Biological invasions are one of two main causes of the rapid loss of species worldwide, and may contribute to the predicted loss of two-thirds of all species by the end of this century. Invasions of nonnative species not only have direct effects (competition, predation), but also indirect effects that cascade through food webs and into other ecosystems. For example, recent research in Japan showed that the indirect effects of nonnative invading rainbow trout on stream food webs reduced the amount of adult aquatic insects emerging from the stream and caused a 65% decline in streamside spiders. By extension, streamside birds also gain about 25% of their energy from emerging insects, so invading trout could affect them as well. This project will study the effect of nonnative brook trout invasions in Rocky Mountain streams, an even more widespread invasion that affects thousands of stream miles. This study will test whether these fish invaders have similar direct effects on native cutthroat trout as rainbow trout do on native fish in Japan. It will also examine whether these effects propagate through the stream food web and reduce emerging aquatic insects that feed streamside spiders. These effects will be measured directly and indirectly by using a stable isotope of nitrogen. This work will be performed in stream sections where nonnative brook trout are removed and compared to those where they have invaded, and along a gradient from central to northern Rocky Mountain streams. This study will have applied implications for the fisheries management of these stream systems.
