Causes of the 27-year Decline in Coral Cover on the Great Barrier Reef

The Great Barrier Reef is the largest coral reef ecosystem in the world, consisting of over 3,000 individual reefs and spanning across an area of 345,000 square kilometers. Although these reefs are considered some of the least threatened reefs in the world, they are degrading at a concerning rate due to severe habitat disturbances such as crown-of-thorn starfish (COTS) predation, coral bleaching, declining growth rates caused by high temperatures, terrestrial runoff, tropical cyclones, and coral diseases. De’ath et al. (2012) show that mean coral cover on the Great Barrier Reef declined from 28–13.8% between 1985 and 2012. Two-thirds of that decrease occurred after 1998, suggesting that coral is dying at an increasing, non-linear rate. Furthermore, rates of coral calcification on the Great Barrier Reef have been declining due to thermal stress and ocean acidification, so reefs have been taking longer to recover from disturbances. At the current rate of disturbance and re-growth, De’ath and colleagues predict that coral cover on the Great Barrier Reef will likely decline to 5–10% by 2022. However, in the absence of cyclones, COTS, and bleaching, these reefs have the potential to increase coral cover by roughly 2.85% each year. Results of this study demonstrate the need to mitigate global warming and ocean acidification in order to maintain the biodiversity and ecological integrity of the Great Barrier Reef.—Kelsey Waite
De’ath, G., Fabricius, K.E., Sweatman, H., Puotinen, M., 2012. The 27–year decline of coral cover on the Great Barrier Reef and its causes. Proceedings of the National Academy of Sciences 109, 17995-17999.

            De’ath and colleagues used coral cover and densities of COTS surveyed around the perimeter of 214 individual reefs throughout the Great Barrier Reef. The data were obtained from the AIMS Long-Term Monitoring Program, which recorded 2,258 reef surveys between 1985 and 2012. All data analysis was done using logistic regression models. The first set of analyses modeled temporal change in coral cover, and how it differed among the northern, central, and southern regions of the Great Barrier Reef. The second set of analyses included the effects of disturbances such as cyclones, COTS, and bleaching.
            De’ath et al. show a significant decrease in hard coral cover, from 28–13.8%, over the last 27 years. This major decline means that tens of thousands of species that live in, or depend on the reefs are losing their natural habitat. Additionally, the rate at which the coral is dying has been increasing substantially since 2006. Disturbance from COTS, cyclones, and massive bleaching events have caused periodic, random fluctuations in variation of coral cover, but have not been linked to any systematic long-term effects in variation over the past 27 years. De’ath and colleagues suggest that the Great Barrier Reef is headed in the same direction as the reefs in the Caribbean, which have been declining at a rate of roughly 1.4% each year (GBR since 2006 is roughly 1.45% each year). The central and southern regions of the Great Barrier Reef are likely to decline to 5–10% coverage by 2022 if there is not a significant change in the rate of disturbances and coral growth. However, in the absence of these disturbances, the Great Barrier Reef has the potential to recover at a relatively fast rate of about 2.85% every year. 

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