Life Under the Snow
The winter landscape can appear essentially barren of wildlife, but a surprising array of life thrives above, within, and beneath blankets of snow.
In this subnivean space, sheltered from temperature flux, shrews devour insect eggs and pupae every day, mice eat seeds, and voles graze on grass. Some rodents live more communally, clustering in dens for warmth, excavating latrine caves to isolate the scent of their feces and urine from their living quarters. When small mammals do surface--sometimes to seek new food sources--they ascend and descend the snowpack in tunnels where radiant heat has thawed the snow along tree trunks and shrubs.
Halfpenny points to a glade of pine trees down the slope. Predators will come along and check every tree trunk, he says. "To a certain extent the same thing is going on with coyotes. But the key predators are martins and most of all weasels, because they go up and down along the tree trunks, too."
The snow conceals mice and voles from many of their summertime avian predators--owls, hawks, and kestrels--and at certain depths and hardnesses, shields them from foxes, coyotes, and bobcats. Weasels move in subnivean space almost as easily as their prey, but their long thin bodies are not efficient for heat storage, so they line their own subnivean dens with fur plucked from their victims to create more warmth for resting.
Sizing up the hole he's dug and data he's collected, Halfpenny knows right away that the snowpack on this day in Yellowstone is isothermal--it's headed for a meltdown. Thawing has progressed enough so that water percolates down through the pack and temperatures are nearly uniform throughout, within a tenth of a degree of freezing. The snow can no longer recover its winter characteristics.
Earlier in the winter, depth hoar--fragile crystals with minute spaces between them--dominates the snowpack. Deer mice, voles, shrews, and weasels can move freely beneath and within it. Grouse often submerge themselves in soft snow as shelter from nighttime cold.
But spring's isothermal conditions are dangerous for mammals below. Because the snow is water-saturated, it's lost many of its insulating properties. A string of too-cold or too-warm days could be disastrous. "Should a real cold front move in, a cross section of snow could freeze and the animals could be trapped in there," Halfpenny says. This late in the winter, food supplies are grazed over. Being trapped in one place by an ice layer could limit the animals' ability to forage, which could be fatal.
A sudden, sustained rise in temperature is equally dangerous. "Since water is percolating down, everything [at ground level] is pretty wet," Halfpenny says. "In a real heavy melt, small mammals can get wet and get hypothermic--or, worse yet, drown. This can be a delicate time of the year for small mammals."
Plants are in a different position, as late winter and early spring's wetter snow transmits more short-wave radiation to the ground. Snowbank buttercups blossom through the drifts, and some grasses begin growing long before the snow melts away. Research suggests that chemical signals in growing grasses trigger reproductive activity in voles, so that they start bearing offspring beneath the snow's protective layer. Halfpenny says the years in which this happens account for significant pulses in vole populations--subnivean litters may survive in conditions of reduced predation and mature to breed in greater numbers, flooding the land with voles.
While many insects either die off in winter or enter a state of suspended animation, some, including snow flies and crab spiders, breed under the snow, feeding on springtails, tiny creatures that appear in such hordes they may make the snow's surface appear coated in soot. Insects that don't remain active survive in a number of ways, including by burrowing into the leaf litter or by "supercooling" their bodies. They pump glycerol into their cells to inhibit ice formation, which would rupture cell walls. Each cell also shrinks when the moisture in it moves toward the ice crystals that form between cells, allowing supercooled creatures to survive in sub-zero environs.
What's happening under the snow among even smaller organisms is far less understood, although in the past decade scientists have made remarkable and important discoveries. When Paul Brooks, now a professor in the department of hydrology and water resources at the University of Arizona, gave his first presentation to the American Geophysical Union in San Francisco in 1993, he summarized two years of research indicating that in midwinter, the ground beneath the snow was thriving with living, breathing, eating, growing microbes.
Afterward, Brooks says, the hall full of scientists was silent. Nobody believed him. "Nobody asked any questions," he says. "One person said I was incompetent, that I clearly didn't know how to measure this stuff. Another person said we all know there isn't life at temperatures that low."
Brooks pressed on and today is a pioneer of subnivean microbial biochemistry. He and others have found that the subnivean world seethes with microbial life--vast mats of fungi and bacterial colonies--many of which were not even known to science 20 years ago.