Print PagePredators do not just kill prey, but can significantly modify their behavior. Experiments suggest that at least in some cases, this behavior alone can prevent local coexistence between multiple similar species. Though community ecology theory has long regarded competition and predation asymmetrically, recent work has demonstrated that they should be considered to have equal potential importance, with each having distinct coexistence mechanisms that may undermine the other. The effects on prey coexistence of the mortality from density-dependent and frequency-dependent predation have been studied extensively. Here we show that prey behavioral responses to predators can affect the coexistence of similar prey species.
We modified a three-trophic level system of Lotka-Volterra equations to include behavioral responses to predators, focusing on coexistence of two species at the middle trophic level. Both focal species interacted with multiple resources and predators, but the strength of the interactions varied. We determined the conditions for stable coexistence of these species, and asked whether predator avoidance behavior changed the outcome of their interactions. Two categories of avoidance behavior were considered: avoidance of all predators regardless of the attack risk, and selective avoidance based on relative danger.
Results/Conclusions
The degree to which two species’ relative fitness must be similar in order to coexist increases with the degree to which they share resources and predators. The region of stable coexistence decreases with avoidance behavior. This occurs because by avoiding predation, uncommon species also gather fewer resources. Under most conditions, if relative fitnessnes are different, exclusion will become more likely. It has a similar effect to increasing the extent to which the prey species’ resources or predators overlap. While it is possible for coexistence to be unaffected by avoidance behavior when growth rates are not linear (i.e. in non Lotka-Volterra models), the precise requirements seem unlikely to be met in nature. Therefore we expect in general that predator avoidance behavior will usually make coexistence more difficult. Finally, the decreased likelihood of coexistence is greater for the case of indiscriminate avoidance, rather than when prey distinguish the actual relative risk of attack.
