Stable isotope analyses reveal impacts of resource availability and interspecies competition on body sizes of California Channel Islands deer mice

Abstract

Island rodent populations have challenged Foster’s rule for insular mammal body size with inconsistent size patterns when compared to mainland populations. Many factors have been implicated in models of island rodent size changes including island area, climate, predation, and competition with other species. Connecting these factors is their influence on resource availability and how rodents preferentially consume different amounts of macromolecules such as carbohydrates and proteins. I studied rodent diet using stable isotope analysis, in the absence of sampling stomach contents or surveying the flora and fauna in the area where the rodents were trapped. Utilizing stable isotope analysis, I examined carbon (13C) and nitrogen (15N) stable isotopes in deer mice (Peromyscus maniculatus) and black rats’ (Rattus rattus) hair collected from recent and historical samples captured on the California Channel Islands. I hypothesized that larger deer mice body size would correlate with greater protein consumption and higher 13C and 15N concentrations. Additionally, I hypothesized that significant variation in deer mice body size and 15N values between islands would be explained by differences in resource availability on islands with or without nesting seabirds and the presence or absence of other rodent species that compete with deer mice for resources. While 13C did not reliably predict the origin of rodent diet components, body mass, body length, and 15N concentration appeared to correlate with availability of protein from seabird materials (eggs and hatchlings) and the absence of competing rodent species. In interpreting the significant differences in body mass and 15N concentration for deer mice on islands with and without seabirds, I considered El Niño Southern Oscillation (ENSO) weather effects on seabird reproductive behavior and species distribution since the deer mice were collected in different years. In the comparison of Anacapa Island deer mice before and after rat eradication, it is possible that artificial selection of larger Anacapa Island deer mice occurred due to the trapping and re-release of a small population of deer mice on Anacapa Island during black rat eradication.