Intraguild interactions and abiotic conditions mediate occupancy of mammalian carnivores: co-occurrence of coyotes-fishers-martens

The widespread eradication of large carnivores and subsequent expansion of top mesopredators have the potential to impact species and community interactions with ecosystem-wide implications. An example of these trophic dynamics is the widespread establishment of coyotes following the extirpation of wolves and mountain lions in eastern North America. Here, we examined the occupancy of three carnivores in northern New York considering both environmental/habitat factors and interspecific interactions. We estimated the co-occurrence of coyotes, fishers, and martens from a landscape-scale winter camera trap survey repeatedly annually for three years. Martens occurred independently of both coyotes and fishers, while fishers and coyotes displayed positive intraguild interactions that were constant across the landscape. Both marten and fisher first-order occupancy was driven by a combination of biotic and abiotic factors, with both species displaying positive associations with forest cover but antithetical responses to average snow depth. The integral and antithetical role of snow depth in driving the occurrence of martens (positive) and fishers (negative) in the landscape indicates that future climatic warming could reduce the availability of current spatial refuges for martens created by severe winter conditions. Climate-driven alterations to established competitive interactions and co-existence patterns between marten and fishers have critical implications for the species' survival and conservation. We provide correlational evidence consistent with the potential for positive top-down effects of dominant mesocarnivores on subordinate species, with fisher occupancy increasing conditional on the presence of coyotes across the landscape. These findings align with the hypothesis that under certain conditions, coyotes may facilitate certain subordinate carnivores. The evidence produced here is consistent with hypotheses on the dynamic nature of trophic niches. We demonstrate the need to consider the interplay between climate, habitat, and interspecific interactions to understand wildlife occupancy patterns and inform wildlife management in a rapidly changing world.

Methods
We conducted camera trap surveys for carnivores from January-March 2016-2018 throughout the Adirondack and Tug Hill regions of northern New York State (Figure S1). The study area is an approximately 32,000 km2 region dominated by contiguous mature stands of deciduous, mixed, and coniferous forests along an elevational gradient ranging from 24 – 1448m. We used a stratified random sampling design to select 15-km2 sample units and a standardized methodology across all surveys which included the same 195 sample units in each of the three years (except for 13 sites that were not sampled in 2017). At each site, a camera trap was deployed randomly within the 15 km2 grid.  Camera traps were secured to trees approximately 1.0-1.5 m above ground. A bait station was placed on a tree opposite the camera trap and secured to the tree using wire mesh. The bait used varied and included beaver (Castor canadensis), moose (Alces alces), white-tailed deer (Odocoileus virginianus), muskrat (Ondatra zibethicus), raccoon (Procyon lotor), beef (Bos taurus), and chicken (Gallus domesticus). At all sites, skunk-based call lures were applied. Cameras were deployed for 3 weeks (21 days) at each location after which cameras were retrieved. Cameras and bait were checked halfway through 3 weeks of sampling (generally day 10 or 11) with batteries and bait replaced and replenished as necessary. Detection records were created for each of the species sampled over the recording period. We used a weekly occasion length, with only one detection of each species being possible per weekly period to ensure independence of detections.