Influence of multiple environments on tadpole phenotypes: plasticity, adaptation, and conservation implications

Phenotypic plasticity is the ability of an organism to alter its phenotype in response to changing environmental conditions. The recognition that phenotypic changes often improve the fitness of individuals (i.e., are adaptive) has expanded interest in plasticity as an important driver of many ecological and evolutionary processes. Predation risk and competition density are important environmental factors that induce adaptive phenotypic changes in many species. However, the specific impact of these factors on an individual's phenotype depends on the foraging behavior employed by the predator, the type of competition experienced (interference vs. exploitative), and other environmental conditions. For anuran larvae (i.e., tadpoles), the presence of vertical habitat structure may affect phenotypic responses by altering: 1) capture success and foraging behavior of predators and 2) the type and level of competition experienced. Using large-scale mesocosm experiments, I found that, compared to simple environments, tadpoles were less likely to express adaptive behavioral and morphological phenotypes in habitats containing greater environmental complexity. For example, tadpole phenotypic responses to changes in habitat structure and competition density were adaptive, but only in the absence of predation risk. However, in a survey of natural ponds, predation risk and competition density, but not habitat structure, explained substantial portions of tadpole phenotypic variance. Even though tadpoles experience changes in multiple environmental variables, predation and competition seem to exert the greatest influence in shaping tadpole phenotypes. Predation risk and competition intensity also interacted to modify the development of a maladaptive tadpole malformity linked to ultraviolet-B radiation overexposure. Behavioral changes of tadpoles induced by predation and competition, such as activity levels, likely mediated this effect. Through this research, I have demonstrated that increasing environmental complexity constrains the ability of organisms to express adaptive phenotypes. Patterns of phenotypic plasticity observed in laboratory conditions do translate to natural environments, but are limited to phenotypic responses to environments that exert strong inductive and selective pressures. Furthermore, phenotypic plasticity can modify the susceptibility of organisms to the effects of environmental stressors.