Threat of predation negates density effects in larval gray treefrogs

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Abstract
While density-dependence is central to most theory regarding population regulation and community structure, specific mechanisms that modify its effects in the absence of changes in consumer-resources ratios (e.g., thinning) are not well understood. To determine if the threat of predation alters effects of density, we investigated the interaction between density of larval treefrogs (Hyla chrysoscelis) and the non-lethal presence of a predatory fish (Enneacanthus obesus). A significant density by fish interaction was consistent for all response variables (e.g., larval survivorship, mass, and time to metamorphosis) driven by a complete lack of density effects in the presence of predators, while predator-free tanks showed classic density-dependent responses. Given that female H. chrysoscelis strongly avoid ovipositing in ponds containing fish, certain larval adaptations are apparently not constrained by maternal behavior and suggest redundancy in response to predators. Our data suggest that non-lethal effects of predators can determine larval performance irrespective of larval density, and that the non-lethal effects of predators can be strong whether lethal effects are strong or weak.