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Friday, May 23, 2008

Greenland Caribou Food Supply Threatened by Warming

Thinning Food Supply
Eric Post |

Jessica Marshall, Discovery News, May 22, 2008

Greenland's caribou are arriving at their spring breeding grounds to find a food supply far past its prime. Caribou in West Greenland migrate inland in spring, to the western edge of the country's inland ice sheet, where they give birth to their calves. There, they feed on freshly emerged plants, which provide the best nutrition.

But two new studies by Eric Post of the Pennsylvania State University and colleagues show that global warming has thrown this system out of whack. Plants are emerging earlier, and all at once across the landscape, so the caribou are arriving to find the plants they rely on are past their prime.

"Because of warming, food is becoming available earlier in the year for caribou. That might sound like a good thing, because caribou come out of the long Arctic winter hungry," Post told Discovery News by e-mail from Greenland, where he is awaiting the spring calving season.

"But we also found that caribou are not adjusting their birth season to help keep up with changes in plant growth. As a consequence, their food is in a sense being taken off the table again before caribou have had a chance to get what they need."

Post documented that over a period of six seasons -- 1993, and 2002-2006 -- the average spring temperature rose by more than 8 degrees Fahrenheit and plants emerged about two weeks earlier, while calf mortality increased fourfold and calf production declined by a factor of seven. These results were published online earlier this month in Philosophical Transactions of the Royal Society B.

In the second study, appearing online today in the Proceedings of the Royal Society B, Post's team showed that warming is also causing plants across the landscape to mature all at the same time. Normally, caribou would start out in the valley where plants emerge first and move uphill, following new plants as the season progresses.

"As it gets warmer, there's less and less difference between the plants in the valley and those on the hilltop. So now, when the caribou goes uphill, counting on finding a good meal, it's out of luck," Post said. "The plants are past their peak."

"Overall, this means that the caribou isn't getting the best diet, and isn't able to provide its newborn calf with the best nutrition," he added. "We think that makes it harder for the calves to survive the first few days of life."

"While we were collecting data on plant growth during those seven years, we also made observations on caribou calving, and found that in years when plant growth started almost simultaneously across the landscape, caribou had fewer calves," Post said.

The mismatch arises because caribou use changing day length in spring to cue their reproduction, while plants rely on temperature.

With global warming, "temperatures rise but day length stays exactly the same," said Marcel Visser at the Netherlands Institute of Technology in Heteren.

This is not the first example where climate change has skewed food supply and demand. Visser was the first to discover this relationship in great tits, whose caterpillar prey emerged earlier because of warming, before the birds' eggs hatched.

But "it's the first time people have shown this in mammals," Visser said.

For the animals to adapt, "they need genetic variation in the population that will push back the earliest date for reproducing," he added. "But that is a slow process. One of the worries is that climate change will occur faster than these changes can occur."

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