| Summary: | Climate-driven invasions of ecosystems by range-extending animals are often mediated by behavioural modifications that increase their chances of establishment in novel biological communities. This creates novel ecological interactions that can also affect the behaviour of native species in recipient ecosystems. However, this key question has seldom been addressed in marine systems, in particular with the additive effect of ocean acidification and the mediating effects of seasonal climate variability. Here, we performed a laboratory experiment to evaluate how novel species interactions, ocean acidification, and future summer vs winter temperatures could affect the behaviour and fitness of a range-extending tropical and co-shoaling temperate fish. Compared to current summer temperatures, tropical fish became more active, more aggressive, and increased their feeding and fitness during future summers in their novel poleward ranges, but reduced their aggression, boldness and feeding during future winters. Tropical fish became more active under ocean acidification. Temperate fish behaviour was unaffected by climate treatment but their growth rates were higher under future winter vs current and future summer temperatures. We conclude that future winter conditions in temperate ecosystems still reduce the performance of range-extending fishes in temperate fish communities and hence slow down tropicalisation of higher latitudes.
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