Invasion of stream fishes into low trophic positions

In the absence of other life-history constraints, fishes that can feed at low trophic levels (i.e., omnivores/detritivores) are predicted to be successful invaders because their food resources during the colonization and integration phases of the invasion are rarely limiting. Accordingly, we hypothesized that trophic position of non-native fishes in five mid-western and south-western U.S. river systems would be lower than native fishes. By standardizing delta N-15 values with a common invertebrate (chironomids) across sites, we were able to evaluate differences in trophic position between natives and non-natives across sample locations and years. Our data tend to support this hypothesis, but there was notable spatial and temporal variation in this pattern. Moreover, three of four species generally fed at lower trophic positions in their introduced ranges than their native ranges. Although many factors influence the success of introduced species, our data suggest that the ability to forage on low-quality resources is a favourable trait for invasive fishes in lotic systems. Because these fishes feed at low trophic positions, it is important to consider how they will influence invaded systems, in contrast to invaders that feed at higher trophic positions. Future studies that evaluate the resistance of communities to invaders from different trophic positions would help understand the mechanisms that control the establishment and spread of species with different life-history traits.