State of the Planet

News from the Columbia Climate School

Can We Save Coral Reefs?

Blue chromis swim over pavona coral in French Polynesia. Photo: Michele Westmoorland.
Blue chromis swim over Pavona coral in French Polynesia. Photo: Michele Westmoorland.

By Christine Evans

This is the second story of a two-part series. 

Coral reefs are immensely important to the health of our oceans. Renowned oceanographer Sylvia Earle refers to them as “a jeweled belt around the middle of the planet,” and they support a staggering amount of the ocean’s biodiversity. They host 32 of the 34 animal phyla that have been discovered, while tropical rain forests support just nine. Nonetheless it is thought that, conservatively, a third of reef-building corals are in danger of extinction.

Soldierfish on a healthy reef in French Polynesia. Photo: Michele Westmoorland.
Soldierfish on a healthy reef in French Polynesia. Photo: Michele Westmoorland.

Local vs. Global Threats

Coral reefs are facing a host of local stressors. Coastal development can harm corals by increasing sediment, pollution and nutrient runoff. Excess nutrients from sources like farming or wastewater can cause algae blooms, which smother reefs. Overfishing has disrupted the complex balance between different species and has dramatically altered marine food webs.

Palmyra Atoll National Wildlife Refuge has some of the most pristine coral reefs in the world. Photo: Kydd Pollock.
Palmyra Atoll National Wildlife Refuge in Kiribati has some of the most pristine coral reefs in the world. Photo: Kydd Pollock.

Apex predators like sharks are critical for keeping the oceans healthy and other species in balance, but their numbers have declined dramatically. In parts of the Caribbean like Belize, this decline has meant a proliferation of grouper, which then eat greater numbers of parrotfish, a key species for keeping corals clear of the algae that can overtake them. Grazers like parrotfish and sea urchins are especially important for corals that are young or recovering from events like bleaching or hurricanes.

So far local threats have proven to be the most damaging. But these impacts can be managed at the local level, unlike global threats like climate change, discussed in part one of this series. Scientists are hoping that better local management, along with new research into reef resilience, might help buy some time for corals.

“Ultimately, if we don’t do something about carbon dioxide concentrations in the atmosphere, local management will not succeed. But effective management actually buys incredibly important time while we figure out how we’re going to deal with the carbon dioxide problem,” says Nancy Knowlton, a coral reef biologist and the Smithsonian Institution’s Sant Chair for Marine Science, who has been studying coral reefs since the early 1970s.

Local Management: Marine Protected Areas

A 2014 report from the Global Coral Reef Monitoring Network of the International Union for Conservation of Nature found that the healthiest reefs in the Caribbean are those that still have large parrotfish populations. Bermuda and Bonaire are prime examples, and both have strong protections for parrotfish.

Families at the beach in Barbuda, an island in the Eastern Caribbean that makes up part of the state of Antigua and Barbuda. Photo: Ayana Johnson.
Families at the beach in Barbuda, an island in the Eastern Caribbean that makes up part of the state of Antigua and Barbuda. Photo: Ayana Johnson.

Barbuda’s Blue Halo initiative seeks to enact similar measures as part of a larger mission to “set some areas aside, protect key species, and prevent habitat damage.”

The initiative was centered on the concerns of local fishermen, who were going farther out and catching less. Said Barbuda’s oldest active fisherman, Papa Joe, “This is no joke. This is a serious serious thing. If you’re a country and you mash up your own livelihood, what is going to happen?” He has watched the reefs decline, but says, “There’s supposed to be a solution to every problem.”

This is what most Florida coral reefs look like today—overgrown with soft corals and algae rather than large, stony corals characteristic of a healthy reef Florida Fish and Wildlife Conservation Commission
A reef overgrown with soft corals and algae—what most of Florida and the Caribbean looks like today. Photo: Florida Fish and Wildlife Conservation Commission.

In Barbuda, close to 80 percent of reefs are covered in algae, and less than 14 percent have living coral. Barbuda now has five no-take marine sanctuaries, covering 33 percent of its coastal area, and has banned the catching of parrotfish and sea urchins. Many hope this can be an example for the rest of the Caribbean, whose fisheries are in a “dire” state, according to one of the scientists involved in the initiative.

Scientists have shown that marine protected areas—especially no-take reserves—tend to see an increase in characteristics like the number of species, species density, biomass, and higher rates of larval dispersal both in and around the reserves. Other studies have found that fish species in protected areas are often more resilient to the impacts of extreme weather and climate change.

Parrot fish on a reef in Bonaire, an island municipality of the Netherlands in the south Caribbean. Photo: Waywuwei/Flickr.
Parrot fish on a reef in Bonaire in the south Caribbean. Photo: Waywuwei/Flickr.

Marine protected areas have a similar impact on coral reefs. A 2006 study of a protected area in the Bahamas found that parrotfish numbers were around five times higher in the reserve than outside, and that they were roughly double in size. As a result, grazing rates doubled, resulting in a fourfold reduction of algal cover—coral’s main competitor. More grazing decreased the seaweed, which increased coral recruitment and growth, increasing fish habitat and further increasing algae consumption. The overall effect of this was to increase the resilience of the ecosystem—both to local and global impacts.

A 2014 study documented the impacts of catastrophic flooding on Australia’s Great Barrier Reef and found that “reserve reefs resisted the impact of perturbation, whilst fished reefs did not.” Furthermore, there was more grazing of algae and more coral recruitment in the protected areas. “The capacity of reserves to mitigate external disturbances and promote ecological resilience will be critical to resisting an increased frequency of climate-related disturbance,” the study concluded.

Sharks swim over a pristine reef at Millennium Atoll in the southern Line Islands, Kiribati. Photo: Enric Sala.
Sharks swim over a pristine reef at Millennium Atoll in the southern Line Islands, Kiribati. “If an alien had just one day on Earth and wanted to see a coral reef, I would show him Millennium Atoll,” says Enric Sala. Photo: Enric Sala.

This has already played out in many marine protected areas. The South Pacific island nation of Kiribati is home to some of the most pristine reefs in the world. The Line Islands in Kiribati—some inhabited and some not—form an archipelago spanning the equator. Enric Sala, a marine ecologist at Scripps Institution of Oceanography and a National Geographic explorer, says seeing these islands is like traveling back in time. The inhabited islands are degraded with tiny fish and no sharks, but as you travel further away from the inhabited islands, the reefs begin to look like they would have perhaps 500 years ago, says Sala in a 2010 Ted Talk.

Coral gardens on Palmyra Atoll in the Phoenix Islands Protected Area. Photo: Kydd Pollock, U.S. Fish and Wildlife Service.
Coral gardens on Palmyra Atoll in the Phoenix Islands Protected Area. Photo: Kydd Pollock, U.S. Fish and Wildlife Service.

The Phoenix Islands Protected Area, also in Kiribati, was established in 2006 and expanded two years later to become the largest marine protected area in the world at the time— roughly the size of California. From 2002 to 2003, El Niño caused massive bleaching, which decimated corals even in this remote area of the Pacific. In assessing the damage, a team of scientists found that “the bleaching had killed all the coral on the lagoon floor, but almost half appeared to be growing back—the fastest recovery any of us had ever seen. The reason seemed clear: abundant fish,” which eat the algae that might otherwise prevent the corals from recovering. “Because fish populations had been protected here, the reefs remained surprisingly resilient even after suffering one of the worst bleaching events ever recorded.”

Coral Resilience and Adaptability

Some scientists have said that corals likely can’t survive above an atmospheric level of 350 parts per million of CO2, a marker which we have already surged past. But more recent research has shown the picture to be more complex, nuanced and hopeful.

A 2014 Ohio State University study found that corals are better able to recover from bleaching events if they have large energy reserves stored as fat and are less choosy about which species of algae they host. “If we conserve reefs that contain coral species with these survival traits, then we’re hedging our bets that we might be able to preserve those reefs for an extra decade or two, buying them enough time to acclimatize to climate change,” says Andréa Grottoli, a professor in the School of Earth Sciences and one of the study’s authors. This means that many species will be lost and that reef diversity will decline, but, continues Grottoli, “We’re actually a bit optimistic, because we showed that there’s acclimation in a one-year window, that it’s possible. In two of our three coral species, we have recovery in six weeks. The paths they took to recovery are different, but they got there.”

A study published earlier this year found that many coral species are already adapting to warmer ocean temperatures. According to study co-author Mikhail Matz, “We show an up-to-10 fold increase in odds of survival of coral larvae under heat stress when their parents come from a warmer lower-latitude location.” He also says, “Corals already have the genetic variance to adapt to changing temperature. It’s just a matter of transporting these mutations from hot locations to locations that will be very hot soon because of global warming.”

Over time, this crossbreeding would happen on its own, but scientists like Matz say humans could help speed up the process by a decade or two. However, “This is not a magic bullet. The existence of this gene variation buys us some time.”

As the predicament of coral reefs demonstrates, their fate—and that of so many of Earth’s species—is still largely in our hands. In a 2009 Ted Talk, Sylvia Earle says, “The next 10 years may be the most important of the next 10,000 years. … We have the capacity to pull ourselves out of this downhill slide in terms of what’s happening to our life support system.”

For coral reefs and life in the ocean, she says, “There’s still time, but not a lot, to turn things around.”

*Coral reef in french polynesia - michele westmorland, photographer and International League of Conservation Photographer Founding Fellow,
A coral reef in French Polynesia. Photo: Michele Westmorland.

Christine Evans is a graduate student in Columbia’s Sustainability Management program. 

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Sanna Persson
Sanna Persson
5 years ago

Hi there,
I am currently working on a paper about shark conservation in Kiribati and would very much like to engage with the author Christine Evans and/or the photographers Kydd Polloc and Enric Sala. If either of you see this, could you please email me on I cannot find your contact details.
I appreciate your time!
Kind regards,
Sanna Persson