Disentangling the influence of abiotic variables and a non-native predator on freshwater community structure

Cause-and-effect interpretations of the apparent impacts of biological invasions are confounded by the coincident occurrence of non-native species and changes to abiotic factors. Native community structure and function can vary greatly in space and time, owing to abiotic variables that could potentially be affected by non-native species. Here, we sought to determine the relative importance of abiotic variables and an invasive predatory fish, the Eurasian round goby (Neogobius melanostomus), on local macroinvertebrate assemblages and benthic algal production in the St. Lawrence River. We used structural equation modeling to evaluate the strength of interaction pathways between hypothesized biotic and abiotic factors driving trophic cascades. Our results indicate that biotic interactions with the round goby drive changes in the abundances of native grazers and non-native dreissenid mussels in the St. Lawrence River. However, the directionality of these effects contradicts previous studies of round goby impacts that emphasized top-down control. We found positive correlations between round goby density and prey (dreissenid and grazer) densities, suggesting complex interactions between round gobies and benthic communities, including strong bottom-up forces. Together with previous studies, our results suggest that, although the interactions between round gobies and benthic communities remain strong, the direction of these interactions varies over space and time. Our results highlight the importance of disentangling the respective roles of biotic interactions and abiotic factors, in order to properly assess the impacts of non-native species as well as the spatiotemporal variability of such impacts.