Damaged coral reefs can recover quickly after restoration work

Restored coral reefs can grow just as quickly as healthy reefs in as little as four years, according to the results of a restoration project in Indonesia. While the rapid recovery is promising, the reefs tend to have less species diversity than undamaged reefs, and more observations are needed to see how they fare over time and in tougher conditions such as heatwaves.

The world’s coral reefs face numerous threats, from rising sea temperatures and ocean acidification to human activity such as overfishing.

Just off the south coast of Sulawesi, Indonesia, reefs were severely damaged about 30 years ago by dynamite fishing, where explosives are dropped into water to kill or stun large numbers of fish.

“There is no natural recovery from the dynamite fishing,” says Tim Lamont at Lancaster University in the UK. “It leaves behind lots of loose fragments of old, dead coral skeletons, which washes around and doesn’t allow for coral to naturally settle on it and grow.”

To help the reef recover, the Mars Coral Reef Restoration programme – part of the Mars corporation’s sustainability plan – has been installing hexagonal sand-coated steel structures on the seabed and transplanting cuttings from healthy corals over the past few years. The structures, known as reef stars, stabilise loose rubble and aid coral growth.

At the same time, Lamont and his colleagues have been monitoring the success of these efforts.

One measure of a coral’s health is to see whether its limestone skeleton develops quicker than it is eroded away. This tells us a reef’s overall rate of growth and is known as its carbonate budget.

“Four years after the restoration process started, reefs had an equivalent growth rate of healthy reefs,” says Lamont. “That’s surprisingly quick.”

But the composition of the restored reefs was different from healthy ones, consisting primarily of branching corals. This is largely due to the restoration method, which uses branching corals that can be extracted from living corals with minimal damage and are easier to attach to the steel structures.

“The difference in community might lead to differences in resilience to future stress events, especially heat stress, as branching corals are generally more sensitive to bleaching,” says team member Ines Lange at the University of Exeter, UK. “We expect to see natural recruitment and recovery of more massive and encrusting corals to restored areas over longer time scales.”

It is encouraging to see that it is possible, given stable climate conditions, to rebuild these vital ecosystems, says Lamont. But longer-term studies are needed to see how well species diversity bounces back, as well as how resilient the reefs are compared with healthy reefs, he says.

However, projects such as this can’t address the biggest threat faced by coral reefs, says Terry Hughes at James Cook University in Australia. “The scale of this study is tiny compared to the amount of corals dying every hot summer as temperatures continue to rise globally,” he says. “For example, you would need to raise and out-plant roughly 250 million adult corals, each the size of a large dinner plate, to increase coral cover on the Great Barrier Reef by 1 per cent.”

“The problem with restoration is not that it doesn’t work at all, and not that it doesn’t restore carbonate budgets,” says Michael Bode at the Queensland University of Technology, Australia. “It’s not even that the species you get back aren’t as diverse as ‘natural’ coral reefs. It is that it’s much too labour and resource intensive to combat the main threat to coral reefs – climate change.”