The Role of Giant Petrels in the Southern Ocean Ecosystem

Summary

Giant petrels are the dominant scavenging seabird species in the Sub-Antarctic and maritime Antarctic ecosystems. They consume large amounts of penguin carrion, but also include significant numbers of seals, Antarctic Krill, Euphausia superba, and small petrels in their diets. Using results of detailed dietary studies at Bird Island, South Georgia, and qualitative data for other localities, notable variation in the quantitative impact of Giant petrels on their prey is revealed. Energy budgets of chicks are calculated from data on meal size and feeding frequency at all stages of growth. Differences in energy requirements between the Northern Giant Petrel, Macronectes halli, and the Southern Giant Petrel, M. giganteus, and between male and female chicks, are demonstrated. Published and unpublished data on numbers of giant petrels at known breeding sites are reviewed, and the present world breeding population is concluded to be ca. 8,600 pairs of M. halli and ca. 38,000 pairs of M. giganteus. Using survival and demographic data for South Georgia, total world non-breeding populations of ca. 26,000 and ca. 113,000 birds are calculated for M. halli and M. giganteus, respectively. The energy consumption of these populations in the breeding season is assessed and, taking into account geographical variation in diet, their impact on local ecosystems is evaluated. The approximations and assumptions of these estimates are discussed.