Alterations of aquatic ecosystems, such as nutrient enrichment and changes in community composition, can potentially have effects that pervade the entire community. We investigated the effects of nutrient enrichment and changes in the presence and density of two species of tadpoles, American toad (Anaxyrus americanus) and Gray treefrog (Hyla versicolor), on freshwater snail abundance (Physella sp.). At low tadpole density, there was no difference in the abundance of Physella sp. among treatments. At high tadpole density, treatments with both species of tadpoles present and nutrient enrichment had a higher abundance of Physella than all other treatment combinations. Mesocosms with high tadpole densities had lower periphyton dry mass than those with low overall tadpole density. At the end of the experiment, increased abundance of Physella was related to earlier metamorphosis in A. americanus and H. versicolor and higher proportions of H. versicolor metamorphosing and surviving. Nutrient enrichment did not independently affect snails. Our results suggest that changes in the composition and density of tadpole assemblages can interact with nutrient enrichment to drive variation in freshwater snail abundances, indicating that ongoing declines in amphibian populations combined with continued anthropogenic nutrient enrichment of freshwater ecosystems may have complex effects on freshwater snail populations.
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