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 here Monday by the National Academy of Sciences.
The West Antarctic Peninsula (WAP) and adjacent Scotia Sea support abundant wildlife populations, many of which were nearly wiped out by humans. This region is also among the fastest-warming areas on the planet, with 5–6 °C increases in mean winter air temperatures and associated decreases in winter sea-ice cover. These biological and physical perturbations have affected the ecosystem profoundly.
One hypothesis to explain the declining numbers of "ice-loving" species is the “sea-ice hypothesis,” which proposes that reductions in winter sea ice have led directly to declines in “ice-loving” species by decreasing their winter habitat, while populations of “ice-avoiding” species have increased. However, 30 years of field studies and recent surveys of penguins throughout the WAP and Scotia Sea demonstrate this mechanism is not controlling penguin populations; populations of both ice-loving Adélie (at left) and ice-avoiding chinstrap penguins have declined significantly.
An alternative, more robust hypothesis that attributes both increases and decreases in penguin populations states that changes in the abundance of their main prey, Antarctic krill is the most likely causation. Unlike many other predators in this region, Adélie and chinstrap penguins were never directly harvested by man; thus, their population trajectories track the impacts of biological and environmental changes in this ecosystem. Linking trends in penguin abundance with trends in krill (at right) biomass explains why populations of Adélie and chinstrap penguins increased after competitors (fur seals, baleen whales, and some fishes) were nearly extinguished in the 19th to mid-20th centuries and currently are decreasing in response to climate change.
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- percent plunge in the flightless birds' populations in the South Shetland Islands.
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.
Krill are tiny crustaceans, 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.
The Adelie penguins, which favor sea-ice habitat during the winter, have declined at a 2.9 percent rate a year over the last decade, while chinstrap penguins, which favor open water, have declined by an even greater 4.3 percent annual rate over the same period, according to the study.
Previously, 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 (right) that thrive under sea ice. According to other recent studies, the krill population in the Southern Ocean has declined by as much as 80 percent 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.
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. (At right: cloud of krill)
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 specialized 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 (right) and Adelie penguins, we know there's a much larger ecological problem."
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 (at left: young penguins with adult chinstrap penguin) 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
Trevilpiece stated, "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."
The study was funded in part by the Lenfest Ocean Program, which supports research on the global marine environment.
IPS News,"Antarctic Penguin Population Declines with Krill", by Jim Lobe, accessed April 12, 2011
William's Page, "Fact pictures of penguins", by William, accessed April 12, 2011
Reuters, "Fewer penguins survive warming Antarctic climate", by Deborah Zabarenko, accessed April 12, 2011
Proceedings of the National Academy of Science of the US, "Variability in krill biomass links harvesting and climate warming to penguin population changes in Antarctica", by Wayne Z. Trivelpiece, et. al, accessed April 12, 2011