Given its extreme conditions, the Antarctic comprises a perhaps surprising diversity of ecosystems. Antarctica is the coldest, windiest, driest and highest continent. Only about 0.4% of the continent is ice-free. Since plants and the invertebrates associated with them and most seabirds require bare rock as growth or breeding habitats, the ice-free areas are important for their survival; consequently, many species are often found breeding close to each other. Antarctica and the Southern Ocean, distinct in their physical properties from other parts of the world, have a large number of endemic species. For example, 100% of the nematodes, 50% of the lichens, and more than 30% of the terrestrial invertebrates on the Antarctic continent are found nowhere else,⁸⁷ and the dominant fish species in the Southern Ocean are endemic to the region.⁸⁸

Marine environments

One of the likely results of climate change is an alteration in the distribution of species, as those adapted to warmer climes expand their ranges south. For example, crabs are not found in the ocean around Antarctica-they became extinct there some 15 million years ago.⁸⁹ Many invertebrate organisms, such as brittle-stars and molluscs, evolved to have only soft shells in the absence of predators. Others, such as some marine snails, have lost their shells altogether. These creatures are no match for shell-crushing invaders, such as king crabs. There is already evidence that king crabs are expanding their range and moving south. In 2004, four specimens of a king crab that is abundant in the coastal waters of New Zealand were found in the northern Ross Sea and near the Balleny Islands (approximately 67°S).⁹⁰ More recently, thousands of these creatures have been found on the shelf slope of the western Antarctic Peninsula.⁹¹ Their presence alone has the potential to extensively modify species diversity in the region.

The pelagic environment

Marine microorganisms form the basis of Antarctic food webs. They include vast numbers of bacteria, phytoplankton (single-celled plants) and zooplankton (single-celled animals). Bacterial communities occur throughout the water column of the Southern Ocean, as well as in the sea ice. These tiny, single-celled organisms provide food for zooplankton, krill, fish and other vertebrates. They are exceptionally numerous and comprise around 90% of the living matter produced in Antarctic waters. The biomass of phytoplankton is estimated to be 5000 million tonnes; there are also about 1200 million tonnes of bacteria and some 600 million tonnes of protozoa. ⁹² The number of species for many groups of organisms is still unknown. The Census of Antarctic Marine Life found many new species that are still being identified; this is particularly true for bacteria.⁹³ The phytoplankton (diatoms, dinoflagellates, cilliates and other protists) in the Southern Ocean comprises 560 known species,⁹⁴ but only a few are widely dominant; their community structure is not constant throughout the Southern Ocean.⁹⁵ One group, the diatoms, is responsible for most of the primary production (fixing of inorganic carbon into organic molecules). Their level of productivity varies greatly with season, being highest in spring and early summer.⁹⁶ Most of their production is consumed or recycled by bacteria and protozoa.⁹⁷

Intense phytoplankton blooms occur in Antarctic waters during spring and summer when increasing sunlight melts the sea ice and warms the ocean. The high light conditions and high nutrient content in the surface waters are ideal conditions for the growth of phytoplankton cells. During photosynthesis, phytoplankton take up carbon dioxide that dissolves in the ocean from the atmosphere. They also produce dimethyl sulfide, a natural aerosol, which is released into the atmosphere. Here it helps cloud formation as it acts as a cloud condensation nucleus,⁹⁸ and increases the reflectance of the sun's heat from Earth. Thus, these single-celled organisms not only support the food web, they also influence the biochemistry of the ocean and play a vital role in affecting global climate by reducing carbon dioxide in the atmosphere and altering global heat balance. In turn, they are affected by anthropogenic changes to the atmosphere. Ozone depletion has increased the damage they experience due to increased ultraviolet B radiation (Box 7.2). Anthropogenic environmental changes are likely to have far-reaching impacts on the Antarctic marine ecosystem.

Zooplankton includes creatures like krill-small shrimp-like crustaceans that rank highly on the menu of many top predators, such as fish, whales, seals, penguins and flying seabirds. Krill in turn feed on phytoplankton and sometimes small zooplankton. Copepods (minute crustaceans) are another grazer on phytoplankton, making up most of the biomass in many pelagic zooplankton communities, and are an alternative food source for higher predators. Krill distribution and abundance were examined in 2006 during a major marine science voyage known as BROKE-West (Baseline Research on Oceanography, Krill and the Environment-West) in the western sector of East Antarctica. Transects were sampled from 30°E to 80°E and krill abundance in this region was estimated to be more than 2.6 million tonnes.¹¹⁴

Both phytoplankton and zooplankton comprise species that build shells made of aragonite or calcite. An increase in the acidity of the Southern Ocean is likely to first affect these planktonic species that form the base of the food web. Carbon dioxide-driven acidification reduces the availability of the carbonate ion that calcium-carbonate, shell-making organisms require for calcification, reducing the ability of these organisms to form shells. The rapid change in the acidity of the ocean is already affecting calcifying organisms-the shells of planktonic organisms known as foraminifera are now about one-third lighter compared with pre-industrial times.¹¹⁵ However, the Australian Antarctic Division-led Southern Ocean Continuous Plankton Survey has observed very large blooms of foraminiferans, especially in the southern summer of 2004-05 when they dominated the surface plankton (up to 80% of abundance) through much of the Southern Ocean south of Africa to Australia.¹¹⁶

It is important to note that different species respond differently to environmental changes. While some are likely to be affected adversely, others might benefit from the changes.¹¹⁷ However, changes at the base of the food web, such as phytoplankton and zooplankton, can potentially radically change the dynamics of the Southern Ocean ecosystem, but it is still unclear whether (or how) higher-order organisms are affected

The benthic environment

The bottom of the Southern Ocean offers rich habitats on hard and soft substrata to a great number of species, many of which grow much slower than their temperate counterparts. Both fixed and mobile species including sponges, molluscs, sea stars and worms are highly diverse and abundant. Bryozoans are particularly diverse and have a high level of endemism.²³ Based on the outcomes of the Census of Antarctic Marine Life, the CCAMLR proclaimed two 'vulnerable marine ecosystems' to protect species assemblages and aid the conservation of biodiversity.¹¹⁸

At depth, environmental conditions are stable and species communities and assemblages appear not to change much. A threat to the biodiversity of the benthos is iceberg grounding. Icebergs break off glacier snouts and ice shelves and often get caught in currents that transport them away from their calving sites. In shallow water, icebergs can become grounded, which stirs up the sediment and crushes benthic fauna in the way. The damage caused by grounding icebergs tends to be local. So far, these grounding events appear to have contributed to the species diversity in the benthic communities by creating a patchwork of areas that are in different stages of recovery. However, an increased rate of iceberg calving may cause more frequent disturbances to benthic areas and not leave sufficient time for populations to recover. Fast-growing organisms are likely to have a better chance to resettle than slow-growing ones. In the long term, while the benthos may not remain scarred and unpopulated, its communities may change in their species composition and some organisms are likely to be lost, at least locally.

2.3.2 Terrestrial environments

Antarctic continent

Antarctica is almost entirely covered in permanent ice. Ice-free areas of exposed rock are rare and account for only about 0.4% of the total area. Most of the exposed rock is in remote mountain ranges; less than 6000 square kilometres is found in small, isolated patches adjacent to the coast but this provides habitat for most of the terrestrial biodiversity of Antarctica. Exposed, ice-free mountain tops exist inland, and about 40-50 species of mosses and lichens survive at elevations of 2000+ metres above sea level. Temperatures range from -30 °C in summer to about -70 °C in winter and the moisture content of the air is typically very low (less than 0.5 kilograms per cubic metre).¹¹⁹ Adélie penguins, and seabirds such as Antarctic petrels (Thalassoica antarctica) and snow petrels (Pagodroma nivea), use ice-free habitats for nesting, and some seals use coastal Antarctic beaches as haul-out areas and fast ice (sea ice adjacent to land) for breeding.

The coastal areas and offshore islands are largely south of the Antarctic circle (66°33'S), which marks the most southerly latitude at which the sun is above the horizon at the winter solstice. The climate here is defined as cold maritime. Milder than the interior, average temperatures can rise above 0 °C in summer but drop to less than -30 °C in winter. The region between 60°S and 70°S is the cloudiest on our planet, with a cloud cover of 85-90% throughout the year.¹²⁰ Winds generated in the interior of the continent drive cold, dense air towards the coast. Smooth ice surfaces on the ice plateau and steep slopes at the coast reduce friction and intensify katabatic winds, which are strongest at the edge of the continent (often 180 kilometres per hour or more). Terrestrial ecosystems are isolated from each other and their floral and faunal communities are less complex than those at lower latitudes¹²¹ or the Arctic region. For example, there are 900 species of vascular plants in the Arctic¹²² compared with 2 species in the Antarctic,¹²³ where lichens and mosses dominate the visible flora.

The microbiotic communities (bacteria and fungi) are species-rich in comparison to other communities and exist in lakes, moss cushions and the soil. Many microorganisms, such as diatoms and cyanobacteria, are endemic to Antarctica.¹²⁴

Lakes and drainage systems are also part of the ice-free areas of Antarctica. Many of these systems exist close to the freezing point of water and their water levels and salinity react quickly to changes in the moisture content of the environment.¹²⁵ Changes in the water chemistry affect life in the lakes, which include bacteria, algae, viruses and some invertebrates, such as copepods and rotifers.

In researching the terrestrial environment, a number of important questions remain unanswered. These concern the species diversity and distribution of soil organisms, such as invertebrates, microbes and algae. New genetic techniques reveal an increasing complexity of species, for example among crustaceans, such as amphipods,¹²⁶ and bacteria.¹²⁷ How these organisms participate in the cycling of nutrients and the flux of carbon through the terrestrial systems is poorly understood. There is also insufficient knowledge about their contribution to the hydrology of terrestrial systems or feedback loops that link them to climate changes.¹²⁸Box 7.3 discusses changes in Antarctic vegetation communities.

Subantarctic islands-Heard Island and McDonald Islands, and Macquarie Island

High-latitude islands and island groups are part of the Antarctic terrestrial environment. Terrestrial ecosystems of the subantarctic islands are very different from those of continental Antarctica. Surrounded by the Southern Ocean and located south of 50° latitude, they are mostly free of permanent ice, although Heard Island, situated south of the Antarctic Polar Frontal Zone has a permanent ice cover. Macquarie Island lies to the north of this zone. The seasonal temperature fluctuations are modest with mean temperatures of around -2 °C in winter and about 8 °C in summer.¹⁴⁵ Species diversity increases with decreasing latitude but is still lower in the subantarctic zone than in subtropical and tropical regions; however, species are often highly abundant. Compared with the terrestrial flora of Antarctica, vascular plants are diverse, with several flowering plants, including megaherbs and grasses; only two flowering plants are found on the Antarctic Peninsula. Mosses and liverworts are a significant component of the landscape.¹²¹ Trees and shrubs are absent from the Australian subantarctic islands, but do occur on other subantarctic islands.

The faunal diversity is dominated by invertebrates and includes microarthropods, such as springtails, and insects including beetles and flies. Many vertebrates, such as flying seabirds, penguins and seals, rely on the ocean for food but depend on the islands for breeding and moulting sites.

Changing environmental conditions may increase the likelihood of alien species establishing themselves in new niches in areas that are, at the moment, too extreme for them to survive.¹⁴⁶ Such changes could become a threat to important areas of biological, scientific, historic, aesthetic and wilderness values.

In the Australian research program, detailed studies of the subantarctic terrestrial environment and environmental change are largely limited to ongoing work at Macquarie Island. A number of vertebrate species have become established on the island. European starlings (Sturnus vulgaris), for example, are self-introduced via New Zealand, while European rabbits, rats and mice arrived with the sealers in the 19th century. Some of these populations, such as the rabbits, have reached vast numbers, which have significantly affected the island's terrestrial environment (Box 7.4). For example, excessive grazing by introduced rabbits destabilised the underlying rock and soil and-in conjunction with high rainfall-led to massive landslides near penguin colonies, killing large numbers of birds.¹⁴⁷ Since similar introductions did not occur on Heard Island or the Antarctic continent, the findings at Macquarie Island are unique to its ecosystem and cannot be extended to other areas.

Vertebrate populations

Antarctic vertebrates encompass a variety of flying seabirds and penguin species, several seal and whale species and numerous fish species. The species diversity, especially on the Antarctic continent, is greatly reduced compared with the temperate and tropical regions and even the Arctic. However, the abundance of many species is very high. All are highly dependent upon the Southern Ocean for food, while their breeding areas include terrestrial, fast ice and marine regions. Many of the large, air-breathing vertebrates are also highly migratory and explore areas far outside the Antarctic and the Southern Ocean.

Status and trend data are available for only a few species, notably the two penguin species on the continent, some albatross and giant petrel populations, and fur and elephant seals at Macquarie Island. Long-term population data do not exist for the ice-breeding seals and whales, most of the flying birds and some of the penguins at Macquarie Island. Hence, trends and status are difficult to establish. Heard Island and McDonald Islands are visited infrequently and data are largely lacking.

Fish

The fish fauna of Antarctica is unique. Their species composition and to a large extent their distribution in the Southern Ocean have been well documented. There are some 322 recognised species in Antarctic waters but only about half (161 species) live in the high Antarctic (i.e. south of the Antarctic Polar Frontal Zone).²⁶ Of those, most (77%) are notothenioids-the most diverse group with 129 species belonging to five families.¹⁴⁸ Their biomass makes up 91% of the Antarctic fish fauna.²⁶ Notothenioids have lived in the Antarctic environment for millions of years and are well adapted to life in a polar ocean. Key to their survival in the freezing temperatures is that these fish evolved to produce glycoproteins that act as antifreeze agents in their blood.¹⁴⁹

In terms of their populations, there is virtually no information on the status and trends of Antarctic fish. This is partly due to the vast area covered by the Southern Ocean that renders population surveys near impossible, especially those frequent enough to estimate abundance and trends. Therefore, formal stock assessments are only available for some of the exploited fish populations.

Historically, vessels from the Soviet Union and other Eastern Bloc countries conducted large-scale fishing operations in the Southern Ocean off the AAT in the mid-1960s. Marbled rock cod (Notothenia rossii) was caught in such quantities that the stock had noticeably reduced by the 1970s and was depleted by the end of the 1980s to a point at which commercial operations were no longer profitable. Off South Georgia, stocks had all but disappeared after only two years of fishing.¹⁵⁰ Currently, only two species of finfish are harvested in the Australian exclusive economic zone at Heard Island and McDonald Islands, and Macquarie Island: the Patagonian toothfish (Dissostichus eleginoides) and the mackerel icefish (Champsocephalus gunnari). The latter is being targeted only at Heard Island and McDonald Islands.

CCAMLR regulates all legal commercial catches, but illegal, unregulated and unreported fishing still occurs in the high seas of the CCAMLR area, albeit at probably lower levels than in the 1980s and 1990s. It is worth noting that in 2010, no illegal, unregulated and unreported fishing was reported in Australia's exclusive economic zone at Heard Island and McDonald Islands, or at Macquarie Island.

Whales

Both toothed and baleen whales are found in Antarctic waters, at least during the southern summer. The former comprise several species, some of them rare, and include killer whales or orcas (Orcinus orca) and sperm whales (Physeter macrocephalus). Baleen whales include the blue (Balaenoptera musculus), Antarctic minke (B. bonaerensis), fin (B. physalus), sei (B. borealis), humpback (Megaptera novaeangliae) and southern right (Eubalaena australis) whales. Most Antarctic baleen whale species spend the summer in the open waters of the Southern Ocean, where they feed extensively as the sea ice recedes. In autumn, they migrate north to warmer waters where they give birth to their young.

By the mid-1900s, a number of great whale species (e.g. blue, humpback and sei whales) living in the Southern Ocean had nearly become extinct after decades of intensive hunting.¹⁵¹ Today, despite efforts to protect them by banning commercial whaling and declaring the Southern Ocean an international whale sanctuary, rates of recovery vary among species and regions, and some populations are still showing no sign of recovery.¹⁵² The reasons for this are largely unknown. Blue, fin and sei whales are listed by the International Union for Conservation of Nature as endangered species, while sperm whales are classified as vulnerable to extinction. Blue whales have not been hunted for 65 years, but so far there are only very limited indications of a possible population recovery.¹⁵³ On the other hand, humpback and southern right whales are comparatively abundant again and are listed as being of least concern. However, the vast abundance of whales from pre-industrial times will in all likelihood remain a thing of the past.¹⁵⁴

Seals

Four species of seal (crabeater, leopard [Hydrurga leptonyx], Ross [Ommatophoca rossii] and Weddell [Leptonychotes weddellii]) inhabit the sea ice zone that surrounds Antarctica and are reliant on the sea ice at critical stages of their lives, particularly in the reproductive and moulting periods. Their populations are difficult to study because these seals are highly mobile, are dispersed over very large and inaccessible regions, spend long periods of time foraging in the ocean where they are difficult to survey, and do not appear to occupy set territories. Sightings are usually of individuals or very small groups. Surveys to estimate their population sizes are infrequent because they are expensive and labour intensive. Consequently, population trends are largely unavailable.

In the past, estimates of the global population of crabeater seals ranged from 2-5 million individuals in the mid-1950s¹⁵⁵ to about 75 million in the early 1970s¹⁵⁶ and 11-12 million in the 1990s.¹⁵⁷ In 1972-73, Laws estimated 772 000 crabeater seals in the Wilkes Land region, East Antarctica,⁷ and postulated that these seals should increase in numbers because of all the 'excess' krill available after many krill-eating whales had been removed from the Southern Ocean. A detailed aerial survey of 1.5 million square kilometres from 64°E to 150°E, roughly coinciding with the area where Laws operated, was conducted in 1999-2000. If Laws' krill surplus hypothesis had been correct, several million crabeater seals could have been expected. However, the survey estimate for crabeater seals yielded fewer than 1 million individuals in the survey area with a range of 0.7-1.4 million.¹⁵⁸ Thus, it appears that crabeater seals are abundant but that earlier estimates were too high. Leopard and Ross seals are probably also abundant, but less so than crabeater seals, with numbers in the tens of thousands.^159-160

Crabeater, leopard and Ross seals inhabit the northern region of the sea ice that consists of ice floes of varying sizes and density - known as pack ice. Weddell seals are found on the fast ice - the sea ice that is attached to the continent. How the pack ice seals respond to environmental stressors may vary among species.¹⁶¹ However, changes in the structure and size of ice floes could lead to the loss of pupping platforms. A reduction in sea ice persistence may decrease the availability of Antarctic krill, an important food source for all pack ice seals - although, if coastal polynyas (ice-free areas) increase in size, crystal krill may become more abundant and may partially offset the loss of Antarctic krill.¹⁶² Leopard seals have the most diverse diet among the ice seals and are likely to be least immediately affected by changes in food availability. However, depending on the rate, kind and magnitude of change, they are likely to be affected eventually.

Antarctic fur seals (Arctocephalus gazella) and southern elephant seals (Mirounga leonina) inhabit the subantarctic islands, but can be encountered as far south as the Antarctic continent. While fur seal populations appear still to be increasing, the numbers of southern elephant seals at Macquarie Island are still in decline (Figure 7.7). The reasons for this are unknown and difficult to investigate, because this species performs long-distance migrations for 8-10 months each year;¹⁶³ however, elephant seals are probably subjected to a number of different pressures throughout the year, as well as at various stages of their lifecycle.

Flying seabirds

Seabirds are typically long lived. They mature late and only lay one or two eggs per year, which are not usually replaced if lost. Also, some albatross species breed only every second or sometimes third year. Although adult survival is usually very high (around 95% of adults return the following year to their colonies), their low reproductive output does not enable seabirds to withstand even small increases above their natural mortality rates.

Most seabird populations in the Antarctic are only infrequently surveyed, because it is difficult to access their colonies. Consequently, few seabird populations have been assessed in a reliable way. Populations comprising fewer than 100 breeding pairs are extremely vulnerable. About one-third of global albatross populations are in this category, including most breeding populations at Australia's subantarctic islands.

The threats that seabirds encounter at sea include competition with commercial fishers for their prey species, death or injury as bycatch in longline and trawl fisheries, intentional shooting, increased dependence on fisheries' discards, and injury from marine pollution. The consequences of global warming and ocean acidification are also likely to threaten many seabirds by affecting the abundance and spatial distribution of their food supply.

On land, seabirds may experience disturbance by humans, loss of breeding habitat, and - because of increased competition for nest sites - exposure to parasites and pathogens. On subantarctic islands, their breeding success can be reduced directly by alien predators, such as cats, rats and mice, as happened on Macquarie Island. Heard Island has so far remained free of introduced vertebrates. Alien species can also have an indirect effect where overgrazing leads to destabilisation of the substratum, which in turn can lead to an increase in landslides.¹⁶⁵

One of the most serious threats to seabirds, particularly those breeding at lower latitudes on the subantarctic islands, is commercial fishing operations. Within the Australian jurisdiction incidental seabird mortality is strictly controlled and regulated. However, seabirds fly enormous distances and often forage in the high seas in international waters where they interact with the pelagic longline fisheries. The seabirds become hooked when they scavenge for food behind the vessels; as the line sinks they drown. Significant research has been undertaken and mitigation methods adopted by CCAMLR as a result have seen the seabird mortality reduced to near zero in the legal fishery. The approach taken is to collaborate with industry members to develop gear that is seabird-safe but does not impact on catch rates of fish (see Box 7.8). CCAMLR continues to monitor fishery interactions with seabirds and has been adjusting the mitigation methods accordingly.

Penguins

In terms of biomass, most Antarctic seabirds are penguins-they make up about 90% of the total avian biomass.¹²⁰ Like all seabirds, penguins are long lived and only produce one or two eggs per year. Penguins often live in large colonies in the coastal areas of subantarctic and Antarctic islands. During the breeding season, the foraging areas of the breeding population are limited, because they need to return regularly to their colonies to feed their offspring. Of the 18 species in the penguin family, 7 live and breed in the AAT at Macquarie Island, but only emperor penguins (Aptenodytes forsteri) and Adélie penguins inhabit colonies in the high Antarctic. Adélie penguins spend the winter months at sea, returning to their breeding colonies during the southern summer, while emperor penguins breed during the winter months and fledge their young in summer. Consequently, these two species are subject to marine and terrestrial processes at different times of the year.

Penguins moult once a year. To prepare for the moult, they feed extensively to lay down sufficient reserves of body fat; during the moult, they cannot fish as their plumage is no longer waterproof. Thus, for several weeks they survive on stored body reserves. With the exception of gentoo penguins (Pygoscelis papua), they forage offshore and are migratory outside the breeding season.¹⁶⁶

The greatest threats for penguins in East Antarctica are likely to be loss of breeding habitat (in the case of emperor penguins) and a reduction in food availability due to global warming and ocean acidification. Changes in sea ice conditions have varied consequences (Box 7.5). For example, a reduction in the sea ice extent potentially shortens foraging distances, but less sea ice also means a reduced production of krill.¹⁶⁷ It is difficult to predict to what extent penguins may be able to adapt to environmental change, particularly as the rate of change is likely to increase once the ozone loss is reversed, making adaptation difficult for these long-lived species.

IUCN= International Union for Conservation of Nature