Carbaryl concentration gradients in realistic environments and their influence on our understanding of the tadpole food web.

Although exposure to pesticides has been correlated with amphibian declines, the mechanism of their role remains enigmatic. Declines have been associated with sublethal exposure, but few outdoor studies have evaluated impacts of low pesticide concentrations. Understanding the effects of a range of pesticide concentrations on amphibians in outdoor mesocosms provides a framework for both direct and indirect effects of exposure. Indirect effects are challenging to glean from lab studies, which typically lack a food web. Our design tested direct and indirect effects of exposure to the insecticide carbaryl on the American toad (Bufo americanus) and the northern leopard frog (Rana pipiens) tadpole survival and growth. We evaluated the effects of five concentrations (2.0, 0.2, 0.02, 0.002, and 0 mg carbaryl/l). Specifically, we predicted a threshold effect in which carbaryl concentrations great enough to reduce zooplankton abundance would have negative indirect effects on tadpoles, but the degree of these effects would not be concentration-specific. Similarly, we predicted that lower carbaryl concentrations (where zooplankton abundance was not reduced) would not differ in effect from controls. We did not observe a threshold effect or any negative effect on tadpoles. The highest carbaryl concentration expedited the time to metamorphosis in the northern leopard frogs by ~4 days. The toads were unaffected by any concentration of carbaryl. Despite significant reductions in zooplankton abundance and increases in phytoplankton abundance, periphyton abundance was unaffected by carbaryl. Taken together, these results suggest that despite evidence from studies using single concentrations of pesticides, trophic cascade models do not sufficiently explain sublethal effects on larval amphibians.