Omnivory, feeding on multiple trophic levels, is widespread in nature, occurring in a variety of taxa from microbes to mammals. Omnivores that prey on species with whom they also compete for resources introduce negative and positive feedbacks that can complicate foodweb dynamics in unexpected ways. Many such species, also called intraguild predators/parasites, are ectotherms whose body temperature depends on the environmental temperature. Temperature variation therefore has a strong effect on their population dynamics and interactions with resources and natural enemies. Given increasing evidence of climate warming effects on natural and managed ecosystems, a mechanistic understanding of how temperature affects the strength of omnivory and its effects on community dynamics is a crucial research priority. I present a mathematical model for intraguild predation/parasitism, which incorporates mechanistic descriptions of the temperature responses of life history and consumption traits based on how temperature affects the underlying biochemical and physiological processes. Because trait responses to temperature are mechanistically derived from first principles of thermodynamics, this framework can accurately predict the effect of omnivory on community dynamics for a wide range of climate change scenarios.
I report two key findings. First, the temperature-dependence of life history (fecundity, development and survivorship) and consumption traits (attack rates and handling times) leads to strong latitudinal patterns in the occurrence of omnivory. Importantly, because the model is mechanistic, it can predict these patterns based solely on information on how temperature affects the underlying traits and therefore completely independently of field data on species abundance patterns and foodweb dynamics. Second, there is a directionality in invasion success. Omnivores adapted to tropical temperature regimes can successfully invade foodwebs in temperate habitats, but omnivores adapted to temperate thermal regimes cannot as easily invade foodwebs in tropical habitats. The key to this directionality lies in the latitudinal differences in the relationship between omnivores' thermal optima and the mean habitat temperatures they experience in their native habitats, and the greater phenotypic plasticity exhibited by tropical ectotherms compared to their higher-latitude counterparts.