Review: Bruno JF, Selig ER (2007). Regional decline of coral cover in the Indo-Pacific: Timing, extent, and subregional comparisons. PLoS ONE, 2(8):e711. doi:10.1371/journal.pone.0000711

Feature Paper: Bruno JF, Selig ER (2007). Regional decline of coral cover in the Indo-Pacific: Timing, extent, and subregional comparisons. PLoS ONE, 2(8):e711. doi:10.1371/journal.pone.0000711

Author Abstract: A number of factors have recently caused mass coral mortality events in all of the world’s tropical oceans. However, little is known about the timing, rate or spatial variability of the loss of reef-building corals, especially in the Indo- Pacific, which contains 75% of the world’s coral reefs. We compiled and analyzed a coral cover database of 6001 quantitative surveys of 2667 Indo-Pacific coral reefs performed between 1968 and 2004. Surveys conducted during 2003 indicated that coral cover averaged only 22.1% (95% CI: 20.7, 23.4) and just 7 of 390 reefs surveyed that year had coral cover .60%. Estimated yearly coral cover loss based on annually pooled survey data was approximately 1% over the last twenty years and 2% between 1997 and 2003 (or 3,168 km2 per year). The annual loss based on repeated measures regression analysis of a subset of reefs that were monitored for multiple years from 1997 to 2004 was 0.72 % (n = 476 reefs, 95% CI: 0.36, 1.08). The rate and extent of coral loss in the Indo-Pacific are greater than expected. Coral cover was also surprisingly uniform among subregions and declined decades earlier than previously assumed, even on some of the Pacific’s most intensely managed reefs. These results have significant implications for policy makers and resource managers as they search for successful models to reverse coral loss.


Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.


Review: For the last few weeks we've been discussing various causes of coral decline worldwide. This week we'll look at what happens after a coral reef is already degraded and address issues that managers use to help recover damaged coral reefs.

This paper is important because of its scope: 6001 surveys of 2667 Indo-Pacific coral reefs performed between 1968 to 2004 were analyzed. In Indo-Pacific contains nearly 75% of reef-building coral cover globally. Their analysis showed that coral cover over the last 20 years decreased 1% annually (a larger 2% loss was seen between 1997 - 2003). Coral cover on surveyed reefs decreased from greater than 75% live coral cover to less than 25% live coral cover throughout the study period. 

This paper represents a database analysis, whereby surveys from many different people conducted along similar guidelines (so that comparisons are applicable) are compared over a large geographical area to determine trends... in this case, trends in % live coral cover over time.

The authors "describe a comprehensive analysis of the timing, rate, and geographic extent of the loss of coral cover across the Indo-Pacific," which the authors bound by Sumatra in the west and French Polynesia in the east. 

Throughout their study range, more reefs were found over time with lower % live cover of coral towards the end of the study than at the beginning of the study. Live coral cover is an important measure of reef health because corals are the chief architects of reef structure, thereby providing a home for thousands of organisms. When corals die, many of the organisms associated with corals die as well. Corals also act as breakwaters to waves, protecting coastlines from erosion. Healthy reefs also produce more fish than unhealthy reefs, providing food for millions of coastal peoples worldwide.

Most of the reefs in the database were surveyed only once, though the authors included 651 reefs surveyed two or more times. Some survey regions were studied more intensively than others, with the Great Barrier Reef having the most number of surveys followed by the Philippines and Mainland Asia. 

By 2003 all 10 subregions studied had a mean percent live coral cover below 30% (the region-wide average was only 22.1% live coral cover), compared to the early 1980s when most surveyed subregions had a mean percent live coral cover of nearly 50% (region-wide estimates are 42.5% mean live coral cover between 1980-1983 for the Indo-Pacific). As the authors note, "our results indicate that coral cover on Indo-Pacific reefs is currently lower and far more uniform than expected."

The authors note that percent live coral cover is just one measurement (or proxy) of reef health, but use it because they did not have abundant data on other important measures of reef degradation, "in particular the abundance and diversity of reef inhabitants." Also, while the authors note that between 1984 and 1996 coral cover was slightly lower in eastern Indonesia than on the Great Barrier Reef, it is known today that eastern Indonesia and Papua New Guinea have the highest total species diversity of corals. Therefore it is important to understand that high percent coral cover does not necessarily mean high diversity of species. There are over 800 species of reef-building (hermatypic) corals in Papua New Guinea compared to less than 50 species of reef-building corals in the Hawaiian Islands, yet some areas of the Hawaiian Islands have over 90% live coral cover. Sometimes, certain species overgrow and outcompete other species.

Therefore, keep in mind that percent live coral cover is one proxy of reef health, but not necessarily a proxy of reef diversity, which depends on a variety of regional factors. 

Nevertheless, the take-home message is well-known... most reefs worldwide are declining in live coral cover. This means that while corals may still be around in 100 years, coral reef communities are likely to look a lot different. Even a pristine and well-managed reef like the Great Barrier Reef in Australia showed a shift towards more study sites with lower % cover of live coral than sites with high live cover. The authors found that only 7 of 390 surveyed reefs in 2003 with greater than 60% live coral cover. Something in management will have to change to protect coral reefs for the coming century.


Now that we know that coral loss has been occurring steadily since the 1970s, we'll address the major approach aimed at halting reef degredation: Marine Protected Areas. Next we'll look at another Bruno and Selig paper that discusses whether marine protected areas (MPAs) have been effective globally in preventing coral loss.