The effects of wood frogs (Rana sylvatica) on forest floor communities: Investigating invertebrate community changes, leaf decomposition and wood frog diet

Background/Question/Methods

The effects of amphibians on ecological communities are often investigated using the aquatic environment with larval stage amphibians. This approach, while extremely useful, only focuses on the aquatic life stage of the amphibian and potentially leads to underestimation of effects mediated through the food web or persisting over multiple life stages. Moreover, environmental pollutants like road salt may alter how metamorphosed organisms interact with the terrestrial environment. If road salt has negative effects on survival, terrestrial density of wood frogs may also change. We tested whether wood frogs (Rana sylvatica) exposed to road salt as larvae or altered juvenile density could affect forest floor invertebrate density and community composition. We conducted an experiment on wood frogs using outdoor mesocosms and terrestrial pens. Larval wood frogs were raised in high (500 mg/L salt) or low (no salt added) salt concentrations until metamorphosis. Juvenile frogs emerging from the mesocosms were then placed in terrestrial pens at high (5 frogs/m²) and low (2 frogs/m²) terrestrial densities. After 18 weeks, we measured juvenile growth (mass, snout-vent length (SVL), relative limb lengths), leaf decomposition rates, juvenile frog diet and invertebrate abundance and richness measures in each pen.

Results/Conclusions

Larval road salt exposure did not affect post-metamorphic frog growth. Low terrestrial density significantly increased frog mass and SVL. An interaction revealed exposure to road salt increased relative hind-limb length among low-density terrestrial juvenile frogs. Road salt did not affect leaf decomposition but high densities of juvenile frogs lowered decomposition rates. Fifty-five percent of juvenile frogs had invertebrates (typically beetles, flies and earthworms) in their stomachs. Larval treatment did not affect frog diet. Frogs in the high-density treatments were significantly more likely to have at least one invertebrate in their stomach than frogs from low-density treatments.  Neither road salt nor terrestrial density affected invertebrate richness or abundance in frog stomachs. Beetles, nematodes, and spiders were the most abundant invertebrates captured in Berlese funnels. Neither road salt nor terrestrial density affected invertebrate richness or abundance in Berlese funnels. Finally, invertebrates from Berlese funnels did not significantly predict stomach contents. Because previous research often focuses on aquatic amphibians, we have a relatively poor understanding of the effects of amphibians post-metamorphosis. Our results suggest salt exposure interacts with density to affect wood frog hind-limb length.  Furthermore, frog density affected diet and terrestrial ecosystems through changes in leaf decomposition, especially at high densities.