Coral mass mortalities in the Chagos Archipelago over 40 years: Regional species and assemblage extinctions and indications of positive feedbacks.

The global decline of reef corals has been driven largely by several marine heatwaves. This has greatly reduced coral cover but has reduced coral diversity also. While there is a lack of data in most locations to detect coral species losses, reefs of the Chagos Archipelago, central Indian Ocean, have long term monitoring data extending back to the late 1970s. Severe declines in cover have occurred since the 1970s, with regional extinctions of some species and key species assemblages. There is a severe decline in coral settlement, along with a substantial loss of habitat quality which has reduced the habitat available for settlement. This is a clear precursor to positive feedback. Regional species extinctions here occur mainly when total coral cover is <10% of pre-warming levels. Climate models predict more frequent and more severe marine heatwaves, and even if this ecosystem recovers it will contain fewer species.