Behavioural plasticity and species interactions as key drivers of tropical fish range-extensions on temperate reefs

Species invasions have historically driven many natural communities around the world into collapse. As climate changes, species are shifting their distribution to regions with more tolerable conditions, intensifying species invasion rates. By feeling the heat, many marine tropical species are moving towards cooler environments. When extending their ranges they are forced to interact and share habitats and resources with native species, potentially triggering modifications in the structure and functionality of temperate communities. For at least two decades, hundreds of tropical fish species have been recruiting to Australian temperate reefs, but successful colonisation depends on how they respond to and cope with unfamiliar environmental conditions. Temperate ecosystems represent a potential hostile environment for tropical species, due to novel prey, predators, competitors, habitats, and suboptimal temperatures. This thesis provides empirical evidence that behavioural traits and biological interactions are underpinning mechanisms controlling the success of tropical fish establishment in temperate waters, and demonstrates some degree of resistance by temperate fish communities to tropical invasions. By assessing a wide range of behavioural responses of temperate and tropical fishes across a temperature gradient from low to high latitudes along the eastern Australian coast, I reveal that behavioural plasticity, generalism and segregation of tropical and temperate species at their leading and trailing edges, respectively, might facilitate coexistence in temperature mixing zones (Chapter 2). In contrast, tropical fish are more riskaverse at leading edges and such behaviour was associated with reduced foraging, possibly compromising their permanent establishment on temperate reefs under current warming (Chapter 3). I further reveal that physical (low temperature and/or unfamiliarity with temperate environmental conditions) and biological (behaviour and abundance of temperate competitors) effects reduce the efficiency of tropical fishes in their food acquisition, which may compromise their fitness delaying their establishment in temperate environments nowadays (Chapter 4). Whilst warming has led to the expansion of sea-urchin barrens in temperate regions, which creates suitable habitats for tropical fish colonisation, future ocean acidification may inhibit the formation of such habitats, reducing the probability of successful recruitment of tropical fishes and the emergence of novel fish community structures (Chapter 5). Although some biological and physical processes may slow the pace of tropicalisation, under future ocean warming tropical fishes may no longer struggle for survival in coolertemperate water, which has historically been acting as the main barrier against the tropicalisation of temperate ecosystems. Yet, ocean acidification can slow down the process of tropicalisation at the leading edges of species ranges.Thesis (Ph.D.) -- University of Adelaide, School of Biological Science, 202