Urban intensity predicts eastern gray squirrel (<i>Sciurus carolinensis</i>) and fox squirrel (<i>Sciurus niger</i>) occupancy and detection in St. Louis green spaces

Abstract Resource partitioning allows ecologically similar species to coexist, but human-altered landscapes such as cities change the availability and quality of resources. Tree squirrels are common in cities, yet the mechanisms that drive coexistence in urban habitats remain unclear. We explored how landscape features affect occupancy and detection of eastern gray squirrels (Sciurus carolinensis) and fox squirrels (Sciurus niger) along an urbanization gradient in St. Louis, Missouri. We conducted presence/absence surveys in 34 green spaces using seasonal camera trapping from fall 2018 to summer 2019. We examined the relationship between urbanization metrics (NDVI, % impervious surface, human population density) and estimates of occupancy and detection. We also investigated mean daily detection frequency among urban, suburban and rural green spaces and across seasons. Autologistic occupancy models with an urban intensity covariate yielded higher estimates of occupancy and detection for gray squirrels than fox squirrels. Fox squirrel occupancy and detection were negatively correlated with urban intensity, whereas gray squirrel occupancy and detection were positively correlated with it. Gray squirrel mean daily detection estimates were higher in urban sites than suburban and rural sites while fox squirrel mean daily detection rates were higher in rural than suburban sites. Both species had the highest mean daily detection rates in the fall and the lowest in the summer. Our results indicated that gray squirrels thrive in urban environments whereas fox squirrels are limited to more rural areas. Our results offer insight into the mechanisms influencing urban species’ distribution and interactions, which are important for understanding the broader impacts on biodiversity.