Abstract
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.
Highlights
Non-native species invasions are commonly cited as a major cause of ecosystem transformation (e.g., Vitousek et al 1996, Mack et al 2000), in many situations it is unclear whether they are drivers of ecosystem change or merely ‘‘passengers’’ along for the ride (Didham et al 2007)
We evaluated how varying abiotic conditions affected the relationship between round goby density and the benthic macroinvertebrate community, by testing for potential synergistic interactions
The second axis (20%) incorporates an ion richness gradient; namely, sites with higher conductivity and calcium concentrations diluted by tributary inputs
Summary
Non-native species invasions are commonly cited as a major cause of ecosystem transformation (e.g., Vitousek et al 1996, Mack et al 2000), in many situations it is unclear whether they are drivers of ecosystem change or merely ‘‘passengers’’ along for the ride (Didham et al 2007). PAGNUCCO AND RICCIARDI population caused by another external driver such as habitat alteration (Bauer 2012); whereas, in the driver model non-native species are primarily responsible for native species decline, even where habitat alteration has occurred (Light and Marchetti 2007, Hermoso et al 2011). We may consider an additive interaction model, in which the invader’s impact adds to that of an external stressor and both are drivers of native species declines (Hermoso et al 2011). Abiotic conditions can affect the functional response of invasive species, with impact scaling disproportionately with invader abundance, leading to synergistic effects (Didham et al 2007)
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