Every year since 1979, marine biologist Wayne Trivelpiece and his wife, Susan, both of the National Oceanic and Atmospheric Administration's Antarctic Ecosystem Research Division in San Diego, California, have braved frigid temperatures and wind speeds that average 40 kilometers per hour to track the feeding, breeding, and migrating of chinstrap and Adelie penguins. During this time, populations they studied on the West Antarctic Peninsula and in the nearby Scotia Sea have declined drastically, and a few have gone extinct, victims of a warming planet that deprives them of their sea ice habitat. Now, in a compilation of over 30 years of data collected from numerous bases around Antarctica, the researchers conclude that the penguins are not only running out of room but also starving.
In 1992, the pair published a paper with ecologist William Fraser, now president of the privately owned Polar Ocean Research Group based in Sheridan, Montana, proposing what they deemed the "habitat hypothesis," the idea that melting sea ice along the Antarctic coast was harming penguins. The average temperature in the region of the Scotia Sea, one of the most rapidly warming places on the planet, has increased by 5˚C to 6˚C over the past 50 years, drastically reducing the amount of sea ice present and the length of time that the ice exists. The researchers proposed that losing their habitat was what was killing Adelie penguins, which need ice to survive. By contrast, the numbers of chinstrap penguins, which avoid sea ice as much as possible, were booming.
But by incorporating data from land-based stations and tourist ships that moonlight as penguin-counting research vessels, the researchers have expanded their data set and reexamined it. In a paper published today in the Proceedings of the National Academy of Sciences, the Trivelpieces and their colleagues propose that a decrease in krill—shrimplike crustaceans that are a staple food for penguins—is to blame for the decline in Adelie penguins and chinstrap penguins, whose populations are now shrinking by 2.9% and 4.3%, respectively, each year. As it turns out, krill larvae are as dependent on sea ice as Adelie penguins are, feeding on algae that grow on the underside of ice packs; krill numbers have dropped by 80% since 1981. Independent of whether they like ice, Adelie and chinstrap penguins like to eat krill.
Back in the 1980s, Wayne Trivelpiece says, when researchers first started noticing a rapid decline in penguin populations, it was unclear what the reason was. To check their hunch that the cause was a food shortage, they began forcing penguin parents returning from the sea to vomit by inverting them over a bucket and pushing on their stomachs. Chinstrap penguins, they and other researchers found, eat krill exclusively; Adelie penguins are dependent on it as well. Continuing research through the 1990s showed that the size of the krill was uniform, suggesting that only a few krill populations were maturing over the years and were available for penguins to eat. Satellite data showed that krill numbers fell in areas where sea ice dwindled. "As aggravating as it was to see penguins declining, it was rewarding to have finally figured out the correlation," Wayne Trivelpiece says.
He believes that the shrinking population is due to the deaths of baby penguins. After their parents leave them to fend for themselves, young penguins stand around before venturing into the sea to search for a decreasing number of krill. Without any guidance, their probability of encountering a krill and knowing what to do with it is very low. Some years, only 10% of the young penguins return, down from 50% in the 1970s. By contrast, Gentoo penguins take their young on hunting trips before abandoning them; their numbers haven't fallen as severely.
Penguins, Fraser says, are a bellwether for how global warming will harm species across the globe. "If what we've seen in 35 years is just the precursor to what occurs across the planet, it's reason to be very concerned," he says. Although he still believes that sea ice loss is responsible for penguin decline in at least some areas, he calls the new study "one of the best papers I've read in quite a while so far as providing a description of the complexity and issues involved" in tracking food webs in the Antarctic.
Fraser calls the data set "formidable" evidence for long-term warming trends, adding that Antarctic research is the longest database in the world of population trends in large animals. "It's a great piece of work and I'm thankful for scientists like them who make such a commitment," adds oceanographer Oscar Schofield of Rutgers University in New Brunswick, New Jersey.