Ontogenetic and fertilizer effects on stable isotopes in the green frog (Rana clamitans)

Abstract

Anurans undergo dramatic morphological and physiological changes at metamorphosis, accompanied by shifts in diet and habitat. Frogs provide an ideal model organism for studying ontogenetic shifts in diet due to their biphasic life history and widespread distribution throughout their range. Analysis of stable isotopes of carbon (C) and nitrogen (N) in green frog tadpoles and adult frogs was used to describe ontogenetic shifts in diet and habitat. Adult and larval green frogs were collected from four wetland locations in Nova Scotia, Canada and analyzed for stable isotopes. Differences in isotopic ratios were observed among tadpoles and adults. Tadpoles from wetlands with high nitrate concentrations exhibited greater δ15N values than adults, while tadpoles from wetlands without N enrichment had lower δ15N values. These enriched nitrogen signatures were probably the result of larval exposure to nitrogenous pollution originating from fertilizers and illustrate the difficulties in using isotope analysis to quantify trophic structure in biphasic organisms occupying dramatically different habitats.