Metamorphosis of two amphibian species after chronic cadmium exposure in outdoor aquatic mesocosms.
Pesticides are widely used by humans to eliminate or reduce populations of unwanted species. These pesticides often cause collateral damage by killing nontarget species and altering biological communities. Our study examined the relative importance of direct and indirect effects of the insecticide carbaryl on southern leopard frog tadpoles, Rana sphenocephala, in a simple aquatic community. We did not detect any significant direct effects of carbaryl on the anuran life-history traits examined in this study. Rather, environmentally relevant concentrations of carbaryl indirectly affected R. sphenocephala life-history traits by causing changes in the intensity of competition and predation within the community. Carbaryl generally increased the survival of R. sphenocephala tadpoles. However, the increase in survival was greatest in mesocosms containing predators, largely due to pesticide-induced mortality of the predators. Carbaryl also exacerbated the effects of competition by decreasing periphyton abundance, which resulted in smaller metamorphs. Thus, we conclude that investigations of the effects of pesticides on a species of interest should go beyond standard toxicological protocols and examine the effects of the pesticide within the context of the community, placing special emphasis on understanding how competition and predation mediate indirect effects.