- It’s long been known that snowshoe hare numbers in North American forests rise and fall dramatically in a predictable 10-year cycle. A year or two later, Canada lynx populations follow the same pattern.
- After decades of research, the dominant view is that the hare cycle is largely driven by predation, though there are still many mysteries to uncover.
- New research is shedding light on the lynx’s hunting behaviors and the asynchronicity of population cycles from region to region.
- Researchers are also looking at how human causes, including forestry practices, climate change and escalating wildfires, may be impacting lynx-hare cycles.
Open any ecology textbook and you’ll find the Canada lynx, the snowshoe hare, and their wildly oscillating population cycles offered as a classic example of the intimate relationship between predator and prey.
The Canada lynx (Lynx canadensis) is a handsome, medium-sized felid, with a thick coat, tufted ears and large paws — an adaptation to deep snow. Its distribution and abundance in North America has long been inextricably linked to that of the snowshoe hare (Lepus americanus), its primary prey. The relationship also fascinates the public, with photos of the cat chasing the hare across snowy landscapes a staple of wildlife photo essays.
Snowshoe hare numbers rise and fall dramatically in a regular cycle of eight to 10 years. A year or two later, Canada lynx numbers follow. This enduring relationship is borne out by Indigenous knowledge and centuries of fur trapping data, and is integral to boreal forest science. Now, researchers are questioning if human activities, including forestry practices, climate change and wildfires may be impacting lynx, hares and this natural cycle.
An enduring ecological relationship
Stan Boutin, a professor at the University of Alberta in Canada, has been watching the lynx-hare drama play out in Kluane, in Canada’s Yukon Territory, for nearly 50 years. Over this time, hare populations have exploded and crashed five times, with lynx populations following behind.
“We’re on a heavy increase phase now. So the prediction is [hare numbers] will still be quite high this spring, maybe one more summer of good breeding, and then, boom,” Boutin says. “It’s always exciting,” and he remains hooked.
Boutin and his colleagues have analyzed the cycle from many angles: vegetation and food availability for the hares; predation pressure; reproduction and maternal stress; and other factors. After decades of observation and experimentation, many researchers believe the hare cycle is generally driven by predation, though food availability and even stress could also play a role. But there are still plenty of mysteries.
“That’s what drives you. You say, ‘I’ve got to get this answer.’ And that’s why you want to go back,” Boutin says. “And when you get … new technology, [and] really creative students coming in to look at these things in different ways, that’s what makes it so much fun.”
Now, with climate change and other major changes sweeping across the cat’s range — which stretches from Alaska, through most of Canada (except for the high Arctic), and into parts of the Rocky Mountains in the United States — Boutin and other researchers are trying to figure out what that may mean for hares, lynx and other predators, now and in the future.
Climate change enters the picture
In Kluane, the maximum snow depth has declined by a third over the past two decades, according to a 2020 Nature Climate Change study, on which Boutin was a co-author.
Lynx and hare are pretty evenly matched in all types of snowy conditions, Boutin says, as both, with their big, hairy footpads, are adapted to float atop deep powder.
That’s not the case with another snowshoe hare predator, according to the 2020 study. Coyotes (Canis latrans) have smaller footpads, so tend to sink in deep snow, slowing them down. When snowpack is thin, as in recent years, coyotes can gain traction, making them more effective at hunting, thereby putting more pressure on hares, the study found.
On the other hand, once the snow fully melts, hares get the upper hand over both lynx and coyotes; few adult hares die in summer, with the carnivores forced to hunt a wider variety of prey, including hare babies (called leverets) and red squirrels, as an alternative.
Climate change is also affecting the snowshoe hare’s ability to stay camouflaged. The hares molt twice a year, sporting a white coat in snowy winters and a brown one in summer. With shorter winters due to climate change, there can be a mismatch between the timing of snow cover and molt; that means you can get white hares on a mostly brown landscape in early spring and late fall. However, research in the Yukon found that a mismatch in molt and snow timing didn’t affect predation in spring, and actually gave hares an advantage in fall, according to a 2022 study published in Ecology, on which Boutin was also a co-author.
Taken together, the different impacts of a couple of extra snow-free weeks on either side of winter could change the amplitude of the predator-prey cycle, Boutin says. But he’s not convinced climate change is going to have a major effect on the essential relationship between lynx and hare in the core of their range — at least not yet.
“My feeling is that right now, it’s likely the cycle won’t change. It’ll still go up and down, particularly in the heart of the boreal forest. It might have a bit of difference at the edges of the snowshoe hare’s range, but not in the center,” Boutin says.
That isn’t to say climate change isn’t a concern for lynx, particularly on the edge of their range. A 2020 study in The Journal of Wildlife Management noted that Canada lynx populations in the U.S. state of Washington will be “increasingly challenged by climatic changes,” particularly at lower elevations.
New tech, new insights
Advancing technologies such as biologging, which uses a range of small, animal-attached monitoring devices, have opened up new ways to study the hunt and other interactions between predator and prey.
Much of what’s known about lynx diet and hunting behavior has come from scat analysis and snow tracking. But both techniques have limitations, particularly when it comes to studying small prey.
Borrowing an idea from the Fitbit craze, Emily Studd, whose doctoral research on food webs in Kluane was co-supervised by Boutin, wondered if strapping an accelerometer and a tiny audio recorder onto a lynx could provide new types of data.
But getting those biologging devices on the lynx wasn’t easy, and required trapping the cat. Unfortunately for researchers, trapping is best done in winter, when it’s easier to travel by snowmobile and when hibernating bears rarely steal trap bait.
Of course, this brings hardships: Winter temperatures in Kluane regularly fall to -25° Celsius (-13° Fahrenheit) or lower. And while trapping, Studd and study co-leads Rachael Derbyshire and Allyson Menzies lived in a poorly insulated cabin warmed by a wood stove, where water bottles froze overnight. They had to check each of their 30 traps twice a day, a task taking at least two hours. If they caught a lynx, they’d sedate it, take samples, and collar it with the biologging device.
“There were many days where we would be handling the animal until 1 or 2 in the morning, and then we would have something else in the trap at 6,” Studd says.
All that hard work paid off. Once in place, the accelerometers revealed in exquisite detail how much effort the lynx put into hunting, while audio recordings offered clues to researchers as to what was being hunted and if the cats were successful. “Sometimes we got the scream, and so we could identify the species that way,” Studd says. “And other times it went down to how long it took the [lynx] to eat.” Studd used snow tracking to validate the biologger data.
The new lynx data on hunting behavior and diet also shed light on the forces driving the lynx-hare cycle during the very active and critically important winter period.
The researchers found that snowshoe hares made up 40-80% of a lynx’s kills, according to a 2021 study published in Methods in Ecology and Evolution. The lynx also scavenged hare carcasses, and ate, on average, 1.2 hares per day.
These biologging methods could revolutionize the study of hunting behavior and diet in a number of species, particularly when looking at small prey, Studd says.
Verifying a traveling wave in Alaska
In neighboring Alaska, Ph.D. candidate Derek Arnold delved into another mystery: Do hare and lynx populations all cycle simultaneously, or does the peak move across the landscape?
To find out, Arnold, now a postdoctoral researcher at the University of Alaska Fairbanks, studied data from lynx collared at four sites across Alaska between 2018 and 2022.
Similar to the research in the Yukon, the Alaskan team also trapped and collared lynx in winter, working out of remote backcountry cabins and getting around on snowmobiles in frigid weather.
When they started, lynx were everywhere and readily wandered into traps to take the bait. “We were at that population peak,” Arnold says, “so we were throwing out all the collars we had.”
Things soon changed.
The first sign of the impending population crash was a drop in lynx reproduction rate, starting in the easternmost study sites, a discovery made as researchers checked dens to document litter size. At the study’s start, they found up to eight kittens in a litter, Arnold says. The following year, litter sizes had been halved; the year after, there were no kittens at all.
The lynx didn’t starve as the hare declined, but many started moving out and moving on. “They’re kind of like, ‘Oh no, this is not great here. Let’s go and see if we can find better, greener pastures,’” Arnold says.
To Arnold’s surprise, GPS collar data showed that the animals dispersed in all directions, traveling thousands of kilometers.
The results of the study, published in 2024, support the hypothesis that lynx population cycles move in a traveling wave, with the peak shifting over time from east to west, Arnold says. Mathematical modeling showed that the lynx’s remarkable ability to disperse long distances shapes the curve and direction of the wave, suppressing hare populations over extended periods of time so that they can’t immediately recover.
This could be one reason why lynx populations farther south, in places like Montana and Colorado, don’t cycle to the same degree, Arnold says: There just isn’t enough connected habitat for lynx to disperse over long distances.
Forestry and fires bring species stress
Wildfire is an integral part of boreal forest ecology, clearing out older forest and creating large patches of different-aged stands.
But in central British Columbia province, decades of fire suppression and forestry practices have transformed the once continuous forest into small cut blocks checkerboarding the landscape “like some kind of rash,” says Donald Reid, emeritus scientist at the nonprofit Wildlife Conservation Society Canada.
Starting in the 2000s, commercial fur trappers in the region voiced concern that snowshoe hare cycles had dampened down, with lynx less abundant. To determine if forestry practices were to blame, Reid and colleagues looked at hare and predator abundance in different-aged forest stands, first in central BC and later in the southeastern Yukon, near the BC border.
“Hares really need two things” to survive, Reid explains. “They need to be sure that the raptors who are sitting up in the trees or flying over can’t readily just drop down and get them. And they need to know there’s lateral cover on the forest floor [hiding them] from the main mammalian predators, mainly the lynx, but also coyotes and sometimes marten, sometimes wolverine.”
In central BC, that ideal forest habitat is found in mid-seral — 15-to-40-year-old — stands, Reid says. Only landscapes with large patches of mid-seral stands support big hare, and by extension lynx, cycles, according to a 2024 study in Forest Ecology and Management, on which Reid was lead author. Those patches need to be bigger than 1,000 hectares (about 2,500 acres), similar to what’s found after a medium to large wildfire. But most cut blocks in the region were small, the study found.
The problem is that “When you introduce fire suppression, in order to keep your timber harvesting supply high, you wipe out anything in terms of a large-scale disturbance,” Reid explains. If fire suppression is to continue, forestry methods need to change by doing a better job of mimicking natural disturbance processes (especially fire), that are specific to each region. With proper planning, this change could allow large forest patches, including large stands of old growth, to persist, supporting hares, lynx and other species.
Likewise, global warming is adding a new challenge for both lynx and hare by changing the nature of wildfires. While fire has long shaped boreal habitat, if wildfires flash through an area too often, or are so hot and intense they burn up the soil seed, those blazes can change the forest types that grow back, altering habitat and the lynx-hare cycle.
In some places, that’s already happening, Reid says. For example, in some spruce-dominated areas of Canada’s far north and Alaska that have been impacted by such fires, young conifers aren’t regenerating and deciduous trees such as aspen are growing instead.
That could mean more plentiful food for hares, but not enough cover to hide from predators, especially in winter. Researchers in the north aren’t yet sure what these changes will mean for lynx and hare populations, but it’s something to keep an eye on, Reid says.
The southern limit
The reintroduction of Canada lynx to the U.S. state of Colorado began in 1999. Jacob Ivan got involved in lynx research there in 2005, at the tail end of the reintroduction. Back then, he had no inkling how much the landscape would be transformed in the next two decades.
“I can remember sitting there with our longtime crew leader, looking off across the valley, and I remember saying to Grant, ‘Are those trees over there brown?’ And he’s like, ‘No, I think that’s just haze.’ … Neither he nor I had any idea of what was coming.”
That was in 2010, and those brown trees were the start of a massive bark beetle epidemic that would leave vast stands of dead conifers in its wake — an event brought on by prolonged drought and warmer temperatures. These conditions meant more beetles survived over winter, with drought-stressed trees even more susceptible to the pests.
The bark beetle population explosion, along with urban expansion and increasing wildfires, has utterly transformed Colorado’s forested mountain landscape. Ivan and other researchers are trying to figure out what that means for lynx.
So far, their research shows bark beetle outbreaks aren’t necessarily bad news for hares or, by extension, lynx, Ivan says. The beetles kill a lot of older trees. But the understory remains healthy, and young saplings get more sun in the more open canopy and regenerate, giving hares needed food and cover.
Ivan’s main concern is that beetle-killed stands don’t provide enough food for red squirrels, another important food for lynx, particularly when hare populations crash. So far, though, the lynx seem to be doing fairly well, Ivan says. But he has other concerns.
“Fire is the thing that probably worries me the most and keeps me up at night, because it’s a much, much more severe impact, and over a huge area,” Ivan says.
Studies definitively show that fires in the U.S. West are becoming more widespread, frequent and intense, and are the kind of blazes that burn up the soil seedbank, leaving nothing but ash in their wake. Those burnt-out swaths don’t provide food or cover for hares, and so are of no value to lynx, an explanation backed up by observations. “We’ve seen that with a lot of our [lynx] location data. They … come up to the edge of the [burned area] and then stop,” Ivan says.
He says the reintroduced population has fanned out and probably now occupies all suitable areas, since this wildcat has limited habitat in Colorado. In future, worsening climate change and fires could wipe out huge swaths of habitat, almost overnight, leaving lynx with fewer places to hunt.
Ivan says he’d love to sit down with the Forest Service, silviculturists, recreation managers and land managers, and ask them if there are techniques available “to protect these really important areas from wildfire. Can you do that? Or is that … off the table? Are we at the mercy of whatever happens?”
Banner image: With thick coats and broad paws, Canada lynx are supremely adapted to deep snow. Image by Keith Williams via Wikimedia Commons (CC BY 2.0).
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Citations:
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Kennah, J. L., Peers, M. J., Vander Wal, E., Majchrzak, Y. N., Menzies, A. K., Studd, E. K., … Boutin, S. (2022). Coat color mismatch improves survival of a keystone boreal herbivore: Energetic advantages exceed lost camouflage. Ecology, 104(2). doi:10.1002/ecy.3882
King, T. W., Vynne, C., Miller, D., Fisher, S., Fitkin, S., Rohrer, J., … Thornton, D. (2020). Will lynx lose their edge? Canada lynx occupancy in Washington. The Journal of Wildlife Management, 84(4), 705-725. doi:10.1002/jwmg.21846
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