Living environment

Antarctic species occupy diverse habitats and ecosystems that include ice-covered areas, ice-free vegetated areas, ice-free rocks, salt and freshwater lakes and streams, intertidal areas, sea ice, middle and deep water, and benthic regions of the Southern Ocean (the benthic zone is the ecological region at the bottom of the ocean, including the sediment surface and some subsurface layers). Permanent ice and snow cover almost the entire Antarctic continent, and permanent or seasonal ice-free areas of exposed rock are rare (Bockheim 2015, Burton-Johnson et al. 2016).

Species that have made Antarctica their home have evolved over many millions of years. The Drake Passage opened about 30 million years ago, isolating Antarctica from other continents. Antarctic terrestrial species have adapted physiologically to cope with the significant environmental challenges they encounter: freezing temperatures, extensive ice cover, extended periods of reduced sunlight and poor soil quality. In addition, despite its vast ice cover, Antarctica lacks persistent free-flowing water, so that most, if not all, terrestrial vegetation freeze-dries to survive winter when liquid water is not available for life (Kawarasaki et al. 2019).

Antarctic plants (mosses, lichens and algae), microorganisms and invertebrates comprise a generally highly specialised flora and fauna that are less diverse than in temperate regions but often locally abundant. In Antarctica, the most diverse and abundant terrestrial biodiversity is invisible to the human eye and contained in the soils and water bodies. Because of their high specialisation, many species are endemic (found nowhere else) (Velasco-Castrillón & Stevens 2014, Biersma et al. 2018, Biersma et al. 2020). Metagenomics or environmental genomics – the study of genetic material obtained from a mixture of microorganisms collected from the environment – is rapidly improving our understanding of microbial community structure and composition, and the extent of biodiversity of the microcosm. Genetic analyses using mitochondrial or genomic DNA are used in studies of higher organisms such as seabirds, and are detecting more differentiation or population structure among species than previously assumed (e.g. Frugone et al. 2018, Rexer-Huber et al. 2019).

Terrestrial species                                          

In the Antarctic terrestrial environment, environmental gradients, such as salinity and ice cover, are more pronounced than anywhere else on Earth. Ecosystems are also often highly isolated, especially in the terrestrial environment where areas are snow- and ice-free (Convey et al. 2014). In 2012, Antarctica’s ice-free areas were classified into 15 Antarctic Conservation Biogeographic Regions (ACBRs) based on differences in biodiversity and landscape features (Terauds et al. 2012); in 2016, a 16th ACBR was added (Terauds & Lee 2016). Biogeographic regions have become an important management tool for the conservation of biologically distinct areas.

Many Antarctic terrestrial species depend on the ice-free areas of the continent and its offshore islands, but only approximately 54,200 km², or 0.44%, of the total Antarctic land mass is ice-free (Brooks et al. 2019). Ice-free areas are either isolated mountain peaks, mountain ranges, dry valleys, exposed coastal fringes or offshore subantarctic islands. Most ice-free areas occur in the Transantarctic Mountains, which extend over 3,500 km and separate East from West Antarctica, and the Antarctic Peninsula.

In some ice-free areas, freshwater and saline lakes exist that range in size from small tarns, which freeze solid in winter, to large and deep lakes in which water remains liquid throughout the year. The species composition varies in freshwater, brackish and saline water bodies. The organisms inhabiting saline lakes originated mainly from marine species. The surface of lakes is typically frozen; in coastal areas, this layer of ice can melt in summer.

One of East Antarctica’s few oases (coastal ice-free areas) is the Vestfold Hills area. Here occurs the greatest concentration of permanently stratified (meromictic) water bodies in Antarctica, and possibly the world (Gibson 1999). Meromictic lakes are usually deep, with steep sides. Since the water layers do not intermix, different light, oxygen and salinity levels occur in different zones. Biodiversity can be high among the different, specialised communities in the aerobic and anaerobic zones (Gibson 1999, Lauro et al. 2011). The 2 main habitats in Antarctic lakes are the water column and the benthic mat. The latter is a mixture of algae, bacteria and mosses that covers the bottom of lakes and supports a community of grazing microinvertebrates.

Human activities mostly affect ice-free areas, as these are often in areas where people build Antarctic stations and other infrastructure. Some 81% of all Antarctic buildings are currently located in coastal, ice-free areas (Brooks et al. 2019). Human presence has led to physical disturbance and permanent modification of habitats (Brooks et al. 2019), reduction of aesthetic values (Summerson & Bishop 2012) and changes in geomorphology (Campbell et al. 1994). Even in areas where human activities have ceased, tracks of vehicles were still discernible 60 years later (O’Neill et al. 2013).

Climate change is likely to enlarge the ice-free areas in Antarctica – possibly by as much as 25% – by the end of the century. Most of this gain will occur in the Antarctic Peninsula region, currently one of the fastest-warming areas. As the physical constraints imposed on terrestrial life by the vast areas of ice are diminishing, the range of certain species will possibly expand. However, other factors potentially limiting distributions, such as water, will also play a role (Lee et al. 2017).

Whereas some native organisms may expand their distributions, others may not survive in the long term as temperatures rise. Changes in the species composition of soil organisms have occurred in the McMurdo Dry Valleys, Ross Sea. The dominant species, a nematode, has decreased, while other less dominant taxa have increased. However, the overall trend indicates a general decrease in the abundance of soil biota (Andriuzzi et al. 2018). Environmental changes may also enable non-native, invasive species to establish (see Non-native species) (Duffy & Lee 2019).

Invertebrates

Multicellular terrestrial animals that live in Antarctica throughout the year are limited to a few types of invertebrates able to withstand the environmental conditions. Many microinvertebrate species live in the benthic mats associated with lakes; some inhabit only the water column; others survive in the adjacent soils. There is some overlap in species that occur in aquatic and soil habitats. The most abundant phyla are rotifers, nematodes and tardigrades, but crustaceans, mites and springtails (microarthropods) occur as well (Gibson et al. 1998, Convey et al. 2008, Nielsen et al. 2011).

Compared with temperate regions, the terrestrial invertebrate species diversity of the region is low. Since Antarctica is an isolated continent, many of the species here are endemic, and most communities have persisted throughout several glacial cycles over millions of years (Iakovenko et al. 2015). More than 520 species of invertebrates occur, belonging to 140 genera in 70 families. Of these, 16 genera and 1 family are endemic (Pugh & Convey 2008). There is only 1 endemic insect, the Antarctic midge (Belgica antarctica) (Potts et al. 2020).

Climatic and hydrological conditions, including snow accumulation, can influence habitats available for soil communities, especially in arid ecosystems. The abundance of soil microinvertebrates varies regionally and depends on the conditions of the local microhabitats, particularly topography, salinity, moisture and vegetation (Kennedy 1993, Nielsen et al. 2011, Velasco-Castrillón et al. 2014).

Invertebrates experience highly variable environmental conditions related to water and temperature on various timescales (daily to annual) and are well adapted to cope with these variations. For example, some invertebrates, such as certain nematodes, rotifers and tardigrades, can dry out (desiccate) when water is extremely limited at low temperatures (Nielsen et al. 2011). Long-term records and experimental data show that the various groups of invertebrates respond differently to a warming environment. For example, nematode densities appear to increase when temperatures rise, whereas collembolans respond negatively, particularly when soil moisture is low. Among mites, responses are more complex and appear to be species-specific (Nielsen & Wall 2013).

Antarctic fungi

More than 1,000 species of fungi have been identified in Antarctica, but the true number is likely to be much greater (Bridge & Spooner 2012). Fungi exist as lichen symbionts but also occur as free-living soil fungi, which play an important role in the decomposition of organic matter and soil formation. Free-living fungi are either filamentous or yeasts. There may be well over 1,000 species of free-living fungi, but the taxonomic composition of soil fungi has not yet been studied in a wide range of Antarctic soils (Newsham et al. 2021). Fungal assemblages comprise both worldwide and endemic species. The latter are recognised as true psychrophilic (cold-adapted) species, in contrast to psychrotolerant (cold-tolerant) species that have adapted to life in polar regions (Rosa et al. 2019). Fungal spores can be trapped in glacial and subglacial ice, where some survive for 10,000–140,000 years (de Menezes et al. (2020); and references therein).

During the summer, temperatures of soil surfaces can exceed 19 °C (Perera-Castro et al. 2020, Robinson et al. 2020). At very high temperatures for Antarctica, the growth of soil fungi may be impaired when water availability, and organic nitrogen and carbon levels increase. For example, under such conditions in the maritime Antarctic, metabolic processes were impaired in the most common soil fungus, Pseudogymnoascus roseus; the fungus’s capacity to extend its hyphae was reduced, and hence its ability to find nutrients. At the same time, organic soil compounds experience reduced decomposition. Ultimately, a reduction or loss of these processes affects the soil ecology and may limit soil development in the Antarctic region (Misiak et al. 2021).

Since Antarctic free-living fungi tend to be cold-tolerant rather than cold-adapted, warming temperatures may not immediately affect their survival. Fifty species of fungi isolated from ornithogenic (formed from bird faeces and debris) soils grew at 37 °C under laboratory conditions (de Sousa et al. 2017). Various Antarctic fungal species are closely related to species that are known pathogens to animals and plants in more temperate regions. There is some concern that Antarctic fungi may have inherent potential to be pathogenic and that they could inadvertently be transported by animals and/or people to other continents (Bridge & Spooner 2012, Rosa et al. 2019).

However, fungi may offer opportunities for bioprospecting (the exploration of natural sources for molecules, and development of genetic and biochemical information into commercially valuable products) – for example, for the development of new drugs. Bioactive compounds with antiviral properties – for example, against dengue and Zika viruses – have been discovered in fungal extracts (Gomes et al. 2018).

In areas where fuel spills and spillage may occur, aromatic and aliphatic hydrocarbons enter the local environment, and can persist in soils and sediments at high concentrations for decades (Gore et al. 1999, Revill et al. 2007, Powell et al. 2010). Whereas some Antarctic mosses and algae are relatively tolerant to diesel contamination (Nydahl et al. 2015), terrestrial invertebrates are expected to be more sensitive (Mooney et al. 2019). Hydrocarbon contamination in soil results in a significant shift in the soil microbial community, favouring copiotrophic species (organisms found in nutrient-rich environments, especially carbon) and known hydrocarbon degraders at the expense of oligotrophic species (organisms living in nutrient-poor conditions) (van Dorst et al. 2021).

Bioremediation of diesel-contaminated soil through biopiling has been successfully demonstrated at Australian Antarctic stations (McWatters et al. 2016a).

Other yeasts have a considerable tolerance for heavy metals and may become useful as an inoculant in wastewater treatments in cold environments (Fernández et al. 2017b).

As air and soil temperatures increase due to climate change, the composition of fungal communities and assemblages is likely to change. The species richness of fungal symbionts of lichens is likely to increase, because they may be able to switch from a state of survival to a state of growth as the availability of free water also increases (Green et al. 2011).

Soil microorganisms

In Antarctica, soils occur only in the small areas that are ice-free. Antarctic soils form very slowly (Mergelov et al. 2020) because the absence of vascular plants and low temperatures limit soil formation. The major soil-forming process is cryoturbation, the constant freeze–thaw cycle that mixes the soil layers (Fisher 2014). Antarctic soils generally form under lichen and moss beds (mineral soils) (Mergelov 2014) where hypolithic communities (on the underside of rocks) contribute to biogeochemical processes (Mergelov et al. 2020), or at seabird colonies where organic matter accumulates to form ornithogenic soils (Bowman et al. 1996). Ornithogenic soils can be hundreds to thousands of years old (Emslie et al. 2014).

Antarctic soils are different from those found on other continents and were added as Gelisols to the United States soil classification system in 1997. Characteristics include (Fisher 2014):

• limited free water
• at times, high salt concentration
• nutrient deficiencies (oligotrophic)
• a thick layer of permafrost
• high ultraviolet radiation in summer that sterilises the surface
• strong winds continuously eroding the landscape.

Antarctic soils are often highly oligotrophic – that is, they lack, or have only low levels of, important nutrients, such as phosphates, nitrates, iron and carbon (Arenz et al. 2014). Soils in different regions also have different properties and characteristics because of differences in environmental conditions in the various parts of the continent (Campbell & Claridge 2009).

Antarctic soils contain diverse and abundant terrestrial biodiversity, comprising microbial communities of bacteria, archaea, dinoflagellates, viruses and algae. The level of endemism is high in these diverse communities – that is, many species are found nowhere else (Hughes et al. 2015). Some Antarctic microbial communities appear to have existed for millions of years. Throughout several glacial cycles, the isolation of areas for very long periods resulted in evolutionary isolation and, hence, distinct bioregions (Convey et al. 2008).

Despite the important roles of Antarctic terrestrial microbial communities, detailed knowledge about them is still relatively limited. Application of new research techniques is enabling documentation of the high levels of diversity of local communities and the complexities of microbial assemblages at different sites.

Although once considered a sterile environment, Antarctica harbours a great variety of bacteria and archaea – microorganisms that often inhabit extreme environments. New species are still being discovered – see, for example, Peeters et al. (2011). Their taxonomy is complex because a large number of new lineages have been identified, and taxonomic affiliations are difficult to resolve (Lambrechts et al. 2019). Microbial biomass comprises a significant part of the total Antarctic biomass. A study in the Dry Valleys, Ross Sea region, determined that cell numbers in the mineral soils were more than 4 orders of magnitude higher than previously thought (3–40 million cells per gram wet weight) (Cowan et al. 2002). Microbial communities are highly diverse and heterogeneous in different environments characterised by, for example, variations in soil pH, level of disturbance and proximity of seabird colonies or seal wallows (Chong et al. 2010).

A recent study suggested that some Antarctic soil microbes may source their carbon and energy from atmospheric hydrogen, carbon dioxide and carbon monoxide. Since samples collected at Robinson Ridge, near Casey Station, and Adams Flat, near Davis Station, provided the same results, it may indicate that this so-called trace gas scavenging is a widespread mechanism in Antarctica’s soils (Ji et al. 2017).

Soil microorganisms affect the weathering of rocks, control soil development and play a major role in nutrient cycling. However, microbial habitats are not well presented in the current estate of protected areas in Antarctica; there is little to no protection of their habitats because they are not visible and often not considered in environmental impact assessments (Hughes et al. 2015).

Climate change puts soil communities at risk, and may alter species abundance and food-web complexities in ways not yet fully understood (Andriuzzi et al. 2018).

Human activities are a major threat to soil organisms. Buildings and roads can compact soils and change the way water flows, and chemical spills and waste disposal impact microbial habitats. Genetic contamination of microorganisms occurs through clothing fibres, equipment, dust, and human skin cells and waste (Sjöling & Cowan 2000, Takashima et al. 2004, Teufel et al. 2010). This may affect the value of the Antarctic microbiome with regard to bioprospecting. Non-native microorganisms introduced to the environment by humans may alter community structure, function and genetic diversity, and eventually lead to the loss of the native soil communities (Hughes et al. 2015).

Antarctic plants

Plant life on the Antarctic continent is limited (Andriuzzi et al. 2018). There are no trees, and the only 2 native flowering (vascular) plants – Deschampsia antarctica (a grass) and Colobanthus quitensis (a cushion plant) – are limited to the northern part of the Antarctic Peninsula (Convey 1996). Plants with root systems do not grow in the soils of East Antarctica (Kudinova et al. 2015). Plants comprising mainly microflora, such as lichens and mosses, dominate the few ice-free areas, and algae prosper under rocks and in snowfields where there is sufficient moisture. In maritime Antarctica, which includes the South Sandwich and South Shetland islands, and the western Antarctic Peninsula, more than 200 species of lichens and more than 100 species of mosses occur, while in continental Antarctica 92 lichens and 25 mosses occur (King 2017).

Environmental factors that affect plants’ growth and ability to survive are freezing temperatures, lack of water and extremely strong winds. However, in summer, temperatures of soil and plant surfaces can be much higher than air temperatures recorded about 2 m above, providing favourable microclimates for growth. Antarctic moss and lichen beds are the forests of Antarctica (Kennedy 1993), offering vital habitats for terrestrial invertebrates and microorganisms (Prather et al. 2019). Since both mosses and lichens depend on ice-free habitat, their distribution is highly fragmented in the mosaic of ice-free areas.

As one of the most successful groups of organisms on Earth, lichens occur worldwide in many different habitats (hot and cold deserts, coastal areas, and high mountains) (Singh et al. 2018). With more than 400 species, lichens are the most successful plants in Antarctica (Pugh & Convey 2008). On the Antarctica Peninsula, there are 269 known species of lichens and 134 species of mosses, compared with 92 lichen and 25 moss species in continental Antarctica. In the subantarctic, there are more than 250 lichen and 335 moss species plus 60 species of flowering plants (King 2017).

Plant diversity decreases with increasing latitude. Whereas there are about 350 lichen species in the northern parts of the Antarctic Peninsula, there are only about 12 species at 87°S (Sancho et al. 2019). Lichens are more widespread and able to occupy drier sites than mosses. They grow slowly and can exist for hundreds of years (Bergstrom et al. 2021). The diversity and growth rates of lichens appear to be sensitive to changes in mean annual temperatures. Lichen observations, in conjunction with molecular studies, may enable researchers to use them as biomonitors of changes in the environment (Sancho et al. 2019).

Mosses have unique survival strategies that enable them to live in cold conditions, such as a high tolerance to desiccation and the capacity to cease all metabolic activities (Turnbull et al. 2009, Cannone et al. 2017). The summer is the growth season of mosses, when the sun frequently shines for 24 hours per day and ambient temperatures are near or above 0 °C. During their growing season, moss beds need free water; therefore, water availability and temperature largely determine their distribution. Mosses often thrive near melt lakes and other areas where free water becomes available in summer, when melting snow and ice produce temporary lakes and streams. Where conditions are favourable, and especially where weathered nutrients are available from ancient penguin colonies (Wasley et al. 2012, Emslie et al. 2014), lush moss beds occur, such as those found in the Windmill Islands region.

The moss growth season lasts only 8–16 weeks (Robinson et al. 2018, Singh et al. 2018). Antarctic mosses grow less than 6 mm per year – much slower than mosses in temperate conditions (Bramley-Alves et al. 2015). Under optimal conditions, moss turfs can reach a thickness of up to 14 cm; individual plants can live for more than 100 years (Clarke et al. 2012, Amesbury et al. 2017). Many moss species photosynthesise optimally when surface temperatures are around 15 °C or above (Lewis Smith 1999). Thus, although Antarctic mosses live in a cold environment, their photosynthetic capacity is geared towards much higher temperatures (19–26.3 °C), similar to temperate or tropical species. They survive by reducing respiration at low temperatures in winter, when they metabolically shut down into a state of dormancy (Perera-Castro et al. 2020).

Mosses occupy the most extensive vegetated areas on the Antarctic continent, but the species composition of mosses is changing, and, in some areas (Robinson et al. 2018), their abundance is decreasing. Reductions in moss beds mean loss of habitat for associated microinvertebrates. Furthermore, construction of new infrastructure may affect moss beds regionally through pollution (Bergstrom et al. 2021).

There is a complex relationship between the amount of free water available to moss beds through increased melt and moss bed growth. Natural drainage patterns can be altered when infrastructure is built, at times with ongoing effects. In addition, trampling of the vegetation, dust from vehicle traffic, and dumping of gravel and snow onto moss beds have negative effects (Bergstrom et al. 2021). Furthermore, warmer temperatures and increased water availability may enable non-native species to establish (see Non-native species). These changes require monitoring through long-term field-based programs.

Snow algae turn the snow cover green or red when they bloom in the summer (Davey et al. 2019, Khan et al. 2021). At the South Shetland Islands, Antarctic Peninsula, bloom areas ranged from 300 to 145,000 m². The largest blooms occur on relatively shallow slopes and near colonies of seabirds or seals whose presence adds nutrients to the environment. Many algal species are not yet identified, and little is known about their dispersal mechanisms. If suitable dispersal mechanisms exist, the coverage of snow algae may increase as the peninsula warms, as long as nutrient sources are available. Snow algae may become an important terrestrial sink for carbon (Gray et al. 2020).

Climate change is likely to drive some marked changes in the Antarctic flora. For example, in the Antarctic Peninsula region, moss banks have responded rapidly to the gradual increase in temperature with increased growth rates (Amesbury et al. 2017). As temperatures rise, ice-free areas will increase (Lee et al. 2017), enabling increased colonisation (both locally and over long distances). Certain local populations are likely to expand, and their biomass will increase (Singh et al. 2018). However, the availability of free water is another important factor, and the responses to environmental change will be species-specific. For example, where water availability decreases, the abundance of submergence-tolerant species decreases, while desiccation-tolerant species become more abundant. This has already been observed in the Windmill Islands, East Antarctica (Robinson et al. 2018, Bergstrom et al. 2021).

Subantarctic plants

At Australia’s subantarctic islands, the vegetation is more diverse than on the Antarctic continent. Macquarie Island supports 91 species of moss, many lichens and liverworts, and 47 species of vascular plants, of which 4 are endemic (Selkirk et al. 1990). Three of these endemic species are listed as threatened. One is the Macquarie cushion plant (Azorella macquariensis), a keystone species of the extensive feldmark vegetation (patchy, low-growing vegetation intermixed with barren, gravelly substrate). Macquarie Island supports by far the largest and best examples of feldmark vegetation in Australia. The Macquarie cushion plant, one of the species of the feldmark, has suffered a catastrophic population collapse triggered by climate change, which is expected to permanently alter this alpine ecosystem. Since 2009, dieback has affected up to 90% of the cushion plants in some locations. The cause of the dieback has been attributed to the alteration of soil conditions related to climate change, and potentially an unidentified pathogen (Skotnicki et al. 2009, Dickson et al. 2019, Dickson et al. 2021).

Macquarie Island is the most southerly location in the world for naturally occurring orchids. Two species of orchids are endemic to the island; both are tuberous, deciduous orchids less than 5 cm tall, and both are listed as critically endangered under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act). The grooved helmet-orchid (Corybas sulcatus) forms loose colonies about 80–150 m above sea level. Only 4 locations, which cover less than 0.3 ha, are known for this species on the eastern side of the island’s plateau (Skotnicki et al. 2009). The windswept helmet-orchid (Corybas dienemus) occurs mainly at the northern end of the island, usually less than 30 m above sea level in waterlogged peat or beneath the megaherb Stilbocarpa polaris (Copson 1984). Its area of occupancy covers about 1.5 ha. The distributions of both species are disjunct and fragmented, and much of their habitats were lost due to grazing and burrowing by European rabbits (Oryctolagus cuniculus). Since the rabbits were eliminated, the cover of native grasses, such as Agrostis magellanica and Deschampsia chapmanii, has increased (Fitzgerald et al. 2021). The megaherbs Macquarie Island cabbage (Stilbocarpa polaris) and silver-leaf daisy (Pleurophyllum hookeri), as well as tussock grass (Poa foliosa), have also recovered substantially. Thus, aside from the cushion plant dieback, the vegetation of Macquarie Island is improving significantly since the eradication of rabbits and rodents in 2011, and grazing and other impacts have ceased (Springer 2018). Overall, the vegetation of the island is in the best shape it has been for more than a century (Visoiu 2019).

Macquarie Island is a World Heritage Area because of its unique exposed oceanic crust, and its wild, natural beauty. With less than 9% of the plant species being introduced weed species (4 extant species), Macquarie Island has likely the most natural floral assemblages of any island in the subantarctic. However, a European native and worldwide weed, the annual blue grass (Poa annua) is the most widespread plant invader in the subantarctic and the world’s most widely distributed species (Chwedorzewska 2008). It is now well established at Australia’s 2 subantarctic World Heritage sites, Heard Island and McDonald Islands, and Macquarie Island (Williams et al. 2018). At Macquarie Island, annual blue grass is perennial and able to survive the winter. It grows quickly when the short growing season commences, enabling it to compete with native plants (Williams et al. 2018).

In the 2013–14 summer, small patches of 2 non-native grass species (Agrostis stolonifera and A. capillaris) were discovered at Macquarie Island. The plants had not yet produced flowers, and because of their limited extent they were removed. Human activities may have inadvertently introduced seeds, but natural transport (e.g. by birds) cannot be excluded (Pertierra et al. 2016).

The vegetation of Heard Island covers the ice-free areas and includes vascular plants (12 species), mosses (44 species), lichens (34 species) and liverworts (17 species) (Hughes 1987, Bergstrom et al. 2002). Heard Island and McDonald Islands became a World Heritage Area because of their unique wilderness, which provides examples of biological and physical processes that occur in an environment undisturbed by humans. As the glaciers are retreating, new habitat becomes available for Poa annua, an invasive grass. The extent of this non-native invader at Heard Island is currently unknown, but it had spread substantially by 2003. In the 2003–04 summer, a single specimen of a small daisy (Leptinella plumosa) occurred at Heard Island where it had previously not been found (Frenot et al. 2005).

Assessment Terrestrial species

2021

Adequate confidence

Macquarie island vegetation is recovering, but Antarctic mosses are deteriorating.

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Assessment Macquarie Island vegetation

2021

Adequate confidence

2016

2011

Since the eradication of rodents and rabbits, the island’s vegetation is overall making a notable recovery. Macquarie cushion plants and many bryophyte species are still suffering some dieback.

Assessment Antarctic mosses

2021

Adequate confidence

Species composition is changing. Increasing mortality causes reduction of abundance in extensively vegetated areas. Loss of habitat for associated microinvertebrates. Some lichen habitat is expanding. Continued loss of moss banks and associated habitat.

Marine vertebrate species

Biodiversity of vertebrates in Antarctic waters is relatively low in comparison with mid-latitudinal or tropical ecosystems, but many species are highly abundant. Fish are the most diverse vertebrate group, with around 200 species, followed by flying seabirds (7 species on the continent and 13 on subantarctic islands) and penguins (2 species in East Antarctica, a further 3 on the Antarctic Peninsula, and 5 on subantarctic islands). Ice-breeding seals (4 species), fur seals (3 species), sea lions (1 species) and elephant seals (1 species) are also part of the Antarctic fauna. In addition, more than 30 species of baleen and toothed whales forage in the Southern Ocean, and some species of toothed whales appear to remain there throughout the year. The Southern Ocean is home to the largest community of warm-blooded (endothermic) predators in the world (Krause et al. 2020).

Status and trend data are available for only a few species (Tables 2, 3 and 4) – notably, the 2 penguin species on the continent, the giant petrel population, some albatross populations, and fur and elephant seal populations 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. Visits to Heard Island and McDonald Islands are infrequent; thus, recent data are lacking. Note that the listing of species frequently varies between national and international assessments, because species are assessed at different scales.

Flying seabirds

Globally, among the 359 seabird species are 22 species of albatrosses, 52 species of petrels and 18 species of penguins (see Penguins). Of the 46 flying seabird species that occur in the Antarctic region, 7 live in the high latitudes (Barbraud & Weimerskirch 2006), while the remainder inhabit subantarctic islands (Woehler & Croxall 1997). High-latitude species include snow petrels (Pagodroma nivea), southern fulmars (Fulmarus glacialoides) and Wilson’s storm petrel (Oceanites oceanicus). Albatrosses, diving petrels, cormorants, shearwaters and other species live in the subantarctic, but may visit Antarctic waters when foraging.

Some 31% of all seabirds (110 species) are globally threatened, including all but one species of albatross, and another 11% (40 species) are listed as Near Threatened (Dias et al. 2019). The biggest threats are invasive, non-native species; bycatch in fisheries; overfishing; and climate change. Of the 22 albatross species, 21 are listed on the International Union for Conservation of Nature (IUCN) Red List. The only albatross listed with a status of Least Concern is the black-browed albatross (Thalassarche melanophris), whose global population appears to be increasing (BirdLife International 2018). All albatross species plus 7 petrel species are covered under the Agreement on the Conservation of Albatrosses and Petrels (ACAP 2021a).

Seabirds typically live for several decades; they mature late and lay only 1 or 2 eggs per year, which usually do not get replaced when lost. Some albatross species breed only every second or sometimes third year. Adult survival is usually very high, and many adults return the following year to their colonies. Because of their low annual reproductive output, seabird populations are unable to withstand even small increases in their natural mortality rates. Since most Antarctic seabird species require bare rock as breeding habitat, the ice-free areas are important for their survival. Consequently, many species breed close to each other, often in very large colonies.

Antarctic terrestrial ecosystems rely largely on nutrients derived from the ocean. Because of their large populations, seabirds play a major role in the transport of marine organic matter onto land, especially during the breeding season. Seabirds deposit large quantities of guano at their colonies and with it carbon, nitrogen and metals, such as zinc and copper. Thus, seabird activities enable biotransportation of trace elements from the marine to the terrestrial environment, and enrich the soils (Castro et al. 2021).

Since many seabird colonies are remote and difficult to access, surveys of most Antarctic seabird populations occur only infrequently, if at all. Furthermore, crevice-breeding birds display cryptic behaviours (an ability to avoid observation or detection) during the breeding season, making it very challenging to obtain population estimates.

The species diversity on the subantarctic islands is different from that on or near the Antarctic continent. The fauna of Heard Island and McDonald Islands includes 4 species of penguin not found on the continent, and 15 species of flying seabirds, including 2 species of albatross, several petrels and skuas (Stercocarius spp.), and the black-faced sheathbill (Chionis minor). Because the islands support various threatened and endangered seabird species, the IUCN has declared them an Important Bird Area. Heard Island is the largest subantarctic island that is free from introduced vertebrate species (Birds Australia 2010).

In 2011, the IUCN also nominated Macquarie Island as an Important Bird Area because of the presence of various threatened and endangered seabird species (BirdLife Australia 2021). Macquarie Island is home to an estimated 3.5 million seabirds, comprising 13 distinct species. These include 4 species of penguins, a variety of small petrels and 4 albatross species: wandering (Diomedea exulans), grey-headed (Thalassarche chrysostoma), light-mantled (Phoebetria palpebrata) and black-browed. Three species of albatross are either increasing (black-browed and light-mantled) or stable (grey-headed). The extremely small breeding population of wandering albatross (5–10 breeding pairs) has been decreasing in the past decade (Cleeland 2018). Globally, the species is listed as Vulnerable, based on relative trends in numbers and survival rates in the past, similar to those observed in the Indian Ocean populations. Longline fisheries elsewhere still pose a risk to some albatross species and grey petrels (Procellaria cinerea), but risks off Macquarie Island have decreased as a result of careful management of fishing activities. The numbers of grey petrels and soft-plumaged petrels (Pterodroma mollis) have increased since the eradication of introduced predators and rodents (McInnes et al. 2019). At Macquarie Island, the successful eradication of rabbits and rodents in 2011 also enabled the recovery of several petrel species (e.g. Antarctic prions – Pachyptila desolata, and white-headed petrels – Pterodroma lessonii), and the recolonisation of others (e.g. blue petrels – Halobaena caerulea, and grey petrels) (Bird et al. 2021). The presence of burrowing petrel species is particularly difficult to detect, but application of genetic techniques (see case study: DNA technology applied in seabird research) has allowed identification of 8 of the 9 petrel species that used to breed on the island (McInnes et al. 2019).

Commercial fishing operations are one of the most serious threats to seabirds, particularly those breeding at lower latitudes on the subantarctic islands. Within the Australian jurisdiction, incidental seabird mortality is strictly controlled and regulated, and is relatively low. However, seabirds fly enormous distances and often forage in the high seas in international waters, where they interact with longline fisheries and net fisheries. Seabirds can become caught when they scavenge for food behind longline fishing vessels; as the line sinks, they drown. Seabird mortalities can also occur from entanglement in nets, or cable strikes in trawl fisheries. Particularly through the efforts of the Agreement on the Conservation of Albatrosses and Petrels (ACAP 2021a), there is progress in developing and improving best practices and procedures for minimising seabird deaths in fisheries. For example, after the introduction of compulsory line weighting in longline fleet fishing in the Convention on the Conservation of Antarctic Marine Living Resources Area, seabird bycatch in that area is at historically low levels. Other efforts include improvements in line weighting in longline fisheries, development of underwater bait-setting devices (which deliver baited hooks out of reach of most diving seabirds) and implementation of new, highly effective, modern techniques to avoid bycatch in trawl fisheries.

One of these techniques is DNA metabarcoding (see case study: DNA technology applied in seabird research). Not every seabird species is inclined to follow fishing vessels. To determine which species are at most risk, DNA metabarcoding has been used to identify the prey ingested by seabirds and compare the results with the target species caught by commercial fisheries near the breeding colonies (McInnes et al. 2017); see case study: DNA technology applied in seabird research).

On land, flying seabirds may experience disturbance by humans, loss of breeding habitat, increased competition for nest sites, and increased exposure to parasites and pathogens. On subantarctic islands, predation by non-native predators, such as cats, rats and mice, has reduced their breeding success. Introduced predators pose a key threat, because they can increase the mortality of adult birds, and reduce breeding success when chicks fall victim to predators. At Macquarie Island, cats (Felis catus) were eradicated in 1998–2000 (Robinson & Copson 2014). Heard Island and McDonald Islands have so far remained free from introduced vertebrates. Introduced species such as rabbits can also have an indirect effect when overgrazing leads to destabilisation of the substratum, which can lead to an increase in landslides across breeding areas (Scott & Kirkpatrick 2008, Bird et al. 2021).

Although removal of invasive introduced vertebrates was important for the avian biodiversity on Macquarie Island, the effects on native predator populations must be taken into consideration (Travers et al. 2021). The eradication efforts negatively affected top-order predators; 762 giant petrels (Macronectes spp.) and 512 brown skuas (Stercorarius antarcticus lonnbergi) died of secondary poisoning when they scavenged on rabbit and rodent carcases. Furthermore, an important food source disappeared with the eradication because these predatory birds used to feed on rabbits. However, at the population level, these losses were not considered a threat to the viability of the affected populations (Springer & Carmichael 2012).

With regard to endangered species, Australia has threat abatement plans that define actions to be undertaken to reduce the impacts of threats to levels where populations of listed species are no longer threatened (DEE 2018). The Threat abatement plan for the incidental catch (or bycatch) of seabirds during oceanic longline fishing operations (2018) deals specifically with key threatening processes in these fisheries, and stipulates required research and management actions. Under the EPBC Act, Australia has also formulated a national strategy for the protection of threatened albatross and petrel species. At the time of writing, the National recovery plan for albatrosses and petrels 2021 (the third such plan) was open for public comment until 27 August 2021; it is anticipated to be finalised in early 2022 (DAWE 2021a).

Penguins

Penguins are estimated to comprise about 90% of the biomass of seabirds in Antarctica (Bargagli 2005), but, as noted above, populations may be difficult to assess accurately. Like all seabirds, they are long-lived and produce only 1 or 2 eggs per year. They often inhabit large colonies in the coastal areas of Antarctica and subantarctic 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. This can increase their vulnerability to localised prey depletion through commercial fishing. Of the 18 species in the penguin family, 7 live and breed in the Australian Antarctic Territory and Macquarie Island, but only emperor (Aptenodytes forsteri) and Adélie (Pygoscelis adeliae) penguins inhabit large colonies in East Antarctica; fewer than 30 pairs of chinstrap penguins (P. antarctica) occur at the Balleny Islands (Macdonald et al. 2002). Adélie penguins spend the winter months at sea and return to their breeding colonies during the summer, whereas emperor penguins breed during the winter months and fledge their young in summer.

Recent surveys show that the total population of Adélie penguins in East Antarctica has increased by 69% over 30 years, from about 520,000 to approximately 878,300 breeding pairs (Southwell et al. 2015b, Southwell et al. 2015a). However, the increase did not occur in all surveyed populations. For example, at the Scullin and Murray monoliths, Adélie penguin populations appear to have remained stable or decreased somewhat (Southwell & Emmerson 2019). Populations elsewhere – for example, on the Antarctic Peninsula – had significantly decreased over 50 years (Dunn et al. 2016).

Emperor penguin colonies are largely located on the land-fast sea ice near the coast of Antarctica. Many colonies are remote from research stations, making estimations of their size challenging. Long-term population data are available for only a few colonies. In 2009, a satellite-based synoptic survey attempted to estimate the size of the global population of emperor penguins, excluding nonbreeders and juveniles (Fretwell et al. 2012) (see case study: Satellite technology reveals more than the distribution of emperor penguins). A major caveat is that very high-resolution satellite imagery is only available late in the breeding season of these penguins, when both partners of a breeding pair are hunting food for their offspring. This can lead to significant underestimates of the size of the breeding population.

The future of emperor penguins depends on the extent to which Earth warms in the coming decades. If global temperature rise can be limited to 1.5 °C, about one-third of the global emperor penguin population will be lost, and two-thirds of the currently existing colonies will be quasi-extinct. If the temperature increases by 4 °C, some 92% of the current population will no longer exist, and 9 of 10 colonies will be quasi-extinct. Major forces driving these changes are loss of sea ice and increased frequency of extreme events (Jenouvrier et al. 2021).

The greatest threats for penguins in East Antarctica are likely to be loss of breeding habitat (in the case of emperor penguins), a reduction in food availability because of climate change or potentially the resumption of the krill fishery, and habitat loss (Trathan et al. 2015). Precautionary catch limits for krill amount to 2.53 million tonnes (t) in the area 30°E to 80°E, and a further 440,000 t from 80°E to around 150°E (Nicol et al. 2012). Any increase in human activities in the area has the potential to disturb wildlife if not managed carefully (Brooks et al. 2019). Changes in sea ice conditions have varied consequences. For example, a reduction in the land-fast sea ice extent could shorten foraging distances, but a reduction in pack ice would reduce krill production (Bretagnolle & Gillis 2010). It is difficult to predict the extent to which 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 and other long-lived species.

The subantarctic Heard and Macquarie islands are rare areas of terrestrial ‘real estate’ in the Southern Ocean. King (Aptenodytes patagonicus), gentoo (Pygoscelis papua) and southern rockhopper (Eudyptes chrysocome) penguins occur in both locations, while royal penguins (E. schlegeli) are found only on Macquarie Island, and macaroni penguins (E. chrysolophus) occupy Heard Island and other subantarctic islands. These islands are critically important breeding areas for these species. The most recent published estimate of the breeding population of royal penguins is 750,000 pairs in 2016 (Salton et al. 2019).

A recent study examined the phylogeographic structure of the Eudyptes species, and questioned the classification of royal and macaroni penguins as separate species due to the lack of significant genetic and phylogeographic structure. However, the authors stated that their findings were not fully conclusive and that further investigations are required. The same study confirmed the separation of rockhopper penguins into 3 species (Frugone et al. 2018).

The most recent census of gentoo penguins on Macquarie Island in 2017 recorded the fewest breeding pairs since counts began, with 2,527 ± 66 pairs in total. Island-wide, breeding pairs have decreased by 1.8 ± 0.4% per year over the past 34 years (Pascoe et al. 2020). Macquarie Island remains an important site for many penguin species, including king and southern rockhopper penguins.

Seals (Pinnipedia)

Both true seals and eared seals inhabit the Antarctic region and are assessed under the EPBC Act and by the IUCN (Table 3).

True seals (Phocidae)

The Antarctic pack ice zone may be the home of about 50% of the world’s true seal populations. Three species of seals inhabit the pack ice zone that surrounds Antarctica: crabeater (Lobodon carcinophaga), leopard (Hydrurga leptonyx) and Ross (Ommatophoca rossii) seals. They rely on the sea ice at critical stages of their lives, particularly in their reproductive and moulting periods. Their populations are difficult to study because these seals are highly mobile, disperse across large and inaccessible regions, and spend long periods foraging in the ocean. Some species also do not appear to occupy set territories. Sightings are usually of individuals or very small groups. Surveys to estimate population sizes are infrequent, because the studies are expensive and labour-intensive. Consequently, population trends are largely unavailable.

Leopard, crabeater and Ross seals have a circumpolar distribution. Crabeater seals are relatively abundant, whereas leopard and Ross seals disperse widely, distribute sparsely and are very difficult to study. Leopard seals typically occupy the Antarctic pack ice zone where they breed. However, these seals also travel to areas north of the Antarctic Polar Frontal Zone, including to subantarctic islands, South America, South Africa and Australia (Staniland et al. 2018). Similarly, Ross seals sometimes travel to subantarctic islands but, like leopard seals, depend on the sea ice as a breeding platform (Arcalís-Planas et al. 2015). Post-moult, Ross seals migrate north (Wege et al. 2021).

The last time circumpolar population estimates were obtained for these species was in the late 1990s during the Antarctic Pack Ice Seal (APIS) survey (Southwell et al. 2012). Estimates of the global population of crabeater seals have ranged from 2 to 5 million individuals in the mid-1950s (Scheffer 1958) to about 75 million in the early 1970s (Erickson et al. 1971) and 11–12 million in 1990 (Erickson & Hanson 1990). In 1999–2000, the APIS survey, a detailed aerial survey, covered an area of 1.5 million km² from 64°E to 150°E. The survey estimated fewer than 1 million individuals in the survey area, with a range of 0.7–1.4 million. Thus, crabeater seals appear to be abundant, but earlier estimates were probably too high. Analysis of satellite imagery shows that leopard and Ross seals also appear to be abundant, with numbers in the tens of thousands, but fewer than crabeater seals (Southwell et al. 2012). Ross seals are the rarest of the pack ice seals, comprising only about 1% of Antarctic seals. They differ from the other seals in that they breed and moult in the pack ice but forage in the open ocean most of the year (Wege et al. 2021).

How the pack ice seals respond to environmental stressors may vary among species (Ainley et al. 2015). Changes in the structure and size of ice floes could lead to the loss of their breeding habitat. Furthermore, a reduction in sea ice persistence may reduce the availability of Antarctic krill, an important food source for all pack ice seals, especially crabeater seals, whose diet can comprise 90% Antarctic krill (Hückstädt et al. 2012). In the western region of the Antarctic Peninsula, major changes in wind strength, sea ice extent and duration, and the characteristics of the ACC are expected to alter the distribution of krill and therefore the distribution of crabeater seals. As the Southern Ocean continues to warm, krill stocks will decrease in the areas they currently occupy. Optimal conditions for krill are predicted to shift south, and whether krill predators such as crabeater seals will be able to adapt or not will determine their future (Hückstädt et al. 2020). Leopard seals have the most diverse diet among the ice seals and may be the least likely to be affected immediately by changes in food availability. However, depending on the rate, kind and magnitude of environmental changes, these changes will affect them eventually.

Weddell seals (Leptonychotes weddellii) occur in the pack ice but breed on the nearshore fast ice that surrounds the Antarctic continent. Their biology is comparatively well studied. There may be around 730,000 individuals, which remain in the Antarctic region throughout the year. Weddell seals have a high level of fidelity to their breeding sites, and have been observed to travel more than 300 km in winter (Heerah et al. 2016). They dive on average to 125 m but can dive to more than 700 m (Goetz et al. 2017). Their breeding season generally commences in October–November; females give birth to pups on the sea ice near breathing holes and nurse them for 5–6 weeks. Warming temperatures may reduce the energetic cost for seals but also reduce access to suitable breeding habitat and sufficient food resources (Guo 2020).

Southern elephant seals (Mirounga leonina) have a circumpolar distribution; they breed on subantarctic islands, including Macquarie Island – this is the only breeding population in the Pacific sector of the Southern Ocean (Volzke et al. 2021). Their major foraging habitats are the Antarctic continental shelf and the Kerguelen Plateau in the southern Indian Ocean (Hindell et al. 2016). The species is highly dimorphic – males are significantly larger than females – and exhibits gender-related difference in foraging strategies and diet. For example, females tend to forage in the open ocean and mainly consume small fish, whereas males are more likely to forage on the continental shelf where they hunt fish, squid and krill (Labrousse et al. 2017).

Southern elephant seals, particularly young males, have numerous haul-out sites on the Antarctic continent, where they spend most of the summer moulting. On Macquarie Island, about 155,000 southern elephant seals may have been present in the 1950s; however, there has been a long-term decline in the population since, which appears to be continuing. The reasons for this are difficult to investigate as this species migrates over long distances for 8–10 months per year (Learmonth et al. 2006). However, elephant seals probably experience different pressures throughout the year, as well as at various stages of their lifecycle. For example, there appears to be a link between sea ice extent in the summer foraging areas of females and the survival of their offspring. Extensive sea ice may exclude the seals from high-quality foraging areas, which in turn affects the survival of their pups (van den Hoff et al. 2014). The decline of the southern elephant seal population has recently been linked to poor foraging success because changing oceanic conditions have reduced prey availability (Clausius et al. 2017). Occasionally, elephant seals are entangled and drowned in longline gear set for toothfish. Low levels of mortality are very unlikely to lead to significant population impacts (Clausius et al. 2017, van den Hoff et al. 2017).

The complexity in the ecology of this wide-ranging species, which occupies different habitats, makes it challenging to predict how climate change will affect the population. Changes in habitat quality will depend on the region; hence, responses of populations and individuals will vary (Hindell et al. 2016).

Eared seals (Otariidae)

Fur seals (Arctocephalus spp.) inhabit the subantarctic islands and occur as far south as the Antarctic continent, where they are infrequent visitors. Several fur seal populations still appear to be increasing, albeit at varying rates depending on location. On Macquarie Island, the very small breeding populations of Antarctic (Arctocephalus gazella), subantarctic (A. tropicalis) and New Zealand (A. forsteri) fur seals appear to be stable.

Whales (Cetacea)

The Southern Ocean is the prime feeding ground for baleen whales (Mysticeti). Nine species and subspecies of these large whales occur in the Southern Ocean: the Antarctic blue (Balaenoptera musculus intermedia), pygmy blue (B. musculus brevicauda), fin (B. physalus), humpback (Megaptera novaeangliae), dwarf minke (B. acutorostrata), sei (B. borealis), minke (B. bonaerensis), southern right (Eubalaena australis) and pygmy right (Caperea marginata). Some species, such as blue and fin whales, hunt throughout the summer in the Southern Ocean, and migrate north in winter to mate and calve in low-latitude waters (Shabangu et al. 2020). Key baleen whale species are assessed under the EPBC Act and by the IUCN (Table 4). At least 22 species of toothed whales (Odontoceti) also occur regularly in the Southern Ocean (Van Waerebeek et al. 2010). The 2 most commonly sighted toothed whale species are killer whales (Orcinus orca) and sperm whales (Physeter macrocephalus).

In the Southern Hemisphere, baleen whales travel long distances from their feeding areas in the Southern Ocean to their nursery areas at lower latitudes. For 80–90% of the world’s whales, the Southern Ocean Whale Sanctuary, established in 1994, is the major feeding ground in the summer (IWC 1995). As major krill consumers, baleen whales depend on lower levels of the food web (i.e. their survival is linked closely to levels of primary productivity) (Leaper et al. 2006). Whales play an important role in biogeophysical cycling of iron. Whale defecation deposits large quantities of iron into the marine environment and promotes ocean productivity – for example, in the otherwise iron-poor waters of the Southern Ocean. This biological fertilisation is likely to increase krill abundance and even fishery yields (Lavery et al. 2014).

Knowledge about the size and structure of whale populations is very limited. Estimating the abundance of whale populations is a complex and complicated process. Issues can arise with species identifications, the timing of surveys, the areas covered and even the response of species to the presence of vessels (Leaper et al. 2008). Many species have a circumpolar distribution, but their species-specific behaviours differ significantly. For example, although sperm whales use the Southern Ocean during summer, generally only the large males visit the regions south of the Antarctic Polar Frontal Zone (Leaper et al. 2008).

Although various whale populations have shown signs of a slow recovery following the prohibition of whaling, and certain protections are now in place, in the face of a warming climate, baleen whales may be particularly at risk (Constable et al. 2014). Models predict a marked slowdown in the further recovery of whale populations (Tulloch et al. 2018). Whales require stable environments; their life histories are linked tightly to water temperatures and food availability, making whales particularly sensitive to climate change (Tulloch et al. 2018). Species feeding in the mid-latitudes (40–60°S), the regions around the ACC, are likely to be particularly affected by climate change. There are strong interactions between temperature, krill density, body condition and breeding success; for example, food availability during the early weeks of gestation affects breeding success of southern right whales (Seyboth et al. 2016).

Baleen whales (Mysticeti)

The largest creatures on Earth, Antarctic blue whales can reach lengths of up to 27 m. They and various subspecies were commercially hunted from 1904 to 1972. The remaining population is only 3% of the pre-exploitation population. We still understand little about their biology, and attempts to estimate their population size have large uncertainties associated with them. Hunting ceased only in 1972, although it was prohibited in the Southern Ocean in 1965–66. Currently, the greatest threats these whales face are declining food sources associated with ocean warming and increasing ocean acidification (Cooke 2018).

The most comprehensively studied whale is the humpback whale; its distribution and stock abundance are probably the best known of any whale species. The International Whaling Commission distinguishes 7 separate breeding stocks of humpback whales in the Southern Hemisphere, and an eighth that occupies the northern Indian Ocean but does not migrate to Antarctic waters (Branch 2011). The various breeding stocks of humpback whales vary in size (Branch 2006). The largest stock is probably breeding stock D, which migrates annually from summer feeding grounds in Antarctica to northern Western Australia for winter (Kent et al. 2012). This stock has a long history of exploitation (Chittleborough 1965). However, since whaling operations ceased, humpback whales have made a remarkable recovery. Recent surveys indicate that the stock is increasing to a point that its delisting as a threatened species under Australian legislation was proposed in 2016 (Bejder et al. 2016); the possibility of delisting humpback whales from Australia’s EPBC Act is currently being reviewed (DAWE 2021b).

Since 1966, humpback whales have been protected legally from commercial whaling. However, they can still be killed under Article VIII of the International Convention for the Regulation of Whaling – including in Antarctica – provided that the government of a member nation has issued a permit (IWC 2021).

Toothed whales (Odontoceti)

Killer whales are the largest species of the dolphin family. Once regarded as a single worldwide species, they were divided into various ecotypes because of marked differences in their morphology, ecology and vocalisation. In Antarctica, 3 types are recognised based on appearance and diet (Pitman & Ensor 2003). The high level of diversity in a species makes its taxonomy challenging but, based on the known variability and genetic evidence, (Morin et al. 2010) suggested elevating these types to species.

Killer and sperm whales depredate toothfish from longlines, including in Australia’s fleet that fishes at Heard Island and McDonald Islands (van den Hoff et al. 2017, Tixier et al. 2020). Because of the intelligence of these species, population increases and the desirability of toothfish as a rich source of calories, mitigation of this activity has proven very difficult (Richard et al. 2020).

Sperm whales rank among the least understood whale species. Commercial activities severely reduced their population, particularly from 1945 to 1975. The most recent assessment concluded that, globally, sperm whales are currently at 32% of their pre‑whaling population levels (Whitehead 2002). A survey off Western Australia also documented that this species shows no sign of recovery (Carroll et al. 2014).

Fish

The Southern Ocean covers about 35 million km², or 10% of Earth’s oceans. In some regions, the ocean is more than 5,000 m deep; shallower waters occur in coastal regions and at submarine ridges. Of the roughly 20,000 different marine fish species, only 322 (1.6%) used to be known from the Southern Ocean, but the level of endemism (i.e. species are found nowhere else) is about 3 times higher than in other isolated marine areas (Eastman 2005). New species are still being identified. For example, the Biogeographic atlas of the Southern Ocean (Duhamel et al. 2014) lists 374 species. Fish occupy a range of habitats in the water column that extends from the surface to the ocean floor. Most species (about 63%) live on the ocean floor (benthic), and about 26% inhabit the water column at 200–1,000 m depth (mesopelagic) (Eastman 1993).

Antarctic fish evolved in an environment that remained stable for millions of years (Navarro et al. 2019). Antarctic fish are highly stenothermal – that is, they can survive in only a narrow temperature range. Temperatures in the Southern Ocean are close to the freezing point of salt water (–1.9 °C) and vary annually by less than 1 °C (Hunt et al. 2003).

Relatively little is known about the capacity of Antarctic fish to adapt physiologically to climate change impacts, such as increasing ocean temperatures and acidification. Icefishes have no haemoglobin to carry oxygen in their bloodstream and, hence, rely on dissolved oxygen levels. Thus, they may be affected as oxygen saturation levels decline with rising temperatures (Constable et al. 2014). Experimental research has shown that heat stress can cause changes in metabolic processes and enzyme activity. The effects depend on the level of temperature change, the duration of the exposure and the species (Forgati et al. 2017). For example, rock cods (Notothenia spp.) experience oxidative damage to their lipid tissues and antioxidant systems after long-term exposure to increased temperatures (Klein et al. 2017).

Cod icefish (notothenioids) make up about one-third of the known 374 fish species in the Southern Ocean (Duhamel et al. 2014), and many species of this group are endemic to the region (Cheng et al. 2003). Since Antarctic fish species evolved over millions of years in subzero temperatures, they have many physiological and biochemical traits (e.g. antifreeze in their blood) that enable them to thrive in their chronically frigid environment (Beers & Jayasundara 2015).

Among the fish species that are important prey for higher Antarctic predators, such as penguins and Antarctic toothfish (Dissostichus mawsoni), is the Antarctic silverfish (Pleuragramma antarctica). The Antarctic silverfish has a circumpolar distribution and is a mesopelagic notothenioid that feeds mainly on Antarctic and crystal krill (Euphausia crystallorophias). The silverfish occurs throughout the Southern Ocean, and is a key species and major forage fish in the pelagic ecosystem on the Antarctic continental shelf (Mintenbeck & Torres 2017).

Like all notothenioid species, Antarctic silverfish lack a swim bladder. Many notothenioids live on the ocean floor and, thus, do not need a swim bladder, whereas silverfish live in the water column, and have achieved near-neutral buoyancy through an increase in fat deposits, and a reduction in skeletal mass and body density (Wöhrmann 1998, Voskoboinikova et al. 2017). Like many fish in the Southern Ocean, Antarctic silverfish are long-lived, mature late, grow slowly and have a relatively low fecundity (Mintenbeck et al. 2012). They spawn underneath the coastal sea ice, probably in late winter. Because sea ice is important for its reproduction, this species is vulnerable to the effects of climate change. Furthermore, Antarctic silverfish are adapted to stable conditions and a water temperature of −2 °C. In the waters off the western Antarctic Peninsula, a rapidly warming region, the stocks of the silverfish appear to have collapsed. Polar species such as the Antarctic silverfish may be affected once the conditions in their current area of distribution are no longer favourable. As the ocean grows warmer, a recovery of the stock is unlikely, and an important prey of many species may be lost (Mintenbeck & Torres 2017).

Another important element of the fish fauna is the Antarctic toothfish, a large notothenioid, with a circumpolar distribution that extends north to the Antarctic Convergence and into subantarctic waters (Hanchet et al. 2015). This species is commercially exploited (see Commercial fisheries). Adults measure around 160 cm in length; the longest Antarctic toothfish caught measured 210 cm and weighed 120 kg. Although they lack a swim bladder, adult toothfish appear to forage through the entire water column and may acquire near-neutral buoyancy through the accumulation of dietary lipids. Buoyancy changes with level of maturation, diet and migration (Hanchet et al. 2015).

It appears that Antarctic toothfish, which have antifreeze glycoproteins in their bloodstream, occur continuously along the continental slope and shelf of the Antarctic continent and reach areas up to 57°S. In comparison, the Patagonian toothfish (D. eleginoides) mainly occupies the insular and continental shelves in the subantarctic and does not have antifreeze glycoproteins. In the waters off the South Sandwich Islands, on the BANZARE Bank and around Bouvet Island, the Antarctic and Patagonian toothfish species overlap; occasional vagrants of both species are observed far to the north of their typical distributions (Hanchet et al. 2010, Péron et al. 2016, Yates et al. 2019).

Antarctic toothfish are generalist predators, and large adults are at the highest trophic level (i.e. highest in the food chain) of fish off East Antarctica (Park et al. 2015). Antarctic toothfish compete with Weddell seals for prey, but the seals also consume toothfish (Ainley et al. 2021), as do sperm and killer whales, and colossal squid (Mesonychoteuthis hamiltoni) (Hanchet et al. 2015). The energy density of Antarctic toothfish is twice as high as that of the seals’ common prey, Antarctic silverfish and dusky rockcod (Trematomus newnesi) (Goetz et al. 2017), and the body mass of large toothfish (around 120 kg) is up to 3 orders of magnitude higher than that of silverfish. Weddell seals may consume large toothfish that are half the seals’ length (Ainley et al. 2021).

Another fish species of importance to predators such as king penguins and some commercial fishers is mackerel icefish (Champsocephalus gunnari). This small, pelagic, schooling fish occurs mainly at depths of 20–250 m and performs a diurnal migration. In late winter, it is an important prey item for king penguins; as much as 17% of the chicks’ diet can be mackerel icefish. This oily fish provides sustenance to the chicks at the end of a long winter, during which they receive food only intermittently. Unlike many other Antarctic fish, mackerel icefish reach maturity relatively early (age 3–4 years). The fish spawn in coastal areas in late winter; females produce 10,000–20,000 eggs that hatch after having spent about 3 months near the seabed (AFMA 2015).

Assessment Marine vertebrates

2021

Limited confidence

The condition of Antarctic marine species varies between species. Populations of fish, whales and penguins are in a good condition, but elephant seal and seabird numbers are poor and deteriorating. The trend for many species is unclear because of insufficient data.

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Assessment Albatrosses

2021

Adequate confidence

2016

2011

All but one albatross species are endangered. Seabird deaths in Australian fisheries and in the Conservation of Antarctic Marine Living Resources area are greatly reduced, but seabirds continue to interact at unsustainable levels with fisheries outside Australia’s jurisdiction.

Assessment Other flying seabirds

2021

Limited confidence

2016

2011

Status and trends vary with species and location. Population data, if available, cover only limited areas. At Macquarie Island, several petrel species have returned to the island since eradication of non-native predators.

Assessment Penguins

2021

Somewhat adequate confidence

2016

2011

Penguins are relatively well studied. Many populations appear to be stable or increasing, but, at Macquarie Island, gentoo penguins appear to be decreasing.

Assessment True seals

2021

Limited confidence

Because of their distribution and behaviours, it is difficult to assess populations of true seals in Antarctica. The last comprehensive seal survey occurred in 1999–2000.

Assessment Eared seals

2021

Adequate confidence

2016

2011

Status and trends vary with species and location. At Macquarie Island, the small populations of fur seals appear to be stable, but elephant seals are still in decline.

Assessment Baleen whales

2021

Limited confidence

2016

Some species have recovered well since whaling ceased in the Southern Ocean, whereas for others there are no signs of recovery. For many species, population data are difficult to acquire.

Assessment Toothed whales

2021

Limited confidence

Killer whales are the best studied toothed whales. However, uncertainties about their taxonomy make population assessments difficult.

Assessment Patagonian toothfish

2021

Adequate confidence

Regular pre-recruit surveys, tagging studies and biennial stock assessments provide relatively high levels of information on current status and trends of fished stocks in Australia’s jurisdiction. Current stock levels are above (Macquarie Island) or just below (Heard Island and McDonald Islands) the long-term target level prescribed in the harvest strategy.

Assessment Antarctic toothfish

2021

Limited confidence

Data for this species are limited. Declines in illegal, unreported and unregulated fishing, and recent research in an exploratory fishery indicate that stocks off the Australian Antarctic Territory are healthy.

Assessment Mackerel icefish

2021

Adequate confidence

Annual surveys and stock assessments provide relatively high levels of information on current status and trends of this species at Heard Island and McDonald Islands. Current fishing levels are sustainable.

Marine invertebrates

The Southern Ocean is home to more than 5,000 invertebrate species. Antarctic invertebrate communities form a significant part of the marine food web because they form habitat, and cycle and sequester carbon and other nutrients. Marine organisms sequester inorganic carbon by converting it into organic carbon and storing it in their bodies (blue carbon). There are strong interactions between blue carbon and loss of sea ice, leading to a negative (mitigating) feedback on climate change effects. The growth season of phytoplankton occurs in summer when less sea ice covers the Southern Ocean (including through the retreat of ice shelves), allowing more light to penetrate the ocean waters than in winter. Through increased primary production by phytoplankton, more carbon reaches the seabed. Benthic communities consume plankton that has died and sunk to the bottom of the ocean. As benthic communities grow, age and die, the carbon is sequestered into the sediments and stored (Barnes et al. 2018). This occurs mostly in deeper shelf waters, but also in some shallow areas. Thus, climate change can lead to a greater carbon drawdown in Antarctica through this benthic–pelagic coupling and the growth of the benthic communities.

Where glaciers retreat and ice shelves disintegrate, new habitat opens that is likely to be colonised by algal and invertebrate communities, increasing the drawdown and storage mechanism. However, as Antarctica warms, more glaciers and ice shelves calve off icebergs. Icebergs can scour the substrate (strike the seabed) in coastal shallows, or further offshore, destroying benthic communities and making the organisms available for scavengers in the lower water column (Dunlop et al. 2014). Consequently, carbon sequestration is reduced. Scouring may be constrained to areas where there are many icebergs, and where winds drive them into shallow waters (i.e. much of the Antarctic Peninsula). Giant icebergs can persist for many years; for example, iceberg A23A (1,760 km²) calved off the Filchner Ice Shelf, Weddell Sea, in 1986 and moved only about 200 km in 30 years. Scouring by giant icebergs can destroy nearly all benthic organisms; it may take decades to hundreds of years for these communities to recover, provided no further major disturbances occur (Barnes 2017). Although scouring is detrimental to benthic organisms, it also mobilises carbon, effectively fertilising affected areas and increasing primary production, particularly in areas where productivity was low (Barnes et al. 2018). At the western Antarctic Peninsula, a region where some of the most significant environmental changes are occurring, scouring may mobilise about 80,000 t of carbon per year to the marine environment. However, carbon storage is disrupted where benthic communities do not have the opportunity to recover – for example, as a result of ongoing iceberg calving and continued scouring (Barnes 2017).

Benthic and pelagic marine invertebrates will be affected by warming ocean temperatures, acidification, sediment input from melting glaciers and changes in nutrient cycling associated with climate change; they will also be affected by commercial fishing activities and invasive species (Brasier et al. 2021).

Benthic invertebrates

The bottom of the Southern Ocean offers rich habitats for many species, which grow much more slowly than their temperate equivalents. Invertebrate taxa living on the continental shelf (0–1,000 m deep) and in the deep ocean (more than 1,000 m deep) encompass 7,210 known species (De Broyer & Danis 2011). At depth, environmental conditions are stable, and species communities and assemblages do not appear to change much. Sea spiders, sea urchins, marine worms, molluscs, sponges and other creatures are highly diverse, with a substantial percentage of endemic species (Brandt et al. 2006, Rapp et al. 2011). The benthic invertebrate communities of Antarctica, especially those living outside the intertidal zone – for example, in the high Antarctic – exist in a very stable environment where temperatures fluctuate as little as 1.5 °C throughout the year (Peck 2005). These stenothermal environments (i.e. with a narrow temperature range) came into existence about 4–5 million years ago as the waters surrounding Antarctica cooled (Pörtner et al. 2007).

It is difficult to predict how warming of the ocean may affect organisms that have adapted to live in a very narrow temperature range. Many invertebrates die or cannot perform crucial biological activities when temperatures are raised 5–10 °C (Pörtner et al. 2007). However, these results are based on experiments during which temperatures are increased rather quickly compared with rates of change expected in nature. The more gradually environmental change occurs, the better are the chances for at least some species to adapt to the changing conditions.

Ocean acidification will have different effects on the various species of invertebrates (Dupont et al. 2010, Hancock et al. 2020). Experimental work on temperate marine organisms has demonstrated a wide variety of responses, ranging from potentially positive effects, such as increased metabolic rates in autotrophs (organisms that produce their own food from inorganic sources) to negative effects, such as decreased growth rates and increased mortality (Hendriks et al. 2010, Hancock et al. 2020, Brasier et al. 2021).

For example, fertilisation of the Antarctic nemertean (ribbon) worm (Parborlasia corrugatus) may not be affected by higher acidity, and experimental work showed that egg development appeared resilient when seawater pH was reduced to neutral. However, abnormalities occurred at a later (blastula) stage of the embryos’ development (Ericson et al. 2010). Although the pH changes that produced the abnormalities are not predicted to occur soon (i.e. by 2100), they are expected if the oceans continue to acidify in the long term (i.e. beyond 2100) (Ericson et al. 2010). Other factors, such as temperature and nutrient availability, also play a part.

Plankton

The deep, turbulent, cold Southern Ocean separates Antarctica from the other continents. Although upwelling currents around the continent deliver nutrients from the deep to the surface waters, plankton (pelagic organisms) plays a vital role in the biological pump, a mechanism through which atmospheric carbon is sequestered and eventually delivered to the ocean floor. Extraordinarily huge masses of phytoplankton (photosynthetic microbes) convert about 90 gigatonnes of dissolved carbon dioxide per year into organic carbon (Cavan et al. 2019).

Krill is among the largest and most ecologically important species of pelagic invertebrates (zooplankton). Near the surface of the ocean, grazers such as Antarctic krill consume phytoplankton and other microorganisms, and are themselves eaten by higher-order predators. These abundant food resources sustain large numbers of predators, including whales, seals, seabirds, fish and squid.

Krill species also play an important role in the nutrient cycling of the Southern Ocean, simply because of their extraordinary abundance. Although the total biomass of Antarctic krill is not easy to estimate, combining acoustic and trawl data provides estimates of krill biomass of about 215 million tonnes (Atkinson et al. 2009). The movement and action of this mass – including daily vertical migration, fast swim speed, rapidly sinking faecal pellets and huge grazing capacity – make krill a critical influence in the stimulation of primary productivity and movement of nutrients through the water column.

Krill populations are already under pressure through increasing ocean temperatures and changes in sea ice cover. Juvenile (larval) krill continues to feed during winter to survive and recruit to the stock during the following spring. The main food source comprises algae that grow on the underside of the sea ice. The dependence of juvenile krill on sea ice algae makes new cohorts particularly vulnerable to reductions in sea ice extent and duration (Bernard et al. 2019). During summer, krill still consume sea ice algae but also use other carbon sources such as pelagic diatoms. This is particularly true for the crystal krill, an omnivorous krill species. Thus, although krill may be more adaptable to changing summer conditions, in winter their reliance on food resources linked to sea ice is a major limitation to their survival (Kohlbach et al. 2019). The combined effects of these pressures and an increase in ocean acidification could significantly compromise krill recruitment within a century (Kawaguchi et al. 2013).

With regard to commercial krill fishing operations off East Antarctica, broadscale surveys of Antarctic krill are conducted at roughly 10-year intervals, and enable the assessment of status. However, the assessment of trends is challenging. Although catches are currently well below sustainable levels, future impacts of climate change will need to be considered.