Warm Winter Worries

What a weird winter! In a region where people love to talk about the weather, the chatter has been constant. Skiers and snowmobilers (and the business they support) are particularly grumpy, but many people are trying to make the best of pleasant temperatures, despite the lack of snow. Humans are lucky. We have temperature-controlled homes and clothing that can be changed at the drop of a hat.

But what about non-human beings? How are they faring as temperatures fluctuate from 50 degrees to the teens with barely a skim of snow on the ground? This week, I combed back through my many articles about animals in winter to try and understand what some of the impacts may be.

In a typical winter, when we have at least six inches of snow, The Subnivean Zone forms. Because of the insulating qualities of snow and leftover summer heat from the Earth, a thin zone opens up under the snow, right at the surface of the ground, which stays at a pretty stable 32 degrees Fahrenheit.

Ruffed grouse make use of this warm blanket. When the snow is deep enough, they “roost” by doing a swan dive, leaving no tracks that would lead a predator to their warm bed. Without snow, or with icy crust, grouse may struggle to find a warm roost.

Small mammals like mice, voles, and shrews lose more heat for their body size than big ones, and therefore must generate more heat to maintain a healthy body temperature of about 98 degrees. Dr. Paula Anich from Northland College has estimated that shrews burn twelve times more energy per unit of body mass than an elk. They are constantly racing toward the edge of starvation, and all of the little beings must eat pretty much constantly to survive. Without snow, they face the choice of staying safe and warm in a burrow, or risking being eaten or getting too cold while they forage for food. “To the mouse, snow means freedom from want and fear,” wrote Aldo Leopold in A Sand County Almanac.

Even with snow, mice still need to fear American martens. These small weasels hunt along the log-lined runways where red-backed voles, mice, shrews, and squirrels travel. The snow is an excellent blanket for this lean mammal, who stores little fat and burns lots of fuel to stay warm. Snow also provides cover from bigger predators. The diets of foxes, fishers, and bobcats overlap with martens’ diets, and those larger carnivores will kill martens to eliminate competition.

Martens’ smaller cousins, least weasels, short-tailed weasels, and long-tailed weasels, all hunt under the snow as well. The subnivium is so important to Mustela nivalis, the least weasel, that their scientific name means “weasel of the snow.” These three fierce hunters need snow for an additional reason: their fur turns white in the winter, and a shift in the timing of snow cover can leave them vulnerable to their own predators.

Short-tailed weasels need snowy winters to match the camouflage of their white fur, and to provide safe, warm hunting grounds. Warm winters like this one will make it harder for them to survive. Photo by Emily Stone.

Autumn leaves can provide insulation before snow accumulates, and then continue to augment our fluffy white blankets. This protects plants’ roots from freeze damage. Dead leaves in the subnivium also provide essential habitat for woolly bear caterpillars, mourning cloak butterflies, luna moth cocoons, the eggs of red-banded hairstreaks, bumblebee queens, spiders, snails, millipedes, mites, and more. Some of these beings are frozen, some of them are active.

Wood frogs take refuge in leaf litter and very carefully allow themselves to freeze solid while the subnivean zone forms around them. Once frozen, their metabolisms are shut down, which preserves their limited energy stores. The moderating effects of the subnivium buffer them from energetically costly freeze-thaw cycles, and also reduce the potential for lethally low temperatures. After warmer winters, female wood frogs lay fewer eggs.

Like wood frogs, goldenrod gall fly larvae can freeze solid. They spend the winter in little round homes built inside goldenrod stems. While they can endure multiple freeze-thaw cycles over the winter, warmer temperatures increase their metabolism, and reduce their body size. When they hatch in the spring, the resulting adults – who do not feed – will not be able to lay as many eggs.

Beyond getting a break from shoveling, it’s not easy to find a silver lining for this weird weather. Here’s one possibility, though: deer ticks might suffer if a hard freeze comes while they are exposed without snow. If mice populations drop as well, then maybe there will be a decline in the spread of Lyme disease next summer. One can hope.

Jon Pauli, a winter ecologist from UW-Madison, once explained to me that “there's a complex ecosystem of interacting microbes, insects, plants and animals that we can't see but are active throughout the winter.” In fact, says one of his research papers, the subnivium is where the “majority of biodiversity in northern temperate areas spends the winter.” That’s why it’s so devastating when The Subnivean Zone doesn’t exist.

If you’d like to learn more about The Subnivean Zone, just wait until May 1st. Winter will be over, but the Museum’s new exhibit, Anaamaagon: Under the Snow, will give you an inside look at this special place.

Emily’s award-winning second book, Natural Connections: Dreaming of an Elfin Skimmer, is available to purchase at www.cablemuseum.org/books and at your local independent bookstore, too.

For more than 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. Our exhibit: “The Northwoods ROCKS!” is open through March 9. Our Winter/Spring Calendar of Events is ready for registration! Follow us on Facebook, Instagram, YouTube, and cablemuseum.org to see what we are up to.