Study says corals are in trouble but not at risk of extinction

• The study examined 318 species across 900 reefs in the Pacific Ocean, from Indonesia to French Polynesia, and found half a trillion coral colonies.
• The study's authors are calling for a revision of the IUCN Red List, which states that a third of all reef-building corals are at some risk of extinction.
• At the same time, the new study emphasizes that localized extinctions and the loss of ecological function are real and present threats.

1. A new study  counting half a trillion coral reef colonies between Indonesia and French Polynesia reveals that many reef-building corals are not in imminent danger of disappearing from the planet.
2. Desolate reefs, ravaged by heatwaves and eroded by ocean acidification, are the stuff of ecological nightmares. But coral reefs are not homogeneous collections; they are conglomerates of various coral species. For these species, salvation may lie in their abundance and diversification.
3. Andreas Dietzel, leader of the study, says: “Most species have such large populations that even if 90% of them were lost, they could still be found in some areas and persist for some time. This gives us the opportunity to bring some stressors like climate change under control and hopefully restore the biodiversity we have seen for millions of years.”
4. Dietzel and colleagues are calling for a revision of the IUCN Red List, which states that a third of all reef-building corals are at some risk of extinction. The list is based on a landmark 2008 study that assessed the status of 704 species for which sufficient data were available.

Pocillopora 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, examined 318 species across 900 reefs in the Pacific Ocean, 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 this region because of the availability of species data and because the area is home to the majority of existing coral species.
• The corals that make up these reefs reside on enormous platforms of their own creation. These reefs are veritable hives for marine life, providing abundant food and shelter. Coral polyps, cousins ​​of jellyfish, are the organisms that create the underlying calcium carbonate structure. These polyps form colonies and are connected to each other in a way that defies differentiation.
• Marine scientists often use coral cover, the area occupied by living corals on a reef, to assess their condition. However, this method doesn't consider the condition of each individual coral species. Establishing the size of a population is complicated. Therefore, scientists like Dietzel use a coral colony connected by connective tissue that reproduces, grows, and dies together as a unit of measurement.

Map showing the scope of the study. Shading indicates coral species richness. White triangles indicate the locations of the 15 islands in the five regions where species abundance was measured. Blue circles show the locations of coral cover measurements. Black circles indicate the locations of coral reef habitat studies.

• About a quarter of the species considered in the study are listed as threatened by the IUCN. Dietzel, a researcher at James Cook University in Australia, emphasizes that "We rely on the application of the IUCN criteria, which in fact aren't designed for species like corals; they're actually designed for mammals, birds, and reptiles, whose populations are typically quite small."
• Unlike many endangered terrestrial species, most corals are found across vast areas of tropical oceans and number in the millions. A dozen coral species classified as endangered by the IUCN have been found to have populations exceeding one billion. Porites nigrescens is one of the most abundant species found in this group. Despite this, the IUCN considers it to be globally threatened.
• At the same time, some less numerous species are classified as "Least Concern." For others, there is insufficient data to determine their status. The authors write, "Our limited understanding of their ecology and abundance reflects their rarity and the fact that their extinction risk may be relatively high and unrecognized."

A reefscape in the Pacific. Image by Christopher Brunner.

1. The new study highlights that localized extinctions and the loss of ecological function are real and present threats. Tabulate corals are an example of relatively rare corals. If they were to become locally extinct, the butterflyfish ( Chaetodon trifascialis ), which feeds on these corals, would be in trouble. Even abundant corals that occupy large areas can disappear from a reef or region.
2. Dietzel says, "The study doesn't say the reefs are doing well. On the contrary, they're really struggling. A species could lose 95% of its members; in that case, the reefs will never be the same again and won't function as they once did. But these species won't go extinct entirely."