Is a trout a trout? A range-wide comparison shows nonnative brook trout exhibit greater density, biomass, and production than native inland cutthroat trout

摘要

Frequently nonnative species invade habitats occupied by a similar, even closely related, species, and it is often assumed that the two have functionally redundant ecological roles. However, the consequences of these types of invasions are rarely studied. Eastern brook trout are the most widely distributed and abundant nonnative fish throughout western North America and are assumed to be analogous in terms of ecological function to the native cutthroat trout they replace. However, based on results of previous studies conducted on a few streams, we hypothesized that the two species differ ecologically, and we predicted that brook trout would exhibit greater density, biomass, and annual production than cutthroat trout at the onset of invasion (i.e., in sympatry) and once replacement has occurred (i.e., in allopatry). We compiled data sets (sympatry: n = 169; allopatry: n = 687) from across the historical range of inland cutthroat trout and made comparisons for all sites combined (western scale) and among regions. In sympatry, at the western scale we observed that brook trout exhibited 3.1 times greater density and 2.5 times greater biomass than cutthroat trout, but no difference in production. In allopatry, density and biomass were 1.5 times greater and annual production 1.9 times higher for brook trout than cutthroat trout. Results of comparisons by region were similar, though more variable. In addition, we found that trout density and biomass were correlated with landscape features such as elevation and gradient, but such watershed variables did not account for the differences we observed between the trout species. These results suggest a general pattern of greater density, biomass, and production when brook trout invade and replace cutthroat trout, and highlight the need for more studies that elucidate the mechanisms responsible for this pattern and that investigate the broader consequences of a nonnative species replacing a closely related (and often presumed to be similar) native species.