Resource specialization by tadpoles determines their developmental responses to leaf litter quality and shade

Background/Question/Methods

Amphibian larval development is a plastic process that is influenced by energy and nutrient inputs from both the aquatic and terrestrial environments. In woodland ponds used by many amphibians larvae, terrestrial subsidies in the form of leaf litter provide both energy (carbon, C) and limiting nutrients (nitrogen, N and phosphorous, P) necessary for tadpole growth, development and survival. However, tree cover also limits light availability to primary producers and lowers water temperature, both of which can limit amphibian growth and development.  To our knowledge, no study has previously teased apart the influence of these opposing effects (light availability and litter quality) of tree cover on larval amphibian development across multiple species of amphibians. We hypothesized that amphibian species would respond more strongly to the most abundant resource type (leaf litter or algae) found in their typical habitat. To test this hypothesis, we raised tadpoles of a closed-canopy specialist (Wood frog, Lithobates sylvatica) and an open-canopy generalist (American toads, Anaxyrus americanus) in outdoor experimental mesocosms with varying levels of leaf litter quality (litter mixture C:N ratios) and light availability (shade). We predicted that wood frog and toad development would depend more strongly on leaf litter quality and light availability, respectively.

Results/Conclusions

In support of our predictions, we found that wood frogs responded most strongly to litter quality, while American toads responded most to light. Wood frogs in high quality litter treatments grew larger, developed faster, and survived better, whereas American toads grew larger, developed faster, and survived better in high light conditions.  Additionally wood frog growth was nonlinearly related to litter quality, matching predictions of resource-dependent growth models developed for aquatic invertebrate consumers. The results of this study reveal species-specific developmental effects of the forest canopy and indicate that litter quality and light should be analyzed independently when evaluating larval amphibian performance.