2.4 Biogeographical Processes
Biodiversity
The Great Barrier Reef is very biodiverse. It includes:
- approximately 2000 fish species
- 500 coral species
- approximately 300 hard coral species>
- approximately 4000 molluscs
- more than 400 sponge species
- twenty-three species of marine mammals
- eleven species of sea snakes
- six species of turtles
- 215 species of birds
- many tropical and migrating whale species.
Many of the marine species are endemic. Examples are that the ecosystem provides the nesting grounds for endangered green and loggerhead turtles. Raine Island has the largest nesting populations of green turtles in the world and it is Australia's most significant seabird rookery.
The GBR ecosystem has the richest faunal biodiversity on the planet. By contrast, coral reefs in New Caledonia contain 1000 fish species and 300 coral species. Whereas The GBR contains a significant proportion of the global marine biodiversity.
The GBR ecosystem has examples of every environmental relationship-symbiosis, parasitism, predation and competition. These create the conditions that are necessary for evolution to occur. While there are some common species that are found in reefs around the world, there is a high degree of specialisation and endemic species. There are still many species yet to be discovered, which is why some of the numbers of species listed above are approximate.
The recent discovery of deepwater Halimeda beds or `meadows' in the Great Barrier Reef World Heritage Area illustrates that our knowledge of the ecosystem is far from complete. Halimeda (marine plants similar to seagrass) beds are found on the outer continental shelf in deep water outside the GBR. They are believed to support many organisms, including undiscovered species.
For years, commercial trawler workers have known about wonky holes-upwellings of freshwater far out to sea. In 2002, these unique features received serious scientific study, which revealed that they may play a significant role in the enrichment of nutrients in this relatively low-nutrient ecosystem. In addition, the wonky holes have unique biodiversity that is yet to be fully studied.
Succession
- Succession is the changes in a plant community over time until it reaches a state of balance with its environment.
- Organisms invade, colonise, grow, reproduce, and die on surfaces that were blank (primary succession) or replace plants and animals fromareas that have been disrupted (secondary succession). The succession of plants and animals continues until stability is reached.
- plant succession occurs on the many exposed sand bars on the reef which form the many islands along it.
- Coral reefs are build up of old coral on top of each other. A succession process that allows this build up is coral spawning.
- Coral Spawning Video
Coral
Spawning
Corals are able to colonise new environments through sexual reproduction. Divers that have experienced the synchronised spawning of coral describe it as 'an upside‑down snowstorm.' As the waters warm in spring, eggs and sperm develop rapidly. In some corals, all polyps are either male or female but more often coral colonies are hermaphrodites and contain both male and female gonads. Synchronicity may ensure the maximum survival of coral offspring because predators are overwhelmed by the sheer overabundance of food. It is essential that a sedentary (stay in one spot) animal such as coral is able to populate effectively. Corals are also able to expand their hold over existing habitats by asexual reproduction with polyps being replicated many times over.
Invasion
Crown-of-thorns Starfish Infestation
One of the most commonly known sea stars that has invaded the Great Barrier Reef is the crown-of-thorns starfish.
- This is one of a few animals that feed on living coral tissue. It gets its name from the dense covering of long sharp spines covering its upper surface.
- The crown-of-thorns is a predator; that is,it eats other living animals.
- After finding suitable coral, the crown of thorns pulls its stomach out through its mouth (a process known as stomach eversion) over the coral polyps and releases digestive juices onto the coral, breaking down the polyp tissue into a readily absorbed ‘polyp soup’.
- It leaves only a white coral skeleton which is soon invaded by algae, worms, boring molluscs or reef settling organisms.
- Researches have shown that chemicals released during the digestion of corals can actually attract other starfish to a feeding site. As the number of starfish feeding in an area increases, the ‘smell’ of digested coral may increase and so recruit still more starfish into the feeding group.
- The crown of thorns phenomenon is not unique to the Great Barrier Reef.In recent years crown of thorns aggregations have caused large scale coral destruction in other areas of the Pacific.