Abstract

Invasive species threaten global biodiversity, but multiple invasions make predicting the impacts difficult because of potential synergistic effects. We examined the impact of introduced beaver Castor canadensis, brook trout Salvelinus fontinalis, and rainbow trout Oncorhynchus mykiss on native stream fishes in the Cape Horn Biosphere Reserve, Chile. The combined effects of introduced species on the structure of the native freshwater fish community were quantified by electrofishing 28 stream reaches within four riparian habitat types (forest, grassland, shrubland, and beaver-affected habitat) in 23 watersheds and by measuring related habitat variables (water velocity, substrate type, depth, and the percentage of pools). Three native stream fish species (puye Galaxias maculatus [also known as inanga], Aplochiton taeniatus, and A. zebra) were found along with brook trout and rainbow trout, but puye was the only native species that was common and widespread. The reaches affected by beaver impoundments had significantly higher puye densities than all other reaches in this study. These results are comparable to those reported for other streams in southern Chile. The presence of trout reduced the abundance of puye, but only in beaver-affected areas; all three natural riparian habitat types had uniformly low puye abundance and were unaffected by the presence or absence of trout. The data suggest that one invasive species, the beaver, enhanced puye habitat and thereby increased the abundance of that species, which, in turn, helped moderate the negative impacts of invasive trout. © American Fisheries Society 2009.