Is a trout a trout? Nonnative brook trout invade at greater density, biomass, and production than native cutthroat trout they replace in the interior western U.S.

Benjamin, Joseph

Frequently nonnative species invade habitats occupied by a similar, even closely related, species, and it is assumed that they have functionally redundant ecological roles. However, the consequences of these types of invasions are rarely studied. Throughout western North America nonnative brook trout are assumed to be analogous to the native cutthroat trout they replace. We predicted that brook trout would exhibit greater density, biomass, and annual production than cutthroat trout at both 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 for each cutthroat trout subspecies (regional scale). 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 was 1.5 times greater and annual production 1.9 times higher for brook trout than cutthroat trout. These patterns were consistent at the regional scale but greater variation occurred. In addition, we found that trout density and biomass were correlated with landscape features such as elevation and gradient. These results suggest a general pattern of greater density, biomass, and production when nonnative brook trout invade and replace native cutthroat trout, and highlights the need for more studies that examine the broader consequences of a nonnative species replacing a closely related (and often presumed to be similar) native species.