Carnivory and resource‐based niche differentiation in anuran larvae: implications for food web and experimental ecology

Abstract

Summary1. Food webs represent the paths of material and energy flow through organisms in an ecosystem. Anuran larvae are important components of pond food webs: they are abundant, consume large quantities of food and serve as prey for many organisms. However, there are very basic uncertainties about the feeding ecology of anuran larvae; for instance, as to which trophic level they belong and whether species differ in resource use. Because anuran larvae have been employed in model systems in experimental ecology for decades, these uncertainties could lead to misinterpretation of published experiments, or inadequate designs of experiments directed at general, conceptual issues in ecology.2. Using13C and15N stable isotope and gut content analyses of free‐ranging and enclosed tadpoles of four ranid species (Lithobates sylvaticus,L. pipiens,L. clamitans,L. catesbeianus) in the food webs of six wetlands, we tested the following null hypotheses: (i) that anuran larvae are strict primary consumers; (ii) that they are non‐selective feeders and therefore exhibit little feeding niche differentiation; (iii) that they are opportunistic consumers and (iv) that their diet remains unchanged through ontogeny.3. All four species consumed and assimilated substantial amounts of animal food; bullfrog larvae, in particular, appear to be predatory. Significant feeding niche differentiation among species occurred with respect to the sources of carbon, consumption of animal matter and nutritional quality of food ingested. We further documented opportunistic feeding habits and ontogenetic shifts in diet.4. Collectively, these studies revealed complex trophic relationships that might require a reconsideration of the role of anuran larvae in pond food webs, as well as a reinterpretation of results of previous studies employing anuran larvae in model experimental systems.