On the evolution of omnivory in a community context

Background/Question/Methods

An individual’s ability to assimilate and obtain one food resource is constrained by trade-offs between foraging for alternate resources. Investment in the ability to exploit one resource may make it more difficult to exploit a second alternate resource, thereby disfavouring the evolution of generalists. Across the continuum of feeding classes, from specialists on animal prey (carnivory) to specialists on plants (herbivory), the generalist strategy of omnivory should thus be a rare strategy yet omnivores are common in nature; so is it possible to reconcile observation with theory? We consider the evolution of omnivory in the context of interactions between foragers and their environment. We use an evolutionary simulation model to consider a community of individuals that are described by their feeding strategy, broadly classified as three feeding types – herbivores, omnivores, and carnivores – and ask how the intrinsic properties of these foragers and their environment determines the prevalence of omnivores (and other feeding types). We examined the traits associated with omnivores as well as the other feeding types and further examined the environmental factors (i.e., quality and availability of plant and animal prey in the environment) that lead to an increase in omnivore prevalence.

Results/Conclusions

Our analysis demonstrates that the phenotype (i.e., the proportion of plant items to prey items in the diet) of all feeding classes (including omnivores) is both readily predictable and explainable based upon the underlying intrinsic abilities (i.e., trait complexes) of an individual and its environment. Omnivores tend to have nutritional traits that are intermediate in value to carnivores and herbivores, but that are biased towards the acquisition and assimilation of animal prey. The trait values that describe an omnivore (or other feeding types) differed as a function of the environment, suggesting that how we describe omnivores is dependent on both the intrinsic characteristics of the organism, interactions with others, and the availability and quality of resources.

Our model suggests that omnivores are ubiquitous across all environmental conditions, although for the most part are not the dominant feeding type. Omnivores are most prevalent when the ratio between plants and animal prey numbers is low, and to a lesser degree, when habitat productivity (i.e., absolute resource quality and availability) is high.