Counting Sheep, the ’driving force’ of Denali

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Dall sheep—an iconic species in Denali National Park—are the subject of an ABoVE project studying the impact of climate change on the animals and their habitat. Credit: National Park Service

Dall sheep—an iconic species in Denali National Park—are the subject of an ABoVE project studying the impact of climate change on the animals and their habitat. Credit: National Park Service

by Kate Ramsayer / DENALI NATIONAL PARK, ALASKA /

On Monday, July 11, as the green shuttle bus stopped at a campground to pick up drenched passengers, a murmur spread. Fingers pointed out the left side of the window, necks craned, binoculars raised to eyes and sure enough, there they were—three Dall sheep perched on a craggy ledge, far above the road in Denali National Park.

What many on the bus might not realize, said Bridget Borg, Denali wildlife biologist, is that those animals spurred the formation of the park 99 years ago. Concerned that market hunters were killing too many sheep to sell to Alaskan miners, Charles Sheldon led the charge to protect Dall sheep and their habitat.

“They’re the reason the park was created,” Borg said. “A lot of people want to come see these charismatic carnivores—the bears and wolves—but sheep are really the driving force behind Denali.”

Sheep on a road

Dall sheep in Denali National Park. Credit: National Park Service

And they’re the subject of one of the research efforts in the NASA-funded Arctic Boreal Vulnerability Experiment, or ABoVE. The ABoVE campaign is looking at changes in Alaska and Northwest Canada in a warming climate, including changes to wildlife species and their habitat.

Borg and colleagues across Alaska and Canada are surveying sheep as a part of a project led by Laura Prugh, of the University of Washington, to examine how changes to snow cover and vegetation at high elevations are impacting Dall sheep.

“We’re one aspect of a really big project,” Borg said.  She and her colleagues at Denali conduct two types of sheep surveys: aerial surveys to estimate the population numbers (current count is about 2,000 in the park), and ground surveys to see how many ewes give birth to lambs in a given year.

In the summer, the biologists have a pretty easy time of spotting sheep for the ground surveys, she said. They hike up from the park road to get a view of good sheep habitat—rocky areas where predators can’t climb—and then look for white spots amongst the grey rocks. It’s a little harder in the late spring, she noted, when the snow has melted into sheep-sized patches on a mountain. Shiny rocks can trick visitors on the bus into yelling out a sheep-sighting as well.

Scientists count sheep.

Denali National Park biologists survey sheep both from the ground and the air. Visitors can often see them from the shuttle bus that drives the one road into the 6-million-acre park. Credit: National Park Service

When the researchers spot sheep, they train a spotting telescope on the area and count the lambs, ewes and rams. They estimate the rams’ ages based on the completeness of the horn curl, and watch an area for a while to make sure they didn’t miss any lambs. Lambs are quite good at hiding behind their mom, Borg said. The lamb-to-ewe ratio is a good indication of the health of the population; a ratio of 30 lambs to 100 ewes is reasonable, she said, but for a few years recently that number was down around 10 lambs to 100 ewes.

“We noticed a decline in productivity—it’s really prompted some questions going forward,” Borg said.

Borg and a colleague count a herd of sheep in Denali National Park. By comparing the lamb-to-ewe ratio from year to year, scientists can see changes in the health of the population. Credit: National Park Service

Bridget Borg (right), wildlife biologist with Denali National Park, and a colleague count a herd of sheep in Denali National Park. By comparing the lamb-to-ewe ratio from year to year, scientists can see changes in the health of the population. Credit: National Park Service

With ABoVE, she hopes to address some of those questions. The sheep study group will use remote sensing maps of snow cover, as well as computer models, to study how that can impact population health. They’ll also look at whether shrubs growing higher and higher in elevation, up into rocky sheep habitat, affects the populations.

“Shrubs are moving up the slope, and the sheep really need those rocky slopes, both for the forage and the escape terrain,” Borg said. Warmer temperatures could also be changing the quality of the forage available to sheep.

With sheep so important to Alaskans and to park visitors, these are key questions to ask, Borg said. “Will future generations be able to come to this park, and see the reason it was created?”