Humboldt Martens Need Old Growth

Distribution and Habitat Associations of the Humboldt marten (Martes americana humboldtensis), and Pacific fisher (Martes pennanti pacifica) in Redwood National and State Parks

Humboldt Marten.
Humboldt Marten.

It’s likely that Pacific fisher (Martes pennanti pacifica) populations are well distributed in Northern California’s Redwood National and State Parks (RNSP) for the same reason that Humboldt martens (Martes americana humboldtensis) have disappeared, according to research done by Keith Slauson, William Zielinski, and Gregory Holm. Second-growth forest habitats that cover a majority of the park are fishers’ sweet and martens’ sour.

Fishers and martens, members of the weasel family Mustelidae, live in conifer or mixed conifer-hardwood forests and eat small rodents, such as mice and voles. The Pacific fisher is a subspecies of fisher that lives in western North America. The Humboldt marten is one of two marten subspecies known to inhabit California. Heavy trapping for their fur caused fisher and marten population declines as early as the 1920’s. Now, they are both listed as Species of Special Concern by the California Department of Fish and Game and Sensitive Species by the U.S. Forest Service.

The researchers of this study recorded fisher and marten distribution in RNSP by combining track plates with food bait—chicken and the commercial lure GUSTO—and systematically spreading them across the park. The pattern of prints on a track plate indicated which animal visited that site. At the end of the study, no martens had left their prints on the plates. While they do not rule out martens inhabiting RNSP, these results are enough to say that the park is not home to a significant population of martens.

Historical records of Humboldt marten distribution show that the subspecies lived almost exclusively in old-growth forests from present-day Del Norte to Sonoma counties. Researchers considered the prognosis for discovery and recovery of the population bleak after a 1996 survey that detected no martens in coastal Northwest California. Then, in 1997, William Zielinski found martens in two locations within the historic range of the Humboldt marten, although until genetic analysis has been done, we cannot be sure they are members of the Humboldt subspecies.

The population Mr. Zielinski found occupies old-growth Douglas-fir habitat to the east of RNSP, in the Smith River National Recreation Area. Because second-growth forest covers the majority of RNSP, restoration of old-growth characteristics could play a major role in marten conservation. Researchers note that increasing the number of martens is especially important because the longer a population remains small, the greater the chance is it will lose genetic variation, an important element for species survival.

The Pacific fisher, on the other hand, seems to be fairly well distributed in RNSP. This is a paradox because its population in the western United States has declined enough for it to be considered a candidate for listing on the Endangered Species List.

So why might fishers succeed in RNSP? Possibly for the very reason Humboldt martens have struggled. Most of the track plates trampled upon by fishers were in second-growth forests. Because fishers visit track plate stations while foraging, it can reasonably be assumed that they choose second-growth forest for the habitat characteristics that improve their hunting success. Sparse shrub cover and an abundance of rodent prey are two second-growth characteristics that facilitate fisher hunting.

The Humboldt marten population direly needs rejuvenation. The best way to do this, the researchers say, is to connect patches of old-growth forest within and between parks. Removing selected trees in crowded second-growth forests allows more light through the canopy, accelerating the development of individual trees and thickening the shrub cover below. In addition to restoring old-growth, managers have the difficult task of addressing two major barriers to marten recolonization of the park—the Klamath river and Highway 101.

The researchers’ report was published in the Journal of Mammalogy under the title “Distribution and Habitat Associations of the Humboldt marten (Martes Americana humboldtensis), and Pacific fisher (Martes pennanti pacifica) in Redwood National and State Parks”

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