Distribution and habitat use of a recently discovered population of Humboldt martens in California

The Humboldt marten (Martes caurina humboldtensis) has declined from over 95% of its historic range in California, with only two populations remaining. In response to the forthcoming listing of the Humboldt marten a conservation assessment and strategy was developed to address the most important conservation needs for this species. This assessment identified an area near the California – Oregon border as the second extant population area in California based on a small number of recent detections. However little else was known of this population, and this prompted my investigation to determine 1) the distribution and potential population size and 2) habitat use by Humboldt martens in this area. This study addresses a key information need identified in the conservation strategy. Between May – August of 2017 and 2018, I used a 2-km systematic grid to sample 51 sample units using baited remote cameras and track plates and detected martens at 20 (39.2 %) sample units. Using an occupancy modeling approach, I found that a combination of elevation and amount of forest habitat with large diameter trees (size class 5, ≥ 60.0 cm QMD) measured at the home range scale (1-km radius, 314 ha) influenced marten occupancy. Marten occupancy was highest in low elevation sample units (mean = 614.6 m, SE = 35.6 m) with an average of 65.3 ha (20.1% of 314 ha, SE = 12.0 ha) of forest habitat in the largest tree size class. The limited number of detections precluded evaluating models with \u3e 3 habitat variables, as well as assessing finer scale habitat use; however, univariate results suggested stream density may also be influential at the home range scale. Consistent with results from the larger California population, managers interested in promoting marten conservation in the California – Oregon extant population area should maintain and increase large patches of forest habitat with large-diameter trees. A novel finding for this population was the importance of low-elevation forest habitat dominated by size class 4 (27.9 – 59.9 cm QMD), suggesting the combination of home-range sized areas with these two habitat compositions is capable of supporting marten occupancy in this region