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.

“Effects of predator avoidance behavior on the coexistence of competing prey”

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Pacifica Sommers and Peter Chesson (May 2019)

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General rules of how predator avoidance behavior alters coexistence of similar species with shared predators and resources

If you have ever tried to catch a lizard or other small creature, you know that animals go out of their way to avoid potential predators. While busy hiding or escaping, however, an animal cannot also search for food and could starve to death from spending too much time avoiding predation. In this way, predators affect their prey not only by killing them, but by scaring them into changing their eating habits. A growing number of studies have shown how scaring prey can affect the stability of the whole ecosystem. Now researchers have shown how predator avoidance behavior affects a classic puzzle in ecology: why there are so many species. The puzzle goes like this: if similar species are competing for resources, why have a few super-competitors not taken over the world? One reason is that all species face trade-offs, and differentiate themselves in some way, finding their own unique ecological niches which result in a diversity of species.

New research published in The American Naturalist uses mathematical models to derive general rules for when avoiding predators increases biological diversity, and when it decreases diversity. In the model, those unique niches of each species can overlap to various degrees in terms of the resources they use and the predators that hunt them. Hiding from the predators reduces the effect of predator overlap on coexistence. For two species with the same predators that rely on very different resources, avoiding predators increases the species’ niche partitioning, generally promoting biodiversity. On the other hand, if two species rely on the same resources and predator partitioning would maintain their coexistence, then hiding from predators undermines diversity by making the superior competitor more likely to exclude the other species. Which competitor is superior also depends on the avoidance behavior, and the authors derive the conditions under which each species benefits more from hiding. The study derives general rules for how a common behavior affects a classic puzzle about biodiversity and competition. These rules can be applied to a variety of ecosystems. As humans continue to change landscapes and eliminate predators, understanding general rules of how biodiversity may be affected will help predict the impacts – and maybe suggest future solutions.


Predator avoidance behavior, in which prey limit foraging activities in the presence of predation threats, affects the dynamics of many ecological communities. Despite the growing theoretical appreciation of the role predation plays in coexistence, predator avoidance behavior has yet to be incorporated into the theory in a general way. We introduce adaptive avoidance behavior to a consumer-resource model with three trophic levels to ask whether the ability of prey, the middle trophic level, to avoid predators alters their ability to coexist. We determine the characteristics of cases in which predator avoidance behavior changes prey coexistence, or the order of competitive dominance. The mechanism underlying such changes is the weakening of apparent competition relative to resource competition in determining niche overlap, even with resource intake costs. Avoidance behavior thus generally promotes coexistence if prey partition resources but not predators, whereas it undermines coexistence if prey partition predators but not resources. For any given case, the changes in the average fitness difference between two species resulting from avoidance behavior interact with changes in niche overlap to determine coexistence. These results connect the substantial body of theoretical work on avoidance behavior and population dynamics with the body of theory on competitive coexistence.