Effects of Introduced Mosquitofish and Bullfrogs on the Threatened California Red‐Legged Frog

Abstract

Abstract: Exotic species have frequently caused declines of native fauna and may contribute to some cases of amphibian decline. Introductions of mosquitofish ( Gambusia affinis) and bullfrogs ( Rana catesbeiana) are suspected to have caused the decline of California red‐legged frogs ( Rana aurora draytonii). We tested the effects of mosquitofish and bullfrog tadpoles on red‐legged frog tadpoles in spatially complex, speciose communities. We added 720 hatchling red‐legged frog tadpoles to each of 12 earthen ponds. Three ponds were controls, 3 were stocked with 50 bullfrog tadpoles, 3 with 8 adult mosquitofish, and 3 with 50 bullfrogs plus 8 mosquitofish. We performed tests in aquaria to determine whether red‐legged frog tadpoles are preferred prey of mosquitofish. Mosquitofish fed on a mixture of equal numbers of tadpoles and either mosquitoes, Daphnia, or corixids until < 50% of prey were eaten; then we calculated whether there was disproportionate predation on tadpoles. We also recorded the activity of tadpoles in the presence and absence of mosquitofish to test whether mosquitofish interfere with tadpole foraging. Survival of red‐legged frogs in the presence of bullfrog tadpoles was less than 5%; survival was 34% in control ponds. Mosquitofish did not affect red‐legged frog survival, even though fish became abundant (approximately 1011 per pond). Two mechanisms may have blocked the effects of mosquitofish on tadpole survival: (1) fish ponds contained fewer predatory invertebrates, and (2) mosquitofish preferred other prey to red‐legged frogs in laboratory trials. Red‐legged frog tadpoles suffered more injuries in ponds with fish, however, and weighed 34% less at metamorphosis. The growth decrease could have been caused by injuries or by lower foraging levels in the presence of fish. Laboratory results showed that young tadpoles were less active in the presence of mosquitofish. Although both mosquitofish and bullfrogs affected red‐legged frogs, the impact of bullfrogs on the survival of red‐legged frogs may contribute more strongly to their decline.