Snow as an ecological factor


Last week, Long Island had its third snowfall of the 2008-2009 winter season. Each of the three was in the 2-inch total accumulation range, and each of these thin covers of snow was gone in less than a week. Here on Long Island where our winter temperatures are greatly moderated by the huge mass of 35-to-40-degree water surrounding it, snow is a novelty. But further north in New York and in much of the world, snow, specifically the amount and duration of snow cover, can have a major influence on people’s daily lives, affecting highway department budgets and wreaking havoc on travel plans, among other things.

It is not surprising that snow also plays a major role in the natural world. Just as we can take advantage of snow’s insulating characteristics and construct igloos or quinzhees for shelter and relative warmth, so do many animals seek protective shelter from harsh winter weather in the snow pack. For many species, snow, and the micro-environment the snow pack creates, is critical to their survival.

Since we associate snow with cold, it seems a contradiction to talk of it in terms of warmth, shelter and 
insulation. Yet the pack of a fresh snowfall contains a large volume of air trapped among the snowflakes, and this characteristic of snow gives it an extraordinary insulation value. In fact, that value (for fresh snow) has been calculated to be nearly that of fiberglass insulation.

One of my professors used the phrase “Nature’s long underwear” to describe the ecological role of snow cover in the north country. I think a more accurate label for the amazing insulative value of snow cover is “Nature’s down comforter,” while leaf litter functions as a protective “long underwear” covering.

So important is the snowpack micro-environment to plants and animals that ecologists have given it a name: the subnivean environment. A two-week study of air and snowpack temperature fluctuations found that, at a snowpack depth of 10 inches, the subnivean temperature remained at 32 degrees despite the outside air temperature dropping as low as -25 degrees! With temperatures fairly stable near freezing, humidity constant, and protection from the chilling wind, animals can greatly reduce their winter energy demands by spending time in the snowpack.

Some animals, such as voles, shrews, and mice, create a network of tunnels and dens and reside full time in the subnivean zone. Beaver and muskrat lodges benefit from a thick covering of snow insulation. Others, such as 
the grouse and ptarmigan, only 
burrow into the snowpack on occasion, perhaps to avoid a particularly cold night. Further north where temperatures are consistently colder than here, these animals may perish without snow.

Snow has also shaped the distribution of many animals, and been the driving force behind some interesting adaptations to cope with it. The white-tailed deer is poorly equipped to deal with deep snow, and its northern range limits are largely determined by average annual snow depths. Deer can survive periodic deep snows by retreating to conifer stands, called deer yards, where the snowpack is shallower and it is easier to get around. But a winter with consistently deep snow spells doom for deer, as the available food in the yards is depleted.

The flexibility and length of the moose’s legs enabled it to travel in deep snow where deer would flounder and perish. Caribou are no larger than deer, but their hooves are bigger and can spread out much wider than a deer’s, enabling them to snowshoe atop the deep snow. And the boundary separating the distributions of snowshoe hare and cottontail rabbit, and lynx and bobcat, is best described as differences in total snowfall. Again, it is the ability of the snowshoe hare and the lynx to move in deep snow, as well as the hare’s ability to change between brown to white with the seasons, that enhance their survival in snow country.

Turkeys and red and gray foxes are not well equipped to travel and forage in deep snow. These animals are tough, resilient and opportunistic, with the ability to digest many different sources of energy and lose as much as 40 percent of their body weight to survive a winter of deep and persistent snow cover.

Mike Bottini is a naturalist and author of “The Southampton Press Trail Guide to the South Fork” and “Exploring East End Waters: A Natural History and Paddling Guide.” Check for Mike’s field naturalist classes.

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