|Jasper Fitzhugh, January 26, 2012
Photos: Mark Bradley, Boreal Nature Photos.
While bushwhacking with friends in east Jasper National Park over the holidays, I almost stepped on a fat little brown vole weaving its way through what was left of a series of tunnels it had built under the snow earlier in the winter. The wind and rain had scoured away the tops, but well-worn paths and some of the crusty white arches remained, evidence of a world that exists beneath the snow.
I’ve always been fascinated by this hidden world. I think it comes from my childhood obsession with the movie The Secret of NIMH. I loved (and kind of feared) the rats of NIMH, with their human-like intelligence and secret, mechanized world hidden underground. Like the farmers in that movie, we would be mistaken to look over the winter snowpack and assume nothing lies beneath.
There is an entire ecosystem at work in and beneath the snow, known as the subnivean. Many critters are busy hunting and foraging. Dead plants and rotten logs host fungal feasts for winter-active springtails and flies, while aphids feed on wintergreen plants. These insects are gobbled up by shrews, which are in turn hunted by larger creatures.
Weasels are at the top of the subnivean food chain, able to hunt within the snowpack. They follow tunnels along tree trunks and shrubs to ferret out the small animals below. Their body shape is especially adapted for this kind of work – they are long, sleek, and designed for snaking down burrows after their prey. As Ben Gadd says in The Handbook of the Canada Rockies, their winter hunting excursions “spread terror through a series of winter runs that small rodents make at the base of the snowpack.”
The winter runs are in the “depth hoar,” the loose, sugary snow closest to the ground. It makes for warmer and easier travel for rodents like mice and voles. There, they graze on grass or insect eggs near the ground. The deep snow helps protect them from birds of prey, coyote and lynx. However, animals like foxes and owls can hear them scurrying around beneath the snow, and easily pounce upon them, especially if the snow is shallow.
Other animals that seek refuge in the snow include birds such as grouse, which stay warm by covering themselves in powder near the surface. Pine martens find much needed rest curled around submerged logs and stumps.
For all the benefits, there are trade-offs to a life in the snow. James C. Halfpenny and Roy Ozanne, authors of Winter: An Ecological Handbook, describe the subnivean world as “both a benefactor and death trap for those living beneath the snow.” The impact of wind, cold and predation are reduced, but moisture, carbon dioxide, available oxygen and darkness are factors limiting survival. The snowpack is not a uniform habitat, but broken up into usable and unusable parts. While wind crusts and dense layers make good foundations for tunnel floors, they can also act as barricades for animals making their way up and down through the snow layers.
Plants, bacteria and fungi that remain active throughout the winter also produce toxic carbon dioxide as a by-product of their respiration. Without adequate venting to the surface, this gaseous build up can be dangerous.
In response, some small mammals are thought to dig tunnels that serve as vents to the surface. In addition, rodents like voles decrease their oxygen consumption, heart rate, and body temperature in an effort to compensate for high CO2 levels. This makes them a little slower, and may make them more at risk of becoming vole steak for a weasel. How they make these trade-offs depends on the winter conditions at the time.
Right now, we don’t have a lot of snow in the valley, and it makes me wonder how the little critters that typically spend most of their winter beneath the snow are doing.
According to Halfpenny and Ozanne, overwinter survival for animals is a product of many things, including the impact of the climate, the availability of food and snowpack conditions.
Depending on the temperature, our voles and mice might be doing just fine. In warmer winter temperatures, they will likely continue to graze on the surface, foraging on grasses and seeds. The little brown vole I saw in the warm temps of early January appeared in good health, quite the little butterball.
The severe cold of last week, however, has likely resulted in the increased mortality of small animals that would normally use the insulation of the snow pack as protection against the cold.
Don’t worry though, the small mammal populations can take it! They are some of the fastest reproducers around. Female meadow voles, for example, pump out three to four litters every year, averaging six pups a shot. They need to produce a lot of little voles, because they are eaten by a plethora of animals across the seasons, from weasels to wolves. Trout or pike will even gobble these little guys up if given the chance.
As you walk along a trail sided by deep snow, look for small holes on either side that act as exit and entry points for rodents boogying across the packed trail. Or maybe you’ll see some tiny tracks across the top of the snow where little critters have had to pop up due to an impassible area beneath. A closer look will reveal some of those NIMH-like secrets of the world beneath the snow. It’s doubtful you’ll see a talking rat wearing a cape, but you never know.