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John Bruno kicks off Part 2 of his Reef Science coral series, excerpting a modified version of an article he published last year on the Earth Portal about Coral Reefs and Climate Change. He covers global patterns of coral loss and several of the mechanisms through which anthropogenic climate change is contributing to this trend including coral bleaching, disease and ocean acidification. Let him know know if there are particular topics you want to hear about. You can reach him at [email protected]

Patterns of coral loss

We know very little about the historical biological baseline of coral reefs because we didn't really begin to study them until they were already being degraded. Based on surveys in the 1960s and early 1970s and recent studies of relatively "pristine" reefs, it appears that historically, coral cover (the percentage of the ocean floor covered by living coral) on undisturbed reefs was 70% or higher. However, even before people began degrading reefs, natural disturbances such as storms and predator outbreaks reduced coral locally. Thus the historical average (i.e., across all reefs in a region, including disturbed and undisturbed reefs) was surely lower and probably closer to 50 or 60%.

Figure 1. Loss of live coral cover on Caribbean reefs since 1977. (from Côté 2008; Modified from Gardner et al. 2003; Photos by I.M. Côté)

There are many examples of coral loss on individual reefs and we can get a rough idea of how coral cover has changed at regional and global scales over time by combining data from many sources. Several such "meta-analyses" have been performed recently by pooling survey data from the scientific literature with unpublished data from governmental and non-governmental monitoring programs. Examples of such organizations include the Australian Institute of Marine Science's Long Term Monitoring Program and Reef Check, which trains volunteer divers to survey hundreds of reefs a year around the world. The resulting picture is of widespread coral loss, even on some of the world's very isolated and intensively managed reefs. For example, coral cover in the Florida Keys declined to only ~ 8% by 2006. Similar losses since the late 1970s have been documented throughout the Caribbean, although some subregions such as the Lesser Antilles still have fairly high coral cover.

The Pacific has generally higher coral cover than the Caribbean, although the picture is not that much brighter. This region, stretching from French Polynesia in the east to the Island of Sumatra in the west, encompasses approximately 75% of the world's coral reefs and includes the center of global marine diversity for several major taxa including corals, fish and crustaceans. But coral cover is still declining in the Pacific, by recent estimates at approximately 1% per year over the last twenty years and 2% annually between 1997 and 2003 (or 3,000 km2 per year). This is approximately five times the net rate of tropical deforestation. Additionally, the spatial patterns of coral reef degradation are very different from rainforest loss in that nearly all reefs have been affected and there are virtually no remaining pristine reefs. Twenty years ago, high cover reefs were common. But today, very few reefs in the Indo-Pacific (only about 1-2%) have coral cover close to the historical baseline of roughly 50%.

Figure 2. Loss of live coral cover (percent) through time, based on a meta-analysis of 6001 quantitative surveys of 2667 reefs performed between 1968 and 2004, originally published in Bruno and Selig 2007. Each green dot represents one surveyed reef in the Indo-Pacific.

Further reading

Link to a nice article in the EoE on the extent and timing of Caribbean coral loss here and to an article on similar patterns of coral loss in the Pacific here.

Literature cited

Bruno, John F., and Elizabeth R. Selig. 2007. Regional decline of coral cover in the Indo-Pacific: timing, extent, and subregional comparisons. PLoS One:e711.

Côté, Isabelle M. (Lead Author); John Bruno (Topic Editor). 2008. Patterns of Caribbean coral loss. In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [Published in the Encyclopedia of Earth December 27, 2008; Retrieved March 2, 2009]. <>

Dr. John Bruno is a marine ecologist and conservation biologist. He's also an associate professor in the Department of Marine Science at the University of North Carolina at Chapel Hill . His research is focused on understanding and conserving the structure and dynamics of marine communities. Dr. Bruno works in a variety of marine habitats including coral reefs, coastal wetland and sand dune plant communities. Read more of his work on The Climate Shifts blog, and check out the Bruno lab home page.

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