Adelie Penguins: Antarctic climatic indicators

Sandra Frantzen


As scientists have become more aware of the factors affecting global climate change, they have found it increasingly necessary to develop methods of determining past climates for comparison with the present. Because the Antarctic is a relatively pure ecosystem, free from excessive direct human involvement, it is an optimal place to study both natural and anthropogenic effects and impacts on the global climate system. Geologists and biologists are using the locations and ages of penguin rookeries in areas of the Antarctic to predict ancient sea level and glacier changes as indicators of global environmental change. Because penguins are relatively sensitive to environmental conditions which limit their spatial distribution, the study of both ancient and modern rookeries and modern penguin populations has implications about the timing of glacier retreat, sea level change, and temperature.

The most widespread and abundant Antarctic penguins, Adelie penguins are studied because they are the most resistant to extreme environmental conditions. Adelie penguins breed in large colonies in stony coastal areas that are ice-free, allowing them access to the sea. 1 The penguins generally return to the same sites to nest each year.2 However, over the thousands of years these penguins have inhabited the Antarctic, several rookeries have been abandoned. The abandoned sites are easily identified because thin layers of sorted pebbles the penguins collected for their nests remain as relics. The age of these sites are evaluated by the amount of lichen cover on the pebbles and by 14C dating of organic remains such as guano, bones, egg fragments, and feathers.3 Because the oldest sites are located at higher elevations than modern sites (up to 30 m), scientists have estimated that sea level was about one to two meters below the ancient nesting sites. This has resulted in an estimated range for the relative sea level curve for the last several millennia: the sea has retreated at an initial rate of 23 mm/yr decreasing to 8 mm/yr in the last millennia.4 In addition, the interval and perpetuity of rookery occupation has been shown to follow glacial movement patterns. During the last glacial maximum, the Terra Nova Bay region (where many of the rookeries are located) was occupied by glaciers much larger than the present ones. The gradual movement of rookeries outwards along the coast indicates that this ice shelf has retreated about 200 km southward in less than 3000 years.

While several rookeries were occupied for thousands of years (the oldest known continually occupied colony is at least 6335 years old), others were used for shorter periods and quickly abandoned.6 The main limiting factors appear to be availability of food, location of sea ice, and climatic conditions. A "penguin optimum", occurring about 3000 to 4000 years ago, has been correlated to the glacial advance in the Northern and Southern Hemispheres found in ice core readings. This peak in rookery numbers indicates the presence of open waters close to the shore in the summer and a reduction (or total disappearance) of a nearby ice tongue. Apparently emulating these environmental conditions on a smaller scale, other abandoned rookeries found along the Victoria Land Coast were dated between 800-1400 A.D., correlating to the Medieval Warm Period.

More recent and shorter-term trends in global climate change have also been observed with Adelie penguin populations. Because persistent sea ice and bad weather during the summer lead to poor breeding success, populations decline in seasons when ice is slow to break and in seasons with December temperature anomalies. Because these population fluctuations have been in phase with contemporary rookeries throughout the Ross Sea region, scientists suggest they are the result of "wide-ranging environmental perturbations affecting the whole of the Ross Sea." 8 Between 1980 and 1990, the total number of breeding pairs of Adelie penguins increased from about 616,500 to 1,082,000 in the Ross Sea area. This dramatic increase has been attributed to the recent warming trend in coastal east Antarctica and the Southern ocean which may have improved nesting success and reduced mortality of breeding adults. Decreasing ice coverage increased the availability of food, improving the chances of younger penguins. The correlation between population and temperature is reasonable since Adelie penguin populations at the Antarctic Peninsula, where the warming trend has been far less conspicuous, have been relatively stable.

It is evident that the use of Adelie penguin rookeries as climatic indicators for both modern and ancient times has become a viable method for scientists to confirm other climatic information sources. While the rookeries do follow predicted trends, it seems unclear, especially for ancient rookeries, whether the locations are definitively the result of global climatic changes or other local environmental or ecosystem changes which are yet undetected. In addition, pinpointing the exact lifetimes of rookeries has been difficult because Antarctic waters are diluted by glacier melt, causing 14C levels in marine organisms to be depleted. Therefore, while it may be useful to look to rookeries for additional information regarding climatic trends, it seems risky to make conclusions entirely on these results. Perhaps rookery locations used in conjunction with the fauna line and other ecosystem relics will give a complete and more accurate picture of the global climate record in the Antarctic.

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