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.

2004. Asymmetric competition between Rana clamitans and Hyla versicolor tadpoles. Á/ Oikos 105: 626 Á/632.When two organisms interact over a potentially limiting resource, several outcomes are possible (e.g. neither affected, both equally affected, one affected more than the other). The outcome depends on a variety of factors, both internal and external to the organism. We performed a laboratory experiment to investigate the outcome of potential competitive interactions between tadpoles of the green frog (Rana clamitans ) and the gray treefrog (Hyla versicolor ), and to examine the factors that may explain the outcome of the interaction. We found that competition between these two species was asymmetric. Hyla versicolor tadpoles were more affected by intraspecific competition than interspecific competition (i.e. grew faster when grown with heterospecifics than conspecifics); whereas, R . clamitans tadpoles were more affected by interspecific competition than intraspecific competition (i.e. grew faster when grown with conspecifics than heterospecifics). Based on behavioral observations, this result could not be explained on the basis of activity levels, a trait previously linked to competitive ability in tadpoles. However, differences in initial body size may help explain the observed results; the gray treefrog tadpoles were larger than the green frog tadpoles.

We investigated the distributions of amphibian larvae within two temporary ponds in central Ohio. Spring Peeper Pond had higher densities of all taxa than did Taylor-Ochs Pond. All amphibian larval distributions were significantly aggregated on all sampling dates in both ponds. Larval abundances changed over time in Spring Peeper Pond, but not in Taylor-Ochs Pond. In Spring Peeper Pond, spring peeper tadpole densities were highest in the shallows, leopard frog tadpole densities were negatively related to temperature, and American toad tadpole densities were not related to any factor. Larval densities were not correlated with any environmental factor in Taylor-Ochs Pond. In both ponds, spring peeper and American toad tadpole densities were positively correlated for some dates and for the overall pooled dataset. Salamander larvae and spring peeper tadpole abundances were positively correlated on one date in Taylor-Ochs Pond. Our study points out that the factors that can explain tadpole distributions vary not only among species, but also among populations of the same species. INTRODUCTIONWhile numerous studies have considered the factors driving among-pond distributions of amphibian larvae, less attention has been paid to within-pond larval distributions, even though such distributions could have significant consequences for the performance of individual amphibian larvae. Within-pond distributions of amphibian larvae have been shown to be a function of water depth, predator presence or absence, presence of aquatic vegetation, substrate type, dissolved oxygen content, and temperature (e.for review).We investigated the distribution of amphibian larvae within two fishless, temporary ponds in central Ohio, USA. We were particularly interested in determining if various physical and biotic characteristics (water depth, dissolved oxygen, water temperature; light at the water's surface, presence or absence of macrophytes) could explain variation in abundance within these ponds.

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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.