Mr. Popper may have had too many penguins, but today Antarctica seems to have too few.

Disruptions in the food supply, caused in part by warming climate, are to blame for shrinking populations of Adélie and chinstrap penguins across the West Antarctic Peninsula, a team of U.S. researchers argues. Rising temperatures alone are bad for these cool, tuxedoed birds, but the penguins’ struggles primarily stem from having too few krill to eat, the group reports online April 11 in the Proceedings of the National Academy of Sciences.

Study co-author Wayne Trivelpiece has kept a close eye on penguins off the tip of the West Antarctic Peninsula, which points fingerlike at Argentina, since the 1970s. Winter temperatures there and in the nearby Scotia Sea have climbed a whopping 5 to 6 degrees Celsius in recent decades.

In the early 1990s, Trivelpiece and his colleagues argued that shrinking ice masses might be a mixed blessing for penguins. Icebound Adélies (Pygoscelis adeliae) were expected to hurt, while the more free-roaming chinstraps (Pygoscelis antarctica) would hunt better with less ice. Instead, from the 1980s on populations of both species plummeted by more than half across the study sites.

As scientists dug into the mystery, it became clear that penguins were just the tip of the iceberg: “It was actually the penguins that pointed us, that said something has radically changed,” says Trivelpiece, an ecologist at the National Oceanic and Atmospheric Administration’s Southwest Fisheries Science Center in La Jolla, California.

In fact, the entire Scotia Sea seems to have had the rug pulled out from underneath it. The rug, in this case, is krill. Numbers of these tiny crustaceans, the bottom-most animals in marine food webs, have dropped by up to 80 percent throughout the region. Some of that has to do with whales and seals — many of these krill-eating species have resurged since the end of Moby Dick–era hunting.

But, Trivelpiece says, the story also comes back to ice. Young krill grow big and fat while hiding under ice masses. Less ice means less krill, and that means both Adélies and chinstraps go hungry. Ironically, the empty buffet is especially bad even for chinstraps, Trivelpiece argues, since — unlike Adélies — these birds don’t live elsewhere in the Antarctic. So although these birds were once thought to represent climate change’s silver lining, he says, today “they’re likely to be one of the more impacted of all.”

Krill are important, but the problem may go deeper, says Oscar Schofield, a biological oceanographer at Rutgers University in New Brunswick, New Jersey. Marine crustaceans, for their part, gorge on tiny photosynthesizing organisms called phytoplankton.

Schofield’s research hints that climate change in the West Antarctic Peninsula may be similarly knocking out this critical bottom rung of the food chain. “Very small changes in the ocean and the atmosphere can have profound impacts on ecosystems,” he says.

Nevertheless, penguins may still be very picky about their ice, says William Fraser, an ecologist at the Polar Oceans Research Group in Sheridan, Montana. He studies penguins living south of Trivelpiece’s broods, and there some small chinstrap populations are growing. These chinstraps may be enjoying global warming’s summer of love, Fraser says.

Still, Trivelpiece is confident that much of the West Antarctic Peninsula’s marine food web is unraveling. If his team hadn’t followed the penguins, the problem may have festered unnoticed for some time, he says. “We’re very fortunate to have shown up about a decade before everything went to hell in a handbasket.”

Image: Scientists say that as krill numbers drop across the West Antarctic Peninsula, Adélie penguins, which depend on the tiny crustaceans for survival, may be going hungry. (Martha de Jong-Lantink/Flickr)

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