Many corals are so abundant that extinction risk is lower than previously feared

March 1, 2021
The plight of coral might not be as dire as previously thought. (David Williamson)

The plight of coral might not be as dire as previously thought. (David Williamson)

The number of reef-building corals across the Indo-Pacific is on par with the number of trees in the Amazon, scientists reported this week, suggesting that despite well-documented population declines among the vulnerable animals, significant extinction among species is unlikely in the near future.

For the new study, which was published Monday in Nature Ecology & Evolution, the researchers combined satellite data and field observations to calculate the total number of coral colonies and the population sizes of more than 300 species across reefs that spanned more than 100,000 square kilometers. They found that roughly half-a-trillion corals live within these shallow-water reefs, and two-thirds of the species had populations of more than 100 million individual colonies.

“Coral species are usually so widely distributed, and their population sizes therefore so large, that we might see devastating loss of corals and a huge decline in population sizes, but we won’t see any significant amount of extinction in the near future,” said the first author of the study, Andreas Dietzel, who recently completed his doctorate at the ARC Centre of Excellence for Coral Reef Studies at James Cook University in Australia. “It gives us a window of opportunity to address the threats to coral reefs [and] get climate change under control.”

Coral populations have been dwindling for decades, with climate change increasing the extent and frequency of mass coral bleaching events and die-offs, Dietzel and his colleagues noted in the study. To get a broad picture of the state of coral reefs, the researchers visited reef slopes, crests and flats in Indonesia, Papua New Guinea, the Solomon Islands, American Samoa and French Polynesia between 1997 and 2006. 

They counted more than 37,000 coral colonies at 60 sites and identified 318 species — approximately half of all those known to dwell in the region. Although corals are found in other parts of the world as well, the region between Indonesia and French Polynesia includes about 70% of the global shallow-water coral reef area and more than three-quarters of the 800 known hard coral species.

The team also drew on satellite imagery that researchers had previously used to map the extent of known coral reefs around the world, as well as more detailed maps from 61 reef locations that revealed what proportion of the seafloor was actually inhabited by corals, versus covered in inhospitable rubble or sand. From all of these sources of data, the researchers created a rough estimate of how many corals of each species likely exist in these regions.

Dietzel and his colleagues found that the six rarest coral species had estimated populations of fewer than 1 million colonies, while one-fifth of the species had population sizes greater than 1 billion colonies, and the eight most common coral species each had populations that actually surpassed the global human population size of 7.8 billion. 

Most of the coral species the researchers examined have ranges that stretch beyond the Indo-Pacific region, indicating that the animals' global population sizes are even larger.

For comparison, about 3,900 tigers, 415,000 African elephants and around 200,000 snowy owls remain in the wild.

Most reefs were dominated by just a handful of species, and 17 especially profuse species accounted for as many coral colonies as the remaining 301 species combined. 

“That essentially tells us that there are certain species that contribute a lot ecologically to coral reefs,” Dietzel said.

The International Union for Conservation of Nature lists about one-quarter of the 318 coral species Dietzel and his colleagues examined as vulnerable to extinction, endangered or critically endangered. However, the researchers estimate that 12 of these 80 species have populations exceeding 1 billion colonies. 

“Overall, [for] most species their population size is larger than 100 million individuals, which again indicates that they probably won’t go extinct anytime soon,” Dietzel said.

The criteria the IUCN uses to determine extinction risk may work less well for corals than they do for terrestrial animals like elephants and tigers, the researchers concluded. Coral larvae can be carried across vast distances by currents, which means they’re less likely to be confined to small ranges than many land animals. Even if a coral species disappears from most of its range, it may still survive in small pockets of reef and slowly recolonize lost habitat or adapt to changing conditions, Dietzel says.

However, he cautions, the new findings should be considered rough preliminary estimates, and it’s unclear how the population of any given species might have changed in the years since the researchers observed it. 

“We can argue that across the entire group of species … the overall extinction risk is probably a lot lower than it was previously estimated, but we lack the ecological data to assess the extinction risk of any particular species,” Dietzel said. “Our study is just a snapshot in time.”

And the findings don’t mean that coral reefs aren’t in trouble; scientists have estimated that the Great Barrier Reef has lost half of its corals in the past quarter-century. Still, Dietzel says, the estimates that he and his colleagues have developed may prove valuable to future coral restoration work by giving researchers a sense of how many corals will need to be transplanted to salvage reefs.

The article, “The population sizes and global extinction risk of reef-building coral species at biogeographic scales,” was published March 1 in Nature Ecology & Evolution. The authors of the study were Andreas Dietzel and Terry P. Hughes, James Cook University; Michael Bode, James Cook University and Queensland University of Technology; and Sean R. Connolly, James Cook University and Smithsonian Tropical Research Institute.

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