The Antarctic environment comprises diverse habitats and ecosystems that include ice-covered areas; ice-free vegetated areas; ice-free rocks; saltwater and freshwater lakes and streams; and the intertidal areas, mid-water, deepwater and benthic regions (the benthic zone is the ecological region at the lowest level of an ocean or lake, including the sediment surface and some subsurface layers) of the Southern Ocean. In the terrestrial environment on the continent, species diversity is low compared with mid-latitudinal or tropical ecosystems; however, many species are very abundant. Species that have made the Antarctic continent their home have evolved over very long timescales so that they are now highly specialised and able to survive in the extreme conditions of the southern continent and the frigid ocean surrounding the continent. Only a few species of terrestrial invertebrates occur and flowering plants are limited in their distribution to small areas at the Antarctic Peninsula. There are no flowering plants in East Antarctica, and lower plants such as mosses, lichens and bryophytes live in the few ice-free areas; algae prosper not only in the marine environment but also in snow fields.
The species composition on the subantarctic islands is quite different from that found on or near the continent. The vegetation of Heard Island, for example, covers ice-free areas of the island and includes a variety of vascular plants (12 species), mosses (44 species), lichens (34 species) and liverworts (17 species).17-18 Macquarie Island supports 45 species of vascular plants and 91 species of moss, as well as a large number of lichens and liverworts.19
Land-breeding vertebrates are represented by only a few species. However, they tend to occur in large numbers on both the continent and on the subantarctic islands. The most diverse vertebrate groups comprise flying seabirds (seven species) and penguins (two species on the continent and five on subantarctic islands); four species of ice-breeding seals, fur seals and elephant seals are also part of the Antarctic fauna.
The abundance of terrestrial invertebrates varies regionally and depends upon the conditions of the local microhabitats and particularly the topography and vegetation.20 Many invertebrates live under rocks or in the relatively moist moss beds, Antarctica's 'forests', where moisture is available.21 The species diversity is low; the most abundant phyla are rotifers (wheel animals), nematodes (worms) and tardigrades (water bears) but mites and springtails are also found.20,22 The terrestrial species diversity of the region pales in comparison with the marine species. Invertebrate taxa living at the continental shelf (0-1000 metres) and in the deep ocean (> 1000 metres) encompass more than 3500 species.23 Creatures such as seaspiders, sea urchins, marine worms, molluscs and sponges are highly diverse with a large percentage of endemic species (i.e. species unique to the region).23-25 The list of species is expected to grow with the publication of a large-scale, international survey of the Southern Ocean - the Census of Antarctic Marine Life of 2007-08.a Antarctic fish are also often endemic and are dominated by notothenioids (icefish), which make up more than half of the 320 fish species known to exist in the Southern Ocean.26