 2001, p. 1) stated that interest in off-highway vehicle use has
increased substantially in recent years and cited several reasons, such
as urban growth in the west, improved capabilities of off-highway
vehicles in accessing previously inaccessible areas, and greater public
interest in unconfined outdoor recreational opportunities.
The Forest Service stated that ``the number of off-highway vehicle
users has climbed sevenfold in the past 28 years, from approximately 5
million in 1972 to 36 million in 2000'' (USFS 2009, p. 2). The Tonto
National Forest, which encompasses a considerable amount of
[[Page 78114]]
Sonoran desert tortoise habitat, receives the highest off-highway
vehicle use of any national forest nationwide, partially due to its
close proximity to the Phoenix metropolitan area. The Arizona State
Land Department recently closed to off-highway vehicle use many of
their lands in Maricopa County (which includes Phoenix), to control
dust pollution, which appears to have shifted off-highway vehicle
access to the nearby Tonto National Forest (USFS 2009, p. 2; USGAO
2009, p. 11). The Tonto National Forest has indicated that soil erosion
appears to be the most significant result from off-highway vehicle use
on their lands and identified ``unmanaged recreation'' (off-highway
vehicle use) as one of four key threats to soil, water, and wildlife
habitat (USFS 2009, p. 1; USGAO 2009, pp. 10, 13).
Off-highway vehicle use is widespread across Arizona, occurring on
Forest Service, BLM, private, tribal, and State Trust lands, and has
been documented on all 17 Sonoran desert tortoise monitoring plots.
Pronounced effects are found on the Four Peaks and Wickenburg Mountains
plots, which are near urbanized areas (greater Phoenix and Wickenburg,
respectively) (AGFD 2010, p. 13).
The Tonto National Forest has proposed to designate approximately
800 mi (1,287 km) of roads as open for use, and close 280 mi (451 km)
of roads which are currently open (due to significant resource damage).
This is a net increase of 520 mi (837 km) of off-highway vehicle trails
and roads on the Tonto National Forest (USFS 2009, p. 3). In addition,
the Tonto National Forest has proposed the designation of five more
off-highway vehicle areas (representing 2,799 ac (1,132 ha)
collectively, or 0.01 percent of its habitat in Arizona) within Sonoran
desert tortoise habitat on the Mesa and Globe Ranger Districts (USFS
2009, p. 3). All other motorized travel not specifically designated
will be prohibited by the Tonto National Forest except as authorized
for dispersed camping access and big game retrieval (USFS 2009, p. 4).
Because of the increase in off-highway vehicle access and subsequent
use anticipated to occur on the Tonto National Forest, associated
threats to the Sonoran desert tortoise and its habitat on the Forest
are expected to increase in scope and magnitude in the immediate
future.
BLM regulations require their lands be designated as open, limited,
or closed to off-highway vehicle use (USGAO 2009, p. 7). As of March
2009, the BLM has nationally designated approximately 32 percent of its
lands as open to off-highway vehicle use, 48 percent as limited-use, 4
percent as closed, and 16 percent of lands have yet to be designated
(USGAO 2009, p. 7). These figures indicate that at least 80 percent of
BLM lands allow for off-highway vehicle use in some capacity. However,
we do not have specific information for BLM off-highway vehicle use in
Arizona. The BLM is taking actions to help manage off-highway vehicle
use on their lands.
Historically, competitive off-highway vehicle racing events have
occurred on a comparatively infrequent basis in Arizona. On BLM lands
in Arizona, these activities are generally restricted from March 31 to
October 15, in consideration of potential surface activity of Sonoran
desert tortoises (USBLM 2010, p. 4). However, similar considerations
may not occur with respect to these events on lands managed by other
agencies, thus making their lands more desirable for planning such
events. For example, a Special Land Use Permit application was recently
submitted to the Arizona State Land Department for the establishment of
a semiannual competitive off-highway vehicle race within Sonoran desert
tortoise habitat, slightly north of Tucson near Mammoth, Arizona (Vega
2010, pp. 1-16).
Competitive off-highway vehicle events can have a variety of
detrimental effects on Sonoran desert tortoises or their habitat. Event
courses have been found to create new destinations for increased, year-
long use, and correspondingly greater impacts to local Sonoran desert
tortoise habitats and higher incidence of illegal route proliferation
(Vega 2010, p. 3). The high rates of speed associated with competitive
off-highway vehicle events significantly increase the likelihood for
damage to burrows or other habitat features (Vega 2010, p. 4). Lastly,
event spectators seeking good views have been found to park their
vehicles indiscriminately along the race course without regard to
vegetation and may crush Sonoran desert tortoises and their burrows, or
start wildfires if parked over dry vegetation (Vega 2010, p. 5).
In his literature review, Boarman (2002a, p. 50) found that, as of
2002, most research on the effect of off-highway vehicles had been
performed in areas of high off-highway vehicle use within the Mojave
desert tortoise distribution. As a result, there are fewer available
data for lightly-traveled areas (Boarman 2002, p. 50).
On the Florence Military Reservation, Grandmaison et al. (in prep.,
p. 16) found that Sonoran desert tortoises use infrequently traveled
gravel roads as movement corridors within their home ranges, placing
individuals at greater risk of mortality from collisions with off-
highway vehicles. Populations that occur in similar areas throughout
their distribution may also be vulnerable to mortality associated with
collisions, or previously discussed indirect effects to their habitat
from off-highway vehicle use.
Effects of off-highway vehicle use on Sonoran desert tortoises are
likely to be more significant within washes that separate steep slopes
and rocky bajadas used by Sonoran desert tortoises, where tortoises are
known to frequent and off-highway vehicle use often occurs (AGFD 2010,
p. 13). For example, ``rock crawling'' (technical off-roading usually
with highly-modified, high clearance, four-wheel drive vehicles),
generally occurs in boulder-strewn washes where Sonoran desert
tortoises are most likely to inhabit. This activity may be uniquely
destructive to Sonoran desert tortoise habitat because: (1) It occurs
on steep slopes and rocky bajadas within Arizona Upland Sonoran
desertscrub where populations reach their highest densities; and, (2)
the intent of rock crawling is to aggressively challenge aspects of a
given landscape that would otherwise clearly represent barriers to
overland travel, which places habitat and tortoises at greater risk.
However, rock crawling activity is presumed to be less popular an
activity than more conventional off-highway vehicle use and, therefore,
likely affects a much smaller percentage of Sonoran desert tortoise
habitat.
Bury (1987, p. 1) studied the effects of off-highway vehicle use on
Mojave desert tortoises in Mojave desertscrub habitat. Some of his
findings included a 60 percent reduction in perennial plant cover, 1.3
desert tortoises per hectare (2.47 ac) in a control plot in which off-
highway vehicles were excluded, versus 0.3 desert tortoises in an area
used by off-highway vehicles, and four times the number of active
burrows in the control plot versus the off-highway vehicle area (Bury
1987, p. 1). Bury and Luckenbach (2002, p. 257) found that there were
1.3 times more live plants, 3.9 times more plant cover, 3.9 times the
number of Mojave desert tortoises, and four times the number of active
burrows in undisturbed Mojave desertscrub as compared to areas where
off-highway vehicles were used. We are not certain whether the areas
studied by Bury (1987, p. 1) and Bury and Luckenbach (2002, p. 257)
were unregulated, or regulated areas with designated routes, but
similar effects to Sonoran desert tortoises and their habitat can be
expected in areas of high off-road
[[Page 78115]]
vehicle use in Sonoran and Mojave desertscrub habitat within Arizona,
particularly in areas of higher accessibility (such as valley bottoms
and lower foothills), such as the Florence Military Reservation in
Pinal County (AIDTT 2000, p. 34; Lutz et al. 2005; p. 22; AGFD 2010, p.
7; Grandmaison et al. in press, p. 4).
Brooks and Lair (2005, pp. 7-8) found that, in Mojave desertscrub,
off-highway vehicle routes can cause a myriad of effects including: (1)
Altering precipitation runoff patterns which promote increased erosion;
(2) producing air-borne pollutants laden with heavy metals that affect
habitat at distances ranging from 65 to 650 feet (20 to 200 m) from the
road; (3) increasing nitrogen deposition in soils, thereby favoring
nonnative plant invasions; and (4) providing a pathway for nonnative
plant species invasions. These impacts degrade Sonoran desert tortoise
habitat as well as their forage base.
Soil disturbance from off-highway vehicle use, development
projects, and other activities can facilitate the invasion of nonnative
plant species by eliminating competition and creating a rougher soil
surface for seeds to lodge and germinate (Hobbs and Huenneke 1992, pp.
329-330). Motorized and mechanical vehicles aid in the dispersal of
plants by transporting seeds of both native and nonnative plant
species. Rew and Pollnac (2010, p. 2) found that trucks and sport
utility vehicles driven off road in dry conditions can pick up as many
as 176 seeds from 50 mi (80 km) of driving, and recreational off-
highway vehicles can pick up as many as 200,000 seeds in 48 mi (77 km)
of off-road driving. Off-highway vehicles are generally transported via
trailer from site to site and may spread nonnative plant species in
subsequent uses. Off-highway vehicle use has also been shown to create
edge effects along trails that generate dust, blanketing adjacent
vegetation, and inhibiting plant growth rates, size, and survivorship,
all of which affect the forage base and available cover for Sonoran
desert tortoises (Ouren et al. 2007, p. 11).
We have documented that off-highway vehicle use poses a threat to
the Sonoran desert tortoise and its habitat in Arizona because it
damages soil, reduces vegetation cover and growth rates, leads to soil
compaction, diminishes water infiltration, diminishes the presence and
impairs the function of soil stabilizers (biotic and abiotic soil
crusts), fragments habitat, facilitates the spread of nonnative plant
species, ignites wildfire, accelerates soil erosion, enhances the
potential for illegal collection (discussed below), and may crush or
injure Sonoran desert tortoises (also discussed below). In addition, we
have documented the tremendous growth in popularity of off-highway
vehicle use in Arizona, as well as compliance deficiencies in off-
highway vehicle licensing programs (and therefore deficient fees
collected that are intended to fund enforcement and environmental
mitigation) and enforcement programs (discussed above and below). This
threat acts synergistically with other threats discussed herein.
Considering the population growth estimates we have documented above
for Arizona, we believe that the popularity of off-highway vehicle use
will continue to grow, leading to an increase in severity and
geographic extent of impacts across the distribution of the Sonoran
desert tortoise in Arizona over time.
Roads and Highways
Foreman (2002, p. 35) estimated that at least 20 percent of land in
the United States has been ecologically affected by roads. Roads and
highways might also adversely affect Sonoran desert tortoises as they
do Mojave desert tortoises. Studies of Mojave desert tortoises suggest
that effects include providing human access to occupied habitat,
facilitating the spread of nonnative plant species, altering movement
patterns, enhancing the genetic fragmentation effect between
populations of Sonoran desert tortoises by acting as barriers, and
contaminating adjacent habitat (Boarman and Sazaki 1996, p. 1; Forman
and Alexander 1998, p. 207; Boarman 2002, pp. 54-55; Edwards et al.
2004, pp. 495, 497; Boarman and Sazaki 2006, p. 95; Andrews et al.
2008, pp. 127, 129-130; Rew and Pollnac 2010, p. 2). Roads that act as
barriers to genetic exchange between Sonoran desert tortoise
populations may increase the risk of inbreeding depression and
population extirpation (Boarman and Sazaki 2006, p. 95). In one
example, biological connectivity between Sonoran desert tortoise
populations of the Harquahala and Wickenburg Mountains is significantly
limited due to several barriers to tortoise movement including highways
U.S. 60 and U.S. 93, the Burlington Northern Santa Fe Railroad, and
urban development, and would be further limited by the proposed
Wickenburg bypass highways which are in the planning phase (Beier et
al. 2006d, p. vi).
The use of dirt or gravel roads by vehicles generates dust which
may adversely affect physiological processes of adjacent plants and
reduce overall primary productivity, whereby affecting the amount and
quality of available forage vegetation for Sonoran desert tortoises
(Sharifi et al. 1997, pp. 844-845).
Construction of major highways planned in Arizona has the potential
to greatly affect certain Sonoran desert tortoise populations. For
example, the Arizona Department of Transportation (ADOT) has proposed
rerouting State Route 95 through the southern and eastern bajada of the
Black Mountains in Mohave County, Arizona (Jacobs Engineering Group,
Inc. 2009, pp. 24, 33; ADOT 2010, p. 3; Goodman 2010, pp. 3-4). The
proposed realignment of State Route 95 is expected to pass directly
through 30 mi (48 km) of a Sonoran desert tortoise population (THS
2009, p. 4; Goodman 2010, pp. 3-4). We expect this new four-lane
highway to eliminate considerable amounts of Sonoran desert tortoise
habitat, become a significant source of mortality, and threaten the
continued viability of the Black Mountains habitat to support the
population of the Sonoran desert tortoise there, if appropriate
mitigation measures are not enacted or are ineffective.
Both the ADOT and the Federal Highways Administration participate
in the BLM's tortoise mitigation program and provide funding for the
acquisition of Sonoran desert tortoise habitat using compensation rates
prescribed for in the BLM's mitigation policy (ADOT 2010, p. 3).
Compensation rates for disturbances in Category I or II habitat are 3-
6:1 and 2-5:1, respectively (USBLM 2009, p. 18). To date, 584 ac (236
ha) of Sonoran desert tortoise habitat have been acquired through this
program with ADOT and Federal Highways Administration. Another 98 ac
(40 ha) are scheduled to be acquired as a result of the proposed
rerouting of U.S. Highway 95 through the Black Mountains of Mohave
County (ADOT 2010, p. 3).
Considerable planning efforts for future road and highway
development in Arizona have been afforded to the preservation of
wildlife corridors, or ``linkages.'' Linkage design plans have been
completed for several biological corridor areas in Arizona where
Sonoran desert tortoises may be threatened by construction and
development activities that could become barriers to movement between
populations (Beier and Majka 2006, pp. 1-81; Beier et al. 2006a, pp. 1-
189; 2006b, pp. 1-151; 2006c, pp. 1-88; 2006d, pp. 1-97; 2006e, pp. 1-
135). These linkage design plans are specific to both individual
corridors that may be affected throughout Arizona, and to species
(including the Sonoran desert
[[Page 78116]]
tortoise) chosen as representative ``focal species'' in each individual
assessment (Beier and Majka 2006, pp. 1-81; Beier et al. 2006a, pp. 1-
189; 2006b, pp. 1-151; 2006c, pp. 1-88; 2006d, pp. 1-97; 2006e, pp. 1-
135).
In one example, a series of voluntary conservation recommendations
were proposed in Beier et al. (2006c, pp. 15-16; 2006e, pp. 14-15) to
mitigate effects of major roadways, such as U.S. Highway 60 which
traverses Sonoran desert tortoise habitat in Pinal and Gila Counties,
Arizona. However, the Sonoran desert tortoise was not afforded
consideration in all projects. For example, Sonoran desert tortoise
populations in Rincon and Santa Rita mountains in eastern Pima County,
Arizona, are adversely affected by Interstate 10 and State Highway 83
(known barriers to tortoise movement), yet were not addressed in the
Rincon-Santa Rita-Whetstone linkage design plan (Beier et al. 2006a,
pp. i-ii). In another example, the Sonoran desert tortoise was not
afforded any consideration in the Santa Rita-Tumacacori linkage design
plan, despite the likely adverse effects by Interstate 19, a known
barrier to movement between populations located in the Santa Rita and
the Atascosa-Pajarito-Tumacacori mountains complex in southern Santa
Cruz County, Arizona (Beier et al. 2006b, pp. i-ii). While some
highways have associated structures that prevent or funnel tortoises to
underground crossings, several populations are still affected by
barriers to movement from major roads and highways that have no such
structures.
In our review of the literature, we have documented that roads and
highways pose a threat to Sonoran desert tortoises in Arizona because
they form barriers to movement, whether through direct mortality from
vehicles or from avoidance of roads by tortoises. The effects
associated with barriers are described in detail in the ``Development
as a Barrier'' section above. While several roads or highways have
associated tortoise fencing and or culverts to prevent road-kill of
tortoises and facilitate safe movement, studies have shown that these
devices are often not maintained and, therefore, become ineffective
over time in achieving their desired goal. This threat also acts
synergistically with other influences discussed herein.
Ironwood and Mesquite Harvest
The harvest of mesquite and ironwood trees for charcoal production
and use in wood carvings adversely affects Sonoran desertscrub habitat
in Mexico, both historically and more recently (Bahre 1991, pp. 143-
146). The harvest of mature mesquites from Mexico's Sonoran desertscrub
habitat permanently alters desert ecosystems because these leguminous
(bearing seed pods similar to pea or bean plants) trees are important
anchors for these systems and their associated flora and fauna (Taylor
2006, p. 8). More than 200 plant and animal species depend on mesquite
trees in northern Mexico for survival and reproduction (American
University Database 2010, p. 1). Mesquite and ironwood trees are
ecologically important to Sonoran desert habitat as they serve as
nursery plants (i.e., aiding in dispersal, germination, seedling
development, and survival) for other plant species used as forage for
desert tortoises, and provide valuable shade for temporary shelter
sites for Sonoran desert tortoises (American University Database 2010,
p. 2). In areas where harvest has been concentrated, the loss of
mesquite trees results in the loss of organic matter, fixed nitrogen,
and sulfur and soluble salts, affecting overall habitat quality and
quantity (Rodriguez Franco and Maldonado Aguirre 1996, p. 47).
The demand for mesquite wood, used for cooking, has increased in
the Sonoran Desert region of northern Mexico; one million ac (400,000
ha) have been cleared of mesquite to meet these growing demands
(American University Database 2010, p. 1). The modification of one
million ac contributes to the degradation or possible loss of 4 percent
of tortoise habitat in Mexico; rangewide, 2 percent. Ironwood trees are
also being harvested in the Sonoran desert of northern Mexico, where it
is cherished for its hardness and carving potential in Seri Indian
artwork (American University Database 2010, p. 2). The accelerated rate
of legume tree depletion for charcoal and carvings in Sonora has
affected the health of ironwood populations and associated communities
(Suzan et al. 1997, p. 955). This is evidenced by an increased number
of damaged and dying trees, as well as generally small size classes for
sampled areas (Suzan et al. 1997, pp. 950-955). In the Sonoyta region
of northern Sonora, more than 478,000 ac (193,000 ha) have been
affected by deforestation related to charcoal production, brick
foundries, tourist crafts, and pasture conversion (Nabhan and Suzan
1994, p. 64). The modification of 478,000 ac (193,000 ha) contributes
to the degradation or possible loss of an estimated 2 percent of their
habitat in Mexico; rangewide, 1 percent.
Pressure for fuel wood and crafts materials has been so intense in
Mexico south of Organ Pipe Cactus National Monument that wood harvest,
especially ironwood, has been detected more than a third of a mile
inside the boundary of the Monument, as supplies have been decimated
south of the border (Suzan et al. 1999, p. 1499). The structure of
Sonoran desert tortoise habitat in both washes and upland habitats in
the Monument boundary has been affected by this harvest (Suzan et al.
1999, p. 1499).
In conclusion, the literature documents that harvest of ironwood
and mesquite trees has degraded Sonoran desert tortoise habitat in
Mexico, primarily, by the loss of organic matter, fixed nitrogen, and
sulfur and soluble salts, affecting overall habitat quality and
quantity, which collectively and indirectly affect the forage base and
protective cover for Sonoran desert tortoises in as much as 4 percent
of its range in Mexico. This threat acts in combination with other
threats that affect Sonoran desert tortoise populations in Mexico
discussed in this finding.
Livestock Grazing
Sonoran desert tortoises, livestock, and wild burros potentially
share habitat throughout their distribution in Arizona, with the
exception of lands managed by the U.S. Fish and Wildlife Service or
National Park Service. Wild burro herds range across millions of acres
of Sonoran desert tortoise habitat in Arizona, predominantly on BLM
lands northwest of Phoenix, although the literature is generally
lacking in analysis of potential effects of wild burros on Sonoran
desert tortoise populations or habitat (AIDTT 2000, p. 21).
The Mexican government has designated over 5 million ac (2 million
ha) of Sonoran desertscrub for conversion into grasslands for livestock
production (American University Database 2010, p. 1). Sonoran desert
tortoises are not found in grasslands, and this habitat type is not
considered suitable for the species. The loss of 5 million ac (2
million ha) would constitute an estimated loss of 20 percent of their
habitat in Mexico; rangewide, 10 percent. Livestock grazing began to
expand and modernize in its extent and distribution in Sonora, Mexico,
in 1950, when land considered unsuitable for agriculture was
subsequently used for livestock grazing (Hawks 2003, p. 3). During this
time, new bulls were introduced throughout ranching operations to
improve herd genetics, and artificial seeding of
[[Page 78117]]
pastures also commenced at this time (Hawks 2003, p. 3). By 1970,
buffelgrass was the chosen seed for artificial range supplementation
for a growing rural livestock industry, and pastures were seeded with
the species throughout Sonora, Mexico. In Sonora, buffelgrass has
trended towards a monoculture in many areas, and changed the fire
regime to the detriment of native vegetation (Hawks 2003, p. 4). We
discuss the threat of nonnative plant species such as buffelgrass in
the ``Nonnative Plant Species and Altered Fire Regimes'' section above.
Livestock stocking rates in Sonora have been documented at 2-5
times the recommended rate for resource sustainability (Walker and
Pavlakovich-Kochi 2003, p. 14; University of Arizona 2010, p. 2).
Rorabaugh (2008, p. 25) found that livestock grazing ``* * * is
probably the most widespread human use of Sonora's landscapes'' and
that rangelands in Sonora are often heavily grazed, with effects most
apparent during periods of drought. Livestock production in Mexico is
concentrated in the northern states, and the numbers of livestock have
grown from 10 million in 1940, to 37.5 million in 1983, largely due to
the proximity to the United States, the major importer of Mexican
cattle and beef (Stoleson et al. 2005, p. 60). In Sonora, 79 percent of
agricultural and rangelands are devoted to livestock production
(Stoleson et al. 2005, p. 60). Effects of poorly-managed livestock
grazing observed in Sonora include changes in plant species composition
and vegetation cover and structure, soil compaction, erosion, altered
fire regimes, and nonnative plant species introductions and invasions
(Stoleson et al. 2005, pp. 61-62).
In the United States, however, permitted levels of livestock
grazing have been reduced to 10 percent of historical levels (Bostick
1990, p. 149). Potential effects of livestock grazing in desertscrub
habitat received significant treatment in the literature, with varied
scientific conclusions. Fleischner (1994, p. 631) listed specific
attributes of ecosystems, such as composition, function, and structure,
as vulnerable to the effects of livestock management through a variety
of mechanisms including: (1) Decreasing the density and biomass of
individual species, reducing species richness, and changing biological
community organization; (2) interfering with nutrient cycling and
ecological succession; and (3) changing vegetation stratification,
contributing to soil erosion, and decreasing availability of water to
biotic communities (Waser and Price 1981, pp. 409-410). In Mojave
desertscrub, livestock grazing can increase soil compaction and
decrease water absorption, thereby reducing water availability to
potential Sonoran desert tortoise forage species and subsequently
reducing available forage (Boarman 2002, p. 30). Oldemeyer (1994, pp.
100-101) commented that there remains much uncertainty on the exact
effects of livestock grazing on desert tortoises. Meyer et al. (2010,
p. 42) suggested that the effects of livestock grazing on Sonoran
desert tortoises should be placed in the context of a grazing regime,
effective precipitation, habitat type, topography, Sonoran desert
tortoise behavior, and habitat requirements. Loeser et al. (2007, pp.
93-96) suggested that climatic variation is key in determining the
ecological effects of grazing practices in arid rangelands.
The effects of soil compaction on desertscrub vegetation have been
analyzed. In Mojave desertscrub where Sonoran desert tortoises also
occur, Adams et al. (1982, p. 167) found that soil strength of drying
compacted soils increased at a greater rate than non-compacted soils,
and that even minor compaction produced similar effects to soil
strength. Soil strength was found to be inversely proportionate to
production of summer annual grass species (Adams et al. 1982, p. 167).
Plant species with taproots appeared more vulnerable to the effects of
soil compaction whereas fibrous root systems common in nonnative
species such as Schismus spp. appeared less vulnerable, which indicates
that root structure affects the response of plant species and that
plant species respond differently to soil compaction, potentially
favoring nonnative species in compacted soils (Adams et al. 1982, p.
174).
While the Mojave and Sonoran desert tortoises differ to some degree
in their biology and behavior, research on livestock grazing effects on
Mojave desert tortoises or their habitat does have applicability to
Sonoran desert tortoises (especially where Sonoran desert tortoises
occupy Mojave desertscrub habitat and by virtue of the arid-land
commonality), representing the best scientific information available.
However, because Mojave desert tortoises typically occur in flat or
gently-sloped terrain and construct earthen burrows in soil, they may
be more susceptible to direct effects from livestock grazing. In
comparison, Sonoran desert tortoises typically occur on steeper slopes
and often construct burrows that are reinforced by boulders and,
consequently, less susceptible to direct effects from livestock
grazing.
Observed effects of livestock grazing within Mojave desert tortoise
habitat include dietary overlap and competition for food resources,
destruction of vegetation structure used as temporary shelter sites,
trampling of tortoises, collapsing of tortoise burrows, altering plant
species composition by facilitating the invasion of nonnative plant
species, and compaction of soil which may inhibit the construction of
burrows (Avery and Neibergs 1997, p. 13). Boarman (2002a, p. 32) as
well as Hobbs and Huenneke (1992, p. 329) found that livestock grazing
can import nonnative plant propagules (seeds and other plant parts that
may propagate) into native vegetation and subsequent physical
alterations in vegetation structure and soil disturbance, such as
trampling by livestock hoof-action, may increase germination rates of
seeds through burying and compaction and provide microsites for
establishment of nonnative plant species.
Avery and Neibergs (1997, p. 13) compared Mojave desert tortoise
habitat in both grazed and ungrazed areas (where buffelgrass was not
intentionally planted), and found no significant differences in annual
plant cover, biomass, or density between study areas. The densities and
individual volumes of big galleta (Hilaria rigida), a perennial grass
species, were greater in grazed habitat than within the grazing
exclosure (Avery and Neibergs 1997, p. 13). There was no significant
difference in total cover of perennial plant species within study plots
(Avery and Neibergs 1997, p. 13). Avery and Neibergs (1997, p. 13)
documented livestock nudging and rubbing Mojave desert tortoises,
collapsing (potentially occupied) desert tortoise burrows, and
destroying vegetation shading actively used burrows. The number of
damaged and undamaged burrows in grazed habitat was equal, whereas the
number of undamaged burrows in ungrazed habitat was significantly
higher (Avery and Neibergs 1997, p. 18). Winter grazing appears to
affect a higher proportion of actively used Mojave desert tortoise
burrows. Indirect effects from burrow damage include increased risk of
tortoise mortality, increased energy costs, and altered activity time
budgets as a result of the need to construct new burrows (Avery and
Neibergs 1997, p. 19). The potential for livestock to damage Sonoran
desert tortoise burrows on lower slopes not reinforced with granite
boulders may be similar to the findings of Avery and Neibergs (1997, p.
18), as almost 200 Sonoran desert tortoise burrows were recorded as
trampled during a survey of the East Bajada plot in the Black Mountains
of
[[Page 78118]]
Arizona in 1997 (Woodman et al. 1998, pp. 74-75).
Some degree of overlap was observed in the forage plant preferences
between Mojave desert tortoises and livestock, with both preferring
green annual species when available, and most overlap occurring during
the spring (Avery and Neibergs 1997, pp. 18-19). However, preferences
began to diverge as spring and summer ensued, with Mojave desert
tortoises preferring dried annuals, beavertail cactus (Opuntia
basilaris), and stems and dried flowers of silver cholla (Opuntia
echinocarpa), and livestock preferring California jointfir (Ephedra
californica) and big galleta grass (Avery and Neibergs 1997, p. 18). We
presume similar relationships between preferred forage species of
livestock and Sonoran desert tortoises exist, because of their highly
varied, and often opportunistic, foraging behavior as they take
advantage of both summer and winter rainy seasons characteristic of the
Sonoran desert. This precipitation pattern affords Sonoran desert
tortoises greater access to standing water and, therefore, the ability
to forage on a more varied forage base, compared to the Mojave desert
tortoise.
Studies have shown that livestock grazing may result in varying
effects on plant species richness, composition, and density of the
Sonoran desert tortoise forage base. Blydenstein et al. (1957, pp. 523,
525) found that vegetation density in some perennial species can be
affected by livestock grazing in Sonoran desertscrub, while species
composition and annual plant species density were unaffected. Sixteen
years of rest from livestock grazing in the desert grassland and oak
woodlands in southeastern Pima County in Arizona (at the extreme
periphery of the Sonoran desert tortoise range) showed increases in
plant species richness and significant increases in canopy cover for
midgrass, shortgrass, shrubs, and forbs (Brady et al. 1989, pp. 285-
287). However, there was no statistical difference in total vegetation
cover between grazed land and rested land (Brady et al. 1989, pp. 285-
287).
Features that attract livestock to certain locations within an
allotment may have pronounced effects on desert tortoises and their
habitat. Livestock watering, supplemental feeding, or salt-lick sites
in desertscrub attract higher use by greater densities of livestock in
arid environments. Effects to desertscrub habitat are commensurate with
livestock use of these areas and decrease with increasing distance from
these sources (Avery and Neibergs 1997, p. 19; Boarman 2002, p. 34).
The density of certain nonnative plant species, such as Schismus spp.,
has also been positively correlated to distance to watering sites,
while others, such as red brome, are negatively correlated (Brooks et
al. 2006, p. 139). Native plant species cover and richness has been
shown to decrease with increasing proximity to livestock waters (Brooks
et al. 2006, pp. 140-141). Brooks et al. (2006, p. 138) state that
these effects can be anticipated from 164 to 656 ft (50 to 200 m) from
the edge of the watering site. Juvenile and adult Sonoran desert
tortoises were frequently observed by Meyer (1993, pp. 101-102) using
salt licks provided for livestock. Frequenting salt licks may benefit
desert tortoises (especially hatchlings and small juveniles), but
likely increases risk of being trampled by livestock because the salt
licks can attract higher concentrations of both livestock and tortoises
in actively grazed pastures. Based on the results of a study conducted
by Balph and Malecheck (1985, p. 227), cattle avoid stepping on uneven
surfaces. Desert tortoises will likely be perceived as an uneven ground
surface, therefore, cattle may intentionally avoid stepping on them.
Neff et al. (2005, p. 87) compared the effects to soil geology,
geomorphology, and geochemical characteristics of biological soil
crusts that had been disturbed, and the subsequent wind erosion due to
livestock grazing, to an ungrazed area in arid lands of southeastern
Utah. They found that ``* * * despite almost 30 years without livestock
grazing, surface soils in the historically grazed sites have 38-43
percent less silt, as well as 14-51 percent less total elemental soil
magnesium, sodium, phosphorus, and magnesium content relative to soils
never exposed to livestock disturbances'' and 60-70 percent declines in
surface soil carbon and nitrogen reserves (Neff et al. 2005, p. 87). We
are not certain to what extent the loss of these surface soil nutrients
may affect the forage quality or quantity for Sonoran desert tortoises
in arid habitat. Approximately 46 livestock grazing allotments on the
Tonto National Forest partially or wholly overlap the potential range
of the Sonoran desert tortoise, with several rated as having impaired
or unsatisfactory soil conditions (AIDTT 2000, p. 37).
We observed several instances in the literature that discussed an
inherent partitioning of land used by livestock and that used by
Sonoran desert tortoises. Livestock often take the paths of least
resistance and are unlikely to venture great distances from water.
These behavioral traits of domestic livestock limit, to some degree,
the potential effects from livestock grazing in Sonoran desert habitat,
as livestock are less likely to travel into rough, steep terrain,
instead favoring valley bottoms and water sources (AIDTT 2000, pp. 9,
21). Effects from livestock grazing are expected to be attenuated due
to the relatively steep slopes and rugged terrain often preferred by
Sonoran desert tortoises, but quantitative studies have not been
conducted to confirm this assumption (AIDTT 2000, p. 9; Oftedal 2007,
p. 26). Because of the generalized differences in habitat usage by
livestock (flats, ridge tops, and drainage bottoms) and Sonoran desert
tortoises (steep slopes and rocky bajadas), ecological and dietary
overlap is uncommon, but does occur to some degree (AGFD 2010, p. 6).
Where such overlap is significant, in particular in periods of drought,
the effect of livestock use on Sonoran desert tortoise habitat may be
considerable (AGFD 2010, p. 7). Sonoran desert tortoises may also
selectively avoid grazed areas. While Sonoran desert tortoises are
generally known to use steep rocky slopes and bajadas as their primary
habitat areas, they occasionally occur in more flat terrain, such as
the Florence Military Reservation, where they are 35 percent less
likely to use habitat where livestock grazing occurs (AGFD 2010, p. 7).
Grandmaison et al. (in press, p. 2) examined microhabitat selection by
the Sonoran desert tortoise on the Florence Military Reservation in
south-central Arizona, and found that tortoises most strongly selected
for canopy cover, followed by an absence of cattle activity and
proximity to roads and washes.
Of the 17 long-term monitoring plots, evidence of some degree of
habitat usage overlap with livestock has been observed on 12 plots. On
several plots (Arrastra Mountains, Bonanza Wash, West Silverbell
Mountain, and Tortilla Mountains) extensive overlap with livestock use
has been documented in each year they were surveyed (AGFD 2010, p. 7).
Heavy trampling and destruction of Sonoran desert tortoise burrows has
been documented on the Bonanza Wash plot. One Sonoran desert tortoise
was crushed by livestock trampling on the West Silverbell Mountain
plot, although such extreme reports of livestock-related direct effects
on Sonoran desert tortoises are uncommon in the literature (AGFD 2010,
p. 7).
Sonoran desert tortoises might compete with livestock for high-PEP
plants (for review, see discussion of diet in the Species Information
section above) and therefore may place unique competitive pressure on
Sonoran desert tortoise populations (Oftedal 2002, pp. 235-236). Many
high-PEP plant species
[[Page 78119]]
are found primarily in the transition zone between areas where
livestock and Sonoran desert tortoises compete directly for these plant
species, as noted in several Arizona long-term monitoring plots (East
Bajada of the Black Mountains, Hualapai Foothills, Little Shipp Wash,
New Water Mountains, San Pedro Valley), in addition to similar
observations from studies performed at Ragged Top, Saguaro National
Park, and Sugarloaf Mountain (Oftedal 2007, p. 26). However, Oftedal
(2007, p. 25) hypothesized that in situations where winter
precipitation is modest, high-PEP plant species are in low abundance,
and nonnative annual grass species are in high abundance, ``the
immediate effect of grazing (forage competition with Sonoran desert
tortoise) would be [a] reduction of overall forage biomass, not [a]
change in the quality of tortoise diets. This suggests that cattle
grazing may be less damaging to tortoises in years of modest
rainfall.'' In conclusion, Oftedal (2007, p. 26) found that ``the high
degree of diet selection that occurs during spring leaves (Sonoran)
desert tortoises susceptible to influences that may alter the abundance
of the somewhat scarce high-PEP plants, and thus that may reduce the
overall quality of the diet. Tortoises foraging in summer appear less
susceptible to the impacts of livestock grazing.'' Thus, seasonality
and precipitation levels appear to affect the likelihood of grazing to
adversely affect the forage base of Sonoran desert tortoises, with
spring being a period of elevated sensitivity of Sonoran desert
tortoises to livestock grazing where tortoises and livestock spatially
overlap.
Livestock grazing can influence the microclimate at the ground
surface. Grazing may positively affect soil temperature and, therefore,
benefit desert tortoise burrow temperatures where burrows are not
associated with boulders, but instead constructed in more open habitat
such as underneath shrubs (Boarman 2002, p. 31). Field research in
Mojave desertscrub indicates that when the undergrowth beneath shrubs
is grazed, and the shrub itself is minimally browsed or unaffected by
grazing, underlying soils may cool from effects from wind and shade.
Heavily vegetated undergrowth traps heat and increases soil temperature
(Boarman 2002, p. 31). Alternately, heavily browsed shrubs can increase
soil temperatures (Boarman 2002, p. 31). Lower vegetative ground cover
in northern Sonora, as a response to livestock overgrazing, was found
to increase soil and air temperatures above the levels found in
adjacent grazed lands within the United States (Bryant et al. 1990, p.
243). Increased soil temperatures may impact the Sonoran desert
tortoise in a variety of ways, such as influencing changes in behavior,
lowering survivorship, and skewing the sex ratios of hatchlings (which
are determined by incubation temperatures; see Species Information,
above).
Bostick (1990, pp. 150-151) suggested that high desert tortoise
densities are correlated with high livestock use, citing health
examinations of Mojave desert tortoises that existed in grazing
exclosures in northwestern Arizona. Bostick (1990, p. 149) also
asserted that desert tortoises feed ``primarily on dung,'' inferring
that with more livestock, there would be an abundance of available
tortoise forage. Bostick (1990, p. 151) summarized his conclusions on
the relationship between livestock grazing and desert tortoises with
the following: (1) Desert tortoises have coexisted with cattle for 300
years in California and Mexico and at least 100 years elsewhere; (2)
the highest tortoise densities known occurred at a time when
overgrazing by livestock was the most severe ever known; (3) the fewer
the cattle on a range, the fewer the number of tortoises; and, (4)
excluding cattle for many years endangers the tortoise population.
Boarman (2002, pp. 27, 35, 38) refuted the conclusions made by Bostick
(1990, pp. 149-151) that grazing benefits the desert tortoise. In
addition, we found no information in the scientific literature that
supported the findings of Bostick (1990, pp. 149-151).
Some research has examined the effects of various livestock grazing
regimes to Sonoran desert tortoise populations. Meyer et al. (2010, pp.
20-26) compared the number and density of Sonoran desert tortoises in
study plots exposed to four different livestock grazing regimes:
Yearlong light grazing (plot size 2,279 ac (922 ha)), yearlong moderate
grazing (plot size 3,254 ac (1,317 ha)), yearlong heavy grazing (plot
size 4,634 ac (1,875 ha)), and rest-rotation (plot size 4,758 ac (1,925
ha)). They found that the highest number and density of Sonoran desert
tortoises (266 total individuals; 36.89 individuals per square mile)
was observed in the pastures with yearlong heavy grazing as compared to
rest-rotation (215 total individuals; 28.94 individuals per square
mile), yearlong light grazing (52 total individuals; 14.61 individuals
per square mile), and yearlong moderate grazing (47 total individuals;
9.23 individuals per square mile) (Meyer et al. 2010, p. 23). The study
plots used for this comparison between the number and density of
Sonoran desert tortoises and various livestock grazing regimes were of
unequal size, with the yearlong light and moderate plots being the
smallest. This could affect the number of tortoises observed but not
likely the density of tortoises. Other variables that likely affected
the analysis of Sonoran desert tortoise densities were differences in
vegetation, topography, soil types, and the location of tortoise
populations among study plots (Meyer et al. 2010, p. 38). In addition,
the ability to detect Sonoran desert tortoises is likely to increase
with intensity of livestock use and a subsequent decrease in ground
cover, which could have further biased the number of observations in
the yearlong moderate and heavy grazing study plots. Given the results
of these analyses, Meyer et al. (2010, p. 42) surmised that ``tortoise
densities were affected by soil, topography and vegetation and had
little or no relationship to livestock grazing or grazing systems.''
Additional research examined effects of grazing regimes on fire
behavior and wildlife and vegetation communities, citing beneficial
effects. Bahre (1991, p. 141) compared the relative frequency of
wildfires that occurred in the mid-1900s (carried by nonnative plants),
to fires in more recent times, and suggested that mechanical fuel
reduction by livestock grazing might assist in reducing the propensity
of wildfires in Sonoran desertscrub habitat. Loeser et al. (2007, p.
97) found that in Arizona grasslands ``* * * some intermediate level of
cattle grazing may maintain greater levels of native plant diversity
than the alternatives of cattle removal or high-density, short-duration
grazing practices.''
In an unpublished review of livestock grazing literature, Holecheck
(undated, p. 2) found that ``* * * controlled livestock grazing may
enhance rangeland vegetation by accelerating plant succession,
increasing plant diversity, increasing plant productivity, and reducing
plant mortality during drought. These positive impacts of livestock
grazing are most likely to occur when grazing intensities are light to
conservative.'' Holecheck (undated, p. 2) countered the unanimous
findings of over 30 independent livestock grazing impact studies that
documented that controlled grazing increases compaction, reduces
infiltration, and increases erosion by claiming that ``these impacts
are generally of small magnitude and are ameliorated by natural
processes that cause soil formation, soil deposition, and soil
loosening.''
Some local land management organizations are currently working on
proactive conservations efforts to reduce
[[Page 78120]]
potential impacts of ranching and other activities on the Sonoran
desert tortoise. For example, the Winkelman Natural Resource
Conservation District (WNRCD, a coalition of local livestock ranchers
and grazing lease permittees in the Winkelman area of the lower San
Pedro River in Arizona) has prepared a draft conservation plan for the
desert tortoise within their area (WNRCD 2010, pp. 1-13). This draft
plan proposes conservation and land management prescriptions for land
managers in their area as recommended by the Arizona Interagency Desert
Tortoise Team. However, presently the draft plan has not secured
specific agreements with land managers responsible for Sonoran desert
tortoise habitat, and it lacks financial commitments to carry out the
recommended conservation actions. For example, Pinal County was
identified as having responsibilities for conservation actions but has
since indicated that they are unable to participate in the draft plan
(Pinal County 2010, p. 1). While this draft conservation plan could
further Sonoran desert tortoise conservation in this area once all the
necessary management and financial agreements are in place and the plan
is finalized, it currently provides limited conservation benefit to the
Sonoran desert tortoise.
In consideration of the literature presented above, we conclude
that grazing effects to the Sonoran desert tortoise may occur but are
likely limited in severity and scope in Arizona, because habitat shared
by livestock and Sonoran desert tortoises is not a significant
proportion in most areas in Arizona, and because livestock grazing in
Arizona is actively managed by land management agencies (see Factor D).
We also acknowledge that data generated from research on grazing
effects to tortoises and their habitat are variable, making it
difficult to accurately assess the risk of livestock grazing to the
Sonoran desert tortoise. However, due to limited regulations affecting
livestock management in Mexico, and the information we have examined on
its extent in Sonora, we conclude that livestock grazing likely poses a
threat to the Sonoran desert tortoise in Mexico. We also acknowledge
the potential for livestock grazing effects to act synergistically with
other influences discussed herein.
Undocumented Human Immigration
United States border-enforcement efforts have significantly
increased along the United States-Mexico international border in
Arizona in recent years. Sonoran desert tortoise habitat occurs along
approximately 140 mi (225 km) of the border, from approximately Nogales
west to the California State line. International border fencing
structures and barriers (especially the impenetrable pedestrian
fencing) along the Arizona-Sonoran border pose population-connectivity
problems for the Sonoran desert tortoise, which depends on emigration
and immigration for genetic fitness of regional populations. However,
along most of the border, just vehicle barriers occur, which allow
tortoises to pass through them, and do not pose a barrier to movement
(Cohn 2007, p. 96; Flesch et al. 2010, p. 179; Audsley 2010, p. 5;
Sferra 2010, pers. comm.). The two primary types of barrier devices
that have been constructed, or are planned for construction, are
vehicle barriers and pedestrian fences, the latter of which may be
impenetrable to Sonoran desert tortoises where the fence is buried into
the ground (Audsley 2010, p. 5; Sferra 2010, pers. comm.). Where
pedestrian fences are not buried completely and bollard fences
(barriers formed by a series of vertical posts) are installed, Sonoran
desert tortoises less than 4 in (10 cm) in width may be able to get
through (Audsley 2010, p. 5; Sferra 2010, pers. comm.).
Undocumented immigrants affect Sonoran desert tortoise habitat by
trampling vegetation along well-used routes and cutting wood for
campfires, which affects the quality and amount of forage and also
reduces the number of temporary shelter sites for Sonoran desert
tortoises (Averill-Murray and Averill-Murray 2002, p. 29). Other human
activities along the international border (off-road driving, high-speed
driving, accidentally setting fires from cooking or purposefully for
distraction of law enforcement personnel, and interdiction activities
by the U.S. Border Patrol, U.S. Immigration and Customs Enforcement,
and other enforcement agencies) also impact Sonoran desert tortoises
and their habitat (AIDTT 2000, p. 27; Marris 2006, pp. 338-339; Sayre
and Knight 2010, p. 347).
Historically, border enforcement policies and associated structures
have indirectly channeled undocumented immigration pressure onto the
Cabeza Prieta National Wildlife Refuge (Marris 2006, pp. 338-339; Cohn
2007, p. 96). Analysis has shown there are about 8,000 mi (12,875 km)
of unauthorized routes on the approximate 1,000 sq mi (2,600 sq km)
refuge, mostly in designated wilderness (McCasland 2010, pers. comm.).
These routes are most likely attributable to illegal cross-border
traffic and associated law enforcement response by Border Patrol
(McCasland 2010, pers. comm.). Recently, 33.5 mi (54 km) of permanent
vehicle barriers were installed along the international border within
the Cabeza Prieta National Wildlife Refuge, which has likely reduced
illegal vehicular access to the Refuge (SBBI Incorporated 2010, p. 1).
Along the entire southern boundary of the Buenos Aires National
Wildlife Refuge, a 7-mi- (11.3-km-) long pedestrian barrier has been
constructed (USDHS 2007, pp. 4, Figure 2-1). Because pedestrian
barriers on the border are generally well-fortified, complete barriers
to terrestrial movement, we assume that Sonoran desert tortoises in the
larger juvenile and adult size classes are now prevented from making
trans-border dispersal movements as a result of the barrier
construction in this area.
The border region associated with the Tohono O'odham Nation in Pima
County, Arizona, was recently considered to have one of the highest
rates of attempted crossings, because it is relatively remote (Sferra
2010, pers. comm.). Currently, all but 3 mi (4.8 km) of the 70-mi (113-
km) section of border between the Tohono O'odham Nation and Mexico is
reinforced with a vehicle barrier (Lackner 2010b, pers. comm.). Vehicle
barriers are not constructed where terrain is too steep or rocky, or
where vehicular access is considered impossible (Lackner 2010b, pers.
comm.). The lands of the Tohono O'odham Nation are predominantly
classified as Arizona Upland Sonoran desertscrub. The lands presumably
have significant numbers of Sonoran desert tortoises, although survey
data are generally scarce from that area.
Along the Organ Pipe Cactus National Monument border with Mexico,
vehicle barriers exist across most of the monument, and a potentially
impenetrable pedestrian fence has been erected in Arizona Upland
Sonoran desertscrub on Monument Hill and along 4 mi (6.4 km) of the
border at the Lukeville Port of Entry (Sferra 2010, pers. comm.).
The comparison of 2009 and 2010 apprehension rates of undocumented
immigrants reflects both the number of attempted illegal crossings and
the intensity of enforcement activities within various regions of the
Arizona-Mexico border, as well as areas north of the border (Lackner
2010a, pers. comm.). Within Sonoran desert tortoise habitat,
significant increases in apprehension rates have occurred in the
following areas (percentage denotes change from 2009 to June 2010):
Tohono O'odham Nation (18.37 percent); Organ Pipe Cactus National
Monument (63.8 percent), and the Sonoran Desert
[[Page 78121]]
National Monument (70.69 percent) (U.S. Border Patrol 2010, pers.
comm.). In other areas, the apprehension rates have substantially
decreased over the same time period: Ironwood Forest National Monument
(-47.18 percent), Barry M. Goldwater Air Force Range (-32.02 percent),
and the Cabeza Prieta National Wildlife Refuge (-13.19 percent) (U.S.
Border Patrol 2010, pers. comm.). Over the same time period, and in
total, there have been 79,307 apprehensions made, compared to 71,775
apprehensions in 2009, which represents a 10 percent increase (Lackner
2010a, pers. comm.).
New border- and access-road construction has connected previously
remote and undisturbed habitat to the existing network of Arizona
roads, providing vehicular access to areas previously only accessible
by foot or on horseback (Sayre and Knight 2010, pp. 346-347; Sferra
2010, pers. comm.). An unintended consequence of these new roads is
that they are used not only by U.S. Border Patrol, but by the public
and illegal traffic, increasing the risk of wildfires, invasions of
nonnative plant species, alteration of erosion and water movement
patterns (affecting infiltration and soil stability), and mechanical
damage to vegetation (Sayre and Knight 2010, p. 347; Sferra 2010, pers.
comm.). Many new roads along the border have included cattle guards
built with enclosed concrete pits that have the unintended consequence
of acting as lethal pit-fall traps for reptiles, such as smaller size
class Sonoran desert tortoises (Sayre and Knight 2010, p. 347).
Based on our review of the literature and communications with
resource experts and enforcement personnel, we conclude that Sonoran
desert tortoises and their habitat, both near the international border
and within corridors of heavy undocumented immigrant travel and
enforcement interdiction, are threatened by these activities.
Specifically, off-road route proliferation, high-speed driving, road
construction (providing new access to formerly inaccessible areas),
human depredation of tortoises as food sources, and barriers to
tortoise movement created by pedestrian fencing are recognized as
having serious impacts to Sonoran desert tortoise habitat. The
geographic scope of these threats is relatively small on the landscape,
restricted to the immediate border region, and to undocumented
immigrant migration corridors, such as that recognized through the
Tohono O'odham Nation, extending through Ironwood Forest National
Monument. However, these impacts are significant where they occur.
Summary of Factor A
Our analysis under Factor A identified an array of threats to
Sonoran desert tortoise habitat. The documented invasion and purposeful
cultivation of nonnative plant species within the distribution of the
Sonoran desert tortoise in the United States and Mexico significantly
increases the threat of wildfire in an ecosystem that evolved in the
absence of wildfire. This threat is widespread and, although currently
and comparatively less significant in Arizona, is substantial in
Mexico, and is expected to increase in the future. When including the
total land area adversely modified by ironwood and mesquite harvesting,
an estimated 98 percent of the Sonoran desert tortoises' habitat will
be lost or adversely modified in Mexico in the near future, or 47
percent of the Sonoran desert tortoise's habitat rangewide. It is
important to recognize that while nonnative plant species are expanding
their distribution on the landscape, Sonoran desert tortoise
populations have persisted in affected areas that remain unburned, for
decades. The effect of nonnative plants on Sonoran desert tortoise
populations is most significant after a wildfire has occurred;
effectively giving nonnative species a distinct competitive advantage
over native vegetation, and threatening a type-conversion in habitat.
While we have found evidence of numerous wildfires in occupied
desertscrub, the majority of occupied habitat that has been invaded by
nonnative plants has not yet burned and remains suitable habitat for
the tortoise.
In addition, projections for human population growth and urban
development throughout the species' range are likely to both pose
significant problems for genetic exchange among Sonoran desert tortoise
populations as well as increase the degree and scope of human
interactions with tortoises and occupied habitat, which threatens the
tortoise in a variety of ways. Currently in Arizona, 75 percent of
potentially occupied Sonoran desert tortoise habitat occurs within 30
mi or less of a city or town with a population of 1,000 or more, and
considering future growth projections, it is likely that 100 percent of
occupied tortoise habitat will be affected in the future. Livestock
grazing in Mexico poses significant threats to the Sonoran desert
tortoise habitat there due to ineffective livestock management and
continued overgrazing. Lastly, desertscrub habitat that has been
disturbed takes a very long time to recover, on the order of decades or
centuries, which hinders remediation projects with respect to their
ability to prevent population declines in Sonoran desert tortoises in
the short- or medium-term. Each of these impacts results in significant
cumulative threats to the species' habitat and, based upon our review
of the best commercial and scientific data available, we conclude that
the present or threatened destruction, modification, or curtailment of
its habitat or range is an immediate threat of high magnitude to the
Sonoran desert tortoise, both now and in the foreseeable future.
Factor B. Overutilization for Commercial, Recreational, Scientific, or
Educational Purposes
Illegal Collection
In urban areas of Sonora, Mexico, such as Hermosillo, desert
tortoises have become increasingly common as household pets. They have
been mostly obtained from the wild in adjacent areas (Bury et al. 2002,
p. 103). The sale of desert tortoises in Mexican pet stores ended when
the tortoise was listed as threatened in that country in 1994 (Bury et
al. 2002, p. 103).
Sonoran desert tortoises are a closed season species in Arizona
(Commission Order 43), and therefore cannot be legally taken from the
wild or possessed without special license. In Arizona, the current
possession limit for Sonoran desert tortoises legally held in
captivity, i.e., either obtained prior to season closure or obtained
through the tortoise adoption program, is one per person per household
(AGFD 2010, p. 12). The AGFD allows for disposition of lawfully
possessed tortoises by gift to another person in Arizona, or as
directed by the AGFD (AIDTT 2000, p. 14). Despite collection
prohibitions in Arizona, the Sonoran desert tortoise is a very common
reptile pet in Arizona households and has been so for decades. The
actual number of Sonoran desert tortoises in captivity is unclear
because there are no special licenses or permits required to possess
Sonoran desert tortoises, or laws that prohibit their propagation in
captivity (Jarchow et al. 2002, p. 289; Jones 2008, p. 69). Jarchow et
al. (2002, p. 289) state that the number of captive Sonoran desert
tortoises in Arizona is so large that an outright prohibition of their
possession is both impossible and impractical.
The popularity of Sonoran desert tortoises in captivity, as well as
the various adoption programs around the State, may unintentionally
mislead the public into thinking that Sonoran desert tortoises are not
protected, and may, therefore, be collected from the wild
[[Page 78122]]
(Grandmaison in press, p. 6). For example, the area surrounding the
Hualapai Foothills plot experienced increased development in 2001,
which may have increased human-tortoise interactions and possibly
illegal collection. Declines in tortoise encounters at this plot in
2001 and 2005 may have, in part, resulted from illegal collection due
to that plot's proximity to developed land (AGFD 2010, p. 7).
Arizona's regulations have no provisions requiring permits for
possession of Sonoran desert tortoises, which would aid in
identification of those tortoises that were in lawful possession before
January 1, 1988. In addition, there may be incentive created for the
illegal release of captive tortoises into the wild because of the
number of tortoises breeding in captivity, and the difficulty
associated with finding recipients of offspring within the legal 24-
month window (under Arizona's Commission Order 42). This could result
in a higher number of illegal and indiscriminant releases into the wild
(AIDTT 2000, p. 14). Edwards et al. (2010, pp. 801-807) conducted
genetic testing of 180 captive tortoises from Arizona to discern their
genetic origin (as Sonoran, Mojave, or a hybrid). They found that 45
percent of sampled captive tortoises were not of strictly Sonoran
origin, but rather either pure Mojave, Sonoran-Mojave cross, or Texas
tortoise (Gopherus berlandieri)--Sonoran desert tortoise hybrids
(Edwards et al. 2010, p. 804). These data indicate there may be a risk
of genetic contamination of wild populations when captives are
released. Genetic contamination can weaken the genetic fitness of a
population and render it vulnerable to extirpation. In addition, as
documented in Factor C below, captive Sonoran desert tortoises have
been shown to have a higher incidence of disease, and their release can
place wild populations at risk.
Opportunities to collect Sonoran desert tortoises often result from
incidental observations by motorists while using dirt, gravel, or paved
roads. In a recent study, out of a total of 561 opportunities for
motorist-Sonoran desert tortoise interaction, 1.43 percent resulted in
attempted collection of a live decoy, and 7.4 percent attempted the
collection of an artificial Sonoran desert tortoise decoy (Grandmaison
in press, pp. 8-9). Combining the data, Grandmaison (in press, p. 11-
12) found that collection attempts varied with road type and
approximately 1 in 12 (8 percent) motorists that detect a Sonoran
desert tortoise in the wild may attempt to illegally collect it. Adult
tortoises are the most conspicuous and are likely the most-frequently
collected age class, which could be detrimental to populations,
especially when reproductive females are collected. Grandmaison (2010a,
pers. comm.) stated, ``Illegal collection of desert tortoises is a form
of additive mortality resulting from the impacts of roadways in
tortoise habitat. Given that adult tortoises are the most likely
demographic to be collected (i.e., they are easier to detect than
juveniles or hatchlings), and the sensitivity of tortoise population
growth rates to even small increases in adult mortality, illegal
collection really needs to be considered when discussing the cumulative
impacts of roads.''
While the actual collection of Sonoran desert tortoises detected on
roadways is one form of interaction, a higher percentage of motorists
attempt to move Sonoran desert tortoises off the roadway when they are
detected. Grandmaison (2010a, pers. comm.) found that 28 percent of all
motorists passing a desert tortoise will move the tortoise off of the
road. While moving a Sonoran desert tortoise off the roadway may be
considered well-intended, the stress to a Sonoran desert tortoise that
is created when it is handled may result in intestinal torsion (which
can cause intestinal obstructions), or lead to the tortoise voiding its
bladder. As discussed below, bladder voiding has serious implications,
potentially resulting in decreased survival, especially during late
spring and early summer in the Sonoran Desert, when precipitation is
usually rare or non-existent (Grandmaison 2010a, pers. comm.; in press,
p. 11).
Although removal of Sonoran desert tortoises from the wild has
clear negative effects on wild populations, their popularity as
household pets may provide some educational benefits to the public.
Jarchow et al. (2002, p. 310) provided evidence for potential
conservation benefits from Sonoran desert tortoises that are already in
captivity by stating, ``The captive population of desert tortoises
provides not only enjoyment to their custodians but, more importantly,
opportunities for education of the public and increased awareness of
the species among those who may never see a desert tortoise in nature.
Thus, the captive population may play an important role in mustering
public support for conservation of their wild relatives.''
In conclusion, research suggests that about 1 in 12 motorists in
Arizona who detect a Sonoran desert tortoise will attempt to collect
it, and that the highest incidence of collection is within the adult
age class. The removal of an adult Sonoran desert tortoise from a
population poses a higher threat to that population, because the
survivorship of tortoises in this size class is the highest, and the
odds of a given Sonoran desert tortoise reaching this size class is
believed to be comparatively low, further adding importance to the
maintenance of adults within a population. The removal of an adult
female from a population also removes the opportunity for numerous
clutches of eggs. In addition, nearly one-third of all motorists who
encounter a Sonoran desert tortoise will attempt to move it off the
roadway, which increases the risk of bladder-voiding, which may place
additional physiological stress on moved tortoises and may decrease
their survivorship. We also found data on collection and sale of
Sonoran desert tortoises in Mexico, which is likely less of a threat in
current times, due to the prohibition of commercial sale and to the
demographic trend associated with more people moving to urban areas,
reducing the number of wild encounters with tortoises in Mexico.
Field Research and Physical Manipulation
Field research and monitoring of wild Sonoran desert tortoise
populations has been ongoing since the 1970s, producing invaluable
information for wildlife and habitat managers to make reasoned
decisions with respect to conservation planning. However, some level of
harassment or potential harm from disease transmission or dehydration
is inherent to hands-on manipulation (such as collecting blood samples,
affixing radio transmitters, and conducting health assessments).
One of the more significant risks to Sonoran desert tortoises from
the handling of wild tortoises by researchers is the increased
potential for them to void water reserves stored in their bladder. As a
defense mechanism when threatened, Sonoran desert tortoises may
occasionally evacuate their bladders, releasing valuable water stores
important for survival in their arid habitat, especially during drought
years. Averill-Murray (2002a, p. 430) noted, ``This water loss could
result in serious health threats or compromise normal behavior or
physiology, especially during hot, dry summer months.'' Water loss in
Sonoran desert tortoises can also result in reductions of reproductive
output and survivorship (Averill-Murray 2002a, pp. 430, 433-434).
Averill-Murray (2002a, pp. 430, 434) found that Sonoran desert
tortoises that urinated during field research handling had a 5-13
percent lower survival rate.
[[Page 78123]]
Any kind of handling of tortoises during field research or
monitoring of Sonoran desert tortoise populations during periods of
excessive drought may be stressful to the tortoises (Berry et al.
2002b, p. 436). Berry et al. (2006b, p. 436) recommended that
scientists working with wild desert tortoises recognize abnormalities
in behavior and laboratory data as early warning signs of stress to
modify, delay, or terminate specific field protocols on stressed
populations.
Use of radio telemetry technology on desert tortoises may affect
their behavior, survival, and reproductive success, but available
literature is largely inconclusive (Boarman et al. 1998, p. 26). There
is little doubt that radio telemetry studies have provided many
insightful data on the biology and behavior of Sonoran desert
tortoises, and are therefore more of a benefit than a potential threat.
Jacobson et al. (1992, pp. 238-239) reviewed the recommended
procedures for obtaining blood samples from desert tortoises, including
collection from the heart, jugular vein, brachial vein, ventral
coccygeal vein, orbital sinus, and trimmed toenails, and assessed the
potential risks associated with each collection site. At a minimum, the
collection of blood samples from desert tortoises is considered
relatively invasive and is likely a source of temporary stress to the
animal, potentially leading to bladder voiding and subsequent
dehydration if fluid levels are not replenished before release.
However, we believe the majority of field researchers exercise
appropriate caution when collecting blood samples from Sonoran desert
tortoises, and the literature does not indicate these procedures are an
appreciable source of mortality for wild Sonoran desert tortoises.
Over the years, field protocols have been developed and
standardized to minimize risks to Sonoran desert tortoises while they
are being physically handled. These protocols are outlined in Averill-
Murray (2000, p. 17) and Berry and Christopher (2001a, pp. 433-434). We
believe these field protocols have minimized potential risks to
individual tortoises posed by researchers during their field work.
Summary of Factor B
We identified two possible mechanisms for which the potential
overutilization of Sonoran desert tortoises for commercial,
recreational, scientific, or educational purposes could occur: Illegal
collection and field research. Many desert tortoises exist in
captivity, and are generally available to those who want one as a
household pet, through several channels within the captive population
(discussed further in Factor D). In addition, efforts are being made to
educate the public about the Sonoran desert tortoise, with an emphasis
on leaving Sonoran desert tortoises in the wild when they are observed.
We believe these factors may reduce the likelihood of illegal
collection. However, a recent scientific study found that one in 12
tortoises that is detected by a motorist (mostly adult tortoises) is
illegally collected. We expect that in the foreseeable future,
incidence of collection will likely increase as the human population
grows and more people will use off-road trails, with higher frequency,
within occupied tortoise habitat. Scientists who conduct field research
on and monitoring of wild Sonoran desert populations have identified
the potential risk for bladder voiding and disease transmission during
field manipulation of tortoises, and have now built appropriate
protocols in their field methodology to minimize these risks. Based on
this information, we find that overutilization for commercial,
recreational, scientific, or educational purposes, in the form of
illegal collection, is likely to threaten the Sonoran desert tortoise
now or in the foreseeable future.
Factor C. Disease or Predation
Natural predation of Sonoran desert tortoises occurs as discussed
previously in the Species Information section above. Unnatural sources
of predation, such as from feral, or off-leash dogs, human depredation
for recreation or as food, and as an indirect result of human land uses
(referred to as subsidized predation) also occur. A subsidized predator
is one whose survival in a particular area is facilitated by the
availability of food, water, or other potentially limiting resources
made available by the presence of human activities in that area
(Boarman 1993, p. 192). Common examples of subsidized predators are
coyotes and ravens. Human activity-related resources that provide basic
biological needs for subsidized predators include such things as roads,
landfills, sewage and septic ponds, open dumpsters, agricultural
fields, feedlots, parks, picnic areas, livestock waters, utility poles,
building sites, and overpasses (Boarman 1993, p. 193; Rosentstock et
al. 2004, p. 3; Boarman et al. 2006, p. 259; Webb et al. 2009, p. 72).
For example, Averill-Murray and Swann (2002, p. 1) stated that
urban development adjacent to the Saguaro National Park in Pima County
threatens the Sonoran desert tortoise via several mechanisms, including
harassment and predation by feral or off-leash domestic dogs, and
illegal releases of captive Sonoran desert tortoises and exotic species
that may transmit diseases to wild Sonoran desert tortoises.
Predation by Ravens
Ravens and coyotes are known predators on Mojave desert tortoises,
and possibly on Sonoran desert tortoises, and are most likely to
benefit from anthropogenic subsidization (Boarman 1993, p. 192; Boarman
et al. 2006, p. 259). Ravens turn over hatchling desert tortoises and
pierce through their soft plastrons, or pierce directly through their
carapace, to access their meat and organs. Ravens are often less likely
to emigrate long distances to colonize would-be suitable areas, but
subsidization from human activities on the landscape create
opportunities for rapid population growth of ravens where they formerly
did not occur (Boarman et al. 1995, p. 1; Fleischner et al. 2008, p.
472). Ravens, in particular, have been identified as subsidized
predators on juvenile Mojave desert tortoises, and possibly on juvenile
Sonoran desert tortoises (Boarman 1993, p. 192). Roads and power line
rights of way attract potential avian predators of Sonoran desert
tortoises, such as ravens and red-tailed hawks that use power lines as
nesting and perching sites, and roads can serve as sources of carrion
(Knight and Kawashima 1993, p. 266). Raven populations, and potential
risk of predation of Sonoran desert tortoises by ravens, are both
higher with increasing proximity to human development (Kristan and
Boarman 2003, p. 2432).
Documented reports of raven predation on Sonoran desert tortoises
are rare in the literature, however. One local rancher in southeastern
Mohave County, Arizona, reported an observation of raven predation on a
Sonoran desert tortoise (Dieringer 2010, p. 1). Ravens have also been
observed on the Four Peak monitoring plot on several occasions, but
their predation on Sonoran desert tortoises within this plot has never
been documented (Murray and Schwalbe 1997, p. 33). Mojave desert
tortoises are most commonly associated with valley bottomlands
characterized by relatively open, sparse vegetation communities which
may be advantageous to a purely visual-based predator such as the
raven. In Arizona Upland Sonoran desertscrub, where Sonoran desert
tortoises reach their peak population densities, habitat is a more
complex mosaic of boulders and denser vegetation, which would hamper
the ability of such predators to locate
[[Page 78124]]
prey, in particular, small hatchlings. Some exceptions include habitat
within sparsely vegetated valley bottoms that are used for dispersal
between populations on adjacent mountains or foothills, or similar,
uncharacteristic areas that maintain Sonoran desert tortoise
populations, such as the Florence Military Reservation. The best
scientific and commercial data available indicates that predation by
ravens is significantly less of a concern for Sonoran desert tortoises
than it is for Mojave desert tortoises.
In conclusion, although raven predation has been identified as a
substantial threat to the Mojave desert tortoise, largely because of
the relatively open, valley bottomland where they occur, the risk to
Sonoran desert tortoise populations is relatively low. Very few
observations of raven predation of Sonoran desert tortoises in Arizona
or Sonora have been documented in the literature, leading us to
conclude that raven predation on the Sonoran desert tortoise is not a
concern.
Predation From Feral or Off-Leash Dogs
Feral dogs are known to interact with numerous species of animals,
including desert tortoises and related species, and they may force
Sonoran desert tortoises to use their habitat in an unnatural manner
(Causey and Cude 1978, pp. 94-95; Lenth et al. 2008, pp. 222-223). The
risk of feral or off-leash dog predation on Sonoran desert tortoises is
expected to be highest within the urban-rural interface (a likely
source of domesticated, feral dogs).
Jones (2008, p. 66) documented 35 separate incidences of harassment
by wild or domestic dogs in surveys conducted in high-use public lands
adjacent to the urban centers of Tucson, Phoenix, and Kingman, Arizona
(Pima, Maricopa, and Mohave Counties, respectively), based upon
observed shell damage. These incidences were positively correlated with
increasing proximity to urban centers. Also, three to five packs of
presumably feral dogs were observed in both the East Bajada monitoring
plot in Mohave County and in Saguaro National Park West in Pima County
(Jones 2008, p. 66). Researchers of Sonoran desert tortoises within the
Tucson Mountain District of Saguaro National Park noted a high number
of tortoises with injuries consistent with dog attacks, attributing
these observations to the close proximity of this district to urban
development (Zylstra and Swann 2009, pp. 14-15). The AGFD (2010, pp.
11-12) reported that domestic dogs, their scat, and chew marks on, or
trauma to, Sonoran desert tortoises have been reported in 47 percent of
the monitoring plots. Three such plots occur within 1 mi (1.6 km) of
developed areas. Domestic dogs have been observed attacking and chewing
on Sonoran desert tortoises in the Hualapai Foothills and Bonanza Wash
plots (AGFD 2010, p. 12). Domestic dogs appear to be a significant
problem, which may be worsening, in the East Bajada plot, where in
1997, 53 percent of live tortoises, and in 2002, 78 percent of live
tortoises, exhibited injuries associated with domestic dogs (AGFD 2010,
p. 12). One citizen commented that in 1997 a purebred Rottweiler was
observed roaming freely on the Ironwood Forest National Monument with
an adult Sonoran desert tortoise in its jaws. The tortoise was mortally
wounded from a punctured carapace, suggesting that large, powerful
domestic dog breeds may be able to penetrate the carapace of adult
tortoises and kill them (Coping 2009, p. 7).
Numerous signs of attempted predation (consistent with those from
feral dogs), ranging from mild to severe, were observed in wild Sonoran
desert tortoises examined in Sonora, Mexico (Brown et al. 2006, p. 6).
We are unaware of the locations where these wild Sonoran desert
tortoises were captured by Brown et al., but the proximity to human
settlements, and free-ranging domestic dogs (a common sight in Mexico)
may have been responsible.
In conclusion, the threat of feral dog predation exists in both
Arizona and Sonora, Mexico, and has been shown to be strongly
correlated with distance to urbanized areas in most cases. We found
numerous reports of observed or suspected feral dog predation in the
literature, most in immediate proximity to urban areas. Feral dog
predation has been documented in approximately half of the long-term
monitoring plots in Arizona, and may be a significant cause of
population decline in one plot. As urbanization and human population
growth continues into the future, as described in Factor A, the
incidence of feral dog predation of Sonoran desert tortoises is
expected to also increase.
Human Depredation and Vandalism
Human depredation (intentional killing) of Sonoran desert tortoises
has been documented to occur either as a result of vandalism (most
commonly via gunshot) or as a source of food. The intentional shooting
of Mojave desert tortoises in southern California was reported to be
relatively common, at least before the Mojave population was Federally
listed. Berry (1986b, p. 127) found that 14 percent of 635 carcasses
taken from 11 sites in the Mojave Desert over a 6-year time period
exhibited signs of gunshots. Many of these observations occurred before
the listing of the Mojave desert tortoise, indicating that tortoises
may have been shot simply for misdirected recreational sport or
entertainment, not from politically-driven motives (people disliking
the protections of the Act). Bury and Marlow (1973, p. 11) described
examples of Mojave desert tortoise mortalities in California as a
result of shooting, including eight independent observations of shot
Mojave desert tortoises along two miles (3.2 km) of dirt road; an
individual's confession of using juvenile desert tortoises as skeet
(aerial shotgun) targets; and a report of an individual lining up a
total of 47 desert tortoises and shooting each of them with a shotgun.
Recreational firearms target practice occurs in dispersed fashion
throughout Federal and State lands in Arizona within the distribution
of Sonoran desert tortoises. Some reports of gunshot deaths of Sonoran
desert tortoises on these lands have been made (Hart et al. 1992, p.
120; AGFD 2010, p. 9; Jones 2010, pers. comm.). In some locations,
recreational firearms target practice is highly conspicuous (as
evidenced by large amounts of debris used as targets and left behind)
in densely occupied Sonoran desert tortoise habitat, most notably in
areas near urban population centers, such as at Sugarloaf Mountain on
the Cave Creek Ranger District of the Tonto National Forest. In this
location, two incidences of shot Sonoran desert tortoises have been
reported, although it could not be determined whether these wounds
occurred pre- or post-mortem (Jones 2010, pers. comm.). Another
incidence of shooting was reported in the Hualapai Foothills monitoring
plot (Hart et al. 1992, p. 120). The AGFD (2010, p. 9) reported 13
separate incidences of vandalism on Sonoran desert tortoises on or
adjacent to 7 different monitoring plots; several of the Sonoran desert
tortoises appeared to have been killed by gunshot.
When studying Mojave desert tortoises, Berry (1986b, p. 129) found
that the incidence of gunshot deaths is likely to be higher in areas of
greater vehicular access and in proximity to urban areas. The potential
effect of gunshot deaths on Sonoran desert tortoise populations is not
entirely known, but is likely most significant on the adult size class,
which is the most conspicuous, and this effect may act synergistically
with other threats we have identified. Combined with the relatively low
recruitment rate of juvenile desert tortoises into adult size
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classes, adverse effects to survivorship of populations adjacent to
urban areas might be expected (Berry 1986b, p. 130).
Sonoran desert tortoises are sometimes used as a food source in
Sonora, and likely experience population declines where they occur
adjacent to moderately sized settlements (Fritts and Jennings 1994, p.
52). Bury et al. (2002, p. 102) reported several historical incidences
of Sonoran desert tortoises being used as a source of food by native
peoples in Sonora, but less frequently in current times. According to
12 interviews at 6 ranches in central Sonora, 67 percent of local
people described Sonoran desert tortoises as declining. All but one
interviewee stated they have eaten Sonoran desert tortoise meat at some
point in their lives (Bury et al. 2002, p. 102). However, demographic
trends in Sonora indicate the number of people living on ranches and
ejidos (commonly owned lands used for agriculture and livestock
grazing) have declined, while city populations have increased,
potentially reducing the likelihood of Sonoran desert tortoises being
used for food (Bury et al. 2002, pp. 102-103).
Sonoran desert tortoises have also been documented as a food source
for undocumented immigrants on their journey through the Sonoran Desert
of Arizona, specifically in the Ironwood Forest National Monument.
Coping (2009, p. 4) claims that by the time undocumented immigrants
reach the Ironwood Forest National Monument, many have been abandoned
by their guides and left without food, water, or a sense of direction,
leaving them in intense desperation (Coping 2009, p. 4). In one
instance on June 2, 1997, a small group of undocumented immigrants
approached a resident living within the Ironwood Forest National
Monument. The immigrants had a live Sonoran desert tortoise they had
captured along the way that had a rope tethered to its front leg. They
told this resident that if they did not receive food from him, they
planned to eat the tortoise (Coping 2009, p. 5). In another reported
observation, a livestock grazing permittee on the Ironwood Forest
National Monument stated that he had seen immigrants carrying
tortoises, ``presumably with the intent to consume'' (Averill-Murray
and Averill-Murray 2002, p. 29). Indigenous communities of the Sonoran
Desert historically used Sonoran desert tortoises for food and
medicine, and their shells for ladles, dippers, bowls, and shovels
(Nabhan 2002, p. 356). However, we have no information to suggest these
uses have continued into modern times.
In conclusion, direct human depredation on Sonoran desert tortoises
is most likely to occur via vandalism (i.e., shooting) and utilization
as a source of food. While the deliberate shooting of Sonoran desert
tortoises has been documented in Arizona, reports are comparatively
rare, especially considering the amount of monitoring and survey effort
that has been afforded to wild populations over the past several
decades. However, as the human population continues to grow and
urbanization expands, we expect the incidence of human depredation to
increase. Sonoran desert tortoises have been used for food in Mexico
historically, but these occurrences are suspected to be comparatively
rare in current times. Sonoran desert tortoises may also be captured by
undocumented immigrants as they pass through remote areas of Arizona,
but increasing border-enforcement activities are expected to reduce the
number of undocumented immigrants entering Arizona in the foreseeable
future, reducing this risk.
Upper Respiratory Tract Disease
The threats of mycoplasmosis (or upper respiratory tract disease
(URTD)), and cutaneous dyskeratosis (shell disease) were major factors
in the listing of the Mojave desert tortoise (Berry 1997, p. 91).
Genetic analyses were performed by Brown et al. (1994, p. 4580) on
seven Mycoplasma organisms that were recovered from the upper
respiratory tract of clinically ill desert tortoises. These laboratory
tests led to the discovery and subsequent species description of
Mycoplasma agassizii, the species of bacteria that causes upper
respiratory tract disease in infected tortoises (Berry and Christopher
2001b, p. 413). Although M. agassizii has been studied in Mojave and
Sonoran desert tortoises, as well as gopher tortoises (G. polyphemus),
since the 1980s, its origins are unknown. It may be a naturally
occurring or an exotic pathogen. There are several potential routes of
inoculation of vertebrates by microbiota such as Mycoplasma spp.,
including horizontal (transmission between individuals), vertical
(passed down from parent to offspring), and environmental (passed from
environment to individual) (Belden and Harris 2007, p. 536). Brown
(2002, p. 1340) states that direct contact with infected individuals is
the most likely route of transmission. Brown (2003, p. 1) stated that
M. agassizii is not known to be transferred through the eggshell.
Disease may be spread to wild populations as a result of the
release of captive native or nonnative tortoise species, which can be
carriers of diseases that could affect wild Sonoran desert tortoises
(Howland and Rorabaugh 2002, p. 343). The release of any captive
reptile or amphibian is strictly prohibited by the AGFD. In a study
investigating the relationship between exposure to M. agassizii and an
urban gradient of Greater Tucson, Arizona, Jones (2008, p. 36-37) found
evidence to suggest a positive correlation between the likelihood of
testing seropositive for antibodies to M. agassizii (meaning a tortoise
has been exposed to URTD), and proximity to urban centers. These
results suggest that there may be a relationship between urbanization
and this pathogen. Tortoises from suburban sites are 2.3 times more
likely to test seropositive for antibodies to M. agassizii than
tortoises from other sites in the greater Tucson area. In fact, Sonoran
desert tortoise populations in the Rincon Mountains (adjacent to
Tucson, Arizona) had the highest prevalence of exposure to URTD of any
sites tested in Arizona, with 72.7 percent of sampled Sonoran desert
tortoises identified as seropositive (Jones 2008, p. 93).
Jones (2008, p. 60) also explored the relationship between URTD and
captive and wild desert tortoises from high-use public lands in
Maricopa, Mohave and Pima counties, and found that captive desert
tortoises are 1.8 times more likely to test seropositive for exposure
to M. agassizii than wild tortoises (p. 65). Sonoran desert tortoises
from Pima County (wild and captive) had the highest incidence of
exposure to URTD and were 5.4 times more likely to be seropositive for
antibodies to M. agassizii than those from Mohave or Maricopa Counties
(Jones 2008, p. 65). While clinical signs of URTD are infrequently
observed in wild Sonoran desert tortoises in Arizona, Jones (2008, pp.
37, 74) found that M. agassizii is widespread among captive desert
tortoises in Arizona, suggesting that the captive population may be an
important reservoir of URTD-infected tortoises that can spread the
disease to wild populations if unlawfully released or allowed to
escape.
Even though URTD appears to occur widely and has been documented in
Sonoran desert tortoise populations, no die-offs have been attributed
to URTD in Arizona. Currently, URTD does not appear to be a source of
mortality for Sonoran desert tortoise populations (Hart et al. 1992, p.
120; AIDTT 2000, p. 9; Averill-Murray and Klug 2000, p. 69; Dickinson
et al. 2002, p. 256; Howland and Rorabaugh 2002, p. 343; Jones 2008, p.
22; AGFD 2010, p. 9). Howland and Rorabaugh (2002, p. 343) hypothesized
that if disease does
[[Page 78126]]
become a significant threat to Sonoran desert tortoise populations in
the future, their patchy distribution may limit the spread of disease.
However, because the captive population of desert tortoises may serve
as a reservoir of disease and because captives are unlawfully released
into the wild by the public, monitoring wild tortoise populations that
occur near urban areas will continue to be important (Howland and
Rorabaugh 2002, p. 343; Jones 2008, pp. 6-7, 41, and 72-73).
An indirect effect of disease is that it may also subject
individuals to increased predation. Sonoran desert tortoises that are
exhibiting clinical signs of URTD may be more active during winter
months, in order to increase their metabolism and elevate their body
temperatures. This increase in surface activity might result in a
greater chance of predation or human detection (Jones 2008; p. 105).
Jones (2008, p.103) found that periods of surface activity may increase
in clinically ill Sonoran desert tortoises; however, home range size
did not differ between seropostive and seronegative tortoises (p. 103),
so seropositive tortoises which are more active in winter months do not
appear to be increasing the areas over which they move.
Wild Sonoran desert tortoises in Sonora, Mexico, were tested for
the presence of antibodies to two Mycoplasma species, M. agassizii and
M. testudineum, and were found to be generally unexposed (Brown et al.
2006, p. 5). Twenty-seven of 28 wild Sonoran desert tortoises were
found to be seronegative, indicating they had not been exposed to
Mycoplasma spp.; and one individual was serosuspect (a result
indicating that the antibody level is intermediate between positive and
negative, and is considered inconclusive) for M. testudineum (Brown et
al. 2006, p. 5). However, 11 of 21 captive Sonoran desert tortoises in
Sonora, Mexico, tested seropositive for antibodies, indicating exposure
to M. agassizii; and four were serosuspect for exposure to M.
testudineum. Ten captive desert tortoises had M. agassizii isolated
from nasal flushes, indicating a current infection, suggesting that
disease may be more prevalent in the Sonora captive population (Brown
et al. 2006, pp. 5-6). Nearly all of the captive desert tortoises
exhibited mild to severe clinical signs of URTD. Of the captive
tortoises, six had swollen or draining chin glands and four had
evidence of nasal discharge (Brown et al. 2006, p. 5-6). Once infected
by URTD, tortoises may ultimately die from the disease.
Cutaneous Dyskeratosis
Cutaneous dyskeratosis, a shell disease, was also a major factor
considered in the listing of Mojave desert tortoises. In populations of
Mojave desert tortoises exhibiting clinical signs of this disease,
significant die-offs have been documented, some as high as 70 percent
mortality rate (Jacobson et al. 1994, p. 69). Cutaneous dyskeratosis
may appear on the carapace, plastron, and thickened scales of the
forelimbs, but is most often apparent on the plastron (Jacobson et al.
1994, pp. 70-74). Potential causes of cutaneous dyskeratosis have not
been confirmed, but may be related to deficiency diseases and
environmental contamination (Berry 1997, p. 91).
Cutaneous dyskeratosis has been reported as more prevalent than
URTD within Sonoran desert tortoise populations across Arizona. As of
2000, Sonoran desert tortoises infected with cutaneous dyskeratosis had
been observed in every monitored population, with the exception of the
Wickenburg Mountains plot (AIDTT 2000, p. 9; Averill-Murray and Klug
2000, p. 69). However, noticeable population-level effects have not
been reported in any of the monitoring plots (AIDTT 2000, p. 9;
Averill-Murray and Klug 2000, p. 69; AGFD 2010, p. 9). Of the 36
individual Sonoran desert tortoises sampled from the Little Shipp Wash
and the Harcuvar Mountains from 1990 to 1994, only 5 (all females
presumed to be at least 30 years old) had signs of cutaneous
dyskeratosis, and all lived through the end of the field study. This
prompted Dickinson et al. (2002, p. 258) to suspect that Sonoran desert
tortoises might not be affected by this disease, although they
acknowledged that more research was necessary. As of 2000, the highest
incidence of cutaneous dyskeratosis (62 percent of individuals) was
reported from the East Bajada plot (AIDTT 2000, p. 9). In Sonora,
Mexico, 14 of the 28 wild Sonoran desert tortoises examined exhibited
clinical signs of cutaneous dyskeratosis (Brown et al. 2006, p. 6).
In conclusion, disease has been documented as a serious threat to
the Mojave desert tortoise, and was a primary cause for its listing
under the Act. The two most prevalent diseases that could affect
Sonoran desert tortoise populations are URTD and cutaneous
dyskeratosis. Researchers have speculated that Sonoran desert tortoises
may be able to clear infections of M. agassizii, and no wild Sonoran
desert tortoises have been found to have died from URTD in Arizona,
although it is nearly impossible to document the precise cause of death
in many situations. The literature documents that Sonoran desert
tortoise populations in proximity to urbanized areas are most at risk
of disease (as a result of released captives), because the captive
population (both in Arizona and Mexico) has a significantly higher
percentage of seropositive tortoises and tortoises that have acquired
URTD. Cutaneous dyskeratosis has been documented in virtually all
Sonoran desert tortoise long-term monitoring plots in Arizona, although
no Sonoran desert tortoises have been documented to have succumbed to
this disease, and we conclude that cutaneous dyskeratosis is not a
substantial threat to populations. Disease screening has been a regular
component to field research and monitoring of wild Sonoran desert
tortoise populations throughout their range for many years, and has not
indicated that either URTD or cutaneous dyskeratosis pose a current
threat to the Sonoran desert tortoise.
For additional information on disease in desert tortoises, or
specific disease data from monitored Sonoran desert tortoise
populations, see Hart et al. (1992, p. 120); Berry (1997, p. 91); Brown
et al. (1994, p. 4580; 1995, p. 350; 2002, p. 497; 2006, pp. 5-6);
Jacobson et al. (1994, pp. 69, 70-74); Schumacher et al. (1999, pp.
829-830); AIDTT (2000, p. 9); Averill-Murray and Klug (2000, p. 69);
Berry and Christopher (2001b, p. 413); Averill-Murray and Averill-
Murray (2002, pp. 16, 19, 26); Brown (2002, pp. 1340, 1343; 2003, p.
1); Dickinson et al. (2001, pp. 254-256; 2002, pp. 256, 258, 260-261;
2005, p. 841); Howland and Rorabaugh (2002, p. 343); Tracy et al.
(2006a, p. 1191); Belden and Harris (2007, pp. 536, 538); Wendland et
al. (2007, p. 1190); Jones et al. (2005, p. 1); Boarman and Kristan
(2008, p. 19); Jones (2008, pp. 6-7, 70, 93, 103, 105); Zylstra and
Swann (2009, pp. ix-x); and AGFD (2010, p. 9).
Summary of Factor C
In review of the information presented above, we conclude that
predation from feral domestic dogs and, to a lesser extent, human
depredation and vandalism, in combination with other threats, threaten
Sonoran desert tortoise populations, most notably as a result of the
expansion of urbanization and associated increases in human activity in
remote areas. We conclude this threat to be of moderate magnitude.
Based upon our review of the available literature, disease does not
appear to be significantly affecting the status of wild Sonoran desert
tortoise populations. Therefore, we conclude that disease
[[Page 78127]]
does not pose a significant threat to the Sonoran desert tortoise now
or in the foreseeable future.
Factor D. The Inadequacy of Existing Regulatory Mechanisms
Within its distribution in the United States, the Sonoran desert
tortoise occurs on lands managed by a myriad of Federal and State
agencies and Native American tribes, and on private lands. State
agencies, such as the Arizona Game and Fish Department (AGFD) or the
Arizona Department of Transportation (ADOT), have either direct
management authority over the Sonoran desert tortoise, or could
potentially impact Sonoran desert tortoise populations or habitat
directly or indirectly in carrying out their intended missions.
Internationally, the Sonoran desert tortoise is listed in Appendix I of
the Convention on International Trade in Endangered Species of Wild
Fauna and Flora (commonly referred as to CITES), which requires permits
to transport individuals between member nations (Bury et al. 2002, p.
86; Howland and Rorabaugh 2002, p. 348). Under the International Union
for Conservation of Nature's ``Red List,'' the desert tortoise
(rangewide) is considered ``vulnerable''--meaning it faces a high risk
of extinction in the medium-term (Rorabaugh 2008, p. 27). In our
review, we found that the Sonoran desert tortoise is commonly
considered in conservation planning where it occurs on public or tribal
lands in Arizona. Below we discuss how each agency or entity manages
their land, or otherwise considers the Sonoran desert tortoise in their
planning activities.
U.S. Bureau of Land Management
BLM is very proactive in their conservation management, directly
and indirectly, through three main mechanisms: (1) Sonoran desert
tortoise habitat categorization and compensation (monies derived from
adverse effects to Sonoran desert tortoise habitat for the acquisition
of new habitat, funding research, etc.); (2) resource management
planning; and (3) land designation. The BLM has developed numerous
documents that outline how Sonoran desert tortoise habitat management
goals and objectives are to be achieved and accounted for in their land
use planning.
The BLM developed the document titled ``Desert Tortoise Management
on the Public Lands: A Rangewide Plan'' (authored by Spang et al.
1988), and created the designation of three categories of desert
tortoise habitat throughout the species' range, using four main
criteria to indicate the importance of the habitat: (1) Maintaining
viable populations, (2) resolvability of conflicts, (3) desert tortoise
density, and (4) population status (stable, increasing, or decreasing)
(AIDTT 2000, p. 16; USBLM 2010, p. 1). The BLM categorized habitat
based upon its suitability for the desert tortoise, with Category I
being the most suited, and Category III the least, with the goals of
maintaining viable desert tortoise populations in Category I and II
habitat, and limiting population declines in Category III habitat to
the extent possible (AIDTT 2000, p. 16). However, not all Sonoran
desert tortoise habitat was included in this categorization process.
AIDTT (2000, p. 19) depicts the distribution of the categorized
habitat included in Arizona. In Arizona, there are 723,769 ac (292,899
ha) of Category I Sonoran desert tortoise habitat, 2.6 million ac (1.1
million ha) of Category II habitat, and 3.8 million ac (1.5 million ha)
of Category III habitat, totaling 7.1 million ac (2.9 million ha) of
categorized habitat (AIDTT 2000, p. 18). The 1988 Rangewide Plan also
indentified 14 different management objectives the BLM has defined
specifically for desert tortoise management, each with its own itemized
management action plan. These management objectives include the
following categories: (1) Increased awareness; (2) inventory and
monitoring; (3) cumulative impacts; (4) identification of endangered
populations; (5) coordination and cooperation; (6) research and
studies; (7) management of tortoise habitat; (8) regulation of lands
and realty actions; (9) regulation of off-highway vehicles; (10)
regulation of livestock use; (11) regulation of wild horses and burros;
(12) wildlife habitat management; (13) predator control; and (14)
management of energy and minerals research and extraction (Spang et al.
1988, pp. 14-23; AIDTT 2000, p. 18).
In 1990, BLM's Arizona State Office issued the policy titled
Strategy for Desert Tortoise Habitat Management on Public Lands in
Arizona, Instruction Memorandum No. AZ-91-16. It outlined objectives
and management actions to be implemented, and also established the BLM
Desert Tortoise Mitigation Policy, which was later reissued in 1999
(USBLM 2010, p. 2). In 2009, the BLM finalized the Desert Tortoise
Mitigation Policy, in order ``to articulate mitigation policy including
off-site compensation for the Sonoran desert tortoise and its habitat
on public lands managed by (BLM) in Arizona, in a consistent manner
between District and Field Offices'' (USBLM 2009b, p. 1). The BLM's
Desert Tortoise Mitigation Policy ``establishes policy to mitigate for
impacts to desert tortoises and their habitats including compensation
for residual impacts that cannot otherwise be mitigated. Mitigation,
including compensation must be designed to meet the purposes of the
Rangewide Plan, including maintaining viable populations as well as
maintaining the quantity and quality of Category I and II desert
tortoise habitat'' (USBLM 2009b, p. 1). Compensatory funds derived from
BLM's compensation policy are then used for a variety of conservation
activities to lessen impacts to Sonoran desert tortoises including
protective tortoise fencing, culverts for crossing, land acquisition,
and research (AIDTT 2000, p. 19). Details of this policy can be found
in USBLM (2009b, pp. 1-45).
The BLM implements various objectives and management actions
through resource management plans unique to certain geographic regions
of BLM-managed lands (USBLM 2010, p. 3). Currently, there are eight
individual resource management plans, some recently issued and others
up to 22 years old, representing the areas with potential Sonoran
desert tortoise habitat (USBLM 2010, p. 3). The Phoenix Resource
Management Plan, which directs the management of approximately 440,000
ac (178,000 ha) of Sonoran desert tortoise habitat, does not contain
district-specific management actions, but incorporates management
actions described in the Strategy for Desert Tortoise Habitat
Management on Public Lands in Arizona (USBLM 2010, p. 3). Approximately
1.1 million ac (455,000 ha) in the Yuma, Lake Havasu, Bradshaw-
Harquahala, and Kingman resource management planning areas that were
considered Sonoran desert tortoise habitat have been designated as
``priority habitats,'' meaning that the BLM prioritizes management of
wildlife habitat over other multiple-use activities (USBLM 2010, p. 3).
The BLM can directly or indirectly manage for the Sonoran desert
tortoise through the process of land designation, such as Areas of
Critical Environmental Concern (ACEC) and Wilderness Areas. In the case
of ACECs, those values may pertain to specific species or habitats, or
cultural or scenic values (AIDTT 2000, p. 22). Sonoran desert tortoises
were the impetus for the Poachie and McCracken ACECs, while other ACECs
benefit the Sonoran desert tortoise through broad protections, such as
in the Agua Fria and Ironwood Forest National Monuments (AIDTT 2000, p.
22). Sixteen Arizona ACECs contain Sonoran
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desert tortoise habitat (AIDTT 2000, p. 22). ACEC designations
facilitate the minimization of surface-disturbing activities, such as
vehicular travel, camping, fire use, mineral extraction activities, and
grazing (AIDTT 2000, p. 22). There are also 48 wilderness areas managed
by the BLM in Arizona, including approximately 850,000 ac (344,000 ha)
of Sonoran desert tortoise habitat, through ``reclaiming damaged areas,
reclaiming old vehicle ways and routes, establishing campfire and
camping policies to avoid resource impacts, establishing livestock
grazing use objectives with respect to desired vegetation, setting
objectives for wildlife habitat including the desert tortoise, and
setting prescriptions for wildfire'' (AIDTT 2000, pp. 22-23). In
addition, the BLM manages Sonoran desert tortoise habitat in Wilderness
Areas and National Monuments with an emphasis on maintaining natural
conditions and biological function of these areas (USBLM 2010, p. 10).
Approximately 22 percent of categorized Sonoran desert tortoise habitat
falls under these management prescriptions on BLM lands in Arizona
(USBLM 2010, p. 10).
Livestock grazing is the most widespread land-use activity
permitted on BLM lands, with 273 individual allotments covering
approximately 6 million ac (2.4 million ha), and 74 percent of Sonoran
desert tortoise habitat in the U.S. on their lands (Rosmarino and
Connor 2008, p. 49). A policy was developed by the BLM's Arizona State
Office in 1994, addressing livestock use of upland vegetation growth in
response to significant winter precipitation, ensuring adequate amounts
of forage remained for the Sonoran desert tortoise (and other species)
before and after livestock use. These ``ephemeral'' pastures or
allotments are permitted for 30 days of livestock grazing, with
additional 30-day extensions if monitoring concludes adequate forage
capacity exists (AIDTT 2000, p. 22). AIDTT (2000, p. 22) viewed this
grazing policy as a ``significant protective change that ensured forage
for other animals, such as desert tortoises, and also ensured that
perennial plants would not be damaged due to insufficient ephemeral
growth.'' In 1997, the BLM (USBLM 1997, pp. 1-18) further developed
standards and guidelines for livestock grazing and rangeland health. In
upland sites, the BLM standard is ``Upland soils exhibit infiltration,
permeability, and erosion rates that are appropriate to soil type,
climate and landform (ecological site)'' (USBLM 1997, p. 5). To assess
whether an allotment is meeting this standard, the BLM uses descriptive
criteria that pertain to soil conditions, ground cover, and erosion
rates (USBLM 1997, p. 5).
The BLM generally prohibits mineral material sales (mining
activities) in Category I and II Sonoran desert tortoise habitat, but
requests are evaluated on a case-by-case basis (USBLM 2010, p. 3). For
example, in the Phoenix District, the BLM has denied 11 such mineral
material sales, while others have been denied in the Tucson District,
to prevent potential impacts to Sonoran desert tortoises and their
habitat (USBLM 2010, p. 4).
In summary, the BLM considers the Sonoran desert tortoise in its
land management planning and has denied or altered projects which could
adversely affect the Sonoran desert tortoise or its habitat,
specifically with respect to mining and livestock-grazing activities.
However, we are not aware of specific actions the BLM is taking with
respect to invading nonnative plant species and subsequent wildfire
concerns, vandalism of tortoises, feral dog predation, or management to
counter anticipated climate change. In addition and as discussed below,
BLM management of off-highway vehicle use on their lands is not
protective of Sonoran desert tortoise populations. Therefore, we
conclude that BLM management of the Sonoran desert tortoise and its
habitat is currently inadequate.
U.S. Forest Service
The Sonoran desert tortoise is included on the U.S. Forest
Service's Regional Forester's Sensitive Species List, which means it is
evaluated in all biological evaluations for activities and projects
proposed within its habitat (AIDTT 2000, p. 35). Sonoran desert
tortoises occur on the Prescott (Bradshaw Ranger District), Coronado
(Santa Catalina and Nogales Ranger Districts), and Tonto National
Forests in Arizona (Murray and Schwalbe 1993, p. 39). The Tonto
National Forest manages the most Sonoran desert tortoise habitat of the
three National Forests in Arizona, where they occur in the Cave Creek,
Mesa, Globe, and Tonto Basin Ranger Districts.
Multiple land uses occur on these National Forests, including
recreation, camping, livestock grazing, and off-highway vehicle use.
Approximately 46 livestock grazing allotments on the Tonto National
Forest partially or wholly overlap the potential range of the Sonoran
desert tortoise, with several rated as having impaired or
unsatisfactory soil conditions (AIDTT 2000, p. 37). We are not aware of
the exact number of livestock grazing allotments that overlap Sonoran
desert tortoise habitat on the Coronado or Prescott National Forests.
With the exception of livestock grazing, the majority of land uses that
have the highest potential to affect the Sonoran desert tortoise occur
in districts adjacent to urbanized areas, such as the Santa Catalina
Ranger District on the Coronado National Forest (adjacent to the Tucson
metropolitan area) and the Cave Creek and Mesa Ranger Districts on the
Tonto National Forest (adjacent to the Phoenix metropolitan area).
While the Coronado National Forest does not have specific management
policies for the Sonoran desert tortoise, two policies may serve its
benefit: (1) ``Provide habitat for wildlife populations consistent with
the goals outlined in the Arizona and New Mexico Department of Game and
Fish Comprehensive Plans and consistent with other resource values;''
and, (2) ``Provide for ecosystem diversity by at least maintaining
viable populations of all native and desirable nonnative wildlife,
fish, and plant species through improved habitat management'' (AIDTT
2000, p. 36).
In September 2005, Region 3 of the U.S. Forest Service adopted a
new policy for rangeland adaptive management (USFS 2007, pp. 1-34),
called the Chapter 90 policy. Under this policy, limits on timing,
intensity, frequency, and duration of livestock grazing are set in
Allotment Management Plans. Monitoring and adaptive management are key
attributes of the Chapter 90 policy and are intended to ensure
livestock grazing outcomes meet desired resource conditions which
include the needs of wildlife such as the Sonoran desert tortoise. The
term ``conservative use'' in this policy is defined as forage
utilization on key forage species between 30 and 40 percent or less of
annual forage production by weight for herbaceous perennials, and 50
percent or less on woody browse species (USFS 2007, pp. 26, 30). It is
inherent in the term ``conservative use'' that watershed conditions and
vegetative ground cover will be optimized as appropriate to various
range sites. At no time is excessive use considered acceptable. The
goal is to achieve conservative use in the uplands over successive
years. This strategy recognizes the importance of adaptive management,
and may include adjustments of timing, intensity, frequency, and
duration of grazing to reach resource objectives (USFS 2007, pp. 13-
14).
Implementation monitoring of livestock grazing under conservative
use practices can be done using a variety of methods, and is designed
to provide
[[Page 78129]]
information that will enable decision-makers to practice adaptive
management by making necessary changes needed for plant development and
recovery, and to assess physical improvements to allotments (USFS 2007,
pp. 16-17). Effectiveness monitoring of conservative use practices
documents whether management actions are having the expected progress
toward achieving resource-management objectives, and is used to track
upland vegetative and soil condition over the long term (USFS 2007, pp.
16-17). From a short-term (within-year) perspective, wildlife habitat
and watershed conditions are gauged by monitoring seasonal utilization
on key forage species during the grazing period. Due to a warmer
climate, variable precipitation, and mild winters, seasonal-utilization
monitoring is important because the end of a particular growing season
is not well-defined for all plant communities in Sonoran desert
tortoise habitat on Forest Service lands. In review of this policy, we
conclude that implementation of the Forest Service's rangeland
management strategy is likely to retain physical characteristics
necessary to provide for the necessary forage and shelter requirements
for Sonoran desert tortoise.
In summary, the USFS considers the Sonoran desert tortoise in all
biological evaluations for activities and projects proposed within its
habitat. The USFS has developed a system of adaptive management for
livestock grazing on their lands, using resource monitoring to indicate
when changes in land conditions occur or prescribed use levels are
unsustainable, preventing excessive harm to sensitive Sonoran desert
tortoise habitat. However, we are not aware of specific actions the
USFS is taking with respect to management of invasive, nonnative plant
species and subsequent wildfire concerns, vandalism of tortoises, feral
dog predation, or efforts to counter anticipated climate change. In
addition, and as discussed below, USFS management of off-highway
vehicle on their lands is not protective of Sonoran desert tortoise
populations. Therefore, we conclude that USFS management of the Sonoran
desert tortoise and its habitat is currently inadequate.
Off-Highway Vehicle Management and Enforcement on Public Lands
While both the USFS and BLM have developed broad, strategic plans
to manage off-highway vehicle use, these plans have been found to be
missing some key elements that could improve off-highway vehicle
management, such as results-oriented goals, strategies to achieve the
goals, timeframes for implementing strategies, or performance measures
to monitor incremental progress (USGAO 2009, p. 16). Limitations of the
USFS's strategic plan have resulted from a general failure to address
motorized travel designations on the ground, communicate with the
public, monitor off-highway vehicle trail systems, or enforce off-
highway vehicle regulations (USGAO 2009, p. 16).
In response to public concerns, the BLM developed the ``National
Management Strategy for Motorized Off-Highway Vehicle Use on Public
Lands'' (USBLM 2001, p. 9). This strategy outlines action items that
are to be implemented ``as soon as practical'' (USBLM 2001, pp. 10-21).
However, the U.S. Government Accountability Office (2009, pp. 17-18)
found that ``[d]espite identifying numerous goals and strategies to
achieve the goals, BLM's recreation plan does not identify any
timeframes for implementing the strategies or any performance measures
for monitoring incremental progress * * *. Without performance measures
and timeframes, the BLM cannot ensure that it is making progress on
achieving its goals in a timely manner.''
The BLM generally prohibits competitive off-highway vehicle events
that could adversely affect Sonoran desert tortoises, from March 31
through October 15, but noncompetitive off-highway vehicle activities
are evaluated on a case-by-case basis, and mitigation measures are
implemented to reduce potential impacts to Sonoran desert tortoises
(USBLM 2010, p. 4). Although requests to permit rock crawling events
(defined in Factor A, above) have been denied where they were proposed
in Sonoran desert tortoise habitat (USBLM 2010, p. 4), this activity
still occurs outside of organized ``events.'' Rock crawling is allowed
where it might adversely affect the Sonoran desert tortoise or its
habitat (USBLM 2010, p. 4).
Both the USFS and BLM acknowledge limited staff and financial
resources for off-highway vehicle management (USGAO 2009, p. 37). Off-
highway vehicles that pass over undisturbed desertscrub habitat may
leave tracks which are then noticed by others and subsequently used
until the trail is mistakenly recognized as a designated route; this
process is known as ``route proliferation'' (Brooks and Lair 2005, p.
5). Illegal proliferation of roads and unauthorized use of off-highway
vehicles has left persistent scars in the Sonoran Desert (Abella 2010,
p. 1249). In the Kingman area, between 1994 to 1999, the BLM tracked an
increase of greater than 20 percent of off-highway vehicle use within
the range of the Sonoran desert tortoise, and reported 124 and 123
violations of improper vehicle use Statewide in 1998 and 1999,
respectively (AIDTT 2000, p. 10). The BLM has only 195 law enforcement
officers nation-wide, which means that on average, each officer is
responsible for overseeing approximately 1.2 million ac (490,000 ha) of
land, or 1,875 sq mi (4,856 sq km) (USGAO 2009, p. 38). Law enforcement
of off-highway vehicle use in the Arizona-Mexico border region is
further complicated by increasing demands to address drug smuggling and
other border-related issues (USGAO 2009, p. 39). To address an
inadequate law enforcement presence, the BLM's Phoenix District has
initiated an ``ambassador program'' which recruits volunteers to
``educate users and promote safe, sustainable off-highway vehicle use
in the area'' (USGAO 2009, p. 38). The use of signs is a common method
to enforce off-highway vehicle regulations on Federal lands, but signs
are often vandalized (sometimes within 48 hours of their installation),
and must be frequently replaced (USGAO 2009, p. 40).
In addition to wildlife management (described below), the AGFD also
licenses, promulgates rules for, and assists with regulatory
enforcement of off-highway vehicles use on public lands. In January
2009, the AGFD created an off-highway vehicle decal program, designed
to increase revenues for off-highway vehicle enforcement, education,
and signage on public lands (AGFD 2009, p. 1). However, as of November
2009, only 21 percent of all eligible off-highway vehicles and off-
highway vehicle owners in Arizona were participating in the off-highway
vehicle decal program (AGFD 2009, p. 1).
In review of off-highway vehicle management on USFS and BLM lands
in Arizona, we conclude that the current status of law enforcement is
inadequate to protect Sonoran desert tortoises and their habitat. We
considered the following in making this conclusion: (1) The documented
adverse effects of off-highway vehicle use on Sonoran desert tortoise
habitat (see Factor A); (2) the propensity for off-highway vehicle
users to illegally collect Sonoran desert tortoises in the wild
(discussed in Factor B); (3) the significant, and growing, use of off-
highway vehicles in Arizona (discussed above in Factor A); and (4) the
deficient level of law enforcement staff responsible for regulating the
use of off-highway vehicles on these lands discussed above.
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In addition, we accept the U.S. Government Accountability Office
finding that the USFS and BLM goals and objectives, intended to protect
trust resources from damage associated with off-highway vehicle use,
miss some key elements that could improve off-highway vehicle
management.
Ironwood and Mesquite Harvest
To address ecological problems stemming from wide-ranging mesquite
and ironwood harvesting in northern Mexico (discussed above in Factor
A), the Arizona-Mexico Commission, and state government in Sonora,
Mexico, made it illegal to cut and export these species (American
University Database 2010, p. 4). Additionally, Mexico's Federal
government has protected the ironwood tree, adding additional
monitoring and enforcement to protect remaining ironwood trees
(American University Database 2010, p. 4). Finally, non-profit, bi-
national groups are raising awareness and funds to help stop these
practices in Mexico (American University Database 2010, p. 4). We
consider these regulations effective in reducing the harvest of
ironwood and mesquite in the future, but the land area already
adversely modified by ironwood and mesquite harvesting, as discussed in
Factor A above, constitutes a current threat to Sonoran desert tortoise
habitat.
U.S. Department of Defense
Three prominent Department of Defense-administered lands maintain
populations of Sonoran desert tortoise: The Yuma Proving Ground, Barry
M. Goldwater Range, and Florence Military Reservation. The Yuma Proving
Ground, administered by the Department of the Army, encompasses 840,000
ac (340,000 ha) in LaPaz and Yuma Counties of southwestern Arizona
(AIDTT 2000, p. 32). The majority of land on the Yuma Proving Ground is
closed to public access year-round with the exception of 133,000 ac
(54,000 ha) that are open to hunting access for 6 months per year. The
relative inaccessibility of these lands results in little disturbance
to the Sonoran desert tortoise and its habitat (AIDTT 2000, p. 33). In
addition, the Yuma Proving Ground developed a management plan for the
Sonoran desert tortoise in 1996 (AIDTT 2000, pp. 33-34). We are
uncertain whether or not this management plan is effective in Sonoran
desert tortoise conservation on the Yuma Proving Ground.
The Barry M. Goldwater Range, used for aerial training exercises,
is the largest contiguous portion of Department of Defense lands in
Arizona (1.7 million ac, 690,000 ha), and is jointly administered by
the Luke Air Force Base and Marine Corps Air Station--Yuma, and is
located in portions of Maricopa, Yuma, and Pima Counties (AIDTT 2000,
pp. 32-33). The majority of military training exercises occur over the
valleys where Sonoran desert tortoise densities are low, leaving the
majority of Sonoran desert tortoise populations unexposed to potential
threats from these exercises (AIDTT 2000, p. 34). Outside of training
exercises, the public may access the Barry M. Goldwater Range with a
permit, via designated routes (AIDTT 2000, p. 34).
The Florence Military Reservation encompasses 25,752 ac (10,421
ha), and is jointly administered by the Arizona Army National Guard,
the Arizona State Land Department, and the BLM (AIDTT 2000, p. 34). As
stated previously, the Sonoran desert tortoise population on the
Florence Military Reservation is unique among other populations across
their range, because of the conspicuous absence of boulder outcrops and
use by tortoises of broad alluvial fans and incised washes (Riedle et
al. 2008, p. 418; Grandmaison et al. in press, p. 4). There is
significant public access and multiple land uses allowed on the
Florence Military Reservation, with no specific protections afforded to
the Sonoran desert tortoise (AIDTT 2000, p. 34). Sonoran desert
tortoise home ranges overlap with concentrated military training areas
on the Florence Military Reservation (Grandmaison et al. in press, p.
1). When not used for military training, these areas serve as
recreational areas for camping, hunting, and off-highway vehicle use,
which cumulatively have degraded Sonoran desert tortoise habitat by
removing vegetative cover, which in turn may have led to reduced use of
these areas by Sonoran desert tortoises (Grandmaison et al. in press,
p. 4).
There are few data on the potential effects of military operations
to Sonoran desert tortoises on U.S. Department of Defense lands,
specifically with respect to aircraft operations. However, Bowles et
al. (1999, pp. 19-26) tested the response of Mojave desert tortoises to
simulated aircraft sound and to sonic booms associated with aircraft,
in an attempt to ascertain potential effects to wild desert tortoises
that are exposed to such auditory stimuli within and adjacent to
aircraft flight paths and military training areas. They found that
Mojave desert tortoises could detect these sounds and had somewhat
subdued reactions ranging from ``freezing'' all movements, to bladder
voiding (Bowles et al. 1999, pp. xxii-xxiv). We are not certain whether
Sonoran desert tortoise populations on U.S. Department of Defense lands
are subjected to aircraft noise at similar sound pressure levels, but
we presume they are, because aircraft training occurs on these lands in
Arizona.
In summary, the Barry M. Goldwater Range and Yuma Proving Ground
provide for considerable protection of Sonoran desert tortoise habitat
on their installations as a result of access restrictions or through a
permitting program. The Barry M. Goldwater Range also created a
management plan specifically for the Sonoran desert tortoise in 1996.
In addition, since these lands are unlikely to be developed in the
future, these areas will likely be important in future Sonoran desert
tortoise conservation planning. However, the literature has documented
that current management on the Florence Military Reservation is not
adequate for protecting Sonoran desert tortoises or their habitat. In
discussion under Factors A and B, we discussed several activities that
occur in this area which adversely affect the Sonoran desert tortoise
and its habitat.
U.S. Fish and Wildlife Service National Wildlife Refuges
Sonoran desert tortoises occur on several National Wildlife Refuges
in Arizona. Sonoran desert tortoise populations are highest on the
Kofa, Buenos Aires, and Cabeza Prieta National Wildlife Refuges,
although they also may occur in low densities within the Cibola,
Imperial, and Lake Havasu National Wildlife Refuges along the Colorado
River (AIDTT 2000, p. 31). The mission of the National Wildlife Refuge
System is `` * * * to administer a national network of lands and waters
for the conservation, management, and where appropriate, restoration of
the fish, wildlife, and plant resources and their habitats within the
United States for the benefit of present and future generations of
Americans'' (AIDTT 2000, p. 31). Management on these National Wildlife
Refuges is largely protective of Sonoran desert tortoises, as multiple
use activities such as livestock grazing and off-highway vehicle use
are prohibited (AIDTT 2000, p. 31). However, the U.S. Border Patrol
uses administrative roads, which are closed to public use in these
areas, along the border region of the Buenos Aires and Cabeza Prieta
National Wildlife Refuges, which may affect Sonoran desert tortoises or
their habitat in these areas. For further discussion of the effect of
U.S. Border Patrol operations on Sonoran desert tortoises or their
habitat, see the section on Undocumented Immigration in Factor A of
this finding.
[[Page 78131]]
In summary, we conclude that the regulations establishing the
mission and management of the National Wildlife Refuge system are
consistent with Sonoran desert tortoise habitat management, and are
therefore adequate to protect the tortoise where it occurs on our
lands.
National Park Service
Sonoran desert tortoise habitat occurs on Organ Pipe Cactus
National Monument, Saguaro National Park, and the Lake Mead National
Recreation Area (AIDTT 2000, p. 27). The National Park Service is
mandated by law to ``conserve the scenery and the natural and historic
objects and the wildlife therein and to provide for the enjoyment of
the same in such manner and by such means as will leave them unimpaired
for the enjoyment of future generations'' (AIDTT 2000, p. 26). The
resource-management goals on National Park Service lands are broad in
scope, and include reducing ground disturbance, developing and
implementing inventory and monitoring programs, assessing and
mitigating resource disturbance, and developing environmental
restoration and research programs (AIDTT 2000, p. 26). Livestock
grazing and off-highway vehicle use are not permitted on National Park
Service lands. While the National Park Service has no specific
provision for Sonoran desert tortoise conservation on their lands, all
wildlife inhabiting National Park Service lands in Arizona, including
the Sonoran desert tortoise, are protected, and possession or removal
of wildlife is prohibited (AIDTT 2000, p. 26).
However, where National Park Service lands are adjacent to urban
areas, such as Saguaro National Park outside of the Tucson metropolitan
area, threats to Sonoran desert tortoises have been documented.
Averill-Murray and Swann (2002, p. 1) and Jones (2008, p. 66)
documented threats such as harassment and predation by feral domestic
dogs, releases of captive Sonoran desert tortoises and exotic species
(that may transmit diseases), road mortality, and illegal collection of
tortoises, as affecting the Sonoran desert tortoise population on
Saguaro National Park land.
In summary, we acknowledge that the mission and management of the
National Park Service and their lands is consistent with Sonoran desert
tortoise habitat management, but where Park Service land is affected by
adjacent urbanized areas, adequate regulatory protections for the
tortoise have not been realized.
Arizona State Land Department
Arizona State Trust Land, managed to derive revenues for trust
beneficiaries including educational, health, and penal institutions,
comprises 13 percent of all land in Arizona, much of which contains
Sonoran desert tortoise habitat (AIDTT 2000, p. 15). In general, the
mission of the Arizona State Land Department is to maximize economic
return (AIDTT 2000, p. 16). The Arizona State Land Department has no
broad management practices, policies, or directives that pertain to
Sonoran desert tortoise management, but does coordinate with the AGFD
on some projects to reduce potential impacts to the Sonoran desert
tortoise (AIDTT 2000, p. 16). Four Sonoran desert tortoise monitoring
sites occur partially or fully on Arizona State Trust Lands: Granite
Hills, Little Shipp Wash, Tortolita Mountains, and Picacho Mountains;
two of these sites, Granite Hills (Pinal County) and Little Shipp Wash
(Yavapai County) are long-term monitoring plots (AIDTT 2000, pp. 5-6,
15). Other blocks of Sonoran desert tortoise habitat on Arizona State
Trust Lands occur west of the Upper Burro Creek, Arrastra Mountain, and
Tres Alamos wilderness areas in Yavapai County and from the Tortolita
to the Tortilla Mountains in Pinal County (AIDTT 2000, p. 15).
Recreation on State Trust Lands is generally not monitored and
therefore may not be protective of Sonoran desert tortoises or their
habitat.
The Arizona State Land Department is considering restricting access
to its lands for purposes of conducting wildlife studies. These access
restrictions may prohibit further research due to numerous permit
requirements. These new policies are not yet in place and could be
changed prior to final issuance (Jody Latimer, ASLD, 2010, pers.
comm.). If implemented as described by Latimer (ASLD, 2010, pers.
comm.), these proposed procedures and fees have the potential to limit
Sonoran desert tortoise monitoring and research on Arizona State Trust
lands in the future through new monetary and procedural requirements.
While these new policies and regulations are not yet in effect, even if
they are implemented it appears they will not address conservation and
management of the Sonoran desert tortoise and its habitat, and further,
may have a negative effect by potentially restricting important
research needed for conservation of the tortoise. Furthermore, we are
not aware of specific actions the Arizona State Land Department is
taking with respect to management of invasive, nonnative plant species
and subsequent wildfire concerns, vandalism of tortoises, feral dog
predation, or efforts to counter anticipated climate change. Therefore,
we conclude that Arizona State Land Department management of the
Sonoran desert tortoise and its habitat is currently inadequate.
Arizona Game and Fish Department
The Arizona Game and Fish Department (AGFD) currently classifies
the Sonoran desert tortoise as a Tier 1b ``Species of Greatest
Conservation Need'' AGFD (2006, p. 485). A Tier 1b species is one that
requires immediate conservation actions aimed at improving conditions
through intervention at the population or habitat level. Before April
28, 1989, the AGFD allowed the collection and possession of one
lawfully captured Sonoran desert tortoise per person (AIDTT 2000, p.
14). After this date, under Commission Order 43, the AGFD closed the
season on Sonoran desert tortoises, which prohibited the take of desert
tortoises from the wild, except under special permit (for example,
scientific or educational) (AIDTT 2000, p. 14). Unless otherwise
prescribed in title 17, it is unlawful to [t]ake, possess, transport,
buy, sell or offer or expose for sale wildlife except as expressly
permitted by this title'' (ARS 17-309). It is also unlawful to release
wildlife into the wild except as authorized by the Arizona Game and
Fish Commission or as defined in title 3 (see ARS 17-306). As a closed-
season species, the desert tortoise cannot be taken from the wild or
possessed without special permit (Commission Order 43). As restricted
live wildlife (R12-4-406), they cannot be imported, exported, or
possessed without special license or lawful exemption.
Enforcement of the State closure on collection of Sonoran desert
tortoises occurs when directly observed by law enforcement personnel,
but the remoteness of many Sonoran desert tortoise populations makes
enforcement strategies and techniques problematic (AIDTT 2000, p. 14).
Furthermore, regulations regarding the collection or possession of
Sonoran desert tortoises are poorly known to the public, emphasizing
the importance of education efforts (AIDTT 2000, p. 14). The effect of
illegal collection of Sonoran desert tortoises on wild populations in
Mexico is largely unknown (see Factor B).
The AGFD has led Sonoran desert tortoise conservation in Arizona
through research, guidance provided to the public and other agencies,
and cooperative conservation management
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on public lands. For example, the AGFD (2007a, p. 1) provides
construction and development contractors with guidance, should a
Sonoran desert tortoise be encountered within an area of a development.
In addition, the AGFD (2007b, p. 1) also provides environmental
consultants guidance on proper survey techniques and considerations
when surveying for Sonoran desert tortoises. AGFD (2006, pp. 485-487)
described numerous management priorities with respect to mitigating
potential threats facing the tortoise in Arizona. The recommendations
outlined in these documents are recommended guidance, voluntary in
nature, and no reporting requirements are mandated. Therefore, we are
uncertain whether project proponents implement these recommendations.
Arizona Interagency Desert Tortoise Team
As part of a multi-agency collaborative project, the Arizona
Interagency Desert Tortoise Team (AIDTT) was formed in 1985 to
coordinate research and management of Sonoran desert tortoise
populations in Arizona. Participating agencies in the AIDTT manage
habitat, manage the species, or conduct research, and include the AGFD,
Arizona State Lands Department, U.S. Forest Service, BLM, U.S. Bureau
of Reclamation, U.S. Bureau of Indian Affairs, U.S. Fish and Wildlife
Service, National Park Service, U.S. Geological Survey, and several
U.S. Department of Defense military reservations (AIDTT 1996, Preface;
AIDTT 2000, p. 2). The AIDTT is co-chaired by representatives from the
U.S. Fish and Wildlife Service (Arizona Ecological Services Office) and
the AGFD. Since its inception, the AIDTT has collaborated in the
development of numerous documents addressing conservation of the
Sonoran desert tortoise including ``Survey Protocol for Sonoran Desert
Tortoise Monitoring Plots: Reviewed and Revised'' (Averill-Murray
2000a), ``Status of the Sonoran Population of the Desert Tortoise in
Arizona: An Update'' (Averill-Murray 2000b), ``Guidelines for Handling
Sonoran Desert Tortoises Encountered on Development Projects'' (AGFD
2007a), ``Desert Tortoise Survey Guidelines for Environmental
Consultants'' (AGFD 2007b), and ``Recommended Standard Mitigation
Measures for Projects in Sonoran Desert Tortoise Habitat'' (AIDTT
2008). Available online, the AIDTT (2008, pp. 1-7) offers guidance on
standard types of mitigation for projects that may affect Sonoran
desert tortoises; these measures are voluntary.
The AIDTT's Memorandum of Understanding, signed in 1995,
established specific objectives for the team including: (1) Ensuring
the survival of the species; (2) preventing loss of the species; and
(3) improving the quality of Sonoran desert tortoise habitat in
Arizona, with the team to function as an advocate for the Sonoran
desert tortoise (AIDTT 1996, Preface; AIDTT 2000, p. 2). A management
plan for the Sonoran desert tortoise completed in 1996 called for
improved monitoring protocols, the implementation of threat-
minimization activities, and the creation of Sonoran Desert Management
Areas (AIDTT 1996, pp. 20-26). However, common criticisms of the 1996
plan include: (1) Lack of meaningful goals and objectives; (2) lack of
political willpower without legal protection for the Sonoran desert
tortoise; (3) failure to designate Sonoran Desert Management Areas; and
(4) poor funding (AIDTT 2000, p. 2). Collectively, these recognized
shortcomings hampered the implementation of threat-minimization
activities. In recognition of these shortcomings, the AIDTT is
currently in the process of developing a State Conservation Agreement,
Assessment and Strategy with the goal of identifying reasonable,
obtainable conservation goals and objectives that will contribute to
Sonoran desert tortoise conservation on public lands in a meaningful
capacity.
Mexican Government (Secretaria de Medio Ambiente y Recursos Naturales)
Throughout Mexico, the desert tortoise is listed as ``Amenazadas,''
or Threatened, by the Secretaria de Medio Ambiente y Recursos Naturales
(SEMARNAT) (Bury et al. 2002, p. 86; Howland and Rorabaugh 2002, p.
348; SEDESOL 2008, p. 99). Threatened species are ``those species, or
populations of the same, likely to be in danger of disappearing in a
short or medium timeframe, if the factors that negatively impact their
viability, cause the deterioration or modification of their habitat or
directly diminish the size of their populations continue to operate''
(SEDESOL 2008 (NOM-059-ECOL-2008), p. 5). This designation prohibits
taking of the species, unless specifically permitted, and also
prohibits any activity that intentionally destroys or adversely
modifies its habitat (SEDESOL 2000 and 2001 (NOM-059-ECOL-2001).
However, activities that unintentionally destroy or adversely modify
their habitat do not appear to be specifically prohibited (e.g.,
cultivation of buffelgrass for livestock grazing). In 1988, the Mexican
Government passed a regulation that is similar to the National
Environmental Policy Act of the United States (42 U.S.C. 4321 et seq.).
This Mexican regulation requires an environmental assessment of private
or government actions that may affect wildlife or their habitat
(SEDESOL 1988 (LGEEPA)).
The Mexican Federal agency known as the Instituto Nacional de
Ecolog[iacute]a (INE) is generally considered the Mexican counterpart
to the U.S. Fish and Wildlife Service. INE is responsible for the
analysis of the status and threats that pertain to species that are
proposed for listing in the Norma Oficial Mexicana NOM-059 (the Mexican
equivalent to a threatened and endangered species list), and if
appropriate, the nomination of species to the list. INE developed the
Method of Evaluation of the Risk of Extinction of the Wild Species in
Mexico (MER), which unifies the criteria of decisions on the categories
of risk, and permits the use of specific information fundamental to
listing decisions. The MER is based on four independent, quantitative
criteria: (1) Size of the distribution of the taxon in Mexico, (2)
state (quality) of the habitat with respect to natural development of
the taxon, (3) intrinsic biological vulnerability of the taxon, and (4)
impacts of human activity on the taxon. INE implemented use of the MER
in 2006; therefore, all species previously listed in the NOM-059 were,
in many cases, based solely on expert review and opinion. Specifically,
until 2006, the listing process under INE consisted of a panel of
scientific experts who convened as necessary for the purpose of
defining and assessing the status and threats that affect Mexico's
native species that are considered to be at risk, and for applying
those factors to the definitions of the various listing categories.
In summary, while the desert tortoise is federally listed in
Mexico, we have documented significant threats to its persistence in
that country (see Factors A and C) that are not controlled by the
listing, and therefore conclude that regulations establishing
management of the Sonoran desert tortoise in Mexico do not provide
adequate assurances of its continued existence in that country.
Summary of Factor D
Numerous State and Federal entities have regulations or policies
which implement management of either the Sonoran desert tortoise or its
habitat throughout the species' range in Arizona. In Mexico, the
species is currently listed as threatened. In our
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review of the available information on each entity's management
policies and regulations, we found numerous examples where the Sonoran
desert tortoise is considered in management actions and tortoise-
specific mitigation measures are mandated, or where land activities
that could appreciably threaten Sonoran desert tortoise populations are
prohibited. While several land managers and agencies in Arizona
actively consider the Sonoran desert tortoise in their resource
planning, we found deficiencies in management of off-highway vehicle
use, policies and procedures inconsistent with Sonoran desert tortoise
conservation, and some threats such as invasive, nonnative plant
species and subsequent wildfire concerns, vandalism of tortoises, feral
dog predation, or efforts to counter anticipated climate change were
not addressed by land management control. Lastly, significant threats
we discuss above in Factors A and C are not being adequately addressed
by land managers, including invasive, nonnative plant species and
associated wildfire concerns, vandalism of tortoises, feral dog
predation, and management to counter anticipated climate change.
Although the Sonoran desert tortoise is considered a threatened
species in Mexico, we are not aware of conservation planning or
enforcement of regulations that has occurred because of this status.
Based upon our review of the information pertaining to threats in
Mexico, it is unlikely that protections afforded to the Sonoran desert
tortoise are adequate to ensure conservation for the foreseeable future
in Mexico. As a result, we conclude that the Sonoran desert tortoise is
threatened due the inadequacy of existing regulatory mechanisms, in
combination with the other threats identified in this finding, both now
and in the foreseeable future.
Factor E. Other Natural or Manmade Factors Affecting Its Continued
Existence
Environmental Contaminants
Many sources of potential contamination presently occur throughout
the distribution of the Sonoran desert tortoise. Copper mining in the
Sonoran Desert has occurred in Arizona and adjacent Mexico for
centuries, and many of these sites have smelters (now decommissioned),
which are former sources of airborne contaminants. In Arizona,
historical or current large-scale copper mining operations exist in
Pima, Pinal, Yavapai, Gila, and Mohave Counties, which are sources of
low-level, persistent contaminants in surrounding areas as a result of
fugitive dust, contaminated surface runoff, and other mechanisms
consistent with contaminant fate and transport. Soil contamination
within ephemeral washes from leaching operations associated with mining
activities has occurred throughout the Sonoran Desert, and will likely
continue to occur where these activities take place. Sonoran desert
tortoises that forage in contaminated ephemeral washes may ingest toxic
constituents through soil or contaminated plant matter, but we are not
aware of any specific reports of tortoises that became sick or deceased
from this risk. The mining industry in Mexico is largely concentrated
in the northern tier of that country, with Sonora as the leader for
generating copper, gold, graphite, molybdenum, and wollastonite, as
well as the leader among Mexican States with the most surface area
dedicated to mining (Stoleson et al. 2005, p. 56). The three largest
mines (all copper) are found in Sonora (Stoleson et al. 2005, p. 57).
The sizes of mines in Sonora vary considerably, as do the known
environmental effects from mining-related activities (from exploration
to long after closure), which include contamination and drawdown of
groundwater aquifers, erosion, acid mine drainage, fugitive dust,
pollution from smelter emissions, and landscape clearing (Stoleson et
al. 2005, p. 57).
Rowe (2008, p. 623) investigated potential effects of persistent,
low-level contaminants (e.g., heavy metals, polychlorinated biphenols,
organochlorides) on long-lived vertebrates (such as the Sonoran desert
tortoise). Cadmium and lead are of special concern due to their
toxicity, and because they are persistent, common environmental
contaminants (Mart[iacute]nez-L[oacute]pez et al. 2010, p. 671).
Cadmium may affect turtle gonadal development, and lead may affect an
individual tortoise's susceptibility to infections and disease, because
it may suppress its immune capacity. The latter can potentially affect
the spread of known diseases such as herpesvirus, cutaneous
dyskeratosis, and URTD within and among affected populations
(Mart[iacute]nez-L[oacute]pez et al. 2010, p. 671). As stated
previously, cutaneous dyskeratosis is prevalent within most populations
of Sonoran desert tortoise throughout their distribution in Arizona,
but this disease has not been determined to currently be a significant
threat to Sonoran desert tortoise populations. Another common
environmental contaminant is the heavy metal arsenic, which is
carcinogenic (cancer-causing) and may also already occur in naturally-
high levels in some areas of the American Southwest (Seltzer and Berry
2005, p. 263).
Because the Sonoran desert tortoise is characterized as having a
delayed sexual maturation and a long generation time, potential effects
from persistent, low-level contaminants in the environment include: (1)
Mortality before reproduction, (2) chronic accumulation of contaminants
that may be transferred to offspring upon maturation, (3) reduced size
at maturity reducing offspring quantity or quality, (4) delayed
expression of fitness effects at the population level, and (5) delayed
recovery of populations following abatement of fitness effects (Rowe
2008, p. 626). In several areas of the Sonoran Desert in Arizona and
Sonora, Mexico, mining operations and other human-related activities
can result in remobilization and concentration of elemental toxicants
in the air, on the soil surface, and on the surfaces of forage plants,
both from ground disturbance and from long-range atmospheric deposition
associated with old copper smelter sites, coal-fired power plants, and
fugitive dust from abandoned and active mining sites (Seltzer and Berry
2005, p. 263; Rowe 2008, p. 628). The most likely routes for exposure
of Sonoran desert tortoises to these types of contaminants are through
ingestion of contaminated soil or plant matter, or through inhalation
of contaminated dust or particles, especially when a tortoise
constructs or modifies a burrow (Seltzer and Berry 2005, p. 263; Hinck
et al. 2010, p. 287). We have no specific records of Sonoran desert
tortoises becoming sick or dying from this type of contamination;
effects from these contaminants can be significantly delayed and slow
to manifest. Also, few field researchers are sampling wild tortoises to
test for contaminant exposure.
Conversion of habitat to large-scale agriculture has been
concentrated in Sonora, Mexico, which has provided sources of surface
and groundwater pollution such as salt intrusion due to agricultural
water use extraction; municipal and agricultural discharges; and solid
waste, including cast-off agrochemical containers, winery residues, and
hog farm muck (Nauman 2007, p. 1). The extent to which Sonoran desert
tortoises drink freely from perennial or intermittent streams is not
known, but since tortoises are opportunistic drinkers, we presume they
use streams as a source of water in addition to ephemeral pools
generated by precipitation events, and that they may subsequently
ingest such toxins.
[[Page 78134]]
In conclusion and based upon our review of the best available
scientific or commercial data, little is known of the potential effect
of low-level environmental contamination on Sonoran desert tortoises.
We did ascertain that the risk of environmental contaminants affecting
Sonoran desert tortoise populations is most likely from the presence of
persistent, low-level toxicants such as heavy metals, polychlorinated
biphenols, and organochlorides. However, potential effects of this type
of environmental contamination are often delayed and difficult to
observe in long-lived species such as the Sonoran desert tortoise,
largely because of delayed sexual maturation and long generation times.
We did not find documentation of population-level effects in Sonoran
desert tortoises as a result of environmental contamination. Therefore,
we conclude that environmental contamination of Sonoran desert tortoise
habitat is not currently threatening populations; however, we
acknowledge that further study is warranted to identify whether there
is a risk for population-level impacts, and we recommend that land
managers consider collecting baseline soil data in areas that may be
vulnerable.
Vehicle Strike Mortalities
We expect that the increased use of off-highway vehicles within
Sonoran desert tortoise habitat will increase the likelihood of
encounters with Sonoran desert tortoises which can result in a variety
of potential outcomes for tortoises. According to the Arizona
Interagency Desert Tortoise Team (AIDTT 2000, p. 10), ``[a]n abundance
of anecdotal knowledge indicates that contacts between people and wild
tortoises usually end to the detriment of tortoises (e.g., collection,
handling, vandalism, crushing under vehicle tires, and shooting).''
Averill-Murray and Swann (2002, p. 1) stated that urban development
adjacent to the Saguaro National Park in Pima County threatens the
Sonoran desert tortoise via several mechanisms, including elevated
mortality on roads. The high rates of speed associated with competitive
off-highway vehicle events significantly increase the risk of direct
mortality of Sonoran desert tortoises from vehicle collisions (Vega
2010, p. 4).
Reptiles, including the Sonoran desert tortoise, may be
particularly vulnerable to roads due to the higher risk of mortality as
a result of vehicle strikes (Boarman and Sazaki 1996, p. 1; Boarman et
al. 1997, p. 57; Forman and Alexander 1998, p. 213; Boarman 2002, pp.
54-55; Boarman and Sazaki 2006, p. 98; Dieringer 2010, p. 1).
Anticipated adverse effects of roads on Sonoran desert tortoise
populations are likely related to the level of their use. For example,
Hoff and Marlow (2002, pp. 451-454) found that the impact of roads on
the prevalence of Mojave desert tortoise signs (tracks, scat, etc.) was
commensurate with traffic volume--with the impacts more significant
adjacent to heavily traveled roads. Mojave desert tortoise populations
showed depressed numbers within 1,300 feet (400 m) of highways in the
Mojave Desert (Boarman and Sazaki 2006, p. 98). Similar effects to
Sonoran desert tortoise populations might be expected when heavily used
roads are adjacent to, or are routed through, core Sonoran desert
tortoise habitat such as steep, boulder-strewn slopes within Arizona
Upland Sonoran desertscrub (Dieringer 2010, p. 1; Grandmaison 2010b, p.
3).
Sonoran desert tortoises move slowly and take a relatively long
time to cross roads and highways, which may place them at elevated risk
(Andrews et al. 2008, p. 124). However, we suspect that, due to their
size and shape (particularly in the sub-adult and adult size classes),
drivers may instinctively avoid striking a crossing tortoise because of
their similarity to rocks, and the subsequent damage that hitting a
``rock'' could do to a vehicle. However, intentional vehicle strikes of
Mojave desert tortoises have been reported (Bury and Marlow 1973, p.
11). While unpaved roads traverse 16 of the 17 Sonoran desert tortoise
monitoring plots, the AGFD is only aware of one instance of direct
mortality of a Sonoran desert tortoise from a vehicle on a long-term
monitoring plot, on the East Bajada Plot (AGFD 2010, p. 14).
Increased vegetation adjacent to paved or heavily compacted roads
resulting from increased water runoff may be beneficial to Sonoran
desert tortoises, serving as a means to rehydrate them, but it may also
attract them to these areas, indirectly increasing the likelihood of
adverse interactions from: (1) Tortoises wandering onto the road, (2)
vehicles pulling onto the vegetated shoulder of the road and crushing
tortoises, (3) injury from grading or mowing activities, (4) exposure
to herbicides applied to control growth of weeds along the road
shoulder, and (5) increased potential for observation and collection by
passers-by (Boarman 2002, p. 55). As stated previously, Sonoran desert
tortoises may use infrequently traveled gravel roads as travel routes
within their home ranges (Grandmaison et al. in press, p. 16). This
suggests that low density Sonoran desert tortoise populations observed
adjacent to heavily traveled roads may be the result of mortality from
vehicle collisions and illegal collection rather than road avoidance
behavior (Grandmaison et al. in press, p. 16).
There appears to be a concerted effort to mitigate the potential
effect of several roads and highways on Sonoran desert tortoise
populations and their habitat. Barrier fencing (or tortoise fencing)
and culverts along roads and highways are recognized methods employed
throughout Arizona to reduce potential mortality through vehicle
strikes of Sonoran desert tortoises. Installing tortoise fencing along
roads and highways minimizes the risk of road mortality of tortoises
but may also enhance the barrier effect between populations by
restricting long-distance movements (Boarman and Sazaki 1996, p. 3).
Culverts that pass under roads and highways may provide opportunities
for Sonoran desert tortoises to safely cross roads and highways
(Boarman and Sazaki 1996, pp. 3-4).
The ADOT constructs and maintains roads and highways that comprise
Arizona's transportation system. It routinely implements varied
conservation and mitigation actions with respect to Sonoran desert
tortoise populations that may be affected by these activities. The ADOT
(ADOT 2010, pp. 2-5) listed numerous conservation measures including
those which address standard (voluntary and involuntary) mitigation
measures, education, new construction design, habitat acquisition,
native plant restoration, nonnative plant control, establishment of
wildlife corridors, and research that have been integrated into their
road system planning, construction, and improvement activities.
Tortoise-proof fencing adjacent to highways has been installed along
numerous routes throughout Arizona including 27.6 mi (44.4 km) along
U.S. Highway 93 and 10.8 mi (17.4 km) along State Route 85 (ADOT 2010,
p. 3). Numerous, additional structures that assist Sonoran desert
tortoises to cross roads safely, such as pathways, ramps, and culverts,
have been installed along the U.S. Highway 93 corridor and along a
segment of the U.S. Highway 60 through the Tonto National Forest (ADOT
2010, p. 3).
The ability of tortoise fencing to prevent road mortality of
Sonoran desert tortoises is highly contingent on inspections and
maintenance. Sonoran desert tortoise fencing along 10 mi (16 km) of
U.S. Highway 93 in Mohave and
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Yavapai Counties in Arizona, between mile markers 144 and 155, was
shown to have major deficiencies, including 567 individual fencing
breaches and instances of culvert undercutting, which diminish the
effectiveness of these mitigation techniques (Grandmaison 2010b, p. 3).
Five Sonoran desert tortoise road-mortalities were documented in 2008
in this stretch of highway, though none was documented in 2009
(Grandmaison 2010b, p. 5). A rancher in southeastern Mohave County,
Arizona, reported observations of Sonoran desert tortoises being killed
on U.S. Highway 93, particularly after heavy rains, when adjacent
tortoise barrier fencing along the highway gets washed out, allowing
access of tortoises to the highway surface (Dieringer 2010, p. 1).
Using radio-telemetry, Grandmaison (2010b, p. 6) found that Sonoran
desert tortoises with home ranges within 0.62 mi (1 km) adjacent to
this stretch of Highway 93 did not cross the highway. However,
additional instances of Sonoran desert tortoise mortality on this and
other major routes within the distribution of Sonoran desert tortoises
undoubtedly occurs but is rarely reported.
Many activities undertaken by the ADOT minimize the effect of roads
and highways on tortoise populations. However, we have concern
regarding the lack of ongoing maintenance of protection structures such
as tortoise barrier fencing. Therefore, we conclude that maintenance of
tortoise protection structures is not adequate to meet the desired
objective of these structures in many areas, or to protect Sonoran
desert tortoise populations affected by heavily used roads and highways
in Arizona.
Balloons and Trash
Helium-filled balloons are capable of dispersing great distances
(greater than 164 mi (264 km)) from their release points, and have been
shown to make up the largest percentage of litter types encountered in
desert tortoise field studies (Walde et al. 2007a, p. 148). Desert
tortoises are known to eat trash, such as balloons, plastic, and other
garbage, which may kill them by becoming lodged in the gastrointestinal
tract or by entangling the tortoise (Averill-Murray and Averill-Murray
2002, p. 27; Walde et al. 2007a, p. 148). Balloons and balloon string
can also entangle the tortoise, sometimes leading to induced amputation
of an appendage (Burge 1989, p. 7). Averill-Murray and Averill-Murray
(2002, p. 27) reported 36 balloons found on Ironwood Forest National
Monument in Pima County, Arizona, indicating that opportunities for a
Sonoran desert tortoise to consume, or become entangled with balloons,
exist. However, Averill-Murray and Averill-Murray (2002, p. 29) posited
that while balloons may affect individuals, they are unlikely to cause
population-level impacts to Sonoran desert tortoises.
Illegal dumping in Arizona is ubiquitous throughout the Sonoran
Desert, but most concentrated in areas adjacent to human settlements.
These relatively small but widely dispersed piles of solid and
potentially hazardous waste may also serve as sources of toxicological
contamination of Sonoran desert tortoises in areas where ingestion of
contaminated soils or plant matter can occur.
In conclusion, balloons and trash occur throughout the range of the
desert tortoise. Trash piles are most concentrated adjacent to human
settlements but helium-filled balloons can travel many miles away from
cities or towns and be deposited in remote habitat as they fall from
the sky. We have documented that balloons in particular may pose health
risks to Sonoran desert tortoises and are encountered in monitoring
plots although specific reports of tortoises directly affected by
balloons are rare in the literature. While effects can occur to
individual tortoises, the literature did not indicate that population-
level effects can be expected from such exposure.
Climate Change
There is unequivocal evidence that the earth's climate is warming
based on observations of increases in average global air and ocean
temperatures, widespread melting of glaciers and polar ice caps, and
rising sea levels (IPCC 2007, p. 4). Furthermore, the Intergovernmental
Panel on Climate Change (IPCC 2007, p. 7) summarized the likelihood of
general future trends in several climatic variables, predicting: (1)
Warmer and fewer cold days and nights over most land areas, (2) warmer
and more frequent hot days and nights over most land areas, (3) more
frequent warm spells/heat waves over most land areas, (4) changes in
precipitation patterns favoring an increased frequency of heavy
precipitation events (or proportion of total rainfall from heavy falls)
over most areas, and (5) an increase in the area affected by droughts.
All of these changes are caused by alterations in the energy balance
within the atmosphere and the Earth's surface. The primary factors that
affect this balance are concentrations of greenhouse gases (mainly
carbon dioxide), aerosols, land surface properties, and solar
radiation. These global climate changes will influence climatic
patterns at regional and local scales.
At a regional scale, there is a broad consensus among climate
models that the area encompassing the southwestern United States and
northern Mexico will get drier in the twenty-first century and that the
trend towards a more arid climate is already under way (Seager et al.
2007). Evidence to support such changes in temperature and rainfall in
the southwest deserts is abundant. For example, maximum summer
temperatures in the southwestern United States are expected to increase
over time in response to changes in the climate system (Christensen et
al. 2007, p. 887). Weiss and Overpeck (2005, p. 2075) examined low-
temperature data over a 40-year timeframe from numerous weather
stations in the Sonoran Desert ecoregion of Arizona and California, as
well as the Mexican States of Baja California, Baja California Sur, and
Sonora. They found: (1) Widespread warming trends in winter and spring,
(2) decreased frequency of freezing temperatures, (3) lengthening of
the freeze-free season, and (4) increased minimum temperatures per
winter year. Such changes are likely to have widespread impacts on
Southwestern ecosystems.
While temperatures in the Southwest are predicted to increase,
rainfall patterns will also be affected. The current, multi-year
drought in the western United States, including most of the Southwest,
is the most severe drought recorded since 1900 (Overpeck and Udall,
2010, p. 1642). Numerous models predict a decrease in annual
precipitation in the southwestern United States and northern Mexico.
Solomon et al. (2009, p. 1707) predict precipitation amounts in the
southwestern United States and northern Mexico will decrease by as much
as 9 to 12 percent (measured as percentage of change in precipitation
per degree of warming, relative to 1900 to 1950 as the baseline
period). Christensen et al. (2007, p. 888) state, ``The projection of
smaller warming over the Pacific Ocean than over the continent, * * *
is likely to induce a decrease in annual precipitation in the
southwestern USA and northern Mexico.'' In addition, Seager et al.
(2007, pp. 1181-1184) analyzed 19 models of differing variables to
estimate the future climate of the southwestern United States and
northern Mexico in response to predictions of changing climatic
patterns. All but one of the 19 models predicted a drying trend within
the southwest (Seager et al. 2007, p. 1181). A total of 49 projections
were created using the 19 models and all but 3
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predicted a shift to increasing aridity (dryness) in the southwest as
early as 2021 to 2040 (Seager et al. 2007, p. 1181). While most climate
change models predict less precipitation in the southwestern United
States, a model produced by the Hadley Centre for Climate Prediction
and Research (HadCM2) predicted increased precipitation throughout most
of the United States, and particularly in the southwest (Weltzin et al.
2003, p. 942). While there may be some uncertainty associated with the
predictions of decreased rainfall in the arid deserts, there is broad
agreement that the overall trend will be reduced precipitation.
In addition to increasing trends in aridity, the timing of
precipitation may also be altered as a result of climate change, which
would result in important changes in the vegetation community within
habitat of the Sonoran desert tortoise. The IPCC (2007, p. 20) found
that winter precipitation in the southwestern United States is
predicted to decline by as much as 20 percent as a result of climate
change, while summer precipitation may increase slightly. Precipitation
in Mojave desertscrub occurs predominantly during the cool-season
(winter) months but, depending on location, it may also occur during
the warm-summer months (Hereford et al. 2006, p. 29). Perennial plant
species in Mojave desertscrub are most affected by changes in winter
precipitation, as increases in winter precipitation increases
germination and the establishment of new plants (Hereford et al. 2006,
p. 25). In contrast, decreases in winter precipitation substantially
increase mortality in perennial plants, most notably in short-lived
species (Hereford et al. 2006, p. 25). In addition, decreasing winter
precipitation has been linked with a high mortality of drought-
resistant shrubs in parts of the Sonoran and Mojave deserts (McAuliffe
and Hamerlynck 2010, p. 885). A reduction in winter precipitation could
significantly alter the plant communities of the Sonoran and Mojave
deserts.
Arid environments are especially sensitive to climate change,
because the plants and animals that inhabit these areas are near their
physical tolerances for temperature and water stress. Slight changes in
temperature and rainfall, along with increases in the magnitude and
frequency of extreme climatic events, can significantly alter species
distributions and abundance (Archer and Predick 2008, p. 23). In fact,
warming effects may be particularly severe for reptiles and amphibians.
For instance, Walther et al. (2002, pp. 393-394) found that because of
their physiology, reptiles and amphibians are sensitive to climatic
changes, which may result in effects to their development, spatial
distribution, and interactions with other species. Specifically, egg
development, sperm development, and sex ratios may be affected by
climatic changes in temperatures. Increased temperatures may influence
sex ratios within clutches to favor females over males, which may
benefit populations as one male can fertilize several females. However,
if temperatures rise too much, the effect could strongly select for
female-only clutches, significantly skewing the sex ratio within
populations, and posing long-term problems for reptiles such as Sonoran
desert tortoise populations (Walther et al. 2002, pp. 393-394). But as
stated earlier, Sonoran desert tortoises build their nests in burrows
underground, thereby tempering the effects of rising surface
temperatures.
Sonoran desert tortoises may be affected directly by regional
climate change. For example, increasing temperatures may cause desert
tortoises to overheat (Ernst and Lovich 2009, p. 544). Sonoran desert
tortoises are vulnerable to overheating because they heat up 10 times
faster than they can cool down, making them potentially sensitive to
temperature extremes associated with anticipated climate change (Ernst
and Lovich 2009, p. 544). While climate change may directly affect the
Sonoran desert tortoise, most of the impacts of climate change are
anticipated to be indirect effects to the tortoise caused by other
changes in the ecosystem that supports them. The following discussion
describes anticipated indirect effects to the tortoise in response to
predicted climate change effects.
Changes in atmospheric carbon dioxide and soil nitrogen levels are
anticipated to affect the Sonoran desert tortoise through responses
observed in their forage base. The desert ecosystems inhabited by the
Sonoran desert tortoise are also expected to be sensitive to increased
levels of carbon dioxide in the atmosphere. Desert shrub cover may
increase with increasing carbon dioxide, but nonnative species may also
respond positively, out-competing native vegetation (Smith et al. 2000,
p. 79; Loubimsteva and Adams 2004, p. 401), thereby increasing the risk
of fire. In addition, water and nitrogen are the biggest constraints
that influence biological productivity in desert ecosystems (Ramanujan
2009, p. 1). Predicted higher temperatures are expected to cause higher
levels of nitrogen to escape as a gas from desert soils, leading to a
decrease in soil fertility (Ramanujan 2009, p. 1). Murphy et al. (in
prep., p. ii) expect these responses in the vegetation community to
adversely affect the quality of forage for Sonoran desert tortoises,
leading to dietary nitrogen deficiencies.
Desert tortoises are likely to be affected by decreases in
precipitation due to climate change. Rain is the single most important
climatic factor that drives desert ecosystems because it ultimately
determines recruitment rates, growth and reproduction rates, nutrient
cycling, and net ecosystem productivity, resulting in these ecosystems
being the most vulnerable to changes in precipitation levels (Weltzin
et al. 2003, p. 944; Huxman et al. 2004, p. 254; Hereford et al. 2006,
p. 25). Peterson (1996a, p. 1831) highlights the importance of rain for
desert tortoises: ``Energy acquisition and expenditure in desert
tortoises are strongly constrained by the contingencies of rainfall,
both indirectly through effects on availability and quality of food,
and directly through reliance on freestanding water for drinking, which
is apparently necessary for achieving a net annual energy profit.''
Desert tortoises evolved in arid conditions, and possess numerous
physiological and behavioral adaptations to survive some degree of
drought (Schmidt-Nelson and Bently 1966, p. 911; Peterson 1996b, p.
1325; Christopher 1999, p. 365; Duda et al. 1999, p. 1188; AIDTT 2000,
p. 4; Berry et al. 2002b, pp. 443-446; Dickinson et al. 2002, pp. 251-
252). Peterson (1996a, p. 1831) found desert tortoises have a very low
field metabolic rate when compared to other desert reptiles, which may
provide them an advantage in drought conditions. However, a decrease in
winter precipitation may disproportionately affect reproductive females
because they are highly dependent upon springtime forage. A decrease in
winter precipitation is expected to adversely affect the quantity and
quality of their forage. This, in turn, is likely to directly affect
reproductive output of Sonoran desert tortoise populations (Hereford et
al. 2006, p. 25). Persistent drought, and subsequent changes in the
tortoise forage base, can affect blood chemistry and water metabolism,
reduce or eliminate the thymus and fat stores, and result in skeletal
muscle and liver atrophy in desert tortoises (Berry et al. 2002b, pp.
443-446; Dickinson et al. 2002, pp. 251-252).
Seasonal changes in rainfall may contribute to the spread of
invasive species, such as Sahara mustard and exotic grasses, which are
capable of explosive growth, and able to quickly
[[Page 78137]]
out-compete native species (Barrows et al. 2009, p. 673). As explained
in Factor A, invasive species displace the native vegetation, reducing
forage for tortoises, and increasing the threat of wildfires in desert
ecosystems, resulting in further reduction of forage plants for the
tortoise.
Droughts, which are likely to be more frequent and severe as a
result of climate change, have been suggested to have caused in
declines in local Sonoran desert tortoise populations. Periodic times
of drought are not uncommon in the Southwest, and tortoises have
evolved with drought. However, future drought conditions may be more
severe and long-lasting than previously recorded droughts (Cook et al.
2004, p. 1016). The effects of drought have been shown to have
significant population-level impacts on Mojave desert tortoises, as
exhibited by the observed declines in their populations during years of
drought-induced reductions in annual plants (Longshore et al. 2003, p.
169). As stated previously, Sonoran desert tortoises strongly benefit
from the bimodal precipitation pattern characteristic of the Sonoran
Desert region, specifically from precipitation received during the
summer monsoon. However, the monsoon is characterized by highly-
localized rainfall events of short duration and high magnitude, and can
be spatially unpredictable. Therefore, while some Sonoran desert
tortoise populations may receive satisfactory amounts of summer
precipitation, others may be exposed to reduced monsoon precipitation
totals, and potentially zero precipitation in a given year. This seems
to have been the case during the late 1980s in the Maricopa Mountains
near Phoenix, Arizona. The precipitous loss of 226 Sonoran desert
tortoises in the Maricopa Mountains plot, which occurred between 1987
and 1990, is believed to have resulted from severe, localized drought,
when no measurable rainfall occurred in that area in 1989. This
indicates that even Sonoran desert tortoises may succumb to excessive
drought conditions (Schwalbe 2010, p. 2). Subsequent surveys have shown
that survivorship within this population has improved, and there is
evidence that reproduction has resumed in this population. Also, a lack
of additional carcasses found on the plot indicates that population
declines have stabilized, and the population might be rebounding (AGFD
2010, p. 4). Drought conditions also apparently played a significant
role in a decline of new Sonoran desert tortoise captures between 1988
and 1990 in the San Pedro Valley (Meyer et al. 2010, p. 11). Localized
cases of population declines as a result of drought could be more
common in the future, due to decreasing rainfall caused by climate
change.
Another way to evaluate the threats to a species is the use of
vulnerability assessments. The results of one assessment, conducted by
Galbraith and Price (2009, p. ii) concluded that the desert tortoise
within the United States was ``highly vulnerable'' to extinction as a
result of climate change. The framework used by Galbraith and Price
(2009, pp. 80-82) considered numerous factors including: (1) Current
population size and trends, (2) range trends, (3) likely future
stressor trends, (4) individual replacement time, (5) likely future
vulnerability to stochastic events, (6) future vulnerability to policy/
management change, (7) likely future vulnerability to natural
stressors, (7) physiological sensitivity to temperature and
precipitation change and to extreme weather events, (8) dispersive
capability and potential rate of increase, (9) habitat specialization,
(10) likely event of future habitat loss due to climate change, (11)
ability of habitats to shift in response to climate change, and (12)
dependence on temporal inter-relations and other species. They
summarized: ``Over the last three or four decades, these populations
(Mohave and Sonoran) have come under high degrees of stress due largely
to human activity (particularly urbanization and recreational
intrusion) * * * Climate change may be a significant new stressor,
causing even more habitat loss and exacerbating an already difficult
situation. Together, existing stressors and the direct and indirect
effects of climate change could result in desert tortoises being put at
even greater risk of population reduction and extinction in their U.S.
range.''
Galbraith and Price (2009, pp. 79-80) estimate that at least 20 to
50 percent of habitat today will not be available to desert tortoises
by 2020 as a result of climate change and, to a much lesser extent,
anticipated development. However, in their analysis, Galbraith and
Price (2009, pp. 74-84) largely disregarded the fact that the Sonoran
desert tortoise ranges into Mexico (which represents approximately half
of its total distribution), which should be factored into their
vulnerability analysis. They also often misapplied or gave
disproportionate influence to specific research on the Mojave desert
tortoise in addressing the desert tortoise in the U.S. as a whole.
While we found certain attributes of Galbraith and Price (2009, pp. 74-
84) to be accurate, these identified shortcomings provide an incomplete
picture of the status of the desert tortoise and its vulnerability to
the effects of climate change.
Weiss and Overpeck (2005, p. 2074) disagreed with Galbraith and
Price (2009, pp. 74-84). Accelerated increases in temperature projected
as a result of climate change will potentially result in changes to the
current geographical boundaries of the Sonoran Desert, as well as the
distribution of associated plant species (Weiss and Overpeck 2005, p.
2074). Specifically, Weiss and Overpeck (2005, p. 2074) predicted that
the current geographic boundary of the Sonoran Desert will contract in
its southeast portion and expand in distribution and rise in elevation
in the eastern and northern portions, thus potentially expanding areas
of suitable habitat for the Sonoran desert tortoise. Weiss and Overpeck
(2005, p. 2075) and Galbraith and Price (2009, p. 80) agreed that
observed changes to the fire regime of the Sonoran Desert favor
nonnative plant species, and may impede the trajectory or degree of
potential expansion of the Sonoran Desert.
With the differences in predicted climate change under different
scenarios, and the uncertainty of those effects on the tortoise, it is
difficult to come to a definitive conclusion as to the potential
impacts of climate change on the Sonoran desert tortoise. All, none, or
a combination of these predictions may actually be realized in the
future within the distribution of the Sonoran desert tortoise, which
adds uncertainty to how the tortoise may respond to any given
combination of these predictions. For example, warmer average
temperatures may affect the Sonoran desert tortoise positively by
lengthening annual surface-activity periods which may enhance
reproduction potential and survivorship. Increased frequencies in heavy
precipitation may provide more opportunities for rehydration of Sonoran
desert tortoises and promote the production of forage species, whereby
reducing daily foraging periods to both avoid excessive high
temperatures and, as a consequence, lessen predation risks. However,
higher temperatures coupled with drought conditions could also
negatively affect the Sonoran desert tortoise by increasing metabolism
rates, foraging needs, and associated foraging time, therefore
increasing predation risk. Higher temperatures coupled with drought
conditions could also reduce forage availability of plant species that
depend on higher frequencies of precipitation events for growth (annual
plant species that respond to monsoon storms).
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The temporal aspect of anticipated changes in climate and their
effects on the Sonoran desert tortoise and its habitat must be
considered in context with the rate of evolutionary adaptation of the
species. Skelly et al. (2007, pp. 1353-1355) examined preferred
temperature ranges and thermal maxima, and suggested that some species
with short generation times might evolve to meet the demands of a
changing climate. The Sonoran desert tortoise has much longer
generation times (approximately 12 to 15 years) and may therefore be
more vulnerable to the effects of climate change, because they are
unlikely to be able to rapidly adapt to environmental changes.
Specifically, we do not expect their evolutionary processes to keep
pace with the relatively fast-paced changes predicted as a result of
climate change in the near- or mid-term.
Perhaps the most important aspect of projected changes in climate
is the relative irreversibility of these changes into the future.
Solomon et al. (2009, p. 1704) state that the effects of climate change
will be irreversible for approximately 1,000 years, even if carbon
emissions dropped to zero in current times, as a result of the
longevity of atmospheric carbon dioxide and feedbacks associated with
ocean warming (Solomon et al. 2009, p. 1709).
Summary of Factor E
Our review of the best scientific and commercial data available
indicated that Sonoran desert tortoises may be vulnerable to the
effects of environmental contamination: Ingestion of trash, including
balloons; and substances from illegal solid waste dumps. However the
literature did not indicate these threats were currently affecting
populations and specific reports of affected individual tortoises were
rare. Vehicle strike mortalities have been documented, and may have
some local sub-population effects in close proximity to more heavily
traveled roads and highways, but again, these effects are more
localized and not rangewide, and thus do not appear to have overall
population-level effects. Further, while management and mitigation
actions are being implemented, such as the construction of barrier
fences and culverts, these devices are generally not maintained and
appear to be ineffective in helping to reduce these individual
mortalities.
Climate change may also affect Sonoran desert tortoises. The
combined effects of global and regional climate change, along with the
effects of long-term drought, will play a role in the long-term
persistence of the species. However, we are not able to quantify, with
certainty, how the direct and indirect effects of climate change will
affect Sonoran desert tortoise populations. Tortoise habitat may shift,
native vegetation may change depending on rainfall patterns, increasing
temperatures may affect the growth of native vegetation, the quality
and quantity of desert tortoise forage may be affected, precipitation
patterns will likely affect desert vegetation, and tortoises may
experience physiological effects that could result in changes in
reproduction and overall survival. We conclude that climate change may
be a significant stressor that exacerbates current threats, both
directly (physiological effects to the tortoise) and indirectly
(habitat loss and fragmentation). As such, climate change, in and of
itself, may affect Sonoran desert tortoise populations, but the
magnitude of the impacts to the Sonoran desert tortoise remains
uncertain. Climate change is not currently a threat to the Sonoran
desert tortoise, but it has the potential to be a threat in the
foreseeable future. Impacts from climate change in the future will
likely exacerbate the current and ongoing threat of habitat loss caused
by other factors, as discussed above.
Finding
As required by the Act, we conducted a review of the status of the
Sonoran desert tortoise DPS and considered the five factors in
assessing whether the DPS is threatened or endangered throughout all or
a significant portion of its range. We examined the best scientific and
commercial information available regarding the past, present, and
future threats faced by the Sonoran desert tortoise. We reviewed the
petition, information available in our files, and other available
published and unpublished information, and we consulted with species
experts, land managers, and numerous stakeholders including Federal,
State, and Tribal agencies.
In considering what factors might constitute threats, we must look
beyond the mere exposure of the species to the factor to determine
whether the species responds to the factor in a way that causes actual
impacts to the species. If there is exposure to a factor, but no
response, or only a positive response, that factor is not a threat. If
there is exposure and the species responds negatively, the factor may
be a threat and we then attempt to determine how significant a threat
it is. If the threat is significant, it may drive or contribute to the
risk of extinction of the species such that the species warrants
listing as threatened or endangered as those terms are defined by the
Act. This does not necessarily require empirical proof of a threat;
however, reasonably strong data-based inferences are the minimum
standard for considering a threat significant. The mere identification
of factors that could impact a species negatively is not sufficient to
compel a finding that listing is appropriate; we require evidence that
these factors are operative threats that act on the species to the
point that the species meets the definition of threatened or endangered
under the Act.
Despite the history of conservation and management efforts afforded
the Sonoran desert tortoise in Arizona, our review of the literature
identified threats to the Sonoran desert tortoise attributable to all
Threat Factors (A-E). The primary threats to the Sonoran desert
tortoise from habitat modification and destruction (Factor A) include
the: (1) Current and ongoing invasion of nonnative plant species
resulting in an unnatural, destructive wildfire regime in portions of
the species' distribution; (2) cumulative, anticipated indirect effects
to habitat and individual tortoises from increased human activity tied
to urbanization and human population growth; (3) current and
anticipated creation of barriers to genetic exchange among populations
from urbanization and associated infrastructure; (4) high and growing
use and popularity of OHV use in Sonoran desert tortoise habitat; (5)
mesquite and ironwood tree harvest in Mexico; (6) improper livestock
grazing in Mexico; and (7) undocumented human immigration and
interdiction activities. The primary threat to the Sonoran desert
tortoise from overutilization for commercial, recreational, scientific,
or educational purposes (Factor B) is illegal collection. The primary
threat to the Sonoran desert tortoise from predation (Factor C) is the
increase in feral or off-leash domestic dog predation and human
depredation associated with anticipated increases in urbanization and
human population growth. The Sonoran desert tortoise is also threatened
by the inadequacy of regulatory mechanisms (Factor D). In our review of
the available information, we found numerous examples where the Sonoran
desert tortoise is considered in management actions and tortoise-
specific mitigation measures are mandated, or where land activities
that could appreciably threaten Sonoran desert tortoise populations are
prohibited. However, significant threats we have identified in Factors
A, C, and E (primarily invading nonnative plant
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species and subsequent wildfire concerns, vandalism of tortoises, feral
dog predation, and climate change) are not being adequately addressed
by land managers or other regulatory mechanisms. The primary threats to
the Sonoran desert tortoise from other natural or manmade factors
affecting its continued existence (Factor E) include the threats from
vehicle strike mortality due to unmaintained structures intended to
prevent tortoise mortality along heavily traveled routes through core
Sonoran desert tortoise populations. In addition, anticipated effects
from climate change are likely to exacerbate the ongoing threat of
habitat loss and degradation by other factors, but we were unable to
conclude that climate change, by itself, currently threatens the
Sonoran desert tortoise. We have documented adverse effects of many of
these threats on existing Sonoran desert tortoise populations, both
historically and currently, and note that many threats act in
synergistic combination in their effects to the tortoise. The factors
that are the primary drivers of these threats, such as urbanization,
human population growth, and drought, are predicted to increase in the
foreseeable future.
As a result of the numerous threats to the Sonoran desert tortoise
identified above--which have occurred historically, continue today, and
are predicted to continue in the foreseeable future--the tortoise has
lost appreciable amounts of habitat to the collective footprint of
urban development, agriculture, and infrastructure on the landscape.
Collectively, these land changes have not only destroyed former Sonoran
desert tortoise habitat, but have fragmented remaining populations,
threatening long-term genetic fitness of the tortoise and precluding
their recolonization ability in the event of population extirpations.
In Mexico, significant areas of former Sonoran desert habitat have been
significantly altered by the cultivation and natural colonization of
invasive, nonnative plant species, and in combination with other
threats, have likely greatly affected the viability of the Sonoran
desert tortoise in that country.
Available monitoring data are not adequate to accurately determine
how the Sonoran desert tortoise historically responded to the loss of
habitat or how populations have individually responded to threats, but
we are reasonably certain that there are fewer Sonoran desert tortoises
currently than historically, and that populations have become
significantly fragmented over time. Currently within Arizona,
approximately 75 percent of potential Sonoran desert tortoise habitat
is within 30 mi (48 km) or less of human populations of 1,000 people or
more. The factors that have resulted in the loss or degradation of
habitat, or threaten the tortoise directly, are predicted to worsen in
the foreseeable future as the footprint of development and
infrastructure grows and human population growth ensues. Some
populations may disappear altogether, while others become smaller and
more contracted; each of these scenarios exacerbates isolation and
genetic and demographic exchange. Therefore, we reasonably anticipate
that the Sonoran desert tortoise DPS is in danger of extinction in the
foreseeable future throughout all or a significant portion of its
range.
On the basis of the best scientific and commercial information
available, we find that the petitioned action, to list the Sonoran
desert tortoise is warranted. In making this finding, we gave
significant deference to the irreversible effect of threats as they are
anticipated to occur in the foreseeable future. We will make a
determination on the status of the species as threatened or endangered
when we do a proposed listing determination. However, as explained in
more detail below, an immediate proposal of a regulation implementing
this action is precluded by higher priority listing actions, and
progress is being made to add or remove qualified species from the
Lists of Endangered and Threatened Wildlife and Plants.
We reviewed the available information to determine if the existing
and foreseeable threats render the species at risk of extinction at
this time such that issuing an emergency regulation temporarily listing
the DPS under section 4(b)(7) of the Act is warranted. We determined
that issuing an emergency regulation temporarily listing the species is
not warranted for this species at this time because we have not
documented any significant population extirpations. However, if at any
time we determine that issuing an emergency regulation temporarily
listing the Sonoran desert tortoise is warranted, we will initiate this
action at that time.
Listing Priority Number
The Service adopted guidelines on September 21, 1983 (48 FR 43098)
to establish a rational system for utilizing available resources for
the highest priority species when adding species to the Lists of
Endangered and Threatened Wildlife and Plants or reclassifying species
listed as threatened to endangered status. These guidelines, titled
``Endangered and Threatened Species Listing and Recovery Priority
Guidelines'' address the immediacy and magnitude of threats, and the
level of taxonomic distinctiveness. The system places greatest
importance on the immediacy and magnitude of threats, but also factors
in the level of taxonomic distinctiveness by assigning priority in
descending order to monotypic genera (genus with one species), full
species, and subspecies (or equivalently, distinct population segments
of vertebrates). As a result of our analysis of the best available
scientific and commercial information, we assigned the Sonoran desert
tortoise a Listing Priority Number of 6, based on the high magnitude
and non-imminence of threats. One or more of the threats discussed
above are occurring in virtually every known population throughout its
range. These threats are ongoing, and will continue to occur into the
foreseeable future and, in some cases (such as nonnative plant species
invasions and climate change effects), are considered irreversible. Our
rationale for assigning the Sonoran desert tortoise an LPN of 6 is
outlined below.
Under the Service's LPN Guidance, the magnitude of threat is the
first criterion we look at when establishing a listing priority. The
guidance indicates that species with the highest magnitude of threat
are those species facing the greatest threats to their continued
existence. These species receive the highest listing priority. Threats
to the Sonoran desert tortoise vary in their magnitude. We found the
most significant threats to the Sonoran desert tortoise to be the
expansion of range and increase in number of nonnative plant species,
urban development and associated human population growth in Arizona,
and the highly popular and growing use of OHVs in Arizona. These
threats have both direct and indirect effects to the Sonoran desert
tortoise and its habitat. The area of land affected by nonnative
species is widespread and, although currently and comparatively less
significant in Arizona, it is substantial in Mexico. It is also
expected to increase in the foreseeable future in both countries. When
including the total land area adversely modified by ironwood and
mesquite harvesting, it is projected that an estimated 98 percent of
the Sonoran desert tortoises' habitat in Mexico (47 percent of habitat
rangewide) will be lost or adversely modified in the foreseeable
future. Additionally, there is currently no viable solution to the
threat posed by the increase in nonnative plants on this landscape. The
projected human population growth and urban development throughout this
DPS are
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likely to both pose significant problems for genetic exchange among
Sonoran desert tortoise populations. This will increase the degree and
scope of human interactions with tortoises and occupied habitat, which
threatens the tortoise in a variety of ways that we discuss in detail
above. Currently in Arizona, 75 percent of potentially occupied Sonoran
desert tortoise habitat occurs within 30 mi (48 km) or less of a city
or town with a human population of 1,000 or more, and, considering
future growth projections, it is likely that 100 percent of occupied
tortoise habitat will be affected in the future. The ever-expanding
human population in Arizona is also likely to lead to commensurate
increases in OHV use. As of 2007, 385,000 off-highway vehicles were
registered in Arizona (a 350 percent increase since 1998), and 1.7
million people (29 percent of the Arizona's public) engaged in off-road
activity from 2005 to 2007. We identified significant threats from OHV
use in Sonoran desert tortoise habitat, including habitat destruction,
increased illegal collection of tortoises, and significant problems
with law enforcement of OHV users. Despite problems associated with OHV
management, several land management agencies responsible for Sonoran
desert tortoise habitat have plans to expand OHV use on their lands.
These three major threats operate in combination with other threats
which, by themselves, might not be as serious, but acting together,
cause a more serious cumulative impact. These threats include improper
livestock management in Mexico, illegal collection and release of
tortoises, undocumented human immigration and associated interdiction
activities, predation from feral or off-leash dogs, vehicle strike
mortality from unmaintained, roadside mitigation devices, and
anticipated possible effects from climate change. In their totality,
these threats are high in magnitude because of the amount of habitat
that is likely to be affected and the irreversible nature of the effect
of these threats in sensitive habitats that are slow to rebound.
Under our LPN Guidance, the second criterion we consider in
assigning a listing priority is the immediacy of threats. This
criterion is intended to ensure that the species that face actual,
identifiable threats are given priority over those species for which
threats are only potential or that are intrinsically vulnerable but are
not known to be presently facing such threats. The threats are non-
imminent because they are not ubiquitous throughout the range of the
Sonoran desert tortoise where they occur. Some are acting currently in
some areas, but not the whole DPS; some threats are likely to expand
geographically over time; some are stabilized or even reducing in
impact. Although we reviewed and discussed the numerous ways that
individual Sonoran desert tortoises are affected by various threats,
there is currently no evidence that any existing population is
threatened with extirpation in the near future. So while some of the
threats are happening now, impacts to tortoise populations are not
likely to be evident in the immediate future.
For example, we have documented that the invasion of nonnative
plants is most significant in Sonora, Mexico, because of active
planting for livestock grazing purposes. However, while there were
historic practices of planting nonnative plant species as forage for
livestock in the United States, these activities have ceased, leaving
only slower, natural mechanisms to facilitate the expansion of
nonnative plant species in this country. Thus, comparatively less
habitat area is significantly altered by nonnative plant distribution
and abundance in Arizona, representing approximately half of the
Sonoran desert tortoises' range. Additionally, monitoring data indicate
that Sonoran desert tortoise populations persist in habitat that is
unburned, but where nonnative species have become established. As
stated in Factor A, wildfire is an important trigger, capable of making
nonnative-invaded habitat unsuitable for Sonoran desert tortoises. The
majority of nonnative-invaded Sonoran desert tortoise habitat remains
unburned in the United States; however we are less certain about the
occurrence of wildfire in nonnative-invaded habitat in Mexico. In both
cases in Arizona and Mexico the ongoing conversion of habitats to
nonnative grasses are not expected to impact tortoise populations in
the very immediate future. Therefore, the actual impacts on tortoise
populations from these and similar threats, such as climate change, are
more likely to occur in the mid- to long-term future and are not
considered imminent.
Also, many of the threats we discuss above are linked to
urbanization and human population growth. In Arizona, we have observed
significant development and human population growth over the past
several decades, but a weakened economy has slowed growth in recent
years. We documented that the Sun Corridor Megapolitan is expected to
nearly double the human population of southern and central Arizona by
2030. However, much of the urbanization that has already occurred
replaced agricultural land that was not usable Sonoran desert tortoise
habitat. Additionally, our evaluation of Sonoran desert tortoise
population monitoring data has not indicated that any monitored
population has been extirpated and less than one-third of monitored
populations have shown declines, indicating that impacts on Sonoran
desert tortoise populations are not currently imminent. These actual,
identifiable threats are covered in detail under the discussion of
Factors A through E of this finding and currently include habitat
destruction, modification, and fragmentation, overutilization,
predation from unnatural sources, inadequate regulatory mechanisms, and
other natural and manmade factors.
The third criterion in our LPN guidance is intended to devote
resources to those species representing highly distinctive or isolated
gene pools as reflected by taxonomy. The Sonoran desert tortoise is a
valid taxon at the DPS level, and therefore receives a lower priority
than species in a monotypic genus. The Sonoran desert tortoise faces
high magnitude, non-imminent threats, and is a valid taxon at the DPS
level. Thus, in accordance with our LPN guidance, we have assigned the
Sonoran desert tortoise an LPN of 6.
We will continue to monitor the threats to the Sonoran desert
tortoise, and the species' status on an annual basis, and should the
magnitude or the imminence of the threats change, we will revisit our
assessment of the LPN.
Work on a proposed listing determination for the Sonoran desert
tortoise is precluded by work on higher priority listing actions with
absolute statutory, court-ordered, or court-approved deadlines and
final listing determinations for those species that were proposed for
listing with funds from Fiscal Year 2011. This work includes all the
actions listed in the tables below under expeditious progress.
Preclusion and Expeditious Progress
Preclusion is a function of the listing priority of a species in
relation to the resources that are available and the cost and relative
priority of competing demands for those resources. Thus, in any given
fiscal year (FY), multiple factors dictate whether it will be possible
to undertake work on a listing proposal regulation or whether
promulgation of such a proposal is precluded by higher-priority listing
actions.
[[Page 78141]]
The resources available for listing actions are determined through
the annual Congressional appropriations process. The appropriation for
the Listing Program is available to support work involving the
following listing actions: Proposed and final listing rules; 90-day and
12-month findings on petitions to add species to the Lists of
Endangered and Threatened Wildlife and Plants (Lists) or to change the
status of a species from threatened to endangered; annual
``resubmitted'' petition findings on prior warranted-but-precluded
petition findings as required under section 4(b)(3)(C)(i) of the Act;
critical habitat petition findings; proposed and final rules
designating critical habitat; and litigation-related, administrative,
and program-management functions (including preparing and allocating
budgets, responding to Congressional and public inquiries, and
conducting public outreach regarding listing and critical habitat). The
work involved in preparing various listing documents can be extensive
and may include, but is not limited to: gathering and assessing the
best scientific and commercial data available and conducting analyses
used as the basis for our decisions; writing and publishing documents;
and obtaining, reviewing, and evaluating public comments and peer
review comments on proposed rules and incorporating relevant
information into final rules. The number of listing actions that we can
undertake in a given year also is influenced by the complexity of those
listing actions; that is, more complex actions generally are more
costly. The median cost for preparing and publishing a 90-day finding
is $39,276; for a 12-month finding, $100,690; for a proposed rule with
critical habitat, $345,000; and for a final listing rule with critical
habitat, the median cost is $305,000.
We cannot spend more than is appropriated for the Listing Program
without violating the Anti-Deficiency Act (see 31 U.S.C.
1341(a)(1)(A)). In addition, in FY 1998 and for each fiscal year since
then, Congress has placed a statutory cap on funds which may be
expended for the Listing Program, equal to the amount expressly
appropriated for that purpose in that fiscal year. This cap was
designed to prevent funds appropriated for other functions under the
Act (for example, recovery funds for removing species from the Lists),
or for other Service programs, from being used for Listing Program
actions (see House Report 105-163, 105th Congress, 1st Session, July 1,
1997).
Since FY 2002, the Service's budget has included a critical habitat
subcap to ensure that some funds are available for other work in the
Listing Program (``The critical habitat designation subcap will ensure
that some funding is available to address other listing activities''
(House Report No. 107-103, 107th Congress, 1st Session, June 19,
2001)). In FY 2002 and each year until FY 2006, the Service has had to
use virtually the entire critical habitat subcap to address court-
mandated designations of critical habitat, and consequently none of the
critical habitat subcap funds have been available for other listing
activities. In some FYs since 2006, we have been able to use some of
the critical habitat subcap funds to fund proposed listing
determinations for high-priority candidate species. In other FYs, while
we were unable to use any of the critical habitat subcap funds to fund
proposed listing determinations, we did use some of this money to fund
the critical habitat portion of some proposed listing determinations so
that the proposed listing determination and proposed critical habitat
designation could be combined into one rule, thereby being more
efficient in our work. In FY 2011 we anticipate that we will be unable
to use any of the critical habitat subcap funds to fund proposed
listing determinations.
We make our determinations of preclusion on a nationwide basis to
ensure that the species most in need of listing will be addressed first
and also because we allocate our listing budget on a nationwide basis.
Through the listing cap, the critical habitat subcap, and the amount of
funds needed to address court-mandated critical habitat designations,
Congress and the courts have, in effect, determined the amount of money
available for other listing activities nationwide. Therefore, the funds
in the listing cap, other than those needed to address court-mandated
critical habitat for already listed species, set the limits on our
determinations of preclusion and expeditious progress.
Congress identified the availability of resources as the only basis
for deferring the initiation of a rulemaking that is warranted. The
Conference Report accompanying P.L. 97-304, which established the
current statutory deadlines and the warranted-but-precluded finding,
states that the amendments were ``not intended to allow the Secretary
to delay commencing the rulemaking process for any reason other than
that the existence of pending or imminent proposals to list species
subject to a greater degree of threat would make allocation of
resources to such a petition [that is, for a lower-ranking species]
unwise.'' Although that statement appeared to refer specifically to the
``to the maximum extent practicable'' limitation on the 90-day deadline
for making a ``substantial information'' finding, that finding is made
at the point when the Service is deciding whether or not to commence a
status review that will determine the degree of threats facing the
species, and therefore the analysis underlying the statement is more
relevant to the use of the warranted-but-precluded finding, which is
made when the Service has already determined the degree of threats
facing the species and is deciding whether or not to commence a
rulemaking.
In FY 2010, $10,471,000 is the amount of money that Congress
appropriated for the Listing Program (that is, the portion of the
Listing Program funding not related to critical habitat designations
for species that are already listed). Therefore, a proposed listing is
precluded if pending proposals with higher priority will require
expenditure of at least $10,471,000, and expeditious progress is the
amount of work that can be achieved with $10,471,000. Since court
orders requiring critical habitat work will not require use of all of
the funds within the critical habitat subcap, we used $1,114,417 of our
critical habitat subcap funds in order to work on as many of our
required petition findings and listing determinations as possible. This
brings the total amount of funds we had for listing actions in FY 2010
to $11,585,417.
The $11,585,417 was used to fund work in the following categories:
compliance with court orders and court-approved settlement agreements
requiring that petition findings or listing determinations be completed
by a specific date; section 4 (of the Act) listing actions with
absolute statutory deadlines; essential litigation-related,
administrative, and listing program-management functions; and high-
priority listing actions for some of our candidate species. For FY
2011, on September 29, 2010, Congress passed a continuing resolution
which provides funding at the FY 2010 enacted level. In 2009, the
responsibility for listing foreign species under the Act was
transferred from the Division of Scientific Authority, International
Affairs Program, to the Endangered Species Program. Therefore, starting
in FY 2010, we use a portion of our funding to work on the actions
described above as they apply to listing actions for foreign species.
This has the potential to further reduce funding available for domestic
listing actions. Although there are currently no foreign species issues
included in our high-
[[Page 78142]]
priority listing actions at this time, many actions have statutory or
court-approved settlement deadlines, thus increasing their priority.
The budget allocations for each specific listing action are identified
in the Service's FY 2011 Allocation Table (part of our administrative
record).
Based on our September 21, 1983, guidance for assigning an LPN for
each candidate species (48 FR 43098), we have a significant number of
species with an LPN of 2. Using this guidance, we assign each candidate
an LPN of 1 to 12, depending on the magnitude of threats (high or
moderate to low), immediacy of threats (imminent or nonimminent), and
taxonomic status of the species (in order of priority: monotypic genus
(a species that is the sole member of a genus); species, or part of a
species (subspecies, distinct population segment, or significant
portion of the range)). The lower the listing priority number, the
higher the listing priority (that is, a species with an LPN of 1 would
have the highest listing priority).
Because of the large number of high-priority species, we have
further ranked the candidate species with an LPN of 2 by using the
following extinction-risk type criteria: International Union for the
Conservation of Nature and Natural Resources (IUCN) Red list status/
rank, Heritage rank (provided by NatureServe), Heritage threat rank
(provided by NatureServe), and species currently with fewer than 50
individuals, or 4 or fewer populations. Those species with the highest
IUCN rank (critically endangered), the highest Heritage rank (G1), the
highest Heritage threat rank (substantial, imminent threats), and
currently with fewer than 50 individuals, or fewer than 4 populations,
originally comprised a group of approximately 40 candidate species
(``Top 40''). These 40 candidate species have had the highest priority
to receive funding to work on a proposed listing determination. As we
work on proposed and final listing rules for those 40 candidates, we
apply the ranking criteria to the next group of candidates with an LPN
of 2 and 3 to determine the next set of highest-priority candidate
species. Finally, proposed rules for reclassification of threatened
species to endangered are lower priority, since as listed species, they
are already afforded the protection of the Act and implementing
regulations. However, for efficiency reasons, we may choose to work on
a proposed rule to reclassify a species to endangered if we can combine
this with work that is subject to a court-determined deadline.
With our workload so much bigger than the amount of funds we have
to accomplish it, it is important that we be as efficient as possible
in our listing process. Therefore, as we work on proposed rules for the
highest priority species in the next several years, we are preparing
multi-species proposals when appropriate, and these may include species
with lower priority if they overlap geographically or have the same
threats as a species with an LPN of 2. In addition, we take into
consideration the availability of staff resources when we determine
which high-priority species will receive funding to minimize the amount
of time and resources required to complete each listing action.
As explained above, a determination that listing is warranted but
precluded must also demonstrate that expeditious progress is being made
to add and remove qualified species to and from the Lists of Endangered
and Threatened Wildlife and Plants. As with our ``precluded'' finding,
the evaluation of whether progress in adding qualified species to the
Lists has been expeditious is a function of the resources available for
listing and the competing demands for those funds. Although we do not
discuss it in detail here, we are also making expeditious progress in
removing species from the list under the Recovery program in light of
the resource available for delisting, which is funded by a separate
line item in the budget of the Endangered Species Program. During FY
2010, we have completed two proposed delisting rules and two final
delisting rules. Given the limited resources available for listing, we
find that we made expeditious progress in FY 2010 in the Listing
Program. This progress included preparing and publishing the following
determinations:
FY 2010 and FY 2011 Completed Listing Actions
----------------------------------------------------------------------------------------------------------------
Publication date Title Actions FR pages
----------------------------------------------------------------------------------------------------------------
10/08/2009....................... Listing Lepidium Final Listing 74 FR 52013-52064.
papilliferum (Slickspot Threatened.
Peppergrass) as a
Threatened Species
Throughout Its Range.
10/27/2009....................... 90-day Finding on a Notice of 90-day 74 FR 55177-55180.
Petition To List the Petition Finding,
American Dipper in the Not substantial.
Black Hills of South
Dakota as Threatened or
Endangered.
10/28/2009....................... Status Review of Arctic Notice of Intent to 74 FR 55524-55525.
Grayling (Thymallus Conduct Status
arcticus) in the Upper Review for Listing
Missouri River System. Decision.
11/03/2009....................... Listing the British Proposed Listing 74 FR 56757-56770.
Columbia Distinct Threatened.
Population Segment of
the Queen Charlotte
Goshawk Under the
Endangered Species Act:
Proposed rule.
11/03/2009....................... Listing the Salmon- Proposed Listing 74 FR 56770-56791.
Crested Cockatoo as Threatened.
Threatened Throughout
Its Range with Special
Rule.
11/23/2009....................... Status Review of Notice of Intent to 74 FR 61100-61102.
Gunnison sage-grouse Conduct Status
(Centrocercus minimus). Review for Listing
Decision.
12/03/2009....................... 12-Month Finding on a Notice of 12-month 74 FR 63343-63366.
Petition to List the petition finding,
Black-tailed Prairie Not warranted.
Dog as Threatened or
Endangered.
12/03/2009....................... 90-Day Finding on a Notice of 90-day 74 FR 63337-63343.
Petition to List Petition Finding,
Sprague's Pipit as Substantial.
Threatened or
Endangered.
12/15/2009....................... 90-Day Finding on Notice of 90-day 74 FR 66260-66271.
Petitions To List Nine Petition Finding,
Species of Mussels From Substantial.
Texas as Threatened or
Endangered With
Critical Habitat.
12/16/2009....................... Partial 90-Day Finding Notice of 90-day 74 FR 66865-66905.
on a Petition to List Petition Finding,
475 Species in the Not substantial
Southwestern United and Substantial.
States as Threatened or
Endangered With
Critical Habitat.
12/17/2009....................... 12-month Finding on a Notice of 12-month 74 FR 66937-66950.
Petition To Change the petition finding,
Final Listing of the Warranted but
Distinct Population precluded.
Segment of the Canada
Lynx To Include New
Mexico.
[[Page 78143]]
1/05/2010........................ Listing Foreign Bird Proposed Listing 75 FR 605-649.
Species in Peru and Endangered.
Bolivia as Endangered
Throughout Their Range.
1/05/2010........................ Listing Six Foreign Proposed Listing 75 FR 286-310.
Birds as Endangered Endangered.
Throughout Their Range.
1/05/2010........................ Withdrawal of Proposed Proposed rule, 75 FR 310-316.
Rule to List Cook's withdrawal.
Petrel.
1/05/2010........................ Final Rule to List the Final Listing 75 FR 235-250.
Galapagos Petrel and Threatened.
Heinroth's Shearwater
as Threatened
Throughout Their Ranges.
1/20/2010........................ Initiation of Status Notice of Intent to 75 FR 3190-3191.
Review for Agave Conduct Status
eggersiana and Solanum Review for Listing
conocarpum. Decision.
2/09/2010........................ 12-month Finding on a Notice of 12-month 75 FR 6437-6471.
Petition to List the petition finding,
American Pika as Not warranted.
Threatened or
Endangered.
2/25/2010........................ 12-Month Finding on a Notice of 12-month 75 FR 8601-8621.
Petition To List the petition finding,
Sonoran Desert Not warranted.
Population of the Bald
Eagle as a Threatened
or Endangered Distinct
Population Segment.
2/25/2010........................ Withdrawal of Proposed Withdrawal of 75 FR 8621-8644.
Rule To List the Proposed Rule to
Southwestern Washington/ List.
Columbia River Distinct
Population Segment of
Coastal Cutthroat Trout
(Oncorhynchus clarki
clarki) as Threatened.
3/18/2010........................ 90-Day Finding on a Notice of 90-day 75 FR 13068-13071.
Petition to List the Petition Finding,
Berry Cave salamander Substantial.
as Endangered.
3/23/2010........................ 90-Day Finding on a Notice of 90-day 75 FR 13717-13720.
Petition to List the Petition Finding,
Southern Hickorynut Not substantial.
Mussel (Obovaria
jacksoniana) as
Endangered or
Threatened.
3/23/2010........................ 90-Day Finding on a Notice of 90-day 75 FR 13720-13726.
Petition to List the Petition Finding,
Striped Newt as Substantial.
Threatened.
3/23/2010........................ 12-Month Findings for Notice of 12-month 75 FR 13910-14014.
Petitions to List the petition finding,
Greater Sage-Grouse Warranted but
(Centrocercus precluded.
urophasianus) as
Threatened or
Endangered.
3/31/2010........................ 12-Month Finding on a Notice of 12-month 75 FR 16050-16065.
Petition to List the petition finding,
Tucson Shovel-Nosed Warranted but
Snake (Chionactis precluded.
occipitalis klauberi)
as Threatened or
Endangered with
Critical Habitat.
4/5/2010......................... 90-Day Finding on a Notice of 90-day 75 FR 17062-17070.
Petition To List Petition Finding,
Thorne's Hairstreak Substantial.
Butterfly as Endangered.
4/6/2010......................... 12-month Finding on a Notice of 12-month 75 FR 17352-17363.
Petition To List the petition finding,
Mountain Whitefish in Not warranted.
the Big Lost River,
Idaho, as Endangered or
Threatened.
4/6/2010......................... 90-Day Finding on a Notice of 90-day 75 FR 17363-17367.
Petition to List a Petition Finding,
Stonefly (Isoperla Not substantial.
jewetti) and a Mayfly
(Fallceon eatoni) as
Threatened or
Endangered with
Critical Habitat.
4/7/2010......................... 12-Month Finding on a Notice of 12-month 75 FR 17667-17680.
Petition to Reclassify petition finding,
the Delta Smelt From Warranted but
Threatened to precluded.
Endangered Throughout
Its Range.
4/13/2010........................ Determination of Final Listing 75 FR 18959-19165.
Endangered Status for Endangered.
48 Species on Kauai and
Designation of Critical
Habitat.
4/15/2010........................ Initiation of Status Notice of 75 FR 19591-19592.
Review of the North Initiation of
American Wolverine in Status Review for
the Contiguous United Listing Decision.
States.
4/15/2010........................ 12-Month Finding on a Notice of 12-month 75 FR 19592-19607.
Petition to List the petition finding,
Wyoming Pocket Gopher Not warranted.
as Endangered or
Threatened with
Critical Habitat.
4/16/2010........................ 90-Day Finding on a Notice of 90-day 75 FR 19925-19935.
Petition to List a Petition Finding,
Distinct Population Substantial.
Segment of the Fisher
in Its United States
Northern Rocky Mountain
Range as Endangered or
Threatened with
Critical Habitat.
4/20/2010........................ Initiation of Status Notice of 75 FR 20547-20548.
Review for Sacramento Initiation of
splittail (Pogonichthys Status Review for
macrolepidotus). Listing Decision.
4/26/2010........................ 90-Day Finding on a Notice of 90-day 75 FR 21568-21571.
Petition to List the Petition Finding,
Harlequin Butterfly as Substantial.
Endangered.
4/27/2010........................ 12-Month Finding on a Notice of 12-month 75 FR 22012-22025.
Petition to List petition finding,
Susan's Purse-making Not warranted.
Caddisfly (Ochrotrichia
susanae) as Threatened
or Endangered.
4/27/2010........................ 90-day Finding on a Notice of 90-day 75 FR 22063-22070.
Petition to List the Petition Finding,
Mohave Ground Squirrel Substantial.
as Endangered with
Critical Habitat.
5/4/2010......................... 90-Day Finding on a Notice of 90-day 75 FR 23654-23663.
Petition to List Hermes Petition Finding,
Copper Butterfly as Substantial.
Threatened or
Endangered.
6/1/2010......................... 90-Day Finding on a Notice of 90-day 75 FR 30313-30318.
Petition To List Petition Finding,
Castanea pumila var. Substantial.
ozarkensis.
6/1/2010......................... 12-month Finding on a Notice of 12-month 75 FR 30338-30363.
Petition to List the petition finding,
White-tailed Prairie Not warranted.
Dog as Endangered or
Threatened.
6/9/2010......................... 90-Day Finding on a Notice of 90-day 75 FR 32728-32734.
Petition To List van Petition Finding,
Rossem's Gull-billed Substantial.
Tern as Endangered or
Threatened.
6/16/2010........................ 90-Day Finding on Five Notice of 90-day 75 FR 34077-34088.
Petitions to List Seven Petition Finding,
Species of Hawaiian Substantial.
Yellow-faced Bees as
Endangered.
6/22/2010........................ 12-Month Finding on a Notice of 12-month 75 FR 35398-35424.
Petition to List the petition finding,
Least Chub as Warranted but
Threatened or precluded.
Endangered.
6/23/2010........................ 90-Day Finding on a Notice of 90-day 75 FR 35746-35751.
Petition to List the Petition Finding,
Honduran Emerald Substantial.
Hummingbird as
Endangered.
6/23/2010........................ Listing Ipomopsis Proposed Listing 75 FR 35721-35746.
polyantha (Pagosa Endangered
Skyrocket) as Proposed Listing
Endangered Throughout Threatened.
Its Range, and Listing
Penstemon debilis
(Parachute Beardtongue)
and Phacelia submutica
(DeBeque Phacelia) as
Threatened Throughout
Their Range.
[[Page 78144]]
6/24/2010........................ Listing the Flying Final Listing 75 FR 35990-36012.
Earwig Hawaiian Endangered.
Damselfly and Pacific
Hawaiian Damselfly As
Endangered Throughout
Their Ranges.
6/24/2010........................ Listing the Cumberland Proposed Listing 75 FR 36035-36057.
Darter, Rush Darter, Endangered.
Yellowcheek Darter,
Chucky Madtom, and
Laurel Dace as
Endangered Throughout
Their Ranges.
6/29/2010........................ Listing the Mountain Reinstatement of 75 FR 37353-37358.
Plover as Threatened. Proposed Listing
Threatened.
7/20/2010........................ 90-Day Finding on a Notice of 90-day 75 FR 42033-42040.
Petition to List Pinus Petition Finding,
albicaulis (Whitebark Substantial.
Pine) as Endangered or
Threatened with
Critical Habitat.
7/20/2010........................ 12-Month Finding on a Notice of 12-month 75 FR 42040-42054.
Petition to List the petition finding,
Amargosa Toad as Not warranted.
Threatened or
Endangered.
7/20/2010........................ 90-Day Finding on a Notice of 90-day 75 FR 42059-42066.
Petition to List the Petition Finding,
Giant Palouse Earthworm Substantial.
(Driloleirus
americanus) as
Threatened or
Endangered.
7/27/2010........................ Determination on Listing Final Listing 75 FR 43844-43853.
the Black-Breasted Endangered.
Puffleg as Endangered
Throughout its Range;
Final Rule.
7/27/2010........................ Final Rule to List the Final Listing 75 FR 43853-43864.
Medium Tree-Finch Endangered.
(Camarhynchus pauper)
as Endangered
Throughout Its Range.
8/3/2010......................... Determination of Final Listing 75 FR 45497-45527.
Threatened Status for Threatened.
Five Penguin Species.
8/4/2010......................... 90-Day Finding on a Notice of 90-day 75 FR 46894-46898.
Petition To List the Petition Finding,
Mexican Gray Wolf as an Substantial.
Endangered Subspecies
With Critical Habitat.
8/10/2010........................ 90-Day Finding on a Notice of 90-day 75 FR 48294-48298.
Petition to List Petition Finding,
Arctostaphylos Substantial.
franciscana as
Endangered with
Critical Habitat.
8/17/2010........................ Listing Three Foreign Final Listing 75 FR 50813-50842.
Bird Species from Latin Endangered.
America and the
Caribbean as Endangered
Throughout Their Range.
8/17/2010........................ 90-Day Finding on a Notice of 90-day 75 FR 50739-50742.
Petition to List Brian Petition Finding,
Head Mountainsnail as Not substantial.
Endangered or
Threatened with
Critical Habitat.
8/24/2010........................ 90-Day Finding on a Notice of 90-day 75 FR 51969-51974.
Petition to List the Petition Finding,
Oklahoma Grass Pink Substantial.
Orchid as Endangered or
Threatened.
9/1/2010......................... 12-Month Finding on a Notice of 12-month 75 FR 53615-53629.
Petition to List the petition finding,
White-Sided Jackrabbit Not warranted.
as Threatened or
Endangered.
9/8/2010......................... Proposed Rule To List Proposed Listing 75 FR 54561-54579.
the Ozark Hellbender Endangered.
Salamander as
Endangered.
9/8/2010......................... Revised 12-Month Finding Notice of 12-month 75 FR 54707-54753.
to List the Upper petition finding,
Missouri River Distinct Warranted but
Population Segment of precluded.
Arctic Grayling as
Endangered or
Threatened.
9/9/2010......................... 12-Month Finding on a Notice of 12-month 75 FR 54822-54845.
Petition to List the petition finding,
Jemez Mountains Warranted but
Salamander (Plethodon precluded.
neomexicanus) as
Endangered or
Threatened with
Critical Habitat.
9/15/2010........................ 12-Month Finding on a Notice of 12-month 75 FR 56028-56050.
Petition to List petition finding,
Sprague's Pipit as Warranted but
Endangered or precluded.
Threatened Throughout
Its Range.
9/22/2010........................ 12-Month Finding on a Notice of 12-month 75 FR 57720-57734.
Petition to List Agave petition finding,
eggersiana (no common Warranted but
name) as Endangered. precluded.
9/28/2010........................ Determination of Final Listing 75 FR 59645-59656.
Endangered Status for Endangered.
the African Penguin.
9/28/2010........................ Determination for the Notice of 12-month 75 FR 59803-59863.
Gunnison petition finding,
Sage[dash]grouse as a Warranted but
Threatened or precluded.
Endangered Species.
9/30/2010........................ 12-Month Finding on a Notice of 12-month 75 FR 60515-60561.
Petition to List the petition finding,
Pygmy Rabbit as Not warranted.
Endangered or
Threatened.
10/6/2010........................ Endangered Status for Proposed Listing 75 FR 61664-61690.
the Altamaha Endangered.
Spinymussel and
Designation of Critical
Habitat.
10/7/2010........................ 12-month Finding on a Notice of 12-month 75 FR 62070-62095.
Petition to list the petition finding,
Sacramento Splittail as Not warranted.
Endangered or
Threatened.
----------------------------------------------------------------------------------------------------------------
Our expeditious progress also includes work on listing actions that
we funded in FY 2010 and FY 2011, but have not yet been completed to
date. These actions are listed below. Actions in the top section of the
table are being conducted under a deadline set by a court. Actions in
the middle section of the table are being conducted to meet statutory
timelines, that is, timelines required under the Act. Actions in the
bottom section of the table are high-priority listing actions. These
actions include work primarily on species with an LPN of 2, and, as
discussed above, selection of these species is partially based on
available staff resources, and when appropriate, include species with a
lower priority if they overlap geographically or have the same threats
as the species with the high priority. Including these species together
in the same proposed rule results in considerable savings in time and
funding compared to preparing separate proposed rules for each of them
in the future.
Actions Funded in FY 2010 and FY 2011 But Not Yet Completed
------------------------------------------------------------------------
Species Action
------------------------------------------------------------------------
Actions Subject to Court Order/Settlement Agreement
------------------------------------------------------------------------
6 Birds from Eurasia......... Final listing determination.
[[Page 78145]]
Flat-tailed horned lizard.... Final listing determination.
Mountain plover \4\.......... Final listing determination.
6 Birds from Peru............ Proposed listing determination.
Pacific walrus............... 12-month petition finding.
Wolverine.................... 12-month petition finding.
Solanum conocarpum........... 12-month petition finding.
Desert tortoise--Sonoran 12-month petition finding.
population.
Thorne's Hairstreak butterfly 12-month petition finding.
\3\.
Hermes copper butterfly \3\.. 12-month petition finding.
------------------------------------------------------------------------
Actions With Statutory Deadlines
------------------------------------------------------------------------
Casey's june beetle.......... Final listing determination.
Georgia pigtoe, interrupted Final listing determination.
rocksnail, and rough
hornsnail.
7 Bird species from Brazil... Final listing determination.
Southern rockhopper penguin-- Final listing determination.
Campbell Plateau population.
5 Bird species from Colombia Final listing determination.
and Ecuador.
Queen Charlotte goshawk...... Final listing determination.
5 species southeast fish Final listing determination.
(Cumberland darter, rush
darter, yellowcheek darter,
chucky madtom, and laurel
dace) \4\.
Altamaha spinymussel......... Final listing determination.
Salmon crested cockatoo...... Proposed listing determination.
CA golden trout.............. 12-month petition finding.
Black-footed albatross....... 12-month petition finding.
Mount Charleston blue 12-month petition finding.
butterfly.
Mojave fringe-toed lizard \1\ 12-month petition finding.
Kokanee--Lake Sammamish 12-month petition finding.
population \1\.
Cactus ferruginous pygmy-owl 12-month petition finding.
\1\.
Northern leopard frog........ 12-month petition finding.
Tehachapi slender salamander. 12-month petition finding.
Coqui Llanero................ 12-month petition finding.
Dusky tree vole.............. 12-month petition finding.
3 MT invertebrates (mist 12-month petition finding.
forestfly(Lednia tumana),
Oreohelix sp.3, Oreohelix
sp. 31) from 206 species
petition.
5 UT plants (Astragalus 12-month petition finding.
hamiltonii, Eriogonum
soredium, Lepidium ostleri,
Penstemon flowersii,
Trifolium friscanum) from
206 species petition.
2 CO plants (Astragalus 12-month petition finding.
microcymbus, Astragalus
schmolliae) from 206 species
petition.
5 WY plants (Abronia 12-month petition finding.
ammophila, Agrostis rossiae,
Astragalus proimanthus,
Boechere (Arabis) pusilla,
Penstemon gibbensii) from
206 species petition.
Leatherside chub (from 206 12-month petition finding.
species petition).
Frigid ambersnail (from 206 12-month petition finding.
species petition).
Gopher tortoise--eastern 12-month petition finding.
population.
Wrights marsh thistle........ 12-month petition finding.
67 of 475 southwest species.. 12-month petition finding.
Grand Canyon scorpion (from 12-month petition finding.
475 species petition).
Anacroneuria wipukupa (a 12-month petition finding.
stonefly from 475 species
petition).
Rattlesnake-master borer moth 12-month petition finding.
(from 475 species petition).
3 Texas moths (Ursia furtiva, 12-month petition finding.
Sphingicampa blanchardi,
Agapema galbina) (from 475
species petition).
2 Texas shiners (Cyprinella 12-month petition finding.
sp., Cyprinella lepida)
(from 475 species petition).
3 South Arizona plants 12-month petition finding.
(Erigeron piscaticus,
Astragalus hypoxylus,
Amoreuxia gonzalezii) (from
475 species petition).
5 Central Texas mussel 12-month petition finding.
species (3 from 475 species
petition).
14 parrots (foreign species). 12-month petition finding.
Berry Cave salamander \1\.... 12-month petition finding.
Striped Newt \1\............. 12-month petition finding.
Fisher--Northern Rocky 12-month petition finding.
Mountain Range \1\.
Mohave Ground Squirrel \1\... 12-month petition finding.
Puerto Rico Harlequin 12-month petition finding.
Butterfly.
Western gull-billed tern..... 12-month petition finding.
Ozark chinquapin (Castanea 12-month petition finding.
pumila var. ozarkensis).
HI yellow-faced bees......... 12-month petition finding.
Giant Palouse earthworm...... 12-month petition finding.
Whitebark pine............... 12-month petition finding.
OK grass pink (Calopogon 12-month petition finding.
oklahomensis) \1\.
Southeastern pop snowy plover 90-day petition finding.
& wintering pop. of piping
plover \1\.
Eagle Lake trout \1\......... 90-day petition finding.
Smooth-billed ani \1\........ 90-day petition finding.
Bay Springs salamander \1\... 90-day petition finding.
32 species of snails and 90-day petition finding.
slugs \1\.
42 snail species (Nevada & 90-day petition finding.
Utah).
Red knot roselaari subspecies 90-day petition finding.
Peary caribou................ 90-day petition finding.
[[Page 78146]]
Plains bison................. 90-day petition finding.
Spring Mountains checkerspot 90-day petition finding.
butterfly.
Spring pygmy sunfish......... 90-day petition finding.
Bay skipper.................. 90-day petition finding.
Unsilvered fritillary........ 90-day petition finding.
Texas kangaroo rat........... 90-day petition finding.
Spot-tailed earless lizard... 90-day petition finding.
Eastern small-footed bat..... 90-day petition finding.
Northern long-eared bat...... 90-day petition finding.
Prairie chub................. 90-day petition finding.
10 species of Great Basin 90-day petition finding.
butterfly.
6 sand dune (scarab) beetles. 90-day petition finding.
Golden-winged warbler \4\.... 90-day petition finding.
Sand-verbena moth............ 90-day petition finding.
404 Southeast species........ 90-day petition finding.
Franklin's bumble bee \4\.... 90-day petition finding.
2 Idaho snowflies (straight 90-day petition finding.
snowfly & Idaho snowfly) \4\.
American eel \4\............. 90-day petition finding.
Gila monster (Utah 90-day petition finding.
population) \4\.
Arapahoe snowfly \4\......... 90-day petition finding.
Leona's little blue \4\...... 90-day petition finding.
------------------------------------------------------------------------
High-Priority Listing Actions \3\
------------------------------------------------------------------------
19 Oahu candidate species \2\ Proposed listing.
(16 plants, 3 damselflies)
(15 with LPN = 2, 3 with LPN
= 3, 1 with LPN = 9).
19 Maui-Nui candidate species Proposed listing.
\2\ (16 plants, 3 tree
snails) (14 with LPN = 2, 2
with LPN = 3, 3 with LPN =
8).
Dune sagebrush lizard Proposed listing.
(formerly Sand dune lizard)
\3\ (LPN = 2).
2 Arizona springsnails \2\ Proposed listing.
(Pyrgulopsis bernadina (LPN
= 2), Pyrgulopsis trivialis
(LPN = 2)).
New Mexico springsnail \2\ Proposed listing.
(Pyrgulopsis chupaderae (LPN
= 2).
2 mussels \2\ (rayed bean Proposed listing.
(LPN = 2), snuffbox No LPN).
2 mussels \2\ (sheepnose (LPN Proposed listing.
= 2), spectaclecase (LPN =
4),).
8 Gulf Coast mussels Proposed listing.
(southern kidneyshell (LPN =
2), round ebonyshell (LPN =
2), Alabama pearlshell (LPN
= 2), southern sandshell
(LPN = 5), fuzzy pigtoe (LPN
= 5), Choctaw bean (LPN =
5), narrow pigtoe (LPN = 5),
and tapered pigtoe (LPN =
11)).
Umtanum buckwheat (LPN = 2) Proposed listing.
\4\.
Grotto sculpin (LPN = 2) \4\. Proposed listing.
2 Arkansas mussels (Neosho Proposed listing.
mucket (LPN =2) &
Rabbitsfoot (LPN = 9)) \4\.
Diamond darter (LPN = 2) \4\. Proposed listing.
Gunnison sage-grouse (LPN = Proposed listing.
2) \4\.
------------------------------------------------------------------------
\1\ Funds for listing actions for these species were provided in
previous FYs.
\2\ Although funds for these high-priority listing actions were provided
in FY 2008 or 2009, due to the complexity of these actions and
competing priorities, these actions are still being developed.
\3\ Partially funded with FY 2010 funds and FY 2011 funds.
\4\ Funded with FY 2010 funds.
\5\ Funded with FY 2011 funds.
We have endeavored to make our listing actions as efficient and
timely as possible, given the requirements of the relevant law and
regulations, and constraints relating to workload and personnel. We are
continually considering ways to streamline processes or achieve
economies of scale, such as by batching related actions together. Given
our limited budget for implementing section 4 of the Act, these actions
described above collectively constitute expeditious progress.
The Sonoran desert tortoise will be added to the list of candidate
species upon publication of this 12-month finding. We will continue to
monitor the status of this DPS as new information becomes available.
This review will determine if a change in status is warranted,
including the need to make prompt use of emergency listing procedures.
We intend that any proposed listing determination for the Sonoran
desert tortoise will be as accurate as possible. Therefore, we will
continue to accept additional information and comments from all
concerned governmental agencies, the scientific community, industry, or
any other interested party concerning this finding.
References Cited
A complete list of references cited is available on the Internet at
http://www.regulations.gov and upon request from the Arizona Ecological
Services Office (see ADDRESSES section).
Author(s)
The primary authors of this notice are the staff members of the
Arizona Ecological Services Office.
Authority
The authority for this section is section 4 of the Endangered
Species Act of 1973, as amended (16 U.S.C. 1531 et seq.).
Dated: November 23, 2010.
Rowan W. Gould,
Acting Director, Fish and Wildlife Service.
[FR Doc. 2010-31000 Filed 12-13-10; 8:45 am]
BILLING CODE 4310-55-P