Trophic niche partitioning and incorporation of anthropogenic food by generalist mesopredators in an urbanizing landscape

Molly Hardesty-Moore, Marisa Christopher and Douglas J. McCauley, Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, Santa Barbara, CA, Douglas J. McCauley, Marine Science Institute, University of California, Santa Barbara, Santa Barbara, CA

Background/Question/Methods
Human activities have altered ecosystems, creating novel environments with vastly transformed species composition and community dynamics. Species that not only survive the transition to an urban or heavily modified ecosystem but learn to thrive in these new ecosystems are considered synanthropic species. This study aimed to determine how three synanthropic mesopredators – Virginia opossums, raccoons, and striped skunks – interact with one another and the urban landscape by examining trophic niche partitioning and use of anthropogenic food resources. We analyzed the δ13C and δ15N stable isotope values for historic and modern opossums, raccoons, and striped skunks to examine how a developing landscape might alter isotopic niche space and overlap between these seemingly similar generalist mesopredators. Hair samples were gathered from modern individuals (2017-2019, n=85) as part of an ongoing raccoon movement study and from historic museum specimens (1946-1985, n=34), covering over 70 years of human development in Santa Barbara County, California. We calculated isotopic niche width and overlap using the program SIBER in R and estimated the proportion of human foods consumed by each species using a stable isotope mixing model in R (package simmr).
Results/Conclusions
Our results demonstrate marked differences between the three mesopredator species: raccoons were determined to be somewhat of "synanthropic specialists," their isotopic niche contracting to contain mostly anthropogenic foods in the modern time period (>50% of their diet); opossums experienced very little change in niche width and proportion of human foods consumed over time (human foods consistently ≤25% of diet); and striped skunks expanded their niche to incorporate anthropogenic foods while still consuming natural food sources. There is evidence for both increased and decreased trophic niche partitioning through time, depending on the species. Our results highlight the heterogeneity in synanthropic species' use of the urban landscape and provide unique insight into urban ecosystem community dynamics and intraguild interactions, with implications for urban ecosystem management.