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Grief on the coral reef

POSTED 21 SEPTEMBER 2006

Death on the reef
All Australia is grieving for Steve Irwin, Crocodile Hunter of TV fame, who was killed Sept. 4, when a stingray's barb pierced his heart on Australia's Great Barrier Reef. The photogenic, enthusiastic reptile-wrestler, died on the reef he loved at age 44.

In its obituary, The New York Times wrote that Irwin's "fearless exploits ... included leaping on the backs of crocodiles, wrestling with boas and mastering poisonous snakes and spiders." Ironically, Irwin was not putting a hammer lock on the deadly stingray when he died.

Irwin, the conservationist who popularized "Crikey, mate!" simultaneously downplayed and highlighted the fearsome claws, Dead branches of coral, overrun with pod-like growths. fangs and toxins of the Australian outback. Irwin was wildlife's opponent in "roll camera!" wrestling matches, but wildlife's advocate in dramatizing the perilous condition of wildlife in the modern world.

Suitably, he died on a biological wonder that is itself threatened with death: the coral reef.

Found in Australia's Great Barrier Reef, these dead branching corals are now home to colonial ascidians (those little sphere shapes you see). Photo: Australian Institute of Marine Science

Coral reefs are stony structures built by a symbiotic relationship between coral, an animal, and single-celled algae -- a plant. The coral houses the algae, which use photosynthesis to make sugar that helps feed the coral. The coral's waste products, including nitrogen and phosphorus compounds, fertilize the algae. Live coral polyps grow on the dead bodies of their ancestors, allowing coral to build up to tremendous depths if left alone.

Coral reefs are a win-win biological mutualism of the tropical and subtropical oceans -- you scratch my back; I scratch yours. But reefs also protect shorelines by blocking waves. They supply building material and food, and are magnets for tourist boats and bucks.

Reefs are also home to maximum biodiversity. About 25 percent of marine organisms rely on coral reefs for some stage of their lives. Significantly, 32 of the 34 animal phyla live on reefs, compared to "just" nine phyla in tropical rain forests. (A phylum is a large group of organisms sharing similar structure -- chordata, for example, contains all animals with backbones).

Yellow tang swims past finger coral in Hawaii.
Coral reefs are considered the rain forests of the oceans: with a dramatic level of biodiversity. A yellow tang swims past finger coral in Hawaii. ©Courtesy L.G. Medaris, Jr.

But coral reefs are under threat from forces that can seem difficult or impossible to control. For example:

little fishOverfertilization: Nitrogen and phosphorus from agriculture and sewage stimulate excess growth in the coral's algae, changing its metabolism so it makes more protein and less of the sugar that coral craves.

little fishOverfishing: If plant-eating fish are fished out, seaweed can mushroom, shading the reef and slowing growth of the symbiotic algae.

little fishDisease: A range of bacteria, fungi and probably viruses are injuring, or sometimes killing, the coral animals.

little fishExotic species: The huge crown-of-thorns starfish, for example, is a big problem on the Great Barrier Reef.

little fishMining: People remove coral for use in aquariums or as building material. Divers and their boats can also destroy the very coral that attracts them to the reef in the first place.

little fishGlobal warming: When seawater warms, coral expels the symbiotic algae, losing its major source of food. Because the algae lends the coral its color, this causes coral "bleaching." If the algae do not revive or return, bleached coral can die in weeks or months, and an entire reef can wither away. As this large coral reef dies, a photographer documents its decline

little fishOverpopulation and coastal development: As more people move to the coasts, they bring more sewage, more lawn fertilizer and more destruction.

The Carysfort Reef off the Florida coast has declined dramatically in the past 25 years. The coral that was healthy in 1975, was visibly sick by 1985, and was dead and broken by 1995. Photos: NOAA and Phillip Dustan, College of Charleston

Just as fish, coral, algae and hundreds of other species interact in building a reef, the destructive forces also interact in wrecking a reef. Global warming, for example, is adding carbon dioxide to the atmosphere, which warms the seas, and also raises ocean acidity. And coral grows slower in acid water.

Excessive water temperature causes coral to expel its symbiotic algae. This causes bleaching, which can kill the coral if it goes on for too long. Bleaching is the major direct impact of global warming on coral reefs. Photo: OAR/National Undersea Research Program (NURP)
Small yellow fish swim by bright white fan-shaped coral in turquoise water

Oldies, but goodies
Coral reefs have been around for hundreds of millions of years, which proves that they can adapt to changing environments. In some reefs, you can drill down hundreds of meters and still hit limestone that was secreted thousands or millions of years ago by coral polyps and their algal co-conspirators. Many corals live close to the surface, where big storms can knock them over, but they have obviously "learned" to survive this annoyance.

Nonetheless, reefs are under threat around the world, especially in the heavily populated Caribbean Sea. "Looe Key, in the Florida Keys, was considered the number-one scuba-diving destination," says Brian Lapointe, a marine ecologist at Harbor Branch Oceanographic Institution in Fort Pierce, Fla. "It was magnificent, with a great number of fish and coral species ... and it has lost more than 95 percent of the chief reef-forming coral, the elkhorn."

A yellow-hued coral branches on the sandy floor of the deep blue ocean
Drastic coral declines in the last 25 years have put two species, staghorn and elkhorn corals, on the U.S. Endangered Species Act, in the endangered category. Photo: Earthwatch Institute

Typically, live coral grow on top of the skeletons of their predecessors, but only 10 percent of the area of Caribbean reefs is now sheathed in live coral, compared to 50 percent just 30 years ago (see p. 27, "Coral Reefs and Global Climate Change..." in the bibliography).

Similar problems affect reefs almost everywhere, and a good number of experts blame climate change. "The decline of elkhorn and staghorn corals is likely the symptom of a problem that is impacting coral reefs around the world -- that problem is global warming," said John Rollino, principal investigator of Earthwatch's Bahamian Reef Survey project, in a press release.

Reefs prove the ecological maxim: "everything is related," and in studying reef decline, it is tough to isolate causes. As a 2004 report concluded, "Climate and localized non-climate stresses interact, often synergistically, to affect the health and sustainability of coral reef ecosystems."

Blue swirl around green landmasses indicating worldwide distribution of coral reefs
General worldwide distribution of coral reefs, with contours indicating the number of genera (clans of related species). The area inside the 50 genera contour is essentially the high-diversity Òcoral triangleÓ Ð the Southeast Asian center of coral diversity. The blue area outside of the controu lines represents regeions with at least 10 but fewer than 25 genera. Graphic: Pew Climate

What is destroying the coral reefs?

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Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

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