Indirect effects of nonnative trout on an alpine-nesting passerine bird via depletion of an aquatic insect subsidy

Nonnative species introduced into one ecosystem can affect adjacent ecosystems that normally receive nutrients or prey from that system. This research shows that the Gray-crowned Rosy-Finch (Leucosticte tephrocotis dawsoni, "Rosy-Finch"), a generalist songbird typically thought of as a seed eater, is indirectly affected by the introduction of fish to naturally fishless habitats. These indirect effects were mediated by competition for different life stages of the same aquatic invertebrates, specifically mayflies. My research objectives were to quantify (1) mayfly abundance and the aggregative response of Rosy-Finches to mayfly emergences at fish-containing versus fishless lakes; (2) the relative importance of the aquatic subsidy in the Rosy-Finch diet; and (3) landscape-scale spatial distributions of breeding birds relative to those of fish and mayflies. Based on benthic invertebrate sampling at 12 fishless lakes and 12 fish-containing lakes, I found mayflies to be 50 times more common at fishless lakes than at fish-containing lakes. As a result, Rosy-Finches foraged preferentially at fishless lakes than at fish-containing lakes. During mayfly emergence nearly six times more Rosy-Finches were observed at fishless lakes than at fish-containing lakes. Observational studies and stable isotope analyses confirmed that mayflies were heavily exploited by the Rosy-Finch; up to 38% of its diet consisted of this resource when it was available. However, at the landscape scale, I did not find convincing evidence that this species preferentially breeds in regions with higher proportions of mayfly-rich, fish-free habitat. Instead, Rosy-Finch distributions reflected the historic distribution of mayflies as would have been observed prior to the introduction of fish. A spatial mismatch between Rosy-Finches and their mayfly prey may occur because when birds select breeding habitat they do not have accurate information concerning the distribution of mayflies as affected by nonnative fish; lakes are frozen at that time. Despite the protections afforded to Rosy-Finch breeding habitats, including national parks and wilderness areas, the availability of an important food resource has been negatively altered by a recreational fishery, with potentially adverse consequences for Rosy-Finch populations. Although the evaluation of potential impacts to aquatic ecosystems are increasingly an integral part of fish stocking programs, my research indicates that these evaluations should be broadened to include indirect effects on terrestrial consumers.