By Caitlin Martin Newnham

Coral reefs are being pushed to the brink. For the past two years, many of the world’s reefs have been hammered hard by “the longest and most widespread coral bleaching event on record,” according to the US National Oceanic and Atmospheric Administration. Since the 1970s, climate change, El Niño events, overfishing, pollution, and other pressures, have caused coral to decline at an unprecedented rate. As coral continue to be assailed from all sides, the question becomes: what happens to a coral reef when the coral disappear?

To get an idea, says University of Queensland ecologist Peter Mumby, look to Jamaica. In the 1970s, the Caribbean nation’s vibrant coral populations died. In their place grew seaweed forests.

Jamaica’s coral reef collapse was a complex process that started with decades of heavy fishing. In the decades leading up to the 1970s, overfishing depleted the region’s fish, including those that eat seaweed. With the fish gone, urchins gorged on the sudden abundance of algae.

“If you were to take a step on a reef in Jamaica at that time, it would be very hard to do that without stepping on a sea urchin,” says Mumby.

But in 1980s, disease struck the urchin population. The urchins died en masse, leaving too few herbivorous animals to keep the seaweed in check. Over the span of a year, the seaweed population soared and began smothering the coral, which were also declining because of disease.

“If you have lots of seaweed, the coral is expending so much energy battling away with the seaweed that it just fails to grow,” Mumby says.

Even a healthy reef will sometimes collapse because of a natural disaster such as a tropical cyclone. In such cases, an “algal turf”—a layer of small algae—begins to grow over the dead coral. In healthy environments, fish will return to the destroyed reef to feed on the algae and, after a few years, the coral will recover.

But in a recent experiment, Mumby and his team studied what happens to a damaged reef when herbivorous fish are unable to repopulate the area, which is what happened in Jamaica’s coral collapse. They found that when large, algae-eating fish such as parrotfish are prevented from recolonizing the reef, the growth of new corals is decreased by 700 percent. When all fish are excluded from returning to the reef, coral growth drops 1500 percent. And in the corals’ place: seaweed.

Seaweed forests lack the complex physical structures of coral that fish need to thrive. Without coral reefs, fish can’t hide from predators and become easy prey.

“If you were to leave the fish alone, it takes something between six and ten years for those fish who consume seaweeds to recover,” Mumby says. But overfishing continues in Jamaica, and even after 30 years the corals have yet to recover.

The transition from vibrant coral reef to seaweed forest experienced in Jamaica foreshadows the large-scale deterioration of coral reefs that could result if global and local pressures such as pollution, overfishing, and ocean warming continue. Theoretically this ecosystem transition is reversible, but only if the pressure that caused the decline goes away.

One of the prominent threats to the future of coral reefs is ocean warming caused by anthropogenic climate change, which triggers coral bleaching events and slows coral growth. With such a persistent pressure, it becomes more difficult for coral to recover.