Nitrogen pollution as a result of agricultural runoff and atmospheric deposition is a major challenge to aquatic ecosystems, and is likely to increase in the future. Nitrogenous pollutants are potential stressors of amphibian larvae through their toxicological impacts on individuals; however, they may also increase primary productivity. We examined how such an anthropogenic stressor could influence the interactions between two potentially competing species of tadpoles (Rana clamitans and R. catesbeiana). In a 42 d mesocosm experiment, R. catesbeiana survival, but not final mass, was reduced by nitrate addition. Rana catesbeiana survival was lower when in competition with R. clamitans than when only experiencing intraspecific competition. For R. clamitans, survival was not affected by nitrate addition or competition type. Rana clamitans in nitrate addition mesocosms were heavier than tadpoles from no nitrate mesocosms, and were heavier in intraspecific than in interspecific mesocosms. Our results suggest that nitrate can influence the performance of amphibian larvae, and that its effects could have complex effects on aquatic ecosystems.

The ability of prey to respond to novel predator cues may depend on the generality or specificity of the response to predator cues. We used laboratory behavioral experiments to examine the ability of tadpoles of three species of anurans (American toad, Bufo americanus; bullfrog, Rana catesbeiana; and green frog, R. clamitans) to respond to the presence of two native potential predators (bluegill, Lepomis macrochirus; and largemouth bass, Micropterus salmoides) and one non‐native potential predator (goldfish, Carassius auratus). We also examined the effect of tadpole size on the behavioral responses of American toads and green frogs to predator cues. All three species of tadpoles responded to the presence of predator cues, although the specific responses varied among species. American toads and green frogs reduced activity in the presence of at least some fish cues, but bullfrog tadpoles did not change their activity. Bullfrogs decreased use of vegetation in the presence of some predator cues, whereas American toads and green frogs did not. American toads only responded to the presence of bluegill cues but not the other fish predator cues, whereas bullfrogs and green frogs responded more generally to the fish predators. In both American toads and green frogs, tadpole size affected behavior. For American toads, activity increased, as did the use of the vegetated side of the aquarium, in larger tadpoles. Not only did size affect American toad behavior, but it also influenced the responses of the tadpoles to predator cues. For green frogs, activity decreased in larger tadpoles. Our results suggest that behavioral responses of tadpoles to predator cues can be influenced by both the identity of the predator and the prey, as well as the size of the potential prey.

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Nitrate is a potential stressor of amphibian larvae. Previous research has shown variation in the effects of nitrate among species and even populations of amphibians. However, relatively few species and populations of amphibians have been examined for nitrate tolerance, especially from populations in the agricultural regions of the United States. Using a 15-day laboratory experiment, we investigated the effects of nitrate on the larvae of two species of anurans (Rana catesbeiana and R. clamitans) that are common throughout the agricultural Midwest. Survival of R. catesbeiana and R. clamitans tadpoles was negatively affected by higher concentrations of nitrate (20 mg L(-1)), with R. clamitans being more affected than R. catesbeiana. The final mass of R. catesbeiana tadpoles was affected by nitrate concentration, with tadpoles exposed to intermediate concentrations of nitrate (5 mg L(-1)) being the heaviest. The final mass of R. clamitans tadpoles was not affected by nitrate concentration. These experiments suggest that these two species differ in their responses to the toxic effects of nitrate and, along with previous results on Ranids, suggest there is a great deal of variation in nitrate tolerance in this family.

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