by Erling Friis-Baastad
By following lynx and snowshoe hare tracks scientists have determined that only about 20 per cent of lynxes’ hare pursuits are successful, says zoologist Charley Krebs.
But forget about some smug hare hiding under a fallen spruce and celebrating her good fortune as one of the 80 per cent who get away. Stress takes its own toll among those who manage, moment by moment, to evade carnivore jaws and beaks.
When it comes to hare mortality, predators rule, says Krebs. And when it comes to the snowshoe hare’s famous 10-year-cycle, it appears these carnivores have plenty of say as well.
Krebs, now professor emeritus at UBC, has been conducting research in the Yukon since 1973. Passing through Whitehorse in late September, he talked about some of what he and his colleagues have learned over the past 40 years of studying hare cycles at the Kluane Research Station.
“Why does a snowshoe hare die? It dies because of a predator,” he says. Predation accounts for 95 per cent of snowshoe hare deaths. “If you take them into captivity they’ll live, three, four even five years, but in the wild if they live to be two years old it’s almost a miracle.”
The boreal forest is full predators. And snowshoe hares do not burrow or climb trees. If they’re lucky they find a fallen tree to hide under. Such vulnerable animals would have to reproduce quickly to maintain their population. They do – often with five to six babies in each litter and four litters in a summer.
That seems straight forward, but there’s more to it. “When we got details of the hare cycle down, several things were puzzling,” says Krebs. The death rate does go up when there are more predators – more lynx, more coyotes, more great horned owls, and so on. However, at the same time, or even just before the death rate climbs, when numbers are near their peak, the hares lower their reproductive rate.
“The cycle goes down to its low phase, but it’s entirely variable how long that is and you go out in the woods sometimes and for two years there are hardly any hares, and sometimes there are hardly any for four years,” he says.
The offspring of stressed females apparently inherit something called “a maternal effect.” This can apply to many animals, from pigs to people, and is a major area of study these days, says Krebs.
The inheritance is not genetic, but entirely physiological, he adds. Meanwhile, do the stressed offspring of stressed mothers pass stress to their offspring, and if so how many generations does this affect? How long does stress keep the birth rate down? When does the maternal effect finally disappear from a lineage? These are questions our colleagues and their students are now studying, says Krebs.
There has been plenty of media coverage of the collapsing hare cycles, he says. The so-called “collapse” has been blamed on climate change especially – “the universal explanation for everything these days” – as well as disease, parasites, moon phases, sunspots and habitat loss. It is known that there are peak highs and peak lows, and then higher lows and lower highs in the cycles. It seems that habitat loss in concert with predation is the major factor.
When their prey disappears, predators have two choices, beyond starvation, says Krebs. They can eat one another. Coyotes eat lynx. Lynx eat coyotes. Wolverines climb trees and prey on great horned owls…. “It’s dog eat dog out there,” he says.
But something else can occur. Predators “go walkabout.” Collared lynx have been known to travel 800 kilometres in a year. The species is “panmictic” across boreal North America, which means that lynx genes in one area are identical to those in another. Alaskan lynx and Nova Scotia lynx successfully mate.
Why go walkabout? Just for romance? Or could they be following prey?
“You read in books that hare cycles are synchronous across Canada,” says Krebs. Not so. “The hare-cycle peak begins, so to speak, in central B.C., then it marches north and about a year later it’s hitting southern Yukon and a year later northern Yukon and then it moves east into N.W.T. about a year later and Alaska, a year or two after that.”
When hares start to breed up in a region, predators start catching up. If the predators begin from their own low phase, it takes longer to for them to catch up than if there are many predators around when the hares start to proliferate. Where there are plenty of lynx, coyotes, foxes and great-horned owls, etc., they catch up to the hares pretty quickly and the hares have a “low-amplitude cycle.”
“So far this data is consistent here and in Alaska, so again it appears that predators are driving whether we have a super cycle or a modest cycle,” says the zoologist.
The cycles are not disappearing. They are going through peak peaks and low lows. “Now all this has to be qualified,” he says. “It takes 10 years to get one observation of the system and we’ve been working in Kluane for 40 years. We’ve got only four cycles nailed down.”
Where is all this going? “In terms of conservation of ecosystems the message is really simple,” says Krebs. “You have to have a large-scale ecosystem to keep the whole thing going where snowshoe hares do occur.” A large, contiguous boreal forest allows hares protective cover. Breaks in that cover leave the animals exposed.
What are we talking about when we talk about large enough ecosystems? Kluane National Park and Reserve is “just a postage stamp” in the scheme of things, says Krebs. “You’ve got to think about wildlife management on an enormous spatial scale if you want to protect an ecosystem like a boreal forest.”
From the northern U.S. up through Alberta and British Columbia forest cover is being broken up by agriculture and other development. A hare trying to dash from one patch of protective forest to another is a sitting duck. Out in the open they find themselves in “a landscape of fear.|”
“That is to say, what does a snowshoe hare worry about?” says Krebs. Does it worry about food? It’s surrounded by food. Does it worry about the weather? It doesn’t give a hoot about the weather, except that they eat more in the cold. What it worries about is predators. ‘Am I going to be eaten today?’”
This column is co-ordinated by the Yukon Research Centre at Yukon College with major financial support from Environment Yukon and Yukon College. The articles are archived at http://www.yukoncollege.yk.ca/research/publications/your–yukon