Differential consumption and assimilation of leaf litter by wetland herbivores: alternative pathways for decomposition and trophic transfer

The process of decomposition has received much attention in terrestrial and stream ecosystems, but our understanding of the factors that contribute to this process in wetlands remains relatively poor. Many macroconsumers in wetlands are classically labeled as herbivores, but increasing evidence suggests that they also contribute to the breakdown of dead plant litter depending on the nutritional quality (i.e., nutrient content, density, and toxicity) of the litter. We examined the relative contributions of 2 common North American temperate wetland consumers, the green frog tadpole (Lithobates clamitans) and the Ram's Horn snail (Planorbella trivolvis), to the decomposition of 5 chemically variable plant litter species. Based on anatomical differences (e.g., mouth parts, digestive structures), we hypothesized that snails would have higher consumption rates than tadpoles, but that tadpoles would have higher assimilation efficiency. We also predicted that consumption rates and assimilation efficiency would vary with litter nutritional quality. Overall, consumers exhibited substantial detritivory and caused up to 62% litter mass loss relative to treatments with only microbes. As hypothesized, snails consumed more than tadpoles, but this difference was largely explained by differences in consumer mass. Contrary to our hypothesis, snails and tadpoles exhibited similar assimilation efficiencies. Both litter mass loss and assimilation efficiency by consumers differed among litter species treatments. Litter mass loss tended to be negatively correlated with litter C : N and C: P, whereas assimilation efficiency had no detectable correlation with any measured litter characteristic. Our study demonstrates that studies of energy and nutrient budgets in wetlands should consider both consumer type and litter species to describe ecosystem function fully.