Effect of pH Variation of Interspecific Competition Between Two Species of Hylid Tadpoles

Abstract

When species differ in their ability to cope with varying abiotic conditions, a change in an abiotic factor can potentially alter the outcome of biotic interactions. The purpose of this study was to examine this phenomenon. We tested the effects of an abiotic factor (pH) and two biotic factors (intraspecific and interspecific competition) on survival and growth to metamorphosis by larvae of two anurans (Hyla gratiosa and Hyla femoralis) in outdoor tanks. Two levels of pH (4.5 and 6.0) and three levels of density (0, 30, or 60 embryos per 580 L tank) of each species were arranged in a fractional factorial design and replicated threes times. Larvae of H. gratiosa exhibited typical patterns of intraspecific density—dependent responses: as density increased, survival and size at metamorphosis decreased and larval period increased. The interactions of the abiotic factor and the biotic factors were complex in this experiment. The addition of H. femoralis at high pH caused decreased survival and increased larval period of H. gratiosa, but at low pH these negative effects were not detected. Variation in pH had no effect on the size at metamorphosis of H. gratiosa. Interspecific competition from H. femoralis caused decreased size at metamorphosis of H. gratiosa only at the high initial densities. Some of the responses of H. femoralis were different than those of H. gratiosa. In this species, intraspecific competition affected only the size at metamorphosis. Low pH increased the survival of H. femoralis, but had no effect by itself on any other trait. Increased conspecific density and pH interacted only in their effect on the size at metamorphosis of H. femoralis. Interspecific competition resulted in decreased size at metamorphosis but only increased larval period at the high initial densities. Interspecific competition at the high density interacted with pH to increase survival and size at metamorphosis at low pH in contrast to those results at high pH. Overall, the larval period was the response most strongly affected in H. gratiosa while for H. femoralis the size at metamorphosis was the most strongly affected. Hyla gratiosa had more negative impact on H. femoralis than vice versa. This may be due to the larger body size of H. gratiosa. Low pH enabled H. femoralis to compete more effectively, because the negative impact of H. gratiosa was lessened. Even in a relatively simple simulation, complex interactions occurred between biotic and abiotic factors. This finding demonstrates the importance of multi—factor analysis in furthering our understanding of the structure of assemblages.