In Hot Water

Half of the world’s reef-building corals have already died,” says John Bruno. Even where humans try their best to tread lightly.

Bruno, a marine ecologist at Carolina, and graduate student Elizabeth Selig came to this conclusion after two years of compiling and analyzing 6,001 scientific surveys of 2,667 coral reefs in the Pacific Ocean from Indonesia to Hawaii. They found that nearly six hundred square miles of these ecologically diverse underwater forests have disappeared every year since the mid-1980s. That’s about 1 to 2 percent of the world’s coral reefs dying each year, nearly twice the rate of tropical rainforest loss.

The Indo-Pacific is home to 75 percent of the world’s coral reefs, which are composed of tiny marine animals. The reefs support thousands of fish species and other aquatic life, and coastal communities depend on healthy coral reefs for fisheries, tourism, and protection from storm surges. “These reefs are incredible buffers,” Bruno says. “You can literally have fifty-foot waves break on a reef during a cyclone, and the waves on the beach are one or two feet high.”

When corals die, they leave skeletons that erode quickly. Fish go away; fisheries suffer. And sea walls disappear. “And the next time there’s a hurricane,” Bruno says, “those fifty-foot waves will slam on the shore.”

Bruno and Selig tracked the decline of coral cover — the measure of how much live coral covers the ocean floor in a given area. Coral cover is a key indicator of reef health in the same way that canopy cover is indicative of tropical rainforest health. In places where coral grows, it has historically tended to cover about 50 percent of the ocean floor. But Bruno and Selig found that coral cover throughout the Indo-Pacific declined from 40 percent in the early 1980s to 20 percent by 2003. Today, only 2 percent of Indo-Pacific reefs come close to the 50 percent baseline.

This same rate of decline exists in Hawaii, Indonesia, Australia, and all points in between. And Bruno and Selig say that such a region-wide decline is the most surprising result of their research because not all coral reefs face the same dangers. Some coral live in tight quarters, which means that viruses can more easily infect and kill nearby coral. Some reefs are over-fished and face more pollution and sediment runoff from coastal development and agriculture. Other reefs are isolated and pristine.

The Great Barrier Reef off Australia’s northwest coast, for instance, is a protected area. Scientists had assumed that coral cover would be better there. They were wrong.

“Coral cover on the Great Barrier Reef was not significantly greater than reefs in the Philippines, where reefs are often thought to be highly threatened and poorly managed,” Bruno says. Coral cover in Hawaii is just as good — or bad — as coral cover in Australia, despite the particular protection policies of the two areas and the fact that the Great Barrier Reef is much more ecologically diverse.

“The Great Barrier Reef is in the coral triangle,” Bruno says. “So it’s the center of biodiversity for the whole ocean. It’s where most of the fish species are, and where most of the different types of invertebrates such as crabs and lobsters live.”

There are hundreds of different species of coral off Australia’s coast. There are about fifty in Hawaii.

“Ecologists had thought that the Indo-Pacific reefs were dying at a slower rate than say, Caribbean Sea reefs, which don’t support as many species and face at least as many threats,” he says. “We assumed that the Indo-Pacific was better off. But it’s just not, at least in terms of coral cover.”

All this leads to a troubling conclusion: coral decline is likely due to large-scale stressors.