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Freshwater ponds inhabited by larval amphibians can become encroached by forest vegetation. Surveys reveal that gradients formed by canopy cover can act as a selective sieve for the distribution of amphibian larvae among ponds. In eastern North America, many species, including spring peepers (Pseudacris crucifer), are usually absent from ponds where forest canopies have closed over the pond basin. Relatively fewer species, including wood frogs (Rana sylvatica), are commonly present in both closed and open canopy ponds. Long-term observations reveal that canopy development is associated with extinction of spring peeper populations and persistence of wood frog populations. We evaluated performance of spring peeper and wood frog larvae in a set of experiments designed to assess three predictions: (1) that conditions in closed canopy ponds (e.g., lower temperature) are associated with depressed growth rates, (2) that the impact of canopy cover will be more severe on an open canopy specialist vs. a canopy generalist species, and (3) that abiotic differences between closed and open canopy ponds may provide a sufficient explanation for performance patterns. In support of the first prediction, a field transplant experiment showed that both species grew slower in closed vs. open canopy ponds. We found some support for the second prediction during two separate experiments. During the field experiment, the impact of canopy on spring peeper growth was similar to the effect on wood frogs. However, in a common garden experiment (in which physical conditions tended to coverge), spring peepers raised on substrates and water from closed canopy ponds grew substantially slower than conspecifics in the presence of water and substrate from open canopy ponds. By contrast, wood frogs grew faster in the closed canopy treatment. Finally, while recognizing that abiotic variation appears to play an important role in performance across the canopy gradient, we rejected the third hypothesis. We found that a biotic manipulation (food addition) partially mitigated depressed growth rates in closed canopy ponds. This result suggests that biotic factors (e.g., variation in abundance and composition of periphyton) also contribute to performance differences across the canopy gradient.

Ectotherms use behaviour to buffer effects of temperature on growth, development and survival. While behavioural thermoregulation is widely reported, localized adaptation of thermal preference is poorly documented. Larval amphibians live in wetlands ranging from entirely open to heavily shaded by vegetation. We hypothesized that populations undergo localized selection leading to countergradient patterns of thermal preference behaviour. Specifically, we predicted that wood frog (Rana sylvatica) larvae from closed canopy ponds would be more strongly temperature selective and would prefer higher temperatures than conspecifics from populations found in open canopy ponds. In a study of six breeding ponds in north-eastern Connecticut, USA, these predictions were upheld. The countergradient, microgeographical variation in thermal preference documented here implies that wood frog populations may have diverged rapidly in the face of contrasting selection pressures. Rapid, behaviourally mediated responses to changing thermal environments have important implications for understanding population responses to climate change.

The vegetation in and around the basins of ephemeral wetlands can greatly affect light environments for aquatic species such as amphibians. We used hemispherical photographs to quantify the light environment in terms of the global site factor (GSF), the proportion of available solar radiation that actually strikes the wetland. We compared GSF to the distribution and performance of two amphibian species (Pseudacris crucifer and Rana sylvatica) within 17 ephemeral wetlands in northeastern Connecticut, USA. We found that P. crucifer is restricted to lighter wetlands (GSF >0.34) and that its abundance is proportional to GSF. By contrast, R. sylvatica is found across the light gradient and its abundance is unrelated to GSF. For both species, GSF is a strong predictor of larval developmental rate. In addition, P. crucifer growth rate is higher in lighter wetlands. Through thermal effects, changes in resources, or other influences, light appears to be an important predictor of the distribution and performance of amphibians. Because the structure of canopies can change rapidly, and because amphibians can be strongly impacted by these changes, vegetation mediated effects on wetland light environments may be critical to understanding the dynamics of amphibian populations within forested biomes.