By Virginia Morell, *Science*NOW
Lemmings don't commit suicide by running blindly off cliffs—that's a myth. But lemming populations in their Arctic tundra home can rise and fall dramatically in just a few short years. And the sudden collapse of a lemming population can mean hard times—or even extinction—for their predators, a new study shows.
Populations of collared lemmings—rotund rodents with small ears and short legs—are driven by predators and snow conditions. Their numbers follow a characteristic 4-year cycle, rising and falling with such regularity that the pattern has long fascinated scientists. But at the turn of the millennium, the lemming cycle collapsed in northeastern Greenland and has not recovered, most likely due to the increased warming of the Arctic, scientists report today in the Proceedings of the Royal Society B. And as lemming numbers have dwindled in the region, so have those of their predators; in fact, some populations are likely facing extinction.
As a result, the authors note, the warming of the Arctic is not only causing some species to shift their ranges northward, but is even adversely affecting those that stay put. The discovery is one of the first to suggest how extensive the ecological effects of global warming are apt to be—with entire communities of animals and plants subject to change as the loss of a keystone species is felt throughout a food web.
Scientists have tracked lemming populations at two Greenland sites, Traill Island and Zackenberg, since 1988 and 1996, respectively. The Traill Island project was originally launched to understand the effect of predators—primarily stoats, snowy owls, Arctic foxes, and long-tailed skuas—on the lemming cycle. The project at Zackenberg, 250 kilometers away, was started "to track the effect of climate change on the lemmings," says Niels Schmidt, a population biologist at Aarhus University in Denmark and the lead author of the new study. To monitor lemming numbers, Schmidt and his colleagues each year tally up the rodents' winter nests, which are visible after the snow melts. The scientists also estimate the number of predators by counting their nests and dens, and keeping track of the number of fledged owls and skuas.
By keeping such close tabs on the lemmings, the team was able to identify exactly when the lemming cycle in these two regions collapsed. On Traill Island, populations peaked at more than 10 rodents per hectare in 1990, 1994, and 1998—but since then, there have been scarcely more than two lemmings per hectare in each 4-year cycle. The story is even grimmer at Zackenberg.
"When there were plenty of lemmings, the predators helped themselves," Schmidt says. In each cycle, the predators would take 75% to 80% of the population, and then the rodents would spend the next 3 years rebuilding. Now that the lemmings have virtually vanished, the local predators are struggling because there are no other rodents in Greenland for them to pursue. "We expect we will lose the snowy owl, skua, and stoat," he says, noting that these three species are dependent on the lemmings to feed their young. Without enough food for their offspring, "they are locally doomed. Only the Arctic fox may survive because it can live on anything from fish that wash ashore to musk ox carcasses." Already, snowy owls have largely stopped breeding on Traill Island, and the population of stoats at Zackenberg has plunged.
Schmidt and his colleagues have not yet fully determined why the lemming cycle has collapsed, but they suspect that changing snow patterns and conditions are largely to blame. Lemmings thrive during long winters under thick, stable layers of snow—beneath the snow's protective cover, they can move safely and raise their young. Since 2000, however, there have been generally "shorter periods of snow cover," Schmidt says, "and that could be a reason for the lemmings' low numbers."
"The study nicely confirms what had been previously suspected—that the collapse of the lemming population cycles in some parts of the Arctic may have very serious consequences for the specialized predators of the tundra," says Gilles Gauthier, an ecologist at Laval University in Quebec City, Canada. The effects are likely to extend far beyond these particular predators, adds Kyrre Kausrud, an evolutionary ecologist at the University of Oslo. "There is some evidence already that this is affecting the multitude of migratory birds that breed in the short Arctic summer; they become alternate prey," mainly for the Arctic fox, he notes.
Losing the lemmings could lead to a "substantial transition in the entire ecosystem, including the vegetation," says Chris Thomas, an ecologist at the University of York in the United Kingdom. "It is a beautiful, if worrying, example of how a subtle change to the interactions between species may suddenly cause a switch in an ecological system from one state to another." With the Arctic experiencing record levels of snow melt this summer, the plight of the lemmings—and an ecosystem that depends on their numbers—is not likely to improve soon.
*This story provided by ScienceNOW, the daily online news service of the journal *Science.
