Coral reefs are complex living systems, supporting coral fishes and many other organisms. They have been in existence in their modern form for over 60 million years, but now they are threatened by human activities. Over 50 per cent of the world's coral reefs are at risk of serious, and possibly even irreparable, damage. Some threats come directly from overexploitation of the resources that reefs provide, food fishes in particular, or from unregulated tourism. Others result from industrial pollution, agricultural practices or the impact of building development at nearby coastal sites. More disturbing still is speculation about the likely consequences of global climate change, affecting reefs in terms of raising sea levels and temperature, as well as causing unpredictable and extreme fluctuations in weather patterns.
Coral fishes have long been an important source of food in developing countries, but fishing needs to be monitored and managed to avoid undue pressure on the resource. Coral reefs may be particularly vulnerable to intensive fishing because of the specialized lifestyles of many fishes there, and the enclosed nature of the ecosystem. Certain methods used on reefs, such as dynamiting and cyanide poisoning, also cause indiscriminate damage. Overfishing affects reefs in different ways. In extreme cases, as in some parts of the Philippines and Jamaica, it may cause wholesale decline and destruction, although it must be borne in mind that this is driven by the needs of local people to support themselves. Less severe fishing may upset the balance within reef-fish communities because it is selective. Popular food fishes are often the top predators, and their removal leads to a relative increase in the smaller fish species that they prey on; fishing also tends to deplete older, larger individuals. Alternatively some fisheries concentrate more on herbivorous or plankton-eating fishes than on the predatory species. The aquarium trade takes a heavy toll on some reefs and is also selective, concentrating on conspicuous and attractively coloured species, such as butterflyfishes.
A coral reef and its inhabitants flourish in stable conditions--corals are intolerant of environmental change. (Image: Linda Pitkin)
The health of a coral reef depends on the survival of its inhabitants but most critical of all are the corals themselves: damage to them leads to the decline of the habitat on which the entire reef community ultimately depends. Corals are very sensitive to changes in temperature, light and water quality, and thus have the potential to indicate global environmental change. Pollution by toxic chemicals, sewage and sediment from run-offs harm reefs close to land more than oceanic reefs, but no reef is entirely immune to the spread of pollutants.
Because of the complexity of environmental factors affecting reefs, long-term monitoring is necessary to identify the genuine changes superimposed on natural fluctuations. In the meantime, far more needs to be done to limit the damage from pollution and irresponsible land-use, and to ensure that fishing practices and tourism are managed so that they are sustainable, allowing the reef communities to replenish and repair themselves. The stewardship of coral reefs is the responsibility not just of the nations to which they `belong', but of all humanity. Nations worldwide must accept a duty to contribute and offer support according to their means if reefs and coral fishes are to have a healthy future.
Various natural disasters have affected coral reefs throughout their existence, but their resilience has enabled them to recover. Storms often damage shallow regions of the reef, but the dominant corals there normally re-grow quickly, in comparison with the slow-growing corals of deeper, more sheltered waters. However, recovery from storm damage and from sedimentation caused by run-offs is less rapid when reefs are under stress from several environmental factors at the same time. The increasing pressures resulting from human disregard for the environment may tip the balance unfavourably for coral reefs and their inhabitants. Certain phenomena might be symptomatic of this stress. Two in particular have attracted media attention in recent decades: coral destruction in the Indo-Pacific by the crown-of-thorns starfish Acanthaster planci, and coral bleaching. Widespread death of corals affects fishes that feed on them; those that depend entirely on corals suffer most, but those that have a broader diet manage better. For example, the guineafowl pufferfish Arothron meleagris has a preference for coral, but if there is a dearth it will eat coralline algae (a seaweed with a hard coral-like structure) instead.
Crown-of-thorns starfish Acanthaster planci. (Image: Linda Pitkin)
Outbreaks of the coral-eating crown-of-thorns starfish are natural, but human activity may indirectly lead to an increase in their incidence. Various hypotheses have been put forward that might account for this, at least in certain places. Localized death of corals, following the warming effect of El Niño, could reduce the feeding grounds of starfish in the area, leading them to congregate on the remaining healthy patches. Death of the starfish's predators, as a result of human-induced damage to the reef, could lead to a true increase in numbers. Another possible explanation is that some of the same environmental conditions that adversely affect the health of the reef, such as increased organic matter from river run-offs, could be beneficial to the larvae of the starfish, increasing their survival rate as they settle on the reef.
A small rise in temperature can have a devastating effect on corals, causing 'bleaching'. This is the expulsion by corals of their zooxanthellae, resulting in severe cases in the death of the colony. Shallow-growing, branching corals are most susceptible, and the skeleton left behind is white until the bare surface becomes masked by a growth of fine algae. The warm current El Niño is responsible for many bleaching events in the Pacific, but coral bleaching occurs in the tropical Atlantic too, beyond the reach of El Niño. Temperature increase is probably the main factor, but other stresses--such as reduced salinity or exposure at exceptionally low tides--may also cause bleaching. Extreme and widespread bleaching events have been known only in the last couple of decades, with two notable El Niño occurrences in 1982-3 and 1997-8.
Can a value be placed on coral reefs? A recent estimate tried to do just that, valuing the world's reefs at US$375,000,000,000 per year in terms of their potential to provide us with economic benefits. Whether this figure is meaningful or not is open to argument. What is certain, though, is that they are a resource we cannot afford to lose. Coral reefs are of importance for fisheries, tourism and coastline protection, but their intrinsic worth, as centres of biodiversity on a par with rainforests and as ecosystems forming an environmental link between land and ocean, is immeasurable.