Countless studies have chronicled the importance of Wyoming’s mule deer migrations. They’re some of the longest in the world, stretching, in some instances, across hundreds of miles.
Scientists know from GPS collar data, trail-camera images and tireless work that those migration paths are so precisely defined that animals often skirt the same bluff, travel the same ridgeline and cross the road at the same spot year after year.
The fidelity evidenced in these well-worn patterns may be causing deer more harm than good in the face of a landscape changed by humans, according to a new paper out of the University of Wyoming and several other universities.
For some species, in fact, this loyalty to what once were the places with the best food and nesting sites might actually be maladaptive.
“Mule deer, especially, are impressively faithful and maybe somewhat unique in that matter compared with other ungulates,” said Kevin Monteith, a UW professor and longtime mule deer researcher. “And their world has been literally changing under their feet for decades, and we’re observing the consequences of that based on the population counts over the years.”
The paper, published Jan. 11 in Frontiers In Ecology and the Environment, is authored by UW professors Jerod Merkle and Anna Chalfoun, along with Western Ecosystem Technology’s Hall Sawyer and several others.
Human-caused climate change altering habitats is bad news for wildlife, but it doesn’t guarantee mass die-offs, the authors say. Given enough time, some species may be able to find new places with food and breeding sites by following “innovators” — individuals that are more inclined to try new things.
But for others, like mule deer, fidelity to migration routes and seasonal ranges is often greater than their ability to adapt — making it all the more important to conserve critical habitat.
Merkle has thought about where animals move for years. A decade ago, as a Ph.D student in Saskatchewan, Canada, he studied site fidelity in a free-ranging herd of bison.
“Those bison had a circuit,” he said. “They moved through meadows the same way, and they showed up at the same week every year.”
When he came to Wyoming and learned about mule deer doing the same things over and over again, he said, “it struck me that animals are relying nearly 100% on their past experiences.”
Whether mule deer learn from their mothers, from others in the herd or some basic instinct, researchers still aren’t sure. But they do know from millions of data points that mule deer largely follow the exact same paths, around the same time, each year.
And that consistency has long served as an evolutionary boon. The longest migrators leave winter range early and follow greening plants to take advantage of the choicest bits of nutrition. Other deer may leave a little later and not go quite as far. Yet more will only go a dozen or so miles. All of those experiences mean herds like the one in the Wyoming Range can support tens of thousands of animals by taking advantage of the variability on vast stretches of wild land.
Many migrating birds return to the same places year after year — a fact that likely stems from a practical reason: If a bird knows a certain tree has just the right nesting hole, or a certain acre of sage brush has just enough food, it doesn’t need to spend valuable time and energy looking around every year for a new location.
Birds nesting in Wyoming are less studied than species like mule deer, said Chalfoun, who is also an assistant unit leader at the USGS Cooperative Fish and Wildlife Research in Laramie, but the trends are still worrisome. She can’t pinpoint exactly where, say, the Brewer’s sparrow goes in the winter when it migrates to the southwestern U.S. and northern Mexico. But she can say that in western Wyoming, the small songbird tends to return to the same region, even the same acre, to nest each spring.
For the most part, migration as a survival strategy has worked. Species thrived and evolved slowly with the landscape until the last couple hundred years, and particularly the last half century.
“Mule deer could literally go up to their summer range, and while they’re gone, a 2-acre well pad could be put in a place they used to forage in the past,” Merkle said. “When they come back, they would say ‘OK, there’s a new thing that’s where I used to live’ and for an animal that bases its life on where it used to live, that’s a hard thing to digest. They’re not used to trying new things and going new places.”
Birds like lesser snow geese continue returning to traditional feeding grounds even when food resources decline significantly and brood sizes drop, according to the paper.
It’s likely the same for the Brewer’s sparrow when well pads harboring predatory deer mice encroach on nesting areas.
Some species simply adapt better than others. Elk, for example, will sometimes follow similar routes, but if one year their winter range is altered, they’ll go somewhere else, Monteith said.
Mule deer, however, just don’t.
A 2017 study by Sawyer showed that the Sublette mule deer herd declined by about 40% over 15 years. They “made fine scale-behavioral changes to avoid habitat close to infrastructure, yet they continued to return to the same winter range year after year,” the paper reads. Less winter range meant less food, which meant fewer animals could survive.
That’s why some proposed solutions, such as off-site mitigation for energy development, may not work, the researchers say.
The best solution, Merkle said, is to conserve existing migration routes for future generations.