Storms, predators and global climate change
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Reefs are frequently disturbed by natural events. Hurricanes and tropical
storms can reduce reefs to rubble. Corals are often affected by diseases,
such as black-band disease, which spreads over colonies, progressively
killing the polyps. The crown-of-thorns starfish feeds voraciously on
corals and, if present in large numbers, can reduce a reef to a mass of
dead coral skeletons in just a few weeks.
Global climate change, or the enhanced greenhouse effect, may cause increases in sea temperature and sea level, as well as changes in ocean current patterns, that could damage coral reefs. For example, corals are very sensitive to changes in temperature. Sea water that becomes too warm causes corals to turn white, or bleach, a reaction that occurs if coral polyps are stressed. Often they recover, but they are also known to die. If the oceans warm up as a result of global climate change, corals may have increasing difficulty recovering from bleaching episodes.
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Changes in land use, such as forest clearance and intensification of
agriculture, cause increased erosion. Soil is washed into rivers and out
to sea, silting up reefs along the coast. The water becomes murky and
stops sunlight, vital for coral growth, reaching the reef. If sediment
settles on top of corals, it may smother them. This can also happen during
the dredging of channels and harbours.
Sewage and agricultural fertilisers, both of which often reach the sea, can damage reefs as well. They increase nutrient levels in sea water, encouraging seaweeds to grow at the expense of corals. Many other wastes from factories, towns and ships are also discharged at sea where they may poison corals and other reef life. Numerous activities, taking place even far inland or on the high seas, may have a deleterious impact on reefs. Managing reefs thus means improving forestry and agriculture methods and disposing of waste and effluent in better ways.
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Coral reefs are being over-exploited on a global basis. It is increasingly
difficult to make a living from fishing, as first the big, commercially
valuable species such as groupers disappear, and then the smaller ones.
Often damaging harvesting methods such as poisons and dynamite are used.
Over-fishing tends to be followed by a change in the ecological balance
of the reef. For example, it becomes overgrown with algae if grazing species
are removed.
Establishing no-fishing areas is one approach to managing tropical fisheries. These not only protect reef species and their habitats, but, by protecting breeding populations, they may improve catches for local fishing communities in surrounding areas.
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Corals and shells are collected all over the world to sell as souvenirs
or to make into jewellery and other handicrafts. Several kinds of mollusc
are now rare on reefs because of over-collecting. In many countries, coral
is also mined from the reef and used to build houses and make roads, or
is burnt to make lime. Where this happens, reefs are often stripped bare.
Sustainable management of reefs will mean regulation of these industries
and better enforcement of existing legislation to ensure that both the
resources and the people who depend on them have a long-term future.
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Millions of tourists visit coral reefs each year and, in many countries,
reef-related tourism is now the main source of foreign exchange. But corals
are easily broken by trampling when people walk out to the reef; snorkellers
and divers may kill polyps simply by touching coral colonies; and anchors
and ship groundings can destroy large areas of reef.
Though tourism can damage reefs, carefully managed, it can play a major role in saving them. The tourism industry can promote and assist in the development of marine parks, mooring buoy programmes to prevent anchor damage, public awareness campaigns, and local initiatives to promote sustainable coastal management.
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