Forest canopy and the performance of larval amphibians

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.