Anthropogenic stressors, such as contaminants or pollutants, can change the outcome of competition in a variety of communities. We examined potential competitive interactions between American Toad (Anaxyrus americanus) and Gray Treefrog (Hyla versicolor) tadpoles by manipulating the presence and absence of each species and overall tadpole density and how they may be affected by anthropogenic nutrient enrichment by manipulating the presence and absence of nitrate and phosphate. Interactions with Gray Treefrogs did not affect American Toad survivorship, mass at metamorphosis, or time to metamorphosis. Nutrient enrichment reversed the relative effects of interspecific and intraspecific conditions on Gray Treefrog survivorship. Namely, in enriched mesocosms Gray Treefrog tadpoles living with American Toad tadpoles had higher survivorship than those in Gray Treefrog–only communities, whereas the opposite was true in no-enrichment mesocosms. Nutrient enrichment accelerated metamorphosis in American Toads but did not affect Gray Treefrogs. Overall tadpole density had no effect on Gray Treefrogs, but American Toads metamorphosed earlier and at a larger size at high tadpole densities. Our results provide further confirmation that the alteration of the environment by humans can have a variety of effects, both directly through their effects on each species and indirectly by altering the interactions among species.
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Vol. 68 • No. 2