It is a conundrum that has fascinated professor Rudy Boonstra and his colleagues for years. The boreal forest wreathes the top of Canada, covering some five million square kilometres. Every ten years, the dominant herbivore in this forest – the snowshoe hare – cycles like clockwork.
In the southwestern Yukon, where Boonstra studies this species, their numbers peak at 200-300 per square kilometre and then rapidly decline to about seven over a period of two or three years. “We know that the cause of the decline is their predators – they are the primary food of the Canada lynx, coyotes and great horned owls," says Boonstra, a physiologist and zoologist at UTSC.
“Eventually, this predator population, which also cycles, builds up and kills off the majority of the hare population, causing the decline. What baffled us is why the subsequent low phase for hares then lasts between two and four years."
After all, he notes, snowshoe hares can live up to the reputation rabbits (to which hares are related) have of breeding quickly and in great numbers. Why, then, does it take them so long to recover from the decline, following the death of most of their predators, and repopulate the boreal forest?
Boonstra’s hypothesis, now widely accepted as fact by the scientific community, is that the snowshoe hares are chronically stressed during the decline because of high predation risk. The effects of this chronic stress may then impair both the reproduction of survivors and of their offspring, delaying recovery during the low phase.
“When humans experience prolonged, intense stress from, for example, a divorce or the death of someone very close, you don’t eat as much, you don’t sleep well, and you lose interest in sex. As a result, reproduction cannot take place. The same happens with the hares. In their increase and peak years, they produce as many as 19 babies in a summer. During the declining years, the number drops to seven."
Boonstra showed through a series of studies that hares breeding during the decline suffer from a kind of post-traumatic stress syndrome. “The overwhelming anxiety caused by years of being hunted constantly by the lynx programs this stress syndrome into their brains." He believes that this anxiety affects the hares for a few generations afterward, as it is passed on from mothers to their offspring.
This type of research has been central to Boonstra’s 30-year focus on ecology, which also includes field work on the Arctic coast of the Beaufort Sea, in the Rocky Mountains and in southern Ontario. It addresses biological questions that are essential to the broad issue of environmental change: How are populations of mammals regulated? Is stress related to the aging process? What are the environmental forces acting on mammals?
“If we are going to understand the changes in the earth’s environment, such as global warming and the melting of the polar ice caps, then we need to understand how these changes impact species on the planet. The world is one big ecosystem. Everything is connected. The shifts in the boreal forest and the reproduction rates of northern mammals such as snowshoe hares may seem trivial, but they are essential in understanding life on this planet."