Multiple predator effects on juvenile prey survival.

Predicting multiple predator effects (MPEs) on shared prey remains one of the biggest challenges in ecology. Empirical evidence indicates that interactions among predators can alter predation rates and modify any expected linear effects on prey survival. Knowledge on predator density, identity and life-history traits is expected to help predict the behavioral mechanisms that lead to non-linear changes in predation. Yet, few studies have rigorously examined the effects of predator-predator interactions on prey survival, particularly with marine vertebrate predators. Using an additive-substitutive design, we experimentally paired reef piscivores with different hunting mode [active predator, Pseudochromis fuscus (F); ambush predators, Cephalopholis boenak (B), Epinephelus maculatus (M)] to determine how behavioral interactions modified their combined impacts on damselfish prey. Results showed that behavioral patterns among predators matched those predicted from their hunting mode. However, it was the identity of the predators what determined the strength of any positive or negative interactions, and thus the nature and magnitude of MPEs on prey survival (i.e., risk-enhancing effects: treatments BB, MM and FM; risk-reducing: BM; and linear effects: FF, FB). Given the specificity of predator-predator interactions, none of the predators were fully functionally redundant. Even when two species seemed substitutable (i.e., predators F and M), they led to vastly diverse effects when paired with additional predator species (i.e., B). We concluded that knowledge of the identity of the predator species and the behavioral interactions among them is crucial to successfully predict MPEs in natural systems.