ALMOST 200 000 species that live on coral reefs could die out in the next 40 years, according to a zoologist from the University of Maryland. Reefs are shrinking faster than ever as development accelerates around tropical coastlines.
Marjorie Reaka-Kudla calculates that there are probably around 950 000 species on the world鈥檚 reefs, and concludes that in 40 years鈥� time, when a predicted 70 per cent of reefs will have been lost, 175 796 species will have gone with them. 鈥淭hese are the first estimates and are based on the best information we have,鈥� she told the AAAS last week.
Reaka-Kudla borrowed equations from biogeographers who have worked out the relationship between the area of an island and the number of species it is likely to support. She knew that reefs cover 0.1 per cent of the planet and how many species are known to live in the oceans, as well as what proportion are likely to live in the tropics, and inhabit reefs. From this she calculates that the world鈥檚 reefs contain 93 000 known species. These probably represent less than 10 per cent of the true number that live there, Reaka-Kudla estimates.
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Ten per cent of reefs have already been lost. If the destruction continues, another 60 per cent will disappear in 20 to 40 years. Reaka-Kudla estimates that 2400 known species have already died out. In 40 years鈥� time another 17 203 will have joined them, she says. Extrapolating from her figures to include the species that have yet to be discovered gives her the total of 175 796.
Although the most obvious species on reefs are the corals themselves and the multitude of brightly coloured fish, most reef-dwellers are tiny worms, sponges and other well-hidden animals that live in holes and crevices created by burrowing animals such as molluscs and sea urchins.
Most of the organisms that die out will be small species such as these. In general, says Reaka-Kudla, the smaller the organism, the fewer eggs it lays and the shorter the distance its larvae travel before settling on the reef. 鈥淓veryone assumes that small species have mobile larvae that are spread widely but this is not so,鈥� she says. 鈥淎s a result small species are especially in danger of extinction.鈥�
The main threats to reefs are overfishing, being buried under a blanket of sediment washed from coastal lands, and an overdose of nutrients from agricultural fertiliser and sewage. Deforestation of coastal hillsides and road cutting leads to a massive influx of sediment into coastal waters which smothers the coral. 鈥淪ediment is the number one enemy of coral reefs,鈥� says Christopher D鈥橢lia, a colleague of Reaka-Kudla鈥檚 at the University of Maryland. Added nutrients cause fleshy algae to grow over the corals and trigger blooms of plankton that blot out the corals鈥� light. They also upset the balance between the coral polyps and the symbiotic algae that live in their tissues, says D鈥橢lia.
Although human development in Asia and Oceania poses the biggest threat, global warming and the extra ultraviolet radiation that reaches the ground through the thinning ozone layer may be an added stress. Already there has been an increase in outbreaks of 鈥渃oral bleaching鈥�, in which a coral鈥檚 algae desert their host when the sea temperature rises about a degree above its usual summer value.
鈥淭hese estimates are the first to give some sort of prediction of how much we might lose very soon. They clearly place us within the range of the mass extinctions seen in the fossil record,鈥� says Reaka-Kudla.