Sea Change: People have affected what penguins eat

The eating habits of Adélie penguins in Antarctica changed significantly about 200 years ago, according to chemical analyses of the birds’ eggshells. Scientists attribute the shift in diet to whaling and other hunting in the region.

The ratios of carbon and nitrogen isotopes in an animal’s tissues—including bones and eggshells—can provide a wealth of information about its eating habits, says Steven D. Emslie, a paleontologist at the University of North Carolina in Wilmington. Recently, he and William P. Patterson, a geochemist at the University of Saskatchewan in Saskatoon, looked at the chemical composition of Adélie penguin eggshells laid during the past 38,000 years to see whether the birds’ dietary habits had changed.

Surprisingly, says Emslie, climate change 10,000 years ago, at the end of the latest ice age, didn’t significantly affect the birds’ diet. In the past 200 years, however, the chemical composition of the penguins’ eggshells made a dramatic shift to lighter isotopes of carbon and nitrogen. Because animals higher in a food chain hold greater concentrations of heavy isotopes, the change is clear evidence that the penguins’ diet shifted from primarily fish to prey such as krill.

The dietary change boils down to the availability of prey, Emslie and Patterson speculate in the July 10 Proceedings of the National Academy of Sciences. During the 19th century, the population of krill in southern seas exploded after Antarctic fur seals, prodigious consumers of krill, were hunted nearly to extinction. That slaughter, followed by widespread killing of krill-eating whales during the 20th century, enabled the tiny crustaceans to proliferate nearly unchecked, says Emslie.

“It’s rare to see such catastrophic changes [in diet] in such a short period,” says Keith A. Hobson, an ecologist at Environment Canada in Saskatoon. The changes “point to a large shift in the ecosystem,” he notes. Even so, Hobson adds, it’s not clear why abundant krill would cause penguins to suddenly shift from fish to what had previously been a secondary food source.

The team’s results are “very compelling evidence of a terrific change” in penguin diet, says Charles H. Peterson, a marine ecologist at the University of North Carolina’s Institute of Marine Sciences at Morehead City. “It’s a cosmic irony of food-web ecology that a rare species is only rare because it’s kept in check by predators,” he adds. “Maybe krill was one of [the penguins’] favorite foods all along.”

Modern-day fishing around Antarctica has depleted fish stocks there. Meanwhile, krill populations have declined as much as 80 percent in the past 2 decades. Understanding why penguin diets changed 2 centuries ago may be vital for their future survival, says Emslie.