The number of Adelie and chinstrap penguins living on the Antarctic Peninsula has plummeted by more than half during the past 30 years.
Scientists once believed that climate change would create a stark contrast between the two species. Ice-loving Adelies, which winter on sea ice, would see their numbers dwindle as their habitat warmed, the thinking went. Ice-avoiding chinstraps, which prefer open water, would thrive.
But the new study suggests that picture was only half right. Populations of both penguin species have plummeted in recent years, which the research blames on the loss of the tiny, shrimp-like krill that are a staple food for both birds.
The amount of krill near the Antarctic Peninsula has fallen by 38 to 81 percent over the past three decades as the region has warmed. With less food available, fewer young Adelie and chinstrap penguins alike survive to maturity, said the new study's lead author, wildlife biologist Wayne Trivelpiece of the National Oceanic and Atmospheric Administration.
"As krill stock started to decline dramatically with the lack of sea ice in the late 1980s, so did the survival of these young," said Trivelpiece, whose work will be published online this week by the Proceedings of the National Academy of Sciences.
The study builds on decades of work by scientists studying Adelie and chinstrap penguins in the West Antarctic Peninsula and the adjacent Scotia Sea.
Trivelpiece, who began studying the birds in 1976, said the relationship between the penguins and their habitat appeared simple at first.
"Whenever we had a year when Adelie numbers went way up, chinstrap numbers dropped -- and vice versa," he said. That made sense to scientists, since they observed Adelies wintering on pack ice, while chinstrap penguins preferred the open water north of that pack ice.
"We proposed that these two species were responding to changes in pack ice," Trivelpiece said, "ice-loving versus ice-avoiding. The population swings were very radical and in equal amounts."
As climate change became a concern, researchers assumed that warming would favor the ice-avoiding chinstrap penguins, sending the Adelies into decline as more and more of their winter sea-ice home disappeared.
But when both species began to decline in the late 1980s, the scientists had to re-examine those assumptions. Data from banded birds eventually revealed that the rise and fall of both penguin species tracked, roughly, with the amount of krill in nearby waters.
'A whole ecosystem turned upside down'
During the last decade, the population of Adelie penguins in the studied area has declined by 2.9 percent per year, while the number of chinstrap penguins has fallen by 4.3 percent per year.
The scientists believe that sea ice is still at the heart of the story. Rising temperatures have reduced the area's sea ice cover, which serves as an important habitat not just for Adelie penguins but also for krill.
"One of the things that's always struck me is how large an effect we have from what is basically a fairly small temperature change, 5 or 6 degrees Celsius," Trivelpiece said. "We're having a whole ecosystem turned upside down. It surprised us. I suspect we have more surprises like that out there."
The relatively small temperature increase has had that outsized effect, the scientist said, because the Antarctic Peninsula "is right on the knife edge between freeze and thaw."
But climate change is not the only pressure on the region's krill stocks, which now face heightened demand from resurgent populations of krill-munching whales and seals and the rise of a commercial krill fishery in Antarctic waters.
At the same time, earlier studies hint that the penguins' feeding choices have dwindled, Trivelpiece said. Archaeological studies suggest that krill didn't become a major food source for the two penguin species until 100 to 200 years ago. But overfishing has reduced the numbers of the fish the penguins once preferred, leaving krill as the birds' major prey.