Non-native species often arrive coincidentally with environmental disturbances, confounding cause-and-effect interpretations of apparent impacts. While some authors argue that invasive species are a main driver of native species declines, others view their proliferation and biodiversity loss as coincident consequences of changing abiotic conditions (e.g., caused by anthropogenic disturbance). This study sought to determine the relative importance of abiotic factors and an invasive predator, the Eurasian round goby (Neogobius melanostomus), in the alteration of macroinvertebrate assemblages and subsequent benthic algal production. We measured goby density, macroinvertebrate diversity, and algal abundance at 25 sites along the St. Lawrence River. We measured 15 environmental variables to characterize local abiotic conditions at each site. Principal Components Analysis identified abiotic gradients, which were then used as predictors in an information theoretic approach (structural equation modeling) to evaluate the strength of interaction pathways between biotic and abiotic factors.
Results/Conclusions
Contrary to previous experimental studies, we found a negative relationship between goby density and benthic algal abundance. This is surprising given that on small spatial and temporal scales, gobies may have an indirect positive effect on algae through the consumption of macroinvertebrates (i.e., a trophic cascade). However, we hypothesize that this positive effect is exceeded by a negative effect of goby predation on attached mussels (zebra and quagga mussels, Dreissena spp.), whose shells provide an attractive colonisable surface for periphyton.
Among abiotic variables, turbidity had a strong negative effect on both goby density and algal abundance. However, goby density was the best single predictor of macroinvertebrate diversity loss and had the highest Akaike weight among predictor variables. Abiotic factors did not significantly influence the effect of round gobies on macroinvertebrates, suggesting that round gobies are playing a major role in altering macroinvertebrate assemblages in the St. Lawrence River. These results demonstrate the importance of disentangling the respective roles of interspecific interactions and abiotic factors in order to understand the impact of non-native species.