American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Prey responses to exotic predators: effects of old risks and new cues”

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Sean M. Ehlman, Pete C. Trimmer, and Andy Sih (Apr 2019)

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New theory on prey responses to exotic predators offers insights and surprises

Behavioral ecological theory on the interactions of predators and prey commonly assumes that predators and prey have coevolved to respond appropriately to one another. Increasingly in the modern world, however, prey must face exotic, invasive predators to which they are not well-adapted. This work develops a predictive, cue-based theory of prey responses to exotic predators. The authors ask how differences in preys’ evolutionary histories with native predators might explain variation in their success with exotic predators: Does the degree of similarity between native and non-native predator cues affect prey responses? Usually. Does the frequency of predation in a prey’s past affect responses to exotic predators? A bit, but sometimes in the opposite direction of previous work. Does the degree to which prey generalize among predator types affect their perception of risk with exotic predators? Yes, in some interesting ways. For empiricists working towards understanding variation in prey responses to exotic predators, this theory offers insights and testable predictions.


Exotic predators can have major negative impacts on prey. Importantly, prey vary considerably in their behavioral responses to exotic predators. Factors proposed to explain variation in prey response to exotic predators include the similarity of new predators to familiar, native predators, the prevalence and diversity of predators in a prey’s past, and variation in a prey’s innate ability to discriminate between predators and safety. While these factors have been put forth verbally in the literature, no theory exists that combines these hypotheses in a common conceptual framework using a unified behavioral model. Here, we formalize existing verbal arguments by modeling variation in prey responses to new predators in a state-dependent detection theory (SDDT) framework. We find that while some conventional wisdom is upheld, novel predictions emerge. As expected, prey respond poorly to exotic predators that do not closely resemble familiar predators. Furthermore, a history with more abundant or diverse native predators can lessen effects of some exotic predators on prey; however, under some conditions, the opposite prediction emerges. Also, prey that evolved in situations where they easily discriminate between safe and dangerous situations can be more susceptible to novel predators.