1. Non-native trout have been stocked in streams and lakes worldwide largely without knowledge of the consequences for native ecosystems. Although trout have been introduced widely throughout the Sierra Nevada of California, U.S.A., fishless streams and their communities of native invertebrates persist in some high elevation areas, providing an opportunity to study the effects of trout introductions on natural fishless stream communities.
2. We compared algal biomass and cover, organic matter levels and invertebrate assemblages in 21 natural fishless headwater streams with 21 paired nearby streams containing stocked trout in Yosemite National Park.
3. Although environmental conditions and particulate organic matter levels did not differ between the fishless and trout streams, algal biomass (as chlorophyll a concentration) and macroalgal cover were, on average, approximately two times and five times higher, respectively, in streams containing trout.
4. There were no differences in the overall densities of invertebrates in fishless versus paired trout streams; however, invertebrate richness (after rarefaction), evenness, and Simpson and Shannon diversities were 10–20% higher in fishless than in trout streams.
5. The densities of invertebrates belonging to the scraper-algivore and predator functional feeding guilds were higher, and those for the collector-gatherer guild lower, in fishless than trout streams, but there was considerable variation in the effects of trout on specific taxa within functional feeding groups.
6. We found that the densities of 10 of 50 common native invertebrate taxa (found in more than half of the stream pairs) were reduced in trout compared to fishless streams. A similar number of rarer taxa also were absent or less abundant in the presence of trout. Many of the taxa that declined with trout were conspicuous forms (by size and behaviour) whose native habitats are primarily high elevation montane streams above the original range of trout.
7. Only a few taxa increased in the presence of trout, possibly benefiting from reductions in their competitors and predators by trout predation.
8. These field studies provide catchment-scale evidence showing the selective influence of introduced trout on stream invertebrate and algal communities. Removal of trout from targeted headwater streams may promote the recovery of native taxa, community structure and trophic organisation.