Threats to Coral Reefs

Despite their considerable importance to humans, many coral reefs around the world are in decline. Some scientists estimate that 58 percent of the world’s coral reefs are at risk from human damage. The most threatened reefs are those of Southeast Asia, where some 82 percent of reefs are at risk. About 61 percent of the Caribbean reefs are also under considerable threat. In many cases these reefs are not simply threatened by one thing, but by a combination of natural and human-caused stresses.

• Natural Threats to Coral Reefs

Reefs have always been subject to natural threats. Storms and hurricanes regularly sweep across some tropical areas, bringing massive waves capable of smashing corals even at depths of 10 m (33 ft) or more. Heavy rainfall can also harm corals by bringing fresh water and sediments onto the reefs.

Certain creatures eat coral. This is usually a natural process on a coral reef, but one creature, the crown-of-thorns starfish, sometimes reaches plague proportions and can destroy all living coral on a reef in a few weeks. These plagues may be natural, and reefs can certainly recover. However, many scientists believe that overfishing of the natural predators of these starfish may be worsening the problem. S tarfish populations may also be growing because they benefit from increased fresh water and sediments.

Crown-of-Thorns Starfish (Acanthaster planci)

Corals appear to be susceptible to a number of diseases. Huge declines in certain types of coral, particularly in the Caribbean, have been associated with particular coral diseases, notably white-band and black-band diseases. Very little is known about most coral disease, but fungi and bacteria may be responsible.

• Human Threats to Coral Reefs

The great bulk of human threats to coral reefs falls into four categories: pollution, sedimentation, overfishing, and global warming. Corals are susceptible to various forms of pollution, but probably the most common is simply the flow of nutrients from untreated sewage and from agricultural land, especially where there is heavy use of pesticides or fertilizer. These nutrients cause large increases in algae, both in the plankton and in algae growing on the seafloor, which then smother the corals or block out the light.

The amount of sediments reaching the sea has increased rapidly over the past hundred years or more. This can be linked to the loss of forests and the development of new agricultural techniques that lay the land open to rain and erosion. The sediments are swept into the oceans through rivers and streams and, like the algae, can smother and kill the coral.

Fishing has been going on around coral reefs for thousands of years. Problems arise when too many people try to catch fish from the reefs, or when, often out of desperation, they turn to destructive measures to capture fish. The rapid growth of human populations, combined with the development of increasingly efficient fishing methods, means that reefs in many areas are overfished. The reefs have fewer and smaller fish than in the past, a disaster both for the reef ecosystem and for the fishers. In a few places, notably in Southeast Asia, some fishers use explosives to capture fish. These explosives kill all fish within a certain area, as well as destroying the corals nearby. It takes many years for these damaged areas to recover.

Even reefs remote from humans have been affected by fishing as certain target species, such as large fish, giant clams, and sharks, have become so valuable that fishing vessels are willing to travel considerable distances to capture the remaining individuals. Today it is rare to find a reef with its full complement of these magnificent creatures.