All penguins are meat-eaters and hunt krill, a tiny shrimplike animal, and fish.
But commercial overfishing of the seas around Antarctica, in particular, is putting increasing pressure on fish stocks and thereby reducing food for penguins. Scientists warn that increased harvesting of Antarctic krill and fish could trigger a catastrophic collapse in the entire marine ecosystem.
Paul Ponganis, a one-time National Geographic Society grantee, studies emperor penguins in the Antarctic.
"Fish stocks are under enormous pressure from commercial fishing, and their depletion will in turn affect the survival of many Antarctic species," said Ponganis, a physiologist at the Scripps Institution of Oceanography in La Jolla, California.
Unafraid of Humans
Not all penguin species are threatened, however.
"For king penguins, which live in the sub-Antarctic but not Antarctica, population numbers, if anything, are on the increase," said Lewis Halsey, a behavioral physiologist at the University of Birmingham in the United Kingdom.
That is partly because the king penguins eat myctophids, or lantern fish, which are not targets of commercial fishers.
"So to some degree there is not a clash between king penguins and fishermen," Halsey said.
In Happy Feet the penguins are scared of humans, who are thought of as aliens.
In real life, penguins generally pay little attention to human visitors.
"Penguins have generally evolved in places where humans haven't lived, particularly those species that live on Antarctica or sub-Antarctic islands," Halsey said. "They're easy to study, because you can walk right up to them."
But this doesn't mean penguins don't experience stress when humans get close.
"On Crozet Island in the sub-Antarctic, where there is a small human presence, due to research and the transport of cargo on and off the island, there is a large king penguin colony [that is] showing signs of decreasing population, while king penguins in general are showing an increase," Halsey said.
Ponganis, the emperor penguin expert, says increased tourism may have negative effects on penguin populations.
"The Antarctic is an extremely fragile environment, and the mere presence of thousands of people visiting colonies during the breeding period has the potential for damage to the population," Ponganis said.
"Most people don't realize that merely viewing wild animals can cause them stress, which in turn might affect their chick-rearing abilities," he said. "If you add in the risk for introducing pathogens, such as bird flu, you can see that uncontrolled ecotourism to a pristine environment, such as the Antarctic, is an accident waiting to happen."
Penguins are also particularly vulnerable to oil spills. Miller, the Happy Feet consultant, says tens of thousands of penguins die every year from oil pollution that may occur when ships clean out their bilges.
"Penguins are quite vulnerable to the toxic effects of oil," Miller said. "They get it on their feathers and end up eating quite a lot of it. If it doesn't kill them, it might make them more susceptible to disease."
The greatest threat to the long-term survival of the penguins, however, may be human-made global warming.
A report released this week by the international conservation nonprofit WWF warned that unchecked climate change could force up to 72 percent of bird species, including penguins, in some areas into extinction.
The report put Galápagos penguins, which are listed as endangered under the U.S. Endangered Species Act, at particular risk.
Some regions close to the Earth's Poles have seen temperatures rise by several degrees Fahrenheit in the last hundred years. Even the slightest climate change affects seawater temperatures, ice cover, and the availability of food sources.
Scientists believe that global warming may have caused the population of emperor penguins to decline 50 percent over the past 50 years.
"Emperor penguins are particularly vulnerable to changes in ice," Miller said. "They nest on sea ice, put an egg on their feet, and then stand around for months. But if that sea ice isn't sturdy enough to last until New Year's [when chicks are ready to head out to sea], they're not going to be able to raise their chicks."
Ponganis, of Scripps, says the emperor penguin is an animal of extremes.
"Pound for pound, the emperor penguin is one of the world's champion divers," he said.
"On a single breath of air, these birds can dive as deep as 500 meters [1,640 feet] and as long as 20 minutes—a feat that few other species, and no other bird, can match.
"A world without emperor penguins in it," Ponganis said, "would be a lesser one."
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