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Corals are struggling, but they’re too abundant to go extinct, study says

  • A study has found that most reef-building coral species are not in imminent danger of being wiped off the planet because they are abundant and occupy vast ranges.
  • It looked at 318 species across 900 reefs in the Pacific Ocean, from Indonesia to French Polynesia, and found half a trillion coral colonies in the region.
  • The study authors are calling for a revision of the IUCN Red List, according to which a third of all reef-building corals face some degree of extinction risk.
  • At the same time the new research underlines the fact that local extinctions and the loss of ecological function are real and present threats.

new study that counted half a trillion coral colonies between Indonesia and French Polynesia found that most corals are not in imminent danger of being wiped off the planet.

Desolate reefs ravaged by heat waves and eroded by acidic ocean waters are the stuff ecological nightmares. But coral reefs are not homogenous wholes; they are conglomerations of coral species. For these species, there might be safety in numbers and wide ranges.

“Most species have such large population sizes that even if they lose 90% of individuals, they will still be found in some pockets and persist for the time being,” said Andreas Dietzel, lead author of the study, “it gives us a chance to get stressors like climate change under control, and hope that the diversity that we’ve seen for millions of years will recover.”

Dietzel and his colleagues are calling for a revision of the IUCN Red List, according to which a third of all reef-building corals face some degree of extinction risk. The list draws on a landmark 2008 study that assessed the status of 704 species for which enough data were available.

Pocilloporid coral in the Coral Sea off the northeastern coast of Australia. Image by Andreas Dietzel.

The new study published in the journal Nature Ecology & Evolution focused on 318 species across 900 reefs in the Pacific Ocean, stretching from Indonesia to French Polynesia. The data used in the new analysis conducted over five years were collected between 1997 and 2006. The researchers chose the region because of the availability of species-level data and because the area hosts most of the world’s coral species.

Reef-building corals sit on massive platforms of their own creation. These reefs are hives for marine life, providing abundant food and shelter. Coral polyps, cousins of jellyfish, are the organisms that create the underlying calcium carbonate structure. Together these polyps form colonies and are interconnected in ways that defy differentiation.

Marine scientists often use coral cover, the area occupied by live corals on a reef, to gauge their condition. However, this method does not capture how each coral species is faring. Arriving at population sizes is tricky. So, scientists like Dietzel use a coral colony bound by a connecting tissue that reproduces together, grows together, and dies together as a measurement unit.

A map showing the extent of the study. Shading indicates species richness of corals. White triangles indicate the location of the 15 islands across five regions where species abundances were measured, blue circles show the locations of coral cover measurements and black circles indicate the location of coral reef habitat surveys.

About a quarter of the species considered in the study are listed as endangered by the IUCN. “It’s based on the application of IUCN criteria which aren’t really designed for species like corals; they are designed for mammals and birds and reptiles, which usually have fairly small population sizes,” Dietzel, a researcher at James Cook University in Australia, noted.

Unlike many endangered terrestrial species, most corals are found across large swaths of tropical oceans, numbering in the millions. A dozen coral species considered under threat under IUCN classification were found to have populations of over 1 billion. Porites nigrescens was one of the most abundant species found by the team. Yet, the IUCN lists it as vulnerable to global extinction.

At the same time, some species with low abundance fall under the IUCN’s “least concern” category. There are others for which sufficient data are not available to determine their status. “Our poor knowledge of their ecology and abundance is reflective of their rarity, and that their extinction risk may be relatively high and unrecognized,” the authors write.

A Pacific reefscape. Image by Christopher Brunner.

The new research underlines the fact that local extinctions and the loss of ecological function are real and present threats. Tabular corals are one example of coral that are relatively rare. If they went locally extinct, the population of butterflyfishes (Chaetodon trifascialis) that feed on these corals would be in trouble. Even corals that are abundant and occupy vast ranges can vanish from a reef or a region.

“The study is not saying that reefs are doing fine. Reefs are really struggling,” Dietzel said. “A species might lose 95% of individuals, in which case reefs will not look anything like they used to, and they won’t function the same way as before. But those species will still not go globally extinct.”

(Banner Image: Fish belonging to the Amphiprion genus on a coral reef. Image by David Williamson.)

Citations:

Dietzel, A., Bode, M., Connolly, S. R., & Hughes, T. P. (2021). The population sizes and global extinction risk of reef-building coral species at biogeographic scales. Nature Ecology & Evolution. doi:10.1038/s41559-021-01393-4

Carpenter, K. E., Abrar, M., Aeby, G., Aronson, R. B., Banks, S., Bruckner, A., … Wood, E. (2008). One-third of reef-building corals face elevated extinction risk from climate change and local impacts. Science, 321(5888), 560-563. doi:10.1126/science.1159196

Malavika Vyawahare is a staff writer for Mongabay. Find her on Twitter: @MalavikaVy

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