Shrinking Coral

Thursday, August 16th, 2007--

Seventy-five percent of all reef-building corals live in the Indo-Pacific Ocean, which spans roughly from west Indonesia to the Hawaiian Islands, and includes the Great Barrier Reef off of Australia. A recent study in PLoS ONE found that the coral cover in the Indo-Pacific, which includes some of the most-intensely managed and protected coral reefs, has dropped by more than twenty percent in the last few decades.

John Bruno and Elizabeth Selig, of the University of North Carolina at Chapel Hill, analyzed thousands of surveys tracking the changes in living coral on the sea floor. “It’s a really key indicator of reef health because the corals are essential for all the other inhabitants of reefs—the fish and invertebrates. They rely on corals just the way birds and insects rely on trees in a forest,” says Bruno, a marine ecologist in the department of marine sciences.

The data indicates that fifty years ago living coral covered, on average, fifty to sixty percent of the sea floor in the Indo-Pacific. “But frankly it’s impossible to know precisely what it was. So the fifty to sixty percent is our best estimate based on talking with lots of local Indo-Pacific reef experts and based on our earliest studies from the early- to mid-1960s.” The researchers found that across the Indo-Pacific coral cover dropped from around fifty percent to 23 percent in the last thirty years. “What that means is that we’ve lost more than half the world’s reef-building corals just in the last couple of decades,” Bruno explains.

This is the first long-term regional-scale study for the Indo-Pacific and required mining 6000 surveys—some by scientists, others by sport divers, local conservation groups and others. Bruno says: “A lot of people have done surveys and they don’t even have the data in electronic form; they’ll kind of mail you photocopies of their chicken scratch results from the 1980s.”

The Indo-Pacific includes some of the most aggressively managed reefs in the world; yet, the researchers found that the loss was uniform. The study shows that locally-managed coral on the Great Barrier Reef have suffered just as much in the last few decades as the reefs in the Philippines, for example, which have not been protected at all. "There’s no quantitative difference in their coral cover,” Bruno says.

This study provides more evidence that climate change is driving the loss of corals. Climate change affects reefs in a few ways: first, as water warms, the coral expels its symbiotic animals (zooxanthellae). The coral bleaches and dies. Coral diseases also seem to thrive in warmer water. Finally, climate change has caused the ocean to become more acidic.

“Climate change doesn’t just warm the ocean, it changes ocean chemistry,” Bruno says. “It actually makes it harder for corals to secrete the calcium carbonate skeletons that they sit on top of.” To keep pace with erosion, coral must constantly build new skeletons.

If declines in coral cover are the same at protected and unprotected reefs alike, is there value in local protection of reefs? “I’m hesitant to be too critical of management or local managers,” Bruno says, adding that over fishing of top predators and sediment run-off are important to manage locally because they may make reefs more susceptible to stresses associated with climate change. “But what it does suggest is that global-scale stressors that managers just can’t cope with are, at least in part, driving coral decline in the Indo-Pacific. Obviously managers can't manage water temperature, so there's not much they can do about that.”

-Flora Lichtman

What did you think of the story? Send us some feedback.

--Flora Lichtman