Effects of forest disturbance, snow depth, and intraguild dynamics on American marten and fisher occupancy in Maine, USA

Human land use is a driving force of habitat loss and modification globally, with consequences for wildlife species. The American marten (Martes americana) and fisher (Pekania pennanti) are forest-dependent carnivores native to North America. Both species suffered population declines due to loss of forested habitat and overharvest for furs, and continued habitat modification is an ongoing threat. Furthermore, the smaller marten may be susceptible to intraguild exclusion where the larger fisher are abundant, and both habitat modification and climate change may reduce spatial refugia available to marten. A detailed understanding of co-occurrence patterns of marten and fisher in landscapes subjected to intense forest disturbance represents a key knowledge gap for wildlife ecology and management. Maine, in the northeastern United States, supports populations of both these species. It is an extensively forested state, and the vast majority is managed as commercial timberland. We designed a large-scale field study to understand the relative importance of three sets of predictions for marten and fisher occupancy patterns where commercial silviculture is widespread: (1) The intensity of forest disturbance primarily determined both marten and fisher occupancy rates, (2) fisher occupancy was limited to areas of shallower snow and marten limited by fisher presence, or (3) both species responded to the composition of tree species within forested habitat. We collected data to test these nonmutually exclusive hypotheses via camera-trap surveys, using an experimental design balanced across a gradient of forest disturbance intensity. We deployed 197 camera stations in both summer and winter over 3 years (2017-2020). We tagged over 800,000 images and found marten at 124 (63%) and fisher at 168 (85%) of the stations. By fitting multiseason occupancy models to the data, we found that the degree of habitat disturbance negatively influenced detection, occupancy, and temporal turnover for both species. Contrary to our expectations, however, we found no evidence of interspecific competition and instead support for positive associations with detection probabilities both spatially and temporally. Both species were positively associated with forest stands containing deciduous trees. Our findings further illustrate the impact that land use has on the occupancy dynamics for these forest-dependent carnivores.