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Peter H. Raven


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Coral Reefs

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oral reefs are among the most ancient and biologically rich of the planet's ecosystems. Often called "the rainforests of the oceans", they first emerged more than 200 million years ago. A number of living coral reefs are believed to be more than 2 million years old, though most began growing within the last 10 000 years. Their rich fish stocks alone feed a billion people annually, but they now face the combined threats of local assault from destructive fishing methods and coastal development, and the global phenomenon of climate change.

The coral polyp is a tiny invertebrate creature related to the jellyfish. For most of its food and energy it depends on algae that live inside it, and when it dies its skeleton forms the calcium carbonate structure on which new coral grows. Over hundreds of years, this symbiotic relationship has created the vast coral reefs that could cover as many as 600 000 square kilometers of the world's oceans. Many of these are in warm tropical waters, but deep, cold waters have their own coral reef systems, such as those recently discovered in the North Atlantic. [Add]

The global area of the most biologically productive near-surface reefs has been estimated at 255 300 square kilometers [1]. These are among the most biologically diverse ecosystems on Earth, calculated to contain more than a million species. Around a quarter of all the world's sea fish feed, grow, spawn and hide from predators in their labyrinths. Hotspots for fish include most of the Philippines and much of Indonesia, as well as Tanzania and the Comoros in Africa, and the Lesser Antilles in the Caribbean [2]. [Add]

For millennia humans have taken fish from reefs without destroying them. But conventional nets get torn on reefs and more destructive fishing methods have become widespread. Some fishers dynamite reefs to capture fish; others use a cyanide solution to catch live fish for East Asian restaurants. This stuns the target fish, such as the large grouper, but also kills many of the surrounding invertebrates and smaller fish. In the past three decades an estimated million kilos of cyanide have been deposited onto the reefs of the Philippines alone.

Coral reefs face many other threats from human activity. They are dismembered by souvenir-seeking divers, mined for building materials and damaged by the anchors of cruise ships. Silt from dredging, deforestation and urban sewage smothers and kills coral, or feeds the growth of suffocating and sometimes toxic algae, which now cover almost all Jamaican reefs. [Add]

Attempts to identify the world's threatened coral reefs have found a strong correlation between risk of damage and coastal population density. Most species-rich coral reefs in Southeast Asia face the gravest threats from rising populations, growing reef tourism and rapidly expanding exports of reef fish. Where coastal populations are generally low, however, the risk of physical assault is also lower. Around 60 percent of reefs in the Pacific Ocean -- including Australia and atoll nations such as Kiribati and Tuvalu -- fall into this category [3].


[reefs, disease, and bleaching]


[area and threats / top countries]

But there are also remote threats. Dust storms from Africa, spread on the winds across the Atlantic, may have introduced bacterial infections from soils to Caribbean reefs [5]. On a global level, no reef can escape the threat posed by the build-up of greenhouse gases in the atmosphere. This works in three ways. Firstly, higher concentrations of carbon dioxide in the air make surface waters more acidic and reduce coral growth rates [6]. Secondly, a warming of the oceans could cause sea-level rise at a rate that coral reefs cannot match as they grow -- threatening the survival of atoll nations. Thirdly and most immediately, the rise of ocean temperatures by half a degree or more in recent decades has already placed many reefs at the top end of temperature ranges they can tolerate without undergoing "bleaching." [Add]


Bleaching occurs when high temperatures expel the algae in coral, removing their distinctive color -- hence the coral appears bleached. If bleaching persists and new algae do not appear, the coral will starve and die, and the reef will become brittle and break up.

As a result of an epidemic of bleaching in the 1990s, which peaked with the El Niño induced warming of 1998, more coral is believed to have died in the last few years of the 20th century than from all human causes to date [7]. A US State Department study in 1999 concluded that two thirds of all the world's coral reefs were deteriorating [8].

Until recently scientists believed that reefs in good general health and remote from human activity were not vulnerable to bleaching. But that view was thrown into question when one of the largest, most remote, pristine and biodiverse coral atolls, at the Chagos Islands in the Indian Ocean, was found extensively bleached [9]. Investigators found an area the size of New Jersey strewn with dead and broken coral. Most of the reef fish had disappeared. [Add]

Coral reefs are a major global biological and economic resource for both fisheries and tourism, and because they protect vulnerable coastlines from wave action and storms. Countries such as Barbados, the Maldives and the Seychelles rely on reef tourism for much of their foreign income. Florida's reefs attract annual tourism revenues of US$1.6 billion. One estimate puts the global annual value of coral reefs in fisheries, tourism and coastal protection at US$375 billion. That is US$60 for every member of the human race [10]. [Add]

Worldwide, there are more than 400 protected coral reefs. The overwhelming number are in Australia and Indonesia, the two countries with the most reefs overall. But most reserves are small and at least 40 countries with reef systems -- in both the industrialized and developing world -- lack any marine protected areas. Nonetheless, there are several examples of good management and planning. Bermuda, for example, closed its pot-fishing industry for the benefit of biodiversity and the lucrative reef-based tourism. The Philippines has organized locally managed marine reserves to protect reefs from cyanide fishing on Apo Island, and developed scuba-diving tourism. Australia's Great Barrier Reef World Heritage Site has imposed "no take" fishing zones as well as local bans on mining and tourism infrastructure [11]. [Add]

The disappearance of species

One of the many threats to coral reefs is the overfishing of target species. Entire populations can be eliminated by fishers who "clear" a reef before moving on to a new area. In the Philippines in the late 1980s the sea urchin Tripneustes gratilla, which had thrived throughout the 24-square-kilometer seagrass bed of a flat reef in Bolinao, became the target of traders from China. By 1995 the urchin was believed to have disappeared from the reef [4]. [Add]

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