Adult wolverine, displaying "golden mantle." © U.S. National Park Service
Recently, the only known wolverine in Michigan passed away. Unlike the life and death of most wild animals, which unfold without human fanfare, her life and death were noticed. Scrutinized even. Rare species catch our attention. Rare species persisting outside their normal range even more so. Wolverines are often characterized as solitary creatures, thinly distributed across their range. But just what do wolverines require for habitat?
There’s been some chatter of Gulo gulo expanding southerly in the lower-48 states. They are comparatively more common in Canada, but some sightings have been reported in California’s Sierra Nevada mountains, Oregon and Washington, as well as Colorado and Idaho. Now, a group of researchers has compiled a strong argument that wolverine distribution may be best described as a function of spring snow cover coupled with a summer heat limit of less than 71.5 degrees Fahrenheit. J.P. Copeland of the U.S. Forest Service’s Rocky Mountain Research Station in Montana and 16 other international researchers have published their hypothesis in the Canadian Journal of Zoology. {1} They examined satellite imagery of spring snow cover at the end of the wolverine’s denning period (last week of April to mid-May) to determine wintry limits of good wolverine habitat in North America and Eurasia. Mother wolverines dig dens out of snow tunnels, or carve them out under downed trees or beneath snow-covered boulders. So it would follow that areas where the snow cover melts off before mid-April probably define the edge of where wolverines will not be found. But areas where snow stays deep enough for wolverine dens until mid-April to mid-May, well, now we’re talking skunk bear territory. Gulo gulo does quite well in Arctic and sub-Arctic areas as well as in alpine and sub-alpine boreal forests of Eurasia and North America. The author’s write:
If persistence of wolverine populations is linked to the availability of suitable reproductive den sites (Banci 1994), snow cover that persists throughout the denning period may be a critical habitat component that limits the wolverine’s geographic distribution.
To test whether or not wolverines are limited at the broadest spatial scale by snow cover lasting through to mid-May, Copeland and his team compared telemetry data for wolverines from 10 recent studies in western North America and Scandinavia, with models of spring snow cover developed from seven years of data (2000 – 2006) and compiled from 1,2000 MODIS images. They then mapped maximum summer temperatures (71.5 F)along with the snowfall data to determine the best climatic habitat for G. gulo, which is known in more southerly latitudes to seek higher elevations (and associated cooler temps) in the summer months. This is the map they produced, showing favorable spring snow cover in blue, and maximum summer temperatures in orange:
Fig. 1, from paper: "Fig. 1. The circumboreal range of the wolverine. The black outline represents the wolverine’s putative current geographic distribution developedfrom existing range maps and local expert knowledge. The wolverine’s hypothesized bioclimatic envelope defined by the spring snow coverage (blue gradient) is overlaid on the summer temperature coverage (orange areas). The gradient in the spring snow coverage depicts the number of years out of seven (2000–2006) in which snow cover was present from 24 April to 15 May, and the summer temperature coverage delineates the areas with average maximum August temperatures >22 C from 1950 to 2000." {I've asked Copeland for permission to use this but have not yet received a response back; if it's in conflict with the journal or the researchers I'll take it down.}
They also documented known wolverine den sites in North America (British Columbia and Ontario in Canada; and Alaska) as well as in Norway, Sweden, Finland and Russia. While a small proportion of den sites were located in areas with that fell outside the satellite imagery classified spring cover areas,* the vast majority were sited where spring snow was recorded for six or seven years out of the seven-year study period. Their illustrations of the den sites and snow cover almost don’t need quantification, the correlation is so strong its readily discernible at a glance. But numbers help, of the 562 den sites they mapped across Eurasia, 97.9 percent of the den sites were overlaid on pixels that received spring snow cover for at least one of the seven years of the study period. Scandinavian wolverines, therefore, overwhelmingly like den sites protected by consistent spring snow cover. North American wolverines showed a strong preference too — though not quite as overwhelming as the Nordic skunk bears — 69 percent made dens in sites that received snow six or seven years out of the whole period. (* Ground investigations of these dens determined they were snow dens, but the area of snow fall was less than the 500m x 500m needed for the satellite imagery resolution.) They also mapped temperature data from 1950 to 2000 to test whether or not wolverine distribution was limited by a maximum heat tolerance, and they compared the upper temperatures known from sites used by wolverines tagged with radio-transmitters.
These strong correlations between necessary spring snow cover and maximum heat tolerance do more to explain wolverine distribution than do prey availability models or human presence. Some wolverines have been documented to avoid low-land habitats with suitable prey that lacked people, which puzzled some researchers. But a lack of year-to-year persistent spring snow in these low-laying spaces may explain the wolverine’s preferences.
If the animal’s reproductive behavior is indeed snow-dependent, then this may be another species to keep our eyes on with regards to climate change. And even though I wasn’t a big fan of this year’s incredibly long and cold winter, perhaps it was actually a boon for wolverine mothers and their offspring.
NOTES:
{1} Copeland, J., McKelvey, K., Aubry, K., Landa, A., Persson, J., Inman, R., Krebs, J., Lofroth, E., Golden, H., Squires, J., Magoun, A., Schwartz, M., Wilmot, J., Copeland, C., Yates, R., Kojola, I., & May, R. (2010). The bioclimatic envelope of the wolverine (Gulo gulo): do climatic constraints limit its geographic distribution? Canadian Journal of Zoology, 88 (3), 233-246 DOI: 10.1139/Z09-136
