Cumulative thermal stress predicts functional change in coral reef fish communities

Marine heatwaves are altering coral reef fish communities through a combination of direct impacts of thermal stress and indirect impacts that degrade coral habitats. Impacts may vary for species in different functional groups (e.g. herbivore replacement of corallivores), altering the functional composition of fish communities that is critical to reef resilience. However, our understanding of trait-based responses of fish communities to marine heatwaves is inadequate, and mostly inferred from local scales and low numbers of species. Here, we use a Bayesian joint species distribution model based on data for 500 fish species from 286 sites surveyed across northern Australia to evaluate the impacts of marine heatwaves on species probability of occurrence and their interactions with four functional traits. The effects of degree heating weeks and coral loss on individual species probability of occurrence had the highest total and proportionally negative responses, respectively. Thermal affinity's positive response to cumulative heat stress (i.e. degree heating weeks) was the strongest trait effect. Community-level analyses predicted losses in habitat specialists, large-bodied fishes, corallivores and pelagic feeders along a gradient of marine heatwave stress. The loss and restructuring of functional diversity suggest that heatwaves will continue eroding fish-dependent ecological roles and associated ecosystem services.