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Humboldt Marten
Martes americana humboldtensis

General Information

Official Status: Species of Concern, the Humboldt marten does not have any special status under the Endangered Species Act at this time, but is considered a mammalian Species of Special Concern by the State of California.

Date Listed: Not listed.

Critical Habitat: Critical habitat has not been designated for the American marten.

Recovery Plan: The American marten does not have a recovery plan.

90-day Finding, Petition to List:


Humboldt Marten, Six Rivers National Forest Photograph
Humboldt Marten

Photo Credit: Six Rivers National Forest

 

Identifying Characteristics:

The American marten, a carnivorous mammal about the size of a mink, has a long, slender body with rounded ears, short limbs, and bushy tail.  Martens have triangular faces with muzzles less pointed than those of foxes.  The tail constitutes about one-third of the total body length.  Each well-furred paw includes five toes.  Their total length ranges from 20 and 24 inches, and adults weigh 1.2 to 3.4 lbs, depending on sex and subspecies.  Males are 20 to 40% larger than females.  The color of the long, silky, dense fur ranges from pale yellowish buff to tawny brown to almost black.  The color of the head is usually lighter than the body, and the legs and tail are darker.  A characteristic throat and chest bib ranges in color from pale straw to vivid orange.  The Humboldt marten is one of 14 recognized subspecies of the American marten. 

Current Geographic Range:

The American marten occurs in forested habitats throughout boreal North America, reaching its southernmost extent in the Sierra Nevada of California and the southern Rocky Mountains of New Mexico.  Three distinct subspecies of the American marten occur in the western United States.  The Humboldt marten is known from coastal northwestern California.  The Sierra marten occurs from the Salmon-Trinity Mountains east to the Cascades and south throughout the Sierra Nevada.  The canyon of the Klamath River occurs between the two subspecies, suggest that the Klamath River and the less hospitable xeric forest types in the river’s canyon may be barriers to movement for martens.  At the northern boundary of the coast redwood zone, the Humboldt marten is replaced by the coastal marten, which occurs in Oregon, Washington, and British Columbia.  The Humboldt subspecies historically occurred in California chiefly within the coast redwood zone from the Oregon border south to Sonoma County.  Since 1995, surveys for the Humboldt marten have been conducted in much of the subspecies’ historic range.  Results of these surveys suggest that the Humboldt marten no longer occurs in much of its historic range.  Currently, the Humboldt marten is known only from southern Del Norte County and northern Humboldt County, less than 5% of its historic range.

Life History:

Martens mature at one year of age, but breeding may not occur before three years of age.  Martens produce one litter per year, with an average of slightly less than three young per female.  Mating generally occurs in July or August; birth occurs in late March or April.  This extended gestation period is due to delayed implantation of the embryo, in which the embryos remain in a state of partial development for several months.  Natal dens are typically located in cavities in very large logs, snags, or live trees.  Kits completely depend upon their mothers at birth, but are weaned by about 42 days of age.  The young leave the company of their mother and disperse in late summer or autumn.  Martens may disperse 20 miles or more from their natal areas.

The American marten is an opportunistic predator with a diverse diet that includes mammals, birds, carrion, eggs, insects, and vegetation (fruits, berries, nuts, fungi, lichens, etc.).  Voles, squirrels and chipmunks are important food items for martens across their range.  In the Sierra Nevada of California, mammals were the most important food item, with microtine rodents the most frequent prey throughout the year, and chipmunks and squirrels increasing in importance during the summer.  Seasonal variation in diets is universal with the importance of soft mast peaking in the fall.  In the diet of Humboldt martens, mammals (93% of scats analyzed) and berries (85%) were the most frequently occurring items, followed by birds (21%), insects (20%), and reptiles (7%).  Squirrels and voles  were the most common mammal species in the diet.  The frequency of berries and birds in the diet of the Humboldt marten is the highest reported in studies of the American marten.

Martens use a home range area larger than is typical for a mammal of their size.  The estimated summer-fall home range size for five radio-collared adult male Humboldt martens was 1,321.7 acres; for a single adult female with one kit, 315 acres; and for three juvenile females, 1,490.8 acres.  In the Sierra Nevada of California, the size of the annual home range varies from 420 to 1,811 acres for males and from 173 to 1,433 acres for females.  The quality of the habitat apparently influences the overall acreage used by martens.  In Ontario, Canada, male home ranges averaged 840 acres in uncut forest and 1,236 acres in cutover areas, whereas females occupied 247 acres in uncut forest and 766 acres in cutover forest.  Humboldt martens apparently select the largest available patch sizes of late-successional stands or serpentine habitat.

General Habitat Characteristics:

American martens are typically associated with closed-canopy, late-successional, mesic coniferous forests with complex physical structure near the ground.  Complex ground structure provides protection from predators and protective thermal microenvironments.  Structure near the ground may be provided by the lower branches of living trees, tree boles in various stages of decay, coarse woody debris, shrubs, and rockfields.  Dense understory brush of shade-tolerant species such as Rhododendron, salal and evergreen huckleberry, is especially noteworthy.  In the western United States, martens are strongly associated with late-successional coniferous forests, but they may occur in earlier seral stages that contain remnants of late-successional forest such large logs and stumps, and dense shrubs.  Martens generally avoid nonforested areas including prairies and clearcuts that lack overhead cover.  Historical records of the distribution of Humboldt martens suggest that the subspecies was closely tied to coast redwood forests.  However, the one known remnant Humboldt marten population occurs in the north-central portion of the described range in an area dominated by Douglas-fir and tanoak.  Coast redwood associations occur on the western edge of the occupied range and white fir occurs at the higher elevations.  This population uses two structurally distinct fog-influenced forest types, one on serpentine soils and one on more productive non-serpentine soils.  The combination of interstitial spaces created by rocks and dense shrub cover may allow Humboldt martens to use stands with highly developed shrub communities on serpentine sites.

Population and Habitat Status:

Historical information and recent survey data indicate that the Humboldt marten population has declined significantly during the past 100 years.  Humboldt martens were relatively common in the early twentieth century, as reported from historic trapping data.  The population decline of Humboldt marten was noted as early as 1937 by noted biologist Joseph Grinnell and his associates:  “…of rather sparse occurrence, though in earlier years it was more generally distributed and fairly numerous.”  Subsequently, trapping of martens in California was outlawed in 1946.  Recent population estimates based on surveys within the known, current range of the Humboldt marten were approximately 60 individuals in 2000-2001 and 40 individuals in 2008.  Female occupancy showed the most substantial declines the from 2000-2001 to 2008.  Since these surveys did not cover the full known range of the Humboldt marten, the subspecies may be more numerous than these estimates.  However, it is likely the entire Humboldt marten population contains fewer than 100 individuals.

Within the historic range of the Humboldt marten, timber harvest has eliminated most late-successional forests on private lands, and much of this forest habitat on Federal and State lands,  in northwestern California.  About 2.7 million acres of late-successional coast redwood forests were present in California during the early to mid-1800s.  Currently, approximately 70,000 ac of late-successional coast redwood forest remain in California, representing about 2.6% of the original late-successional coast redwood forest.  This remaining late-successional coast redwood forest occurs primarily in reserves on State and Federal land where it is protected from future timber harvest. 

Threats:

Loss, modification, and fragmentation of habitat are significant ongoing threats to the remaining Humboldt marten population.  Martens have specialized habitat requirements that include large diameter live trees, snags and logs, especially within late-successional habitat.  These habitat features may take centuries to develop.  Little habitat with the necessary structural characteristics to support Humboldt martens is expected to regenerate over the next few decades.  Without a management strategy to maintain key habitat elements it is unlikely lands available for timber harvest will support a viable marten population.  Humboldt martens and their habitat in the remaining occupied area are patchily distributed.  Further loss or degradation of suitable habitat could appreciably reduce the likelihood of survival of this subspecies. Wildfire has the potential to greatly alter habitat essential to martens when it removes structural components including overstory canopy, large logs or dense understory shrubs.  Projects to reduce risk of wildfire are potentially beneficial to martens by reducing overall risk of large-scale, intense wildfire, yet need to be carefully planned to minimize the loss of essential habitat components or fragmenting existing suitable habitat.  Roads may fragment suitable habitat and provide corridors for movement of potential predators such as bobcats and coyotes.  While direct trapping of martens in California has not been legal for several decades, incidental capture of martens while targeting other species may still occur, and should be monitored to assess the risk.  Trapping of martens remains legal in Oregon.  Management activities that encourage populations of other carnivores, such as bobcats, fishers and cougars, may place additional pressure on the remaining marten population, as several of these species, especially fishers, may opportunistically kill martens when encountered. 

Conservation Needs:

A conservation strategy for the Humboldt marten should increase the size of the current population so that genetic, demographic, and environmental uncertainties are less threatening, and establish multiple populations so that a single catastrophic event (such as large wildfire) cannot eliminate the subspecies.  Specific measures to conserve the Humboldt marten may include:

    • Maintain currently occupied habitat.
    • Restore habitat to increase and reconnect habitat patches near the known population.
    • Increase the overall size of suitable habitat patches.
    • Restore functional landscape connectivity to enable recolonization of suitable, but currently unoccupied habitat.
    • Establish high priority restoration areas.
    • Restore or maintain dense, productive understory shrub layers and reduce road densities.
    • Protect currently suitable resting and denning structures.
    • Establish additional populations within the historical range.
    • Monitor the existing population to determine population trend.
    • Conduct additional research to investigate conservation needs of Humboldt martens.
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Last updated: January 18, 2012

Arcata Fish and Wildlife Office, 1655 Heindon Road, Arcata, California 95521, USA
Tel: (707) 822.7201 Fax: (707) 822.8411