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Identify and assess habitat
Gulf of Maine watershed habitat analysis technical report
Arnold Banner, Ph.D. and Sue Schaller
Gulf of Maine Program
March 2001
Abstract
The USFWS Gulf of Maine Program mapped habitats of 64 federally listed
Endangered and Threatened species, declining neo-tropical migrants,
shorebirds, waterfowl, anadromous and inter-jurisdictional fishes, throughout
the United States portion of the Gulf of Maine watershed. The resulting
digital maps can be displayed for individual species, or as combined
data highlighting localities having high species richness and high aggregate
habitat value. These products may be of use to biologists and managers
in Field and Regional offices, state agencies, and non-government entities
to evaluate impacts from development activities, to assess wildlife
resources at a landscape level, and to focus conservation activities.
Introduction
Conservation planning depends on information on the distribution of
species, their habitats, and the ecological factors which affect them.
While site inspection gives a most precise view of local conditions,
at the time of the visit, biologists generally may benefit from information
disclosing longer term conditions and having a landscape or ecosystem
perspective. Project biologists and managers do not routinely have time
or resources to assemble such background data. This watershed-wide study
is intended to provide a comprehensive analysis, narrative descriptions,
and display of habitats based on environmental characteristics and available
occurrence information, including that from the scientific literature
and from unpublished surveys. Habitat maps for individual species have
been further processed into composite maps highlighting areas of highest
resource value. This type of information has proven useful for screening
permits in the Southeast (USFWS Region 4), for refuge comprehensive
planning in the Northeast (USFWS Region 5), and for land acquisition
with state and non-government partners.
Methods
Selection of Species to Map
A major aspect of the watershed study is the selection of the species
for habitat mapping. Our list of evaluation species was created in a
series of steps, starting with a comprehensive survey of species of
high national importance occurring within USFWS Region 5. This was developed
by combining lists of all federally listed Threatened and Endangered
species, 'non-game birds of Management Concern', and waterfowl, shorebirds,
anadromous and inter-jurisdictional fishes (inshore species of concern
to USFWS and NOAA) which have significantly and persistently declined
in abundance. In each case we adopted lists of such species developed
by experts/agencies focusing on those groups (see Regional components).
From this overall regional list we extracted those species for which
the Gulf of Maine (GOM) watershed offers habitat (based on occurrences
in the watershed of those species as shown by Breeding Bird Survey and
Christmas Bird Counts, and/or evidence from technical literature, or
expert advice, identifying distribution of the species). All of the
nationally important species which regularly occur in the watershed
were included in our study.
To accommodate concerns of conservation partners with more localized
perspective, we created a supplementary species list for the watershed,
consisting of species which are both federal trust responsibilities
(migratory birds, migratory fishes) and which are designated as Threatened
or Endangered by 2 or all of the 3 states in the Gulf of Maine watersheds
(see Supplement to the Regional List).
The species for the analysis, then, consisted of all plants and animals
from the regional and supplementary lists and which have a relatively
significant presence in the study area (more than accidental/occasional
occurrences); see the 'SPECIES' column in the table GOM WATERSHED SPECIES
LIST.
Study Area Boundary
The GOM watershed boundary was constructed by selecting the outer
boundaries of all smaller watersheds in Massachusetts, New Hampshire,
and Maine which flow into the Gulf of Maine. These were identified using
USGS 1:24,000 and 1:100,000 hydrology coverages. The boundary was extended
into the Gulf at Cape Cod and eastern Maine (see figure, below).
Gulf
of Maine Watershed Study Area
Use of GIS for habitat mapping and biological decision making. Geographic
information systems (GIS) - are well adapted to handle large geographic
databases, to identify and display associations between base themes
and species occurrences, and to conduct analyses based on combinations
of base layers and/or occurrence information. The digital files may
be in the form of grid-cell, point, line, or polygon features and linked
databases, which can be displayed and plotted as maps. These maps may
represent areas of relatively high habitat value, or areas which we
have relatively great confidence are suitable habitat for the species
of interest.
GIS often is used just to map and display locations at which geographic
features or organisms were observed. Inferences about habitat may be
made from such occurrences, particularly by using them with coincident
environmental themes, such as land cover or soils types. Alternatively,
GIS can be used to create new themes, based on features or combinations
of features from one or more existing themes. If these features represent
the environmental preferences or tolerances of a species, the resulting
theme may depict the local habitat distribution for that species. Such
a habitat map can be tested for accuracy or relevancy by comparing the
mapped habitats to the locations known to be used by the species. The
GIS characterization of habitats based on a species' environmental preferences
is a type of habitat modeling.
Approaches to Habitat mapping. Habitats are the places where organisms
live (Odum 1959). Therefore, habitats may be identified directly by
observing the use of a locality by a species, or indirectly by habitat
modeling. Habitats vary in quality, and this can affect the number of
individuals which can be supported per unit area, or the frequency or
likelihood of their occurrence at a site. We used the observed level
of use and the apparent relative suitability of environmental conditions
to assign numerical scores reflecting gradations in habitat value. Such
scores reflect our level of confidence that an area represents suitable
conditions for the species. It must be noted that mapped habitats, whether
developed from occurrence information or from models, are necessarily
based on data representing interpretations of past conditions. Even
an accurate depiction of suitable habitats cannot guarantee that the
species will regularly be found at those sites. Occupancy of many habitats
is highly seasonal and also may be associated with variations in population
levels. Our use of models and occurrence data in mapping habitat for
each species is listed in the column "basis of final map" in
the table GOM WATERSHED SPECIES LIST.
Use of Occurrence Data. For species subject to recent, comprehensive
biological surveys (e.g., aerial surveys of bald eagle nests) we mapped
habitats based on these occurrences and scored these areas as 'optimal'.
Otherwise, where a survey was less extensive (e.g., International Shorebird
Survey sites), we mapped habitats using a model, and used the occurrence
information to score as 'optimal' the localities known to be used, placing
our highest confidence in the value of those habitats. Where surveys
characterized only a subset of the potential habitat (e.g., Breeding
Bird Survey stops for many of the species) we used these observations
just to test habitat maps derived from models. The primary occurrence
information available to us is listed in the table "Occurrence
Data Used in Developing Gom Watershed Habitat Maps, or for Testing Modeled
Habitats".
Use of Habitat Models. We developed simple habitat models, similar
to the Fish & Wildlife Service habitat suitability index models
(USFWS 1980), for use within our GIS. Habitat suitability index models
are hypotheses correlating environmental conditions to some measure
of a species' biological 'performance', such as abundance or reproductive
success. Because of the limited information on those measures for most
of our species, our models generally used the probability of occurrence
as a measure of performance. That is, our assignment of higher habitat
suitability index values represents increased confidence that the species
will make use of the site.
Model development included review of the literature and discussions
with experts to: 1) identify environmental factors affecting potential
use of the study area, and 2) estimate the relative suitability of such
habitat features as land cover types, minimum patch size, edge effects,
water depths, or soil types. The suitability of each factor was expressed
as an index, ranging from 0 (least suitable) to 1.0 (most suitable,
in this locality). These individual suitability index (s.i.) values
were combined, into an overall habitat suitability index (HSI) value.
For example, a preferred substrate type might be accorded an s.i. of
1.0. However, if this was coincident with a totally unsuitable vegetation
cover (s.i. = 0), and use of the area was limited by the minimum of
these 2 factors, the overall HSI would be 0. HSI may be based upon one
to many factors and may consider requirements of various life stages.
Models were used to improve the accuracy of some data sets. For example,
the Maine Department of Inland Fisheries and Wildlife (MDIFW) mapped
shorebird habitats as polygon features broadly encompassing the areas
being observed, with a numerical database listing the abundance of birds
at each observation site. Therefore, the polygons surround used habitat
and often have inclusions of non-habitat areas. Using a geographic information
system (GIS), we applied supplementary environmental data (substrate,
land cover, water depth) in order to delete some of the unsuitable portions
of these areas.
Model Validation and Adjustment. Draft models for many of the species
developed and later were tested using different sets of occurrence data.
For testing, we compared the proportion of known occurrences coinciding
with mapped habitat to the proportion of a set of random points coinciding
with mapped habitat. Where the agreement with occurrences was significantly
different (Chi-square; .01 criterion) from the agreement with random
points, we regarded the mapped habitats as being a useful predictor.
For some species, where the literature on habitat needs was rudimentary,
or focused on factors not distinguishable in our environmental data
sets, we developed several models and retained the one having the best
association with known occurrences. This test encouraged development
of parsimonious yet accurate models, since a high level of agreement
tended to be prevented by errors of omission (habitat not covering the
occurrences) or of commission (habitat too extensive and so covering
more random points).
Habitats mapping accuracy is controlled by the quality of the models
and of the environmental themes. Narrative descriptions of habitat needs
and the resulting species models, as well as metadata for the GIS coverages,
is accessible in the Table of Models and Metadata. This table also links
to descriptions of some of the major environmental themes used in portraying
habitats.
Results
Data analysis was performed using ArcInfo and ArcView software. Data
were processed as shape files and grids (proprietary vector raster spatial
data formats, respectively). Habitat components were developed in successive
iterations, and final products were in grid formats.
Groupings of Individual Species
Software limitations prevented our combining all species data into
one grid. Therefore, species grids were grouped into five general categories.
These were: songbirds (grid name [SONGBIRDS]), shorebirds ([SHOREBIRDS]),
waterbirds (including waterfowl, seabirds, etc.; grid name [WATERBIRDS]),
Federally listed threatened or endangered species ([FED_LISTED]), and
the remaining species, mostly fishes and hawks ([FISH_RAPTORS]). The
habitat scores for each of the evaluation species were retained as attributes
of the five resulting grouped grids, allowing the user to display maps
by species. Alternatively, 'clicking' on a cell when one or more of
the grouped grids are active, reveals the scores in that cell for each
of those species. We have developed an ArcView script to allow tabulation
of habitat values for all species within a parcel of interest, and comparison
of data for a parcel with average habitat values throughout the watershed.
The species groupings and associated field names are listed in the Table
of Models and Metadata.
Sums of All Habitat Scores
Habitat suitability scores from grids of the individual species also
were added on a cell by cell basis to allow users to display the landscape,
shaded by overall habitat value. The scores of this grid (GOMSUM) reflect
both the number of species using each cell, and the relative habitat
suitability for those species.
Habitat Scoring
by General Land Cover Type
The grid [GOMSUM] was subdivided according to four generalized cover
types (grass/shrub/bare, forested, freshwater, and saline). The top
increments of habitat values (top 25% and 50%) then were calculated
for each of the four resulting grids, considering both the summed habitat
scores and total areas of the grids at each score. This habitat quality
times area product is equivalent to the habitat unit values of the Habitat
Evaluation Procedures (USFWS 1980), and is intended as a measure of
carrying capacity. Thus, one acre of habitat with a score of 50 should
offer the same value as five acres with a score of 10. Protection of
the highest value habitat, therefore, confers maximum conservation benefit
and efficiency. The top 25 and 50% increments of the four types were
reassembled into the grids [GOMTOPQUARTER] and [GOMTOPHALF], respectively.
These grids are useful for viewing the landscape when there is a preference
for certain cover types or the user wishes to isolate the more highly
valued areas.
Limitations of the Data. As mentioned above, maps of habitats for
the individual species are limited by the accuracy of the data sets
used in developing them (both occurrence data and environmental themes)
and the validity of the models used to interpret those data.
Maps showing some highest increment of habitats (e.g.,[GOMTOPQUARTER]),
while providing a useful visual display, may totally omit habitats or
habitat components for some species which do not coincide with those
of other species, and so never reach a high 'summed score'. Such maps
also may leave out lower-scored areas needed to support or buffer the
highly scored habitats which are retained.
Artifacts of grid-cell mapping of riverine themes may in some cases
cause habitat omissions, and in others aquatic habitat values in primarily
upland areas. Fish habitats were gridded from continuous polygons/arcs
derived from USGS hydrology coverages. Single line arcs were necessarily
converted into strings of cells, each with a minimum width of 30 m (the
cell dimensions). Even where the dominant land cover of a cell was upland,
we retained the habitat value for a stream passing through the cell.
As a result, some upland areas will display habitat value for anadromous
fishes.
The grid process left discontinuities when converting narrow (< 30
m wide) polygon features, such as small rivers, where only part of a
cell was crossed by aquatic habitat. Moreover, where National Wetlands
Inventory polygons representing wetlands and water bodies differed from
those of USGS, the only fish habitat we retained was that which corresponded
to aquatic classes in the former. Therefore, fish habitats and migratory
pathways may not appear as continuous extents of habitat, or extend
to the boundaries of contiguous wetlands in all cases.
Suggestions for Updates. We recommend that this project be periodically
updated and enhanced. This includes improving the format for ease and
accuracy of use and interpretation, and accuracy of the habitat maps.
The latter are limited both by our knowledge of the habitat needs of
the species, and the quality of the environmental themes. Revisions
and corrections should be based on further information from the biological
literature, from field checking of the maps and from use of newer and
more accurate species survey and environmental themes, particularly
land cover and soils.
Acknowledgments
We wish to acknowledge the assistance and information provided by
the following individuals and organizations, without which this project
could not have been completed.
Robert Barlow (SUNY, NY)
Frank Biasi (The
Nature Conservancy, MA)
John Bidwell, Joe McKeon, Graham Smith, Jed
Wright, Jean Fujikawa, Robert Houston, Susi vonOettingen, Doug Forsell,
Drew Major, Mike Amaral, Sylvia Schmidt, Ron Joseph, Anne Hecht, Janith
Taylor, Jennifer Casey, Ann Roy, Kim Sprankle (USFWS)
Mark L. Botton (Fordham Univ., NY)
Randall Boone (Colorado State University)
Ken Buja (NMFS-SEA)
Katie Callahan, Bob Estabrook (NH DES)
Dave Capen, Sean McFaden (University of Vermont)
Dan Coker (Maine Natural Resources Information
and Mapping Center)
Diane Deluca, Chris Martin (NH Audubon Society)
Dave Evers (BioDiversity Research Institute,
ME)
Don Field (NOAA, CCAP)
Lew Flagg, Seth Barker, Peter Thayer (ME
DMR)
Sue Gawler, Molly Docherty (Maine
Natural Areas Program)
John Greenwood, Sara Cairns, Ed Robinson,
Claire McBane (NHF&G);
Peter Grose (NOAA NOS)
William Hansen (Florida Power and Light,
ME)
Brian Harrington (Manomet Bird Observatory)
Karsten Hartel (Harvard University)
Charlie Hodgson (Friends of Taunton Bay,
ME)
John B. Holt, Jr. ( MA)
Greg Horton, Norm Dube (Atlantic Salmon Commission)
Bill Huber (Quantitative Decisions, Merion
Station, PA)
Keith Johnston, Linda Alverson, Susan Hitchcox
(Maine
Audubon Society)
Barry Keim (University of New Hampshire)
Harley Knebel, Stephen M. Howard, Keith Pardieck,
BH Powell, Dennis Jorde, Rich Signell, Ian Thomas (USGS)
William Krohn, Jeffrey Hepinstall, Raymond
O'Connor, Ken Oliviera, Andrew Gilbert (University of Maine)
Marc Loiselle (MGS, ME)
Maine Office of GIS
Massachusetts Office of GIS
Jim McDougal (Essex County Greenway, MA)
Scott Melvin, Brad Blodget, H. Heusemann,
Steve McCrae, Amy Maher (MA Fish and Wildlife);
Isabel Morin (ENSR, Acton, MA)
New Hampshire GIS (GRANIT)
Murray Olmstead (MA)
Yvette Ortega (USDAFS)
Dave Sczeback (MA Natural Heritage Program)
Greg Shriver (SUNY, NY)
Carl N. Shuster, Jr. (VA)
Jackie Sones, Kathy Tuxbury (Wellfleet Bay
Audubon Sanctuary, MA)
Allen Starr, Charley Todd, Tom Hodgman, John
Kenney, Andrew Weik (MDIFW, ME)
Kyle Stockwell, Nancy Sferra (TNC, ME)
University of Southern Maine Library and
Interlibrary Loan Department
Peter Vickery (Massachusetts Audubon Society)
Dave Wilkinson (NRCS, ME)
Holly Yachmetz (NMFS, MA)
Citations
Odum, E.P. 1959. Fundamentals of Ecology. W.B. Saunders Co.
546 pp.
USFWS. 1980. Habitat Evaluation Procedures (HEP). U.S.D.I.
Fish & Wildlife Service. Division of Ecological Services. ESM
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