Sp. Lawler et al., Effects of introduced mosquitofish and bullfrogs on the threatened California red-legged frog, CONSER BIOL, 13(3), 1999, pp. 613-622
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.