<b>Range-extending fish become competitive dominants under ocean warming but not heatwaves or acidification</b>

<p dir="ltr">Ocean warming is driving species range-extensions into cooler regions. The direct physiological influence of warming on species performance can accelerate such extensions into novel ecosystems, however, indirect effects of invader–resident interactions in cooler regions may counter these positive effects. Here we examined the foraging performance and densities of competing warm-water and cool-water fishes across a latitudinal temperature gradient spanning 1,500 km from tropical to temperate reefs subjected to rapid ocean warming in the southern hemisphere, and across natural analogues of temperate, tropicalized and acidified reef localities in the northern hemisphere, and during a severe marine heatwave at a temperate reef. While current levels of ocean warming have allowed the warm-water fish to extend their ranges into temperate ecosystems at both hemispheres, their foraging performance was reduced at both the cold- and warm-temperate reefs compared to the (sub)tropical reefs. However, at the (warmer) tropicalized reef the warm-water fish had higher foraging performance and maintained densities, even under extreme pH reduction, compared to the temperate reef. In contrast, the cool-water species struggled at the warmer tropicalized and extreme reefs with reduced foraging performance and lower population densities compared to temperate reef. Contrastingly, the severe heatwave experienced at the temperate reef did not alter the foraging behaviours of either species. We suggest that ocean warming boosts the foraging performance of the range-extending warm-water fish and impairs that of their cool-water competitor at temperate reefs, irrespective of acidification and heatwaves, leading to a shift in dominance hierarchies on temperate reefs. We conclude that warming-driven increases in foraging performance of the warm-water species may alleviate foraging limitations and enhance its establishment at its leading range edges under climate change, to the detriment of its cool-water competitors.</p>