Old lemming puzzle gets new answer.

What makes lemming numbers boom and bust? The answer to this question--literally the oldest in the study of population cycles--is food supply, claims an international research team.

The lemming population cycle, with peaks 1,000 times as high as its valleys, was the first discovered, explains Peter Turchin of the University of Connecticut in Storrs. Biologist Charles S. Elton pointed it out in 1924, elaborating on a notion that struck him while browsing in a Norwegian nature book. He couldn't read the words, but the pattern in the numbers jumped out.

Researchers have proposed two basic explanations for the pattern, says Turchin. In one, a lemming increase brings more predators, which then nearly wipe out the lemmings. Alternatively, the lemmings themselves might be considered predators, dwindling when they overwhelm their food supply of moss.

Now, Turchin and his colleagues in Sweden and Finland suggest a new way to choose between the explanations: Check the shape of the population curves.

Population theories predict that prey numbers rise to blunted peaks, whereas predator numbers spike sharply. That's because prey surge to the maximum density their environment permits and hover at that level while predators catch up. However, when predators finally get so numerous that they overeat their prey, starvation quickly undercuts predator numbers.

"The new insight is you can use this for a diagnostic test," says Turchin. He and his colleagues plotted lemming-population data from three study sites during 3 decades. The lemming numbers definitely spike, the researchers report in the June 1 NATURE. That pattern suggests that the numbers plunge because the lemmings overgraze the moss.

In contrast, vole populations at three sites traced the blunter curves typical of prey, the researchers report. That profile agrees with recent experiments in which vole populations practically stopped cycling when researchers removed weasels.

Decades after the discovery of such cycles, experiments are finally identifying the driving forces, Turchin notes. In the late 1990s, studies manipulating populations revealed that predators control some famous cycles: lynx for snowshoe hares, predatory beetles for bark beetles, and perhaps parasites for red grouse. "The field now has a critical mass of theory and a critical mass of data," Turchin says.