Biodiversity series, Paper no. 2
Department of the Environment, Sport and Territories, 1994
6. Biodiversity of Australia's external territories
Australia's external territories are geographically diverse. Cold climate areas include Heard Island and the McDonald Islands near the Antarctic Convergence in the Southern Ocean and the Australian Antarctic Territory, which covers just under six million square kilometres, or about 42 per cent of Antarctica. Temperate and tropical habitats include: the Territories of Christmas Island; the Cocos (Keeling) Islands and Advance and Cartier Islands in the Indian Ocean; the Coral Sea Islands near the Great Barrier Reef; and the Territory of Norfolk Island in the Pacific Ocean.
Apart from the Australian Antarctic Territory, all of Australia's external territories are small, relatively isolated islands. As such, they have several distinguishing features. These include their limited space, restricted habitats, impoverished fauna and flora compared with continental areas, and often significant levels of endemism due to isolation. The biological significance of islands lies in the uniqueness of island biota, due to the presence of endemic, relict and/or specialised species, their value as refuges; and the ability of islands to act as reservoirs for the conservation of genetic material 142.
The subantarctic islands (including Macquarie Island which is governed by Tasmania), and the Australian Antarctic Territory share the additional feature of harsh physical conditions, and accordingly a relatively low diversity of terrestrial species. The value of these territories as breeding and feeding areas for seabirds and mammals, however, cannot be underestimated.
External territories – summary
Australia's external territories are geographically diverse and include the Australian Antarctic Territory and subantarctic islands; Christmas Island, the Cocos (Keeling) Islands, Advance and Cartier Islands in the Indian Ocean; the Coral Sea Islands near the Great Barrier Reef; and Norfolk Island in the Pacific Ocean.
The Australian Antarctic Territory and subantarctic islands have highly diverse marine communities that are unique to the areas (endemic), particularly in bottom-dwelling communities close to the shore. The islands also provide some important breeding sites for seabirds and seals.
The tropical islands also have their own endemic flora and fauna. The landbirds of Christmas Island have been isolated for so long almost all are recognised as endemic species or subspecies. Many of the tropical islands also provide nesting sites for seabirds. For example, North Keeling Island is so popular as a nesting site that it is considered vital to the stability of Indian Ocean bird populations. Christmas Island is the only known nesting site in the world for the rare and far-ranging Abbott's booby.
6.1.1 Rich and endemic plant and animal communities in Australian external territories
Marine biodiversity in the subantarctic islands and the Australian Antarctic Territory is highly significant for both species diversity and endemism, particularly in the near-shore bottom-dwelling communities 172. The Antarctic marine flora for example consists of more than 700 species of benthic macroalgae 65.
A number of Australia's island territories have a high proportion of endemic flora and fauna. For example, approximately 10 per cent of plants, 8 per cent of the land crabs, 50 per cent of the birds, and more than 50 per cent of the reptiles on Christmas Island are endemic 5 25. The land birds of Christmas Island have been isolated from other populations for so long that almost all are recognised as endemic species and subspecies. Endemic animals include an undescribed gecko Cyrtodactylus species; the metallically coloured skink Cryptoblepharus egeriae 25; the fruit bat Pteropus melanotus; the Christmas Island imperial pigeon Ducula whartoni; the Christmas Island emerald dove Chalcophaps indica natalis; and Abbott's booby, a bird which now nests only on Christmas Island 44. Christmas Island is also the home of the golden bosun-bird (Phaethon lepturus fulvus) which is recognised as a distinct subspecies of the whitetailed tropicbird. P. lepturus is found in tropical waters around the world, but only possesses a golden sheen on Christmas Island 44. Four species of orchid are endemic to Christmas island, of which one may be extinct 44.
Norfolk Island has 51 endemic plant species or subspecies recorded (of which 44 per cent are rare or extinct due to clearing and logging), and 58 per cent of the bird species are endemic 77.
While low endemism is to be expected on the purely coralline oceanic island group of the Cocos (Keeling) Islands which provide little variation in terrestrial habitats, of special interest is the problematic grass genus Lepturopetium. The genus was previously known only from the type collections of its two named species – L. kuniense from Isle of Pines, New Caledonia, and L. marshallense from Eniwetok Atoll, Marshall Is. The recent collection (1986) from West Island in the Cocos (Keeling) Islands remains specifically unplaced 153.
As mentioned in the previous section, nearshore benthic endemism in the Antarctic and subantarctic regions is very high. Seventy per cent of the 55 species of Rhodophyta found in the subantarctic for example, are endemic to that region 65 and faunal endemism ranges from 57 per cent in the Polychaeta to 90 per cent in the Amphipoda and 95 per cent of fish 172. While terrestrial endemism is generally low, Heard Island is the home of an endemic cormorant, Phalacrocorax nivalis. This species is considered vulnerable as it has a population of less than one hundred breeding pairs, and only two known breeding sites 36 97.
6.1.2 Unusual biota of Christmas Island
Christmas Island has many unusual and outstanding biological features. Land crabs are a dominant component of the island's fauna. The endemic red crab (Gecarcoidea natalis) is the most abundant crab on the island, dominating the forest floor and influencing the development of the unique structural characteristics and species composition of the island vegetation. The sparse forest floor vegetation with areas almost clear of herbs, shrubs and ferns appears to be at least partly due to the effects of the red crab.
Although the island is not rich in plant species, the structural simplicity of the taller rainforest is unusual 66 60. Several of the dominant tree species of the rainforest appear 'out of place'. Syzygium nervosum, for example, appears to attain a much larger size on Christmas Island than in other areas and unusually large specimens of Terminalia catappa also occur. It is also unusual for T. catappa to occur naturally above the shoreline 46 45.
Hosnie's Spring, Christmas Island, supports a forest of mangrove trees that are amongst the largest of their kind ever recorded. The mangroves grow some 120 metres inland of the seaward cliff, and approximately 24-37 metres above sea-level on an inclined surface, also unusual for a mangrove forest. The two dominant species, Bruguiera gymnorhiza and B. sexangula, are normally confined to coastal mangrove swamps and attain a height of 40 metres at Hosnie's Spring, a size that is unusual for both species 44. Conditions favourable for mangrove establishment do not appear to have existed since the last interglacial period and the stand is thought to have persisted in its present site for up to 120 000 years 6.
6.1.3 The refuge value of Australia's external territories
Many of the external territories provide extremely valuable bird nesting sites. North Keeling Island, for example, probably attracts the greatest variety of seabirds of any island in the Indian Ocean and is thus considered to be vital to the stability and cohesion of the Indian Ocean bird populations. Red-footed boobies (Sula sula rubripes), white terns (Gygis alba) and frigate birds (Fregata species), for example, breed on North Keeling Island in large numbers. In fact, North Keeling is only exceeded in significance for seabirds by Aldabra and Christmas Islands, both of which, unlike North Keeling Island, have suffered human settlement impacts 78.
Ashmore Reef and Cartier Island are also important breeding colonies for a diversity of ocean-going seabirds such as the brown booby (Sula leucogaster), common noddy (Anous stolidus) and lesser frigate bird (Fregata ariel). Large flocks of migratory waders, including whimbrels (Numenius phaeopus) and ruddy turnstones (Arenaria interpres), have also been recorded on reefs adjacent to the islands 78.
Several of Australia's external island territories provide nesting sites for vulnerable or endangered species. Christmas Island is the only known nesting habitat in the world for Abbott's booby, which, although a far ranging species, is also extremely rare 60. Heard Island provides an important and undisturbed habitat for a number of burrowing petrel species, and is particularly significant because of the absence of cats, rats and rabbits.
Australia's subantarctic islands are of great biological importance as they are the only breeding sites for seabirds and seals in a vast oceanic expanse. This is reflected in the enormous breeding populations found on the islands. The colonies of the macaroni penguin (Eudyptes chrysolophus) on Heard Island and the McDonald Islands represent some of the greatest concentrations of seabirds in the world with some 2 million breeding pairs. Other avian species with large populations on these islands include the gentoo penguin (Pygoscelis papua), the southern rockhopper penguin (Eudyptes chrysocome), the Antarctic prion (Pachyptila desolata) and the South Georgia diving petrel (Pelecanoides georgicus). A 1988 census also showed a fur seal (Arctocephalus gazella) population of more than 15 000 animals 36. The Heard Island and McDonald Islands group is the only major subantarctic island group whose terrestrial ecology has not suffered irreversible impacts by human-introduced biota, and as such, provides a valuable benchmark in the region 36. South of the subantarctic islands, Antarctica itself also includes significant breeding and feeding grounds for birds, seals and whales.