This technical report summarises three sets of data analysis based on monitoring of shallow reef biodiversity around the entire Australian continent. It is relevant to policy makers, managers and the research community. Key findings include a very clear impact of climate change on Australian reefs in the form of changing compositions of reef fishes and coral communities, and declines in some populations of temperate mobile invertebrates and tropical reef fishes. Relevant components have already been reported to the State of the Environment team in a national reefs case study and contributions to other relevant assessments. Follow up work using these results will be relevant to the Australian Government (e.g. with respect to threatened species and Australian Marine Parks) and the Great Barrier Reef Marine Park Authority. Improved models are required to quantify the relative impact of gradual warming and marine heatwaves on coral and rocky reef ecosystems.

Human activities are altering the structure of ecological communities, often favouring generalists over specialists. For reef fishes, increasingly degraded habitats and climate-driven range shifts may independently augment generalization, particularly if fishes with least-specific habitat requirements are more likely to shift geographic ranges to track their thermal niche. Using a unique global dataset on temperate and tropical reef fishes and habitat composition, we calculated a species generalization index that empirically estimates the habitat niche breadth of each fish species. We then applied the species generalization index to evaluate potential impacts of habitat loss and range shifts across large scales, on coral and rocky reefs. Our analyses revealed consistent habitat-induced shifts in community structure that favoured generalist fishes following regional coral mortality events and between adjacent sea urchin barrens and kelp habitats. Analysis of the distribution of tropical fishes also identified the species generalization index as the most important trait in predicting their poleward range extent, more so than body or range size. Generalist tropical reef fishes penetrate further into subtropical and temperate zones than specialists. Dynamic responses of reef fishes to habitat degradation imply loss of specialists at local scales, while generalists will be broadly favoured under intensifying anthropogenic pressures. An increased focus on individual requirements of specialists could provide useful guidance for species threat assessments and conservation actions, while ecosystem and multi-species fisheries models should recognize increasing prevalence of generalists.