Individual dietary specialization in a generalist predator: A stable isotope analysis of urban and rural red foxes

Some carnivores are known to survive well in urban habitats, yet the
underlying behavioural tactics are poorly understood. One likely
explanation for the success in urban habitats might be that carnivores are
generalist consumers. However, urban populations of carnivores could as
well consist of specialist feeders. Here, we compared the isotopic
specialization of red foxes in urban and rural environments, using both a
population and an individual level perspective. We measured stable isotope
ratios in increments of red fox whiskers and potential food sources. Our
results reveal that red foxes have a broad isotopic dietary niche and a
large variation in resource use. Despite this large variation, we found
significant differences between the variance of the urban and rural
population for δ13C as well as δ15N values, suggesting a habitat-specific
foraging behaviour. Although urban regions are more heterogeneous
regarding land cover (based on the Shannon index) than rural regions, the
dietary range of urban foxes was smaller compared to that of rural
conspecifics. Moreover, the higher δ13C values and lower δ15N values of
urban foxes suggest a relatively high input of anthropogenic food sources.
The diet of most individuals remained largely constant over a longer
period. The low intra-individual variability of urban and rural red foxes
suggests a relatively constant proportion of food items consumed by
individuals. Urban and rural foxes utilized a small proportion of the
potentially available isotopic dietary niche as indicated by the low
within-individual variation compared to the between-individual variation.
We conclude that generalist fox populations consist of individual food
specialists in urban and rural populations at least over those periods
covered by our study.