Context. Mule deer (Odocoileus hemionus) are a species of social, economic and cultural importance that are experiencing a population decline throughout much of their range. Factors such as disease, landscape-level changes in nutritional output of forage species, winter habitat degradation, habitat fragmentation, competition for resources with both domestic and wild ungulates, and predation have all been implicated in the decline. Recently, summer habitat quality in Oregon has been identified as a potentially strong limiting factor for mule deer population performance. Competition and habitat degradation from feral horses (Equus caballus) have been proposed as factors potentially exacerbating existing habitat limitations for mule deer.

Aims. The aim of this study was to investigate factors affecting mule deer summer range occupancy within a study area that experienced both wildfires and the presence of feral horses.

Methods. We deployed 72, 77 and 75 camera traps throughout the Murderers Creek and Northside wildlife management units located in north-eastern Oregon during summer 2019, 2020 and 2021 respectively. We used an occupancy modelling framework to build and evaluate models of mule deer summer range occupancy related to competition, vegetation and abiotic variables.

Key results. Our final model set included four covariates: (1) an index of feral horse use; (2) whether the site was disturbed by wildfire; (3) distance to forest edge; and (4) eastness of the site. Model averaging indicated that mule deer probability of occupancy was negatively related with increased feral horse use and distance to forest edge, and positively related to eastern slopes and sites within a wildfire perimeter.

Conclusions. Our results add to the growing body of literature indicating that wildfires in forested ecosystems benefit mule deer and add to the limited body of literature indicating that feral horse use of a site negatively impacts mule deer.

Implications. Managers should incorporate fire into forest restoration projects when possible to benefit mule deer. Where feral horses and mule deer overlap, increased management of feral horse herds could potentially benefit mule deer. Lastly, our research showcases the utility of camera traps coupled with occupancy modelling to answer research questions that, in the past, were mainly answered through radio collaring.