"A group of us have been following mouse and vole numbers in the Kluane region since the early 1970s," says Charles Krebs, a researcher at the Arctic Institute's Kluane Research Base. "The summer of 2002 is certainly the year of the mouse and vole for southwestern Yukon."

Numerous species of voles and mice are known to inhabit the Yukon, but only a few are commonly seen.

"The two most common species in our area are the deer mouse (Peromyscus maniculatus) and the northern red-backed vole (Clethrionomys rutilus), and of these the deer mouse is the worst pest in houses."

"In addition, there are eight vole species-mostly field voles (Microtus) in grassy habitats -- including the singing vole you can sometimes hear in the woods or at higher elevations," adds Krebs.

Gardeners in particular have felt the effects of what many consider a rodent 'plague'.

"They've eaten almost everything from the garden. They even eat chick weed!" says Neil Rollinson. "My lawn looked like Iraq this spring, and my dog spent most of his waking hours hunting them down."

Wildlife viewing biologist Bruce Bennett has received a number of calls about mice and voles this year. "People see them crossing the highway and around their homes. We've been getting lots of reports on them."

"I've been expecting this since last year, when we experienced a huge shepherdia (bearberry) crop," says Bennett. "It's one of their main foods."

Bennett has also observed an increase in short-eared and northern hawk owls, and wonders if this is partly due to the surge in rodents.

According to Krebs, nearly all Yukon mouse and vole species reach high numbers in the same years. These cyclical peaks have occurred in 1973, 1984 and again in 2002.

"What strikes us is that these three major peaks in mouse and vole numbers have generally occurred during the decline phase of the ten-year snowshoe hare cycle," Krebs explains. Kluane hares have peaked in 1971, 1980-81, 1989-90 and in 1998. "We're interested to know why snowshoe hares and mice numbers are correlated."

There's been one exception -- the 1989-90 hare peak was not followed by a high mouse year. That means it's been eighteen years since the territory experienced such high numbers of voles and mice.

Krebs suggests, and then dismisses, two possible explanations for the connection between hares and rodents-that the species compete for food or have common predators. "Neither of these holds water," he says, "Snowshoe hares don't eat the same kinds of plants as voles and mice, so it's unlikely there's direct competition for food."

"And hare predators like lynx and great-horned owls are not generally major predators of mice." Coyotes, weasels and martens benefit when mice are populous, but Krebs goes on to explain that mouse abundance, at least in the Kluane region, is not high enough to support a wide range of predators.

"We think that mouse and vole numbers are driven by an indirect effect from the hare cycle on soil nutrients," Krebs explains. "We've suggested that hare browsing and grazing at high hare numbers releases nutrients that are taken up by berry-producing plants."

The result is a good berry crop two or three years after the hare peak. In turn, mice and voles thrive on the bumper crop and their numbers grow.

Since 1997, Krebs and his colleagues have been measuring berry crops at Kluane to see if their hypothesis works. Because the cycles are far apart it takes a longer time to recognize associations in the data, so it may be a few more years before their research is conclusive.

Krebs points out that there are other factors that can contribute to berry production. "It's probably related to climate as well as the hare cycle," he adds.

For Yukoners who have been grappling with high numbers of mice and voles in gardens, attics and sheds, Krebs assures them that 2003 will be different.

"I'm pretty sure they won't have to worry about mouse pests for another eight to ten years."