Effects of introduced mosquitofish and bullfrogs on the threatened California red-legged frog

Exotic species have frequently caused declines of native fauna and may cont ribute to some cases of amphibian decline. Introductions of mosquitofish (G ambusia affinis) and bullfrogs (Rana catesbeiana) are suspected to have cau sed 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 hatchlin g red-legged frog tadpoles to each of 12 earthen ponds. Three ponds were co ntrols, 3 were stock ed with 50 bullfrog tadpoles 3 with 8 adult mosquitofi sh, and 3 with 50 bullfrogs plus 8 mosquitofish. We performed tests in aqua ria to determine whether red-legged frog tadpoles are preferred prey of mos quitofish. Mosquitofish fed on a mixture of equal numbers of tadpoles and e ither 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 mosqu itofish to test whether mosquito fish interfere with tadpole foraging. Surv ival 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-leg ged frog survival, even though fish became abundant (approximately 1011 pet pond). Two mechanisms may have blocked the effects of mosquitofish on tadp ole survival: (1) fish ponds contained fewer predatory invertebrates, and ( 2) mosquitofish preferred other prey to red-legged frogs in laboratory tria ls Red-legged frog tadpoles suffered more injuries in ponds with fish, howe ver, 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 pre sence of mosquitofish. Although both mosquitofish and bullfrogs affected re d-legged frogs, the impact of bullfrogs on the survival of red-legged frogs may contribute more strongly to their decline.