The big picture
Broad-scale study suggests sea ice not driving changes in penguin populations
Posted February 17, 2012
The story of how three little penguin species are coping with significant changes in climate around the Antarctic Peninsula has followed a familiar narrative in recent years.
Colonies of ice-dependent Adélie penguins along the western side of the peninsula are blinking out of existence. A subantarctic species called gentoos that disdain ice is thriving, pushing farther south. The third species of brushtail penguin known as the chinstrap is somewhere in the middle.
All of this is under way in a region considered one of the fastest warming in the world. It’s where average year-round temperatures are 3 degrees Celsius higher over the last 60-plus years — and about double that during just the winter season.
A driving force in the success and failure of the different penguins revolves around sea ice, its extent and duration shrinking dramatically in recent decades. Such a scenario means that the true Antarctic penguin, the Adélie, is losing habitat to the subantarctic gentoos and chinstraps.
That’s the story. But it’s not the whole picture, according to scientists who have recently published a paper in the journal Ecology that synthesized penguin census data from a 31-year period at 70 sites around the Antarctic Peninsula.
The findings suggest that site-specific long-term studies may not accurately reflect regional trends because population responses to climate change vary spatially. By looking at a large number of populations along the Antarctic Peninsula, the authors find that they call the “classic sea ice hypothesis,” while likely still a factor in the dynamics of the food web, “misses many of the key details that emerge from a more comprehensive regional-scale analysis.”
“This study really captures what we’re trying to do, which is to understand the broad-scale picture of penguin population dynamics, not necessarily looking at any one site in particular. We’re trying to get a bird’s eye view of what is going on,” explained Heather Lynch , lead author of the paper.
More than five years ago, while a post-doctoral fellow with William Fagan at the University of Maryland, College Park , Lynch joined forces with Ron Naveen and his nonprofit conservation organization, Oceanites Inc. In 1994, with funding from theNational Science Foundation (NSF) , Naveen had initiated the Antarctic Site Inventory, an ambitious program to census the seabird populations around the Antarctic Peninsula.
Naveen had the data. Lynch and Fagan had the analytical tools to create models that would pinpoint trends over time and space. For the Ecology paper, the researchers also drew upon other long-term datasets, including those by National Oceanic and Atmospheric Administration (NOAA) scientists working in the South Shetland Islands off the northwest tip of the peninsula.
The analysis found that Adélies were in significant decline. No surprise there. Out of 24 breeding sites, Adélies were down at 18 sites and increasing significantly at only three colonies. But chinstraps — contrary to some previous research that implied they were mostly thriving alongside gentoos, as both species are not dependent on sea-ice like their cousin — are also failing. The 29 chinstrap sites surveyed found significant declines at 16 sites and increases at seven.
“If you look at the big picture, chinstraps are declining rapidly and regionally,” Lynch said, even more so than Adélies in the last decade.
But none of the changes affecting chinstraps or Adélies appear to be directly related to sea ice, according to the research. That finding supports a paper published last year by NOAA scientists in the journal Proceedings of the National Academy of Sciences.
“We suggest that sea ice no longer drives trends in penguin populations through direct, physical effects on habitat. Rather, sea ice is one of several factors that mediate prey availability to penguins,” wrote the authors, led by Wayne Trivelpiece , a scientist with NOAA’s National Marine Fisheries Service .
The work by Lynch, Naveen and their colleagues also suggest a correlation to food, at least for the peninsula Adélies, which rely almost exclusively on shrimplike krill. They used chlorophyll a as a proxy, or as a way to estimate biological activity, because actual estimates of krill biomass in the Southern Ocean are unavailable on the scales covered by the research.
Chlorophyll a is a green pigment in phytoplankton, microscopic plantlike organisms that float in the ocean. Krill feed on phytoplankton, hence the connection. Scientists believe the little crustaceans are also reliant on sea ice as both a habitat and source of food, as juvenile krill feed on algae that grow under the ice. So sea ice is still in the equation.
The scientists found that neither chlorophyll a nor changes in spring sea ice were correlated to the spatial pattern of chinstrap population declines.
In contrast to the other two Pygoscelis species, gentoo penguin populations are significantly increasing at 32 of 45 sites and significantly decreasing at only nine sites, the researchers reported. Lynch and colleagues found that gentoo colonies are restricted to areas with less than 50 percent sea ice cover in November. Their southward march has been rapid, thanks to the decline in sea ice at that time of year.
“I think we have to rethink the paradigm of population change in all three species,” said Lynch, now an assistant professor at Stony Brook University .
Naveen said the disparate findings suggest that researchers still have much to learn about what is driving the ecosystem.
“We want to try to understand more precisely why we’re seeing different responses by these three species in what is a vastly warming ecosystem. It could be food related. It could be oceanography. It could be something else,” he said.1 2 Next