The Great Barrier Reef is the world's largest reef system composed of over 2,900 individual reefs and 900 islands stretching over an area of 344,000 square kilometres. The reef is located in the Coral Sea, off the coast of Queensland in north-east Australia. Approximately two million people visit the Great Barrier Reef each year.
The Great Barrier Reef is the world's biggest single structure made by living organisms. This reef structure is composed of and built by billions of coral polyps. This reef supports a wide diversity of life, and was selected as a World Heritage Site in 1981. The Queensland National Trust named it a state icon of Queensland.
A large part of the reef is protected by the Great Barrier Reef Marine Park, which helps to limit the impact of human use, such as fishing and tourism. Other environmental pressures on the reef and its ecosystem include runoff, climate change accompanied by mass coral bleaching, and cyclic population outbreaks of the crown-of-thorns starfish. The crown-of-thorns starfish (see photo below)) preys on coral polyps. Large outbreaks of these starfish can devastate reefs. In 2000, an outbreak contributed to a loss of 66% of live coral cover on sampled reefs in a study by the RRC (Reefs Research Centre.) Outbreaks are believed to occur in natural cycles, worsened by poor water quality and overfishing of the starfish's predators.
The reef is a very popular destination for tourists, especially in the Whitsunday Islands and Cairns regions.
The Great Barrier Reef hosts 30 species of whales, dolphins, and porpoises, including the dwarf minke whale, Indo-Pacific humpback dolphin, and the humpback whale. Large populations of dugongs live there.
6 species of sea turtles come to the reef to breed – the green sea turtle, leatherback sea turtle, hawksbill turtle, loggerhead sea turtle, flatback turtle, and the olive ridley. The green sea turtles on the have two genetically distinct populations, one in the northern part of the reef and the other in the southern part. Fifteen species of seagrass in beds attract the dugongs and turtles, and provide fish habitat. The most common genera of seagrasses are Halophila and Halodule.
Saltwater crocodiles live in mangrove and salt marshes on the coast near the reef. The salt water crocodile population covers a wide-range but at a low density. Around 125 species of shark, stingray, skates or chimaera live within and around the reef. Close to 5,000 species of mollusc have been recorded on the reef, including the giant clam and various nudibranchs and cone snails. 49 species of pipefish and nine species of seahorse have been recorded. At least seven species of frog inhabit the islands.
215 species of birds (including 22 species of seabirds and 32 species of shorebirds) visit the reef or nest or roost on the islands, including the white-bellied sea eagle and roseate tern. Most nesting sites are on islands in the northern and southern regions of the Great Barrier Reef, over 1 million birds using the sites to breed. The islands of the Great Barrier Reef also support 2,195 known plant species; three of these are endemic. The northern islands have 300–350 plant species which tend to be woody, whereas the southern islands have 200 which tend to be herbaceous; the Whitsunday region is the most diverse, supporting 1,141 species. The plants are propagated by birds, wind and the ocean.