Busy highways and fences are seen as some of the greatest obstacles to the survival of migrating North American big game animals like caribou, deer, antelope and elk, cutting them off or redirecting them around traditional feed grounds.
But a recently published study claims that even in the wilderness-like setting of Yellowstone National Park, migratory elk are declining because of ecological factors — namely predation and drought.
“This is one of North America’s wildest and best-protected landscapes, where elk and other ungulates still retain their long-distance seasonal migrations — and yet it is the migratory elk that are struggling while their resident counterparts thrive in the foothills,” said Arthur Middleton, who led the study as a University of Wyoming doctoral student. He is now a postdoctoral fellow at the Yale School of Forestry and Environmental Studies.
Published this month in the journal Ecology, the study was conducted by the Wyoming Cooperative Fish and Wildlife Research Unit — a joint program involving the U.S. Geological Survey, the University of Wyoming and the Wyoming Game and Fish Department.
The study found a 70 percent decline in the recruitment of elk calves by the 4,500-member Clarks Fork elk herd over 21 years. The elk spend winter in the Absaroka Mountains of Wyoming and migrate 25 to 40 miles in the summer into the higher elevations of the east-central region of Yellowstone, including the upper Lamar River drainage.
Over the four years of the recent study, only 70 percent of the migratory cow elk were getting pregnant compared to 89 percent of the elk that wintered in the same area northwest of Cody, Wyo., that didn’t migrate into Yellowstone.
“A lower pregnancy rate reduces the number of calves that are born in the first place, then predation seems to reduce the number of migratory calves that survive the first few months of life,” said Matthew Kauffman with the USGS.
Elk and other animals migrate for many reasons — including a desire to reach better forage and to avoid harsh weather and predators. Yet resident elk have settled into irrigated croplands where forage is better than the vegetation now found in Yellowstone which, along with the rest of the West, has suffered through extended drought.
What’s more, the elk that stay closer to human activity have encountered four times fewer predators, since bears or wolves that venture into agricultural areas are often shot or trapped and relocated to avoid run-ins with livestock or humans.
So why don’t the migratory elk just stop migrating? There has been a significant decline. A study in 1979-80 found 81 percent of the herd was migratory. By 2009 that number had dropped to 47 percent. Those still migrating are doing it for the simple reason that as offspring, that’s what they learned from their parents.
The study’s authors said the migratory elk will not disappear, but there could be a long-term shift underway in the number of migratory versus resident elk in the area.
One of the upshots of having so many elk calves killed off by predators is that the migrants tend to be an older herd on average than the resident elk. That also could affect the number of animals migrating as older elk die off and aren’t replaced in the population.
One factor not addressed in the study is hunting and the resulting impacts to the different herds.
Montana has been struggling with ways to encourage private landowners to allow more public hunting as a way to reduce herds that seek refuge during hunting season, and then after the season is over raid adjacent landowners’ crops. Wyoming may be seeing the same scenario played out with this resident Clarks Fork herd.
“Most immediately, these trends have meant lost hunting opportunity in the backcountry areas frequented by migratory elk, and increasing crop damage and forage competition with domestic livestock in the front-country areas where resident elk are expanding,” Middleton said.