Urbanization alters environments in many ways that impact abundance and behavior of urban animals. Fewer natural predators and greater food availability - as well as increased contact with humans - may lead to altered stress responses, increased boldness, and decreased vigilance compared to non-urban populations. We investigate how the Eastern gray squirrel (Sciurus carolinensis) differs in relative abundance and flight initiation distance (FID) upon human approach across common urban habitats that vary in human presence and environmental disturbance. We specifically addressed: (1) variation in abundance, (2) variation in FID, and (3) variation in local and landscape-scale habitat relevant to abundance and FID. We sampled 17 sites in the St. Louis (MO) metropolitan region (5 neighborhoods, 5 parks, 3 cemeteries, 3 golf courses, 1 urban forest). For FID measurements, we recorded both the distance at which the individual was spotted and the distance at which the individual fled (FID). Across all sites we observed 337 squirrels and measured FID for 118 squirrels in the two habitat types in which they were most abundant (neighborhoods and parks). Understanding how animals behaviorally respond to urban environments will help explain the ecological consequences of rapid urbanization and has implications for evolutionary responses.
Results/Conclusions
Relative abundance (accounting for sampling effort) varied by habitat type: parks had the greatest abundance, followed by the urban forest, neighborhoods, cemeteries, and golf courses. Between neighborhoods and parks, habitat use and FID differed as well. More squirrels were observed on the ground in parks than in neighborhoods and squirrels in parks had significantly greater FID compared to neighborhoods (i.e., fled at a farther approach distance). The extremely low relative abundances in cemeteries and golf courses was surprising, and we address landscape-scale differences between each of the five habitat types to gain insight into these patterns of abundance. Variation in abundance and FID may also be attributable to differences in human habituation. Individual squirrel responses were correlated with their immediate surrounding habitat but this response differed in parks and neighborhoods. In parks, squirrels had shorter FID (allowed closer approach) in habitat that had greater impervious surface coverage and was more open, suggesting increased habituation in more disturbed portions of parks. In contrast, squirrel FID did not vary with habitat features in neighborhoods. Together, our results suggest that both abundance and behavioral responses to urbanization are heterogeneous within the metropolitan region and may reflect variation in habituation.