The effects of extirpation of frogs on the trophic structure in tropical montane streams in Panama

Amphibian populations are in global decline. Species that have stream dwelling tadpoles and inhabit upper montane environments are disproportionately affected. Tadpoles are keystone herbivores in the streams so their removal is expected to have a wide range of ecosystem effects. This inspired the collaborative TADS (Tropical Amphibian Declines in Streams) project which investigated the spectrum of ecosystem effects of the frog extirpation. The study was conducted in the uplands of Panamá at two sites that were differentially affected by the die offs. This arm of the TADS project used stable isotopes to investigate the changes in trophic structure in the streams. During the field season, a massive die off event occurred at the healthy site. This provided a unique opportunity to study the changes in trophic structure as they were transpiring. Several interesting trends were elucidated. The ultimate source of nitrogen and carbon are different at both sites. The food web was truncated in the absence of the tadpoles, resources that were thought to be important in the system proved not to be and fractionation of 15N and 13C varied between the sites. In keeping with the trends in the current literature, the IsoSource mixing model software was used to evaluate the trophic linkages. IsoSource which is used to determine relative contributions of multiple sources to a consumer, proved to be ineffective for pioneer studies such as this one. It is clear from the data however, that the trophic structure in both locations is significantly different and that the tadpoles provided an important subsidy to the food web. Some compartments of the food web in El Copé have already begun to approach prevailing conditions in Fortuna, but further studies are required to determine the length of time required for the entire system to equilibrate and approximate to the current conditions in Fortuna.Ph.D., Environmental Science -- Drexel University, 200