Coral and Coral Reef

Coral reefs are sometimes disturbed by natural forces, such as extreme rain events that dilute seawater, waves associated with hurricane-force winds, volcanism, earthquakes, and thermal stress from unusually warm water (such as El Niño events). These natural conditions rarely destroy entire reefs, and the ecosystem can recover over time.

The crown-of-thorns starfish (Acanthaster planci) has caused severe damage to coral reefs in the Pacific Ocean and elsewhere. These large, bottom-dwelling invertebrates feed on the corals, destroying them in the process. This damage has been well documented on the Great Barrier Reef of Australia, almost one-quarter of which was destroyed by a crown-of-thorns infestation in the 1980s. When corals are destroyed by starfish, algae and bacteria grow over the surface and inhibit the establishment of new coral.

Another threat to coral reefs, observed in early 1980's in the Caribbean and the Florida Keys, was a blight that decimated the population of spiny sea urchins. These invertebrates are important because they feed on benthic algae, preventing them from overgrowing the corals.

A phenomenon known as coral bleaching is caused when coral polyps expel their zooxanthellae so that the coral becomes pale or white. Without their algae corals become weak, and after several weeks may die. For some years marine scientists have been observing major bleaching events in many parts of the world; the bleaching event of 1998 was the worst ever observed, affecting most of the world's coral reefs simultaneously. In some cases, severe damage has been caused to the coral-reef ecosystem (e.g., 80% coral dieoff). Scientists believe that unusually warm water temperatures are responsible for these catastrophic bleaching episodes. The cause (or causes) of these unusually warm temperatures is not certainly known, but a majority of scientists believe that global warming caused by human alteration of the atmosphere (especially increases of atmospheric carbon dioxide [CO₂]) are responsible. Atmospheric CO₂—expected to increase to twice its natural level (i.e., its level just prior to the Industrial Revolution) by 2065—is also expected to harm coral reefs by changing the chemistry of seawater in such a way as to make calcium carbonate less available to reef-building organisms.

Marine biologists also suspect that other devastating infectious coral diseases are also becoming more common. These diseases have names such as "black band," "white plague," and "white pox." They are capable of wiping out much of the coral in an afflicted reef.

Events such as crown-of-thorns population explosions, spiny sea urchin population collapses, and coral diseases can all be considered natural events. However, in many cases, marine scientists suspect that human influences, such as pollution, ozone depletion, or global warming may ultimately be to blame.