Managing human-altered ecosystems requires identifying the factors that promote species coexistence in trophic webs and understanding how such stabilizing mechanisms are altered by anthropogenic disturbance, such as introduction of a novel species. Adaptive, antipredator defense has been shown theoretically to enhance coexistence of multi-predator communities when employed by shared prey. Yet to date, all such theory has assumed that prey maintain accurate perception of predation risk and appropriate antipredator responses, assumptions that may not be justified when considering a novel predator.
We therefore consider the effects of predator novelty on community coexistence, species abundance, and the relative strength of alternative energy pathways in an intraguild predation (IGP) module. We separately consider the effects of an introduced omnivore and those of an introduced consumer, examining the conditions that lead to the exclusion of the existing native predator. For each case, we describe two invasion scenarios: suboptimal defense, whereby an invader is recognized as a predation threat yet elicits an ineffective antipredator response; and naïveté toward an unfamiliar invader, for which prey fail to accurately estimate predation risk. Because of the role of productivity in shifting the relative importance of competition and predation and determining coexistence conditions in IGP settings, we examine naïveté and suboptimal defense across a basal enrichment gradient.
Results/Conclusions
Our model predicts that predator novelty can weaken the stabilizing effect of adaptive defense, causing exclusion of native predators that would otherwise persist. Community coexistence is predicted to be more sensitive to the effects of suboptimal defense (compared with naïveté) because it erodes the effectiveness of antipredator defense even when maximally employed by prey. Contrary to simple intuition, suboptimal defense differentially leads to exclusion of native predators in highly productive environments and when defense costs are low. In addition, moderate novelty of an omnivore can increase resource density — via strengthening of the trophic cascade — while consumer novelty is predicted to reduce the abundance of the shared resource in all cases. When the direct energy pathway dominates, consumer novelty leads to omnivore exclusion; in contrast, when the primary energy flux is indirect (at very low resource productivity) increasing novelty of the intermediate consumer can facilitate three-species coexistence by providing a subsidy to an otherwise excluded native omnivore. Our results suggest that models of adaptive defense are sensitive to assumptions regarding predator-prey eco-evolutionary experience and that predator novelty has significant implications for broader food web stability.