Inside the Outdoors: WSI is a game of inches and days
If you asked a stranger on the street what a "winter severity index" might measure, the answers could be pretty creative. Perhaps it would measure the number of Minnesota schools that close for a snow or extreme cold event. Or the number of roadside assistance calls for tows and jump-starts during the recent "polar vortex" that descended on our state from the Arctic.
In fact, the winter severity index—or WSI—is a measure used by wildlife managers as they anticipate the effects of winter weather conditions on wildlife. And, in Minnesota—not surprisingly—the concern of wildlife managers is especially directed toward whitetail deer, the bread-and-butter big game animal of our state. Among the threats to whitetail deer—threats that include predation, disease and collision with our cars, trucks and SUVs—winter can be the most devastating, as well as highly unpredictable.
On Sunday, during the brief warmup after we experienced some of the coldest temperatures in a generation, my Labrador retriever and I took advantage of the pleasant weather and took a long afternoon walk. Our route took us through a forested stretch where tall pines and spruce make the light subdued on even a sunny day. Now and then a deer trail crossed, as evidenced by heart-shaped prints in the skim of snow on the road shoulder. Then, typically, those trails plunged into the drifts among the trees.
I could tell the snow was deep without having to wade out there under those stately conifers. I knew because a deer trail in those places was not defined by distinct, separate footsteps. Instead, the tracks were connected by the drag of the deer's forelegs as it took each step, and in some places, the deer's chest plowing a shallow trough as it moved through the deep snow.
When deer are forced to fight deep snow, they expend more energy when they travel to feed, or—less often, but more critical—they flee a pursuing predator. As we all know, energy conservation is critical to the wildlife that stay here with us and endure the rigors of a Minnesota winter. Their chances of surviving to see the coming spring are greatly improved if they have enough to eat, and can limit the energy they must expend.
So how, exactly, is this winter severity index calculated? Wildlife managers count the days when snow depth equals or exceeds 15 inches, and the number of days when the temperature drops below zero. The two numbers—days with 15 or more inches or snow, and days below zero—are added together. If the total does not exceed 100, the Minnesota Department of Natural Resources considers it a mild winter. If it reaches 180 or more it's considered severe, when some deer will almost certainly die of winter's effects. And of course, there is a middle ground between these extremes.
One of these measures, low temperature, has an effect on virtually all wildlife in winter. No matter how you slice it, low temperatures require all creatures to expend more energy to maintain critical body temperature. Snow depth, on the other hand, is closely linked to the survival of deer. Let's face it: the top wildlife management priority in Minnesota is the whitetail. So it's not much of a surprise that the WSI measures a weather factor that directly affects deer survival.
When snow gets deep enough, deer will "yard up," limiting their movements to an area where they trample the snow down enough to make travel easier. But a consequence is that the food supply in this restricted area can eventually be depleted. If snow depth doesn't diminish soon enough with warming temperatures, deer can face starvation. Or, if not starvation, then weakened condition. This can result in failure to bear healthy fawns, and greater vulnerability to predators, like gray wolves and even coyotes.
It's ironic that the same deep snow that can threaten the survival of deer can be a benefit to some wildlife. One obvious example is an upland game bird that shares the northern wooded habitat with the whitetail. That creature, of course, is the ruffed grouse. While some songbirds that remain with us in winter will "hole up" within cavities in trees, and pheasants will take shelter in dense cattail swamps, the ruffed grouse will use deep snow to its advantage.
A grouse will burrow completely under the surface of soft snow, whose insulating properties can yield a temperature that is 20, 30 or more degrees warmer than the air above the snow. This allows grouse to conserve energy and spend less time foraging to find food. This, in turn, means less exposure to predators and harsh winter elements.
So far this winter, almost the entire state is considered to be down in the mild WSI range, with 50 or fewer days having temperatures below zero or a snow depth of 15 or more inches. Only the far Northeast corner of the state, the region known as the Arrowhead, has local areas with an index of 80 to 90. But there could easily be 60 or more days—reaching into early April—before snow depths in the northern part of the state decline appreciably. Remembering that both snow depth and sub-zero temperatures contribute to the WSI, it's way too early to say there will be no parts of the state that end the winter with severe readings.
It's ironic that snowy winter conditions that favor the ruffed grouse—not to mention Minnesota skiers and snowmobilers—will in contrast always pose a potential threat to our premier big game animal, the whitetail deer. Which brings to mind that favorite Minnesota saying: "A lot of people talk about the weather, but nobody does anything about it."