Ecological Responses to Climate Change on the Antarctic Peninsula
The peninsula is an icy world that's warming faster than anywhere else on Earth, threatening a rich but delicate biological community
The top predators of the Antarctic Peninsula—the seabirds and seals—are highly sensitive indicators of ecosystem change. Because they are long-lived and wide-ranging, their life histories integrate the effects of marine environmental variability over long periods and large areas. From a more practical point of view, these species are also good candidates for the role of canary in the coal mine because they are abundant and readily accessible. They breed on land (but feed at sea), and so it is comparatively easy to gather data on their reproductive success, their population, their diet and other aspects of their biology. In some sectors of Antarctica, databases on these predators now span seven decades, making them among the best-studied wild vertebrates on Earth. Importantly, the long time series allow investigators to distinguish genuine trends from mere ecosystem noise.
On the western Antarctic Peninsula, and specifically in the vicinity of Palmer Station, one such study has been going on for more than 30 years. The subjects of the study are three closely related species of penguins, the Adélie, the gentoo and the chinstrap, collectively known as the brush-tailed penguins because of their long, stiff tail feathers.
Although the three penguins are members of the same genus, they have quite different life histories and favor different habitats. Adélie penguins are a true Antarctic species, distributed all around the coast of the continent, though only in areas where sea ice can be relied on to last throughout the winter. Chinstrap and gentoo penguins, in contrast, tend to avoid areas with persistent sea ice; they evolved in sub-Antarctic habitats where conditions favor open water with only minimal sea ice. At Palmer Station, Adélies are in decline, whereas gentoos and chinstraps are growing more abundant.
Adélie penguins have always occupied territory near Palmer Station, but gentoos and chinstraps were unknown there until recent years. The first chinstrap colony was established in 1976, and gentoos arrived in 1994. Thus biologists from the station have been able to observe the entire history of these local populations. Carbon-14 dating of material excavated from local colonies (both active and abandoned) reveals evidence of Adélie occupation going back 700 years, but no hint of the other two brush-tailed species. Thus it appears the environmental conditions promoting the local presence of gentoos and chinstraps are unprecedented in the period covered by this record.
Adélie penguin populations are decreasing throughout the mid- to northern Antarctic Peninsula, and there is wide concurrence that this regional trend is correlated with a gradual decrease in the availability of winter sea ice. However, the exact role played by sea ice in the ecology of the species has remained uncertain, primarily because few winter studies have been conducted. In a recent effort to address this gap in our understanding, the penguins' movements, distribution and foraging were monitored continuously for 24 months using satellite telemetry, at-sea censuses and extensive field collections of diet information.
A key finding of the study is that Adélie penguins find their prey in winter primarily at isolated "hot spots" where the topography of the sea floor creates upwellings of warmer water from the circumpolar current; the upwellings in turn promote congregations of krill and fish. The birds' access to these hotspots requires winter sea ice. Adélie penguins do not forage at night, and hence they cannot travel far to find food during the short days of the polar winter. Only by migrating over the sea ice can they stay close enough to their feeding grounds. As sea ice continues its long-term retreat in the waters of the western Antarctic Peninsula, Adélie penguins will lose access to the most productive winter foraging regions.
A second process implicated in the shrinking of Adélie populations is increasing snowfall over the Antarctic Peninsula. Although it may seem counterintuitive that a polar species would be adversely affected by snow, Antarctica is a polar desert with low average precipitation, and it is in this environment that Adélie penguins evolved. Snow has been increasing over the peninsula since at least the beginning of the 20th century. The loss of sea ice is one factor causing this change in weather patterns. Exposing open ocean to the atmosphere brings higher levels of evaporation and increasing cloud cover. Spring blizzards during the Adélie penguins' breeding season (November) have increased in frequency and severity. The storms kill large numbers of eggs and chicks when the snow eventually melts and floods the nests.
Some of the same factors that are negatively affecting Adélie penguins are helping chinstrap and gentoo penguins to prosper. Both of the latter species have maintained their sub-Antarctic breeding chronologies; by breeding approximately three weeks later than Adélie penguins, they reduce the risk of nest flooding in the aftermath of spring blizzards. In winter, both species forage successfully in ice-free areas. Chinstrap penguins accomplish this by migrating north beyond the sea-ice zone. Gentoo penguins winter at their summer breeding colonies, but they choose sites for these colonies close to areas where fast currents or an upwelling of warmer water ensure that sea ice does not persist.