There are 18 species of penguin in the world, six of which breed regularly on the Antarctic Peninsula and across the Scotia Sea (Figure 1): Adélie [Pygoscelis adeliae], chinstrap [P. antarctica], gentoo [P. papua], king [Aptenodytes patagonicus], emperor [A. forsteri] and macaroni [Eudyptes chrysolophus]. Populations of penguins in this region have been changing over the past century (e.g., 1-2), with marked declines recorded for many colonies of macaroni, Adélie, and chinstrap penguins. For example, macaroni penguins have declined in SubArea 48.3 (3), and while Adélie and chinstrap penguins are stable or increasing in SubArea 48.1 in Marguerite Bay (2,4), they have declined throughout most of the western Antarctic Peninsula (WAP) to the north of Marguerite Bay (2). Adélie penguins are also stable or increasing in the Weddell Sea region, as well as in SubArea 48.4 (5). Gentoo penguins are increasing in abundance on the WAP in 48.1 and at the South Orkney Islands in 48.2 and are expanding their breeding range to the south (2); elsewhere in the region, gentoo populations do not appear to be trending significantly (5), though large interannual fluctuations in gentoo abundance can complicate trend assessment. King penguins have increased in SubArea 48.3 (6) and appear to be extending their range south to the South Shetland Islands in 48.1 (7), and whilst emperors have now disappeared from the Dion Islands on the WAP (8) they continue to breed near Snow Hill Island. These findings highlight considerable spatial heterogeneity in penguin population trends and the importance of comparative work across long-term study sites.
Existing datasets on Antarctic Peninsula and Scotia Arc penguins
Our understanding of penguin population dynamics on the Antarctic Peninsula and across the Scotia Sea comes from a number of long-term studies undertaken by an international community of researchers active in the region; some of these studies now cover more than 20 years (or longer) in duration. Long-term datasets on abundance, diet, demographics, and phenology include studies from King George Island (e.g. 9-11), Livingston Island (e.g. 9), Port Lockroy (12), Anvers Island and Biscoe Point (13), Cierva Point (14), Signy Island (15), Avian Island (16) and Bird Island (3). Operating across the Antarctic Peninsula and South Orkney Islands, the Antarctic Site Inventory program (17) has surveyed penguin nests and chicks (and other seabirds) since 1994 (5,17) with regular visits to many colonies in most years using tourist vessels to provide a broader geographic coverage complementary to that provided by other long-term research programmes.
Drivers of population change
Several hypotheses have been suggested to explain why penguin populations are changing, including: (i) changes in the ecosystem associated with regional climate change; (ii) changes in the marine community associated with the historical harvesting and subsequent recovery of marine mammals; (iii) impacts from the commercial fishery for Antarctic krill [Euphausia superba]; and (iv) impacts from tourism.
Environmental variability and change affect penguin population processes either directly through physiological impact, or indirectly via the distribution or availability of their mid-trophic-level prey. On short time scales (days to years), breeding behaviour, as well as breeding success and survivorship rates, can be affected by random events (such as unusual storm events), seasonal variability, or interannual variability in environmental conditions. Over longer periods of time (years to decades), persistent changes can drive shifts in distribution and abundance, which are easily detected by long-term survey programmes (2,18-19).
Large-scale changes in marine ecosystems resulting from historical sealing, whaling and fishing, make it difficult to distinguish between the influence of climate change and the influence of marine resource extraction (20-21). Indeed, it is highly probable that no unifying solution will be found to account for all observed changes in penguin populations. Different drivers of change may be impacting different species and operating at different temporal and spatial scales, that range from that of a single colony, to the regional scale, or even to the circumpolar scale. Nevertheless, there is some compelling evidence that indicates climate change can have a direct effect on penguin abundance and distribution. Increased snowfall resulting from increased warm, wet conditions may have contributed towards Adélie population declines close to Palmer Station (20), a feature that may also affect populations in the South Shetlands. Colonies that have experienced more snow accumulation have decreased more rapidly than penguin colonies where wind scour abates snow accumulation. Similarly, gentoo penguins breed in areas with <50% sea ice concentration in the austral spring and following the decline in spring sea ice along the WAP, they have expanded their breeding range southward into new areas (2).
Predicting and monitoring future change
Because it is difficult to disentangle the effects of climate change from those of past and present resource extraction, making future predictions about penguin population changes may be more complex than originally envisaged. As climate change might potentially impact penguins at any or all of their breeding sites, foraging sites, moulting sites or wintering sites, a broader understanding of the spatio-temporal dynamics of penguin life-history and their ecological interactions will be necessary. Long-term studies of fixed populations (e.g. Palmer Station’s Long-Term Ecological Research program, CCAMLR’s Ecosystem Monitoring Program, etc.) have played an essential role in our current understanding of changes in penguin population dynamics. In addition opportunistic vessel-based surveys (e.g. 5,17) have provided a wealth of data on penguin abundance and distribution that have been critical for tracking range shifts and colonization/extinction dynamics (2). New technologies, such as remote sensing imagery, unmanned-aerial vehicles (UAVs), animal tracking devices (satellite- or light-based geolocators), and automated individual identification systems (radio frequency identification), as well as remote automated camera systems, are now available and increasingly capable of providing important details about abundance, life cycle, demography and distribution on land and at sea (foraging and migration), at local and regional scales. Integration of these disparate data streams, and their analysis alongside complementary datasets on krill abundance (and availability to foraging penguins) and environmental conditions (e.g. sea ice), will be required to disentangle confounding drivers of change and predict better the future of penguins along the Antarctic Peninsula. Focused study regions, including reference areas where fishing is restricted, would help reduce the complexity of interacting drivers (15).