Susan G. Trivelpiece

Variability in krill biomass links harvesting and climate warming to penguin population changes in Antarctica

The West Antarctic Peninsula (WAP) and adjacent Scotia Sea support abundant wildlife populations, many of which were nearly extirpated by humans. This region is also among the fastest-warming areas on the planet, with 5–6 °C increases in mean winter air temperatures and associated decreases in winter sea-ice cover. These biological and physical perturbations have affected the ecosystem profoundly. One hypothesis guiding ecological interpretations of changes in top predator populations in this region, the “sea-ice hypothesis,” proposes that reductions in winter sea ice have led directly to declines in “ice-loving” species by decreasing their winter habitat, while populations of “ice-avoiding” species have increased. However, 30 y of field studies and recent surveys of penguins throughout the WAP and Scotia Sea demonstrate this mechanism is not controlling penguin populations; populations of both ice-loving Adélie and ice-avoiding chinstrap penguins have declined significantly. We argue in favor of an alternative, more robust hypothesis that attributes both increases and decreases in penguin populations to changes in the abundance of their main prey, Antarctic krill. Unlike many other predators in this region, Adélie and chinstrap penguins were never directly harvested by man; thus, their population trajectories track the impacts of biological and environmental changes in this ecosystem. Linking trends in penguin abundance with trends in krill biomass explains why populations of Adélie and chinstrap penguins increased after competitors (fur seals, baleen whales, and some fishes) were nearly extirpated in the 19th to mid-20th centuries and currently are decreasing in response to climate change.

Ecological Segregation of Adelie, Gentoo, and Chinstrap Penguins at King George Island, Antarctica

Ecological segregation among Adelie, Gentoo, and Chinstrap penguins dur- ing summer results from differences in breeding chronology, foraging behaviors, and life history tactics. To determine the importance of these factors in segregating the niches of the three species, we collected data on their population size, breeding success, breeding chronology, feeding frequency, and foraging range. Gentoo Penguins feed inshore and are deep divers. Their small populations probably reflect the comparatively large amount of food they need to rear chicks and the small foraging range that is dictated by short nest relief schedules and nonfasting behaviors. Their deep diving ability enables them to exploit a niche that is unavailable to their more abundant congeners. Adelie and Chinstrap penguins are shallow-diving, offshore foragers that avoid competition by differences in breeding chronology, prebreeder behaviors, and molting strategies. Adelie chicks fledge just as Chinstrap chicks reach creche age. Different migration times and molting locations further reduce niche overlap. The ultimate factors responsible for this trophic segregation may be unrelated to these proximate factors, however. Nonmigratory behavior, short nest reliefs, nonfasting, and slow growth of chicks may be adaptations to the mild conditions experienced by Gentoo Penguins in their sub-Antarctic range. The differences in Adelie and Chinstrap breeding time may reflect adaptation of Adelies to early breeding in the harsh, short summer of the continental Antarctic, and adaptation of Chinstraps to later breeding in the milder Maritime Antarctic. We found no evidence to suggest that the niches of the Pygoscelis penguins were influenced by competition for food, and suggest that adaptation to conditions in the center of their respective distributions is the primary cause affecting ecological segregation in areas of sympatry.