More than half the coral in Australia's Great Barrier Reef has disappeared since 1985, due to storm damage, crown of thorns starfish and bleaching.
Storms caused the greatest loss of coral, at 48 percent, with the starfish accounting for 42 percent and bleaching ten percent.
"We can't stop the storms but, perhaps we can stop the starfish. If we can, then the reef will have more opportunity to adapt to the challenges of rising sea temperatures and ocean acidification," says John Gunn, CEO of the Australian Institute of Marine Science (AIMS).
The finding is based on the most comprehensive-ever reef monitoring program, which started broadscale surveillance of more than 100 reefs in 1985 and added more detailed information on 47 of them from 1993.
And projections show that, if the coral loss continues, the reef could lose half of what's left by 2022.
"Interestingly, the pattern of decline varies among regions," says AIMS research fellow Dr Peter Doherty.
"In the northern Great Barrier Reef coral cover has remained relatively stable, whereas in the southern regions we see the most dramatic loss of coral, particularly over the last decade when storms have devastated many reefs."
Intense tropical cyclones have caused massive damage, mainly to the central and southern parts of the reef. Population explosions of the coral-eating crown of thorns starfish have hit the whole length of the reef, and two severe coral bleaching events have damaged northern and central parts.
"Our data show that the reefs can regain their coral cover after such disturbances, but recovery takes 10 to 20 years. At present, the intervals between the disturbances are generally too short for full recovery and that's causing the long-term losses," says Dr Hugh Sweatman, one of the study's authors.
Eliminating the crown of thorns starfish offers the best chance of helping the reef recover, says Gunn.
"The study shows that in the absence of crown of thorns, coral cover would increase at 0.89 percent per year, so even with losses due to cyclones and bleaching there should be slow recovery," he says.
"We at AIMS will be redoubling our efforts to understand the life cycle of crown of thorns so we can better predict and reduce the periodic population explosions of crown of thorns. It's already clear that one important factor is water quality, and we plan to explore options for more direct intervention on this native pest."