Characterizing diet components of fisher (Pekania pennanti) in response to tree mortality in Kings River California using DNA metabarcoding of scats

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Kristine L. Pilgrim, Taylor Wilcox and Michael Schwartz, Rocky Mountain Research Station, USDA Forest Service, Missoula, MT, Rebecca E. Green, Pacific Southwest Research Station, USDA Forest Service, Shaver Lake, CA, Kathryn L. Purcell, Pacific Southwest Research Station, USDA Forest Service, Coarsegold, CA, Eric L. McGregor, Institute for Natural Resources, Oregon State University, Coarsegold, CA, Larissa Gleason, The Great Basin Institute, Reno, NV, Samuel K. Wasser, Center for Conservation Biology, Seattle, WA

Background/Question/Methods
The recent tree mortality epidemic in California has dramatically altered the landscape with an estimated 102 million dead trees. Wide-spread tree mortality, particularly of conifers, in the Sierra National Forest of California has been observed in habitat types and elevations used by fisher (Pekania pennanti), a species of conservation concern in the southern Sierra Nevada. The primary objective of our study was to evaluate any potential changes in the diet of this forest dependent carnivore that might be associated with such a dramatic change to their landscape. Fisher scats were collected using trained detector dogs over a systematic sampling grid during eight sampling seasons (four pre- established tree mortality (Tm) in 2011-2013 and four post-Tm collected in 2017-2018) in the Kings River Study Area. We examined the diet composition of 211 fisher scats using DNA metabarcoding.
Results/Conclusions
We observed 51 different diet items across all samples and were able to successfully assign 35 to species, with the remaining 16 being assigned to genus. Species of birds, mammals, and plants of the Ribes genus (gooseberries and currents) comprised the highest proportions of diet items observed. Fisher scats also contained DNA from species of reptiles, insects, snails, and fungus. The number of scats with mammal prey items was higher in scats collected pre-Tm, (91.8%) compared to scats collected post-Tm (52.5%), with a substantial reduction in the number of scats containing Douglas and flying squirrel. Conversely, a higher proportion of scats collected post-Tm (46.5%) contained DNA from Ribes sp. compared to scats collected pre-Tm (14.5%). Our data show that fishers are consuming fewer tree squirrels in the wake of widespread tree mortality and may be shifting their diets to more available but less nutritionally valuable berries.