Climate change erodes competitive hierarchies among native, alien and range-extending crabs

Global warming and ocean acidification alter a wide range of animal behaviours, yet the effect on resource competition among species is poorly understood. We tested whether the combination of moderate levels of ocean acidification and warming altered the feeding success of co-occurring native, alien, and range-extending crab species, and how these changes affected their hierarchical dominance. Under contemporary conditions the range-extending species spent more time feeding, than the alien and the native species. Under conditions simulating future climate there was no difference in the proportion of time spent feeding among the three species. These behavioural changes translated to alterations in their dominance hierarchy (based on feeding success) with the most dominant species under present day conditions becoming less dominant under future conditions, and vice versa for the least dominant species. While empirical studies have predicted either reversal or strengthening of hierarchical dominance in animal species, we suggest that even moderate increases in ocean temperature and acidification can drive a homogenisation in behavioural competitiveness, eroding dominance differences among species that are linked to fitness-related traits in nature and hence important for their population persistence. [Display omitted] •Ocean warming and acidification can alter species behaviour and competition.•We tested this on co-occurring native, alien, and range-extending crab species.•Range-extending crabs outperformed the alien and native crabs in current conditions.•Under future climate conditions, the three crabs no longer differed in competition.•Climate change can drive homogenisation in behavioural competitiveness. Exposure to moderate ocean warming and acidification resulted in the degradation of competitive hierarchies of a native, a range-extending, and an alien crab species, driving a homogenisation in behavioural competitiveness.