United States Department of Agriculture

Forest 

Service

Pacific Southwest 

Research Station

General Technical Report

PSW-GTR-260 February 

2019

A Conservation Assessment and 

Strategy for the Humboldt Marten 

in California and Oregon 

Keith M. Slauson, Gregory A. Schmidt, William J. Zielinski, Phillip J. Detrich, 

Richard L. Callas, James Thrailkill, Brenda Devlin-Craig, Desiree A. Early, 

Keith A. Hamm, Kristin N. Schmidt, Amber Transou, and Christopher J. West 
This publication is available online at www.fs.fed.us/psw/. 

Pacific Southwest Research Station

800 Buchanan Street

Albany, CA 94710

Disclaimer 

The findings and conclusions in this publication are those of the authors and do not 

necessarily represent the view or position of their respective agencies. The use of 

trade or firm names in this publication is for reader information and does not imply 

endorsement by the respective agencies of any product or service. 

Authors 

Keith M. Slauson is a research fellow and William J. Zielinski is a research 

ecologist, U.S. Department of Agriculture, Forest Service, Pacific Southwest 

Research Station, Redwood Sciences Lab, 1700 Bayview Drive, Arcata, CA 95521, 

keithmslauson@fs.fed.us, bzielinski@fs.fed.us; Gregory A. Schmidt is a fish and 

wildlife biologist, U.S. Department of the Interior, Fish and Wildlife Service, Arcata 

Field Office, 1655 Heindon Road, Arcata, CA 95521, gregory_schmidt@fws.gov; 

Phillip J. Detrich is an environmental consultant, PJD Environmental Consulting, 

411 McCloud Avenue, Mount Shasta, CA 96067, fishbirdco@gmail.com; Richard 

L. Callas is a senior environmental scientist, California Department of Fish and

Wildlife, North Region, 601 Locust Street, Redding, CA 96001, Richard.Callas@

wildlife.ca.gov; James Thrailkill is a field supervisor, U.S. Department of the Interior, Fish and Wildlife Service, Roseburg Field Office, 2900 NW Stewart Parkway,

Roseburg, OR 97471, jim_thrailkill@fws.gov; Brenda Devlin-Craig is a wildlife

biologist, U.S. Department of Agriculture, Forest Service, Smith River National

Recreation Area, 10600 Highway 199/P.O. Box 228, Gasquet, CA 95543, bdevlin@

fs.fed.us; Desiree A. Early is a terrestrial biologist and Keith A. Hamm is a conservation planning manager, Green Diamond Resource Company, 900 Riverside Drive,

Korbel, CA 95550, DEarly@greendiamond.com, khamm@greendiamond.com;

Kristin N. Schmidt is a wildlife biologist, Redwood National and State Parks, P.O.

Box 7/121200 Highway 101, Orick, CA 95555, kristin_schmidt@nps.gov; Amber

Transou is a senior environmental scientist, California State Parks, North Coast

Redwoods District, P.O. Box 2006, Eureka, CA 95502, Amber.Transou@parks.

ca.gov; and Christopher J. West is a senior wildlife biologist, Yurok Tribe, Wildlife

Program, 190 Klamath Boulevard, Klamath, CA 95548, cwest@yuroktribe.nsn.us.
Abstract 

Slauson, Keith M.; Schmidt, Gregory A.; Zielinski, William J.; Detrich, Phillip 

J.; Callas, Richard L.; Thrailkill, James; Devlin-Craig, Brenda; Early, 

Desiree A.; Hamm, Keith A.; Schmidt, Kristin N.; Transou, Amber; West, 

Christopher J. 2019. A conservation assessment and strategy for the Humboldt 

marten in California and Oregon. Gen. Tech. Rep. PSW-GTR-260. Arcata, CA: 

U.S. Department of Agriculture, Forest Service, Pacific Southwest Research 

Station. 124 p. 

This report is based on the final version of the Humboldt marten (Martes caurina 

humboldtensis) conservation assessment and strategy submitted on September 30, 

2017 by the Humboldt Marten Conservation Group. This assessment and strategy 

was developed in response to the decline of the Humboldt marten throughout most 

of its historical range in California and Oregon. The first four chapters synthesize 

the state of the knowledge on the Humboldt marten and relevant ecological and 

management aspects of the forest habitats it occupies. The final chapter identifies 

overall conservation goals, evaluates current threats or impediments to achieving 

those goals, and identifies a series of conservation actions and information needs 

that could help to achieve the overall conservation goals. 

Keywords: Martes caurina humboldtensis, Humboldt marten, coastal 

California, coastal Oregon, conservation strategy. 
Summary 

The Humboldt marten (Martes caurina humboldtensis) is a medium-size carnivore 

that historically occurred throughout the coastal forests of northwestern California 

and Oregon. Decades of unregulated trapping followed by extensive habitat loss 

and fragmentation from unregulated timber harvesting are likely the two most 

important historical factors that caused declines in the distribution and abundance 

of Humboldt martens. This subspecies has been extirpated from greater than 

95 percent of its historical range in California, where it is known only from one 

population numbering fewer than 100 individuals and another smaller population recently detected near the California-Oregon border. Recent genetic analysis 

has found that coastal Oregon martens, previously recognized as Martes caurina 

caurina, are most genetically similar to the Humboldt subspecies and should be 

reclassified as M. c. humboldtensis. Two known Oregon populations remain, one in 

the central coast area and one in the south coast. Additional survey effort is needed 

to fully understand the current distribution in coastal Oregon, but the degree of 

range contraction is apparently of similar magnitude to that in California. 

In recognition of the need to improve the status of the Humboldt marten, the 

Humboldt Marten Conservation Group (HMCG) was formed in California in 

2011. In 2013, as new genetic information suggested that the Humboldt subspecies included coastal Oregon, the HMCG was expanded to include that area. The 

HMCG is composed of federal, state, tribal, private, and nongovernmental organizations with an interest in conservation and management of the Humboldt marten 

on public, tribal, and private forests in coastal northwestern California and coastal 

Oregon. The HMCG recognized the need for an integrated regional approach to 

address the immediate research and conservation needs for the Humboldt marten, 

and agreed that developing a conservation assessment and strategy should be the 

first step toward implementing such an approach. 

This conservation assessment is a synthesis of published and unpublished 

scientific literature on Humboldt marten ecology and habitat relationships. Where 

appropriate, it also includes relevant published studies on martens from elsewhere 

in North America. We have included published and unpublished literature through 

2016, and anticipate that future updates will incorporate new information. This 

conservation assessment considers three major topics: (1) biophysical environment, 

(2) human modifications to the environment, and (3) biological information on 

martens. 

Contemporary detections of Humboldt martens have occurred in three habitat 

types: (1) moist Douglas-fir (Pseudotsuga menziesii (Mirb. Franco) associated forest 
types, (2) moist forest types on serpentine soils, and (3) shore-pine (Pinus contorta 

Douglas ex Loudon)-associated dune forests on coastal terraces. A dense, spatially 

extensive shrub layer typically dominated by shade-tolerant ericaceous species is 

a consistent feature within these three forest habitat types. The majority of marten 

detections have occurred in largely unmanaged moist Douglas-fir-associated forest 

types, where martens select large patches (>80 ha) of late-successional forest as 

habitat. Research in more intensively managed forest has found some Humboldt 

martens occupying and reproducing in home ranges composed of a mosaic of 

younger stands, most of which were harvested more than 20 years ago. Many of 

these stands contain residual conifers left uncut in earlier harvests, conspicuous 

numbers of large live and dead conifer trees, and large residual hardwood structures. In this intensively managed landscape, nearly half of the martens monitored 

have been killed by bobcats (Lynx rufus), which select for the young regenerating 

forests that support their two most important prey species, dusky-footed woodrats 

(Neotoma fuscipes) and brush rabbits (Sylvilagus bachmani). This pattern of high 

predation rates by habitat generalist carnivores in intensively managed landscapes is 

consistent with studies of North American martens elsewhere. Because survival is 

the most sensitive vital rate for martens, predation related to landscape composition 

likely represents a limiting factor for Humboldt marten populations. Collectively, 

these results suggest that the management and restoration of suitable habitat for the 

Humboldt marten will require consideration of multiple spatial scales, including 

stand, home range, and landscape, to account for marten habitat needs and to mediate interactions that affect their survival. 

The diet of the Humboldt marten is composed largely of small mammals, and to 

a lesser degree birds, with berries and ground-nesting wasps and hornets consumed 

when they are seasonally available. Sciurids are the most important prey, representing 42 percent of the overall proportion of metabolizable energy, ranging from 

29 percent (spring) to 51 percent (summer). Chipmunks (Tamias sp.) are the most 

important sciurid in the summer and fall; when they are less available in winter and 

spring owing to reduced activity, Humboldt martens increase their use of alternative 

prey of similar or larger body size, including Humboldt flying squirrels (Glaucomys 

oregonensis) and medium and large birds. The importance of sciurids in Humboldt 

marten diets is consistent with diets reported by other marten studies from the 

Pacific States. Collectively, these key prey species use food resources and den 

and cache sites in large standing woody structures (live and dead) associated with 

mature and late-successional forest conditions. Between foraging bouts, martens 

use rest sites to conserve energy and avoid predators, typically selecting the largest 
available live and dead woody structures. Humboldt martens typically rest in cavities and platforms in large-diameter live and dead conifers and hardwoods. Additionally, Humboldt martens rest in cavities and chambers associated with downed 

logs and natural log piles, and less often in slash piles, rock piles, and dense shrub 

clumps. Female martens are obligate cavity users for reproduction, bearing young 

in cavities in large-diameter hardwoods and conifers, and often moving young to 

platforms in trees and snags or cavities near the ground later in the denning season. 

Marten habitat management at the stand scale will involve maintaining, enhancing, 

and recruiting suitable resting and denning structures, as well as providing for the 

needs of important prey. 

The overall goal of this conservation strategy is to establish self-sustaining, 

interacting populations of Humboldt martens within suitable habitat throughout 

their historical range. To achieve this goal, this conservation strategy uses a threepronged approach: (1) protect existing populations and currently suitable habitat, 

(2) reestablish populations where currently suitable habitat is inaccessible owing 

to existing dispersal barriers, and (3) restore suitable habitat conditions in specific 

areas to increase population size and distribution. Our first step in developing 

this strategy was to identify conservation emphasis areas, including (1) extant 

population areas encompassing the distribution of extant populations; (2) population reestablishment areas, including unoccupied habitat capable of supporting ≥5

female marten home ranges; and (3) landscape connectivity areas in currently lowsuitability habitat between extant population areas or population reestablishment 

areas. We mapped these conservation emphasis areas using contemporary survey 

results and a recently developed rangewide landscape habitat suitability model. 

We conducted a threats assessment for the California portion of the assessment 

area to describe and rank potential threats to martens or their habitat, and identified 

specific conservation actions and information needs. In California, threat categories 

with the highest impact levels included large-scale habitat fragmentation, wildfire, and lethal disease. We also identified seven additional medium-level threats 

and 18 low-level threats. We determined that three high-priority conservation 

actions—strategic habitat restoration and management, population reestablishment 

through assisted dispersal, and monitoring of population response to management 

and stochastic events—could ameliorate most high- and medium-level threats. The 

strategy also identifies high-priority short-term information needs, including: 

• Development of a habitat management guide capable of identifying habitat 

characteristics most important for supporting marten occupancy, reproduction, and survival at the stand and home range scales. 
• Continuing surveys to identify new occupied areas and to monitor the distribution and abundance of known populations. 

• Evaluating the potential for maintaining and improving each population 

reestablishment area. 

• Assessing the feasibility of an assisted dispersal program for population 

reestablishment. 

• Determining the habitat associations for key marten predators, and identifying management actions that can reduce predator abundance where 

martens occur. 

• Identifying habitat conditions for successful marten dispersal and developing management recommendations that can maintain or improve these 

conditions where needed. 

In Oregon, contemporary survey efforts are not yet sufficient to guide the identification of conservation emphasis areas or threats assessment processes. The shortterm high-priority information needs for Oregon can be met by: 

• Conducting surveys to determine the size and distribution of known marten 

populations using a sampling design capable of tracking long-term changes 

in size or distribution. 

• Evaluating the landscape habitat suitability model for coastal Oregon and 

refining it with new survey data as necessary. 

• Determining the multiscale habitat needs for marten home range occupancy, reproduction, and survival using standardized vegetation sampling 

methods. 

• Determining the population-level impacts of factors contributing to marten 

mortality, including distribution and frequency of predation, vehicle strikes, 

legal trapping, and rodenticide poisoning. 

Finally, we will seek to maximize the value of new information and management 

actions by using standardized surveys, population monitoring protocols, and 

vegetation mensuration techniques throughout the range of the Humboldt marten. 
Contents 

1 Chapter 1: Introduction 

1 Background 

5 Geographic Scope 

6 Goal and Objectives of the Assessment and Strategy 

7 Chapter 2: Biophysical Environment 

7 Physical Environment 

8 Plant Communities and Natural Disturbance 

10 Natural Disturbance 

13 Implications for Conservation 

15 Chapter 3: Human Modifications to the Environment

15 American Indian Use of Martens 

15 American Indian Use of Fire 

16 Historical and Contemporary Use and Management of Natural Resources 

16 Alteration of Natural Fire Regimes 

18 Habitat Loss and Conversion From Timber Harvest 

19 Current Conditions and Land Use of Assessment Area 

21 Predator Control 

21 Climate Change in the Assessment Area 

22 Implications for Conservation 

25 Chapter 4: Biological Information 

25 Description and Taxonomy 

29 Historical Distribution: Coastal California and Coastal Oregon 

29 Contemporary Distribution and Abundance: Coastal California and Oregon 

33 Life History 

33 Territoriality 

33 Activity Patterns 

33 Food Habits and Habitat-Prey Relationships 

37 Reproduction 

37 Survivorship and Longevity 

40 Parasites and Disease 

41 Dispersal and Recruitment 

42 Population Biology and Dynamics 

vi 
45 Intraguild Predation and Interspecific Competition

47 Multiscale Habitat Use 

48 Microscale Habitat Use: Resting and Denning Structures 

51 Stand-Scale Habitat Use 

53 Home Range-Scale Habitat Use 

56 Landscape-Scale Effects on Habitat Use: Movement, Occupancy,

and Population Dynamics 

59 Contemporary Landscape Habitat Suitability 

63 Implications for Conservation 

65 Research and Management Needs in the Assessment Area 

67 Chapter 5: Development of the Conservation Strategy Foundation 

67 Strategy Goal, Guiding Principles, and Approach 

69 State of the Science for Supporting the Development 

of a Conservation Strategy 

69 Overall Strategy Approach 

70 Conservation Emphasis Areas 

72 Extant Population Areas 

77 Population Reestablishment Areas 

80 Landscape Connectivity Areas 

82 Potential Threats to the Humboldt Marten 

83 Classification of Potential Threats

87 Degree of Potential Threat Impacts 

88 Conservation Actions 

88 Integration of Conservation Actions and Conservation Emphasis Areas 

88 High Priority Conservation Actions 

94 Next Steps and Key Information Needs for Further Development of the 

Conservation Strategy 

97 Acknowledgments 

97 English Equivalents 

98 References 

117 Appendix 1—Humboldt Marten Conservation Group 

119 Appendix 2—Rational for Threats Rankings in California 
Chapter 1: Introduction 

Background 

Until recently, the American marten (Martes americana) was the only marten 

species recognized in North America, occurring in boreal forests, montane coniferous forests, and Atlantic and Pacific coastal forests (Gibilisco 1994). However, the 

Pacific marten (Martes caurina) was recently split from the American marten based 

on genetic and morphological differences (Dawson and Cook 2012). The range of 

the Pacific marten occurs largely in montane and coastal coniferous forests west of 

the Rocky Mountain crest, while the range of the American marten occurs to the 

north and east of the Rocky Mountain crest. The deep genetic split between these 

two species of martens in North America is thought to have originated from the 

persistence of marten populations in two disjunct glacial refugia during the most 

recent glacial period (Dawson and Cook 2012). 

The Humboldt marten (M. c. humboldtensis), one of the recognized subspecies of the Pacific marten (Grinnell and Dixon 1926), was historically distributed 

throughout the coastal, fog-influenced forests of the coast redwood (Sequoia sempervirens (Lamb. ex D. Don) Endl.) region in California from northwestern Sonoma 

County northward to the Oregon border (Grinnell et al. 1937) (fig. 1). Ongoing 

genetic analyses support the conclusion that there is no genetic distinction between 

martens in the range of the Humboldt marten subspecies in California and the M. c. 

caurina subspecies in the Oregon Coast Range and that these populations represent 

a single evolutionary unit (Schwartz et al. 2016, Slauson et al. 2009b). Furthermore, 

based on the combination of nuclear and mitochondrial DNA analyses, coastal 

California and coastal Oregon martens are different from interior California and 

interior Oregon martens representing M. c. sierrae in the Marble and Trinity Mountains of northern California, M. c. caurina in the Cascade Mountains of Oregon, 

and M. c. caurina in the Olympic and Cascade Mountains of Washington. Based on 

these collective findings, the range of the Humboldt subspecies has been expanded 

to include the Oregon Coast Range, historically recognized as that of M. c. caurina 

(Schwartz et al. 2016). For these reasons, this assessment and strategy will focus on 

the collective distribution of the Humboldt subspecies from its historical range in 

coastal northwestern California and coastal Oregon. 

Throughout this assessment and strategy, we have primarily used published 

studies within the range of the Pacific marten. Where information from the range 

of that species is limited or absent, we include relevant references from studies 

within the range of the American marten. However, confusion may arise when 

describing older studies conducted in the portion of the historically described range 

1 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
Washington Clatsop Columbia 

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California

Jackson Klamath 

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Del Norte California

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Figure 1—Historical range (shaded gray area) of the Humboldt marten (Martes caurina humboldtensis) in 

coastal California and coastal Oregon. This geographic extent defines the scope of the assessment area. 

2 

GENERAL TECHNICAL REPORT PSW-GTR-260
3 

of the American marten now recognized as that of the Pacific marten. To reduce 

this potential confusion, we use the current taxonomic designation of Pacific marten when citing studies on the previously designated American marten but which 

occur within the currently recognized range of the Pacific marten. Throughout the 

document, we use the following terms in reference to specific marten taxonomical 

levels: (1) “Humboldt marten” to refer to M. c. humboldtensis, representing the 

subspecies found in the assessment area; (2) “Pacific marten” to refer to the species 

of marten occupying North America west of the Rocky Mountain crest, including 

the Humboldt subspecies; (3) “American marten” to refer to the species occupying 

North America east of the Rocky Mountain crest; and (4) “North American martens” to refer collectively to both Pacific and American marten subspecies. 

In North America, martens have a long history of being trapped for their valuable pelts. Martens were trapped in the coastal forests of California and Oregon as 

early as the late 1800s, and declining harvests of martens were noted in both states 

in the early 1900s (Dixon 1925, Grinnell et al. 1937, Zielinski et al. 2001). Owing 

to these declines, the trapping season was closed indefinitely in California in 1946 

(Zielinski et al. 2001). Despite similar declines in marten harvest in Oregon since 

the 1940s (Zielinski et al. 2001), martens are still legally trapped in that state. 

While successful marten trapping was declining in the early and mid-1900s, 

unregulated harvest of redwood forest was increasing. Regulation of timber harvest 

began in California in the decades following 1960, but by the mid 1990s more than 

95 percent of the redwood-associated forests in the historical range of the Humboldt 

marten had been logged (Fox 1996). A similar magnitude of logging took place in 

the coastal forests of Oregon (Bolsinger and Waddell 1993). The Pacific marten is 

closely associated with late-successional coniferous forests (reviewed in Buskirk 

and Powell 1994, Slauson et al. 2007, and further described below), and North 

American martens are highly sensitive to the loss and fragmentation of mature 

forests (Chapin et al. 1998, Hargis et al. 1999, Potvin et al. 2000). Over 70 percent 

of contemporary detections of Humboldt martens have occurred in large patches 

(>80 ha) of old-growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) forest with dense shrub cover, consistent with the habitat selection patterns 

reported by Slauson et al. (2007). In both coastal California and Oregon, martens 

have also been detected in moist coniferous forests on serpentine soils and forests 

adjacent to coastal dune systems. Both of these forest types often have less tree 

cover than more productive soil types, but have dense shrub layers (Slauson and 

Zielinski 2001, Slauson et al. 2007, Zielinski et al. 2001). Decades of unregulated 

trapping followed by extensive habitat loss and fragmentation from unregulated 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
Chapter 1: Introduction 

Background 

Until recently, the American marten (Martes americana) was the only marten 

species recognized in North America, occurring in boreal forests, montane coniferous forests, and Atlantic and Pacific coastal forests (Gibilisco 1994). However, the 

Pacific marten (Martes caurina) was recently split from the American marten based 

on genetic and morphological differences (Dawson and Cook 2012). The range of 

the Pacific marten occurs largely in montane and coastal coniferous forests west of 

the Rocky Mountain crest, while the range of the American marten occurs to the 

north and east of the Rocky Mountain crest. The deep genetic split between these 

two species of martens in North America is thought to have originated from the 

persistence of marten populations in two disjunct glacial refugia during the most 

recent glacial period (Dawson and Cook 2012). 

The Humboldt marten (M. c. humboldtensis), one of the recognized subspecies of the Pacific marten (Grinnell and Dixon 1926), was historically distributed 

throughout the coastal, fog-influenced forests of the coast redwood (Sequoia sempervirens (Lamb. ex D. Don) Endl.) region in California from northwestern Sonoma 

County northward to the Oregon border (Grinnell et al. 1937) (fig. 1). Ongoing 

genetic analyses support the conclusion that there is no genetic distinction between 

martens in the range of the Humboldt marten subspecies in California and the M. c. 

caurina subspecies in the Oregon Coast Range and that these populations represent 

a single evolutionary unit (Schwartz et al. 2016, Slauson et al. 2009b). Furthermore, 

based on the combination of nuclear and mitochondrial DNA analyses, coastal 

California and coastal Oregon martens are different from interior California and 

interior Oregon martens representing M. c. sierrae in the Marble and Trinity Mountains of northern California, M. c. caurina in the Cascade Mountains of Oregon, 

and M. c. caurina in the Olympic and Cascade Mountains of Washington. Based on 

these collective findings, the range of the Humboldt subspecies has been expanded 

to include the Oregon Coast Range, historically recognized as that of M. c. caurina 

(Schwartz et al. 2016). For these reasons, this assessment and strategy will focus on 

the collective distribution of the Humboldt subspecies from its historical range in 

coastal northwestern California and coastal Oregon. 

Throughout this assessment and strategy, we have primarily used published 

studies within the range of the Pacific marten. Where information from the range 

of that species is limited or absent, we include relevant references from studies 

within the range of the American marten. However, confusion may arise when 

describing older studies conducted in the portion of the historically described range 

1 

GENERAL TECHNICAL REPORT PSW-GTR-260
4 

In California, the 

Humboldt marten has 

been extirpated from 

more than 95 percent 

of its historical range. 

and regulated timber harvesting are likely the most important historical factors that 

caused declines in the distribution and abundance of martens in coastal California 

and coastal Oregon. 

In California, the Humboldt marten has been extirpated from more than 95 

percent of its historical range. Until recently, the subspecies was known only 

from a single population that likely contains fewer than 100 individuals (Slauson 

et al. 2009a). From 2001 to 2008, occupancy at previously occupied sites in this 

population declined by 42 percent during a period when minimal timber harvesting occurred in its range (Slauson et al. 2009a). Following the 2008 population 

assessment, a wildfire burned through approximately 25 percent of this population’s 

range, potentially reducing the population owing to habitat loss or degradation. In 

2012, all locations sampled in 2008 were resampled (Slauson 2012). Preliminary 

occupancy estimates for the 2012 sampling were similar to results from 2008 

(Slauson 2012), suggesting that marten occupancy in northern coastal California 

did not substantially change over that time period. More recently, during 2011–2015, 

martens were detected at more than 10 sites in two adjacent watersheds of tributaries of the Middle Fork of the Smith River near the California-Oregon border, 

suggesting that a small population exists there. 

In the Oregon Coast Range, martens are known from two apparently disjunct 

populations in the central and southern portions of the former range (Moriarty et 

al. 2016, Zielinski et al. 2001). In 2002 and 2017, large wildfires burned through 

portions of the contemporary distribution of the southern coastal Oregon population, likely resulting in further habitat loss in areas with high burn severity. The 

four extant marten populations in coastal California and Oregon are separated by 

distances of 20 to 40 km. Current habitat conditions, degraded by logging, severe 

wildfire, and urbanization, may render them functionally isolated. 

In 2010, the Humboldt marten was petitioned for listing as threatened or 

endangered under the federal Endangered Species Act (CBD 2010). The U.S. Fish 

and Wildlife Service published a “substantial” 90-day finding on the petition to 

list the Humboldt marten on January 12, 2012 (77 FR 1900), but a “not warranted” 

12-month finding on April 7, 2015 (80 FR 18741). In 2015, the Humboldt marten

was petitioned for listing as threatened or endangered under the California Endangered Species Act (EPIC and CBD 2015). In February 2016, the California Fish and

Game Commission accepted the petition and initiated a 1-year status review of the

Humboldt marten in California. During this review period, the Humboldt marten is

considered a “candidate” for state listing in California, and is afforded all the legal

protections provided a formally listed species under California law. In Oregon, the

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
6 

GENERAL TECHNICAL REPORT PSW-GTR-260

augmented by other research conducted on Pacific and American marten. Chapter 5 

presents the rationale for a conservation strategy and develops a threats assessment 

based on the information from chapters 2 through 4. Finally, the “Potential Threats 

to the Humboldt Marten” chapter provides a structure to identify and prioritize 

specific conservation recommendations. 

Goal and Objectives of the Assessment and Strategy 

The overall goal of this report is to inform the development of a conservation strategy that will establish self-sustaining, interacting populations of Humboldt martens 

where possible throughout their historical range. Specific objectives are to: 

• Provide a comprehensive review and reference document that describes the

biology and ecology of the Humboldt marten (chapters 1 and 4), the biophysical environment in which it exists (chapter 2), and the anthropogenic

modifications to that environment (chapter 3).

• Conduct a threats analysis (found in chapter 5) based on the review information in chapters 1 through 4.

• Identify conservation actions that can reduce or eliminate threats (chapter 5).

• Identify key information gaps and research actions to address these information needs (chapter 5).

• Serve as a guiding document for reference by HMCG member organizations (app. 1) to facilitate the implementation of conservation actions to

benefit Humboldt marten.
7 

Chapter 2: Biophysical Environment 

Physical Environment 

We used the generalized historical extent of the distribution of the Humboldt 

marten in coastal California and coastal Oregon (fig. 1), broad-scale ecological 

criteria (Ricketts et al. 1999), and natural biophysical features to delineate the 

Humboldt marten conservation assessment area. From south to north, the assessment area includes three forest ecoregions: northern California coastal forests, the 

coastal edge of the Klamath-Siskiyou forests, and the central Pacific coastal forests 

of Oregon (Ricketts et al. 1999). The northern portion of the assessment area is 

bordered to the east by the nonforested Willamette Valley and in the central and 

southern portions by the distinctive transition from fog-influenced mesic forest 

communities to drier interior forest communities. This transition occurs approximately 30 to 40 km from the coast in the Klamath Mountains and 20 to 30 km from 

the coast farther south in the California Coast Range. 

The topography of the assessment area is dominated by steep mountainous 

terrain dissected by many rivers and creeks. Elevation in the assessment area ranges 

from sea level to 1524 m. The topographic diversity creates many microenvironments that affect the distribution of vegetation composition and structure owing to 

elevation, aspect, and weather. 

The climate is heavily influenced by proximity to the Pacific Ocean. The 

combination of Pacific storms in the winter and clouds and coastal fog in the summer produces persistent moist conditions year-round on most of the coastal slope of 

the assessment area, while inland portions may experience relatively drier summer 

conditions. Strong west-east gradients in climatic conditions create characteristic 

shifts in vegetative communities as conditions become drier with increasing distance from the coast. This distance-from-coast gradient is strongest in south coastal 

Oregon and the California portions of the assessment area, where cool and moist 

summer conditions become most constricted. Temperatures across the assessment 

area are relatively mild, with low variation between summer highs (average 15 °C) 

and winter lows (average 5 °C) (PRISM 2012). As a result of relatively mild winter 

temperatures, most precipitation falls as rain, with average totals ranging from 

100 to 300 cm, generally increasing from south to north in the assessment area. 

Approximately 90 percent of the area lies below the elevation of persistent snow, 

which typically occurs above 914 m. Snowpacks usually form only in the northern 

Oregon Coast Range and along the eastern edge of the assessment area in northern 

California and southern coastal Oregon. Summers are warm but remain moist 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
8 

owing to the presence of clouds and fog. This summer moisture strongly influences 

the structure and composition of coastal forests. 

Plant Communities and Natural Disturbance 

The assessment area is characterized by some of the most productive forests in 

the world. In unmanaged late-seral stages, these forests are typically composed of 

long-lived, large trees, multilayered canopy structures, substantial standing and 

down large woody debris, and abundant ferns, herbs, and shrubs on the forest floor 

(Chappell et al. 2001, DellaSala et al. 2011, Sawyer 2007, Sawyer et al. 2000b). 

Assessment area forests are largely coniferous, typically dominated by redwood 

and Douglas-fir in California and by Douglas-fir, western hemlock (Tsuga heterophylla (Raf.) Sarg.), and Sitka spruce (Picea sitchensis (Bong.) Carrière) in Oregon 

(Ricketts et al. 1999, Sawyer 2007). Subdominant conifers include western redcedar 

(Thuja plicata Donn ex D. Don), Port Orford cedar (Chamaecyparis lawsoniana (A. 

Murray bis) Parl.), and grand fir (Abies grandis (Douglas ex D. Don) Lindl.), with 

sugar pine (Pinus lambertiana Douglas) and white fir (Abies concolor (Gord. & 

Glend.) Lindl. ex Hildebr.) at higher elevations (Chappell et al. 2001, Sawyer 2007). 

Hardwood-dominated stands in the assessment area are uncommon, but hardwoods 

such as tanoak (Notholithocarpus densiflorus (Hook. & Arn.) P.S. Manos, C.H. 

Cannon, & S.H. Oh), golden chinquapin (Chrysolepis chrysophylla Douglas ex 

Hook.) Hjelmqvist), and Pacific madrone (Arbutus menziesii Pursh) are common 

canopy subdominants. In some near-coast locations or postlogging sites, red alder 

(Alnus rubra Bong.) can be an early-successional overstory dominant. Riparian 

forests featuring broadleaf species such as red alder, black cottonwood (Populus 

balsamifera L. ssp. trichocarpa (Torr. & A. Gray ex Hook.) Brayshaw), and bigleaf 

maple (Acer macrophyllum Pursh), and mesic shrub species such as vine maple 

(A. circinatum Pursh), often are distinct within the conifer-dominated forests. 

Dense herbaceous or shrub layers are characteristic structural components of 

the forest floor in unmanaged stands in the assessment area. Species presence and 

dominance is shaped largely by the combination of soil nutrients and moisture, with 

herbaceous species such as western sword fern (Polystichum munitum (Kaulf.) C. 

Presl) dominating on nitrogen-rich or very moist sites, and evergreen shrubs such as 

Pacific rhododendron (Rhododendron macrophyllum D. Don ex G. Don) and salal 

(Gaultheria shallon Pursh) dominating on nutrient-poor or drier sites (Chappell 

and Kagan 2001). Other dominant or codominant understory shrub species include 

evergreen huckleberry (Vaccinium ovatum Pursh), salmonberry (Rubus spectabilis 

Pursh), red huckleberry (V. parvifolium Sm.), and in serpentine habitats (see description below), shrub form tanoak (N. densiflorus var. echinoides (R.Br. ter) P.S. 

GENERAL TECHNICAL REPORT PSW-GTR-260
9 

Manos, C.H. Cannon & S.H. Oh), and huckleberry oak (Quercus vacciniifolia Kellogg) (Chappell et al. 2001, Jimerson et al. 1995, Sawyer et al. 2000b). Many of the 

dominant shrub species are adapted to fire by having lignotubers (basal swellings 

at the interface between the roots and shoots below the soil surface), allowing these 

species to quickly sprout after fire and thus maintain site dominance (Agee 1993). 

Two other forest habitats of particular relevance to martens include coastal 

serpentine habitats and dune forest communities on coastal terraces. Martens have 

been detected in these habitat types recently; however, these types have limited 

distributions compared to the more extensive dominant coastal forest types that 

supported the majority of the historical marten distribution. We use the term “serpentine” to refer to the environment influenced by the unique chemical composition 

of ultramafic rocks, serpentine, and serpentinized peridotite, and the resulting plant 

community structure and species composition it supports (Harrison and Rajakaruna 

2011, Jimerson et al. 1995). 

Forests in serpentine habitats are typically open and rocky with stunted trees 

that contrast sharply with the dense, rapidly growing stands on more productive, 

nonserpentine soils that surround these sites (Jimerson et al. 1995). On the extreme 

coastal edge of the distribution of serpentine habitats within the assessment area, 

increased moisture and summer fog help to support dense, spatially extensive 

shrub layers. Martens have been found in this type of serpentine habitat in both 

south-coastal Oregon and north-coastal California portions of the assessment area 

(see “Multiscale Habitat Use” in the “Biological Information” chapter), but not in 

the more extensive interior serpentine habitats that are more open owing to the low 

amount of shrub cover. The specific serpentine plant communities used by martens 

include a variety of coniferous trees such as Douglas-fir, sugar pine, lodgepole pine 

(Pinus contorta Douglas ex Loudon), western white pine (P. monticola Rydb.), Jeffrey pine (P. jeffreyi Balf.), knobcone pine (P. attenuata Lemmon), and Port Orford 

cedar, and are dominated by mast-producing shrubs such as shrub form tanoak, 

huckleberry oak, and red huckleberry (Jimerson et al. 1995, Slauson 2003). Like the 

shrub community used by martens on more productive soil types, the shrub community in serpentine habitats is composed of long-lived, mast-producing species 

that maintain site dominance, rather than early-seral shrub communities composed 

of species such as Ceanothus sp. Nutt, which only dominate for relatively short 

periods after disturbances. 

We use the term “coastal dune forest communities” to describe the forests on 

stabilized dune terraces that are typically dominated by shore pine (P. contorta 

Douglas ex Loudon var. contorta; a coastal form of lodgepole pine), and in some 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
10 

areas codominated by Sitka spruce. The understory of these forest communities is 

typically dominated by salal and evergreen huckleberry (Chappell et al. 2001). In 

the assessment area, these sites are primarily in coastal Oregon. 

Natural Disturbance 

In the assessment area, natural disturbance forces affect the composition and 

structural characteristics of the forest communities at multiple spatial scales. These 

include fire, windthrow, insects, disease, and mass wasting events such as landslides. Fire is the major coarse-scale disturbance force in all but the wettest coastal 

forests, where windthrow is the major source of natural disturbance. A fire regime 

is the product of the climate, ignition frequency, and the characteristic pattern 

in which fire interacts with the topography and vegetation structure within any 

region (Agee 1993). Fire regimes are typically defined by the following attributes: 

temporal (seasonality and fire return interval), spatial (fire size and burn complexity), and magnitude (intensity, severity, fire type) (Sugihara et al. 2006). In general, 

short fire return intervals are indicated by the presence of multiple fire scars within 

old living trees or recently cut stumps. Many trees survive the low-intensity fires 

characteristic of shorter return intervals. In contrast, long fire return intervals are 

characterized by rare but destructive fires; such past fires are sometimes indicated 

by large stands of old trees of very similar age that regenerated together after a 

catastrophic event. 

In the following discussion, we describe the prehistoric fire regimes that existed 

in the assessment area prior to their interruption following Euro-American settlement. The prehistoric fire regimes included strong influences from many centuries 

of American Indian anthropogenic burning, which are included in this discussion of 

“natural” disturbance, as well as in the “Human Modifications to the Environment” 

chapter. The purpose of this discussion is to provide perspective on the forests present in the assessment area today, which to varying degrees still reflect influences 

from the prehistoric period. 

As elsewhere, ignitions in the assessment area occur from either lightning 

strikes or anthropogenic sources. Lightning strikes are fewest in northern and 

southern portions of the assessment area and greatest in the western edge of the 

Klamath-Siskiyou ecoregion, where the driest conditions and the highest mountains 

occur (Morris 1934, van Wagtendonk and Cayan 2008). 

In coastal portions of the assessment area, conditions that promote ignition 

and spread of fire are moderated by high winter precipitation and by fog, clouds, 

and moderate temperatures during summer (reviewed in Lorimer et al. 2009). 

Strong ocean-to-inland, and to a lesser degree, north-to-south gradients in these 

GENERAL TECHNICAL REPORT PSW-GTR-260
11 

climate-driven moisture conditions are evident across the assessment area. These 

regional gradients result in nonanthropogenic fires occurring more frequently with 

increasing distance from the coast and farther south in the assessment area (Oneal 

et al. 2006, Sawyer et al. 2000a). At finer scales, topographic characteristics such 

as slope position and aspect strongly affect fire frequency and severity (Oneal et al. 

2006). From north to south, the inland extent of moist maritime conditions declines 

and becomes highly dendritic, pushing farther inland primarily within coastally 

oriented watersheds. 

Lightning strikes in the redwood forests of the Northern California Coast ecoregion occur at only 15 to 23 percent of the frequency observed in more interior sites 

in the Klamath Mountains and Sierra Nevada (van Wagtendonk and Cayan 2008). 

Furthermore, lightning strikes in coastal forests are less likely to ignite fuels owing 

to the presence of coastal fog during the months of peak strike activity (reviewed in 

Lorimer et al. 2009). 

In the northern and central Oregon portions of the assessment area, the fire 

regime is classified as stand replacing with very infrequent (i.e., >200 years) return 

interval in the north, transitioning into a low and mixed severity with 35- to 200-

year return intervals in the central region (USDI GS 2012). Within these areas, the 

fire regime differs by forest type. Moist Sitka spruce forests exist in the near coastal 

zone where climatic conditions limit both the frequency and intensity of naturally 

occurring fires (Agee 1993). Fires in Sitka spruce forests were very rare and burned 

under rare weather conditions where fires were driven into the coastal zone by dry 

winds from the interior (Agee 1993). Many fires spreading from interior western 

hemlock forests were naturally extinguished upon entering Sitka spruce forests 

(Agee 1993). 

In moist western hemlock forests of coastal Oregon, extensive stand-destroying 

fires are part of the historical record (Agee 1993). Although large blocks of singleage cohort forests occurred in the northern Oregon Coast Range as a result of large 

and severe fires, the fire regime in the central Oregon Coast Range is a mix of 

high- and low-severity fires with fairly long return intervals averaging >250 years 

(Impara 1997). Although there are some accounts of American Indians using fire in 

the central and northern Oregon portions of the assessment area, Agee (1993) found 

little evidence to suggest that their influence represented a major effect on the fire 

regimes found there. Conclusions from studies using fire scar and age class analysis 

generally agree in this portion of the assessment area (e.g., Impara 1997). 

In the south coastal Oregon and California portions of the assessment area, the 

fire regime shifts to a mix of low- and mixed-severity fires with 0- to 35-year and 

35- to 200-year frequencies (USDI GS 2012). Paradoxically, the most frequent fires

apparently occurred throughout the Redwood Region, including near-coast sites

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
12 

GENERAL TECHNICAL REPORT PSW-GTR-260

and along drainage bottoms where climatic and ignition patterns rarely support the 

natural occurrence of fire. Although fire scar studies report frequent fires (e.g., <26 

years) (Norman 2007) in the Redwood Region, existing stand structures suggest 

infrequent high-severity events (e.g., >500 years) (Veirs 1982). This mismatch 

between fire ignition rates and fire frequencies indicate a large influence of regular 

anthropogenic fire in these areas. 

Lorimer et al. (2009) reviewed fire history studies conducted throughout the 

Redwood Region of the assessment area and concluded that the majority of presettlement fires were likely of American Indian origin and therefore that estimating 

the nonanthropogenic occurrence of fire was not likely possible from fire scar 

analysis. Annual and near-annual burning by American Indians to keep village 

sites and travel routes clear and to improve resources such as acorn crops, forage for 

game, and plants for basketmaking materials was widespread, with more frequent 

burning associated with forest conditions with higher moisture levels (Anderson 

2006, Lake 2007, Stuart and Stephens 2006, Vale 2002). Trends of fire frequency 

increasing with drier conditions along both west-east and north-south gradients are, 

however, present in fire scar data for the Redwood Region and likely represent the 

overall variation in fire frequency (Lorimer et al. 2009). Agee (1993) characterized 

the Redwood Region as having a moderate-severity fire regime, owing to its ability 

to resist all but the most extreme fire events. Nonanthropogenic fire frequency most 

likely differs along moisture and climatic gradients consistent with centuries-long 

return intervals in the nearest coastal locations, to several decades in locations 

where redwood forests border more fire-prone habitats. 

In the interior areas of the California and southern Oregon portions of the 

assessment area, mesic coastal forest types transition into largely Douglas-fir-dominated forest types. In these forest types, both fire scar and age class studies corroborate low- and mixed-severity fire regimes with 35- to 200-year frequencies. A 

significant influence from American Indian burning is also evident in these forests 

(Lake 2007, Lewis 1993). The assessment area includes the most mesic portion of 

the Douglas-fir forest types (which extend farther inland toward more xeric conditions), including stands that are on the longer end of the range of fire frequency for 

this type. However, geologic and topographic diversity within the Klamath-Siskiyou Mountains have produced substantial variation in fire frequencies and burn 

severities, precluding generalizations across the region (Agee 1993). For instance, 

the presence of fire-dependent species such as lodgepole pine and knobcone pine 

along serpentine-soil-dominated ridges suggests along serpentine soil dominated 

ridges that fire frequency may be very complex and site specific across this portion 

of the assessment area. 
13 

Windthrow is another important disturbance factor that may complement other 

disturbances such as fire and disease, or may represent the dominant disturbance 

factor in mesic forest types where fire is rare (e.g., Sitka spruce forests) (Ruth and 

Harris 1979). Windthrow is typically recognized as a relatively fine-scale disturbance factor, producing single and small clusters of partial or entire tree breakage 

and uprooting, respectively. However, topographic position can play a role in 

uncommon large-scale windthrow events that create uneven-age structure in coastal 

forests over many decades (Harcombe et al. 2004). Severe windthrow events are 

episodic but can dramatically alter forest structure and composition. Windthrow 

can also increase structural complexity in trees by snapping tops and stimulating 

regrowth of multiple tops that provide unique habitats for flora and fauna (Sillett 

and Van Pelt 2007). Limb and leader breakage from windthrow can also allow for 

fungal pathogens and insects to attack trees, starting the process of cavity and snag 

creation. 

Insects, disease, and landslides typically foster small-scale disturbances at 

the level of individual trees or small patches of trees. However, the pathogens that 

cause Port Orford cedar root rot (Phytophthora lateralis) and sudden oak death 

(Phytophthora ramorum), two recently established nonnative disease agents, have 

the potential to greatly alter the composition and function of forests within the 

assessment area (Hansen et al. 2000, Rizzo and Garbelotto 2003). Port Orford cedar 

root rot affects only a single species, but causes high mortality rates (USDA FS 

2006). Sudden oak death affects many oak species and has the potential to result in 

large-scale mortality of species such as tanoak, chinquapin, and shrub oaks that are 

common components of marten habitat (see the “Biological Information” chapter) 

(USDA FS 2006). The variety of oak species in forests of the assessment area 

provide key resources for martens and their prey in the form of annual mast crops 

and cavity formation that provide resting and denning locations. 

Implications for Conservation 

• The assessment area is biophysically diverse and includes substantial variation in dominant forest types and their relationships with key disturbance

factors. Conservation actions and measures must be developed to be applicable within the context of this variability.

• Prehistoric fire regimes shaped the forest types present across the assessment area. Fire frequency and severity are dependent on the site-specific

conditions present in each forest type. Nonanthropogenic fire frequency

varies along dominant gradients in summer moisture and temperatures,

increasing in frequency both from north to south and west to east across the

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
14 

GENERAL TECHNICAL REPORT PSW-GTR-260

assessment area. Burning by American Indians was apparently the primary 

source of ignition in some areas, strongly increasing the fire return rate in 

these areas. Managers seeking to use the historical fire regime as a template 

for guiding current management should evaluate the sources of fire return 

interval data to identify sites representing anthropogenic and nonanthropogenic fire histories, and clearly identify where management for anthropogenic fire histories may be desirable. 
15 

Chapter 3: Human Modifications to the Environment

American Indian Use of Martens 

Historically, martens were trapped and their skins used in ceremonial regalia by the 

Yurok Tribe of northern coastal California (K. Slauson, pers. obs., 2002) and tribes 

in coastal Oregon (Jacobs 1934). However, their relatively low prominence among 

other regalia and long use when incorporated into regalia suggests that martens 

were not trapped for these purposes at levels that would significantly affect populations. Tribes located on the eastern edge of the assessment area do not feature 

martens in regalia, but nearby tribes such as the Hoopa Valley Tribe in California 

do prominently feature fishers (Pekania pennanti) (Higley 2008). 

American Indian Use of Fire 

American Indian tribes used fire for cultural fire purposes throughout the assessment area (Lake 2007). The use of fire by American Indians differed from nonanthropogenic fire regimes in five key ways: seasonality, frequency, extent, site, and 

outcome (Kimmerer and Lake 2001). Notably, Kimmerer and Lake (2001) stated 

“scholars of aboriginal fire agree that anthropogenic fire far exceeded the frequency 

of natural lightning strikes.” Within the assessment area, the shortest reported fire 

return interval, 7 years (Veirs 1982), is closest to known village sites; fire return 

intervals increased with distances from such locations (Norman 2007). American 

Indians used frequency of burning and season of burning to minimize the spatial 

extent of fires and to achieve the intended outcomes over the desired area (Lake 

2007). The cultural fire regime was managed to meet objectives at a variety of 

spatial scales, including the site-specific, resource patch, and broadcast burning of 

larger areas (Lake 2007). In locations that were routinely managed by tribes using 

fire, the nonanthropogenic fire regime was overridden by the anthropogenic fire 

regime. Therefore, the reconstruction of fire history from fire scar studies requires 

that the landscape context and degree of nonanthropogenic influence be taken into 

account for each study location. For example, fire scar studies that have reported 

the shortest fire-return intervals in the northern Redwood Region are also those 

in proximity to American Indian village sites; however, village sites represent a 

small proportion of the historical landscape. Creating regional mean fire return 

intervals by simply averaging the results across studies, without weighting them by 

their relative historical spatial influence, may result in negatively biased estimates. 

For instance, the unweighted mean approach was used by Safford et al. (2010) to 

produce a mean estimate of 23 years for the redwood forest typea return interval clearly uncharacteristic for nonanthropogenic ignition regime in this climate, 

especially in the northern range of the redwoods. 

In locations that were 

routinely managed by 

tribes using fire, the 

nonanthropogenic fire

regime was overridden 

by the anthropogenic 

fire regime.

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16 

GENERAL TECHNICAL REPORT PSW-GTR-260

By the early 1900s, 

annual harvest totals 

of martens in the 

assessment area were 

already in decline, 

signaling stress on 

populations from 

trapping. 

Historical and Contemporary Use and Management of 

Natural Resources 

Trapping of martens for the fur trade— 

The first major impact of the arrival of Americans of European ancestry on martens 

was the fur trade. Trappers from British companies on the Columbia River entered 

the Oregon Coast Range in the early 1820s, focusing on beaver and otter, but also 

taking “small furs” (Dillon 1975). By the late 1800s and early 1900s, significant 

trapping efforts were well underway (Anonymous 1914, Grinnell et al. 1937). By the 

early 1900s, annual harvest totals of martens in the assessment area were already in 

decline, signaling stress on populations from trapping (Dixon 1925, Zielinski et al. 

2001). Accounts of individual trappers taking 35 and 50 martens during single winters in localized areas within the California portion of the assessment area (Grinnell 

et al. 1937) suggest the historical abundance of the Humboldt marten and the impact 

of individual trappers on marten populations. The sharp decline in annual harvest 

rates caused Dixon (1925) to call for the closing of the trapping season on martens 

in California in 1925 for fear of their extirpation. Declining trapping harvests led 

to the closure of the marten trapping season in extreme northwestern California 

in 1946 (Zielinski et al. 2001). The trapping season for martens within the entire 

California portion of the assessment area was closed in 1954. 

Trapping records for the Oregon portion of the assessment area peaked in the 

1940s and did not exceed 15 percent of the 1940s levels thereafter (Zielinski et al. 

2001). Martens are still legally trapped in the Oregon portion of the assessment 

area. At about the same time the trapping season was closed in California and the 

peak trapping harvest occurred in coastal Oregon, the era of large-scale clearcut 

logging of coastal old-growth forests was beginning (Haynes 2001, see the “Habitat 

Loss and Conversion From Timber Harvest” section below), compounding the 

negative effects on martens. 

Alteration of Natural Fire Regimes 

Contemporary changes to the fire regimes within the assessment area include 

elimination, suppression, or management of most natural and anthropogenic 

ignitions. Long-established American Indian burning practices have disappeared. 

Currently, burning for grazing, fuel reduction, and forestry site preparation is carried out under prescribed conditions, and most wildfires are suppressed as soon as 

possible. Fire suppression has strongly affected the structure and function of forest 

ecosystems in more interior forest types, such as mixed-conifer forests in the mid 

elevations of the Sierra Nevada Mountains (Miller et al. 2009). However, in portions of the assessment area with summer fog, fire suppression appears to have had 
17 

little effect on the structure and composition of the dominant forest types, and has 

not caused an increase in high-severity fire relative to the historical patterns (Miller 

et al. 2012, Odion et al. 2004). 

Miller et al. (2012), in a study that encompassed four national forests, including 

the Six Rivers National Forest, did not detect a change in the trend of fire severity 

from 1910 through 2008, but found that other parameters such as fire frequency, 

size, and total acres burned had increased. They reported that in forest types largely 

inland from the assessment area, high-severity fire most frequently occurred in 

areas dominated by small-diameter trees. In forest types where fire-return intervals are centuries long, the period of fire suppression may not yet have been long 

enough to accumulate such effects (e.g., Oneal et al. 2006, Veirs 1982). Where 

fire-return intervals are shorter, such as a few decades, forest structural change and 

subsequent wildfires have not produced burn conditions outside those expected in 

unmanaged stands based on the patterns of presuppression burning (Miller et al. 

2012, Odion et al. 2004). Our review suggests that the prevailing notion that fire 

suppression has moved forests out of their natural range of variability, increasing 

their susceptibility to increased fire severity, applies more strongly to interior and 

southern portions of the assessment area than to near-coast forest types that receive 

summer fog. 

Uncertainties regarding the degree of historical wildfire, sources of such fires, 

degree of change in forest structure owing to suppression efforts, and localized 

differences in fire-mediating conditions such as summer fog, create challenges for 

management. Evaluations of changes in forest structure and susceptibility to higher 

severity fire, and actions intended to reduce the risk of high-severity fire, will 

have to be made cautiously based on site-specific information. However, if future 

changes in climatic conditions result in drier and warmer conditions, an increase in 

fire severity and intensity may result, especially in the most inland portions of the 

assessment area. 

In the assessment area, the most dramatic change to the fire regime has been 

the loss of American Indian burning practices from the California coastal ecoregion and the western edge of the Klamath-Siskiyou forest ecoregion where it was 

commonly practiced historically (Lake 2007). Based on the characteristics of the 

climate and nonanthropogenic fire ignition frequency in the assessment area, it is 

most likely that American Indian burning dramatically decreased the nonanthropogenic fire return intervals where it was practiced (Lorimer et al. 2009). The shortest 

fire return intervals (<20 years) in mesic forest types (e.g., redwood) in the southern 

portion of the assessment area are in proximity to village sites (e.g., Norman 2007, 

The prevailing notion 

that fire suppression

has moved forests out 

of their natural range of 

variability, increasing 

their susceptibility to 

increased fire severity, 

applies more strongly 

to interior and southern 

portions of the 

assessment area than 

to near-coast forest 

types that receive 

summer fog. 

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GENERAL TECHNICAL REPORT PSW-GTR-260

Before the era 

of commercial 

timber harvesting, 

vegetation within 

the assessment area 

was predominantly 

composed of oldgrowth coniferous 

forest. 

Veirs 1982) and fire return intervals increase greatly with the distance from village 

sites (e.g., Veirs 1982). This suggests that fire return interval data are best interpreted in the context of gradients that span environmental, ocean-inland distances, 

and anthropogenic activity, and distance from village and key resource sites. Simple 

averages of point estimates of fire return interval studies (e.g., Safford et al. 2010, 

USDI GS 2012) are likely to be misleading until there is a better understanding of 

the extent to which environmental and anthropogenic gradients affected historical 

fire regimes in the southern portion of the assessment area. 

Habitat Loss and Conversion From Timber Harvest 

Before the era of commercial timber harvesting, vegetation within the assessment 

area was predominantly composed of old-growth coniferous forest. Estimates of the 

historical amount of old growth in coastal forests differ depending on the definition 

of old growth and the extent of the area examined, but most estimates of the percentage of area in old-growth forest conditions at the time of European colonization 

were >75 percent for the coastal California region, >50 percent for the KlamathSiskiyou region (Strittholt et al. 2006), and 40 to 85 percent in the Oregon Coast 

Range (e.g., Ripple 1994, Strittholt et al. 2006, Teensma et al. 1991). 

The forests of the assessment area were largely accessible to early logging 

methods by the late 1800s and early 1900s, with wide-scale harvesting increasing 

after World War II. As a result, most of the old-growth forests in the assessment 

area have been logged (Bolsinger and Waddell 1993), causing coastal old-growth 

forests to be considered one of the most heavily affected terrestrial habitats in 

western North America (Ricketts et al. 1999). For example, Save the Redwoods 

League (2015) estimated that approximately 5 percent of the original old-growth 

coast redwood forest remains across the 809 000-ha range of that species (which 

is almost entirely in California). The Oregon Coast Range is estimated to contain 

<10 percent of the historical amount of old-growth (Bolsinger and Waddell 1993, 

Ohmann and Spies 1998), with the highest amount remaining on the Siuslaw 

National Forest. Modeling landscape-scale growth and fire simulations, Wimberly 

et al. (2000) estimated that at any one time during the past 3,000 years, between 25 

and 75 percent of the Oregon Coast Range supported old-growth forests under the 

natural fire regime. 

The spatial distribution of old-growth forest has also changed over the past 200 

years. In the Oregon Coast Range, old-growth patches were historically large (210 

000 to 850 000 ha), but have been reduced to a maximum of 647 ha today (Wimberly 2002, Wimberly et al. 2004). Furthermore, old-growth forest patches were 

more connected historically, with most forest patches >200 years old located within 
19 

1 km of another old-growth patch, whereas today old-growth patches are much 

more isolated from the nearest remnant patch (Wimberly et al. 2004). 

Current Condition and Land Use of Assessment Area 

Land ownership in the assessment area is composed of private industrial timberlands (>50 percent), U.S. Forest Service (USFS) (20 percent), Bureau of Land 

Management (BLM) (10 percent), state and national parks (<5 percent), state forests 

(<10 percent), and small private and tribal lands (<5 percent) (fig. 2). On industrial 

timberlands, clearcut logging, short-rotation silviculture, removal of late-seral structure and hardwoods, and road building have produced forests with highly altered 

structures and compositions. Managed regenerating stands often lack key habitat 

elements used by martens, especially suitable resting and denning structures in 

large, standing, live and dead trees, and dense ericaceous shrub cover (Slauson and 

Zielinski 2007, Slauson et al. 2010). On industrial timberlands, older regenerating 

stands that are developing such conditions may be scheduled for harvest in future 

decades, unless they are part of other conservation agreements for other wildlife or 

fish species. 

Logging on USFS and BLM lands was not as prevalent historically as on 

private lands, and in recent decades has primarily been confined to thinning and 

management of older forest plantations. USFS and BLM lands in California and 

Oregon include more than 202 000 ha of wilderness areas within the assessment 

area. In the majority of these areas, the primary human influence may be stand 

conditions reflecting lack of wildfire, but >25 percent of these areas have burned 

in the past two decades. Redwood National and State Parks total more than 72 000 

ha in the assessment area, with approximately one-third of the area composed of 

old-growth forest. Forests in state and national parks, and some tribal lands, are 

not subject to harvest, except where younger stands logged by previous owners are 

being thinned and managed to develop old-growth characteristics. 

In recent years, federal and state land management agencies in the assessment 

area have focused on forest restoration and fuels management. These efforts have 

included management of young stands in previously logged areas to stimulate stand 

growth toward mature conditions, and thinning of understory fuels in older stands 

to reduce the likelihood of catastrophic fire. Because of limited budgets, these 

efforts have treated a small percentage of the areas where such actions would be 

appropriate. 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
Pacific

Ocean

Land Ownership 

Private 

Forest Service 

State Forest 

Bureau of Land Mgt 

Tribal 

Redwood National and 

State Parks 

Historical Range 

 ̄

0 75 150 

Clatsop Columbia Washington 

Washington Multnomah Hood River 

Tillamook Sherman 

Siuslaw Yamhill National Clackamas Wasco 

Forest 

Polk Marion 

Lincoln Jefferson 

Benton Linn 

Crook Siuslaw 

National 

Forest 

Deschutes 

Lane 

Douglas 

Coos Lake 

Klamath 

Curry Jackson 

Josephine Rogue RiverSiskiyou 

National 

Forest Oregon 

Six 

Rivers Klamath California National 

Forest National 

Del Norte Siskiyou 

Forest Modoc 

Shasta Lassen 

Humboldt Trinity 

Tehama Plumas 

Butte 

Glenn Sierra 

Mendocino 

Yuba Nevada Colusa 

Lake Sutter 

Placer 

El Dorad Yolo 

Kilometers Sonoma Napa Sacramento 

Solano Amad 

Marin SAN JOAQ 

Figure 2—Land ownership within the

historical range of the Humboldt marten 

(Martes caurina humboldtensis) in 

coastal California and coastal Oregon. 

20 

GENERAL TECHNICAL REPORT PSW-GTR-260
21 

Predator Control 

Predator control using a variety of direct and indirect methods was historically 

commonplace in the Western States to improve game populations and protect 

livestock. Although martens may not have been targeted directly, the methods used 

to poison target predators were often not specific to the target species. Two former 

methods, poisoning carcasses of ungulates and aerial broadcasting of poisoned 

baits, were capable of killing martens. On the Olympic Peninsula of Washington in 

the early 1900s, dead martens and fishers were observed in the vicinity of ungulate 

carcasses that had been shot and laced with poison baits (B. Adamire, pers. comm., 

in Zielinski et al. 2001). Records of the distribution and intensities of these activities are difficult to obtain and their historical prevalence in the assessment area 

is unknown. Intentional poisoning as a means of predator control largely ceased 

before the 1970s. 

More recently, widespread use of rodenticides and other toxic chemicals at 

illegal marijuana cultivation sites has emerged as a threat to Pacific martens. In 

California, residues of anticoagulant rodenticides were found in >85 percent of 

dead fishers tested for toxicant exposure, and several fishers have died from acute 

rodenticide poisoning on the Hoopa Valley Indian Reservation, located on the 

southeastern edge of the assessment area (Gabriel et al. 2012b, Gabriel et al. 2015). 

Although predation was the dominant source of mortality for fishers throughout 

California (70 percent compared to 10 percent for toxicants), a greater proportion 

of the fisher mortalities in northern California were attributed to toxicants and 

fewer to predation as compared to the Sierra Nevada population of fishers (Gabriel 

et al. 2015). Given the similarity in foraging and diet composition between fishers 

and martens, and the widespread occurrence of illegal marijuana cultivation in the 

assessment area, rodenticide exposure could be a serious threat to the persistence 

and recovery of Humboldt marten populations in the portions of the range (e.g., 

mainly California) where illegal marijuana cultivation occurs. As of 2014, an ongoing study in California had detected rodenticides in one of six (17 percent) dead 

Humboldt martens tested (Slauson et al. 2014). 

Climate Change in the Assessment Area 

The assessment area includes the southern terminus of temperate rainforests of 

the North American continent. These forests are dependent on a suite of climatic 

conditions that create moist conditions throughout much of the year (DellaSala 

et al. 2011). Overall, climatic conditions that support mesic coastal forests extend 

farther inland in Oregon and are limited to near the coast in the southern Oregon 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
22 

GENERAL TECHNICAL REPORT PSW-GTR-260

and California portions of the assessment area. Global climate change is expected 

to increase average temperatures across the assessment area in coming decades; 

however, the exact magnitude of this change is still in question and depends in large 

part on the continued rate of global carbon emissions (DellaSala 2013). 

Predictions for changes to future precipitation levels differ for the assessment 

area (PRBO 2011). Climate projections suggest that drier conditions will occur 

closer to the coast in the future, resulting in a narrower extent of coastal forest, 

and loss and fragmentation of coastal forests at their southern extent in California. 

Changes have already been observed in a key element of precipitation: summer fog 

frequency has been declining over the past century across the California portion 

of the assessment area (Johnstone and Dawson 2010). This trend could reduce the 

inland and southern extent of coastal forest conditions such as the distribution of 

dense shrub layers that are an important component of marten habitat. However, 

other authors reviewed by PRBO (2011) speculated that fog may increase as a result 

of an increasing temperature differential between land and ocean. 

Under moderate emissions scenarios, within 50 years, current bioclimatic 

conditions are projected to occur reliably only in northern Humboldt and Del Norte 

Counties in the current northern extent of the coast redwood’s range (DellaSala 

2013). In the Oregon portion of the assessment area, contractions of the bioclimatic 

conditions supporting coastal forests are projected to be greatest in the south 

coastal region, south of the Rogue River. Here, conditions supporting coastal forests 

will likely become increasingly limited to a more narrow inland distribution within 

50 years (DellaSala 2013). Projections for impacts of climate change on forests in 

the northern portion of coastal Oregon are less severe. 

Implications for Conservation 

• The magnitude of loss and fragmentation of late-successional and oldgrowth forest habitat in the assessment area limits the locations where

short-term conservation actions can benefit marten population persistence

and recovery. 

• Most remaining late-successional and old-growth landscape-scale restoration of functional connectivity among areas currently suitable as habitat for

martens is an important conservation consideration and will require cooperation from both public and private land managers.

• Fire is an important natural disturbance process and accurately understanding its role in creating and maintaining forest structural and compositional

conditions is critical for informing the management of natural or desired
23 

conditions. In the California and southern Oregon portions of the assessment area, evidence of fire history can include both natural and cultural 

fire regimes. Cultural fire regimes greatly increased fire frequencies to 

produce desired resource conditions in specific areas, but these effects had 

limited spatial distributions within the assessment area. Application of 

reconstructed fire history data to guide management of existing conditions 

will require the careful evaluation of the role cultural fire regimes played 

in the fire history data being used to avoid overrepresenting the influence 

of cultural burning had across the landscapes of the assessment area. The 

evaluation of the historical role fire had on creating site-specific conditions will require characterization of the frequency and the extent to which 

cultural versus natural fire regimes occurred at the specific locations being 

considered for management. 

• Alteration of the natural fire regime in much of the assessment area has not

resulted in conditions promoting an increase in fire severity on account of

excessive fuels buildup that has occurred in more inland areas with more

frequent fire return intervals. Recent fires in and adjacent to the assessment

area have increased in size compared to historical fires, with most highseverity portions of fires occurring in young, largely managed stands. The

trend toward larger fire sizes will likely put more marten habitat at risk in

the short term. Even fires of moderate intensity may have short-term negative impacts of reducing the important shrub layer (see the “Stand-Scale

Habitat Use” section).

• Protection of marten habitat elements may at times appear to be at odds with

other management objectives. Habitat elements that confer suitability for

marten are often also targeted to reduce the risk of high-intensity or large

fires. Management to improve habitat conditions for martens and other species, while reducing the risk of catastrophic wildfire, will have to consider

the tradeoffs between these objectives where they are at odds for specific

habitat features. Note specifically the challenge of maintaining or regenerating a shrub layer when carrying out understory thinning to reduce fuel density. Creative planning and perhaps new science can be applied to explore

the tradeoffs of the possible long-term benefit of effective fuels treatments

against their short-term negative effects on key habitat features.

• The objectives of reducing the risk of severe fire and maintaining marten

habitat elements are not mutually exclusive over the long term; in fact, the

persistence of a sufficient amount of suitable habitat to support marten populations will be dependent, in part, on reducing the impacts of high-severity

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
24 

GENERAL TECHNICAL REPORT PSW-GTR-260

fire. Managers can strive to incorporate known elements of marten habitat as management objectives at various spatial and temporal scales, while 

improving overall forest resilience to severe wildfire and climate change. 

Reducing the risk of severe wildfire through habitat modification in certain 

areas may improve the persistence of marten habitat in neighboring areas 

or across the landscape as a whole. Late-successional and old-growth forest 

habitat in the assessment area severely limits the locations where shortterm conservation actions can benefit marten population persistence and 

recovery. 

• Most remaining late-successional forest occurs on public lands, but land

ownership in the assessment area is dominated by private industrial timberlands. On private lands, the remaining small remnants of old forest are

patchily distributed.

• The illegal use of rodenticides and other toxic chemicals in marijuana cultivation poses a serious threat to marten populations in the assessment area.

Environmental conditions sought by illegal marijuana growers, such as road

access, small streams, and recently harvested timber sites, are widespread

in currently occupied marten habitat in California and southern Oregon.

• The projected effects of climate change will present challenges to the restoration of habitat and populations, particularly in the southern portion of the

assessment area. It appears that the overall extent of favorable conditions

will be reduced, along with connectivity between areas where climate is

favorable for habitat. This emphasizes the value of conservation actions that

restore habitat in the remaining areas with the largest amounts of suitable

habitat that have the potential to support meaningful populations, reduce

threats in such areas, and ensure that these areas are connected to the greatest degree feasible.
25 

Chapter 4: Biological Information 

In this section we review existing biological knowledge of the Humboldt marten. As 

described above, early information primarily came from trapping records and the 

collection of museum specimens. Detection surveys, some incorporating vegetation 

analysis, were underway in the 1990s (Zielinski et al. 2001). Detailed field research 

on the Humboldt marten only began in 2000; thus, many aspects of its biology 

and ecology have not been studied. In this assessment, we have used all published 

and unpublished data available up to 2016 from within the range of the Humboldt 

marten. Where information gaps existed, we drew upon published information from 

outside the range of the Humboldt marten that best characterized aspects of its biology that are consistent across the range of the species. We used unpublished data 

from both inside and outside the range of the Humboldt marten sparingly and only 

to fill gaps in the published literature. 

Description and Taxonomy 

North American martens are characterized by the typical long and narrow mustelid 

body type, overall brown pelage with distinctive gular (throat) coloration varying 

from orange to yellow to cream, large and distinctly triangular ears, and a bushy 

tail that is proportionally equivalent to about 75 percent of the body length (Clark et 

al. 1987, Powell et al. 2003) (fig. 3). The combination of these characteristics can be 

used to distinguish martens from other mustelids, including the larger fisher, which 

usually appears black in the field, with well-rounded ears and a comparatively 

longer tail that is proportionally equivalent to >80 percent of body length (Powell 

et al. 2003) (fig. 3). Martens can be distinguished from the similar-size American 

mink (Neovison vison), which can have either dark or medium brown pelage, by the 

mink’s comparatively small and rounded ears, and shorter, less bushy tail (Lariviére 

1999) (fig. 3). The long-tailed weasel (Mustela frenata), with a smaller body size 

and comparatively more rounded ears than martens, shares the overall brown pelage 

and includes yellow-orange gular coloration but often has white, yellow, or black 

pelage coloration that extends onto the rostrum, between the eyes, and on the sides 

of the face (King and Powell 2006). In addition, the long-tailed weasel has a much 

less bushy tail and a more clearly defined black terminal tip (fig. 3). 

Among the American and Pacific marten species, as many as 14 subspecies 

have been proposed based on variation in morphological characteristics (Hall 1981). 

The merits of these subspecies have been the subject of considerable debate owing 

to the interpretation of patterns of morphological variation observed among subspecies (Dillon 1961, Hagmeier 1958). Recent phylogenetic studies have found little 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
26 Diagnostic photos 

U. S. Fo r e st Se r v ic e U.S. For e s t S e r v ic e 

U. S. Fo r e st Se r v ic e 

U.S. Forest Service K. Slauson Mink. Long-tailed weasel. Marten. Fisher. Remote camera photo 

K. Slauson

Long-tailed weasel.

Mink. 

Marten. 

U.S. Forest Service Forest Service U.S.

Fisher. 

Figure 3—Comparison of photographs of Humboldt martens (Martes caurina humboldtensis) from the similar fisher (Pekania pennanti), American mink (Neovison vison), 

and long-tailed weasel (Mustela frenata). The upper panel includes diagnostic photos. The lower panel includes typical photos from remote cameras for martens, fishers, and 

mink, where identification is best made using the combination of relative tail length-to-body length ratio, ear morphology, and overall pelage coloration. 

GENERAL TECHNICAL REPORT PSW-GTR-260
27 

genetic variation across the continuous mainland distribution of the American marten (Dawson and Cook 2012, Graham and Graham 1994, Kyle and Strobeck 2003), 

with the exception of the Newfoundland marten (M. americana atrata) (McGowan 

et al. 1999), suggesting that the American marten recently expanded from a single 

glacial refugium in eastern North America. 

In contrast, Dawson (2008) found significant geographic variation within the 

Pacific marten, consistent with patterns of genetic variation reported for other 

species that expanded from multiple glacial refugia during the Holocene (Hewitt 

1996, Lessa et al. 2003). These phylogeographic findings suggest that subspecific 

designations for the Pacific marten most likely reflect the distribution of populations and geographic variation resulting from isolation in disjunct forest refugia 

during the last glaciations (Slauson et al. 2009b). Throughout this section, we adopt 

the new species-level nomenclature, but maintain the use of subspecific epithets as 

previously described (e.g., Hall 1981). 

The Humboldt marten (M. caurina humboldtensis) and the Sierra marten (M. c. 

sierrae) are now recognized as the two subspecies of Pacific marten in California 

(Zielinski and Slauson, in press). In Oregon, two subspecies of martens have been 

recognized historically, including M. c. caurina of the Coast Range and Cascade 

Mountains, and M. c. vulpina occurring in the Blue Mountains of northeastern 

Oregon (Hall 1981). 

Compared to the Sierra subspecies, the Humboldt subspecies is darker, with 

a richer golden tone to the underfur, has a smaller and patchier gular patch that is 

more cream colored than orange and yellow (fig. 4), has a smaller skull, smaller 

and less crowded premolars, and a more narrow rostrum (Grinnell and Dixon 

1926). Recent phylogenetic analyses continue to support the distinctiveness of 

the Humboldt subspecies, based on the presence of distinct haplotypes shared by 

historical museum specimens and martens in the extant range (Schwartz et al. 2016, 

Slauson et al. 2009b). Marten populations in coastal Oregon historically described 

as M. c. caurina also share these haplotypes, leading Slauson et al. (2009b) to 

suggest that martens on the Oregon Coast Range may be M. c. humboldtensis. 

More recent mitochondrial and nuclear DNA analyses throughout the Pacific States 

have strengthened the case that coastal California and coastal Oregon populations 

represent a single evolutionarily significant unit, and that coastal Oregon martens 

should be recognized as the Humboldt subspecies (Schwartz et al. 2016). Evolutionarily significant units are populations that are either geographically separated, show 

limited gene flow, or have locally adapted phenotypic traits caused by differences 

in selection. 

Recent mitochondrial 

and nuclear DNA 

analyses throughout 

the Pacific States have 

strengthened the case 

that coastal California 

and coastal Oregon 

populations represent 

a single evolutionarily 

significant unit, and 

that coastal Oregon 

martens should be 

recognized as the 

Humboldt subspecies. 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
28 

GENERAL TECHNICAL REPORT PSW-GTR-260

A 

B 

K. Slauson K. Slauson

Figure 4—Comparison of overall pelage coloration and the extent of gular patches on typical historical museum specimens of the (A) Humboldt marten (Martes caurina humboldtensis) in California and (B) Sierra marten (M. c. sierrae). Specimens were photographed at the 

Museum of Vertebrate Zoology, University of California, Berkeley. 
29 

Historical Distribution: Coastal California and 

Coastal Oregon 

To describe the historical distribution of martens, we used the locations of verifiable museum specimens, trapping records, or published interviews with trappers 

prior to the 1950s, as summarized by Zielinski et al. (2001). The historical distribution of the Humboldt marten in California included the northern Coast Range 

from the Oregon border southward to northwestern Sonoma County, “within the 

narrow northwest humid coast strip, chiefly within the redwood belt from near sea 

level to about 3000 ft” (Grinnell et al. 1937) (fig. 5). Of the 24 historical records of 

Humboldt marten occurrence, 20 (83 percent) occurred <25 km from the coast; no 

records occurred >35 km from the coast (Grinnell et al. 1937). Outside low-elevation coastal forests, early observations of the association of the Humboldt marten 

with dense shrub layers were made by trappers: “All trappers agree that, although 

martens have been taken along streams well down to sea level, martens are found 

mostly on the high ridges. This makes the trapping of them difficult, as the climb in 

winter is heart-breaking, owing to the wet tangle of brush and down timber, higher 

than one’s head...” (Grinnell et al. 1937). In the early 1900s, Grinnell et al. (1937) 

noted that the Humboldt marten was already rather uncommon, though in earlier 

years was fairly numerous. These authors reported a local account of individual 

trappers capturing 35 to 50 martens in one winter within a few miles of the coast. 

The historical distribution of martens in coastal Oregon included the Coast 

Range from the Columbia River south through the coastal portions of the KlamathSiskiyou Mountains to the California border (Zielinski et al. 2001) (fig. 5). Museum 

or trapping records for martens exist for every county within their historical range 

in coastal Oregon (Marshall 1994, Zielinski et al. 2001) (fig. 5). In Oregon’s coastal 

region, >90 percent of historical records of martens are from sites near the coast, 

with few records from the more inland portions of the region (Zielinski et al. 2001). 

Contemporary Distribution and Abundance: Coastal 

California and Oregon 

To describe the contemporary distribution of martens, we used the results of 

surveys conducted since the late 1980s that provided verifiable evidence of species occurrence, such as tracks, photographs, or DNA. Zielinski et al. (2001) and 

Slauson and Zielinski (2004) summarized pre-2005 survey efforts, and we summarized post-2005 surveys from published (Hamm et al. 2012, Moriarty et al. 2016, 

Slauson and Zielinski 2007, Slauson et al. 2009a) and unpublished sources (BLM, 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
30 

Figure 5—Historical range and distribution of historical records of occurrence for the four Pacific marten (Martes 

caurina) subspecies––M. c. humboldtensis (M.C.H.), M. c. sierrae (M.C.S.), M. c. caurina (M.C.C.), M. c. vulpina 

(M.C.V.)––in Oregon and California. Range boundaries and historical records modified from Zielinski et al. (2001). 

GENERAL TECHNICAL REPORT PSW-GTR-260
31 

California State Parks, Green Diamond Resource Company, Hoopa Valley Tribe, 

Humboldt Redwood Company, Mendocino Redwood Company, Redwood National 

Park, Yurok Tribe, USFS Pacific Southwest Research Station, and USFS Six 

Rivers National Forest). In central and north coastal Oregon where survey efforts 

have been sparse, we also included the locations of alternative verifiable evidence, 

including carcasses of road-killed individuals and the recent locations where 

martens have been legally trapped. 

Despite surveys occurring at >3,000 locations within the historical range in 

California, representing >50,000 survey days of effort, the Humboldt marten is 

known only from two disjunct remnant populations occupying <5 percent of its historical range in the state (fig. 6). In 2008, the larger of the two populations occupied 

a 100-percent minimum convex polygon area of 62 700 ha (Slauson et al. 2009a). 

Although this area may appear large, both martens and suitable marten habitat are 

patchily distributed throughout the area. The 2008 estimate of the total population 

size occupying that area was <100 individuals (Slauson et al. 2009a). Survey efforts 

since 2008 in the vicinity of this population have detected martens in a few new 

locations, primarily along the edges of the known occupied area, representing an 

increase of <5 percent in the overall occupied area. 

From 2011 through 2015, multiple marten detections were reported in a new 

area near the California-Oregon border in northeast Del Norte County, California 

(fig. 6). Based on criteria regarding distance between survey stations, these are 

believed to represent 12 independent detections (Slauson 2016). Additional surveys 

will be necessary to determine if this population is isolated from or connected to 

the southern coastal Oregon population to the north or the main California population to the south. 

In coastal Oregon, surveys for martens have begun in recent years, but the 

current distribution of marten populations is not yet well understood, and no 

population size estimates outside the dune forest habitat west of U.S. Highway 101 

are available. Contemporary detections have occurred in two distinct areas: on or 

near the Siskiyou National Forest in the south (Curry County), and on or near the 

Siuslaw National Forest to the north (Lincoln, western Lane, western Douglas, 

northwestern Coos Counties) (Moriarty et al. 2016 Zielinski et al. 2001) (fig. 6). In 

addition, road-killed martens have been found on U.S. Highway 101 in the immediate vicinity of the Siuslaw National Forest and Oregon Dunes National Recreation 

Area (fig. 6). Based on modeled habitat suitability (see the “Contemporary Landscape Habitat Suitability” section), existing conditions in the central and northern 

Oregon Coast Range may only be sufficient to support a single population on or 

near the Siuslaw National Forest. 

Despite surveys 

occurring at >3,000 

locations California, 

the Humboldt marten 

is known only from 

two disjunct remnant 

populations occupying 

<5 percent of its 

historical range in 

the state. 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
32 

Figure 6—(A) Distribution of contemporary survey effort and (B) contemporary verifiable Humboldt marten (Martes caurina humboldtensis) detections from surveys (circles), and roadkill mortalities (n = 14) or trapped (n = 3) individuals (asterisks; central coastal Oregon only) in the assessment area. 

GENERAL TECHNICAL REPORT PSW-GTR-260
33 

Life History 

Territoriality 

Martens exhibit intrasexual territoriality, with males excluding males and females 

excluding females from their home ranges (Powell 1994). Adult females will 

occasionally allow female offspring to establish home ranges in portions of the 

adult’s home range. This strategy might improve female offspring survival during the early dispersal period and ensure that high-quality reproductive habitat is 

passed on through matrilines. Male and female home ranges typically overlap, with 

dominant males maintaining home ranges that encompass one or more female’s 

home range (Powell 1994). Although paternity studies for martens are lacking, we 

assume that males with home ranges encompassing reproductively active females 

usually sire the young compared to males occupying adjacent home ranges, as has 

been demonstrated in the fisher (Aubry et al. 2004). However, males occasionally 

make substantial movements from their home ranges during the breeding season to 

increase their opportunities for reproduction. 

Activity Patterns 

Martens are active year-round, adjusting their activity patterns seasonally to synchronize with those of key prey species (Zielinski et al. 1983). Accordingly, martens 

may be active at any time of day. Peak diurnal activity typically occurs during summer when ground-dwelling sciurids and young birds are key prey. Peak nocturnal 

activity is typically during winter when nocturnal prey species, such as Humboldt 

flying squirrels (Glaucomys oregonensis) or snowshoe hares (Lepus americanus), 

become more important (Zielinski et al. 1983, 2008). 

Food Habits and Habitat-Prey Relationships 

Martens are considered dietary generalists; however, their diet changes with seasonal prey availability, and during particular seasons they may specialize on a few 

prey species (Martin 1994, Zielinski et al. 1983). Overall, mammals dominate the 

diet, but birds, insects, and fruits are important seasonally (Martin 1994, Zielinski 

et al. 1983). In the Sierra Nevada and Cascade Mountains of California, the winter 

prey of Sierra martens is primarily Douglas’s squirrels (Tamiasciurus douglasii), 

snowshoe hares, western red-backed voles (Myodes californicus) and other voles 

(Microtus sp.), and northern flying squirrels (Glaucomys sabrinus) (Slauson and 

Zielinski 2010, Zielinski et al. 1983). In summer, the diet changes to focus on 

ground-dwelling sciurids, although voles continue to be important prey (Martin 

1994, Zielinski et al. 1983). 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
34 

GENERAL TECHNICAL REPORT PSW-GTR-260

Prey body size appears 

to be an important 

factor in prey selection 

by the Humboldt 

marten, with mediumsize prey (85–225 g) 

representing 55 to 66 

percent of summer 

through winter diet, 

and used 2.6 times 

more than small (<40 g) 

and 8.4 times more 

than large (>250 g) 

prey, respectively. 

Slauson and Zielinski (2017) analyzed the diet of Humboldt martens from the 

main California population. (Unless otherwise cited, the following information 

on Humboldt marten diet is based on findings of that study.) They calculated the 

importance of the prey species in the diet from 528 scats collected in all seasons 

(winter: n = 53, 10 percent; spring: n = 41, 8 percent; summer: n = 151, 29 percent, 

and fall: n = 277, 52 percent) using the proportion of metabolizable energy (PME). 

Mammals dominated the overall diet (PME = 72 percent), followed by birds (PME 

= 18 percent), with berries, insects, and reptiles collectively contributing <10 percent 

PME. This analysis indicated that mammals and birds were eaten year-round, while 

reptiles, insects, and berries were eaten only seasonally (fig. 7). While >37 prey 

species were identified in the overall diet, only 11 species contributed >5 percent 

PME, and the majority (59 to 64 percent) of PME during any one season was composed of only four taxa (chipmunks [Tamias sp.], medium-size birds [40 to 200 g], 

western red-backed voles, and either Douglas’s squirrel [summer-fall], Humboldt 

flying squirrel [winter], or large birds [>200 g; spring]). Overall, sciurids were the 

most important prey group, comprising 42 percent of PME of the annual diet and 

varying seasonally: 50 to 51 percent during summer-fall; 29 to 34 percent during 

winter-spring. Chipmunks were the most important single prey species from summer through fall, contributing 23 to 34 percent PME during those seasons (fig. 7). 

Prey body size appears to be an important factor in prey selection by the Humboldt 

marten, with medium-size prey (85–225 g) representing 55 to 66 percent of PME 

from summer through winter, 2.6 to 8.4 times the PME compared to small (<40 g) 

or large (>250 g) prey, respectively. During winter, when important medium-size 

prey (e.g., chipmunks) were consumed least often (likely because of their reduced 

availability), Humboldt martens shifted to alternative medium-sized prey (birds and 

Humboldt flying squirrels) and large prey (fig. 7). 

Identification of bird species in scats is difficult and many samples are not 

taxonomically identifiable beyond class, but body size categories can be assigned 

based on the relative sizes of structures such as flight feather shafts, claws, and 

bills. The majority of identifiable birds taken by martens were passerines, including year-round residents (e.g., Steller’s jay, Cyanocitta stelleri), and summer- (e.g., 

Parulidae [New World warblers]) and winter-migrant (e.g., varied thrush, Ixoreus 

naevius) species. Woodpecker remains (e.g., northern flicker, Colaptes auratus) 

also appeared in the sampled scats. 

The frequency of berries in Humboldt marten scats was higher than that 

reported by Martin (1994) for other North American martens. Of the berries 

identified, over 90 percent were ericaceous species including salal (47 percent), 

evergreen huckleberry (31 percent), red huckleberry (21 percent), and manzanita 
35 

A 

B 

Figure 7—Summer and fall diet composition of the Humboldt marten (Martes caurina humboldtensis) in California by: (A) 

percentage of metabolizable energy for major prey groups, and (B) percentage of metabolizable energy contributed by each 

prey species by season. Only mammal species representing ≥5 percent of metabolizable energy consumed are presented. 

These results are based on 528 scats collected in Humboldt and Del Norte Counties, California, from 2000 to 2014 (Slauson 

and Zielinski, 2017). 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
36 

Many of the Humboldt 

marten’s key prey 

species (red-backed 

voles, flying squirrels, 

Douglas’s squirrels) 

reach their highest 

densities in forest 

stands with mature 

and late-successional 

structural features 

where their key food 

resources—conifer 

seed crops and 

truffles—are most 

abundant. 

(Arctostaphylos sp.) (13 percent). Although insect remains frequently occurred in 

scats, the only scats composed entirely of insects had native wasps (Vespula sp.), 

bald-faced hornets (Dolichovespula maculata), and nonnative European honey 

bees (Apis mellifera). These species comprised 53 percent of all insect prey taken, 

with use peaking in the summer and early fall when raiding hives would yield the 

maximum energy content of larvae from native species, and larvae and honey from 

nonnative honey bees. 

In contrast to much of the marten’s range in North America, in most of the 

assessment area winter snowfall is rare and no persistent snowpack forms. This 

has important implications for seasonal prey availability. In regions where snowpack forms, prey species such as chipmunks, voles, and semifossorial insectivores 

become less available to martens, but owing to the lack of significant snowpack in 

the assessment area, these species may be more available during late fall, winter, 

and early spring. Furthermore, the snowshoe hare, an important winter prey species 

throughout much of the marten’s range, is found only in the northern Oregon Coast 

Range portion of the historical range of the Humboldt marten (Verts and Carraway 

1998). As a result of these factors, winter diet of the Humboldt marten exploits a 

broader range of taxa than other North American marten subspecies. 

Many of the Humboldt marten’s key prey species (e.g., western red-backed 

vole [Hayes and Cross 1987, Rosenberg et al. 1994, Zabel and Waters 1992], flying squirrel [Waters and Zabel 1995], Douglas’s squirrel [Carey 1991, Carey and 

Johnson 1995]) reach their highest densities in forest stands with mature and 

late-successional structural features where their key food resources—conifer seed 

crops and truffles (fruiting bodies of ectomycorrhizal fungi)—are most abundant 

(Luoma et al. 2003, Smith et al. 2002). Also, the density of ericaceous shrub layers 

is positively correlated with chipmunk density (Hayes et al. 1995), Humboldt flying 

squirrel abundance (Carey 1995), and overall small mammal abundance (Carey and 

Johnson 1995) in coastal Oregon. Coarse woody debris is also positively associated 

with overall small mammal abundance (Carey and Johnson 1995), and density of 

large snags is a predictor of abundance of Humboldt flying squirrels (Carey 1995) 

in coastal Oregon. Andruskiw et al. (2008) found American martens’ hunting success to be lower after logging reduced the physical complexity provided by coarse 

woody debris on or near the forest floor. Many bird species preyed upon by martens 

are associated with shrub understories (e.g., Hagar 2003) and use fruits of ericaceous species as residents and migrants (e.g., Tietz and Johnson 2007). 

GENERAL TECHNICAL REPORT PSW-GTR-260
37 

Reproduction 

Martens are polygamous with females solely responsible for raising young. Mating 

is largely known from the behavior of captive animals, where mating occurs from 

late June to early August, with a peak in July (Markley and Bassett 1942) (fig. 8). 

Females typically give birth in March and April (Strickland et al. 1982) and female 

Humboldt martens have a mean parturition date of April 13 (Delheimer et al. n.d.). 

Females do not mate until 15 months of age, and owing to delayed implantation, 

will not produce their first litters until at least 2 years of age (Strickland et al. 1982). 

In an Ontario study, Thompson and Colgan (1987) reported that <25 percent of 

yearlings produced ova, and in Quebec, Fortin and Cantin (2004) reported a range 

of 40 to 76 percent with ova among 83 yearling martens over a decadal period. Not 

all females ≥2 years old give birth in a given year; Thompson and Colgan (1987) 

reported pregnancy rates of 50 percent of females during years of environmental 

stress. However, in the Sierra Nevada of California during 2009–2011, all 20 

captured females ≥2 years old were lactating, indicating that they were all actively 

attempting to raise young (Slauson 2017). This study included both mild and severe 

winters, including 2011, 1 of the top 10 snowfall years in the Sierra Nevada in 

the last century (Slauson 2017). This suggests that in California, regardless of the 

effects of winter conditions on prey populations, adult female Pacific martens may 

be more likely to attempt to produce litters annually than martens in more northern 

portions of North America where prey populations such as snowshoe hares fluctuate more severely. However, annual variation in environmental conditions and prey 

populations will still affect survival rates of various age classes. 

Females are capable of producing from one to five kits per litter, but the 

reported averages are between two and three (Mead 1994, Strickland and Douglas 

1987). Female Humboldt martens have a mean litter size of 1.8 (SE = 0.15; Delheimer et al. n.d.). Further evidence suggests age-related fecundity in martens, with 

the highest number of kits produced by 3- to 5-year-olds and the lowest number 

by first-year breeders and females >5 years old (Fortin and Cantin 2004, Mead 

1994). No data exist on the average number of young martens raised to weaning or 

recruited into the population per breeding female. An understanding of recruitment 

and the effects of annual variation in environmental conditions and prey populations on kit survival are important information gaps. 

Survivorship and Longevity 

Longevity in martens is not well understood because (1) most studies of longevity 

are from trapped populations where the age structure was truncated and captures 

biased toward younger individuals, and (2) few studies of untrapped populations 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
38 

GENERAL TECHNICAL REPORT PSW-GTR-260

Figure 8—Annual reproductive cycle for North American martens (Martes caurina, M. americana). 

have reported age structure. Captive martens have been reported to reach 15 years 

of age, but there is no evidence to suggest many individuals in wild populations 

approach this age. In two marten populations subject to trapping in Ontario and 

Quebec, only about 10 percent of all trapped individuals (including approximately 

1,000 females) were >5 years old (Fortin and Cantin 2004, Strickland and Douglas 

1987). All 22 martens captured from an untrapped population in northeastern 

Oregon were <5 years old (Bull and Heater 2001). Over a 3-year period, age structure among 96 martens from an untrapped population in the Sierra Nevada Mountains of California had relatively consistent proportions of yearling and adult age 

classes, with only 1 of 27 females (4 percent) observed >4 years of age (Moriarty et 

al. 2014, Slauson 2017) (fig. 9). 

Factors influencing survival of untrapped populations of martens are poorly 

understood. Martens coexist in forested ecosystems with larger bodied mammalian 

and avian predators that occasionally prey on them. Therefore, predator avoidance 
39 

A. Lake Tahoe region marten population

B. Lassen region marten population

Figure 9—Age structure for untrapped populations of Pacific martens (Martes caurina) in the (A) Lake 

Tahoe region (n = 96) (Slauson 2017), and (B) Lassen region (n = 54) (Moriarty et al. 2014) of California 

from 2009 through 2012. 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
40 

Collectively, the 

results of these 

studies indicate that 

widespread humaninduced changes 

to forest structure 

are associated with 

increased predation 

risk for martens. 

has likely been an important factor shaping their evolution, leading martens to 

select for highly complex forest structure and avoid areas lacking overhead and 

escape cover. By specializing on structurally complex boreal-montane forests, 

martens minimize their distributional overlap with larger bodied generalist predators such as bobcats (Lynx rufus) that typically use more diverse habitats, including 

younger forest stages and more open habitats such as oak woodlands and meadows. 

Among reported mortality sources, the proportion of predation events varies 

by landscape condition. Predation represented 62 percent (Bull and Heater 2001) 

and 75 percent (McCann et al. 2010, Wilk and Raphael 2018) of mortality events 

in moderately to heavily logged forest compared to 40 percent in a forest reserve 

(Hodgman et al. 1997). The species of predators that killed martens in these studies 

also varied between moderately to heavily logged forests and forest reserves, with 

generalist carnivores, including bobcats and coyotes (Canis latrans), responsible 

for 71 percent (Bull and Heater 2001) and 75 percent (Wilk and Raphael 2018) of 

predation events in intensively logged forests in Oregon versus 40 percent in a forest reserve in Maine (Hodgman et al. 1997). 

Kill site characteristics are rarely reported in the literature, but in two studies 

the sites where martens were killed were associated with reduced escape cover in 

heavily logged stands (Wilk and Raphael 2018) or open shelterwood stands (Ellis 

1998). In an ongoing study of Humboldt martens in intensively managed forests on 

the edge of the main California population, preliminary results have been consistent 

with these patterns. Predation was implicated in all cases where cause of mortality 

could be determined, the dominant predator was a generalist—the bobcat—(Slauson et al. 2014), and the majority of kill sites occurred in logged sites near roads 

(Slauson 2015). Preliminary results of a study on bobcat diet and habitat use in the 

same area show that bobcats prey primarily on herbivorous species such as duskyfooted woodrats (Neotoma fuscipes) and brush rabbits (Sylvilagus bachmani), and 

that bobcats select for regenerating harvested stands ≤30 years old (Slauson 2018) 

where these prey species are most abundant (e.g., Hamm and Diller 2009). Collectively, the results of these studies indicate that widespread human-induced changes 

to forest structure are associated with increased predation risk for martens. 

Parasites and Disease 

Little information is available on disease or parasite prevalence in the Pacific 

marten. Zielinski (1984) discovered antibodies to bubonic plague (Yersinia pestis) in 

4 of 13 martens in the Sierra Nevada. He also found a chipmunk flea (Monopsyllus 

ciliatus) to be the most common ectoparasite, which was also probably the source 

of the plague transmission. Ticks are commonly encountered on trapped martens, 

GENERAL TECHNICAL REPORT PSW-GTR-260
41 

especially on the nape and in the ears during the spring and summer (K. Slauson, 

pers. obs.) and may be potential vectors of disease transmission to martens (Gabriel 

et al. 2012a). Sympatric populations of gray foxes (Urocyon cinereoargenteus) and 

fishers in or near the assessment area are known to have been exposed to viral and 

parasitic diseases such as canine distemper, parvovirus, toxoplasmosis, West Nile 

virus, and rabies, which are also known to be transmittable to martens (Brown et 

al. 2008, Gabriel et al. 2012a). Mustelids are among the species most susceptible 

to canine distemper disease (Deem et al. 2000); in black-footed ferrets (Mustela 

nigripes) distemper infection causes a fatality rate close to 100 percent (Bernard 

et al. 1984). Given the small population sizes of Humboldt martens in California 

(and probably in Oregon), lethal disease outbreaks have the potential for significant 

population-level effects. 

Dispersal and Recruitment 

Natal dispersal by young-of-the-year is generally thought to occur as early as 

August, with fall, winter, and spring dispersal periods also reported (Clark and 

Campbell 1976, Slough 1989) (fig. 8). Natal dispersal in the Humboldt marten 

occurs as early as August and continues at least until the following summer season 

(Slauson 2017). Dispersal has been divided into three phases with respect to the 

decisions faced by juveniles or adults and potential factors influencing those decisions: (1) whether to disperse from the natal area or home range, (2) search behavior, and (3) factors important to settlement (Bowler and Benton 2005, Stamps 2001). 

Incorporating the social or habitat factors in these phases of the dispersal process is 

critical to understanding how they may influence the outcome of dispersal events. 

Although some adult male and female martens leave their home ranges during periods of low prey densities (Thompson and Colgan 1987), few studies have 

reported such behavior. Search behavior and travel distances of juveniles and adults 

are critical factors in understanding the effects of landscape patterns on dispersal. 

Although dispersal distances of >70 km have been reported for martens (e.g., Fecske and Jenks 2002), other studies have found that most juvenile American martens 

in both logged and unlogged landscapes dispersed much shorter distances (≤5 km 

[Broquet et al. 2006]; <15 km [Phillips 1994]; x 15.5 km, SE = 1.01 [Pauli et al. 

2012]). Johnson et al. (2009) compared dispersing juvenile American martens in an 

unharvested forest landscape with others in a regenerating forest landscape that had 

been >80 percent logged 20 to 60 years earlier using a combination of selection and 

clearcut methods. In the regenerating landscape, juvenile martens traveled slower, 

moved shorter distances, had poorer body condition, and suffered twice the mortality rate (49 percent vs. 25 percent). These authors noted the findings of Andruskiw 

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42 

GENERAL TECHNICAL REPORT PSW-GTR-260

Landscape pattern 

can have important 

effects on dispersal 

dynamics, affecting 

both the distance 

dispersers can traverse 

and their success rate 

in establishing home 

ranges and surviving 

to adulthood. 

et al. (2008), who found that martens were more successful in obtaining prey in the 

unlogged versus the logged landscape. Although commercial trapping mortality 

occurred during this study, it was similar in both study landscapes, eliminating this 

concern in the interpretation of their results (Johnson et al. 2009). Landscape pattern can have important effects on dispersal dynamics, affecting both the distance 

dispersers can traverse and their success rate in establishing home ranges and 

surviving to adulthood. 

The annual recruitment rate of juvenile cohorts in untrapped marten populations depends on three factors: (1) the number of young per female that reach the 

dispersal stage, (2) the success rate of dispersing juveniles in establishing home 

ranges, and (3) the effects of annual environmental stochasticity. In an untrapped 

Pacific marten population in the Sierra Nevada Mountains of California, annual 

juvenile recruitment represented approximately 40 percent of the total observed 

population size by the following breeding season and was relatively constant during 

a 3-year study period (Slauson 2017) (fig. 10). 

Population Biology and Dynamics 

Understanding the relative importance of population processes and life stages, such 

as yearling, subadult, and adult, to population growth is key to identifying management and conservation actions likely to benefit species’ persistence (Mills 2007). In 

Figure 10—Percentages of yearling, subadult, and adult age classes by sex occurring annually in an 

untrapped population of Pacific martens in the Lake Tahoe region of California from 2009 through 

2011. Population sampling occurred during May to July of each year (Slauson 2017). 
43 

this section, we review an existing stage-structured population model for martens 

developed by Buskirk et al. (2012) and evaluate its applicability to the Humboldt 

marten. Then we develop a revised version of this model using parameter estimates 

that are more realistic for the Humboldt marten. We then use these models to 

conduct sensitivity analyses to identify the relative effects that age-related survival 

and fecundity have on marten population growth. 

For their model, Buskirk et al. (2012) estimated fecundity and survival rates by 

life stage based on review of marten studies across North America. Most of those 

studies were from marten populations accessible to trappers. Four stages were chosen, representing juveniles (stage 1: 0 to 1 year old), yearlings (stage 2: 1 to 2 years 

old), young adults (stage 3: 2 to 3 years old), and older adults (stage 4: >3 years old). 

Buskirk et al. (2012) estimated the stable age distribution as 53 percent juveniles, 13 percent yearlings, 9 percent 2-year-olds, and 24 percent older animals. 

Elasticities, a measure of the proportional change in population growth rate over 

discrete time steps (lambda) given an infinitesimal one-at-a-time proportional 

change in a demographic parameter (Mills 2007), showed that key transition rates 

were adult (stage 4) and juvenile (stage 1) survival. Adult and juvenile survival 

collectively accounted for a total of 52 percent of total elasticity. In contrast, fecundity accounted for only 21 percent of total elasticity across all stages (Buskirk et 

al. 2012). These results indicated that changes in adult and juvenile survival have a 

much larger relative impact on population growth rate than fecundity at any stage. 

Buskirk et al.’s (2012) population modeling provided a first step in evaluating 

the importance of life stages on marten population growth. However, several of 

their model parameters may not be representative for the Humboldt marten owing 

to their reliance on data from northern marten populations and from populations 

subject to trapping. 

First, fecundity estimates for 1-year-old martens used by Buskirk et al. (2012) 

were higher than those from untrapped California populations in the Sierra Nevada. 

Of 14 yearling Sierra marten females, only 3 (21 percent) showed evidence of 

lactation during 2009–2011 (Slauson 2017). Moreover, female American martens 

breeding for the first time have shown lower overall success rates in raising kits to 

dispersal age (Kartashov 1989, Thompson and Colgan 1987). Therefore, we reparameterized yearling fecundity using a lower pregnancy rate of 0.18, resulting in a 

yearling fecundity rate of 0.16. 

Second, considering multiple sources, Buskirk et al. (2012) chose a juvenile 

survival rate of 25 percent. However, Potvin and Breton (1997) estimated juvenile 

survival at 61 percent, and Johnson et al. (2009) found that 49 percent of juveniles 

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44 

GENERAL TECHNICAL REPORT PSW-GTR-260

dispersing in unlogged forest survived to establish home ranges and become adults, 

even where trapping occurred. Therefore, we chose the rate of 0.50 for juvenile 

survival to better represent the expected juvenile survival rate in untrapped populations in more intact forests. 

Third, both trapped and untrapped populations rarely contain individuals >5 

years of age. If all stages >1 year old have survival rates of 0.75 as suggested by 

Buskirk et al. (2012), the final stage would have more individuals than would be 

expected based on field data of population age structures (Bull and Heater 2001, 

Fortin and Cantin 2004, Slauson 2017). Therefore, we chose a lower survival rate of 

0.70 for all stages >1 year old. We believe these three changes create a more realistic 

model for untrapped or lightly trapped marten populations and, specifically, for 

Humboldt marten populations (fig. 11). 

The stable age distribution from our model includes 42 percent juveniles, 20 

percent yearlings, 13 percent 2-year-olds, and 25 percent ≥3-year-olds (fig. 11). 

Similar to Buskirk et al.’s (2012) results, elasticities revealed that key transition 

rates were for adult (≥3-year-olds), juvenile, and yearling survival (25 percent, 21 

percent, and 19 percent of total elasticity, respectively). Reproduction had the greatest elasticity for adults (≥3-year-olds) (13 percent of total elasticity), but was low 

overall, accounting for a total of 20 percent elasticity across all stages. 

Figure 11—Life cycle diagram for untrapped Pacific marten (Martes caurina) 

populations (including the Humboldt marten) modified from Buskirk et al. (2012) 

by substituting parameter estimates that better characterize untrapped populations. 

Stable age distribution is indicated in each circle, below the age class. Elasticities are indicated in italics below each survival probability (S) or fecundity (F) 

estimate. 
45 

The results of both Buskirk et al.’s (2012) and our matrix models suggest that 

variation in survival across all stages, and especially among adults, has a much 

larger impact on population growth than variation in fecundity. Rates of pregnancy 

and kit production appear fairly constant in field data for untrapped populations in 

California, further suggesting that survival is the most important variable in marten 

population persistence and growth. 

Thus, factors influencing marten survival have important management and 

conservation implications. Rather than expecting that marten populations will 

be capable of growing quickly in temporarily favorable environments, managers 

should anticipate that stable conditions and long time periods will be required for 

population growth and recovery (Buskirk et al. 2012). Analytical elasticities can be 

valuable for evaluating the relative importance of variation in vital rates on population growth, but it also is important to consider how each vital rate actually varies 

in unmanaged and managed environments (Mills 2007). Because a large portion of 

the assessment area is under commercial timber management, and there are strong 

indications of higher predation and lower survival rates in harvested forests, the 

relationship between various silvicultural practices and vital rates is an important 

area for future research. Johnson et al. (2009) made an important link between juvenile survival and the landscape condition resulting from intensive timber harvest, 

demonstrating a 50 percent reduction in juvenile survival in a landscape dominated 

by young clearcut and selectively harvested forest versus a landscape dominated by 

older uncut forest. This magnitude of effect has important implications for reduced 

population growth. How survival in untrapped populations is influenced by less 

intensive silvicultural practices, as well as how and why other vital rates vary, are 

also important areas for future research. 

Intraguild Predation and Interspecific Competition

Martens are susceptible to predation by larger mammalian and avian predators, 

typically habitat-generalist species, including coyote, bobcat, and great horned owl 

(Bubo virginianus) (Bull and Heater 2001, Thompson 1994). In addition, fishers are 

known to kill martens (e.g., McCann et al. 2010), and dense fisher populations can 

limit the distribution of marten populations (Krohn et al. 1997, 2004). Intraguild 

killing in carnivores follows predictable body size relationships, with species pairs 

with 2.5 to 4 times the body mass most likely to participate in killing, with the 

larger species always the winner (Donadio and Buskirk 2006). We adapted the predictions using these body mass relationships for the community of mesocarnivores 

most often co-occurring with martens in the Pacific States (fig. 12). As discussed 

above, marten populations in logged forest landscapes have lower survival rates 

Rather than expecting 

that marten populations 

will be capable of 

growing quickly in 

temporarily favorable 

environments, 

managers should 

anticipate that stable 

conditions and long 

time periods will be 

required for population

growth and recovery. 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
46 

1 

15 

8 

Body Weight (kg) 

Coyote 

n = 34 

Gray fox 

Fisher 

?* 

? 

n = 6 Bobcat 

Marten 

Low High 

Expected Frequency and Intensity of Attacks 

by Larger-bodied Carnivores on Martens 

Based on Body Size Differences 

Because survival 

is the most 

influential parameter 

in population 

performance, predation

rates can have 

meaningful effects on 

marten abundance 

and population growth 

rates. 

Figure 12—Intraguild predation relationships between larger-bodied carnivores and Pacific martens 

(Martes caurina). Solid arrows connect carnivores confirmed to kill Pacific martens from field studies. 

Relative weight of arrows indicates the number of documented cases of intraguild predation on Pacific 

martens (Bull and Heater 2001, Raphael 2004, Slauson et al. 2014). The single dashed arrow indicates 

a likely predator based on body mass ratio (Donadio and Buskirk 2006), but yet to be confirmed by 

field studies. The asterisk denotes a carnivore that has been confirmed to kill American martens (M. 

americana) (Hodgeman et al. 2000, McCann 2010) but has yet to be observed for the Pacific marten. 

from natural causes such as predation than those in uncut landscapes, and the species most commonly reported as predators on martens tend to be habitat generalist 

carnivores and raptors (Bull and Heater 2001, Thompson 1994). In an ongoing study 

of dispersing Humboldt martens in an intensively managed landscape in California, 

9 of 20 (45 percent) radio-collared martens monitored for >6 months were killed by 

bobcats (Slauson et al. 2014). Minimizing predation risk probably shapes martens’ 

daily decisions as to where to forage and rest as well as broader decisions regarding 

dispersal and home range establishment. And, because survival is the most influential parameter in population performance, predation rates can have meaningful 

effects on marten abundance and population growth rates. 

GENERAL TECHNICAL REPORT PSW-GTR-260
47 

Temporal scale 

Gene flow 

Management scales 

Individual 

structures 

Stand 

management 

Watershed/ 

project level 

≥10-year Forest 

planning cycle 

Resting 

Foraging habitat 

& structures 

Reproduction 

Survival 

Seasonal prey 

bases 

Dispersal 

Population 

persistence 

Marten spatial scales

Micro 

Stand 

Home 

range 

Landscape 

Daily Seasonally Annually Decadal 

Multiscale Habitat Use 

Pacific and American martens have been shown to select habitat at three primary 

spatial scales: microhabitat, stand, and home range (fig. 13). These three scales represent three of the four orders of habitat selection proposed by Johnson (1980), with 

the microhabitat scale representing 4th order, stand scale representing 3rd order, and 

home range scale representing 2nd order selection. The distribution of suitable habitat 

at the landscape scale (the 1st order per Johnson 1980) also has important influence 

on marten dispersal, home range scale habitat selection, and population density. 

At the microscale, martens select specific structures for foraging or resting, 

such as large logs, which they run along searching for prey, or cavities in snags that 

provide thermal benefits (Taylor 1993) and reduce predation risk while denning or 

resting. At the stand scale, martens select forest stands with the structural features 

that provide one or more life history requirements (e.g., prey populations, foraging 

structures, and resting structures). At the home range scale, martens position their 

Figure 13—Conceptual relationships between Pacific marten (Martes caurina) life history needs and the spatial and temporal scales at 

which habitat meets those needs. The Z-axis indicates how management scales relate to marten habitat use and life history needs. 

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48 

GENERAL TECHNICAL REPORT PSW-GTR-260

Habitat selection by 

martens appears to 

be strongest amongst 

all scales during the 

process of home 

range establishment, 

where individual 

martens select where 

to locate home ranges 

in order to include the 

resources to support 

their year-round 

life history needs 

and avoid same-sex 

conspecifics.

home ranges to include enough high-quality habitat to provide for life history needs 

(e.g., seasonal prey bases, den sites) and access to mates, while avoiding individuals 

of the same sex (Katnik et al. 1994). At the landscape scale, dispersing individuals 

select suitable portions of the landscape that are unoccupied by individuals of the 

same sex to establish home ranges (Johnson 2008). At the landscape scale between 

populations, marten dispersal maintains adjacent populations (or metapopulation 

structure where it exists) along with gene flow. 

In a recent review of habitat selection by both North American marten species, Thompson et al. (2012) found that habitat selection appeared to be strongest 

amongst all scales during the process of home range establishment, where individual martens select where to locate home ranges in order to include the resources 

to support their year-round life history needs and avoid same-sex conspecifics. 

Furthermore, they reported that martens use managed landscapes “where sufficient 

cover and structures important to fitness are present and where the species can 

exhibit landscape-scale selection to maintain 70 percent or more of home ranges in 

suitable habitat conditions.” 

At the landscape scale, marten population density declines as suitable habitat 

declines from either wildfire or timber harvest. Although martens generally select 

mature and old forests over young regenerating stands, habitat use varies across the 

continent, making generalization difficult at the stand scale. Variation in the selection for older forests appears to be linked to key aspects of marten ecology, including 

the abundance of key prey species, predator distribution, and the development of 

structural complexity near the ground in different forest types (Thompson et al. 2012). 

Consequently, assumptions about stand-scale habitat relationships should be ecosystem-specific and not based on general understandings from throughout their ranges. 

Microscale Habitat Use: Resting and Denning Structures 

Martens use rest structures daily between foraging bouts to provide thermoregulatory benefits and protection from predators (Taylor and Buskirk 1994). Short-term 

reuse rates for individual rest structures are typically low, and selection for structure type changes seasonally to meet thermoregulatory needs (e.g., Spencer 1987). 

Thus, many resting structures meeting various seasonal requirements are required 

across marten home ranges. Large-diameter live trees, snags, and logs provide 

the main types of resting structures for martens (Schumacher 1999, Slauson and 

Zielinski 2009, Spencer 1987). Slauson and Zielinski (2009) reported that among 55 

rest structures used by Humboldt martens in the summer and fall, the most commonly used included snags (37 percent), downed logs (23 percent), and live trees 

(17 percent) (fig. 14). Less commonly used structures included large slash piles with 
49 

A B 

C 

Figure 14—Examples of Humboldt marten (Martes 

caurina humboldtensis) resting structures: (A) cavity 

in Douglas-fir snag; (B) platform on top of broken 

Douglas-fir snag; (C) cavity in downed Douglas-fir 

log. Red arrows identify the resting location in each 

structure. 

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GENERAL TECHNICAL REPORT PSW-GTR-260

Rest structures used 

by Humboldt martens 

averaged 95 cm d.b.h. 

for snags, 88 cm max 

diameter for downed 

logs, and 94 cm 

d.b.h. for live trees;

structures that on

average exceeded 300

years of age.

large-diameter logs (10 percent), natural rock piles (8 percent), and shrub clumps 

(6 percent). Preliminary results of an ongoing study of Humboldt martens on more 

intensively managed lands also indicate that live trees, snags, and downed logs are 

the most commonly used rest structures (76 percent), with rock piles (11 percent) 

and slash piles (11 percent) used to a lesser degree (Slauson 2015). Although martens use human-created structures, such as slash piles, these structures appear to be 

most useful in summer when insulation from cold temperatures or damp conditions 

is not a requirement (Raphael and Jones 1997). 

Martens typically select the largest available structures for resting and denning (Gilbert et al. 1997, Spencer 1987, Wilbert 1992). Rest structures used by 

Humboldt martens in largely unmanaged forest averaged 95 cm diameter at breast 

height (d.b.h.) for snags, 88 cm large-end diameter for downed logs, and 94 cm 

d.b.h. for live trees; structures that on average exceeded 300 years of age (Slauson 

and Zielinski 2009). Preliminary data on Humboldt marten rest structures from 

more intensively managed lands indicate a similar pattern of use of large-diameter 

conifer structures, with 70 percent of structures >70 cm d.b.h. (Slauson 2015). Most 

resting locations (i.e., the actual resting place in the structure) were in tree cavities 

(33 percent), on platforms in broken-top snags or on large live branches (33 percent), 

or in chambers within log piles or rock outcrops (28 percent) (Slauson and Zielinski 

2009) (fig. 14). Rest structures provide cavities or chambers that likely become 

especially important during late fall through late spring, when wet and cold conditions are common. 

Denning structures where females give birth to kits are called natal dens, and 

the locations where they later move their kits are referred to as maternal dens. 

Martens appear to be more selective of habitat conditions at den sites than at resting 

sites (Thompson et al. 2012). Characteristics of den structures and their surrounding 

stands each influence den-site selection (Ruggiero et al. 1998). High-quality foraging habitat near den sites is likely important, allowing females to maximize energy 

gained from foraging during lactation and minimize time spent away from their 

kits, especially when kits are dependent on their mothers for thermoregulation. The 

most common den structures used by North American martens are large-diameter 

live and dead trees with cavities (Thompson et al. 2012). 

As of 2015, nine natal dens and 25 maternal dens had been described for the 

Humboldt marten. The majority (66 percent, n = 6) of natal dens were in cavities in 

large-diameter hardwoods (average d.b.h. = 101.8 cm), with a single snag (114 cm 

d.b.h.), single downed log, and single subterranean location used (Delheimer et al. 

n.d.). Maternal dens were primarily in cavities (85 percent) in 18 live trees (92.9 cm 

average d.b.h.), five snags (145 cm average d.b.h.), one rock pile, and one marten 

box (Delheimer et al. n.d., Slauson and Zielinski 2009). 
51 

Stand-Scale Habitat Use 

Martens select forest stands that provide habitat structure supporting one or more 

life history needs that include foraging, resting, or denning. In addition, stand structure features that may reduce the risk of predation, such as dense overhead vegetation and vertical tree boles, could also be important. In general, the Pacific marten is 

associated with late-successional conifer stands (Powell et al. 2003) characterized by 

an abundance of large downed logs and large, decadent live trees and snags (Buskirk 

and Ruggiero 1994). Also, many key prey species occur in their highest densities in 

forest stands with late-successional structural features (e.g., western red-backed vole 

[Hayes and Cross 1987, Zabel and Waters 1992], northern flying squirrel [Waters 

and Zabel 1995], Douglas’s squirrel [Carey 1991]), and dense ericaceous shrub 

layers (e.g., chipmunks [Hayes et al. 1995]). For reasons previously discussed, coarse 

woody debris is also an important component of stand structure, affecting both prey 

abundance and marten predation success (Andruskiw et al. 2008). 

Analysis of Humboldt marten habitat selection in California revealed that 

they most strongly selected stands of old-growth, conifer-dominated forests with 

dense, ericaceous shrub layers (Slauson et al. 2007) (fig. 15). Late mature stands 

were used in proportion to availability, while stands in earlier successional stages 

were selected against (Slauson et al. 2007). The old-growth and late-mature stands 

selected by martens were most often dominated by Douglas-fir overstories, but also 

had mature hardwood understories composed of either tanoak or chinquapin. Shrub 

layers were dense (>70 percent cover), spatially extensive, and dominated by ericaceous species, including evergreen huckleberry, salal, and rhododendron (Slauson 

et al. 2007) (fig. 15). The majority of detections of martens in south coastal Oregon 

shared these same stand characteristics (Slauson and Zielinski 2001). 

A recent study focusing on martens on more intensively managed lands on the 

western edge of the main California population found some martens using a mosaic 

of managed stands that were mostly harvested several decades ago. Many of those 

managed stands included substantial numbers of residual, large-diameter live and 

dead conifers and hardwoods. This work should provide new insight into compatible forest management alternatives where maintaining marten habitat and timber 

production are both management objectives (Slauson et al. 2014). 

In addition to old-growth stands in highly productive soils, martens in coastal 

California and south coastal Oregon also have used forest- and shrub-dominated 

habitats occurring on less productive serpentine soils, hereafter called serpentine 

habitats. These habitats typically feature conifer-dominated overstories with dominant species, including lodgepole pine, western white pine, and Douglas-fir; but 

they also include dense (>70 percent cover) shrub layers dominated by huckleberry 

Analysis of Humboldt 

marten habitat 

selection in California 

revealed that they 

most strongly selected 

stands of old-growth, 

conifer-dominated 

forests with dense, 

ericaceous shrub 

layers. 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
52 

C 

A B 

Figure 15—Typical habitat structure within mesic 

Douglas-fir-associated stands occupied by Humboldt martens (Martes caurina humboldtensis) in 

coastal California and coastal Oregon. Note the (A, 

C) conifer-dominated overstory, (A) dense shrub

layer, and (B, C) presence of large-diameter live

trees, snags, and downed logs.

GENERAL TECHNICAL REPORT PSW-GTR-260
53 

oak, shrub form tanoak, and red huckleberry (Slauson and Zielinski 2001, Slauson 

et al. 2007) (fig. 16). In contrast to the dense old-growth stand structure martens 

use on productive soils, stands used in serpentine soils include any seral stage and 

with tree canopy closures ranging from sparse (20 percent) to dense (>70 percent; 

Slauson and Zielinski 2001, Slauson et al. 2007) (fig. 16). Serpentine habitats used 

by martens also contain abundant rocky outcrops, providing chambers that martens 

use as resting structures where large woody structures are rare (Slauson and Zielinski 2009). Although serpentine soils are extensively distributed in northwestern 

California and southwestern Oregon, martens have been found only in serpentine 

habitats in mesic, fog-influenced areas near (<30 km) the coast. 

Martens in central coastal Oregon have been detected in surveys and found as 

roadkill (Moriarty et al. 2016, Zielinski et al. 2001) in and adjacent to shore pine 

and transitional shore pine-hemlock-Douglas-fir forests. Shore pine forests near 

these sites had many of the same characteristics as serpentine habitats, such as 

variable tree overstory with dense, spatially extensive ericaceous shrub understories 

(Chappell and Kagan 2001). 

Home Range-Scale Habitat Use 

Martens exhibit strong habitat selection at the home range scale, suggesting that this 

scale of selection directly influences an individual’s fitness (Thompson et al. 2012). 

They establish home ranges to fulfill year-round resource needs as well as to access 

members of the opposite sex during the breeding season. Theoretically, a predator’s 

home range size is a function of prey density and habitat quality; smaller home 

ranges typically represent better quality habitat conditions. Marten home ranges are 

often positioned to maximize the composition of high-quality habitat and minimize 

low-quality habitat (Phillips 1994) (fig. 17). Individual home ranges typically 

include a high proportion (≥70 percent) of suitable habitat (reviewed in Thompson 

et al. 2012). Females, as a result of their solitary role in raising young, have unique 

needs and must have access to reliable nearby prey resources to support the energetic demands of lactation and food provision for kits. 

Amount and spatial distribution of high-quality habitat are related to marten 

home range size. As the amount of low-quality habitat (e.g., recent clearcuts or partial 

harvests) increases, home range size increases (Fuller and Harrison 2005, Potvin 

and Breton 1997, Potvin et al. 2000, Thompson 1994). Accordingly, the largest home 

range sizes for Pacific martens in California and Oregon, estimated at >1000 ha, are 

those of individuals occupying landscapes with extensive logging (Bull and Heater 

2001, Ellis 1998, Self and Kerns 2001). The fitness consequence of larger home 

ranges with higher portions of managed stands (including regenerating clearcut, 

Martens in central 

coastal Oregon have 

been detected in shore 

pine and transitional 

shore pine-hemlockDouglas-fir forests.

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
54 

Figure 16—Typical habitat structure within mesic serpentine habitat 

occupied by Humboldt martens (Martes caurina humboldtensis) in 

California and south coastal Oregon. 

GENERAL TECHNICAL REPORT PSW-GTR-260
55 

Old growth (>200 years)

Late-mature (>125 years)

Pole to mid-mature (35–100 years)

Regenerating clear-cut (<35 years)

A 

B 

Figure 17—Home range position in the landscape, habitat composition, and within-home range 

habitat use by (A) a typical adult female and (B) a typical adult male Humboldt marten (Martes 

caurina humboldtensis). Home ranges represent 100 percent minimum convex polygons in nonserpentine forest habitat. Ninety-seven percent (38 of 39) of the adult females’ and 77 percent (30 of 39) 

of the males’ within-home range resting and active locations occurred in the core old-growth and 

late-mature riparian habitat patches (Slauson and Zielinski, unpublished data). 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
56 

Humboldt marten 

home ranges and 

detection sites often 

include large patches 

(median >150 ha) of the 

most favored habitat: 

old-growth forest, oldgrowth in combination 

with late-mature forest, 

or serpentine habitat. 

partial harvest, and shelterwood) appears to be increased risk of predation, as described in detail in the “Intraguild Predation and Interspecific Competition” section. 

The home ranges of Sierra martens in largely unlogged forest landscapes averaged 300 to 500 ha for males and 300 to 400 ha for females (Simon 1980, Spencer 

et al. 1983). Limited telemetry data from Humboldt martens suggest that adult male 

home ranges (n = 3) are of similar size (300–400 ha) (Slauson 2015). Telemetry 

data and habitat selection analysis at Humboldt marten detection sites indicated that 

home ranges include large patches (median >150 ha) of the most favored habitat: 

old-growth forest, old-growth in combination with late-mature forest, or serpentine 

habitat (Slauson et al. 2007) (figs. 17, 18). 

Landscape-Scale Effects on Habitat Use: Movement, 

Occupancy, and Population Dynamics 

Habitat patterns and composition at the landscape scale affect the ability of martens 

to successfully disperse and find suitable home ranges, their survival and spatial 

occupancy dynamics, and ultimately, population size and persistence. The amount 

and spatial arrangement of high-quality habitat capable of supporting one or more 

home ranges, the distance between patches, and the quality of intervening habitat 

are key factors at the landscape scale (Chapin et al. 1998, Hargis et al. 1999, Kirk 

and Zielinski 2009). 

Figure 18—Use and availability of the largest contiguous patch of old-growth, old-growth plus late-mature, 

or serpentine habitat within a 1-km radius that encompassed locations occupied by Humboldt martens 

(Martes caurina humboldtensis) in California (Slauson et al. 2007). 

GENERAL TECHNICAL REPORT PSW-GTR-260
57 

Marten populations are sensitive to loss and fragmentation of high-quality habitat at the landscape scale. This is not to say that martens cannot persist on managed 

landscapes; with careful management to maintain and regenerate critical structures, 

and if certain thresholds are not surpassed, martens can be maintained along with 

timber production (Thompson et al. 2012). Where clearcut harvesting is practiced, 

reduced populations can be expected for ≥40 years if >25 to 30 percent of the forest

is composed of regenerating stands (Chapin et al. 1998, Hargis et al. 1999, Poole et al. 

2004, Potvin et al. 2000, Thompson and Harestad 1994). Martens are sensitive to the 

spatial distribution of suitable habitat and typically establish home ranges where ≥70

percent is composed of suitable habitat and include large patches (>150 ha) of suitable 

habitat in individual home ranges (Chapin et al. 1998, Potvin et al. 2000, Slauson et 

al. 2007). American martens appear more capable of tolerating partial harvest than 

clearcutting, but are very sensitive to the residual basal area of certain tree species, 

tree sizes, and canopy covers left after harvest (Fuller and Harrison 2005, Godbout 

and Ouellet 2008, Potvin et al. 2000). Threshold values for canopy cover, residual 

basal area, and species composition following timber harvest are ecoregion-specific 

and remain to be developed within the range of the Humboldt marten. 

Beyond the direct impacts of habitat loss and fragmentation from timber 

harvest or severe wildfire, these modifications also affect martens indirectly by 

increasing the abundance of habitat generalist carnivores and raptors capable of 

preying on martens. Bobcats select for young regenerating clearcuts in the assessment area (Slauson 2018 and Wengert 2013) and are important predators on martens 

(Bull and Heater 2001, Slauson et al. 2014). Analysis of more than 400 bobcat scats 

identified brush rabbits and dusky-footed woodrats as the primary prey species in 

the bobcat diet in coastal California forests (Slauson N.d.). In the assessment area, 

dusky-footed woodrats are most abundant in young, regenerating even-aged stands 

(Hamm and Diller 2009). The diet and habitat associations of brush rabbits have not 

been studied in the assessment area, but their reliance on grasses, herbs, and disturbance-dependent shrubs (e.g., Ceanothus sp.) in their diets elsewhere (reviewed in 

Verts and Carraway 1998) suggest their abundance is closely tied to young stands 

regenerating from timber harvest or severe wildfire. Thus, both of the important 

prey species of the bobcat occur in early-seral forests in which bobcats should also 

be expected to occur and focus a high proportion of their hunting activities. 

While bobcats select for and most frequently forage in young regenerating 

stands, they appear sensitive to the proportion of young regenerating forest at the 

Marten populations are 

sensitive to loss and 

fragmentation of highquality habitat at the

landscape scale. 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
58 

To develop effective 

strategies for 

Humboldt marten 

management and 

conservation, 

managers will 

have to consider 

landscape-scale 

habitat conditions 

and account for the 

direct and indirect 

effects of management 

alternatives, including 

the effects of habitatmediated predation on 

martens. 

landscape scale. In landscapes composed of a mix of 40- to 60-year-old regenerating stands and old-growth, or composed largely of unmanaged mid- and late-successional stands, bobcats were nearly absent (Slauson, in prep.). Elsewhere in North 

America, martens occupying remnant suitable portions of landscapes fragmented 

by logging experience higher predation rates and lower survival rates than martens 

occurring in more contiguous, unlogged landscapes (Potvin and Breton 1997, 

Raphael 2004, Thompson 1994). To develop effective strategies for Humboldt marten management and conservation, managers will have to consider landscape-scale 

habitat conditions and account for the direct and indirect effects of management 

alternatives, including the effects of habitat-mediated predation on martens. 

Dispersal is the means by which marten populations maintain and expand their 

distribution and population size. Successful dispersal requires functional connections between habitat patches capable of supporting reproduction. Dispersal is also 

essential to maintain viable metapopulations, should that be the structure of marten 

populations. Johnson et al. (2009) found that in intensively logged landscapes, 

daily and total dispersal distances were reduced by approximately 50 percent, and 

that the success rate of juvenile martens dispersing to establish home ranges was 

reduced from 49 to 25 percent. In combination, reduced foraging efficiency in 

predominantly clearcut stands (Andruskiw et al. 2008), and increased predation risk 

in postlogging early-seral habitat may pose significant impediments for dispersing 

martens. 

Given the energy demands of dispersal, foraging success is likely to influence 

dispersal movements. Dispersing martens use a search strategy that is not random 

or linear, suggesting they are responding to habitat cues and that landscape pattern 

likely influences movement trajectories (Johnson 2008). Search behavior during 

dispersal often involves sharp turning angles to avoid barriers or low-quality habitat 

and reversing direction to return to familiar areas (Johnson et al. 2009). In a local 

example, a dispersing juvenile female Humboldt marten moved along a strand of 

unmanaged stands of various seral stages, but repeatedly used a small (<50 ha) 

old-growth stand prior to returning to her natal area (Slauson and Zielinski 2009a). 

Also, two subadult males that made relatively long dispersal movements across 

intensively managed forest each finally established home ranges in areas with uncut 

forest stands or large numbers of residual structures (Slauson et al. 2014). In addition to the direct and indirect effects that habitat conditions can have on dispersal, 

natural and anthropogenic features—such as major rivers and highways—can 

influence both the decisions to cross and the outcome (e.g., survival or death) of 

those decisions. The combination of the well-documented behavior of avoiding 

entering habitats with reduced escape cover (e.g., Cushman et al. 2011) and the 

GENERAL TECHNICAL REPORT PSW-GTR-260
59 

number of Humboldt marten roadkill observed along U.S. Highway 101 in coastal 

Oregon (e.g., Zielinski et al. 2001) suggest highways may represent important filters 

for movement and dispersal. In addition, major rivers pose the additional challenge 

of the need for the disperser to successfully swim across open water which, at this 

time, has an unknown influence on both the probability that a disperser will decide 

to cross and survive. Overall, the assessment of conditions supporting dispersal will 

need to consider the combination of direct and indirect effects of habitat conditions 

on dispersers as well as how strong natural and anthropogenic features can act as 

filters or barriers to dispersal. 

Contemporary Landscape Habitat Suitability 

Slauson et al. (2018) developed a landscape-scale habitat suitability model for 

marten for the extent of the assessment area where survey data were available up 

to 2010 and applied the model to the full extent of the assessment area (fig. 19). 

The model was developed using the combination of variables that best described 

the distribution of marten detection/nondetection survey data. The survey dataset 

consisted of 1,159 surveys using protocols with sufficient survey effort to yield 

high probabilities of detection at locations from public, private, and tribal lands 

distributed from Mendocino County, California, north to Coos County, Oregon, 

that were surveyed between 1990 and 2010. A total of 29 predictor variables from 

six categories were considered: climate, topography, linear features (roads and 

streams), forest structure and composition, landscape arrangement, and disturbance. 

Variables were assessed at three spatial scales using “moving windows” with 500-, 

1000-, and 3000-m radii to assign each 30- by 30-m pixel a value representing the 

variable characteristics within the windows. 

The forest structure and compositional variables were derived from the gradient 

nearest neighbor (Ohmann and Gregory 2002) gridded vegetation dataset, which 

provided a seamless source for calculating the proportion of the area of the moving 

window at each scale for 13 forest structural and compositional variables. Candidate multivariate models were developed based on hypothesized positive and negative relationships between marten occupancy and combinations of the variables. 

The top ranked models that best fit the data consistently included three variables: old-growth structural index (OGSI) (at the 1-km scale), serpentine habitat (at 

the 3-km scale), and precipitation (at the 3-km scale) (Slauson et al., n.d.). Higher 

average OGSI values at the 1-km scale and higher areas of precipitation at the 3-km 

scale were positively associated with marten occupancy. The OGSI is an average of 

four separate indices representing stand age and four structural features: number of 

large trees (>100 cm d.b.h.), large snags (>50 cm d.b.h. and >15 m tall), volume of 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
60 

GENERAL TECHNICAL REPORT PSW-GTR-260

Figure 19—Landscape habitat suitability 

for the Humboldt marten (Martes caurina 

humboldtensis) throughout its’ historical 

range in coastal California and coastal 

Oregon (Slauson et al., n.d.). OGSI = 

Old-growth structural index. 
61 

large snags, and tree size diversity (Spies et al. 2007). Small and moderate amounts 

of serpentine habitat at the 3-km scale were positively associated with marten 

occupancy, but large areas were negatively associated. In summary, the top models 

identified two habitat types: mesic forest habitat with high OGSI values (most 

widespread habitat type) and mesic forest habitat on serpentine soils with low OGSI 

values (this habitat type occurs in <5 percent of the historical range) (fig. 19). 

The final model averaged the coefficients of the top three models and had 

an overall correct classification rate of 83 percent, with greater skill at correctly 

classifying marten detection locations (91 percent of 75) than nondetection locations 

(82 percent of 1,078) and a true skill statistic of 0.73 (with 1.0 being best) (Allouche 

et al. 2006). Model predictions were stable when evaluated with 5 by 5 cross-validation. The model did not predict well for the shore pine habitats in central coastal 

Oregon where survey data were not available during initial model development 

but martens are known to occur based on roadkill and more recent survey results. 

While this discrepancy will need to be addressed in the next model revision, shore 

pine habitat represents a rare habitat type (<3 percent of historical range), that can 

likely be modeled similar to how serpentine habitat was modeled; thus shore pine 

habitat type is accounted for in this assessment. 

The final model, when applied to the full extent of the assessment area, predicts 

that suitable habitat occurs in <15 percent of the historical range and 91 percent of 

all detections occur in these areas. Three general areas with large patches of highsuitability habitat were identified: one in northwestern California that supports the 

only known populations for the state, and two in Oregon, one on the central coast 

and one in the south coastal area (figs. 19 and 20). The model also predicted some 

habitat that may provide connectivity between the California and south coastal 

Oregon populations, but identified little habitat between the south and central 

coastal Oregon populations. 

The model described above was constructed and evaluated prior to a number of 

new detections of Humboldt martens in California during 2011–2015. These more 

recent detections provided an opportunity to evaluate the accuracy of the model’s 

predictions. The model correctly classified 73 percent (36 of 49) of these new 

detection locations: 17 of 18 new detection locations in the 2012 population monitoring effort, 4 of 9 new detection locations in the area just south of U.S. Highway 

199, and 15 of 22 areas used by radio-collared individuals in the managed forest 

landscape west of the main California population (Slauson et al. 2014). The six 

misclassified marten detection locations from the first two survey efforts occurred 

an average of 825 m from a habitat patch identified as suitable in the model. The 

seven misclassified marten-use areas from the dispersal study had approximate use 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
62 

GENERAL TECHNICAL REPORT PSW-GTR-260

Figure 20—Distribution of marten detection/nondetection survey data through 2010, and resulting landscape habitat suitability 

model for the Humboldt marten (Martes caurina humboldtensis) in coastal California and Oregon (Slauson et al., n.d.). OGSI = 

old-growth structural index. 
63 

area centroids that occurred an average of 1623 m from a modeled suitable habitat 

patch. Marten locations (13 of 49, 27 percent) that occurred outside of modeled suitable habitat patches represent surveys conducted during the dispersal season, and 

therefore may represent the presence of nonresident individuals, or individuals that 

attempted to establish home ranges but were known to have died in these areas. In 

sum, the landscape habitat suitability model performed well at identifying locations 

where the new detections occurred. As such, it should provide a strong foundation 

for conservation planning. However, we recognize that as new information is gathered, updates to the model and reinterpretation of model outputs may be required. 

Implications for Conservation 

• Recent genetic evidence indicates that coastal California and coastal

Oregon marten populations represent a single evolutionary unit. To conserve the genetic diversity expressed in this population, it should be managed to maintain and restore the population connectivity and gene flow

within the current (and ideally the historical) range of this subspecies.

• Humboldt martens occur in a few small, isolated populations. The distribution of the Humboldt marten has significantly declined in both California

and Oregon. Only four geographically disjunct populations are known to

currently exist. The largest California population was estimated to be at

<100 individuals in 2012 (Slauson 2012, Slauson et al. 2009a); its small

size puts it at risk of extirpation. The uncertainty about the sizes of Oregon

populations—which are likely to be smaller than the largest California

population, based on the current knowledge of the sizes and distributions

of occupied habitat patches—is also of significant concern. Efforts to limit

future reductions and increase population size and connectivity would be a

valuable contribution to the conservation of the species.

• The majority of key prey species for martens in the assessment area rely

primarily upon conifer cone crops and the fruiting bodies of ectomycorrhizal fungi as their food sources. In addition, dense shrub cover can influence

the abundance of seasonally important prey species. Management practices

that maintain and enhance these resources are likely to improve foraging

habitat for martens.

• Juvenile dispersal distance and successful home range establishment can

be significantly affected by landscape condition. Long-term planning and

management that affects functional connections between patches of suitable

habitat will have an effect on population size and connectivity.

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
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GENERAL TECHNICAL REPORT PSW-GTR-260

• Adult and juvenile survival are the most important demographic parameters

affecting stability and growth of marten populations. As such, individual or

cumulative factors that reduce or increase survival rates represent significant impacts (negatively or positively, respectively) on population growth

and persistence.

• Larger bodied carnivores, especially habitat generalists, are the most common predators of martens across their range in North America, and predation is expected to be a primary driver of survival, which in turn is a

very influential predictor of marten population growth and persistence.

Predation rates on marten populations are highest in landscapes that have

been fragmented by intensive logging methods, including clearcut, shelterwood, and closely related silvicultural systems. The bobcat has been identified as the most common predator of the California population of Humboldt

martens. Bobcats are strongly associated with harvested stands ≤30 years

of age, but are much less common in landscapes where a high proportion of

harvested stands are ≥40 years old. Indirect effects of management actions

on marten predation risk should be assessed during development of landscape-scale forest management and restoration plans.

• Most known locations for Humboldt martens occur on productive soil

types, where martens show strong selection for large patches of old-growth

conifer-dominated forest with dense ericaceous shrub layers. However, they

also can use more open forest types with dense shrub layers on low-productivity soil, such as shore pine and transitional shore pine-hemlock-Douglasfir forest types or forest types on near-coast serpentine soils. Recently,

Humboldt martens on the edge of the current population distribution have

been found in a mosaic of stands that were harvested several decades ago

and include large-diameter live and dead conifers and live hardwoods. The

effects these managed and low-productivity habitats may have on marten

population performance, survival, and reproduction is unknown. More

information is warranted to understand the role these habitat types could

play in supporting marten conservation objectives.

• Dense, spatially extensive shrub layers are a consistent habitat component

at most locations where Humboldt martens have been detected in coastal

California and coastal Oregon. This unique habitat association has not been

described elsewhere in the distribution of either North American marten

species. These shrub layers are dominated by shade-tolerant ericaceous
65 

species. Retention and regeneration of shade-tolerant ericaceous shrub layers is likely to have important consequences for marten habitat suitability 

and population conservation. 

• Marten population persistence is affected by habitat conditions at multiple

scales. Forest management and restoration planning that includes considerations for marten habitat and population conservation will benefit from

developing management and restoration actions for resting/denning features

and surrounding stands, while maintaining suitability at the home range

and landscape scales. Indirect effects of forest management that influence

the distribution and abundance of marten predators is also an important

consideration in forest management and restoration planning for Humboldt

martens.

Research and Management Needs in the Assessment Area 

• Determine the phylogenetic relationships for all Pacific marten populations

in California and Oregon. This information will better identify how extant

populations could be managed to preserve their genetic uniqueness within

their respective ranges.

• Continue to survey for Humboldt martens in forested habitats at the margins of the known populations. Use consistent survey and monitoring methods to determine population distribution, abundance, and trends throughout

their range.

• Quantify amounts and thresholds of marten denning habitat at the home

range scale. Develop spatially explicit models of denning habitat distribution to inform habitat conservation, maintenance, and restoration planning

and implementation.

• Estimate demographic parameters, including fecundity and stage-specific survival, to support the evaluation of management and conservation

actions. For fecundity estimation, both pregnancy and kit production rates

are needed to better understand the relationships between these two metrics of reproduction. Also, determining rates of reproductive decline among

aging adult females would improve population modeling.

• Determine the landscape-scale habitat characteristics that promote successful dispersal for Humboldt martens. Identify effective approaches for

maintaining or improving the conditions capable of promoting successful

dispersal at both the stand and landscape scales.

• Describe the habitat selected by important marten prey, and identify standlevel management actions that can increase their abundances.

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
66 

• Continue research on predators of the Humboldt marten, habitat conditions that support them, and characteristics of kill sites. Strengthen the

understanding of relationships between various types of silvicultural management and key marten predators. Identify stand- and landscape-level

management actions that would reduce predation risk to martens.

• Use existing information to evaluate characteristics such as timing, speed,

and distance, and environmental factors contributing to successful dispersal.

Incorporate findings into an evaluation of the feasibility of assisted dispersal.

• Evaluate relationships between Humboldt marten distribution and various

timber and fuels management practices at the stand and landscape scales.

Identify silvicultural practices that increase habitat suitability for martens

and their prey. Develop canopy-cover, basal-area, and tree-species composition thresholds for partial and selection harvest and thinning that are

compatible with maintaining martens and shortening the time for restoring

habitat suitability.

• Increase understanding of the linkages between marten fitness parameters

and multiple-scale habitat characteristics. Continue to refine the analytical foundation for evaluating the effects of management and conservation

actions on marten population size and viability.

• Evaluate the risk of inbreeding and depressed fitness for the population

sizes that currently exist. If warranted, identify alternatives for improving

gene flow and effective schedules for implementation.

• Continue to evaluate the impacts of rodenticides on Humboldt martens,

explore actions that are effective in reducing their impacts to martens and

their prey, and share information with law enforcement and regulatory

agencies as appropriate.

• Continue to update and improve the accuracy of the landscape habitat suitability model. Incorporate influences of important predators on the habitat

suitability for martens.

GENERAL TECHNICAL REPORT PSW-GTR-260
67 

Chapter 5: Development of the Conservation 

Strategy Foundation 

In this section, the Humboldt Marten Conservation Group (HMCG) describes the 

process for developing the conservation strategy foundation (fig. 21). Each step of 

the process uses the scientific information in previous sections to identify key conservation actions and research needs. Then each conservation action is prioritized 

within a biologically relevant timespan for marten populations. The HMCG expects 

that the overall conservation strategy will be further developed over time with the 

completion of additional strategy modules, such as the Habitat Management Guide 

and Population Monitoring Protocol (see the “Next Steps” section). The intent is for 

this strategy to be a dynamic product, subject to updating and improvement as new 

information is gathered and synthesized (fig. 21). 

Strategy Goal, Guiding Principles, and Approach 

To establish a solid scientific foundation for the development of the conservation 

strategy, the HMCG has developed an overall goal—to establish self-sustaining, 

interacting populations of Humboldt martens throughout their historical 

range—that defines the overall direction for the strategy, along with guiding principles, and an approach for meeting the goal. The guiding principles set the rules 

by which the strategy will be developed and, if needed, amended, and the approach 

provides the details on how we will achieve our overall goal. 

In interpreting the science to identify effective conservation options, the 

HMCG applied the following “best practices” guidelines to develop the conservation strategy: 

• The HMCG used the best available scientific information on martens.

Where available, information from studies conducted within the assessment

area was used. When using information collected outside the assessment

area, the HMCG relied primarily on findings that were consistent across

different forest ecoregions.

• The HMCG used the principles of conservation biology, landscape ecology,

and forest ecology as the foundation for the conservation strategy. Guiding

principles of conservation biology include the recognition that martens in

the assessment area exist in a metapopulation structure (a group of spatially

separated populations of the same species that interact via dispersal) and

this structure is currently in a nonequilibrium state. The HMCG defines

nonequilibrium as the population processes (e.g., occupancy of suitable habitat, population size fluctuations, dispersal rates) not currently being in a

The HMCG's overall 

goal—to establish 

self-sustaining, 

interacting populations 

of Humboldt martens 

throughout their 

historical range. 

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68 

GENERAL TECHNICAL REPORT PSW-GTR-260

Figure 21—Conceptual model of the development and implementation steps for the Humboldt marten conservation strategy. 

steady state owing to a combination of both natural and anthropogenic factors. Guiding principles of landscape ecology include how quality, amount, 

and spatial arrangement of habitat affects both individuals and subpopulations. In addition, functional habitat connectivity within and between subpopulations will be necessary for the successful movement of individuals to 

support population persistence and gene flow. Guiding principles of forest 

ecology include managing marten habitat and habitat elements within their 

historical ranges of variability and natural disturbance regimes where possible. In addition, many important habitat elements naturally develop over 

decades to centuries and involve growth, disturbance, and decay processes. 

• The HMCG addressed four spatial scales: (1) landscape, (2) home range, (3)

stand, and (4) individual habitat structures (microscale). The conservation

strategy foundation is based on these four spatial scales in which martens

use habitat, leading to a spatial template to guide the application of conservation actions. The HMCG identified and prioritized conservation actions

and information needs anticipated over the next 15 years (see “Timing” in

the “Classification of Potential Threats” section for justification).
69 

State of the Science for Supporting the Development of 

a Conservation Strategy 

The current state of scientific knowledge for the Humboldt marten continues 

to develop; however, the HMCG recognizes that management and conservation 

actions to guide the maintenance and expansion of martens and their habitat are 

needed promptly to take steps toward attaining our overall strategy goal. The 

HMCG recognizes that the development of the conservation strategy will occur in 

stages as new science emerges. Despite some information gaps, The HMCG has 

determined that there is sufficient scientific information at this time to develop a 

conservation strategy foundation that will establish the overall goal, guiding principles, and approach for the strategy and identify its core components, the conservation emphasis areas (see below). In addition, the conservation strategy foundation 

includes a threats assessment, conservation actions to ameliorate significant threats, 

and next steps and information needs to guide the acquisition of new science to 

support the development of future strategy components and conservation actions. 

For example, the HMCG recognizes the need for new science to develop a Habitat 

Management Guide (see the “Next Steps” section on page 94) and the next module 

of the strategy. 

Overall Strategy Approach 

This conservation strategy uses a three-pronged approach: 

• Protect existing populations and currently suitable habitat.

• Reestablish populations in areas of currently suitable but unoccupied

habitat.

• Restore habitat conditions in specific areas to increase population size, distribution, and connectivity.

Conservation actions include maintaining and improving habitat for survival

and reproduction in existing and potential population areas, along with maintenance 

and restoration of functional habitat connectivity between areas of currently suitable habitat. Reestablishment of martens in areas that are suitable, but unoccupied, 

may reduce overall population impacts, should occupied areas be lost in stochastic 

events such as severe wildfire or disease outbreaks. Concurrently, landscape-scale 

habitat restoration efforts that increase the amount and improve the distribution of 

suitable habitat will (1) support long-term expansion and viability of reestablished 

populations, and (2) improve long-term viability of extant populations through 

exchange of dispersers and gene flow. 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
70 

The conservation 

strategy is primarily 

based on identification 

of spatially explicit 

conservation 

emphasis areas where 

conservation actions 

appear to have the 

greatest potential 

to meet the overall 

strategy goal. 

To make the approach for the overall strategy operational, the HMCG: 

• Identified locations that serve important functions in supporting, securing,

and restoring marten populations.

• Identified priorities for each location and actions that are likely to move

each location toward the desired conditions.

• Identified the threats for each location that could inhibit or reverse progress toward desired conditions, and actions that could mitigate against such

impacts.

Conservation Emphasis Areas 

The conservation strategy is primarily based on identification of spatially explicit 

conservation emphasis areas where conservation actions appear to have the greatest 

potential to meet the overall strategy goal. Conservation emphasis areas fulfill the 

three-pronged strategy approach by: 

• Protecting existing populations and currently suitable habitat in order to

anchor the existing populations—through extant population areas (EPAs).

• Reestablishing populations in areas of currently suitable, but unoccupied,

habitat—through population reestablishment areas (PRAs).

• Restoring or improving habitat conditions to increase connectivity between

existing populations and the number of sites able to support populations—

through landscape connectivity areas (LCAs).

In mapping the conservation emphasis areas, the HMCG relied on recent and

verifiable marten detections and the landscape habitat suitability model (figs. 

19, 20). The landscape habitat suitability model relies on the statistical relationships between marten survey results and attributes of vegetation and the physical 

environment represented in the model (see the “Contemporary Landscape Habitat 

Suitability” section). The HMCG recognizes that all models are caricatures of 

reality and are only as good as the data used to create them; the marten model is 

no exception. As described earlier, the model is better at correctly identifying the 

locations where martens were detected than locations where they were not detected, 

and puts the emphasis on higher accuracy for detections, as is desired for habitat 

models for species of conservation concern (Franklin and Miller 2010). Sufficient 

information presently exists to identify areas of relatively high habitat suitability 

(via the output of the habitat suitability model) and general principles appropriate 

for habitat management. 

GENERAL TECHNICAL REPORT PSW-GTR-260
71 

The HMCG suggests that it is prudent to recommend a demarcated set of 

geographical areas representing the state of knowledge about Humboldt martens 

in California through 2016, but recognizes that new information and conservation 

opportunities may require updating the boundaries of these areas and consideration 

of other areas not currently emphasized. 

Conservation emphasis areas are intended to represent the areas with the greatest current potential to achieve the overall marten conservation goal, to establish 

self-sustaining, interacting populations throughout their historical range. There are 

three categories of conservation emphasis areas: 

• Extant population areas—The distribution of extant populations, identified to protect existing populations and adjacent suitable habitat in order to

anchor the existing populations.

• Population reestablishment areas—Unoccupied habitat currently suitable

to support ≥5 female marten home ranges, identified to reestablish populations in areas of currently suitable, but unoccupied, habitat.

• Landscape connectivity areas—Habitat adjacent to or between any combination of the two areas listed above, but currently unsuitable or of low

suitability, identified to restore or improve habitat conditions to increase

connectivity between existing populations and the number of sites able to

support populations.

In the following sections, the HMCG describes the origin, proposed location,

and intended function of the conservation emphasis areas. The HMCG recognizes 

that land managers have multiple objectives to consider when implementing their 

land use plans, and the strategy presented herein does not impose any additional 

planning criteria. Rather, the HMCG’s primary intention with the strategy is to 

inform the objectives of managers when working in and near these areas as to the 

potential consequences of their actions with regard to Humboldt marten conservation. Note that there is variability in vegetation, topography, and other environmental factors that cannot be accounted for in mapping at the scale of the conservation 

emphasis areas. To help guide the design and evaluation of projects, local managers 

will need to apply methods for evaluating habitat management actions at finer 

spatial scales. The forthcoming Habitat Management Guide (see the “Next Steps” 

section) will illustrate a range of conditions suitable for martens at scales from the 

stand level to home ranges, which will provide the linkage between the conservation emphasis areas and project-level guidance that managers can use in evaluating 

current habitat suitability and potential effects of management actions. 

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72 

GENERAL TECHNICAL REPORT PSW-GTR-260

Management actions 

that maintain or 

enhance habitat 

suitable for marten 

occupancy and 

increase the 

capability to support 

reproduction and 

survival will be 

consistent with the 

goal of the strategy. 

Currently suitable habitat, capable of supporting both reproduction and survival, will have the most significant value for martens, and where it exists, receive 

the highest priority for maintenance and protection. Management actions that maintain or enhance habitat suitable for marten occupancy and increase the capability to 

support reproduction and survival will be consistent with the goal of the strategy. 

Meanwhile, areas adjacent to habitat suitable for reproduction and survival represent strategic opportunities for habitat restoration or improvement and connectivity 

to other areas of suitable habitat. Within each conservation emphasis area category, 

strategic habitat restoration areas can be identified to: 

• Increase the amount of suitable habitat capable of supporting individual

home ranges; or

• Increase functional habitat connectivity between patches of currently

suitable habitat.

The HMCG expects that future tools such as the Habitat Management Guide

(see the “Next Steps” section) will help identify areas where potentially adverse 

actions, such as fuels reduction treatments or timber harvests, can be strategically 

located to avoid marten habitat or minimize negative effects on suitable habitat 

while meeting the overall intent of such projects. The HMCG expects that management actions, including habitat maintenance, habitat improvement or restoration, 

and connectivity improvement could occur in each of these conservation emphasis 

areas. The forthcoming Habitat Management Guide will help managers determine 

the most appropriate locations for and the appropriate size of areas to target for 

implementing actions to support the overall strategy goal. 

Area statistics for the conservation emphasis area are presented in table 1. The 

HMCG proposes initial locations for each of the three types of conservation emphasis areas in California and EPAs in Oregon but realizes that landowners, agencies, 

and possibly other stakeholders will ultimately determine the actual application of 

conservation emphasis areas and habitat management actions recommended by this 

conservation strategy. 

Extant Population Areas 

EPAs include the contemporary distribution of marten populations in the assessment area, plus areas of suitable habitat extending 2 km from verified marten 

locations at the periphery of each population. Contemporary distribution is defined 

by the locations where verifiable records of martens occurred from 1980 through 

2016, including survey station detections and telemetry locations. However, the 

margins of the depicted EPA polygons (fig. 22) are based on judgment and the 

aforementioned landscape habitat suitability model, and should not be interpreted as 
73 

Table 1—Area statistics of conservation emphasis areas (CEA) in California 

Percentage 

of California 

historical 

range 

Amount of suitable habitata

Percentage of 

Conservation emphasis areas Total size Occupancy each CEA 

Hactares Hactares 

Extant population areas: 

North coastal California 81,182 3.9 65,616 80.8 

California-Oregon border 17,615 0.9 15,366 87.2 

Population reestablishment areas: 

Smith River 82,583 4.0 71,065 86.1 

Middle Klamath 65,389 3.2 52,104 79.7 

Redwood and Prairie Creek 143,003 6.9 74,563 52.1 

Northern Redwood State Parks 14,419 0.7 981 6.8 

Landscape connectivity area: 

Lower Klamath 33,977 1.6 1,817 5.3 

Total 438,169 21.2 — — 

Undefined areas 1,625,684 78.8 — — 

a

 Amount of suitable habitat for occupancy includes the total acreage of the two highest categories (high and moderate) of 

modeled habitat suitability (green and red in fig. 24) 

formal boundaries. The survey data available to delineate the boundaries for extant 

populations varied considerably across the assessment area. The HMCG aggregated 

marten locations into EPAs by connecting locations within 5 km of each other (the 

typical marten dispersal distance, as described in the “Biological Information” 

chapter) with the shortest possible line through suitable habitat (as defined by the 

landscape habitat suitability model). If the total number of detections in an isolated 

cluster was <5, or a cluster of detections was separated by >5 km from other detection locations, they were not identified as an EPA owing to insufficient evidence 

that the population is self-sustaining. Based on the distributions of recent survey 

efforts and verifiable marten detections, we identified four EPAs in the assessment 

area: (1) North Coastal California, (2) California-Oregon Border, (3) South Coastal 

Oregon, and (4) Central Coastal Oregon (fig. 22). The EPAs in California are based 

on a significant amount of survey information and are regarded as having a high 

degree of certainty in the delineation of their boundaries. The EPAs in Oregon 

are based on a lower level of survey information at the time of their creation and 

therefore have a lower degree of certainty. However, the EPAs in Oregon are 

provisionally identified to guide the strategic survey efforts necessary to refine 

their distribution, estimate population sizes, and identify where martens or suitable 

habitat currently occur. 

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74 

GENERAL TECHNICAL REPORT PSW-GTR-260

 

 

Central Coastal Oregon EPA 

OREGON 

CALIFORNIA 

South Coastal Oregon EPA 

North Coastal California EPA 

CA–OR Border EPA 

Km 

Figure 22—Locations of the four extant population areas (EPAs) of Humboldt martens (Martes caurina humboldtensis) in the assessment area. The two extant population areas in Oregon are provisional until additional 

distributional data are obtained to better guide their delineation. 
75 

North Coastal California EPA— 

Available survey data for this EPA were much more extensive than for other EPAs. 

More than 600 verifiable locations of marten occurrence, including detections and 

radiotelemetry locations, were buffered by 2 km and connected to produce an EPA 

of 81 200 ha (fig. 23). The North Coastal California EPA includes the watersheds 

of South Fork of the Smith River, Blue Creek, Bluff Creek, Camp Creek, Cappell 

Creek, Pecwan Creek, Slate Creek, and Rock Creek, within Del Norte, northern 

Humboldt, and western Siskiyou Counties. 

The administrative units responsible for land or species management in the 

North Coastal California EPA are the California Department of Fish and Wildlife, 

Green Diamond Resource Company, Klamath National Forest, Six Rivers National 

Forest, and the Yurok Tribe. 

California-Oregon Border EPA— 

The first detection of a marten in this EPA occurred in 2011. Subsequent survey efforts from 2012 through 2014 detected martens at five additional locations east of U.S.

Highway 199 near the California-Oregon border in northeastern Del Norte County 

(fig. 23), on the Six Rivers National Forest and adjacent private lands. The spatial extent and number of martens represented by this EPA are currently unknown. 

One additional area in California, Prairie Creek Redwoods State Park, had one 

or two verifiable marten detections in the same general area each year from 2009 

to 2013 but none since. These detections, represented by two black dots outside the 

EPA in figure 23, were approximately 10 km west of the nearest marten location 

within the North Coastal California EPA and were insufficient in number to be 

considered a population using our criteria. Determining the number, sex, and ages 

of martens currently present in Prairie Creek Redwoods State Park is an important 

information need. 

South Coastal Oregon EPA— 

The majority of contemporary detections of martens in the South Coastal Oregon 

EPA occur in Curry County, both north and south of the Rogue River, on the Rogue 

River-Siskiyou National Forest (figs. 6 and 22). A single verified detection exists 

for Cape Blanco State Park (Moriarty et al. 2016), and unverified sightings were 

reported from Humbug Mountain State Park in the late 1990s. Two detections exist 

from western Josephine County, one of which occurred on Medford District BLM 

lands. Most of this EPA was designated based on the distribution of suitable habitat. The spatial extent and number of martens represented by this EPA are currently 

unknown. 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
76 

Km 

Josephine

County

Curry 

County

OREGON 

CALIFORNIA 

Del Norte 

County

Siskiyou

County

Humboldt 

County

Figure 23—Location of the North Coastal California and California-Oregon Border extant population areas (dark 

gray polygons) for Humboldt martens (Martes caurina humboldtensis) in California. Marten detections are indicated 

as solid black circles. 

GENERAL TECHNICAL REPORT PSW-GTR-260
77 

Central Coastal Oregon EPA— 

The majority of contemporary detections of martens in the Central Coastal Oregon 

EPA are from a combination of roadkill locations; survey detections; and, most 

recently, radiotelemetry locations from west of U.S. Highway 101 in coastal dune 

forest habitats in northwestern Coos, western Douglas, and western Lane Counties 

(fig. 6). Single trapping, roadkill, and incidental capture records occur west of U.S. 

Highway 101 in this EPA; and much of the EPA west of U.S. Highway 101 was 

delineated based on the presence of suitable, but heavily fragmented, habitat. The 

largest, most-contiguous patches of suitable habitat in this EPA have yet to be thoroughly surveyed. The spatial extent and number of martens represented by this EPA 

are currently unknown; however, in 2016, fewer than 63 individuals were estimated 

to occupy the dune forest habitat west of U.S. Highway 101 (Linnell et al., n.d.). 

Population Reestablishment Areas 

PRAs include habitat that currently appears suitable, but is not occupied by a 

marten population, or that has too few individuals (<5 females) or detections (<5 

independent locations) to be considered as currently supporting a self-sustaining 

population. To be considered a PRA, sufficient suitable habitat (≥1500 ha) was 

present to support ≥5 female marten home ranges. We used a minimum of five 

female home ranges to define the minimum criterion for identifying a PRA because 

martens are rarely reliably detected in areas without the presence of habitat capable 

of supporting several reproducing individuals. The HMCG acknowledges that the 

exact number of female home range areas necessary to support a viable population 

is unknown and is dependent on factors (e.g., proximity to other areas supporting 

reproduction) other than the number of female home ranges. The HMCG assumes 

that areas with enough suitable habitat to support ≥5 female home ranges represent

a reasonable minimum for considering an area as a candidate for population reestablishment. The HMCG identified PRAs using the landscape habitat suitability 

model (see the “Contemporary Landscape Habitat Suitability” section) and by 

recognizing additional large areas with high old-growth structural index (OGSI) 

value (Spies et al. 2007). The OGSI is a composite index from gradient nearest 

neighbor analysis (Ohmann and Gregory 2002) that integrates stand age, density of 

large conifers, diversity of tree sizes, density of large snags, and volume of down 

wood (Spies et al. 2007). Old-growth structural index, high rainfall areas, and 

serpentine habitat were the three variables included in all top landscape habitat suitability models (Slauson et al. 2018). However, the habitat model valued areas with 

high OGSI less when it occurred at lower elevations. This is most likely due to the 

combination of few contemporary marten detections at low elevations, where there 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
78 

is a more extensive legacy of intensive forest management, rather than a biological 

relationship with marten habitat with high OGSI at low elevations. The majority of 

the assessment area is composed of nonserpentine habitat types (>90 percent), and 

marten detections at nonserpentine sites had the strongest association with high 

OGSI values. Therefore, we also used OGSI measured at the 1-km scale as a standalone variable to provide a more generalizable means for evaluating current habitat 

conditions than the full landscape habitat suitability model for delineating PRAs. 

The HMCG identified PRAs in California but had insufficient detection data to 

identify PRAs in Oregon. Ideally, future survey efforts will yield sufficient data to 

establish PRAs in Oregon. 

In northern coastal California, the HMCG identified four PRAs (fig. 24), based 

largely on the presence of contiguous areas of suitable habitat as predicted by the 

landscape habitat suitability model (fig. 24). The HMCG also included several large 

patches of low-elevation forest with high OGSI values that were not recognized by 

the model (fig. 24). For example, the HMCG included an area of old-growth habitat 

in Redwood National and State Parks on the coast west of the North Coastal California EPA. 

The Smith River PRA (fig. 24) is adjacent to the northern border of the North 

Coastal California EPA and has connectivity to the South Coastal Oregon EPA (fig. 

24). This PRA represents 4 percent of the historical range of the Humboldt marten in 

California (table 1). Based on the landscape habitat suitability model, over 86 percent 

of the Smith River PRA is capable of supporting marten occupancy (table 1). 

The Northern Redwood State Parks PRA (fig. 24) is adjacent to the northern 

margin of the North Coastal California EPA. This is the smallest PRA (<1 percent 

of the historical range in California), but is composed primarily of low-elevation 

habitat with high OGSI values and includes a large block of habitat currently being 

managed to accelerate recruitment of old-growth forest characteristics. This PRA 

may eventually influence the movement of martens between the California and 

Oregon EPAs along the narrow western north-south corridor of currently suitable 

habitat (fig. 24). 

The Middle Klamath PRA (fig. 24) is adjacent to the northeastern edge of the 

North Coastal California EPA and is located at the northeastern-most portion of 

the historical range in California. Over 79 percent of this PRA is predicted to be 

suitable habitat (table 1). However, much of the habitat in this PRA is outside the 

influence of the typical fog zone and thus may support fewer areas with the mesic 

habitat characteristics consistent with occupied marten sites in the neighboring 

North Coastal California EPA. 

GENERAL TECHNICAL REPORT PSW-GTR-260
79 

Figure 24— Locations of the four population reestablishment areas (PRAs) for Humboldt martens (Martes caurina humboldtensis) in California depicted 

with modeled habitat suitability (A) and old-growth structural index (B): (1) Smith River PRA, (2) North Redwood State Parks PRA, (3) Middle Klamath PRA, and (4) Redwood-Prairie Creek PRA. EPA = extent population area. 

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80 

GENERAL TECHNICAL REPORT PSW-GTR-260

The Redwood and Prairie Creek PRA (fig. 24) is the largest PRA (6.9 percent 

of the historical range in California). Sixty percent of the PRA is currently suitable 

habitat or high-value, low-elevation OGSI habitat, and it also contains large tracts 

of young forest being actively managed to accelerate the restoration of old-growth 

forest characteristics. This PRA provides the most significant opportunity for 

southward expansion of the range in California (fig. 25). 

Landscape Connectivity Areas 

LCAs include large areas of forest habitat that (1) occur between or adjacent to 

any combination of EPAs and PRAs, (2) require straight line dispersal distances 

that average >10 km from the 10 nearest verifiable adult female locations, and (3) 

currently do not appear suitable for supporting successful dispersal or reproduction. 

The primary intent of an LCA is to improve functional habitat connectivity, allowing martens to recolonize currently suitable, but unoccupied, habitat and supporting 

the exchange of individuals between existing and reestablished populations into the 

future. Furthermore, connectivity may be enhanced over longer distances or where 

dispersal barriers are present by including areas suitable for supporting marten 

reproduction to increase the probability of successful dispersal and the frequency 

of dispersal attempts. Unfortunately, habitat requirements for successful dispersal 

are poorly understood at present. Areas presently regarded as unsuitable have a 

combination of low landscape habitat suitability value and low home range-scale 

OGSI value; but such areas may be capable of becoming suitable in the future. The 

HMCG assumes that successful dispersal across LCAs will be a function of their 

habitat quality and distance between EPAs and potential PRAs. At the present time, 

available information is sufficient to identify only one LCA in California, although 

ideally new information will be available in the near future to identify LCA locations in Oregon. 

LCAs will continue to be evaluated based on their current and future ability to 

support marten dispersal, their proximity to extant populations, and the potential 

for connectivity with future marten populations. The HMCG intends that the 

forthcoming Humboldt Marten Habitat Management Guide, in conjunction with 

connectivity modeling, will assist achievement of LCA objectives. The HMCG also 

anticipates that developing a specific connectivity improvement plan may be desirable for all recommended LCAs. The HMCG does not anticipate that the entire area 

of any LCA will be completely dedicated to meeting marten connectivity improvement goals. 
81 

Figure 25—Location of the landscape connectivity area for the Humboldt marten (Martes caurina humboldtensis) in 

California. EPA = extent population area; PRA = population reestablishment area; LCA = landscape connectivity area. 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
82 

Lower Klamath LCA—

The Lower Klamath LCA lies between the North Coastal California EPA to the 

northeast and the Redwood and Prairie Creek PRA to the west (fig. 25). Area statistics for this LCA are presented in table 1. The natural expansion of the marten population into the Redwood and Prairie Creek PRA and the maintenance of gene flow 

will likely require that the Lower Klamath LCA contain adequate suitable habitat 

for both successful dispersal and some level of residency, supporting both survival 

and reproduction. However, unassisted natural dispersal across this LCA appears 

unlikely unless areas of suitable habitat are maintained and enhanced owing to the 

presence of features (e.g., U.S. Highway 101, Klamath River) that will challenge 

successful dispersal. Although dispersal across this LCA may only require roughly 

10 km straight-line distance movements, the Klamath River and U.S. Highway 101 

likely represent filters with unknown levels of resistance for dispersing individuals. In addition, the amount of young regenerating forest present in this LCA will 

likely influence the numbers of marten predators and have an indirect influence on 

marten survival and successful dispersal. Thus, the degree to which the Redwood 

and Prairie Creek PRA will be isolated in the foreseeable future depends on both the 

maintenance and enhancement of habitat connectivity and the number of dispersers 

necessary to overcome the resistance to dispersal posed by the river and highway. 

Potential Threats to the Humboldt Marten 

In this section, the HMCG reviews and evaluates past, current, and future threats 

potentially affecting Humboldt marten populations or their habitat within the 

California portion of the assessment area. Potential threats are defined below. This 

assessment should not be construed as an attempt to assess threats under the federal 

Endangered Species Act for the purposes of determining whether a species should 

be listed as endangered or threatened. The purpose of this threats assessment is to 

evaluate potential threats in order to identify conservation actions that the HMCG 

and land managers within the assessment area can take to ameliorate these threats 

and help achieve the overall goal of this conservation strategy. Currently, the threats 

assessment has only been completed for the California portion of the assessment 

area, and the Oregon portion will be completed at a future date. This approach was 

similar to the conservation assessment processes by NatureServe (Master et al. 

2012) and for two threat assessments for the West Coast Distinct Population Segment (DPS) of the fisher (Naney et al. 2012, USDI FWS 2014). To identify potential 

threats, the HMCG reviewed information in previous sections of this conservation 

assessment and relevant literature for other species and subspecies of martens in 

North America. Because of the similarity of natural history and the range overlap of 

GENERAL TECHNICAL REPORT PSW-GTR-260
83 

 

the fisher and Humboldt marten, we also relied on information from the two recent 

fisher threat assessments mentioned above. 

The HMCG defined a potential threat as any activity or process that has caused, 

is causing, or may cause negative effects on marten populations resulting in: 

• A further decline in populations.

• A further decline in the amount of suitable habitat.

• An impediment to restoration of marten populations or suitable habitat

within the assessment area.

Once a threat is identified, it is evaluated to determine its potential impact

to martens or existing and future suitable habitat conditions in EPAs, PRAs, and 

LCAs. Potential threats are evaluated with respect to both the scope (spatial extent) 

and severity (intensity) of their potential effects. Finally, threats are ranked according to the relative impact level (a combination of scope and severity classifications) 

to help prioritize threats that should be addressed by conservation actions and 

to help guide the use of limited resources for the greatest potential conservation 

returns. 

Classification of Potential Threats

The HMCG identified 29 potential threats within four threat categories: (1) habitat 

loss and degradation, (2) climate change effects on habitat loss, (3) reduction in marten survival and reproduction, and (4) small and disjunct populations (table 2). For 

each potential threat the HMCG identified the timing, scope, severity, and impact 

for EPAs, and combined PRAs and LCAs in the California portion of the assessment area. The HMCG summarized the available scientific information for each 

threat regarding its potential direct (e.g., mortality) and indirect (e.g., habitat effects) 

population impacts (see the “Biophysical Environment,” “Human Modifications to 

the Environment,” and “Biological Information” chapters and app. 2 for summary 

information). When empirical evidence or other information was unavailable for a 

potential threat, we used expert opinion to estimate the scope, severity, and impact 

values. If significant information gaps resulted in significant levels of uncertainty in 

the scope and severity values for particular threats, information needed to fill these 

gaps was identified and listed in the “see Next Steps section on page 94.” 

Timing— 

Timing refers to whether the potential threat occurred primarily in the past (historical), is ongoing (current), or may occur in the short-term future (future). To 

provide a temporal extent for assessing most threats, the HMCG chose three marten 

generations, or 15 years. Marten generation time was calculated as follows: Female 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
Table 2—Timing, scope, and severity values for potential threats within the California portion of the assessment area

a 

Potential threat Timing

ScopeEPA

b 

 

SeverityEPA 

ScopePRA 

b

and

LCA 

b

S

P

L

Habitat loss and degradation: 

Wildfire—

Ongoing Large Serious Restricted S

Ongoing Restricted Serious Small S

Ongoing Restricted Extreme Small E

Proportion of area affected by wildfire(s) 

Emergency fire suppression 

Postfire salvage logging 

Lack of fire as a disturbance process Ongoing Small Moderate Small M

Forest management: 

Management of early-successional forests (≤60 years old)—

Ongoing Small Extreme Small S

Ongoing Restricted Slight Small S

Ongoing Restricted Extreme Small S

Ongoing Small Serious Small S

Overstory removal (logging) 

Thinning (precommercial, commercial, restoration) 

Shrub layer reduction 

Reduction of large residual structures (live trees, logs, snags) 

Reduction of hardwoods Ongoing Small Moderate Small M

Ongoing Small Extreme Small E

Ongoing Small Serious Small M

Ongoing Small Extreme Small S

Ongoing Small Extreme Small S

Ongoing Small Moderate Small M

Ongoing Small Extreme Small E

Ongoing Small Extreme Small E

Management of mid-successional forests (60 to 120 years old)—

Human development (urbanization, road construction, recreation, agriculture, 

large reservoirs) 

Cumulative effects of habitat loss and degradation leading to large-scale 

fragmentation of suitable habitat

Ongoing Pervasive Serious Large S

Short term Large Slight Large M

Long term Pervasive Moderate Pervasive S

Climate change effects on habitat loss—

Reduction in marten survival and reproduction—

Ongoing Small Moderate Small M

Legal trapping

c and incidental capture during legal trapping 

Research activities (live-trapping, radio-collaring) Ongoing Small Slight Small S

Overstory removal (logging) 

Thinning (commercial, restoration) 

Shrub layer reduction 

Reduction of large structures (live trees, logs, snags) 

Reduction of hardwoods 

Removal of late-successional forest (>120 to 150 years old) 

Climate change: mid-21st century (2040–2060) 

Climate change: late 21st century (2080–2100)

84
Potential threat Timing

ScopeEPA

b

SeverityEPA 

Scope- S

P

L

PRA and

LCA

b

b

Collision with vehicles Ongoing Small Extreme Small E

Exposure to toxicants (primarily rodenticides from illegal marijuana grows) 

Lethal diseases 

Human-influenced predation (habitat modification favoring marten predators) Ongoing Large Extreme Moderate E

Ongoing Serious Restricted Restricted S

Ongoing Restricted Serious Restricted S

Small and disjunct population threatsFuture Large Moderate NA N

Inbreeding depression 

Demographic stochasticity ( environmental variation that affects survival and 

reproduction) 

Future Restricted Moderate NA N

Table 2—Timing, scope, and severity values for potential threats within the California portion of the assessment area

a

(co

a When information to make an accurate estimate of the scope or severity of a particular threat was lacking, values were estimated using expert opinion. For details on the ra

for each potential threat's classification, see appendix 2. Forest age class-seral stage breaks were based on age-seral relationships from Jimerson et al. (1996) and Sawyer et 

(2000b) 

b EPA = extant population area; PRA = population re-establishment area; LCA = landscape connectivity area. 

c For California, legal trapping refers to the legal trapping for other species using traps that may also capture martens, and trapping on tribal lands that may include 

martens or use of traps capable of capturing martens. 

85 
86 

GENERAL TECHNICAL REPORT PSW-GTR-260

martens typically first give birth to young at 2 years of age (Mead 1994). Buskirk et 

al. (2012) reviewed published studies on marten survival across North America and 

estimated that adult (>2 years old) annual survival was 0.75 for all adult ages. Using 

these numbers for age at first reproduction (α) and adult survival (s), marten generation time (T) is calculated as T = α + [s/(1–s)] or T = 2 + [0.75/(1–0.75)] = 2 + 3. 

Thus marten generation time (T) = 5 years, and three generations equals 15 years. 

For assessing potential threats related to climate change, we selected two temporal 

extents, mid-21st century (2040–2060) and late 21st century (2080–2100), to match 

the temporal extents of existing climate model predictions. 

Scope— 

The scope of a potential threat refers to the estimated proportion of each EPA or 

the combined PRAs and LCAs that reasonably can be expected to be affected by 

a potential threat within 15 years (except for the future potential threats of climate 

change, which is evaluated at longer time frames, as noted above). Scope categories 

include: 

Pervasive: 71 to 100 percent—affects all or most of the suitable habitat or 

individuals. 

Large: 31 to 70 percent—affects much of the suitable habitat or individuals. 

Restricted: 11 to 30 percent—affects some of the suitable habitat or individuals. 

Small: 1 to 10 percent—affects a small proportion of the suitable habitat or 

individuals. 

Severity— 

The severity of a potential threat is the estimated level of impact to habitat or individual martens that reasonably can be expected from the potential threat within 15 

years (except for potential threats of climate change at longer timeframes, as noted 

above). Importantly, severity is nested within the scope of the potential threat, such 

that if the scope estimation is that a potential threat may have an impact on 11 to 30 

percent of the suitable habitat or individuals, severity describes the degree of impact 

that may occur to the 11 to 30 percent of suitable habitat or individuals. Severity is 

estimated by the degree of decline it may cause to martens or their habitat using the 

following categories: 

Extreme: 71 to 100 percent—likely to have an impact on all or most of the suitable habitat or individuals within the scope defined. 

Serious: 31 to 70 percent—likely to have an impact on much of the suitable 

habitat or individuals within the scope defined. 
87 

Moderate: 11 to 30 percent—likely to have an impact on some of the suitable 

habitat or individuals within the scope defined. 

Slight: 1 to 10 percent—likely to have an impact on a small proportion of the 

suitable habitat or individuals within the scope defined. 

Both the scope and severity classifications incorporate uncertainty by designating a range within each category that best fits the current knowledge base for each 

potential threat. The specific rationale and supporting information for assigning 

scope and severity classifications for each potential threat are included in appendix 2. 

Here is an example of scope and severity estimation: within the Wildfire 

potential threat category for the California EPAs (table 2; app. 2), to evaluate the 

scope (“proportion of area affected by wildfire[s]”), we examined the size of recent 

(i.e., from 2000 to 2014, the past 15 years) wildfires within the vicinity of the EPAs 

and determined that a single large wildfire could realistically affect 31 to 70 percent 

(thus, scope = “large”) of the suitable Humboldt marten habitat within the California EPAs over the next 15 years. For the severity estimate, the HMCG determined 

that even a low-severity burn (most large wildfires contain areas with high and low 

burn severity) would likely cause a short-term loss or degradation of suitable habitat 

by removing the extensive shrub layer typically required by the Humboldt marten. 

The HMCG estimated that within the scope of the suitable habitat in the California 

EPAs that could be affected by wildfire over the next 15 years, 31 to 70 percent 

(severity = “serious”) of that suitable habitat could be lost or degraded, because 

wildfires of any severity could result in habitat loss or degradation. 

In another example, within the “reduction in marten survival and reproduction” 

potential threat category, to evaluate the scope for the “collision with vehicles” 

subcategory we presumed that greater vehicle frequency and speed would increase 

the probability of a vehicle striking a marten. But the HMCG determined that only 

small portions (1 to 10 percent, thus scope = “small”) of the California EPAs, PRAs, 

and LCAs contained highways with relatively heavy traffic traveling at relatively 

high speed (i.e., ≥45 mph [72 kph]). For the severity estimate, the HMCG presumed 

that all martens struck by a vehicle would sustain mortal injuries, so we estimated 

that within the limits of the scope, 71 to 100 percent (severity = “extreme”) of all 

martens struck by a vehicle would be killed. 

Degree of Potential Threat Impacts 

The HMCG calculated the impact level for each potential threat by combining the 

scope and severity scores from table 2 into a calculation matrix (table 3) per Salafsky et al. (2008). The combinations of the scope and severity classification ranges 

defined the threat impact level. Timing was not applied in the calculation of the 

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
88 

Table 3—Calculation matrix for potential threat impact level 

Scope 

Pervasive Large Restricted Small 

Severity (71–100 percent) (31–70 percent) (11–30 percent) (≤10 percent)

Extreme: 

(71 to 100 percent) Very high High Medium Low 

Serious: 

(31 to 70 percent) High High Medium Low 

Moderate: 

(11 to 30 percent) Medium Medium Low Low 

Slight: 

(≤10 percent) Low Low Low Low 

The three potential 

threats with the 

highest impact values 

were large-scale 

fragmentation from 

cumulative effects of 

past timber harvest, 

wildfire, and lethal 

disease. 

threats impact level, but was included in the summary assessment. Of the 11 threats 

evaluated, none were rated as “very high” impact, three were rated as “high,” and 

the remainder were rated as “medium” or “low” (table 4). The three potential threats 

with the highest impact values were large-scale fragmentation from cumulative 

effects from past timber harvest, wildfire, and lethal disease. Threats with medium 

or high-impact levels were ranked according to the overall impact of the category 

(table 5). 

Conservation Actions 

For each potential threat, the HMCG identified the conservation actions that may 

reduce or eliminate the potential impacts (table 5). Potential threats were ranked 

according to their impact level from highest to lowest. 

Integration of Conservation Actions and Conservation 

Emphasis Areas 

The conservation emphasis area categories (EPA, PRA, and LCA) identify specific 

areas where protection of existing suitable habitat, restoration of suitable habitat 

conditions, or reduction of potential threats are designed to promote marten persistence and facilitate the increase in population size and distribution. Conservation 

actions were designed to ameliorate threats; however, their impacts will not be 

equal in all conservation emphasis areas. 

High-Priority Conservation Actions 

Table 5 lists conservation actions that address impacts of all threats with moderate 

to high impact levels. However, these actions may not be applicable in all settings, 

GENERAL TECHNICAL REPORT PSW-GTR-260
89 

Table 4—Impact levels for all threat categories evaluated 

Impact level 

Number 

of threat 

categories Threat categories included 

Very high 0 None 

High 3 – Large-scale fragmentation from cumulative effects of

past timber harvest and severe wildfire

– Wildfire

– Lethal disease

Medium 8 – Shrub layer reduction

– Fire suppression

– Postfire salvage

– Exposure to toxicants

– Human-influenced predation

– Climate change: late 21st century

– Climate change: mid-21st century

– Inbreeding depression

Low 18 – Loss of late-successional forest

– Lack of fire as a disturbance process

– Overstory removal: early and mid-successional

– Thinning: early and mid-successional

– Reduction of large residual structures: early and

mid-successional

– Reduction of hardwoods: early and mid-successional

– Legal trapping and incidental capture

– Collision with vehicles

– Demographic stochasticity

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
Table 5—Threats impact level, threat rank, and conservation actions for potential threats evaluated for California

EPAs PRAs/LCA

Threat

rank Threat description Impact Timing Impact Timing Possible conservation actions 

1 Habitat loss and degradation: 

1 Cumulative effects of habitat loss and 

degradation leading to largescale fragmentation of suitable 

habitat 

High Past and 

ongoing 

High Past and 

ongoing 

As appropriate, federal, state, tribal, and 

managers could consider the spatial cate

population areas, population reestablishm

connectivity improvement areas in proj

and where appropriate integrate them int

land management plans. Within these sp

management ensure that currently suitab

protected or enhanced and strategic area

currently suitable are managed to enhanc

ity as soon as possible. Conduct populat

to measure the effectiveness of these acti

adaptive management. 

1 Proportion of area affected by 

wildfire( s) 

High Ongoing Medium Ongoing As appropriate, federal, state, tribal, and 

managers could develop fire-response pr

occupied marten habitat areas and develo

tion prescriptions that reduce the risk of w

maintaining the structural elements requ

Humboldt marten. They could increase t

and number of populations to reduce the 

wildfire to significantly affect population 

1 Shrub layer reduction Medium Ongoing Low Ongoing As appropriate, federal, state, tribal, and 

managers could develop timber harvest, 

reduction, reforestation, and prescribed f

that minimize impacts to ericaceous-dom

layers where possible to maintain suitab

marten. Identify stand management alter

could restore or enhance dense ericaceou

shrub cover where it has been lost or sig

reduced. 

90
Table 5—Threats impact level, threat rank, and conservation actions for potential threats evaluated for California

a

(continu

EPAs PRAs/LCA

Threat 

rank Threat description Impact Timing Impact Timing Possible conservation actions 

1 Emergency fire suppression Medium Ongoing Low Ongoing Federal, state, tribal, and private land staf

detailed maps of important marten habita

dent control teams, to enable considerati

ing critical areas in strategic fire suppres

Maintain large snags and logs during em

suppression in late-successional conifero

the greatest extent possible. Reduce wild

through fuels reduction projects and pres

to maintain large trees and logs postfire. 

1 Postfire salvage logging Medium Ongoing Low Ongoing Federal, state, tribal, and private land man

minimize the removal ofstructural elem

for suitable marten habitat including live 

dead trees oflarge diameter. 

2 Reduction in marten survival and reproduction:

2 Lethal diseases High Ongoing Medium Ongoing Managers could do the following: inocula

tens captured during research activities a

diseases such as CDV and rabies, monito

prevalence and outcomes to determine ex

severity of this threat to marten populatio

the distribution and number of populatio

probability of disease to threaten populat

tence, establish field research protocols t

transmission of lethal pathogens between 

other species. 

2 Exposure to toxicants 

(primarily rodenticides from 

marijuana cultivation sites) 

Medium Ongoing Medium Ongoing Conduct aerial surveys to detect and remov

sites in critical marten habitat. Test all de

exposure to toxicants and apply new met

they are developed, for testing live anima

compounds. 

91 
Table 5—Threats impact level, threat rank, and conservation actions for potential threats evaluated for California (continu

EPAs PRAs/LCA

Threat 

rank Threat description Impact Timing Impact Timing Possible conservation actions 

2 Human-influenced predation Medium Ongoing Medium Ongoing Identify how land management activities c

predation rates on martens by (1) affecti

bution and abundance of marten predator

increasing the likelihood of predation wh

are enabled by habitat degradation. Dev

gest management measures in the Habita

Guide that can reduce both the abundanc

predators and their risk of predation. Eva

tential efficacy of short-term removal of 

tors until more favorable habitat conditio

3 Climate change effects on habitat loss: 

3 Climate change: mid-21st century 

(2040–2060) 

Low Future Medium Future Establish marten populations closer to the 

the potential impacts from climate chang

habitat. Enhance connectivity from inter

areas of current habitat suitability. Coord

eral, state, tribal, and private land manag

the conservation emphasis areas: extant p

areas, population reestablishment areas, 

connectivity areas into their land manage

3 Climate change: late 21st century 

(2080–2100) 

Medium Future High Future Same as previous. 

4 Small and disjunct population effects: 

4 Inbreeding depression Medium Ongoing NA NA Determine the extent of loss of genetic div

populations. Ifloss is significant, develo

translocation ofindividuals from extant 

areas to enhance genetic diversity. 

NA= not applicable. 

92
93 

and may be modified to achieve desired outcomes for martens. The HMCG recognizes that project areas often have unique circumstances and environmental conditions; therefore, design and evaluation of management actions will likely be best 

accomplished on a case-by-case basis. 

Here the HMCG identifies specific conservation actions that could mitigate the 

threats with the highest impact levels. 

California— 

• Strategic habitat management and restoration: Four (40 percent) of the

threats with the greatest impacts on the Humboldt marten (existing largescale fragmentation, wildfire, shrub layer reduction, and human-influenced

predation) could be addressed through the maintenance and restoration of

suitable habitat. Maintenance of suitable habitat is most important where

it exists in EPAs, PRAs, and LCAs. Restoration of suitable habitat is most

needed to counteract the large-scale fragmentation of suitable habitat from

past actions. Restoration would increase the overall amount, patch sizes,

and connectivity of suitable habitat across all conservation emphasis areas.

Young stands can take several decades to grow beyond the stages that support key marten predators and to then develop suitable characteristics for

marten use. Thus, conservation actions that would speed attainment of better stand conditions in key areas are high-priority conservation actions.

• Evaluation of population re-establishment through assisted dispersal:

Three (30 percent) of the threats with the greatest impacts on the Humboldt

marten (existing large-scale fragmentation, wildfire, and lethal disease)

could be addressed by increasing the distribution and number of marten

populations. Even if initiated promptly, habitat management actions may

take decades to produce conditions supporting functional connectivity and

colonization of presently unoccupied areas. Therefore, the HMCG recommends immediate evaluation of the feasibility of translocation of individuals to unoccupied suitable areas in PRAs. Obviously, identification of

donor areas for translocation must consider impacts to extant populations.

However, if areas can be identified that are presently acting as population

sinks, where individuals disperse but contribute little to population growth

or persistence, these areas have the potential for the removal of individuals

with minimal impacts to donor populations.

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
94 

• Monitor populations to detect: (1) changes in distributions and abundance over time, (2) responses to restoration and management actions, (3)

responses to natural disturbances (e.g., wildfire) and disease outbreaks.

This conservation action could improve the understanding of the scope and

severity of threats and provide timely information on the results of restoration and management actions.

Oregon— 

Additional information is needed to identify specific conservation actions that could 

address threats to martens in the Oregon portion of the assessment area; thus, the 

HMCG is not listing any options at this time. In the interim, parties interested in 

proactive management can look to the recommendations for the California portion 

of the assessment area. Many of the important habitat management and restoration 

needs for Humboldt martens will likely be applicable rangewide. 

Next Steps and Key Information Needs for Further 

Development of the Conservation Strategy 

Throughout the strategy development process, the HMCG identified key information gaps and tools necessary for evaluation and implementation of conservation 

actions as well as the development of future conservation strategy modules. These 

include the following (not in order of importance): 

Steps for inventorying, monitoring, and managing populations— 

• Determine and monitor the abundance and distribution of martens in each

EPA to provide baseline information and trends through time. Determine

the number and distribution of martens in the California-Oregon Border

EPA, Prairie Creek Redwoods State Park, and both Oregon EPAs. Conduct

surveys in the largest areas of suitable habitat in all PRAs around EPAs

to identify any additional areas that support martens. Use survey methods

capable of detecting trends in distribution and abundance and of evaluating

the effects of natural disturbance events and management and conservation

actions over time. Include survey design considerations for a survey protocol suitable for long-term monitoring of multiple forest carnivore species’

distribution and population status.

• In Oregon, develop approaches to assess the effects of legal trapping,

roadkill mortality, and other anthropogenic sources of mortality on marten population size, growth rate, and distribution. Develop and distribute

a protocol and collection forms for salvaging marten roadkill in Oregon

GENERAL TECHNICAL REPORT PSW-GTR-260
95 

and California. Identify and implement measures to reduce anthropogenic 

mortality sources: (1) evaluate and implement alternative measures for 

improving the permeability of U.S. Highway 101, and other highways with 

the potential to impact martens, to successful marten crossings in areas 

of Oregon and California where it overlaps populations currently or areas 

where population expansion is planned, (2) close legal trapping season for 

Humboldt martens or restrict trapping for other species in the occupied 

marten range where martens may be captured as “bycatch” until population 

sizes are large enough to sustain mortality from trapping, and (3) evaluate 

risk-reward for new research projects that use capture methods that may 

result in marten mortalities and encourage use of noninvasive methods 

when possible. 

• Determine scale and magnitude of impacts from legal and illegal rodenticides used in both control of mountain beavers (Aplodontia rufa) and marijuana cultivation. If necessary, develop regulatory approaches to address

this mortality factor. Encourage the development of methods to assay toxicant levels in live animals.

• Determine whether extant populations have lost significant genetic diversity or are at risk for inbreeding that could lead to reduced fitness.

• Determine the short- and long-term responses of marten distribution and

demographics, important marten prey, and marten predators to wildfire,

including the effects of variation in the spatial pattern and intensity of

wildfire.

• Develop predictive tools to assess impacts of wildfire on marten habitat to

model future fire scenarios and provide decision support for fire management planning. Identify management actions that could alleviate negative

short- or long-term effects to habitat.

Steps for evaluating scale-specific habitat suitability, predicting habitat 

responses to management actions and natural disturbance— 

• Determine home range size and the habitat composition and patch size

characteristics that most influence marten occupancy, survival, and reproduction at the home range scale across the three main habitat types they

occupy. 

• Develop a Humboldt marten habitat management guide capable of identifying the habitat characteristics most important for supporting marten

occupancy, reproduction, and survival at the stand and home range scale.

Design this guide to help managers identify alternative actions most likely

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
96 

GENERAL TECHNICAL REPORT PSW-GTR-260

to support the maintenance or improvement of suitable habitat conditions 

for martens. 

• Conduct experimental studies to monitor short- and long-term effects

of alternative silvicultural and fuels management approaches on important marten habitat structures and on key prey and predators. Identify the

habitat conditions capable of supporting functional connectivity. Develop

models to identify the specific areas most likely to support marten movement, dispersal, and occupancy between EPAs and PRAs. Develop specific

management approaches to enhance habitat conditions where movement is

predicted to occur between EPAs and PRAs.

• Expand knowledge of the prey and habitat relationships of key marten

predators, such as the bobcat. Determine the distributional response of key

marten predators to various forest management practices at the landscape

scale in the California and Oregon EPAs. Determine the diet and relationship to specific forest management methods for key marten predators.

Identify stand- and landscape-scale management alternatives to reduce the

abundance and distribution of key marten predators, such as the bobcat,

where important.

• Evaluate the accuracy of the marten landscape habitat suitability model

outside the initial development area and make revisions as necessary, using

recently collected detection data to better inform the model. Use recent survey data from the shore pine and shore pine transitional forests to improve

the model’s performance in this habitat type. Determine the appropriate

application of the model and predictions based on its use. Evaluate the efficacy of modeling important marten predator distribution for improving the

model’s accuracy rangewide.

• Determine within-stand characteristics that influence marten selection

for foraging, resting, and denning. Identify stand management approaches

capable of maintaining key marten habitat elements for these behaviors.

Develop consistent methods of vegetation sampling to help facilitate the

comparison of results across sites and studies.

• Determine short- and long-term population responses of martens, important

marten prey, and marten predators to different spatial patterns and intensities of wildfire. Identify management actions that could alleviate negative

short- or long-term effects to habitat.
97 

• Determine marten dispersal characteristics, including distance, timing, and

factors contributing to successful dispersal. Determine the characteristics

of functional connectivity capable of supporting successful marten dispersal, and identify management alternatives capable of maintaining and

improving dispersal where necessary.

Steps for evaluating and implementing methods for population re-establishment— 

• Conduct a feasibility assessment for each PRA to determine (1) amount of

reproductive habitat available, (2) amount of suitable habitat available, (3)

prey resources available, (4) risk of predation, (5) potential distribution and

carrying capacity, and (6) high-priority areas for restoration or improvement of suitable habitat conditions.

• Evaluate the desirability and feasibility of assisted dispersal for martens

from the North Coastal California EPA. If assisted dispersal is feasible,

develop an implementation plan that includes identification of regulatory,

planning, and funding steps necessary for implementation.

• Determine the conditions under which captive breeding may be necessary.

Identify quantifiable measures for when populations may qualify for this

option and identify a schedule of actions needed to initialize this action.

Acknowledgments 

This conservation assessment and strategy would not have been possible without 

the hard work and contributions of the members of the Humboldt Marten Conservation Group. Prominent among those was Dr. Lowell V. Diller, who passed away on 

March 4, 2017. As a member of the Conservation Group, Lowell made significant 

contributions through his knowledge of wildlife ecology and forest management, as 

well as his creative thinking, optimism, and old-fashioned common sense. 

U.S. Equivalents

When you know: Multiply by: To get: 

Centimeters (cm) 0.394 Inches 

Meters (m) 3.28 Feet 

Kilometers (km) 0.621 Miles 

Hectares (ha) 2.47 Acres 

Degrees Celsius (°C) 1.8 °C + 32 Degrees Fahrenheit

A Conservation Assessment and Strategy for the Humboldt Marten (Martes caurina humboldtensis) in California and Oregon
98 

GENERAL TECHNICAL REPORT PSW-GTR-260

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Appendix 1: Humboldt Marten Conservation 

Group 

Executive Group 

Richard Callas, California co-chair, California Department of Fish and Wildlife 

Phil Detrich, project coordinator, PJD Environmental Consulting 

Gregory Schmidt, California co-chair, U.S. Department of the Interior, Fish and 

Wildlife Service, Arcata Field Office 

Keith Slauson, science coordinator, U.S. Department of Agriculture, Forest 

Service, Pacific Southwest Research Station 

Russ Stauff, Oregon co-chair, Oregon Department of Fish and Wildlife 

James Thrailkill, Oregon co-chair, U.S. Department of the Interior, Fish and 

Wildlife Service, Roseburg Field Office 

William Zielinski, science advisor, U.S. Department of Agriculture Forest Service, 

Pacific Southwest Research Station 

Participants 

California State Parks, Northcoast Redwoods District: Amber Transou, Jay Harris 

Confederated Tribes of the Siletz Indians of Oregon: Jeanne Spaur 

Green Diamond Resource Company: Keith Hamm, Desiree Early 

Hancock Forest Management: Tim McBride 

Lowell Diller Consulting: Lowell Diller 

National Council for Air and Stream Improvement: Jake Verschuyl 

National Park Service, Redwood National Park: Dave Roemer, Kristin Schmidt 

Oregon Department of Fish and Wildlife: Rod Krahmer 

Oregon Department of Forestry: Nick Palozzotto, Randy Smith, Jennifer Weikel 

Oregon Parks and Recreation Department: Vanessa Blackstone 

Plum Creek Timber: Jeff Light, Ron Stiener 

Roseburg Forest Products: Rich Klug 

Save the Redwoods League: Richard Campbell 

U.S. Department of the Interior, Bureau of Land Management: Scott Hopkins, 

Rex McGraw, Randall Miller, Roy Price, Robin Snider 

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U.S. Department of the Interior, Fish and Wildlife Service, Arcata Field Office: 

Bruce Bingham, Kathleen Brubaker 

U.S. Department of the Interior, Fish and Wildlife Service, Oregon Fish and 

Wildlife Office: Sue Livingston 

U.S. Department of Agriculture, Forest Service, Pacific Northwest Research 

Station: Katie Moriarty 

U.S. Department of Agriculture, Forest Service, Siskiyou National Forest: 

Dave Clayton 

U.S. Department of Agriculture, Forest Service, Six Rivers National Forest: 

Brenda Devlin-Craig 

U.S. Department of Agriculture, Forest Service, Siuslaw National Forest: 

Deanna Williams 

Weyerhaeuser Company: Matt Hane, Mike Rochelle 

Yurok Tribe: Kent Barnes, Jim Erler, Chris West 

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Appendix 2: Summary of threats to the Humboldt marten, 

rationale for scope and severity rankings in California, 

and supplemental information used for their evaluation 

Potential threat 

Habitat loss and degradation: 

Wildfire—

Proportion of area affected by wildfire(s)—

Scope: Recent fires have been large enough that a single fire in the extant population area (EPA) 

could burn 31 to 70 percent of the area, and one or more fires in the population reestablishment areas/landscape connectivity area (PRAs/LCA) could realistically affect 11 to 

30 percent of the habitat over the next 15 years. 

Severity: Low-intensity fires have the potential to degrade marten habitat over the short term 

through the reduction of shrub cover. High-severity fires or the proportions of fires with 

high severity have the potential to cause long-term habitat degradation by causing high 

or complete overstory mortality in suitable habitat. All fires have the potential to cause at 

least short-term habitat degradation. 

Emergency fire suppression—

Scope: Decisions made during efforts to contain wildfire may result in further loss or degradation 

of habitat, for example, through back burning and containment line creation. Based on 

the sizes of recent fires and areas included in back burning suppression efforts, 11 to 30 

percent of the EPAs and 1 to 10 percent of the PRAs and LCAs could be potentially affected over the next 15 years by suppression efforts. Wildfires burned approximately

17 percent of the moderate- and high-suitability Humboldt marten (Martes caurina 

humboldtensis) habitat within the North Coastal California EPA between 2000 and 2014 

(i.e., the previous 15 years), with a single large fire (the Blue 2 Fire in 2008) (MTBS 

2015) responsible for nearly all burned suitable habitat. No large fires burned within the 

California-Oregon Border EPA during the past 15 years (i.e., 2000–2014) (MTBS 2015); 

however, a single large fire in 1987 (the Longwood Fire) (MTBS 2015) burned approximately 12 percent of the moderate- and high-suitability Humboldt marten habitat within 

the California-Oregon border EPA. 

Severity: The severity ranks mirror those of wildfire owing to their similar effects on habitat.

Postfire salvage logging—

Scope: There is some uncertainty in the potential scope for this threat, owing to variable use of 

postfire salvage following recent fires. We chose the scope value based on the potential 

amount of area that could be affected if salvage logging were to match the areal extent 

of salvage in the Biscuit Fire area. Note that under the California Forest Practices Rules, 

“substantially damaged” private lands can be completely salvaged after wildfire; the 

extent to which this might occur in the assessment area is unknown. 

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Potential threat 

Severity: The removal of large-diameter dead or declining trees represents a high degree of habitat 

degradation. 

Lack of fire as a disturbance process—

Scope and severity: In most of the EPAs, PRAs, and LCAs that are exposed to summer fog, fire has not been 

excluded long enough to have resulted in any significant losses of the functional role fire 

plays in creating key marten habitat elements such as snag recruitment. In more inland 

areas outside the influence of summer fog, one or more fire-return intervals may have 

been missed, resulting in the accumulation of fuels that may increase the risk of fires 

with higher intensity and larger area. 

Forest management: 

Management of early-successional forests (≤60 years old)—

Overstory removal (logging): 

Scope: Harvest of early-successional stands is likely to only occur in a small proportion of the 

EPAs, PRAs, and LCA over the next 15 years. 

Severity: While early-successional forests are not considered suitable marten habitat, harvesting 

early successional stands using even-age management increases the length of time before 

suitable habitat can be recruited. Where this occurs adjacent to currently suitable habitat 

or in areas of important connectivity, the action degrades the adjacent suitable habitat 

and areas of habitat connectivity. 

Thinning (precommercial, commercial, restoration)— 

Scope: Thinning is likely to occur in many areas of young forest in both the EPAs and PRAs, and 

LCA over the next 15 years. 

Severity: Most thinning of young dense forest stands is likely beneficial for marten habitat recruitment and thus has the lowest severity rank. 

Shruba

 layer reduction— 

Scope: Shrub layer reduction, for either fuels reduction or for reducing competition with conifer 

regeneration, is likely to occur in limited areas of the EPAs, PRAs, and LCAs over the 

next 15 years. 

Severity: The loss or reduction of the shrub layer is a significant degradation of habitat.

Reduction of large residual structures (live trees, logs, snags)— 

Scope: The harvest of residual large live conifers, through selection harvest, is likely to occur in a 

small proportion of the EPAs, PRAs, and LCA over the next 15 years. 

Severity: Removal of large-diameter structures is a significant degradation of habitat.

Reduction of hardwoods— 

Scope: Activities that remove hardwoods to reduce competition with conifers are likely to occur 

in a small proportion of the EPAs, PRAs, and LCA. 

Severity: Hardwoods, specifically tanoak (Notholithocarpus densiflorus) Hook &Arn. P.S. Manos 

C.H. Cannon, & S.H. oh and golden chinquapin, (Chrysolepis chrysophylla) are important species for providing marten den and rest sites as well as mast for prey species. The

reduction of hardwoods below their natural levels of abundance, based on site conditions,

represents a degradation of habitat.

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Potential threat 

Management of mid-successional forests (60 to 120 years old)— 

Overstory removal (logging)— 

Scope: Harvest of mid-successional forests is likely to occur in a small proportion of the EPAs, 

PRAs, and LCA over the next 15 years. 

Severity: Where it occurs adjacent to older stands or in important areas of habitat connectivity, midsuccessional habitat can be used by martens. This successional stage also is important for 

future recruitment into highly suitable late-successional habitat. Loss of mid-successional forest in key areas of the EPA, PRAs, and LCA represents significant habitat degradation. 

Thinning (commercial, restoration)— 

Scope: Thinning is likely to occur in a small proportion of the EPAs, PRAs, and LCA over the 

next 15 years. 

Severity: The severity of the impact of thinning depends on the extent, the proportion of overstory 

removed, the size of trees removed, and the resulting stand structure. Restoration thinning prescriptions that reduce unnaturally dense stands by removing smaller diameter 

trees and create more stand heterogeneity are likely to be beneficial. However, we consider here the impacts of commercial thinning prescriptions, where the largest diameter 

trees are harvested, uniform tree spacing is created, resulting in a homogeneous stand 

structure. Where these outcomes result in association with suitable habitat or in areas 

important for connectivity, they may represent moderate to serious habitat degradation. 

Shruba

 layer reduction— 

Scope: Shrub layer reduction is likely to occur in a small proportion of the EPAs, PRAs, and LCA 

over the next 15 years. 

Severity: The loss or reduction of the mesic ericaceous shrub layer represents a significant degradation of habitat. 

Reduction of large structures (live trees, logs, snags)— 

Scope: Reduction of large structures in mid-successional forest is likely to occur in a small proportion of the EPAs, PRAs, and LCA over the next 15 years. 

Severity: Removal of large-diameter structures in mid-successional forest would result in a significant degradation of habitat. 

Reduction of hardwoods— 

Scope: Reduction of hardwoods is likely to occur in a small proportion of the EPAs, PRAs, and 

LCA over the next 15 years. 

Severity: Hardwoods, specifically tanoak and chinquapin, are important species for providing den 

and rest sites as well as mast for prey species. The reduction of hardwoods below their 

natural levels of abundance represents a degradation of habitat. 

Removal of late successional forest (>120 to 150 years old): 

Scope: Logging of late-successional forest is uncommon in most areas of the EPAs, PRAs, or 

LCA. 

Severity: Where logging of late-successional forest occurs, it results in either habitat loss or a high 

level of degradation, depending on the harvest method used. 

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Potential threat 

Human development (urbanization, road construction, recreation, agriculture, large reservoirs)— 

Scope: Human development is likely to occur in a small proportion of the EPAs, PRAs, and LCA 

over the next 15 years. 

Severity: Human development typically results in the direct loss or significant modification of 

habitat. 

Cumulative effects of habitat loss and degradation leading to large-scale fragmentation of suitable habitat—

Scope: The cumulative effects of habitat loss and degradation leading to large-scale fragmentation 

of suitable habitat are likely to continue to affect the majority of the EPAs, and a high 

proportion of suitable habitat in the PRAs and LCA over the next 15 years. We consider 

both past effects and those likely to occur over the next 15 years to be contributors to the 

scope of this threat. 

Severity: The combination of past and future cumulative effects of habitat loss and degradation 

leading to large-scale fragmentation of suitable habitat is likely to result in the continued 

degraded state or increase landscape-scale habitat degradation over the next 15 years. 

Climate change effects on habitat loss:

Climate change: mid-21st century (2040–2060)— 

Scope: Climate change under short-term projections is likely to affect the more inland portions of 

the EPAs, PRAs, and LCA. 

Severity: The potential short-term impacts of climate change may lead to slight changes that include 

increases in temperatures and reductions in moisture, including both rainfall and fog that 

may directly or indirectly contribute to the slight degradation of habitat. 

Climate change: late 21st century (2080–2100)— 

Scope: Over the longer term, climate change is likely to affect the majority of EPAs, PRAs, and 

LCA. 

Severity: Longer-term impacts are likely to be more severe in the inland areas, including much of 

the PRAs, and to be less significant contributors to habitat degradation in the EPAs and 

LCA. 

Reduction in marten survival and reproduction— 

Incidental capture during legal trapping of fur bearers— 

Scope: Martens are not legally trapped in California and few trappers pursuing legally trapped 

species are expected to be active in the EPAs, PRAs, or LCA over the next 15 years, 

based on recent trapping activity trends (R. Callas, California Department of Fish and 

Wildlife, pers. comm.). 

Severity: Incidental capture by trappers using live traps is not likely to cause mortality but may 

involve minor injury. 

Research activities (live trapping, radio-collaring)— 

Scope: Research activities are not likely to be conducted in more than 10 percent of the population over the next 15 years. 

GENERAL TECHNICAL REPORT PSW-GTR-260
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Potential threat 

Severity: While the potential for injury or mortality occurs during live capture, chemical immobilization, and radio-collaring, mortalities or significant injuries attributed to recent research 

activities for the Humboldt marten or elsewhere in California have been rare. 

Collision with vehicles: 

Scope: Small portions of the EPAs, PRAs, and LCA include roads with frequent high-speed traffic.

Severity: Collision with vehicles is likely to result in mortality. 

Exposure to toxicants (primarily rodenticides from illegal marijuana cultivation sites): 

Scope: There is some uncertainty as to the extent to which the EPAs, PRAs, and LCA are affected 

by illegal marijuana cultivation where toxicants are used. Based on known locations of illegal marijuana cultivation, we consider some portion of each area to be affected. Law enforcement personnel within northern coastal California estimated that they find a relatively 

low percentage (approximately 15 percent) of illegal marijuana cultivation sites. Further, 

law enforcement is only able to remediate (i.e., cleaning up toxicants at discovered sites) 

at a fraction of the illegal cultivation sites that are found. To our knowledge, there have 

been no systematic surveys of the distribution or abundance of illegal marijuana cultivation sites within the two EPAs in northern coastal California. The “restricted” scope 

(11–30 percent of a population or suitable habitat affected) assigned to this potential threat 

reflects our assumption that the relatively remote location of the two California EPAs and 

the dense shrub layer found in suitable Humboldt marten habitat would lower the suitability of these areas for marijuana cultivation. 

Severity: Exposure to toxicants is serious and likely involves direct mortality or indirect mortality 

by compromising exposed animal’s immune system or motor function for most individuals 

exposed. To date, seven dead Humboldt martens have been tested for rodenticide exposure 

(all from the North Coastal California EPAs), one of which tested positive with a sublethal 

concentration (K. Slauson, unpublished data). That marten was killed by a bobcat; whether 

the sublethal dose of rodenticide predisposed that marten to predation is unknown. 

Lethal diseases: 

Scope: Lethal diseases, both natural and human influenced, have the potential to affect most of the 

areas of the EPAs and smaller portions of the PRAs and LCA over the next 15 years. 

Severity: Lethal diseases have serious impacts, typically resulting in mortality for most individuals. 

Human-influenced predation (i.e., habitat modification favoring marten predators):

Scope: Predation has emerged as a likely limiting factor for marten population growth and expansion. Marten predators such as bobcats primarily prey on species closely associated with 

early-seral habitat conditions. Even-aged forest management creates and can perpetuate 

the presence of early-seral habitat that supports marten predators. We consider both the 

temporary effects of existing early-seral habitat and the potential for creation of additional 

early-seral habitat in this threat. Human-influenced predation is likely to impact some portions of the EPAs, PRAs, and LCA over the next 15 years. 

Severity: Greater distribution and abundance of marten predators increases predation risk and reduction in survivorship, and is a serious impact where it occurs. 

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Potential threat 

Threats related to small and disjunct populations 

Inbreeding depression: 

Scope: It is uncertain whether inbreeding depression is occurring within the North Coastal California EPAs. However, the small population size and apparent period of isolation from 

other populations provide the conditions for the significant loss of genetic diversity. If inbreeding depression is occurring, it is likely to affect most of the individuals in the EPAs. 

Severity: Due to the uncertainty in the degree of genetic diversity that may have been lost, we estimate that while most individuals may have reduced genetic variability, deleterious effects 

from loss of genetic diversity are only manifesting in some, but not all, individuals in the 

EPAs. 

Demographic stochasticity (i.e., environmental variation affecting survival and reproduction):

Scope: The threat is an inherent risk to small populations with the potential to affect some individuals over the next 15 years. 

Severity: Short-term stochasticity is only likely to affect individuals for a portion of their lives, and 

thus for individuals has a moderate severity. 

a

 Refers to the dense mesic shrub layer required by the Humboldt marten. 

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