Evaluating a Predator-Induced Phenotype in a Mixed Species Context

Abstract

Phenotypic plasticity, a single genotype producing multiple phenotypes in response to environmental change, is crucical to our understanding ecological and evolutionary processes. Adaptive plasticity describes phenotypic response wherein a subsequent fitness benefit is conferred to the plastic individual. Predator-induced plasticity is a well-studied form of adaptive plasticity. For instance, numerous tadpole species exposed to aquatic predators produce more muscular and brightly colored tail fins, which have been shown to improve survival chances in subsequent predator encounters compared to noninduced individuals. However, predator-induced phenotypes can be costly when expressed in a non-lethal environment. Current understanding of the relative costs and adaptive benefits of predator-induced plasticity is based on intraspecific comparisons. However, multiple species differing in their plastic abilities often co-occur and interact with one another in nature. Few studies have evaluated whether the adaptive benefits and relative costs of predator-induced plasticity are retained in multi-species assemblages (a more ecologically-relevant setting). We conducted a two-phased experiment to evaluate the adaptive value and relative costs of a predator-induced phenotype in tadpoles of the Pine Woods Treefrog, Hyla femoralis, in the presence and absence of a congeneric species, the Squirrel Treefrog (Hyla squirella). Hyla femoralis and H. squirella share ecological settings and close evolutionary ties, yet larval H. squirella does not exhibit the same phenotypic response (changes in body/tail morphology) to predation risk as larval H. femoralis does. In Phase I (Induction), single-species assemblages were assigned to one of two predator-exposure (induction) treatments: a non-lethal treatment with a caged dragonfly nymph or a control with no predator. After four weeks, H. femoralis tadpoles from both induction treatments were photographed for morphometric analysis to quantify any plastic responses (change in morphology) to perceived predation risk. We found that larval H. femoralis morphology significantly differed between induced and noninduced populations. In Phase II (Predation Trials), tadpoles from single (H. femoralis only) and mixed-species assemblages (H. femoralis and H. squirella) were exposed to one of two predation treatments: a lethal, free-swimming predator treatment or a no-predator control. Periodic survival estimates were determined for both assemblages in lethal treatments to quantify possible survival advantages conferred by the inducible phenotype in larval H. femoralis. Growth metrics (size at emergence) and a development metric (time to emergence) were collected for both assemblages in control treatments to quantify possible costs associated with induced plasticity. Survival data supports that the adaptive advantage of increased survival in induced H. femoralis tadpoles is retained in mixed-species assemblages. Size at emergence in larval H. femoralis was not affected by induction treatment or assemblage type. Conversely, time-to emergence was significantly impacted by induction treatment. Induced H. femoralis tadpoles in both single and mixed-species assemblages took longer to reach metamorphosis, indicating predators likely have a stronger effect on developmental timelines than the presence of another tadpole species. This study aims to contribute revelatory insights into the ecology and maintenance of adaptive plasticity in natural, complex community systems.