Wyoming’s bighorn sheep are pressed by skiers
Casper, Wyo. • A recent study of an isolated bighorn sheep herd in Wyoming's Teton Range has revealed new insights on how ungulates cope with the loss of migration routes, and how backcountry recreation encroaches on their remaining habitat.
The bighorn sheep population in the Tetons stands at roughly 125, as it has since the 1980s. While biologists say the herd is in no immediate threat of a devastating die-off, the small numbers raise concerns about the herd's long-term viability.
The new information gives wildlife managers a better understanding of the adaptive abilities of bighorn sheep. It will also inform future discussions of whether to enlarge current winter closures that prevent skier incursions into remaining sheep habitat.
Wyoming Game & Fish biologist Alyson Courtemanch completed the GPS collar study during her graduate research at the University of Wyoming. Her research documents how the Teton herd of bighorn sheep has evolved a unique strategy of wintering at extremely high elevations — above 10,000 feet. Rather than drop down with the first snows each autumn, as other bighorn sheep herds do, the sheep climb higher.
The bighorn sheep survive the harsh months of December through March by seeking out patches of rocky, windblown ground high in the Tetons, when skiers at nearby Jackson Hole Mountain Resort frolic in the 500 inches of snow that fall each winter.
As skiers cut runs and eat burgers at the Couloir Restaurant, the sheep eke out a living by grazing meager sprigs of grass and even pine needles, burning off their fat stores and, importantly, avoiding the deep powder the Tetons are known for.
However, the Teton snows and the increasing popularity of backcountry snow sports have conspired to force the bighorn sheep off much of the winter habitat that would otherwise be available to them. GPS data collected by Courtemanch during winters 2008 through 2010 suggests that skiers in high alpine areas cause bighorns to avoid choice habitat, and stay more active, which uses up their energy reserves. Even one skier a week can be enough to drive the sheep off choice locations and onto less ideal habitat.
To those used to seeing bighorn sheep grazing along hiking trails or on the roadside, such skittishness from skiers seems unusual. Courtemanch and others explain that sheep can habituate to predictable movements of people and cars. After being captured by helicopter crews, released sheep may run just 30 meters away before stopping to graze while keeping an eye on the humans.
However, the high speed and unpredictable routes of descending skiers often triggers a strong flight response among wintering sheep, perhaps by resembling a predator attack. If a group of skiers crests a ridge and drops into the viewshed of wintering sheep, the herd is likely to avoid the area entirely.
That puts increasing pressure on this isolated herd perched high in the crown of the Tetons, in the last refuge it can find.
Even though the herd's population is stable and not in decline, the sheep's long-term prospects are not favorable without an increase in habitat. Biologists say the Teton herd's small numbers of 75 in a southern group and 50 in a separate northern group put the sheep at increasing risk of inbreeding and epidemic die-offs.
"The bighorn sheep are a fitting symbol of the Teton Mountain Range and something we shouldn't lose," said Mike Whitfield, director of the Heart of the Rockies Initiative who authored a study on the Teton herd in the early 1980s. "They've survived for thousands of years and we shouldn't kill them off in 200 because we can't coexist with them."
Adapting to migration loss
The Teton sheep herd's pattern of moving up in winter and down in spring is contrary to previously documented ungulate migrations.
Across western Wyoming, mule deer, pronghorn, and elk migrate out of the mountains when the first fall snowstorm dumps more than a few inches of snow on the ground. Such seasonal movements often offer hunters their best shot at big game.
In spring these same animals migrate up from lower elevation, following the green-up of highly nutritious forbs and grasses from the valley floor to alpine peaks. Crucially, the amount of time animals spend eating green forage drives their summer weight gain, and determines how well they will survive the coming winter.
University of Wyoming professor Matt Kauffman and his graduate students at the Wyoming Cooperative Fish and Wildlife Research Unit have confirmed these ancient patterns through several GPS tracking studies. (The public can learn more about these studies and view migration maps at the Wyoming Migration Initiative website.)
In contrast to other ungulates, the Teton bighorn sheep winter high, and in spring they make what Courtemanch calls an "abbreviated migration." They drop down over consolidated snowfields until they reach green forage appearing at the edge of melting snow banks. Throughout spring, they work their way up to higher ground, following the line of green grass, until they are back at the same elevations they wintered, and higher.