Single and interactive effects of malathion, overwintered green frog tadpoles, and cyanobacteria on gray treefrog tadpoles.

Amphibian population declines around the world are associated with invasive species, pesticides, pathogens, habitat destruction, or a combination of factors. Because contamination is widespread, it represents a relevant environmental stress that can affect the ability of organisms to deal with other factors present in the environment. We examined the effects of the insecticide malathion, larger tadpole competitors (green frogs, Rana clamitans), and a toxic cyanobacteria (Anabaena spp.) on tadpoles of Cope's gray treefrog (Hyla chrysoscelis) reared from hatching through metamorphosis in outdoor mesocosms. The response of mass at metamorphosis and time to metamorphosis was significantly affected by exposure to malathion and presence of overwintered green frog tadpoles. Malathion generally led to increased mass at metamorphosis, earlier time to metamorphosis, and increased activity during larval development. These results likely stem from short-term increases in periphyton associated with malathion exposure (although these effects were nonsignificant). Exposure of gray treefrogs to overwintered green frog tadpoles led to an earlier time to metamorphosis without differences in mass at metamorphosis and was associated with increased activity in gray treefrogs. Survival of gray treefrogs was significantly affected by an interaction of green frog and malathion, indicating nonadditive effects of these treatments. Exposure to cyanobacteria had a significant negative effect on green frogs but no effect on treefrogs. Malathion had the strongest effect on the community, but our results indicated that some factors can interact in ways not predicted by single factors alone.