In the Pacific Ocean, the population size of this coral, Acropora cf grandis, is about 100 million.

Andreas Dietzel

A comprehensive survey of corals has turned up billions of colonies across the Pacific Ocean. The work—based on actual head counts, satellite data, and informed estimates—suggests many species are not in immediate danger of extinction, and the census could help conservationists and policymakers make better decisions about how to protect reefs.

The numbers are “incredibly encouraging,” says Nancy Knowlton, a retired coral reef biologist at the Smithsonian Institution’s National Museum of Natural History. “The biggest take-home message is that it’s not hopeless, even if corals have been painted as the canary in the coal mine.” And although some researchers worry this abundance could lead policymakers to ease up on efforts to protect reefs, Knowlton says, “having a clear set of numbers makes it easier to figure out what to do next.”

Over the past several decades, corals have suffered tremendous damage from warming seas, which causes bleaching, a process that causes stressed corals to lose the algal partners they need to survive. Corals are also being assaulted by ocean acidification, which can harm their ability to build their hard frames, as well as by pollution, overfishing, oil spills, and other human activities. In some places, such as the Caribbean, coral numbers have dwindled and, overall, the extent of coral reefs is half what it was in the 1870s. Experts have warned that most coral reefs could be gone by 2100. Already, about one-third of the world’s 6000 known coral species are listed on the International Union for Conservation of Nature’s (IUCN’s) Red List of Threatened Species. On the other hand, some reefs are proving resilient to marine heat waves or have continued to thrive against all odds.

The upshot is that “most people don’t know how worried to be” about corals, Knowlton says. Most estimates of coral numbers are based on expert opinion. A few researchers have counted populations of a single species, or looked at coral populations in a small area, but no one had taken a comprehensive look. “It’s a question many people have wondered about at the pub, but this is the first effort to actually estimate the number,” says John Bruno, a marine ecologist at the University of North Carolina, Chapel Hill. 

The new coral census focuses on a 10,000-kilometer-long swath of the Pacific Ocean between Indonesia and French Polynesia. First, Andreas Dietzel, a coral reef biologist at James Cook University, Townsville, and colleagues used satellite images and maps of coral habitats to determine the extent of coral reef habitat. They also included information from Bruno about coral cover on 900 reefs. Co-author Terry Hughes, also from James Cook, already had head counts of 318 species at 60 sites. In that census, Hughes counted any coral colony at least 2 centimeters across, though some massive colonies stretch many meters.

Based on those numbers, the researchers concluded that about 200 coral species each have more than 100 million colonies. The count topped 1 billion for about 60 species, Dietzel, Hughes, and their colleagues reported Monday in Nature Ecology & Evolution. Eight species outnumber the world’s population of humans (7.8 billion). And even the rarest species topped 1 million. These numbers add up to about a half-trillion corals, on par with the number of birds in the world and the number of trees in the Amazon. “That’s a lot of coral,” Bruno says.

The numbers are estimates and, particularly for rarer corals, may be off by quite a bit, Dietzel warns. Even so, Knowlton says, “they are not going to be wrong enough to it to make any difference.” And, with so many colonies, “most of these species will not go globally extinct in the near future,” Dietzel and his team concluded.

The data shed new light on conservation strategies already in place. IUCN has listed 80 of the 318 species included in the study as either vulnerable, endangered, or critically endangered; yet 12 of those 80 have estimated populations exceeding 1 billion. For example, one of the more abundant species, Porites nigrescens, which forms massive boulders on reef flats, is considered vulnerable, even though it is resistant to bleaching. In contrast, IUCN lists some of the rarer corals as “data deficient”—meaning not enough is known to make a call on the species’ vulnerability—or of least concern. “A major revision of current Red List classifications is urgently needed,” Dietzel and his colleagues wrote.

Actually, a reclassification of corals is already underway, says David Obura, chair of the IUCN Coral Specialist Group. Also, as he and Beth Polidoro, who coordinates the Red List for the group, point out, Red List status “is not determined by the total number of individuals.” As the fate of passenger pigeon has shown “species with extremely high populations have gone extinct in the past,” Polidoro adds.

Bruno, too, is worried. “I fear these results will be misinterpreted by policymakers, conservationists, and other scientists.” The actual numbers of individual colonies, even individual species, may be huge, but in many places, reefs that create rich habitats for sea life and protective barriers for islands are rapidly shrinking. There, “Despite the large populations, corals have become so scarce, they no longer fill that role,” he adds.

And even Dietzel agrees that local stressors have severely damaged reefs in some places, causing local extinctions in places such as the Caribbean. Also, the new numbers are based on data collected before 2006, so coral populations might have shrunk somewhat.

Dietzel notes that corals living in the Pacific Ocean tend to have “geographic ranges and population sizes … [that are] typically much larger than those of Caribbean species.” That doesn’t mean Pacific corals aren’t in danger, he says, but, “We hope that our study inspires a rethinking of what declines in abundance mean for the global extinction risk.”