Seasonal migrations, once viewed as natural and beneficial for wildlife, are not helpful for elk that leave and return to Yellowstone National Park, according to a study published in the journal Ecology.

A long-term study by the U.S. Geological Survey, the University of Wyoming, and the Wyoming Fish and Game Department found that migratory elk returned to the park with fewer calves due to predation and drought while elk that remained on public and private lands outside the park year-round flourished in comparison.

The study describes a long-term decline in the number of calves produced annually by the Clarks Fork herd, a population of about 4,000 elk whose migrants travel annually between winter ranges near Cody, Wyoming, and summer ranges within Yellowstone.

Migratory elk experienced a 19 percent depression in rates of pregnancy over the four years of the study and a 70 percent decline in calf production over 21 years of monitoring by the WGFD, while the elk that did not migrate, known as 'resident elk', in the same herd experienced high pregnancy and calf production and are expanding their numbers and range into private lands outside of the park.

"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 this work as a University of Wyoming doctoral student and is now a postdoctoral fellow at the Yale School of Forestry and Environmental Studies.

Part of the problem, which is reflected in a reduced pregnancy rate of the migratory elk -- 70 percent versus 90 percent of the so-called resident elk -- seems to be tied to climate change.

The study shows that the hotter and dryer summer conditions of the last two decades, coincident with the long-term drought widely affecting the West, has reduced the duration of the spring period when tender new grasses are available to elk. This makes it harder for female migratory elk to find the forage they need to both nurse a calf and breed. Though elk typically bear a calf every year, migratory elk that nursed a calf had only a 23 percent chance of becoming pregnant again in the following year.

Another key to lower calf production is predation, as the migratory elk herds "share their range (in Yellowstone) with four times as many grizzly bears and wolves than resident elk, and both predators are well known to prey on young elk calves," the report noted.

"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 a research wildlife biologist with the USGS and assistant professor at the University of Wyoming.

Globally, wildlife migration is a dwindling phenomenon. Research and management often focus on conspicuous barriers like fences, roads, and other kinds of development that can physically impede migration corridors. While those are important, this study suggests that even in a landscape as well-protected as the Greater Yellowstone Ecosystem, subtler changes in predator management and forage quality on the seasonal ranges of migratory animals will also play an important role. Migration is conventionally understood as a strategy to gain better forage quality while also reducing exposure to denning predators, but in this case, it seems those benefits are instead being realized by the residents.

The study's authors note that their work does not predict that migratory elk will disappear, but rather that there could be a long-term shift underway in the relative abundance of migratory versus resident elk in the system. The study also highlights the perils of characterizing Yellowstone wolf re-introduction as a "natural experiment." Other key factors have changed since wolves were re-introduced, including growth in grizzly bear numbers and recurrent long-term drought associated with reduced snowpack and hotter summers. The authors caution that such factors should be taken into account in the effort to understand ongoing ecological changes in Yellowstone.