Sea freshening may drive the ecological impacts of emerging and existing invasive non-native species

Aim The spread of invasive non‐native species (INNS) will pose major threats to global biodiversity over the coming decades. However, predicting how key effects of climate change will influence the abilities of INNS to establish and exert ecological impact is a major challenge. One overlooked aspect of global change is the expected freshening of certain marine systems, which may interact with INNS and lead to drastic effects on community structure and stability. Location Baltic Sea, Europe. Methods Here, using three predatory amphipod crustaceans, we experimentally assessed how salinity reduction may affect the impacts of the emerging INNS, Pontogammarus maeoticus, relative to an existing INNS, Gammarus tigrinus and a trophically analogous native, Gammarus salinus. We quantified per capita impacts of the three species via the comparative functional response method (prey consumption over a range of prey densities) under a predicted seawater freshening scenario. We then combined amphipod functional responses with their life history traits to compare population‐level relative impact potential (RIP) on prey of the three amphipod species across salinities. Results Freshening substantially altered the predicted relative ecological impacts of both the INNS compared with the native. First, the functional responses of invasive P. maeoticus and G. tigrinus increased under freshening, while that of the native G. salinus decreased. Second, RIP became consistently higher for both the INNS compared to the native with increased freshening. Main conclusions Our methods thus reveal potential for climate change via seawater freshening to drive large shifts in dominance and ecological impacts of INNS compared with natives. With the number of INNS introductions unlikely to saturate in the near future, we highlight the need to assess the impacts of potential future INNS, alongside established non‐natives and native species, in combination with abiotic changes associated with climate change.