PHENOTYPIC VARIATION AND THE OUTCOME OF INTERSPECIFIC COMPETITION IN HYLID TADPOLES.

Abstract

For those amphibians which breed in temporary ponds, larval growth rate is an important parameter in determining successful completion of the larval stage. Wilbur and Collins (1973) and Wilbur (1976) have proposed that growth rate of early larvae is an excellent predictor of the length of the larval period, and their data have supported this prediction for Rada sylvatica and Ambystoma species (but see Smith-Gill and Berven, 1979, for an alternative interpretation). In a temporary pond, whether any offspring of a pair of adults survive depends upon whether any of the tadpoles metamorphose before the pond dries, therefore upon the length of the larval period. Individual larval growth rate is important for successful metamorphosis in a second manner: individuals which grow quickly achieve a size refuge from predation at an earlier age (P. Morin, unpubl. data). Larval growth rate is also a valid measure of an individual's competitive ability (Wilbur and Collins, 1973; Collins, 1975; Smith-Gill and Gill, 1978). If competition becomes important due to either large numbers of young hatchlings or the diminished resources of a drying pond, then variation in competitive ability will lead to variation in larval growth and ultimately variation in metamorphic success among the members of a population. This will be true whether competition is intraor interspecific in nature. Further, if such variation in competitive ability is genetically based, then the outcome of competition would depend upon the intensity of the interaction and the relative frequencies of the different competing genotypes in the population, a natural process of soft selection (Wallace, 1975). Thus larval growth rate is likely to be an important parameter in determining metamorphic success whatever the dominant environmental influence: habitat ephemerality, predation, intraor interspecific competition. One important source for variability in larval growth rate is among the different sets of full sib families which comprise a larval population in temporary ponds. Because any such variation in larval growth rate and resultant variation in metamorphic success among the separate full sib families in the pond would lead to differential reproductive success within a frog population, the presence of such variation is of fundamental interest. In this paper I report the results of an experiment designed to test for the presence of variation among sibships in a population of Hyla gratiosa and the effect of that variation on metamorphic success, both within that population and on individuals of a co-occurring sibship of Hyla femoralis, an ecologically similar species.