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 to 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 to 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.

The dataset relates to a series of functional response experiments across two experimental salinities, featuring a native and two non-native amphipod crustacean species. The data was analysed using the Frair package in R.

Three species listed as G. tig (Gammarus tigrinus - established non-native), G. sal (Gammarus salinus - native) and P. mae (Pontogammarus maeoticus - potential future non-native). Density relates to the number of Artemia franciscana prey offered, with the five experimental salinities being 2, 4, 8, 16 and 32. Treatment of prey consumed, and prey consumed and wounded, outlined in published manuscript.