Western Indian Ocean Coral Diversity

Coral reefs are threatened by climate change, thus effective policies and management require spatial prioritization for conservation investments. Our aim was to develop a spatially explicit ecological model to predict current (2020) and future (2050) numbers of coral taxa at moderate scales (i.e., ~6 km2). Our machine learning predictive models of coral community attributes in 7039 mapped reef cells in the western Indian Ocean were based on 35 spatially complete influential environmental proxies trained with ~1000 field surveys. Four models explored: influences of climate change, water quality, direct human-resource extraction, and variable selection processes on numbers of coral taxa. Two predictive models examined the predictions of all variables and compared them to a variable-restricted climate change (8 commonly used variables) and human influence model (9 variables). The most frequently selected temperature variables in all models were the median, skewness, excess heat, rate of temperature rise, and kurtosis. However, non-temperature variables of observer, depth, wave energy, dissolved oxygen, salinity, chlorophyll-a, calcite concentrations, sunlight, and net primary productivity were frequently as important or stronger. Human influences of national jurisdiction, distance to people, sediments, and nutrients were selected but less influential when compared to the climate or the full variable models. Comparing models indicated the importance of variable pre-selection processes and variable interactions in predicting climate change and human influences on coral diversity. Comparing climate scenarios in the moderate RCP2.6 and extreme RCP8.5 emission scenarios indicated fewer losses in coral taxa (RCP2.6 = 5.2%, RCP8.5 = 8.1% respectively) relative to cover (RCP2.6 = 14%, RCP8.5 = 34%) over the 30 years. Excess heat and rate of temperature rise variables used in the Intergovernmental Panel on Climate Change (IPCC) forecasts predict more negative effects on corals than our four models but shown here to have low to modest effects.