The Great Barrier Reef

Introduction

Off the north-eastern coast of Australia is found the world's largest coral reef system, the Great Barrier Reef. This reef covers an area of 344 400 square kilometres. The Great Barrier Reef has over 1500 species of fish, 400 species of coral, 4000 species of mollusc, 800 species of echinoderm (animals such as sea anemones and starfish) and 240 species of birds. It is also an important breeding ground for animals like fish, turtles, whales and dugongs. See image 1

Coral reefs are found in tropical, shallow waters along migratory paths of birds and fish. They have all the conditions necessary to create a vibrant, diverse ecosystem. Many people have described coral reefs as the tropical rainforests of the sea. Judging by the complex connections between different organisms found in the coral reefs, it seems an appropriate comparison. Like a rainforest, coral reefs are very efficient at cycling nutrients between organisms within the ecosystem and are very biologically dense, filled to capacity with many different plants and animals.

The structure of the reef

The Great Barrier Reef stretches over 2300 kilometres. It is not one, cohesive reef but a series of reefs and islands. It contains about 2900 individual reefs, 618 continental islands and 300 small sand islands called coral cays. Of these 300 cays, about 230 have vegetation on them. See image 2

Life in the reef

Corals are created by tiny animals called polyps. Coral polyps are related to sea anemones and sea jellies, and they secrete a limestone skeleton. This skeleton provides a hard structure that polyps can live on and that protects them from some predators. Coral polyps live in communities that are commonly called coral, which is made up of the limestone skeletons of previous generations. Coral polyps live in symbiosis, or a close, mutually beneficial relationship, with a type of algae called zooxanthellae. These tiny plants provide food for the polyp, while the polyp provides a structure for the algae to live on. See image 4

The coral reef food web is very tightly woven. Crustaceans (which are the closest to decomposers in the coral reef) feed on the detritus (waste products) from plants and animals. These crustaceans are eaten by molluscs like octopuses as well as small fish. These animals are then in turn eaten by large fish, which are then eaten by larger fish and sharks. This is only a small example of the enormous food web that makes up the coral reef ecosystem. In the coral reef, producers include algae and sea plants. Primary consumers are mostly small fish, molluscs and coral polyps, which are eaten by secondary consumers like larger fish and sharks.

Animals in the coral reefs have become masters of camouflage. Predators and prey alike have adaptations that allow them to blend in with the sand, rock, coral and water. Fish have coloured scales that match their environments. Sharks are coloured in such a way that they blend in with the surface of the water when viewed from below. Some fish even camouflage themselves as larger organisms by swimming in schools, or large groups (also called shoals). From far away, some animals might consider them to be one larger animal. The fish have similar repeating patterns such as stripes, making the blending effect even more deceptive. See image 5

Damage to a single part of a coral reef can cause the entire ecosystem to be thrown off balance. If a population of producers or primary consumers is damaged, secondary consumers will have a hard time finding things to eat. If an animal that preys on corals polyps is damaged the corals and other similar animals, like worms and anemones, can overgrow, causing further problems like food shortages and sediment from detritus. Coral reefs have evolved to be very efficient at regulating themselves and any damage to them causes this self-regulation to become out of balance.