Before highways and fences sliced through the West, and towns and energy development sprawled across the prairie, herds of deer, elk, pronghorn and bison reigned supreme.

They moved from valleys and deserts to mountains as seasons changed from fall and winter to spring and summer. They followed the green wave, researchers have started calling it, and the food they found along the way kept herds robust.

While many of those ancient migration routes have been lost to westward expansion and settlement, some still remain.

Efforts have been underway in many Western states to document and then try to conserve at least portions of critical migration routes. Wyoming has, in many ways, been at the forefront of those efforts with the Wyoming Game and Fish Department establishing formal migration route designations. But even with those designations, and a Department of the Interior secretarial order placing a priority on protecting migration corridors, energy leasing in the routes continues at a record-breaking pace — making figuring out how and what to protect even more critical, some say.

New research published in late June in the Journal of Applied Ecology might eventually help with those decisions. In a paper titled “All routes are not created equal: an ungulate’s choice of migration route can influence its survival,” wildlife biologists explain that mortality rates on some routes can, in fact, be as much as three times higher than on others.

While the paper doesn’t address the factors that influence survival, authors say it does let wildlife and land managers know that some routes could be conserved because of their success and other routes may need a little help.


With the right technology and some patience, it doesn’t take biologists long to begin mapping migration routes. Over the course of a year, they can tell where deer or elk move. Add another year and researchers can verify that information.

Figuring out the success of a corridor, on the other hand, is a much bigger undertaking, said Hall Sawyer, the paper’s lead author and a research biologist for Cheyenne-based Western EcoSystems Technology Inc.

“The only way you do a survival analysis is for a lot of animals to die,” he said. “Studies that are over two or three years don’t typically give enough information.”

So Sawyer and the paper’s co-authors, including University of California Berkeley professor Arthur Middleton, began a seven-year project with a deer herd in southern Colorado and northern New Mexico. The deer weren’t Wyoming animals, but the results can be applied to herds across the West, researchers said.

They collared 66 deer from four high-use routes. By the end of the study, 44 had died.

The pathways wandered roughly parallel to each other, sometimes as close as 5 miles and others as far away as 20 miles. On a map it looks like a wide paintbrush stroke running north to south with four thin lines meandering up and down inside its borders.

Depending on which route deer chose, Sawyer and the other authors found, the animals had up to a three times higher chance of dying.

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So then why wouldn’t deer simply choose the better route? Because big game animals like deer learn their migration routes from their mothers, new research has shown, which means it would take generations for them to evolve.

The extreme differences between routes surprised even Sawyer, who has spent more than two decades studying big game migration in Wyoming and the West.

“But it makes sense,” he said. “We have winter ranges that are better or worse and summer ranges that are better or worse, so why wouldn’t we expect migration routes to be?”


Now that Sawyer and Middleton have demonstrated variation among migration routes, they and other researchers can use existing migration data to analyze some Wyoming paths. They can also use that data to potentially pinpoint places where deer are struggling to survive, Sawyer said.

That information will be key for agencies like the Wyoming Game and Fish Department, said Brian Nesvik, the department’s director.

The department’s job is to first work on and help pay for migration research and data collection, he said. Secondly, it is to “provide a wildlife perspective on how federal land management agencies or the Office of State Lands should go about permitting and allowing development in those particular corridors.”

Wildlife agency officials are in the process of establishing two new official wildlife corridor designations, one for mule deer from southeast of Alpine to north of Evanston and another for pronghorn from Jackson to north of Rock Springs.

The designation process is “proven science and good science,” Nesvik said. “I don’t see that changing, but I see based on differences in corridors and how animals use them the policy could be different.”

Sawyer has some guesses on what influences the success of a route, including intensity of fences, highways, predators and energy and residential development, but understanding issues on individual routes will take more study.

“In Wyoming and the West we have to make choices and tradeoffs,” Middleton said.

If people want to have both wildlife and development of natural resources, residential areas, roadways and other infrastructure, he said, then choosing which corridors are most important to herds will be critical.

“In some cases, this are a step toward doing that in a more informed way.”

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