Earlier spring snowmelt in northern Alaska could mean future trouble for wolverines, elusive fur-bearing mammals that hide in snow caves when they are young and roam vast distances when they are adults.
A newly published study examined end-of-May snowpack at wolverine den sites in northern Alaska and in the Rocky Mountains, and it finds the Alaska snowpack to be sparser. High-latitude wolverine tundra habitat may lose its spring snow earlier than low-latitude but high-altitude habitats, and future management should take that trend into account, said the study, published in the bulletin of the Wildlife Society, an 80-year-old international organization.
The study uses a combination of photographic records to compare late-May snow conditions at known den sites in 2016. At 86 percent of the high-altitude Rocky Mountain dens — sites in Idaho and Montana — snow persisted in late May, sometimes in heavy layers, the study found. But at the northern Alaska den sites examined, there was very little snow remaining at the end of May, just small patches scattered over large areas of bare ground, the study said.
Wolverines, the largest land-dwelling members of the weasel family, use dens to give birth and take care of their young. Wolverine kits are born between February and April and are nursed for a little over two months, according to the Alaska Department of Fish and Game's species profile. In Interior and northern Alaska, wolverine dens are carved out of snow and have tunnels that can be up to 60 yards long, the state said.
Just how much snow wolverines need is not fully understood, the study says.
Lead author Audrey Magoun, a Fairbanks-based scientist who did her Ph.D. research in Northwest Alaska, said all the dens she examined in that region were located entirely within the snow column. That includes the tunnels that branched out, Magoun said in an email.
Elsewhere, wolverines are known to use a mix of snow, rocks, downed trees, tundra soil and other material to construct dens, she said. Even when the wolverine dens are made with mixed materials, snow can be important, she said.
"Snow probably provides protection from severe cold even if the den is within the boulders and the young are adequately protected from predators," she said in her email.
Snow is used by Alaska wolverines for more than denning, said Tom Glass of the Wildlife Conservation Society, a co-author of the study.
"The question on how wolverines will be affected by climate change is clearly complex," Glass said in a statement released by the Wildlife Conservation Society. "During our aerial and ground-based surveys on the North Slope, we have observed the use of snow holes for denning, and also by both males and females for caching food, resting, or perhaps shelter from predators such as wolves."
For Lower 48 wolverines, persistent and stable snow depth greater than about 5 feet "appears to be a requirement for natal denning," according to the U.S. Fish and Wildlife Service.
The new study does not identify trends in spring snow cover. But elsewhere, there is documentation of earlier spring snowmelt in northern Alaska and the Arctic, part of a pattern associated with rapid warming in the northernmost regions.
Average "snow-off" and green-up dates in the five national park units in Arctic Alaska have probably advanced by six days over the past eight decades, according to a recent study. The study, published in the journal Remote Sensing, uses satellite imagery of snow and vegetation in five northern Alaska Park Service units, along with historic weather information from Kotzebue in Northwest Alaska.
Arctic-wide, there is evidence that snowmelt is happening much earlier than it did in the past. A 2010 study by Environment Canada researchers found that from 1967 to 2008, pan-Arctic snow cover in May declined by 14 percent; the June decline over that period was 46 percent, according to the study. A follow-up 2012 study by two of the same researchers found a 17.8 percent per decade reduction in June snow cover from 1979 to 2011, a faster rate of decline than recorded for September Arctic sea ice during that period.
Nevertheless, Alaska wolverine populations appear to be in good shape, and that goes for the animals in Northwest Alaska, Magoun said. But by nature, wolverines are distributed in very low densities, she said. The highest wolverine densities ever recorded fall in the range of 10 to 15 animals for every 250,000 acres, she said. In the Chugach National Forest of Southcentral Alaska, the average density is 4.5 to 5 wolverines for every 250,000 acres, according to the Alaska Department of Fish and Game.
Wolverines are also known to travel far to find food, with adult males roaming up to 385 square miles of territory and adult females covering up to 230 square miles, according to Fish and Game.
The health of the Rocky Mountain wolverine population is the subject of debate and study. A proposal for Endangered Species Act protection cited climate change and wolverines' need for snowy dens as one of several justifications for a listing as threatened. Another justification is habitat fragmentation.
The U.S. Fish and Wildlife Service in 2014 rejected the listing proposal. But a 2016 federal court ruling in Montana concluded that the agency's decision was "arbitrary and capricious," driven by politics rather than science. The ruling ordered the agency to revive the listing process for the wolverine.
"No greater level of certainty is needed to see the writing on the wall for this snow-dependent species standing squarely in the path of global climate change," said the ruling by U.S. District Court Judge Dana Christensen, a graduate of the University of Montana School of Law and an appointee of Barack Obama.