Arctic stakeout yields hare-raising results

by Charles Ker
Staff writer

UBC zoologist Charles Krebs has just completed the most ambitious ecological stakeout ever.

The life and death cycle of snowshoe hares in Canada's North has been observed for more than 300 years but remains one of nature's enduring mysteries. In regular nine to 10-year intervals, hares from Alaska to Labrador increase and then die in startling numbers. Populations can plummet from 300 animals per square kilometre to one.

From 1986 until last September, Krebs and his colleagues have been monitoring virtually all hare movement in sections of Yukon spruce forest. The dogged team of technicians and researchers compiled an exhaustive, daily chronicle of hare interactions including what they eat, how they forage, defecate, mate and, most especially, how they interact with predators.

"There is no doubt that predation causes mass paranoia among these hares who are constantly looking over their shoulders during the declining phase of each cycle," says Krebs.

Chronic stress caused by predation is the subject of one of the more than 103 publications and theses produced by the multimillion-dollar project funded by the Natural Sciences and Engineering Research Council (NSERC).

The team tested theories about the hare population cycle by sectioning off two, square-kilometre blocks of wilderness with electrical fencing to keep out lynx, coyotes and other mammal predators. The idea was to observe the effects on hares of reduced predation and additional food, both by themselves and in combination. Fertilizer was also spread around two large blocks of forest to see what effect added plant growth had on hare populations.

To collect their data, researchers trapped and attached radio collars to more than 1,000 hares, red squirrels, and predators like lynx and coyotes and monitored their whereabouts each day. Results from the project showed that while both food and predation play a role in generating hare cycles, they alone are not the root causes. The combined treatment of predator reduction and additional food supplies delayed the decline but did not prevent it.

"If both food and predation are together sufficient to explain population cycles in snowshoe hares, why were we not able to prevent the decline entirely in the combined treatment area?" Krebs asks.

Owls and other raptors are part of the reason since they cause 40 per cent of predation and the enclosures failed to eliminate them. Attempts to string up nets and fishing line to ward off birds of prey were ineffective due to snow buildup.

The study also refutes a previously held theory that food shortage followed by predation gives rise to the cycle. Krebs asserts the phenomenon results from a complex, three-level interaction among herbivores (hares), their food plants, and their predators.

The team found that almost all snowshoe hares in the study area died from predator attack. From 1989 to 1993, predation accounted for 83 per cent of deaths among radio-collared hares and only 9 per cent were attributed to starvation.

The electrical fencing and additional food supplies did produce a dramatic effect in the experimental enclosures. Averaged over both the peak (1989-90) and decline (1990-93) phases, predator exclusion approximately doubled the density of hares, food addition tripled density and the combined treatment increased density eleven-fold.

Krebs' team, including Tony Sinclair, Jamie Smith, Roy Turkington, and Kathy Martin from UBC, and three others drawn from the universities of Alberta and Toronto, worked from a research campsite operated by the Arctic Institute on the boundary of Kluane National Park.

Situated 150 kilometres west of Whitehorse in the midst of virgin spruce forest, Krebs says the project was as much a test of human dynamics as those of the animals they were observing.

"It is a great achievement for those involved to have sustained that level of data collection over such a prolonged period in such a remote location," says Krebs. "It's certainly a benchmark for future large-scale research projects."

Krebs is presently using a Killam Research Fellowship to write up the results of the research in a publication tentatively titled, Vertebrate Community Dynamics in the Yukon Boreal Forest.