Global Warming Threatens Lemmings in Norway

Kate Ravilious
for National Geographic News
November 5, 2008
Climate change may be responsible for shrinking lemming populations in Norway, a new study shows.

As a result, the lack of the small mammals is cascading through the ecosystem, forcing predators to find different food sources.

Lemming populations throughout Scandinavia tend to explode naturally every three to five years, causing huge numbers to go in search of food.

Occasionally this leads the rodents to jump into water and swim to new pastures—the origin of the myth that lemmings commit mass suicide.

When lemmings boom, they're hard to miss. Norwegians have had to use snowplows to clear the squashed rodents off the roads.

In recent years, however, outbreaks have become a rarity in many parts of Scandinavia.

The Wrong Kind of Snow

Kyrre Kausrud, a professor at the University of Oslo in Norway, and his colleagues analyzed lemming boom-bust cycles since 1970 for one site in southern Norway. Their findings are reported tomorrow in the journal Nature.

The data revealed that lemmings in this region have not had a population explosion since 1994.

Climate data collected over the same period suggest that warmer temperatures can explain why the rodents' numbers have remained low for more than a decade.

(Read more about global warming.)

During the winter lemmings live in tunnels under the snow. Warmth from the Earth melts some of the snow near the ground, providing pockets of air and access to food such as moss.

In recent years, warmer temperatures have been changing the structure of the snow—with devastating effects for the lemmings.

Rather than remaining below freezing for most of the winter, temperatures have bounced above freezing a number of times, melting and then refreezing the snowpack.

"This enables water to enter the system, flooding the snow tunnels and then forming ice layers on the ground," Kausrud said.

Many lemmings drown when their burrows are flooded, and those that survive often starve when their food is trapped under an icy layer.

The team's data showed that lemming population explosions were linked to years with colder winters, providing the ideal snow conditions for lemmings to thrive.

They also showed that winters in southern Norway have been warmer since 1994, preventing females from rearing the large broods that lead to lemming outbreaks.

"Their findings present a convincing demonstration of the effects of climate change on lemmings and their wider ecosystem," said Tim Coulson, a population biologist from Imperial College London who was not involved with the study.

Bad News for Predators

Studying census data on animals, Kausrud and his colleagues demonstrated that the lemming scarcity is also affecting the greater ecosystem.

(Related: "Arctic Foxes Put Eggs in 'Cold Storage' for Lean Times" [October 29, 2007].)

Unable to gorge themselves on lemmings every few years, predators like the Arctic fox and snowy owl have had to rely on other food sources, such as ptarmigan (a kind of grouse) and willow grouse.

As a result, the numbers of these ground-nesting birds have been diminishing. "Lemming population explosions help to take the pressure off ptarmigan and willow grouse," Coulson explained.

Although the study looks only at one region in Norway, Kausrud and his colleagues believe the effect could be widespread.

"We might indeed expect similar changes across Scandinavia and in [Canada and Alaska]" said study co-author Nils Stenseth, also of the University of Oslo.

Kausrud and his colleagues think that it is unlikely that climate change will drive lemmings to extinction, but the impact on the ecosystem could be severe.

"As competitive relationships change for predators, prey, and plants, the whole community changes," he said.

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