Conspecific density determines the magnitude and character of predator-induced phenotype

The benefits in survival gained from predator-induced phenotypes often come at a cost to other components of fitness. Therefore, the level of expression of an induced phenotype should mirror the level of risk in the environment. When a predator exhibits a saturating functional response the risk of mortality to a given prey decreases as prey density increases. Therefore, for a given predator threat, investment in defense should be lower in prey at high density relative to those at low density. In this study, I test whether the magnitude of predator-induced morphological plasticity decreases with increasing conspecific density by exposing pine woods tree frog (Hyla femoralis) tadpoles at three different densities to predators (present or absent) in a factorial experiment. Tadpole morphology was not affected by changes in density in the absence of predators. However, predators had a significant, density-dependent effect on tadpole morphology. Specifically, the magnitude of morphological response was graded and larger for animals in the low density (high risk) environment. This study demonstrates that tadpoles can modulate phenotypic plasticity in response to mortality risk as a function of both the density of conspecifics and chemical cues from predators, which suggests that they are able to detect and respond to fine-scale changes in the threat environment. In addition, this study highlights the need for analytical approaches that allow morphological plasticity studies to elucidate allometric relationships in addition to simply quantifying size-corrected traits.