Interspecific differences in susceptibility to competition and predation in a species-pair of larval amphibians
Fundamental issues in the study of predator-prey interactions include addressing how prey coexist with their predators and, moreover, whether predators promote coexistence among competing prey. We conducted a series of laboratory experiments with a freshwater assemblage consisting of two predators that differd in their foraging modes ( a crayfish, Procambarus sp., and the western mosquitofish, Gambusia affinis) and their prospective anuran prey (tadpoles of the narrow-mouthed toad, Gastrophyrne carolinensis, and the squirrel treefrog, Hyla squirella). We examined whether competition occurs within and between these two prey species and, if so, whether the non-lethal presence of predators alters the outcome of competitive interactions. We also asked whether the two species of prey differ in their susceptibility to the two types of predators and whether interspecific differences in predator avoidance behavior might account for this variation. Our results indicated that Gastrophyrne was a stringer competitor than Hyla at high densities of surviving conspecifics that metamorphosed. However, the presence of mosquitofish did not alter the outcome of this competition, nor did either type of predator affect the density-dependent responses of Gastrophryne. In laboratory foraging trials, the number of tadpoles of each species that was killed, but not completely consumed by mosquitofish, was similar for Gastrophryne and Hyla. Yet, significantly more individualsof Gastrophryne than of Hyla were killed and completely eaten by mosquitofish at the end of the experiment. The two species of prey did not differ in their spatial avoidance of either type of predator, suggesting that this behavior did not play a significant role in the differential vulnerability of the prey to predation. By reducing the abundance og G. carolinensis, the potential exists for predators, such as mosquitofish to ameliorate this species' competitive impact on other species. In this way, predators may promote coexistence of species withi some assemblages of amphibians.