COMPETITION, PH, AND THE ECOLOGY OF LARVAL HYLA-ANDERSONII

I investigated the effects of an abiotic factor, pH, and a biotic factor, interspecific competition, on breeding success in a low-pH tolerant anuran, Hyla andersonii. I tested whether low pH would indirectly facilitate H. andersonii by lessening the impact of competition from two less acid-tolerant species. I also tested whether the competitive effect of an allotopic, closely related species (Hyla versicolor) would be greater than that of a syntopic, more distantly related species (Rana sphenacephala). I exposed H. andersonii sonii tadpoles to two levels of pH (3.9, ambient) and three levels of competition (H. versicolor, R. sphenocephala, and none). Reduced pH had no direct effect on any of the species and did not influence the outcome of interspecific competition. Competition from X. sphenocephala decreased survival and mass of H. andersonii, whereas competition from H. versicolor only decreased mass, without affecting survival of H. andersonii. These results suggest that some other correlate of low pH in natural ponds limits H. andersonii to acidic ponds. Further, the assumptions that co-occurrence or level of relatedness can be used as a gauge of competitive strength are shown to be unreliable.