Pesticide has asymmetric effects on two tadpole species across density gradient.

Northern leopard frogs (Rana pipiens) have been disappearing throughout their range. The causes for extirpations have been elusive, although habitat alteration appears to be a major factor. Pesticides have been implicated in declines in western amphibians, but no mechanism has been identified. We tested the hypothesis that leopard frog tadpoles would be eliminated from insecticide-exposed ponds when they were less abundant than a competitor (American toads, Bufo americanus) by manipulating toad and leopard frog density (16, 50, or 150 of each species) with or without the insecticide carbaryl. Insecticide exposure did not interact with density on either species, indicating that contaminant effects were similar despite drastic differences in competition. The survival of both species decreased with increasing conspecific density, but not heterospecific density. Leopard frogs produced more metamorphs at intermediate densities, whereas toads produced more metamorphs at high density. Leopard frog mass at and time to metamorphosis responded negatively to density. Carbaryl did not affect leopard frog survival but positively affected mass at and time to metamorphosis. In contrast, toad survival was reduced after exposure, but mass at metamorphosis was greater. Pesticide-induced survival reductions may not affect population size if reductions are slight (as in toads, whose fitness may be compensated by increased mass). Management strategies aimed at ameliorating the effects of carbaryl in systems with these two species may not need to consider density.