Energy dynamics within food webs are not necessarily confined to conventional habitat boundaries. Instead, adjacent food webs (e.g., lakes and terrestrial uplands) may be linked by cross-boundary subsidies. The movement of resources from more productive donor systems can subsidize consumers in adjacent and less productive recipient systems. Introduced predators can have indirect effects on the organisms in recipient systems via direct effects on the subsidizing organisms within the invaded communities. We present results of a study on the role of introduced trout in altering lake-derived resource subsidies for passerine birds in the Sierra Nevada alpine ecosystem (California, USA). Nonnative fish have been introduced to the majority of naturally-fishless lakes throughout the Sierra Nevada alpine zone. Previous food web studies in this landscape have demonstrated adverse direct and indirect effects of nonnative fish on native aquatic communities, but have not examined effects on terrestrial species. We examined indirect effects of nonnative fish on Gray-crowned Rosy Finches (Leucosticte tephrocotis) through competition for different life stages of large-bodied aquatic invertebrates, especially mayflies. The abundances of emerging aquatic insects and foraging birds were compared at fish-containing versus fishless lakes across three spring-summer seasons. Every one to two weeks we simultaneously conducted avian point counts at paired, fish-containing and fishless lakes. The aquatic insect emergence was sampled concurrent to each avian point count. In the first, second, and third years of the study, we observed finch abundance and insect emergence at four lakes (two replicates), eight lakes (four replicates), and 24 lakes (twelve replicates), respectively. We analyzed the data using a repeated measures mixed model.

Results/Conclusions

Our results show a significant treatment effect of fish on bird numbers (p<0.05), with more Gray-crowned Rosy Finches observed foraging at fishless lakes than at fish-containing lakes. Furthermore, these results show a significant difference (p<0.05) in the average number of observed finches with respect to the timing of large-bodied aquatic insect emergences. These results support the prediction that in California’s Sierra Nevada alpine ecosystem, nonnative fish diminish the strength of an aquatic to terrestrial ecosystem subsidy and thereby indirectly affect terrestrial predators such as the Gray-crowned Rosy Finch. This work also supports the need for inclusive, cross-system considerations of effects to native biota in the management of nonnative aquatic species. A re-examination of traditional habitat boundaries may provide a new paradigm with which to manage food webs affected by nonnative species.