Regional comparison of native cutthroat and nonnative brook trout and their food web effects

Brook trout (BK) are the most widely distributed and abundant nonnative fish in the western U.S., and have excluded cutthroat trout (CT) from much of their native range.  We compared density, production, and food web interactions of CT and BK between two regions in Idaho and Colorado.  Because of differences in life history and feeding behavior, we hypothesized that BK exhibit greater density and production and cause greater depletion of benthic invertebrates, reducing emerging insects and riparian spiders compared to CT.  We also hypothesized that population and food web effects differ across regions because of variation in the habitat template or evolutionary history.  A regional analysis (>1000 locations) showed that BK exhibited at least 44% greater density and 49% higher production than CT, regardless of region.  A field comparison of 10 pairs of streams, 5 in each region, showed that flux of emerging insects was similar between both species in Idaho, but that dipterans were lower in BK streams in Colorado.  Riparian spider abundance was greater along CT streams than BK streams in both regions, though the differences were not significant.  This research will contribute to more accurate predictions of both the effects and true costs of nonnative trout.