Non-lethal predator effects resulting from adaptive foraging: Empirical studies and predictive framework

Behavior (or other traits) that benefits an individual consumer’s growth often comes at the cost of increased predation risk. An adaptive response to changes in factors (e.g. resource level or predation risk) that affect this tradeoff is to modify behavior. Nonlethal (also termed trait-mediated or nonconsumptive) predator effects can result if the consumer’s responses to changes in predator density affect the fitness of the consumer or species it interactions with. For example, trait-mediated indirect interactions in the form of trophic cascades result when a predator indirectly affects a resource through induced changes in consumer (prey) behavior. Here we review experimental and observational studies of nonlethal predator effects. In particular we identify the responses that are typically measured and recent trends. We then present a graphical framework that clarifies the conditions under which adaptive responses to predation risk are predicted to have large effects on consumer fitness. To illustrate the utility of the framework, we use it to derive predictions of the relationship between predator foraging mode and the magnitude of nonlethal predator effects on the consumer. We further apply the framework to an experimental and field study of a Great Lakes zooplankton system (Bythotrphes - Daphnia). The framework predicts strong nonlethal effects in the zooplankton system, which we observe empirically, and clarifies how changes in environmental factors could alter this prediction. Finally, we discuss some areas of future experimental research that will further the understanding of the connection between adaptive phenotypic responses of individuals, and population and community level patterns.