Starfish Did It

Great Barrier Reef Loses Half Its Coral in Just 27 years

A new study blames mostly the Crown of Thorns killer starfish for the decline in coral cover from 28 percent to 13.8 percent in less than three decades.

By Nick Collins

A comprehensive survey monitoring 214 of the individual reefs along Australia’s Great Barrier Reef, a World Heritage site, found that coral cover decreased from 28 percent in 1985 to 13.8 percent this year.

If replicated across the entire Great Barrier Reef, which runs the length of Queensland and stretches 155 miles from shore, the figure equates to a loss of coral across almost 19,300 square miles of reef—more than twice the area of Wales.

One of the key factors in the deterioration of coral was population explosions of Crown of Thorns starfish, which researchers held responsible for more than two fifths of the overall loss.

Under the right conditions, Crown of Thorns can each produce tens of millions of larvae, triggering mass population booms that have been shown to strip entire reefs of coral.

Outbreaks of the starfish can be sparked by the runoff of fertilizers from the shore into the sea, and coastal development has increased their frequency from a rate of about two per century to one in every 15 years.

The giant predators, which have up to 21 arms covered in poisonous spikes, can each consume up to 107 square feet of living coral per year and will eat virtually any type that lives on the reef.

Although slow-moving, the starfish can devour vast amounts of coral by descending on it and pushing their stomach out through their mouth, allowing them to digest an area equal to their own 17-inch diameter in one swoop.

Other major factors affecting the reef are cyclone damage, which caused around half of all coral death, and bleaching due to rising sea temperatures, which was responsible for 10 percent.

Although storms were found to have the biggest overall impact, Crown of Thorns outbreaks are the most easily preventable threat, experts said.

At its current rate of decline, the Great Barrier Reef could lose another half of its remaining coral in a decade, but without starfish outbreaks coral cover would begin to slowly increase, a new study in the Proceedings of the National Academy of Sciences journal reported.

John Gunn, chief executive of the Australian Institute of Marine Science, who led the study, said: “We can’t stop the storms, but perhaps we can stop the starfish.

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“Our data show that the reefs can regain their coral cover after such disturbances [as Crown of Thorns outbreaks or cyclones], but recovery takes 10-20 years.”

More studies are needed to understand what causes population explosions of starfish and find ways of intervening to protect the reef from the predators, he added.

Professor Ove Hoegh-Guldberg of Queensland University, lead scientist on the Catlin Seaview Survey, a project to survey the deeper levels of the Great Barrier Reef, said the threat of a new outbreak was looming this year after the record flooding of Queensland in 2010.

He said: “When you have a flood along the coastline, or disturb the flow of nutrients into the sea, these starfish tend to outbreak, so they go from being a minor player that you can hardly ever find on a reef to being one or two per square meter.

“Suddenly you have got millions of them. Two years ago Queensland had record flooding along the coast, and two years after that we might have a really large Crown of Thorns outbreak developing across the central Great Barrier Reef.”