June 12, 2000

he periodic explosions in the population of Norwegian lemmings, followed by times of scarcity, have more to do with the availability of food than the widespread belief that the furry rodents commit suicide en masse when their numbers grow too large.

That's the conclusion advanced by Peter Turchin, an associate professor of ecology and evolutionary biology at UConn, and colleagues at universities in Sweden and Finland in this week's issue of the journal Nature.

Lemming suicide is fiction, the researchers say. The little mammals are "predators" who "prey"on moss. As their numbers grow, lemmings deplete their forage in arctic and alpine habitats more rapidly than the slow-growing mosses can replenish themselves. Faced with a desperate shortage of food, the lemmings attempt to migrate in search of areas where food may still be remaining.

"It's a completely natural strategy," notes Turchin. "If they stay in the same place they will starve. They have nothing to lose.

"When they migrate, however, many do drown while crossing rivers and lakes," he adds. It is this that gave rise to the myths associated with lemmings.

The great boom and bust cycles of lemmings - their numbers can change by a factor of 1,000 in one year, Turchin says - is what makes the species

fascinating to ecologists interested in population dynamics. Ironically, although the explosive growth and mass migrations of lemmings has attracted attention from laymen and naturalists since the Middle Ages, the underlying causes for their dramatic population oscillations have long remained a mystery.

To address this issue, Turchin and colleagues applied a battery of statistical tests to three previously collected datasets of lemmings, each at least 20 years long, and contrasted the lemming data with comparable data for voles. Vole populations, also native to northern Scandinavia and closely related to lemmings, also fluctuate dramatically and cyclically.

The researchers plotted the data on graphs and used mathematical models to interpret it.

The vole populations lingered around their maximum density before their numbers dropped, whereas the lemming populations showed a far more violent rise and fall in their number. The different patterns in the data point to very different causes for the population fluctuations in the two species.

"The primary food of lemmings," says Turchin, "is moss, which all grows on top of stones. Once lemmings have nibbled moss down to the rock, they are out of food for a long time, and must either starve or move in search of greener pastures.

"Voles, on the other hand, can't destroy their 'larder'," he says. They eat grass, most of which is underground, in the form of roots. As voles graze on the parts above ground, the plants simply keep regrowing.

But their population cycle is driven by the predatory attention of weasels. "Voles have no defenses against weasels," says Turchin. Eventually the number of weasels grow so large, they kill their prey faster than the voles can reproduce, and the vole population crashes.

"Our findings suggest that the different food habits of lemmings and voles have important consequences for their population dynamics," notes Turchin.

The results have broader implications. "As we gain a better understanding of the dynamics of lemming populations," says Turchin, "we also advance the general theory of population ecology. And general theory is already applied to such diverse issues as managing fisheries, controlling destructive insect pests, or helping to preserve endangered species."

David Bauman