Wildlife populations have been exposed to change in their environment throughout the history of our planet. Some extreme events led to mass extinctions. However, other natural changes—for example, changes in atmospheric CO2—have taken place slowly for centuries or longer, enabling certain vertebrate species to evolve adaptive traits (Würsig et al. 2002). In contrast, current climate change is occurring at an unprecedented rate, leaving many species vulnerable because their capacity to adapt operates much more slowly than the rate of climate change. Also, the changes are not constant, but often vary with region, and may differ in their timing and scale.
Species differ significantly in their ability to adapt because of differences in their physiology, generation time, longevity, reproductive output and success, and more. It is difficult to predict with certainty how species will react to changes in their environment, or to changes in interspecies interactions. Birds, for example, tend to breed most successfully when their food supply is abundant at a certain time of year. If food becomes less available at key times (e.g. onset of breeding, fledging or weaning of young), the survival of the species can be affected, particularly when this change in availability occurs in the long term. Sudden changes in habitat promoted by extreme events may, in some cases, advantage a population. For example, a study of Adélie penguins in the Ross Sea suggests that this species can be advantaged by reductions in sea ice cover (Lescroel et al. 2014). In contrast, extensive sea ice concentration greatly reduced the breeding success of several flying seabird species (Barbraud et al. 2015). In general, inherent differences between species, plus a lack of understanding about how various environmental factors may interact, currently make it difficult to predict the fate of particular species and populations with certainty (Würsig et al. 2002).
With the level of climate change predicted under a business-as-usual scenario, organisms are likely to encounter significant changes to their environment, which may alter species composition and abundance in various regions. Organisms can react to their changing environments in 3 main ways:
- Species shift to areas where the conditions are similar to those they encountered previously and where adaptations are not required. Movement of species on the Antarctic Peninsula is possible: as the northern parts become warmer, affected species may move further south. However, the size of the Antarctic continent and access to food limit how far they can go. In the southern Indian Ocean, wildlife populations breeding on the subantarctic islands have far fewer options to move south, because there is no intermediate location between the islands and the Antarctic continent. Thus, if they were to shift their distribution, they may have to endure colder conditions than they have so far experienced.
- Species adapt to live under warmer and perhaps more marginal conditions at their current breeding locations. This might require a shift in their behaviour and/or physiology to allow them to adjust—for example, the timing of their breeding season, the growth rate of their offspring or even the age of first breeding. Some of these changes would require a change in their genetic makeup. Which strategy species ultimately choose depends on their degree of adaptability, as well as the rates of change of the various parameters (Lagos et al. 2015).
- If species fail to move or to adapt to their altering environment, they will become extinct (Learmonth et al. 2006). This would be an undesirable outcome because biodiversity would be reduced. Some species are clearly more threatened by environmental changes than others.
A further threat to marine organisms is the occurrence of plastic in their environment. Plastic debris has been polluting Earth’s oceans for decades and has even been detected in the sediments of the deep ocean (Van Cauwenberghe et al. 2013). The total amount of debris afloat is difficult to estimate, but beach surveys on subantarctic islands—for example, Macquarie Island—have indicated that monthly accumulation rates of plastics are increasing (Eriksson et al. 2013). Entanglement in, and ingestion of, plastics have long been recognised as serious hazards to wildlife, but the extent to which they occur is largely unknown and requires further investigation (Bravo Rebolledo & Franeker 2015). A technical report recently published by the Secretariat of the Convention on Biological Diversity demonstrated that 80 per cent of wildlife encounters with marine debris involved macroplastics (pieces of plastic more than 5 millimetres in diameter) and 11 per cent involved microplastics (pieces of plastic less than 5 millimetres in diameter) (Secretariat of the Convention on Biological Diversity 2012). Macroplastics can harm, and often kill, wildlife. But the effects of microplastics tend to be less obvious. Because microplastics are very small, they are bioavailable (i.e. they can be ingested), mainly by marine invertebrates. Thus, microplastics enter the food web near its base and can be transferred through the web by secondary ingestion by higher organisms (Seltenrich 2015). Microplastics can leach toxins and can concentrate certain persistent organic pollutants (POPs) (Wright et al. 2013). In seabirds in the north Pacific Ocean, chemicals originating from plastic particles appear to have been transferred into the birds’ tissues (Tanaka et al. 2013). The potential impacts of transfer of microplastics and their accumulation requires further study.
POPs and heavy metals are among the tens of thousands of pollutants that have reached the world’s oceans. Some POPs have reached the Southern Ocean through long-range transportation through the atmosphere (Galbán-Malagón et al. 2013), but Antarctic stations are also likely sources (Wild et al. 2014). Heavy metal contamination can also occur because of human activity on stations. For example, mercury, lead and other metals (probably originating from paint) were found in the tissues of Antarctic clams (Laternula elliptica) (Vodopivez et al. 2015). Like microplastics, heavy metals and POPs can enter the food web. For example, 33 different POPs were detected in the blood of wandering albatrosses from Crozet Island in the southern Indian Ocean (Goutte et al. 2013, Carravieri et al. 2014). A variety of POPs were also found in organisms as diverse as krill, snow petrels (Pagodroma nivea), benthic organisms and fish off the coast of Adélie Land, East Antarctica (Goutte et al. 2013).
Ocean acidification is likely to have severe biological impacts within decades, and could dramatically affect the structure and function of marine ecosystems (Feely et al. 2004, Doney et al. 2009a, Hutchins et al. 2009, Orr et al. 2009, Dupont et al. 2010, Ericson et al. 2010). Such changes would have profound effects on ecosystem services, including the productivity of fisheries. These changes are most pronounced in the polar regions, where the acidity of the water is changing twice as fast as in the warmer tropical and subtropical regions.
Antarctic invertebrate communities form a significant part of the marine food web. They drive geobiochemical cycles and detoxify the marine environment. The various species of invertebrates will be affected differently by ocean acidification. 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 in sea urchins (Hendriks et al. 2010).
Ocean acidification affects the life stages of organisms in different ways (Dupont et al. 2010). 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 the pH of sea water (which is normally alkaline) was reduced to neutral (Ericson et al. 2010). However, abnormalities occurred at a later stage (blastula stage) of the embryos’ development (Ericson et al. 2010). Although the pH changes that produced the abnormalities are not predicted to occur by 2100, they are expected if the oceans continue to acidify beyond 2100 (Ericson et al. 2010). Other factors, such as temperature and nutrient availability, also play a part.
Another factor potentially influencing microbe communities is increased ultraviolet B (UVB) radiation because of ozone depletion (UNEP 2016). Different species appear to respond differently to UVB stress. In the Southern Ocean, UVB penetrates to a depth of approximately 12 metres (Tedetti & Sempere 2006). Thus, plankton communities in shallow waters are subject to higher UVB radiation than those in deeper waters. Debate still exists about whether this exposure may lower primary production in the Southern Ocean (Moreau et al. 2016). Under increasing UVB radiation, the microbial species composition has shifted towards more UVB-resistant species, causing changes in the composition of microbial communities and production. This may influence the food availability of organisms at higher trophic (food web) levels. Furthermore, under experimental conditions, the concentration of phytoplankton and their cell sizes decreased when they were exposed to high UVB stress (Davidson & Belbin 2002). In the Southern Ocean, phytoplankton is largely protected from radiation by a layer of snow or ice, even where that layer is thin. As sea surface temperatures rise, early melting of the ice will expose phytoplankton to higher levels of solar radiation for longer periods (Hader et al. 2007).
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 (those 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 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.