A stable isotope study of a neotropical stream food web prior to the extirpation of its large amphibian community

Rapid and massive amphibian population declines have been reported throughout upland areas of the Neotropics. The abundance and species richness of Neotropical amphibian communities suggest that losses of this magnitude are likely to have strong effects at the ecosystem level. To improve understanding of the implications of their loss we used stable isotope analysis to examine trophic relationships in an ecosystem in which amphibians arc dominant in a second-order forest stream at 750 m asl in Parque Nacional Omar Torrijos Herrera, Panama. We analysed delta(13)C, delta(15)N and C:N ratios of major biotic components (basal resources, invertebrates, amphibians, fish and reptiles) in the stream and of the adjacent riparian food web. Tadpoles (mean delta(15)N = 4.49 parts per thousand) and adult amphibians (mean delta(15)N = 5.45 parts per thousand) were intermediate links in the aquatic and terrestrial food web respectively. High delta(15)N signatures identified fish as top predators in the aquatic food web and snakes and the toad Bufo as top predators in the terrestrial food web. Isotopic signatures clearly distinguished between trophic groups of tadpoles: microbial feeders (Centrolenidae, delta(15)N range = 0.91-3.05 parts per thousand), herbivores (Rana and Hyla, delta(15)N range = 4.74-5.15 parts per thousand) and neuston feeders (Colostethus, delta(15)N range = 5.31-6.40 parts per thousand). Dependence on autotrophic production was indicated by enriched signatures of carbon isotopes in pool dwellers versus those that reside in faster-flowing sections of the stream. High nitrogen concentrations in detrital matter (average 0.8%, C:N = 10.3) suggested that grazing tadpoles enhanced nitrogen fluxes and improved the quality of organic matter available to detritivores.