Condition

The Great Barrier Reef is part of a global centre of coral diversity located in the Indo-Pacific and includes more than 70 hard coral (Scleractinia) genera. Coral reefs are a significant component of the Great Barrier Reef World Heritage Area with over 2900 individual coral reefs stretching for more than 2300 kilometers. Some 350 coral species inhabit the Great Barrier Reef compared with the global maximum of about 450 species found in Indonesian and Philippine waters. Most of the hard coral species on the Great Barrier Reef are also found on coral reefs elsewhere in the Indo-Pacific, but 10 species are considered endemic, being found only on the Great Barrier Reef. Soft corals (Alcyonaria – octocorallia) are also an important component of many reefs, however their taxonomy is not well documented and less is known about their ecology and condition.

Condition: Hard Corals

Hundreds of hard coral species inhabit the Great Barrier Reef, but reef communities vary from one place to another

The cross-shelf pattern is correlated with an increase in wave exposure and light availability from inshore to offshore reefs. In terms of species diversity, the inner-most mainland fringing reefs or platform reefs within a few kilometres of the coast have the lowest diversity (100-150 species), but this rises rapidly away from the coast so that fringing reefs around high island groups such as the Palm Islands have over 300 species. Platform reefs further offshore in mid- and outer-shelf areas have high cover but somewhat lower species counts. Coral cover is extremely variable between reefs but surprisingly, highest cover is often found on pristine nearshore reefs.

North-south variations in hard corals also exist but are less conspicuous than the cross-shelf differences. In particular, species diversity tends to decrease from north to south along the eastern Australian coast, with up to 343 species found in the northern and central Great Barrier Reef, but only 244 recorded in the reefs of the Capricorn-Bunker group in the south.

Each blue line represents coral cover for one reef between 1993 and 2002

Surveys conducted by the Australian Institute of Marine Science (below) do not show any single, reef wide trend in hard coral cover through time, however this is not unexpected given the high natural variability of coral reefs (see “Variation on coral reefs”). Between 1993 and 2002, coral cover on offshore reefs of the Capricorn Bunker group in the southern Great Barrier Reef has continued to increase from very low levels caused by storm damage in 1988. Similarly, hard coral cover in both inshore and offshore reefs of the Cooktown/Lizard Island region in the northern Great Barrier Reef has also increased from low levels caused by severe storms a decade ago. Since 1995, coral cover on inshore reefs of the Cairns and Townsville sections, and mid-shelf reefs of the Townsville and Whitsunday regions have declined due to several major disturbance events including the reef wide coral bleaching event of 1998, Cyclone Justin in 1997, and outbreaks of crown-of-thorns starfish since the late 1990’s.

Condition: Soft Corals

Soft corals are commonly found throughout the Great Barrier Reef, however they have received relatively little scientific attention compared to hard corals. Due to the difficulties in identifying soft corals to species level, species distributions are currently not available and soft coral diversity is discussed here in terms of genera.

Although they are usually less abundant than hard corals, soft corals are important and visually spectacular members of the reef community

Surveys conducted between 1996 and 1999 by AIMS scientists have increased our understanding of soft coral abundance, diversity and distribution in the Great Barrier Reef. During these surveys, the spatial distribution of 40 genera was estimated and mapped but a further 21 rare genera were not included in the analysis as the data were incomplete. Generally, soft coral diversity is highest in the mid-shelf regions of the Great Barrier Reef with fewer genera recorded in inshore and offshore regions. The greatest diversity of soft corals is found between 11⁰S and 13⁰S latitude and like hard corals, fewer genera are present in southern regions of the Great Barrier Reef.

Soft coral cover is usually less than 10% of the reefal benthos but can vary considerably between reefs. In shallow water less than five meters deep, soft coral cover tends to be higher on inshore reefs than on mid-shelf and outer reefs. However, this trend is less pronounced at deeper depths where similar levels of soft coral cover are found across inshore, mid-shelf and offshore reefs. In some circumstances, soft corals may form the dominant benthic group and some coastal reefs are inhabited by soft coral assemblages that cover up to 70% of the available space. It has been suggested that after episodic disturbance events such as cyclones, soft corals may out-compete hard corals, replacing them as the dominant benthic group. However, this has not been observed on the Great Barrier Reef where soft coral cover on reefs has remained remarkably consistent over time. Surveys have demonstrated that there was no shift from hard coral assemblages to soft coral assemblages up to ten years after removal of hard corals by crown-of-thorns starfish. Instead, the disturbed areas remained dominated by turf algae until fast growing hard corals re-colonised these sites.

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