Ecological causes and consequences of variation in tadpole behavior

Background/Question/Methods

Theory predicts that intraspecific trait variation can have important ecological impacts, yet we have a poor understanding of the causes and consequences of trait variation in natural systems. Behavior is often particularly variable and has recently drawn increased interest. Natural selection may generate among-population trait differences by local adaptation to divergent ecological communities, and these adaptive variants may have cascading effects on the rest of the ecosystem through interactions of these organisms with other species. Trait differences may also arise non-evolutionarily by ecological factors acting withinpopulations; certain ecological conditions may yield greater variation among individuals. We used wood frog tadpoles to explore 1) how adaptation to different ponds produces population differences in behavior, 2) how ecological factors directly impact within-population variation in behavior, and 3) the consequences of variation at each of these scales for pond ecosystems. We reared tadpoles in pond mesocosms and manipulated the source population, tadpole density, presence of predators, and spatial heterogeneity in resources to determine which factors affect behavioral variation. We also measured tadpole growth rates, periphyton biomass, and predator growth to determine the ecological consequences of different levels of variation.

Results/Conclusions

We found that behavioral responsiveness to predators increased with predation risk in source ponds, indicating that predation may drive ecological differences among populations. More active tadpoles were expected to have stronger negative effects on periphyton, and less responsive tadpoles would thus result in smaller indirect effects of predators on periphyton biomass. Increased responsiveness of tadpoles was surprisingly not associated with greater increases in periphyton. This may indicate that foraging behavior and overall activity are decoupled more than has typically been recognized. Several ecological factors were associated with behavioral variation within populations, apparently mediated by their impact on size structure. This variation had minimal effects on other trophic levels, however. Future work should continue to explore the ecological causes of behavioral variation in order to situate it in a predictive framework. These findings highlight the potentially important role of ecological differences among communities in shaping, and being reciprocally shaped by, intraspecific behavioral variation.