Yukoners with freezers full of blueberries and cranberries probably think that the rainy weather last summer is responsible for this year's bumper crop of berries. But there could well be another reason, a most basic reason; it could be a matter of lots of bunnies pooping in the woods.

Researchers working with the Kluane Boreal Forest Ecosystem Project have studied snowshoe hare cycles in the boreal forest for more than 25 years. Hare populations increase and decrease in approximately ten-year cycles, skyrocketing as much as 300 fold over the course of some cycles.

When hare numbers are high, there can be on average about 70 bunny pellets per square metre in the boreal forest. That's a lot of fertilizer, says Mark O'Donoghue, a biologist who worked with the Kluane project for eight years.

The researchers speculate that nutrients from the pellets flush into the soil, giving blueberries, kinnikinnick and other dwarf shrubs a big boost. At this point, O'Donoghue describes this connection between bunnies and berries as a set of ideas that has not been tested, but it serves to illustrate the huge number of interrelationships that this project has teased out.

While the Kluane project focused on hares, it also looked at many of the other animals that are part of the same food web. Lynx and coyotes, the two main predators of hares, also have population cycles linked to those of the hares.

As time went on, the researchers noticed cycles in other animals such as red-backed voles, whose numbers peak two to three years after those of hares. These voles are typically the most abundant small mammal in the Kluane study area, but little is known about population cycles in this species in North America.

In Scandinavia, red-backed voles are known to have three to four year cycles, a time frame that makes them easier to study. "It took us 25 to 30 years to accumulate information on what is happening with the hare cycle here, and to see that a parallel thing is happening with voles," says O'Donoghue.

In the Kluane area, the researchers used several different study plots to try and figure out why vole populations might cycle. For 10 years they trapped voles in the different plots and tested a range of possibilities.

Predators were kept out of some plots, but this move had little effect on the numbers of voles. In other plots researchers looked at whether hares and voles were competing for food, but found that they ate different plants.

In plots where they added fertilizer to stimulate plant growth, the number of red-backed voles declined a bit. The researchers found that on these plots, the increased growth of grasses caused a decline in dwarf shrubs on which these voles depend.

Weather also had some influence; after rainy summers, there were usually lots of voles the following spring, but the effect was not strong enough to fully explain the vole cycles.

The most noticeable changes occurred with the shrubs in the area; they grew the highest during and right after the peak of the hare cycle, suggesting that all those bunny pellets were acting as fertilizer for the plants.

From there, the berries provide the link in the story. It takes two years to produce a good berry crop, so two years after hares are at their peak, a bumper crop of berries follows.

Voles eat seeds, and during good berry years they have a plentiful food supply, much of which lasts right through the winter. Late springs mean that the berries are protected even longer from foraging birds. When lots of voles survive the winter and mate in the spring, a population boom follows.

From hares to pellets to berries to voles; it is no wonder that it takes a long-term study to start making some of the connections. O'Donoghue, now the regional biologist in Mayo, says that most research projects follow this pattern: the more you find out, the more questions you end up having.

"You definitely get a whole lot more humble about what you do and do not know," he says.

Last spring a book on the results of this project was released. Titled "Ecosystem Dynamics of the Boreal Forest: The Kluane Project," it is edited by Charles Krebs, Stan Boutin and Rudy Boonstra.

Mark O'Donoghue, the Northern Tutchone Regional Biologist, can be reached in Mayo at 996-2162 or mark.odonoghue@gov.yk.ca.