Coral reefs, seaweed
2 June 2009
A global survey of coral reefs has shown that, while reefs face many threats, fears of a takeover by seaweed have so far not been realised.
AIMS scientist Dr Hugh Sweatman, along with Dr John Bruno, Dr Elizabeth Selig and Dr Virginia Schutte from the University of North Carolina at Chapel Hill and Florida-based marine ecologist Dr William Precht, have published a paper* that questions a common view that many reefs that were once lush coral communities are now overgrown by seaweed.
"Phase shifts" in the coral reef environment mean the replacement of one stable community of animals and plants by an alternative stable community, in this case the replacement of coral by seaweed. Once seaweeds become abundant they persist because they hinder coral recovery.
According to the scientists, while it was certainly the case that coral had decreased worldwide in recent years, in many locations seaweed had not increased correspondingly.
The scientists' survey examined reefs from around the world, including the Great Barrier Reef, and found that replacement of coral by seaweed was less common than previously assumed. In the Indo-Pacific region, macroalgae dominated only one per cent of the reefs that were surveyed.
The Great Barrier Reef has a special place among reefs of the world because direct human pressures are low compared with many other reefs and it is actively managed. Its herbivorous fishes are not targeted and persistent phase shifts have been rare, although they can be made to occur experimentally.
"Australia has been relatively lucky so far but we cannot afford to be complacent," Dr Sweatman said. There is no general trend to seaweed dominance on the Great Barrier Reef, according to this survey, although reef scientists and managers will continue monitoring it closely.
Elsewhere there have been several dramatic examples of such phase shifts, with one of the most widely known and striking occurring in the Caribbean in the 1980s. Following a series of damaging events, including hurricanes and disease, coral cover on several reefs in Jamaica plummeted from about 70 per cent to less than 10 per cent. Seaweeds rapidly became the dominant life form because of widespread overfishing of herbivorous fishes and the local extinction of a seaweed-grazing sea urchin. As a result, there has been little coral recovery on Jamaican reefs in the past 30 years.
The study team said that while their analysis suggested the incidence of reefs that were dominated by seaweed had been overstated, examples such as the degradation of Jamaican reefs were powerful warnings of the consequences of subjecting reefs to multiple natural and human-induced disturbances.
In the wake of these dramatic losses of Caribbean coral, scientists became concerned about how widespread this phenomenon might be in the rest of the world.
The study published today 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.
Until now there has been no generally accepted threshold at which a phase shift can be said to occur. "This has led to substantial confusion about the causes, generality, severity and management implications of phase shifts on reefs," the scientists said. The team came up with a "phase-shift index" to determine the state of each reef and apply a consistent measure to the level of seaweed versus coral cover.
They found that while there were moderate local increases in seaweed cover over the study period, only four per cent of reefs worldwide were dominated by macroalgae – that is, more than 50 per cent of the reef surface was covered in seaweed – and most of these were in the Caribbean.
"Macroalgae such as seaweed have taken over and are dominating some structures, but those incidences are less common and less widespread than assumed," Dr Bruno said.
Over the period of the study, the greatest losses of coral on the Great Barrier Reef were due to the crown-of-thorns starfish and, to a lesser extent, cyclones. However, the coral that was lost was not replaced by persistent communities of seaweeds.
"In practice the amount of coral on a reef depends on the number of new coral colonies replacing established colonies that die because of crown-of-thorns starfish, storms, warm water bleaching and diseases," Dr Sweatman said. "If conditions do combine to reduce the amount of coral, our study shows that the shift is most often from high coral cover to low coral cover, not from high coral cover to high cover of seaweed."
The low incidence of phase shifts on reefs where the fish and urchin grazers have not been damaged reinforces the idea that the causes of coral mortality should be the prime concern.
"I hope this study leads to a clearer definition of what coral-algal phase shifts are and broadens our perspective on the serious loss of corals in many parts of the world," Dr Sweatman said.
*The paper, written by John Bruno, Hugh Sweatman, William Precht, Elizabeth Selig and Virginia Schutte, is titled "Assessing evidence of phase shifts from coral to macroalgal dominance on coral reefs" and has been published in the journal Ecology (http://www.esajournals.org/toc/ecol/90/6).
For further information, please contact:
Dr Hugh Sweatman , AIMS
Phone: 07-4753 4470, 0419 986 746
Ms Liz Tynan , AIMS
Phone: 07 4753 4235, 0400 773 691
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