There is a strong connection between climate change and penguin populations, as a warming planet causes declines in krill, a key food source for the birds [AP]
Two species of Antarctic penguins have declined sharply over the past 30 years as their chief food source has been devastated by a combination of other predators, over-fishing, and rapidly melting sea ice caused by global warming, according to a new study released on Monday by the National Academy of Sciences.
Based on studies of Adelie and chinstrap penguins and the ecosystems that have sustained them dating back to the 1970s, the report found that dramatic declines in krill, the shrimp-like creatures that depend on sea ice for reproduction, are chiefly responsible for the more than 50 per cent plunge in the flightless birds' populations in the South Shetland Islands.
The Adelie penguins, which favour sea-ice habitat during the winter, have declined at a 2.9 per cent rate a year over the last decade, while chinstrap penguins, which favour open water, have declined by an even greater 4.3 per cent annual rate over the same period, according to the study.
Some scientists had predicted that the decline in sea-ice habitat in the Antarctic caused by warming air and water temperatures would have a more negative impact on the Adelie penguin populations given their greater dependence on sea ice as a habitat.
Under that so-called "sea-ice hypothesis", the chinstrap penguins were expected to increase their population, at least relative to their Adelie cousins.
But the study found that the abundance - or lack - of krill appears to be playing a greater role in reducing the two species' populations.
Krill feed on photoplankton that thrive under sea ice. According to other recent studies, the krill population in the Southern Ocean has declined by as much as 80 per cent since the 1970s.
"For penguins and other species, krill is the linchpin in the food web", according to Dr Wayne Trivelpiece, the lead author and a seabird researcher with the National Oceanic and Atmospheric Administration's (NOAA) Antarctic Ecosystem Research Division.
"Regardless of their environmental preferences, we see a connection between climate change and penguin populations through the loss of habitat for their main food source", he said. "As warming continues, the loss of krill will have a profound effect throughout the Antarctic ecosystem."
The Antarctic is among the fastest warming ecosystems on Earth. Mean winter air temperatures have increased by five to six degrees Centigrade since the 1970s.
The warming has reduced both the extent and duration of winter sea ice on which photoplankton and thus krill - and ultimately penguins - depend.
"If warming continues, winter sea-ice may disappear from much of this region and exacerbate krill and penguin declines", according to the study.
The decline in krill, however, is not due to the disappearance of sea-ice alone, according to the report, which also cited commercial fishing for krill by specialised trawlers beginning nearly 40 years ago and growing competition for krill by recovering whale and fur seal populations.
Indeed, populations of both Adelie and chinstrap penguins grew steadily between the 1930s and the 1970s as a result of the losses sustained by the two sea mammals hunted by humans.
"Penguins are excellent indicators of changes to the biological and environmental health of the broader ecosystem because they are easily accessible while breeding on land, yet they depend entirely on food resources from the sea", according to Trivelpiece.
"In addition, unlike many other krill-eating top predators in the Antarctic, such as whales and fur seals, they were not hunted by humans", he said. "When we see steep declines in populations, as we have been documenting with both chinstrap and Adelie penguins, we know there's a much larger ecological problem."
Juveniles in both the Adelie and chinstrap penguins have proved the most vulnerable to the loss of krill biomass, according to the report.
While in the mid-1970s, about 50 per cent of young penguins would survive their first year, only 10 percent survive now.
A version of this article first appeared on the Inter Press Service News Agency.