Changes in coral species

Key drivers
Temperature
Ocean acidification
Storms and waves
What has happened

In BIOT, major coral mass bleaching events were recorded in 1998 and 2015/16, associated with elevated SST over sustained periods. Following the 1998 mass bleaching and before the 2015 heatwave, BIOT corals had mostly recovered their previous levels of extent and species diversity. However, the two intense marine heatwaves that followed in rapid succession between 2015 and 2016, led to further significant mass bleaching that caused extensive loss of corals and profound changes in terms of coral cover and species composition. 

The extensive damage to the reefs through these more frequent and severe warming events is compromising coral recruitment in BIOT. Data from 2017 indicate that the density of newly settled coral recruits (less than 1 year-old) have reduced by approximately 90% since 2013. Growth rates for several coral species were also low in 2018 and 2019, suggesting prolonged negative effects of heat stress on coral physiology, and an increase in the bioeroding sponge Cliona species has reduced the area suitable for new coral settlement.

Since the late 1970s, several coral species and key species assemblages in BIOT have been declared regionally or functionally extinct, including the endemic Chagos brain coral (Ctenella chagius), which is considered rare. Profound changes across the BIOT reefs are reflected by a major shift from highly diverse reefs composed of competitive and sensitive coral species to reefs dominated by fewer stress-tolerant, massive corals.

CONFIDENCE LEVEL
HIGH

High evidence, high agreement

Research has been focused on building a strong long-term baseline of coral data by gathering samples and observations from key reef areas over multiple years. Expert input from scientists who have worked on coral reefs in BIOT for up to 45 years, including as yet unpublished data.

What could happen

Climate projections for BIOT suggest a significant increase in the frequency of severe annual bleaching events in the coming decades, even under lower greenhouse gases emission scenarios in line with the Paris Agreement. Ultimately, the primary control on coral-reef recovery and survival will be the frequency and magnitude of future heat stress events. 

It remains to be seen whether BIOT reefs will follow the same recovery trajectories as after 1998, or whether recovery will differ between locations, and whether some sites may regime-shift to other states such as fleshy-algae dominated communities. Around Salomon atoll, recent observations of the return of Acropora corals offers some cause for optimism. 

The presence of the MPA, if effectively enforced or complied with, will at least mean future recovery trajectories are not impeded by fishing of functionally important species, which would otherwise limit the ability of the reefs to recover between climate disturbance events.
 

CONFIDENCE LEVEL
HIGH

High evidence, high agreement

Research has been focused on building a strong long-term baseline of coral data by gathering samples and observations from key reef areas over multiple years. Expert input from scientists who have worked on coral reefs in BIOT for up to 45 years, including as yet unpublished data.