Everybody loves snowshoe hares – especially for dinner. Coyotes, great-horned owls and lynx catch the speedy adult hares. Baby hares fall victim to squirrels, weasels and various birds of prey. Marten will eat any size of hare, large or small.
With so many animals snacking on them, it’s a wonder snowshoe hares survive at all. But they do – sometimes in startling numbers. Snowshoe hares are famous for their population cycles, from boom to bust and back to boom, all within eight to 10 years.
The last few population booms, however, have been far from impressive. In the early 1980s in the Kluane area, at the peak of a population cycle, researchers estimated an autumn hare density of about 4.5 per hectare. There were hares everywhere, says Jean Carey, who helped live-trap hares for the count.
“For several months we would set 50 traps and catch over 40 hares, night after night. Talk about rewards for our efforts!”
The next couple of peaks in the hare cycle were considerably lower in the same area. In the late 1980s, populations topped out at about 2.5 hares per hectare, and the peak in the late 1990s crept only a fraction higher. The most recent population peak, in 2006-2007, was the lowest yet, with a maximum of about 1.2 per hectare.
Since then, numbers have dropped by half. However, that doesn’t mean there are no hares out there or that snowshoe hares are in danger of disappearing, says Mark O’Donoghue, Environment Yukon’s regional biologist in Mayo.
“In the bush, at least in this area, you can still find some good pockets of hares, as evidenced by tracks, but overall they’re very low right now.”
He’s also not sure that the pattern of diminishing population peaks is anything out of the ordinary in the longer term. Historical records, mainly based on fur harvests, hint at a wide variation in the scale of the population cycle, but those records have limitations.
“We don’t have good data on actual hare numbers back any further than about the 1970s, and those data are from scattered studies,” O’Donoghue says. The Kluane studies are actually the longest complete time series of population studies he knows of. They began as part of a study by university researchers looking at the ecology of Kluane’s boreal forest and have been continued in recent years under an ecological monitoring program supported by several agencies.
Why bother counting hares? They might not look it, but snowshoe hares are important. They are what is called a “keystone species”- a vital link in moving the boreal forest’s energy resources from plants to other animals. Hares nibble away at forest shrubs, and the forest’s larger predators nibble away at the hare population and everybody prospers.
When there are plenty of hares, shrubs are mowed down, especially in winter when the hares hone in on the dwarf birch and willow shoots that project above the snow. At the same time, winter predators eat hearty, and local lynx, coyote and owl populations boom. When the hare population drops, birch and willow bounce back and the predator populations shrink. Lynx populations usually fluctuate with roughly the same frequency as hare populations. Exactly how these interlinked population cycles work is a puzzle researchers are still working on.
“The main cause of mortality of hares is clearly from predators in all of the cycles we’ve observed, but you hear stories of finding large numbers of hares apparently starved to death after decimating their food supplies,” says O’Donoghue. “Not recently though.”
While being killed and eaten certainly limits a hare’s productivity, that’s not the only way predation affects the hare population. The stress created by high numbers of predators reduces the reproductive rate of female snowshoe hares, and they pass that reduced reproductive rate on to their offspring, adding to the downward pressure on the population.
Once the hare population is low, however, other factors can come into play. More food is available because fewer hares have been eating it. And predator populations have dropped, so more hares can survive.
Of course, it’s not that simple. There are plenty of other things that can tip the balance one way or the other, including temperature, snow depth, fires, plant growth and changes in the populations of predator species and other prey species. O’Donoghue says it’s hard to tell what might produce a super-peak in hare populations. “Maybe all the stars just line up right (figuratively, that is).”
Juvenile hares are preyed upon by small predators like squirrels, he says. But squirrels also eat conifer cones, which are a lot easier to find and catch.
“If you had super cone crops during a cyclical increase in hares (leading to the squirrels eating cones instead of baby hares) and low numbers of other predators like weasels, marten, and raptors, maybe the hare numbers are able to take off, and it just takes longer for the effects of predators to catch up.”
On the other hand, high numbers of hungry smaller predators might cause a slow increase in hare numbers that predators are more able to keep up with, leading to a low peak. “I’m just speculating though,” he adds.
O’Donoghue is involved in the Community Ecological Monitoring Project, which has expanded the Kluane monitoring to several locations in the southern and central Yukon. Snowshoe hare monitoring is part of its program. By establishing the natural range of variation in numbers and the causes, he says, we’ll be better equipped to spot changes in the system and understand why they are occurring.
For more information about snowshoe hares, follow the link at http://www.environmentyukon.gov.yk.ca/wildlifebiodiversity/mammals.php, or contact an Environment Yukon regional biologist.
The Your Yukon column is co-ordinated by the Northern Research Institute at Yukon College. This column is sponsored by the CircumArctic Rangifer Monitoring and Assessment Network. A full list of funders and all past articles are available at www.taiga.net/yourYukon.