Coral reefs deteriorate around the world

Sean Connolly, Senior Lecturer, Australia

Coral reefs are the world's most biologically rich marine ecosystems, harboring some of the world's most beautiful organisms. They provide the principal source of protein for over 10 million people worldwide. Reef-based activities (principally fishing and tourism) form the economic livelihood of millions more. Clearly, the human costs of a worldwide breakdown of these ecosystems are enormous.

Yet global deterioration of coral reefs is severe and ongoing. Wholesale disintegration of reef ecosystems has occurred in some places, and collapse on a worldwide scale is a real risk.

But there is some good news, too: we know what steps the international community can take now to protect and restore reefs' "resilience" -- their capacity to maintain integrity in the face of the environmental fluctuations that are a natural part of life in any ecosystem. We must mediate the severity of global warming, while simultaneously conserving the resilience of coral reefs.

Historically, the principal agents of reef degradation have been over-fishing and pollution, not global warming. In healthy ecosystems, when prey numbers decline, predators become malnourished, and their numbers decline, too, giving their prey a chance to recover. But human predators are different. When our prey numbers decline, their economic value tends to increase, so fishing intensifies. This means that once predator species become depleted, fishing pressure shifts towards plant-eating fish species, leading to precipitous declines in the numbers of herbivores on coral reefs.

Herbivorous fish are key players on coral reefs. When coral populations decline in the aftermath of cyclones, diseases, and other disturbances, it is the herbivores that keep seaweed in check, and allow coral populations to recover. Without them, fast-growing seaweed quickly monopolizes space on the reef, preventing restoration of healthy quantities of coral cover.

Because corals provide the habitat structure on which other reef organisms depend, the decreases in coral cover lead to big decreases in a reef's biodiversity. Pollution by nutrients and toxins from adjacent land areas further vitiate coral populations' ability to recover, giving seaweed an even greater competitive edge.

Corals are formed by a symbiosis between an animal and a one- celled plant. The animal provides shelter and nutrients for the plant; the plant converts sunlight into energy, which it shares with the animal. For reasons that are not yet fully understood, this partnership breaks down when corals experience unusually high temperatures. The plant is expelled, and corals turn a brilliant white color. If temperatures soon return to normal, the partnership can be restored. But if corals remain ``bleached'' for too long, they die.

For many species of coral, this bleaching threshold is usually only a couple of degrees centigrade above the typical maximum temperature for a given location. Interpreted in light of even the most optimistic global warming scenarios, this is disturbing news. If coral bleaching thresholds remain steady, local summer temperatures will exceed those thresholds regularly within a few decades. Corals grow slowly, so reefs sustaining severe bleaching will not recover before they bleach again.

Fortunately, there is evidence that bleaching thresholds evolve. Most coral species have broad geographical distributions, and bleach at different temperatures depending on location. Many species that bleach at 28 degree or 29 degree centigrade on Australia's Great Barrier Reef routinely tolerate temperatures of 34 degrees or more in the Arabian Sea. Unfortunately, we do not know how long it takes for such adaptation to occur.

Two things can be done to protect coral reefs. The first is obvious: minimize global warming by, say, honoring the Kyoto Treaty's emissions targets. Second, we must restore coral reefs' capacity to cope with environmental change--their resilience--by protecting the fish stocks that keep seaweed in check, and thereby facilitate the recovery of coral populations from bleaching. These recovery phases enable adaptations to higher temperatures to spread. Without fish, seaweed dominance will prevent that recovery.

The catastrophic collapse of fish stocks around the world--on coral reefs and elsewhere--has provided hard lessons about managing fisheries. Many intensively regulated fisheries have collapsed along with the unregulated ones. So there is a growing consensus that standard techniques for managing fisheries must be complemented with a system of ``no-take zones''--areas in which fishing is prohibited altogether.

As fish populations in these no-take zones recover, they create ``spill-over'' effects, with catches often increasing in areas outside the no-take zones. More importantly, they provide a refuge for fish populations, a kind of insurance policy against stock collapse. On coral reefs, these no-take zones can help insure that a portion of coral reef habitat sustains a healthy fish community, and secure the coral reef resilience that this brings.

Scientific evidence indicates that, to be effective, 30-50% of available habitat should be set aside as no-take zones. This figure is far higher than levels of protection in even the wealthiest countries that harbor significant coral reefs, the US and Australia, where current levels of protection are less than 5%.

Given the magnitude of the threat to the world's coral reefs, the international community's response has been frighteningly slow. But there is some encouraging news: in June, Australia's government proposed a major increase in no-take areas within the Great Barrier Reef to over 30%. If adopted, this policy would set a new global standard and perhaps prompt other nations to follow suit. The future of tropical marine ecosystems, and the millions of people whose lives are linked to them, depend on it.

-- Sean Connolly is a Senior Lecturer in the Department of Marine Biology, and a research scientist at the Center for Coral Reef Biodiversity, at James Cook University in Queensland, Australia. Dr. Connolly recently co-authored a review of the evidence on global warming and coral reefs for the journal Science.