Penguin Decline Due to Global Warming?
for National Geographic News
|September 13, 2004|
The Earth is getting warmer, according to most scientists. In recent
years that phenomenon has prompted researchers to investigate what
effect rising temperatures are having on cold-loving penguins and other
Les Underhill directs the Avian Demography Unit at the University of Cape Town in South Africa. He suspects that global climate change may be responsible for declining penguin populations on South Africa's Prince Edward Islands.
The islands dot the Indian Ocean some 1,000 miles (1,770 kilometers) off the South African coast.
Most of the islands' penguin colonies are dwindling. According to Underhill, one reason for the decline may be a climate-induced southward shifting of food-rich waters. The change may have forced the seabirds to swim farther to forage.
Underhill and his colleagues will soon begin to test this idea. The researchers plan to place electronic tracking devices on the islands' penguins to record when they go out to sea to get food for their chicks and when they return.
"We suspect that one of the consequences of global climate change is that, with warmer seas, the journeys will become longer," he said in an interview with the radio program Pulse of the Planet.
The Prince Edward Islands sit near the southern boundary of the Antarctic Circumpolar Current (ACC). It is in these food-rich waters that the islands' penguins are thought to forage.
Considered the greatest of all ocean currents, the Antarctic Circumpolar Current mixes waters from the Indian, Pacific, and Atlantic Oceans. It swirls about 140 million cubic meters (4.9 billion cubic feet) of water per second around Antarctica.
The southern edge of the current is marked by a boundary separating it from the cold coastal waters along the Antarctic continental shelf. Waters to the north of the boundary are several degrees Celsius warmer than those to the south.
"The southern boundary of the ACC is noted for being a food source," said Eileen Hofmann, an oceanographer at Old Dominion University in Norfolk, Virginia.
But the boundary is not fixed. It shifts north and south within a range of about 6 to 12 miles (10 to 20 kilometers). This may be due, in part, to westerly winds that circle what many call the Southern Oceanthose parts of the Indian, Pacific, and Atlantic Oceans that surround Antarctica.
"The question is whether or not the current core, the whole system, shifts into a different state," Hofmann said.
Hofmann added that there is currently no evidence to suggest the whole system is shifting. Rather, the influence of a climate change-induced shift is a topic of scientific speculation.
Underhill, the University of Cape Town researcher, speculates that a southward trending boundary could be adversely influencing penguin colonies on the Prince Edward Islands. In response, Hofmann said it is possible that southward movement of the current could be hurting penguins, if that area is indeed where the penguins go to forage.
"It's documented that a lot of Southern Ocean whale populations are associated with the boundary and feed along the boundary," she said. "It wouldn't be too surprising if penguins do the same thing."
Antarctic Ice Changes
Wayne Trivelpiece directs Antarctic seabird research for the U.S. Antarctic Research Division at the Southwest Fisheries Science Center in La Jolla, California. He said changes in Antarctic ice cycles are influencing the Adélie penguin colonies he studies on the Antarctic Peninsula.
Average wintertime temperatures on the peninsula, Trivelpiece said, have risen by about 9 degrees Fahrenheit (5 degrees Celsius) in the last 50 years. As a result, pack ice that once formed every winter now has a cyclical nature to it.
"For a couple of winters in a row there's ice in these areas, then there's three to five years with no ice," he said.
Trivelpiece's research shows a correlation between pack ice and the availability of algae for krill, a main staple to the Adélie penguin diet. In winter, algae accrete onto pack ice. Later, melting ice makes the algae available to krill during the spring breeding time. Penguins, in turn, eat the krill.
But when there is no ice, there is no algaea phenomenon that disrupts the food chain.
During years that followed winters with lots of ice, Trivelpiece and his colleagues have found ample small krill. But in years of no sea ice, what krill the researchers found were significantly larger. In each successive year without winter sea ice, the krill get progressively larger.
This cycle is reflected in the rate in which young Adélie penguins survive to reach breeding age on the Antarctic Peninsula. With less krill to go around during the ice-free years, fewer penguins survive to adulthood.
In the last decade Adélie penguin populations have declined by almost 50 percent.
Trivelpiece cautions that this finding is specific to the Antarctic Peninsula. In more southerly regions of Antarctica, penguin populations are actually getting larger, owing to the warming temperatures.
For example, in the Indian Ocean region of Antarctica, ice still forms each winter. But owing to the warmer temperatures, the ice breaks up earlier, giving penguins access to their primary food source earlier in the breeding season. This, in turn, has led to higher reproductive success and increasing Adélie populations in this region.
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