What's stopping you? Variability of interstate highways as barriers for four species of terrestrial rodents

Abstract Landscape features that form a barrier for one species may not be a barrier for others, depending on how each species perceives the feature. Barriers may not be equivalent, due to unique characteristics at different locations, which may ameliorate barrier effects. In this study, we employed population genetics techniques to investigate how four Rodentia species with differing life history characteristics (fox squirrels [ Sciurus niger ], eastern gray squirrels [ Sciurus carolinensis ], eastern chipmunks [ Tamias striatus ], and white‐footed mice [ Peromyscus leucopus ]) responded to interstate highways as barriers to movement at seven locations with differing site‐specific landscape features. For comparative purposes, we included control sites that lacked heavily trafficked roads. We detected genetic differentiation relative to interstate highways among the gray squirrels, chipmunks, and white‐footed mice, but signals for differentiation among those three species varied by site, suggesting that site‐specific features are important considerations for species responses to high‐traffic roads. Chipmunk and gray squirrel populations, two forest specialist species that avoid open areas, exhibited more genetic structure corresponding with highways than either white‐footed mice (a nocturnal generalist) or fox squirrels (a larger‐bodied specialist that traverses open areas more readily than the other species in this study). Fox squirrels did not exhibit genetic differentiation at any site, whereas gray squirrel study populations evinced no population differentiation at sites with bridges spanning the interstate highway. Chipmunks also exhibited population differentiation at two of the control sites, indicating that less obvious barriers in the landscape may be as important as roads for sensitive species. We interpreted variation in genetic differentiation of species across sites in the context of how interstate highway features (i.e., the presence and condition of drainage culverts and bridges near study sites), combined with species‐specific life history characteristics, influence the barrier potential of highways. Culvert presence generally corresponded with a low or nonexistent barrier effect; however, an open, large culvert (>8 m in height) may create a semi‐permeable barrier for species that avoid open areas. Bridge presence appeared to facilitate gray squirrel and chipmunk dispersal across highways.