Modeling the effects of dispersal and patch size on predicted fisher (Pekania [Martes] pennanti) distribution in the U.S. Rocky Mountains

Climate change impacts many species through shifts in habitat. The intensity of this impact will depend on the dispersal rates of the species, the patchiness of the environment, and the velocity of habitat change. Here we examine how dispersal affects projected future habitat availability for a threatened carnivore, the fisher (Pekania [Martes] pennanti). We used non-invasive genetic sampling to detect fisher across their historical distribution in Montana and Idaho. This survey included 4846 non-invasive hair snares, of which 288 identified fishers through mitochondrial DNA analysis. We modeled the distribution of fisher across western Montana and northern Idaho using a suite of vegetative, topographic, and climatic variables. We modeled future distribution using a global climate model and two climate change scenarios (high emissions [A2] or reduced emissions [B2]) and three time steps (2030, 2060, and 2090). We incorporated the effects of dispersal ability and habitat patch size into our model by varying the distance and enforcing a minimum patch size at which newly created habitat could be colonized. We found that the probability of current fisher occurrence was highest given the presence of mesic forest types with tall trees, high annual precipitation, and mid-range winter temperatures. Future predictions show an increase in area of high-probability habitat under most dispersal assumptions. Interestingly, we found a large contrast in results when minimum patch size and species dispersal capabilities were considered. Our distribution model with full dispersal and no limits on patch size predicted a 24.5% increase in fisher habitat by 2090, whereas a dispersal limit of 1 km through non-habitat (agricultural fields and urban zones) and a minimum patch size yielded a loss of 25.8% of fisher habitat under this same scenario. Varying dispersal appears to limit habitat availability more than minimum patch size under most scenarios.