By Deborah Zabarenko, Environment Correspondent
WASHINGTON (Reuters) - Two of the most well-known penguin species in Antarctica -- chinstraps and Adelies -- are under pressure because a warmer climate has cut deeply into their main food source, shrimp-like creatures called krill.
Fewer of the juvenile penguins survive what scientists call their "transition to independence" because there isn't enough krill to go around, according to a study published on Monday in the Proceedings of the National Academic of Sciences.
The study found only 10 percent of young penguins survive the first independent trip back to their colonies from their winter habitat, said lead author Wayne Trivelpiece, a sea bird expert at the National Oceanic and Atmospheric Administration's Antarctic Ecosystem Research Division.
When the study began, back in the mid-1970s, the chances that a two-to-four-year-old penguin would survive the trip was about 50 percent, Trivelpiece said in a telephone interview.
"What's changed is young penguins surviving their transition to independence," he said. "They're no longer able to do that anywhere near the way they used to do, and we think that's directly related to the fact that there's 80 percent less krill out there now."
Initially, scientists figured one of these two penguin species might actually benefit from climate change, since Adelies love ice and chinstraps avoid it. They theorized that the chinstrap penguins might flourish with less ice and more of the open water they favor.
But this latest research suggests it is less a question of vanishing penguin habitat -- though this is also occurring -- than it is a matter of vanishing habitat for krill.
Krill form the basis of the marine food web, supporting organisms ranging from fish and penguins to whales. Krill feed on phytoplankton -- basically, ice algae -- that grow lushly on the undersides of ice floes.
These tiny crustaceans are specially adapted to graze for the tiny plants among the ice crystals. But in the last few decades, winter ice has formed later in the season and has covered less area and spring melt comes earlier. Without ice, krill's feeding is disrupted and populations fall.
Trivelpiece's research focused on an area that is experiencing some of the most extreme climate warming on the planet: the Antarctic Peninsula and the islands around it.
Mean winter temperatures have risen 9 to nearly 11 degrees F (5-6 degrees C) in that area since the mid-20th century, compared to the rise in world mean temperatures of less than 2 degrees F (about 0.74 C) for all of the 20th century.
The study was funded in part by the Lenfest Ocean Program, which supports research on the global marine environment.