Relations among Fishers, Snow, and Martens in California: Results from Small-scale Spatial Comparisons

A recent publication hypothesized that deep snowfall can limit fisher (Martes pennanti) populations, and high fisher populations limit marten populations (M. americana). These hypotheses were evaluated by comparing the historic (1919-1924) and current (1989-1994) distributions of both Martes species in California to current snowfall distributions and forest types presumed to be habitats for each species. Data on historic distributions of fishers and martens came from the locations of trapped animals reported by Grinnell et al. (1937), whereas current distributions came from detection surveys (sets of track-plates, cameras, and both) conducted primarily by personnel of the US Department of Agriculture (USDA) Forest Service. Information on forest types came from a computerized database created by the USDA Forest Service from satellite imagery. In both time periods, areas occupied predominantly by martens were closely associated with forested areas with the deepest snow (>23 cm per winter month), areas occupied predominantly by fishers were forested areas with low monthly snowfall (<13 cm), and overlap zones with both species were in forested areas of intermediate monthly snowfalls ( ≥ 13 to 23 cm). In contrast, there was poor spatial correspondence in both time periods between the occurrences of fishers and martens and the small-scale distributions of forest types. Although we continue to recognize the importance of forest structure and prey populations on the distribution of both species of Martes, these findings suggest that climatic and interspecific factors deserve more attention as factors potentially affecting fishers and martens.