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Collasping corals
10 JULY 2008

Extinction looming among reef corals
In a new and disturbing study issued today, an international group of scientists reported that one-third of coral reef species, including some of the most common and important reef-builders, could face extinction. The assessment is grim news for marine biodiversity, says first author Kent Carpenter, of Old Dominion University (Norfolk, Va.). "We know that a huge percentage of the biodiversity in the oceans is found on coral reefs." Previous studies have focused on reefs as a whole, rather than the corals that comprise them, he says. "But if the coral species no longer exist, then reefs will not exist, and the potential loss of biodiversity in a cascading effect is tremendous."

Coral reefs are home to numerous species of fish, shellfish and other marine organisms, and also play an essential role in protecting shorelines from the ocean.

Acropora "Reefs are the most biodiverse ecosystems in the oceans, and if you remove the foundation of the ecosystem, the potential for loss of biodiversity is tremendous."

The researchers looked at all 845 known species of symbiotic reef-building corals. These animals provide a stony house for algae which, in return, perform photosynthesis and release nutrients that nourish the coral. The researchers used data on population changes and geographic range to classify extinction risks according to categories from the International Union for the Conservation of Nature, ranging from "least concern" (very low probability of extinction) to the "threatened" categories ("vulnerable," "endangered," and "critically endangered").

Adequate information existed to support classification of 704 species, and about 33 percent of them were found to be in one of the threatened categories, the researchers found. That was a shock, Carpenter says. "We initially went in knowing that corals ... have very wide distribution. ... We were really overwhelmed at the percentage of species that fall into the threatened category, and these are also the most common corals on many reef ecosystems."

Seeing the reefs and the corals
It's no secret that many coral reefs are in bad shape, due to a witches-brew combo of overfishing, disease, warming oceans, and pollution due to erosion, sediment and coastal development. In 1997-98, during a huge "bleaching event," warm water and perhaps other causes forced the coral to expel their essential symbiotic algae, and "Large tracts were totally wiped out," Carpenter says, "lost, for all time, apparently."

A few fish float over brown and gray landscape beneath the ocean.
Coral bleaching at Kelso Reef, on the Great Barrier Reef in Australia. Bleaching occurs when corals expel their symbiotic algae, and it can kill the colony. Image © Cathie Page

A small percentage of the bleached reefs have recuperated, Carpenter says, generally those that "were the most isolated from human impacts." Sediment can deprive the symbiotic algae of needed sunlight, while overfishing can cause indirect harm. In the Caribbean, for example, removing the fish that feed on non-symbiotic algae can allow them to flourish and smother the coral. Coral are a breeding and spawning ground for many fish species, and so the decline of the Caribbean corals endangers the local food supply as well as a prime tourist attraction. The Caribbean had the largest percentage of species in the highest risk categories, reflecting damage from intense coastal development.

Shallow water blues
Around the world, many near-surface coral -- which is where most reefs occur -- are threatened as global warming raises water temperatures and promotes bleaching. Although deeper corals are less affected, "The bottom line," Carpenter says, "is that if CO2 continues to increase, and causes more and more coral bleaching and disease events, then there are many corals that will not be able to replenish themselves."

As an additional hazard, billions of tons of atmospheric carbon dioxide is absorbed in ocean water each year, which becomes more acidic and attacks corals and other animals, such as clams, that build their structure from calcium carbonate.

Black bands across blue and yellow coral.
Coral diseases are another cause of decline among reef-building species. Black band disease strikes a colony of Favia speciosa at the Great Barrier Reef, Australia. (Photos of healthy Favia speciosa.) Image © Cathie Page

Coral reefs are just one victim of greenhouse warming, but reefs will still benefit from local regulations, even if the level of carbon dioxide in the atmosphere continues to rise, Carpenter says. "We can help those corals that still exist get through the crisis by enacting as many other conservation efforts as we can," including "strict fishing regulations, and strict reductions in other human effects, such as sedimentation, pollution and coastal development."

As intended, the study offered a big-picture view of coral, and the big picture is that a higher percentage of coral species are imperiled than any large group of land animals except amphibians. "We have all heard about the effects of climate change on terrestrial organisms," says Carpenter, "but we are beginning to understand that one of the most important habitats in the ocean is most at risk."

- David J. Tenenbaum

Related Why Files
Declining reefs

• One Third of Reef-Building Corals Face Elevated Extinction Risk From Climate Change and Local Impacts, by K.E. Carpenter et al, Science, 11 July 2008.

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