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The Great Barrier Reef is the world’s largest structure built from organisms, spanning 2,600 km and made up of nearly 3,000 individual coral reefs. It is home to numerous species and is under threat from climate change, tourism, water pollution, and overfishing. Preserving it will require careful management and conservation.
The Great Barrier Reef is the largest barrier reef system in the world, spanning 2,600 km (1,600 mi) over an area of approximately 344,400 square km (133,000 sq mi). Made up of nearly 3,000 individual coral reefs, the Great Barrier Reef is sometimes called the world’s largest organism, but is more accurately the world’s largest structure built from organisms. The reef is made up of 900 islands, created when sand piles up on top of the coral just below the surface. The reef is located just off the northeastern coast of Australia and can easily be seen from the air. Much of it is protected as part of the Great Barrier Reef Marine Park and has been named a state icon for Queensland, the adjacent Australian state.
This reef was created over thousands of years by coral polyps, stationary organisms similar to sea anemones that leave behind a hard calcium carbonate skeleton when they die. New polyps build on the old ones, creating an endless cycle of expansion. Reproducing by budding, large colonies of related coral polyps are often genetically identical, being considered a single organism. Corals occasionally change their genetics through sexual reproduction with other corals of the same species by releasing sperm into the water. Colony expansion is kept in check by predators, food restrictions, and severe storms, which can break off large chunks of the reef if they are poorly attacked.
The growth of each coral polyp is relatively slow: a single polyp can increase in diameter by 1 to 3 cm (0.39 to 1.2 inches) per year, while growing vertically by 1 to 25 cm (0.4 to 12 inches) per year. year. The Great Barrier Reef, along with all other coral reefs, grows on a stable area of continental shelf dotted with small underwater hills. These keep the coral elevated and give it a foundation. There is evidence that some of the skeletal material in the present reef dates back 600,000 years, although the present living structure of the reef is thought to be 6,000 to 8,000 years old, making it one of the oldest organisms in the world if considered as a whole.
Corals require tropical levels of heat to grow. For this reason, coral growth is not thought to have begun around the area until about 25 million years ago, when Australia moved into tropical waters. Since then, the growth has been punctuated by changes in ambient temperature and sea level. During the Last Glacial Maximum, when sea levels were 200-400 feet lower than today, many parts of the site of the present Great Barrier Reef would have been above water or very shallow. Unlike other reef systems, the Great Barrier Reef lacks atolls, making it difficult to study. Although the structure has been known to Aboriginal Australians and Torres Strait Islanders for tens of thousands of years, it was not until 1770 that the reef was discovered by James Cook, whose ship suffered extensive damage when it ran aground on the reef.
In places, the reef extends from the surface down to 150 m (490 ft) deep, limited only by the lack of sunlight. Coral polyps are cnidarians, like jellyfish. They use stinging cells, called nematocysts, to capture prey, ranging from plankton to small fish, and to defend themselves against predators such as starfish. These nematocysts consist of a sharp venomous tip that can shoot at a target in as little as 600 nanoseconds, achieving an acceleration of about five million G.
Corals, such as those that make up the Great Barrier Reef, have a close symbiotic relationship with algae of the genus Symbiodinium. These algae live directly on the surface of the coral polyps, obtaining security in exchange for the food they provide to the coral through photosynthesis. Many corals get their beautiful, distinct color from the algae that inhabit them. Occasionally, due to algae stress, corals expel their symbiotic partner, causing a temporary loss of coloration. When this process occurs on a large scale, it is referred to as coral bleaching, due to the accompanying loss of color. Mass coral bleaching occurred in the summers of 1998, 2002 and 2006, due to the increase in water temperatures due to global warming.
Numerous species make their home on the reef, including thirty species of whales, dolphins and porpoises, six species of sea turtles, 125 species of sharks, rays, skates or chimaeras, 215 species of birds, 17 species of sea snakes, 1,500 fish species and more than 400 coral species. Some of these species are endemic to the Great Barrier Reef and are found nowhere else. The great biodiversity present on the coral reef has made parts of it frequented by diving spots, visited by the greatest underwater photographers in the world. Divers sometimes wear special protective suits to protect them from dangerous jellyfish in the area.
The Great Barrier Reef has been called one of the seven natural wonders of the world. It is an ecosystem in its own right, a biologically active place where thousands of plant and animal species coexist, fluttering in and out of the complex structures created by the coral polyps. It’s hard to think of an underwater ecosystem as diverse in its species or as stunning in appearance as the Great Barrier Reef. Coral reefs have existed on and off in subtropical and tropical waters for hundreds of millions of years.
Currently, the reef is under threat from climate change, tourism, water pollution and overfishing of keystone species such as the giant clam Triton. Warmer temperatures disrupt the precious balance between corals and their symbiotic algae, causing mass bleaching events that are far more frequent than would otherwise occur. Agricultural fertilizer runoff from Australian farms causes algal blooms, which absorb nutrients and leave little residue for living coral reefs and the animals that inhabit them. Overfishing of keystone species causes coral predators like the crown-of-thorns starfish to reproduce too quickly, so they continue to eat far more living coral than they might have in decades past. Preserving the Great Barrier Reef for the Earth and future generations will require careful management and conservation of the environment.
