Adélie penguin populations have dropped precipitously over the last 50 years due to a reduction in their major food source — krill. Young krill feed on algae living on the underside of sea ice. Reduced sea ice means fewer krill. (Photo by Mike Polito, Woods Hole Oceanographic Institution)

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Adélie penguin populations have dropped precipitously over the last 50 years due to a reduction in their major food source -- krill. Young krill feed on algae living on the underside of sea ice. Reduced sea ice means fewer krill. (Photo by Mike Polito, Woods Hole Oceanographic Institution)