Adélie penguin population change in the pacific sector of Antarctica: relation to sea-ice extent and the Antarctic Circumpolar Current

Abstract

One of the longest continuing data sets involving a marine organism in the Antarctic is that of annual estimates of breeding population size of Adélie penguins Pygoscelis adeliae at colonies on Ross Island, Ross Sea, 1959 to 1997. The sizes of these colonies have displayed significant interannual variability during the 29-yr period. We hypothesized that changes are related to natural environmental factors; and used path analysis to analyze annual variation in population growth in relation to physical environmental factors during that part of the record with comparable sea-ice satellite imagery from 1973 to 1997. The Ross Sea sector of the Southern Ocean lying north of Ross Island, from 150°E to 130°W, comprised our study area. Annual population growth measured during summer was explained best, and inversely, by the extent of sea-ice in the study area 5 winters earlier, and in some way related to the Southern Oscillation. Analysis of a subset of the sea-ice data from 1979 to 1997 indicated strong correlations to ice conditions in the eastern portion of the study area (174 to 130°W), and virtually no correlations to the western half (150°E to 175°W). This result supported other indirect evidence that the Ross Island penguins winter in the eastern Ross Sea/western Amundsen Sea. A demographic model indicated that variation in survival of juveniles and subadults might account for the observed population variation, and would also explain the 5-yr lag as 5 yr is the average age of recruitment to the summer breeding population. Extensive sea-ice during winter appears to reduce subadult survival, expressed subsequently when these cohorts reach maturation. We hypothesize that extensive (more northerly) sea-ice limits access of penguins to productive waters known to occur south of the southern boundary of the Antarctic Circumpolar Current, with starvation or increased predation disproportionately affecting less-experienced birds. The observed patterns of penguin population change, including those preceding the satellite era, imply that sea-ice extent has changed significantly over recent decades.

Contrasting population changes in sympatric penguin species in association with climate warming

“…Persistent climate anomalies reducing the winter/spring sea ice extent appear to affect the recruitment and biomass of the krill exploited by penguins (see also Wilson et al, 2001;Fraser & Hoffmann, 2003); penguin population responses are also consistent with the duration of the anomalies that affect krill; these usually become manifest during the breeding season although they may operate before arrival. For example, impacts upon the number of breeding pairs could result from interactions that affect survival and/or foraging success over the winter period prior to the breeding season (Trathan et al, 1996); this may act over a number of weeks (or months) to impact upon body condition and subsequent ability to breed.…”

“…The strongly coupling of the atmosphere-ocean-sea-ice systems and the precession of ENSO anomalies reflect on sea-ice anomalies in different regions of Antarctica according to the dominant cyclical signal. These anomalies appear to affect the populations of Adélie penguins in the Ross Sea (Wilson et al, 2001). Therefore, the direct attribution of penguin trends in some regions to climate change cannot be ruled out.…”

“…Trends in Adélie penguins appear to have increased in other locations where food chains may be less Antarctic krill-dependent and local conditions may be more stringent than in the Antarctic Peninsula: in East Antarctica, in the Indian Ocean, many of the colonies near Syowa Station appear to have increased (Kato et al, 2002); and at the Ross Island, Ross Sea, colonies have also increased (Wilson et al, 2001). In these areas, Adélie penguin increases are attributed to improving spatial and temporal access to nesting and foraging habitats because of earlier sea ice breakout and the formation of polynyas close to land margins.…”

“…Extensive ice cover in late spring and early summer is a physical barrier preventing the access of penguins with lower ice tolerances to the breeding colonies (Lishman, 1985;Fraser et al, 1992;Trathan et al, 1996;Ainley et al, 1998;Wilson et al, 2001;Rombolá et al, 2003;Lynnes et al, 2004). On the other hand, regional sea ice variability limits the abundance of Antarctic krill (Euphausia superba), a dominant component of the penguin diet in some regions (Fraser et al, 1992;Fraser & Trivelpiece, 1996;Fraser & Patterson, 1997;Lynnes et al, 2002, Fraser & Hoffman, 2003Lynnes et al, 2004).…”

“…The three pygoscelid penguins, Adélie (Pygoscelis adeliae), chinstrap (P. antarctica), and gentoo (P. papua) penguins, are all affected by changes in sea ice extent (Fraser et al, 1992;Trathan et al, 1996;Ainley et al, 1998;Wilson et al, 2001;Clarke et al, 2002;Kato et al, 2002;Smith et al, 2003). The gentoo penguin is mainly subAntarctic with a subspecies confined to the Antarctic Peninsula.…”