Ecological generalism and physiology mediate species biogeographic ranges under ocean warming

Climate-driven species redistributions are often facilitated by niche modifications that increase a species’ chances of establishment in novel ecological communities. It is relatively well-understood how range-extending species adjust individual niche traits when entering novel environments, yet it remains unclear how modification of ecological niche traits collectively may alter the pace of species range-extensions or contractions. Here, we quantified habitat niche width, species abundance, and physiological performance of range-extending coral reef fishes and co-existing local temperate coastal fishes along a 2,000-km latitudinal gradient from tropical to temperate reefs in a global warming hotspot. We then assessed the dietary and behavioural niches and establishment potential of the same species to understand whether ecological generalism facilitates successful range-extension of coral reef fishes. At its temperate cold-range edge, one coral reef species (<i>Abudefduf vaigiensis</i>) that increased their habitat, dietary and behavioural niches collectively showed stronger establishment, but experienced decreased physiological performance (increased oxidative stress) compared to its core tropical range. Contrastingly, the three other coral reef species that showed unmodified habitat, dietary or behavioural niches showed lower establishment potential in their novel ranges. One temperate species showed decreased abundance, habitat niche width, and physiological performance at their warm-trailing edge compared to their core temperate range, while the other two temperate species showed contrasting responses. We conclude that ecological generalists might be more successful than ecological specialists during the initial stages of climate change, with increasing future warming further strengthening this pattern by physiologically benefitting tropical generalists but disadvantaging temperate specialists.