PS 76-174
Do introduced fish negatively affect California pond communities by changing amphibian and invertebrate dynamics?
Introduced species play a significant role in global amphibian decline. In particular, introduced predatory fish have caused declines in many amphibian species. Introduced predatory fish can affect amphibians through direct mortality as well as by altering the aquatic community amphibians interact with. We investigated these effects on two native California amphibians: the Pacific chorus frog (Pseudacris regilla) and the California newt (Taricha torosa). The Pacific chorus frog is less vulnerable to decline as a widespread, generalist species. However, Pacific chorus frogs exhibit changes in morphology and behavior in response to introduced fish, changes which increase survival in the presence of fish but decrease survival in the presence of predatory invertebrates. While California newt larvae are known to suffer direct mortality from commonly introduced fish species in the lab, field surveys have had inconclusive results on the effects of fish on California newts in natural ecosystems. We sampled ponds with and without introduced fish in northern California from 2006-2010 and assessed the abundance of amphibian larvae and predatory aquatic insects to answer two questions. First, do fish reduce the abundance of these two amphibians? Second, do fish reduce the abundance of the invertebrates which prey upon these amphibians?
Results/Conclusions
Ponds occupied by introduced fish had significantly fewer Pacific chorus frog tadpoles than ponds without fish, and marginally significantly fewer California newt larvae. However, predatory invertebrates were equally common in ponds with and without introduced fish, indicating that fish are added to, but do not replace the predators faced by amphibian larvae. This combination of predators may add new demographic pressures to amphibians, particularly those which have different behavioral and morphological defenses to predatory invertebrates than they have for predatory fish. We found a positive correlation between the abundance of Pacific chorus frog tadpoles and the abundance of California newt larvae. There was also a positive correlation between the abundance of California newt larvae and the abundance of predatory invertebrates (Dytiscidae, Aeshnidae and Belostomatidae). However, neither of these positive correlations existed in the absence of fish. These results indicate that introduced predatory fish are causing novel interactions between aquatic species in CA pond ecosystems, interactions which might make populations more vulnerable to perturbation.