Humboldt Marten Review
Humboldt Marten Review illustrates an example of the complexity and delicate balance that the legal cannabis industry faces with the environment in northern California. This status review report contains the most current information available on the Humboldt marten (Martes caurina humboldtensis) and serves as the basis for the California Department of Fish and Wildlife’s (Department) recommendation to the California Fish and Game Commission (Commission) on whether to list the species as threatened or endangered under the California Endangered Species Act. The Environmental Protection Information Center and the Center for Biological Diversity, as joint petitioners, submitted a “Petition to List the Humboldt Marten (Martes caurina humboldtensis) as an Endangered Species under the California Endangered Species Act” (Petition) to the Commission on June 8, 2015. At its scheduled public meeting on February 11, 2016, the Commission considered the Petition and based in part on the Department’s petition evaluation and recommendation found that sufficient information existed to indicate the petitioned action may be warranted and accepted the Petition for consideration.
Upon publication of the Commission’s notice of its findings, the Humboldt marten has designated a candidate species on February 26, 2016. Humboldt martens are currently designated a California Species of Special Concern, a non‐regulatory designation intended to focus attention on animals at conservation risk, stimulate research on poorly known species, and achieve conservation and recovery of these animals before they meet criteria for listing as threatened or endangered under the California Endangered Species Act (CESA; Fish & G. Code, § 2050 et seq.). Additionally, Humboldt martens throughout their range in California and Oregon are currently under review for listing as threatened or endangered under the federal Endangered Species Act by the U.S. Fish and Wildlife Service. Species Description, Biology, and Ecology ‐ Martens have yellowish to dark brown fur with a contrasting lighter chest patch, the long, sleek body from typical of members of the mustelid (weasel) family, a relatively long bushy tail, and typically weigh 0.4‐1.25 kg (0.88‐2.76 lbs.).
Humboldt martens in California have subtle physiological differences from Sierra martens (M. caurina sierra) which also occur in California. Within California, Humboldt martens historically occupied near‐coastal forests from Sonoma County north to the Oregon border; however, the current distribution within the state is limited to two small areas of Del Norte, northern Humboldt, and western Siskiyou counties, a small fraction of the historical range. Humboldt martens breed once per year and females typically first give birth at two years of age and reach peak productivity from three to five years of age, although not all females attempt to breed each year. Kits are born in natal dens where they remain completely dependent on the mother for seven to eight weeks, after which the mother typically moves them to one or a series of maternal dens until the kits disperse, typically in late summer.
Dispersal distances of Humboldt martens are largely unknown but likely similar to distances of other North American martens, which typically average less than 15 km (9.3 mi.). Available information suggests that home ranges of Humboldt martens are similar to Sierra marten home range sizes in California, 70 – 733 ha (173 – 1,811 ac.). In California, Humboldt martens subsist on a diet composed primarily of small mammals (squirrels, chipmunks, and voles) and birds, and to lesser degree reptiles, fruits, and insects. Known predators of martens in North America include bobcats, coyotes, foxes, fishers, and great‐horned owls, with bobcats being the primary predator of Humboldt martens in California.
Humboldt martens in California are associated with two distinct habitat types: late‐successional coastal redwood, Douglas‐fir, and mixed conifer forests with dense mature shrub layers; and serpentine habitats with variable tree cover, dense shrub cover, and rock piles and outcrops. Consistent among the two habitat types is the requirement for structures for denning, resting, and escape cover. In late-successional forests, structures used include tree cavities, defects, snags, and logs; while in serpentine habitats rock piles and outcrops are commonly used in addition to tree structures. Humboldt martens also rely on extensive stands of dense shrub cover in both habitat types.
Status and Trends ‐ There are no historical Humboldt marten population estimates available, but anecdotal evidence from early naturalists and trapping records suggest the species was far more common and widespread in the early 20th century than they are today. The California population is currently estimated to number fewer than 200 individuals. It appears the Oregon population also numbers fewer than a few hundred individuals and there is uncertainty about the degree to which animals in Oregon and California interact and exchange genes. Humboldt martens historically ranged from the coastal forests of northwestern Sonoma County north to Oregon’s Curry County within the narrow humid coastal zone (Figure 1).
The historical described range was roughly 22,000 km2 (8,500 mi2 ). By the 1940s, a significant decline in Humboldt marten trapping returns was noted and a retraction of the southern end of the range appears to have occurred. In California, over the last 25 years Humboldt martens have only been detected in Del Norte, northern Humboldt, and extreme western Siskiyou Counties; suggesting a range reduction greater than 93% (Figure 2). Threats ‐ The Department has identified multiple potential threats to the long‐term persistence of the California Humboldt marten population. Some of these threats are largely the result of historical practices while others are ongoing.
Ongoing threats include habitat loss from timber harvesting, wildfires, urbanization, cannabis cultivation; elevated predation rates; exposure to toxicants and diseases; climate change; and risks inherent to small populations. Historical accounts suggest that trapping pressure on Humboldt martens in the late 19th and early 20th centuries was intense, and declines in the population were noted by the early 1900s. Marten harvest continued essentially unchecked until 1946 when the Commission instituted the first Humboldt marten trapping closure. Historical trapping, coupled with habitat loss, was likely the cause of the dramatic reduction in the marten’s California range.
Today, trapping of Humboldt martens is prohibited in California and no longer poses a significant threat to the population. In addition, the probability of unintended bycatch is extremely low due to current trapping regulations and the low number of active trappers within the extant range. Humboldt marten populations have been negatively impacted by historical and ongoing habitat degradation and loss from timber harvesting and other silvicultural treatments of older forests, wildfires and associated salvage logging, development of coastal forests for human settlement, as well as the clearing of forests for the cultivation of cannabis.
Forest conditions in the range of the Humboldt marten today have largely been shaped by a legacy of over 100 years of logging and timber management which has reduced the area of old growth conifer forest in the Pacific Northwest by an estimated 72% since European settlement. In recent decades the logging of old growth forest stands on private and public lands has dramatically slowed from peaks in the second half of the 20th century due to more restrictive regulations and market conditions. However, it will likely take decades for the forest stands degraded and fragmented from historical logging to recover the dense shrub cover, and centuries to recruit the large tree structures needed to restore high-quality Humboldt marten habitat conditions.
Humboldt marten habitat suitability may be reduced under commonly used timber harvest methods through reduction of overstory canopy cover and the loss of dense shrub cover. Shrub layers can be destroyed or degraded through post‐harvest stand management treatments such as burning, mechanical clearing, herbicide application, and through competitive exclusion by densely planted conifers in plantations which shade out understory shrubs. Shrub cover has been found to be more patchily distributed in thinned stands than in old-growth stands on federal forest lands, and decades are required to restore dense shrub layers following harvests. Wildfires and the associated salvage logging of damaged trees can threaten the already small Humboldt marten population by reducing and fragmenting the remaining habitat.
On federal lands in north coastal California, there was a net 5.6% loss of old forest habitat over the period of 1993‐2012, primarily attributed to wildfires, despite gains from forest succession. Connectivity between old forest stands was found to have decreased over the same period, mainly due to fragmentation caused by wildfires. Large fires in southwest Oregon (the 2002 Biscuit Complex Fire and 2017 Chetco Bar Fire) have likely isolated the California – Oregon border Humboldt marten population from the two extant Oregon population areas (Figure 7). Additionally, vegetation management activities designed to reduce the risk of wildland fire by removing shrubs, reducing canopy cover, and removing snags and logs can degrade marten habitat and contribute to habitat fragmentation. Humboldt marten researchers and land managers consider wildland fire a serious threat to the extant population in California, estimating that a single large fire could eliminate 31% to 70% of the currently occupied habitat.
Humboldt Marten Review
The negative impacts of wildland fires on marten habitat vary with the intensity of the burn and include the removal of large tree structures and dense shrub layers as well as the fragmentation of habitat. The number of fires, mean fire size and annual area burned in northwestern California were all found to have increased over the century from 1908 – 2008, suggesting the threat to Humboldt martens from wildland fires is increasing. Habitat loss and degradation from human settlement and residential development rapidly increased in the 1850s, and several portions of the historical range have been converted from forests to urban areas, primarily in and around Crescent City, Humboldt Bay, Fortuna, Fort Bragg, and Willits; and much of the historical range south of Del Norte County has been parceled and occupied by very low-density housing.
However, the core population areas currently occupied by Humboldt martens in California are almost entirely unoccupied by humans, with the exception of some areas adjacent to the Klamath River on Yurok Tribal lands. While further human development of the historical range will likely continue into the future, a modeled analysis of future land conversions under several human population growth scenarios found the probability of significant conversions to urban and agricultural uses in the northwest California coast region to be very low for the remainder of this century.
Large‐scale marijuana cultivation in remote forests throughout California has increased since the mid‐ 1990s, coinciding with the passage of California laws legalizing certain uses of cannabis. Cultivation can impact Humboldt martens through the clearing and fragmentation of forests and the application of pesticides, including highly toxic anticoagulant rodenticides. Humboldt and Del Norte counties are known centers of legal and illegal cannabis cultivation in California due to the remote and rugged nature of the land and abundant water sources, although to date a relatively small percentage of the land area has been impacted by these practices. The extent to which land clearing for legal and illegal cannabis cultivation will contribute to future Humboldt marten habitat loss and degradation is unknown.