Studying urban wildlife is key to understanding the ecological shift towards a mesocarnivore dominated ecosystem and the potential negative human health consequences. No previous studies have quantified the influence of anthropogenic food sources on raccoon and Virginia opossum diets, which would clarify the role of bottom-up regulation in urban environments and provide a comparison of two urban generalists. Stable isotope analysis (SIA) of consumer tissues and their potential food sources can be used to reconstruct consumer diet. Corn and sugar cane are both C4 plants and common in processed products. C4 plants are not abundant in western Washington state, and have carbon isotope values that are distinct from naturally occurring C3 plants, thus SIA can be used to infer the influence of anthropogenic products on wildlife diet. We analyzed hair samples from 40 raccoons and 26 opossums from 7 Seattle parks and the urban matrix in Tacoma to investigate differences among populations and species. We used δ13C and δ15N values of mesocarnivores and their diet to estimate the relative proportions of anthropogenic, natural terrestrial, and natural marine resources. We also analyzed potential diet items (arthropods, plants) and soil samples to characterize the isotopic environment.
Raccoons in all locations have purely terrestrial diets, although the contribution of anthropogenic resources is highly variable among individuals (range = 13-65%). Raccoon diets varied among Seattle parks, which may reflect access to and/or availability of novel human food sources near each park. Sometimes these differences were counter-intuitive; for example, raccoons from the largest Seattle park have the most human influenced diets. Seattle opossums also have terrestrial diets, but with lower inter-individual variation and overall anthropogenic contribution (range = 10-21%) than Seattle raccoons. Trophic separation between Seattle raccoons and opossums suggests that raccoons have a higher proportion of animal-based food sources in their diet. Tacoma opossum diets are similar to raccoons, with high variability in anthropogenic resource use among individuals (range = 28-52%), and little trophic separation. Despite differences in sampling locations within Tacoma and Seattle (urban matrix versus park), similarities in raccoon diet at a population level are compelling. Our results are consistent with previous behavioral research suggesting raccoons are less sensitive to human proximity than opossums. Future work will focus on collecting raccoon samples from Tacoma parks, and a larger opossum sample to further investigate the diet and behavior of urban mesocarnivores.