Competitor-Induced Plasticity in Tadpoles: Consequences, Cues, and Connections to Predator-Induced Plasticity

Phenotypically plastic responses to environmental change are typically compartmentalized by the type of environmental cues that cause the induction (e.g., temperature, light), but different types of environmentally induced responses might very well be related to each other. I used a number of experiments to document competitor-induced behavior and morphology in wood frog tadpoles (Rana sylvatica), to quantify how competitor-induced changes affected subsequent competitive ability, and to investigate the cues responsible for the competitor-induced response. Competitors induced wood frogs to increase their activity and generally develop larger bodies and smaller tails. Further, wood frogs were able to discriminate between intraspecific and interspecific competitors. Whereas behavioral responses to competitors are well documented, the widespread morphological responses are a new discovery; these responses are particularly intriguing because they are in the opposite direction of predator-induced responses in tadpoles. The competitor-induced phenotypes are not simply an allometric effect, but appear to be adaptive responses to competitors; competitor-induced wood frogs experienced higher relative growth rates than noninduced wood frogs in subsequent performance trials, but past studies suggest that the competitor-induced phenotype should experience higher predation risk. When responding to competition, wood frogs were not responding only to reduced food. They were able to sense changes in both the per capita food levels and changes in density (per unit volume) that were independent of per capita food level. Thus, these animals have an amazing ability to sense changes in their environments and respond in very precise ways. When one considers competitor-induced responses in light of past studies on predator-induced phenotypes in larval anurans, one sees that predator-induced traits provide individuals with increased predator resistance but decreased competitive ability, whereas competitor-induced traits provide individuals with increased competitive ability but decreased predator resistance. This suggests that predator- and competitor-induced plasticity have evolved an intricate link that presents a trade-off between competitive ability and predator resistance ability. This trade-off is common across species and makes sense in light of the environments in which wood frogs live, ranging from ponds with high densities of predators and few tadpoles competing to ponds with few predators and extremely high densities of tadpoles competing. Further, there is evidence that this type of plasticity trade-off might be quite common in other phenotypically plastic taxa.