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Are coral reefs of the world being overgrown by seaweeds 2

New corals growing on an old Acropora skeleton. Image: AIMS Long-term Monitoring Program.

"Phase shifts" in natural ecosystems mean the replacement of one stable community of animals and plants by an alternative stable community. In the case of coral reefs, anthropogenic disturbance can lead to the replacement of corals by seaweeds. Once seaweeds become abundant they persist because they hinder coral recovery.

There were some spectacular examples from the Caribbean in the 1980s, where a combination of damage by hurricanes and low numbers of weed-eating fishes (because of over-fishing) led to reefs being covered by large seaweeds. Once seaweeds become established they can block the re-establishment of corals by shading and brushing over small coral colonies and slowing their growth. But is this happening world-wide?

A recent review of information from reefs in many parts of the world by AIMS scientist Dr Hugh Sweatman, along with Dr John Bruno, Dr Elizabeth Selig and Virginia Schutte from the University of North Carolina at Chapel Hill and Florida-based marine ecologist Dr William Precht, has found that only four per cent of reefs are dominated by seaweeds in the sense of having more than half their surface covered with seaweeds.

According to the scientists, it was certainly the case that coral had decreased worldwide in recent years, but in many locations seaweed had not increased correspondingly.

Even taking the conservative figure of 25 per cent of reef surface covered with macroalgae, less than 20 per cent of reefs worldwide are dominated by seaweeds and the majority of these were in the Caribbean. For the Indo-Pacific region, including the Great Barrier Reef, the figure is just one per cent.

The study also found that, worldwide, coral cover has decreased in recent years but macroalgae have not increased. This suggests that the primary problem is coral decline and its causes; establishment of macroalgae is an important but secondary issue.

The study is the first global-scale analysis of thousands of surveys of individual reefs, involving more than 3,500 examinations of about 1,800 reefs made between 1996 and 2006.