Antarctica's wildlife in a changing climate
Despite being one of the coldest, most inhospitable places on Earth, Antarctica hosts a wealth of biodiversity, and its remoteness and extreme climate have lent a certain amount of protection to the many species that call the continent and its surrounding waters home. The Antarctic has not been immune to human influences, however, and its fauna is suffering from a range of human impacts, both past and present.
In the late 1700s, early explorers decimated many seal populations, almost wiping out species like the Antarctic fur seal. Large-scale whaling in Antarctica began in the early 1900s and similarly saw the populations of many species nearly become extinct. These species are still recovering and Antarctica's ecosystems continue to be shaped by the massive human-induced population declines that occurred in the past.
Since the 1950s, overharvesting of fish and krill has also altered these populations, and ongoing overharvesting remains of large concern.
Other stressors include pollution in the form of persistent organic pollutants (POPs), particularly DDT, which, even though released into ecosystems thousands of miles away and in many cases decades ago, concentrate in polar regions and bio-accumulate up the food chain. The impacts of these pollutants on Antarctic species is unknown, but they have been shown to have deleterious affects on humans and wildlife.
Invasive species are also of large concern, as is climate change. The polar regions are warming faster and experiencing more dramatic changes than anywhere on the planet. In Antarctica, climate change is having a range of impacts, from ocean acidification (which is more pronounced in colder waters) to warming temperatures (the Western Antarctic Peninsula is one of the fastest warming places on the planet), and loss of sea ice.
Changing Sea Ice
Sea ice serves as the basis for Antarctic ecosystems and "most organisms' life cycles in this region are attuned to ice seasonality," according to a 2013 paper. The large phytoplankton blooms that occur in the spring and summer attract species from around the globe to feed on the krill that these blooms support. In Antarctica, a wide array of species is dependent on krill, including penguins, seabirds, seals and whales.
Krill are tiny shrimp-like crustaceans that act as a keystone species in Antarctica, meaning that they are critical to ecosystem functioning, but a 2010 study found that the density of krill populations has declined 80 percent since the 1970s. This is due to a variety of factors, including over-harvesting, sea ice loss, and ocean warming and acidification.
Krill and other Antarctic species are highly dependent on sea ice, which has declined dramatically in some areas of the Antarctic Ocean. Says Hugh Ducklow, a biological oceanographer at Columbia University's Lamont-Doherty Earth Observatory, "The foundation of the ecosystem is melting away."
Some species, like Emperor and Adélie penguins, are considered to be "ice-loving," or ice-dependent, while others, like Gentoo or Chinstrap penguins, are said to be merely ice-tolerant. And while it is difficult to generalize for all populations of these species, Adélies have decreased in population with the loss of sea ice, and it is possible that Emperors will follow suit. Gentoo and Chinstraps, on the other hand, have increased their range in parts of the Western Antarctic Peninsula.
Adélie penguins—one of four penguin species that breed on the Antarctic continent—are dependent "in a complex way on sea ice for foraging, resting, moulting and migrating."
Changes in sea ice, therefore, may significantly impact penguin populations. According to a 2014 study, for instance, Adélies are having more trouble feeding as sea ice retreats in areas of the Western Antarctic Peninsula. Too much sea ice, however, and the Adélies have to travel further to the coast to find food, resulting in fewer chicks surviving to adulthood.
According to the study, "If summer pack ice disappears in the northernmost Antarctic regions, Adélie penguins will be forced to reduce their geographic range southwards, as is occurring in the rapidly warming Western Antarctic Peninsula region." The Ross Sea is the southernmost marine ecosystem on Earth and may become a "last refuge" for Antarctic penguins.
At the Palmer Research Station on the peninsula, Adélie penguin numbers have declined 65 percent over the past 25 years and are being replaced by Chinstrap and Gentoo penguins.
The whole of this decline cannot be attributed to any one factor but is instead the result of a complex mix of impacts. The findings of a 2013 study, for example, "suggest that models of population change based on food web processes alone may be insufficient to account for the observed variability" in penguin populations in the Palmer Long Term Ecological Research (LTER) region along the western Antarctic Peninsula.
The five Adélie penguin colonies in this area have seen an 83 percent decline in breeding pairs since 1974 and one colony, which had existed for 500 years, went extinct in 2007. The researchers found breeding habitat quality, which can be strongly influenced by weather and climate, to be an important factor in the population decline.
Emperor penguins have seen similar losses as a result of a changing climate. A 2001 study found that Emperor penguin populations had declined 50 percent over the previous 50 years in Terre Adélie, Antarctica, and that an unusually long bout of high temperatures in the late 1970s contributed to the decline.
Emperor penguins are particularly vulnerable to changes in sea ice, as this impacts the availability of krill. Not enough sea ice and there's not enough food. On the other hand, too much sea ice can negatively affect reproduction. When there is more sea ice, female penguins have to travel further to find food for their young. The male is left guarding the egg, but if the female is gone too long, he may abandon the egg or chick and go in search of food to avoid starvation.
A 2014 study of Emperor penguins in Terre Adélie found that they are "fully deserving of endangered status due to climate change," for which they are up for consideration. According to lead study author Stephanie Jenouvrier, a biologist with the Woods Hole Oceanographic Institution, "at least two-thirds of the colonies are projected to have declined by greater than 50 percent from their current size by 2100. None of the colonies, even the southern-most locations in the Ross Sea, will provide a viable refuge by the end of the 21st century."
Similarly to penguins, some species of seals and whales are considered to be ice-dependent for food and reproduction, while others do better in ice-free conditions.
Crabeater, Weddell, Ross and Leopard seals are ice-dependent species, reliant on sea ice for reproduction and foraging. Southern Elephant and Fur seals, however, are not as dependent on the ice for feeding, wintering and breeding.
Whale species like Minke and Killer are closely connected to the ice, while Humpback and Blue whales feed in Antarctica in the austral summer but head north for warmer waters to breed.
While data on whale and seal species remains more patchy than data on penguins, according to a 2012 synthesis paper, "Given both this dependence on krill and life histories that exhibit different affinities to sea ice, it is conceivable that WAP [Western Antarctic Peninsula] marine mammal populations have exhibited trends not too dissimilar from those…for penguins."
This is only visible for seals so far, with large increases in the ice-avoiding Fur and Southern Elephant seals, and declines in ice-dependent Crabeater and Weddell seals. However, there are many factors influence population dynamics and it would be a mistake to attribute these increases or decreases solely to any one factor.
Such changes do underscore, however, the delicate balance that exists on Antarctica and the changes that are likely to come from upsetting that balance.
Antarctica supports a complex web of life due to the nutrient-rich waters surrounding the continent. Twenty-four hours of summer sunlight spur the growth of phytoplankton, which support vast swarms of krill and in turn much of the rest of Antarctica's food web. Even though Antarctica is remote and can seem far removed, the changes that are underway will not only affect the species that live there but, because many of the continent's species are migratory, may have ripple effects for marine biodiversity in oceans around the globe.
William Fraser et al. A Nonmarine Source of Variability in Adélie Penguin Demography, Oceanography (2013). DOI: 10.5670/oceanog.2013.64
Christophe Barbraud et al. Emperor penguins and climate change, Nature (2001). DOI: 10.1038/35075554
Stéphanie Jenouvrier et al. Projected continent-wide declines of the emperor penguin under climate change, Nature Climate Change (2014). DOI: 10.1038/nclimate2280