Study: Mule deer, other species struggle to adapt to changed habitats | News
Countless studies have chronicled the significance of Wyoming’s mule deer migrations. They are among the longest in the world, stretching, in some cases, for hundreds of kilometres.
Scientists know from GPS collar data, trail camera footage, and tireless work, that these migration routes are so precisely defined that animals often walk along the same cliff face, walk the same ridge line, and cross the road. in the same place year after year.
The fidelity shown in these well-worn models may cause deer more harm than good in the face of a human-modified landscape, according to a new paper from the University of Wyoming and several other universities.
For some species, in fact, this fidelity to what were once the best foraging and nesting sites might actually be maladaptive.
“Mule deer, in particular, are incredibly loyal and perhaps somewhat unique in this respect compared to other ungulates,” said Kevin Monteith, a UW professor and longtime mule deer researcher. “And their world has literally changed under their feet for decades, and we’re seeing the consequences of that based on population numbers 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 Hall Sawyer of Western Ecosystem Technology and several others.
Human-caused climate change that alters habitats is bad news for wildlife, but it doesn’t guarantee mass mortalities, the authors say. Given enough time, some species may be able to find new places with food and breeding sites by following “innovators” – individuals more willing to try new things.
But for others, like mule deer, fidelity to migration routes and seasonal ranges often outweighs their adaptive capacity, making the conservation of critical habitat all the more important.
Merkle has been thinking about animal movements for years. Ten years ago, as a doctoral student in Saskatchewan, Canada, he studied site fidelity in a herd of free-ranging bison.
“These bison had a circuit,” he said. “They moved through the meadows the same way and they arrived the same week every year.”
When he came to Wyoming and learned that mule deer do the same things over and over again, he said, “It struck me that the animals rely almost 100% to their past experiences.
Whether mule deer learn from their mother, other herd members, or a basic instinct, researchers are still unsure. But they know from millions of data points that mule deer largely follow the exact same paths at around the same time every year.
And that consistency has long been an evolutionary boon. The longest migrants leave the wintering area early and follow the greening plants to take advantage of the best nutrients. Other deer may leave a little later and not go as far. However, others will only go a dozen kilometers. All of these experiences mean that herds like the one in the Wyoming Range can support tens of thousands of animals by taking advantage of the variability over large swaths of wilderness.
Many migrating birds return to the same places year after year – a fact that probably stems from a practical reason: if a bird knows that a certain tree has just the right nesting hole, or that a certain acre of scrub has just enough food, he does not need to spend time and energy searching for a new location every year.
Birds that nest in Wyoming are less studied than species like mule deer, said Chalfoun, who is also deputy unit chief at USGS Cooperative Fish and Wildlife Research in Laramie, but the trends are still concerning. She cannot determine exactly where, for example, Brewer’s Sparrow goes in winter when migrating to the southwestern United States and northern Mexico. But she can tell that in western Wyoming, the little songbird tends to return to the same area, even the same acre, to nest each spring.
Overall, migration as a survival strategy worked. Species have thrived and evolved slowly with the landscape until the past two centuries, and especially over the past half century.
“Mule deer could literally move up into their summer range, and while they were gone, a 2-acre well could be put in where they used to feed in the past,” Merkle said. “When they came back they were like ‘OK, there’s a new thing where I used to live’ and for an animal that bases its life on where it used to live, that’s hard to digest. They didn’t used to trying new things and going to new places.
Birds like lesser snow geese continue to return to traditional feeding grounds even when food resources drastically decline and clutch sizes decrease, according to the article.
The same is likely true for 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 changes, they will go elsewhere, Monteith said.
Mule deer, however, simply do not.
A 2017 study by Sawyer showed that the Sublette mule deer herd has shrunk by about 40% in 15 years. They “made small-scale behavioral changes to avoid habitat near infrastructure, but they continued to return to the same wintering grounds year after year,” the paper says. Less wintering area meant less food, which meant fewer animals could survive.
That’s why some proposed solutions, such as offsite mitigation for energy development, may not work, the researchers say.
The best solution, Merkle said, is to conserve existing migration routes for future generations.
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