As snow goes, where go the animals?

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Snowy winters: latest casualty of global warming
bright orange fox with black feet/legs in mid pounce above snowy bank
Photo: Colorado, Eric Magnuson
A red fox hunts for a mole living beneath the snow in the subnivium.

Globally, 2012 was one of the warmest 10 years on record, but the term “global warming” is misleading: It’s not just about temperature, but also about melting ice and rising sea level, animal pollinators and seed formation. It’s about agriculture and the economy, about wildfires and storms.

And global warming is also about snow. Across the northern hemisphere, site of 98 percent of snow accumulation, snow is arriving later and leaving earlier. Wet, dense snow is replacing, dry, powdery snow.

In 2012, according to State of the Climate, 2012, snow cover on land in the Northern Hemisphere averaged roughly 24.6 million square kilometers, about 0.3 million square kilometers below the 1967-2012 average. 2012 ranked as 12th lowest in snow area during that period.

And 2012 was part of a long-term slide.

So what happens to the animals that live in the snow? As global warming continues, will declining snow become yet another face of the environmental disaster caused by the glut of greenhouse gases?

June snow cover, assessed as the difference from the 1971–2000 average

7 graphs
(Blue: above-average cover; orange: below-average cover.) The map shows a long decline in the amount of snow across the snowiest parts of the North. Despite the uproar over the melting Arctic Ocean, the decline in June snow cover was much greater between 1979 and 2011. According to NASA, June snow cover dropped 17.8 percent per decade, compared to 10.6 percent per decade for September sea ice area.

Snow is difficult to measure. It comes and goes. A little accumulates, or a lot. It may fall cold and fluffy, then melt, turning wetter and denser. When you need the global picture, snow is hard to measure from space, and expensive to measure from the ground.

And regional trends can mock the global ones: A recent analysis of snow records in California, reported “… no statistically significant trends” in either the whole period (133 years) or in the most recent 50 years in the areas with the best data.

June snow cover trend 1967 – 2012

graph showing the june snow cover in the North Hemisphere has been declining with fluctuations from about 10 millions of square kilometers in 1970s to 5 millions in 2010s.
The decline in June snow cover, a reflection of global warming, accelerated after 2003. Between June 2008 and June 2012, North America set three records for low snow cover. In Eurasia, each successive June from 2008 to 2012 set a new record for the lowest snow cover extent. Data from June is chosen to sum up precipitation and temperature over the entire cold season.

Built for snow!

You could guess that limiting an animal’s habitat will cause it difficulty, and the limited number of studies do show that snow shortages affect mammals that are highly dependent on snow.

The starting place for investigation is those animals that are snow specialists. The wolverine, for example, is a carnivore that travels atop snowy landscapes with huge feet. “With those big feet; they can float on snow,” says Kevin McKelvey, leader of the monitoring and disturbance ecology team at the U.S. Forest Service Rocky Mountain Research Station in Montana. “They have figured out how to live on the snow.”

In the winter of the inland Canadian West, after wolves have headed south to follow the caribou migration and the Arctic fox have moved north to the Arctic Ocean, the wolverine is left by itself “in the middle of a great white barren, but they have figured out a way to exist in a niche where nobody else wants to exist,” McKelvey says.

a wolverine holding a tree branch
The wolverine is the king of the snowy, wintry barrens in the Canadian West. Notice those claws?

While some mammals, like the weasel, chase rodents below the snow, the wolverine “is too big to play those games, but it is adept at finding caribou carrion below the snow,” McKelvey says.

  A 2010 study paints a perilous present and fearsome future for the ferocious wolverine: “The results presented here reveal patterns consistent with the hypothesis that declining snowpack is negatively affecting the population dynamics of a snow-dependent carnivore, the wolverine. Wolverine populations are declining most quickly in provinces where snowpack levels are declining the fastest.”1

Climate models predict more of the same, McKelvey told us: wolverine habitat is expected to shrink to 37 percent of the current area by the end of this century, causing genetic isolation and reproductive problems.2

a white hare on the ground covered with brown branches and withered grass.
Courtesy L. Scott Mills, University of Montana.
Animals adapt to survive their usual environment, and for rabbits and hares, survival is all about camouflage. If snowfall is delayed in the fall, the snowshoe hare still turns from brown to white on schedule. “There are these white things against a brown background,” says McKelvey. “The leaves are gone and you can see them in the corner of your eye. Being hares, they have no idea they are white, they go run under bush, hunker down, and think, ‘I’m hiding. I’m a big white rabbit!'”

A shorter winter could mean that caribou would return to the barrens earlier in the summer, but McKelvey says, “we have found no evidence for this in historic data and fossil records.” In reality, he says, the wolverine is always associated with arctic conditions, even including a fossil from Spain showing a wolverine living “in the depths of the ice age.”

Already, wolverines are scarce, and isolated to snowy mountains, which ecologists call “islands.” One study of three mountain ranges in Montana put the total population at 12 or 13, but only about half were of reproductive age.

To avoid inbreeding and a decline in fitness, these wolverines must travel to find mates, but here again, the scarcity of snow makes life difficult. With continued warming, the islands “are going to go away, or get smaller and further apart,” McKelvey says, “and so we will have fewer dispersers [moving between groups], with less total area to live in, and less success in dispersing. It’s not really rocket science.”

a sleeping bear hidden in a cave with eyes open.
Photo: Rosswood, British Columbia, Canada, .sarahwynne.
A Kermode bear (a subspecies of the North American black bear sometimes called the “spirit bear”) gazes up from hibernation.

Underground story

While the wolverine survives a murderous wind-chill on the windswept barrens, a much mellower scene transpires at the base of the snow. Wind is absent, snow insulates, earth heat is trapped, temperatures hover around freezing, and often, fungi decay dead organic stuff.

“The majority of biodiversity and ecological function in temperate system occurs below the snow, although it’s not as apparent as deer or wolverines,” says Jonathan Pauli, an assistant professor of wildlife ecology at the University of Wisconsin-Madison.

Geothermal heat causes water to evaporate and crystalize, “so you have a pocket of loose granular snow, and there can even be enough light for plants to actively photosynthesize” as spring progresses, Pauli says.

Voles and shrews can survive, and voles may even reproduce, in this cool, damp and rather dark world. “We have this elaborate ecosystem in the subnivium that we just ski over,” says Pauli. “We don’t see it, but it’s a pocket of space that organisms exploit.”

Snow job

The whole sub-snow-system is reliant on deep, fluffy snow that “makes a great, stable thermal blanket,” says Pauli. “We are seeing rain at times [in the winter] when it has never been seen before,” Pauli says, and so global warming offers a two-prong attack to the under-snow world: The snow cover doesn’t last as long, and rising temperatures and rainfall raise its density and reduce its insulating value.

Snow cover compared to average:
November 1966 – December 2012

North America. Roll over to see Eurasia.

  As a result of both trends, “the subnivium is becoming much more variable in temperature, and colder,” Pauli says. “It’s counterintuitive, it’s warmer in winter, but that changes the physical properties of snow, so the highs are higher, and lows are much lower, so we project that we will see more freeze-thaw cycles in the subnivium.”

Snow helps protect plant roots from damaging freeze-thaw cycles.

An eight-year study of a spruce forest in Sweden3, showed the dramatic way that snow cover affects soil and plants. When the researchers removed the snow, the soil frost lasted 118 days (compared to 57 days when the snow was left in place). “Understory species composition was strongly altered,” with an 82 percent decline in the dominant dwarf shrub, and less than half of the vegetation survived.

At colder temperatures, animals require more food, but there are other dangers. The wood frog secretes glucose in a complex strategy to avoid dehydration, stabilize proteins in the cells, and allow the heart to stop beating. If temperatures are too unstable, the glucose stores may be exhausted, causing death or near starvation in the spring. The antifreeze fails if the temperature falls below -8°C, and the frog freezes to death.

Because the subnivium is a seasonal refuge for species that are adapted to a stable environment, “What the changes are going to mean for some species is pretty obvious; they will have some real problem facing the altered environment,” Pauli says.

tracks and holes on the snow cover
Cottontail rabbit and long-tailed weasel tracks in fresh snow.

  Not every species faces an immediate threat from the loss of snow cover, says Ben Zuckerberg, an associate professor of wildlife ecology at UW-Madison. “In our work, snow cover never comes out as critical to most winter birds in northern latitudes.” However, predators such as the great horned owl and various hawks eat small mammals that live under the snow and so could be harmed indirectly by environmental change.

Studies of the subnivium must occur within the context of ongoing changes, Pauli says. “There is a general, northward progression of species. Around the Great Lakes, small mammals from the south have been replacing northern small mammals. We are interested in understanding how relevant the snow changes are to these movements, but it is all predicated on knowing what the snow is like, and what drives those changes.”

And it’s not easy even to know the background condition, the big picture on snow cover through history. “The biggest questions are where the snow is, what its conditions are,” says Pauli.

Focusing on a habitat that seems hostile and barren exposes some of the emerging subtleties of climate change. “People often relate warming to heat waves, drought, extreme events like hurricanes or tornadoes,” says Zuckerberg.

But in fact, warming can cause cooling in refuges beneath the snow. Above the snow, it can deprive animals like the wolverine of their competitive advantage — the ability to master harsh, arctic conditions, and could possibly hasten their extinction.

– David J. Tenenbaum

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Terry Devitt, editor; S.V. Medaris, designer/illustrator; Yilang Peng, project assistant; David J. Tenenbaum, feature writer; Amy Toburen, content development executive

Bibliography

  1. Nonlinear responses of wolverine populations to declining winter snowpack, Jedediah F. Brodie & Eric Post, Popul Ecol (2010) 52:279–287.
  2. Climate change predicted to shift wolverine distributions, connectivity, and dispersal corridors, Kevin McKelvey et al, Ecological Applications, 21(8), 2011, pp. 2882–2897.
  3. Absence of snow cover reduces understory plant cover and alters plant community composition in boreal forests, Juergen Kreyling et al, Oecologia (2012) 168:577–587.
  4. Subnivean zone: Life under the snow
  5. Snow Cover Extent Declines in the Arctic
  6. Wolverine: Chasing the Phantom
  7. Climate Change Leaves Hares Wearing The Wrong Colors
  8. Rising Temperatures Threaten Fundamental Change for Ski Slopes
  9. Above, within, and beneath blankets of snow