Environmental determinants of invasion hotspots and refugia at a river scale: implications for conservation of native biodiversity

Introductions of exotic invasive species are a global disturbance for natural habitats, native species and their related ecological services. The severity of invasions and impacts, however, can greatly vary from local to global scales. This variation can be observed in both invasion hotspots, exhibiting disproportionate invasion and impacts, or refugia, exhibiting lower than expected invasion and impacts. The latter are believed to be habitats where ecological, social or environmental conditions create deterrence of invaders. Dissolved ions and presence of wetlands have been suggested as environmental conditions that buffer the impacts of Ponto-Caspian invaders, notably the round goby (Neogobius melanostomus). In this study, we used a generalized additive model (GAM) to analyze the effects of local water conductivity and wetland presence on the density of the round goby in a large-scale study (> 1300 sites), spanning the major part of the non-tidal freshwater portion of the St. Lawrence River (Canada). Our results showed that, even in long-invaded regions, round goby density was null in sites with water conductivity under 100 μS/cm and increased toward a probable biological optimum of water conductivity ~300 μS/cm. Additionally, the presence of wetlands, even if they did not completely disrupt surpass the effect of conductivity, appeared to decrease the round goby densities along the conductivity continuum. Our study demonstrated that low water conductivity (