Global Warming Shortens Spring Feeding Season for Mule Deer in Wyoming

Migratory animals are facing new challenges as climate change reshapes landscapes; protecting migration corridors may be the key to survival for some.

Mule deer starting their spring migration in the Colorado Rockies browse for new growth just below the snowline. Credit: Bob Berwyn
Mule deer starting their spring migration in the Colorado Rockies browse for new growth just below the snowline. Credit: Bob Berwyn

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Global warming presents a new and growing threat to lands where deer and antelope play. Droughts across the mountains and plains of Wyoming can cut the spring growing season from four months to two. That dries up nutrient-rich green grasses and shrubs just when they are needed most by migrating mule deer to replenish body fat after the winter, and to rear their young.

“Climate change is drastically affecting their food supply during migration,” said University of Wyoming researcher Ellen Aikens, who analyzed 19 years of drought records going back to 2001, and mule deer migration data from a shorter period to document the impacts. “In dry years, they get only 50 percent of the benefit that they get in an average year,” she said. 

The findings were published June 11 in Global Change Biology.


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Drought periods have intensified and become more persistent during the past 20 years across the West, and Wyoming is no exception, the National Drought Monitor shows. Climate scientists say human-caused warming is driving long-term regional aridification, potentially signaling the emergence of a megadrought. During the past 17 years, mule deer abundance has declined by 36 percent in Wyoming.

In the long run, persistent drought spells trouble for the West’s mule deer herds, which have been central to Native American culture for thousands of years, for food and clothing, and as a spirit animal that represents fertility. They are now managed as an economically and culturally important game species for hunters and wildlife watchers. 

Studies of other migrating ungulates like elk and moose suggest that a lack of protein-rich spring greens can—along with other factors—affect their survival the following winter, as well as the health of fawns and calves.

Some of Wyoming’s mule deer migrate 150 miles each year, starting in the Red Desert in the southern part of the state in March, and ending up in the mountains of the Yellowstone region two to three months later. Aikens said the animals surf a green wave, but the ride shortens considerably in dry years.

“One thing that is really striking is just how ephemeral spring green-up is in Wyoming, especially in the stopover sites that animals forage in during migration,” she said, referring to how quickly plants could grow and then disappear. 

In related research, she visited migration corridors week after week for three years, documenting plant growth, taking clippings to analyze the nutritional value of the food and collecting fecal samples for deer diet analysis.

“By visiting these sites basically every week, you can see the plants spring up and then quickly dry down, especially in lower elevation habitats,” she said. In dry years, the vegetation dried up even in the higher elevations by July or August. “So my experience in the field really solidified in my mind how narrow a time window animals have to forage in Wyoming, even in the best of years.”

Another important finding was that none of the migration routes, even those across shady, north-facing slopes, were buffered from drought impacts. Instead, the routes with the most abundant forage were most severely impacted by drought, she said.

Key Corridors

The new study’s findings are important because they show drought and climate change effects on animal migrations across the mid-latitude landscapes of North America and Europe, said Matthew Kauffman, director of the Wyoming Cooperative Fish and Wildlife Research Unit and a co-author. 

“Patterns of energy gain and loss are clearly established for ungulates,” he said. “During winter they are definitely burning their fat reserve. The tender, juicy spring salad mix is critical because they can digest it quickly and turn it into fat.” The fat level, in turn, determines the size of the fetuses and how much milk does can produce to feed their young, he added.

“It’s well-known that drought has a negative effect on ungulates. Ellen’s study drills down and shows one of the mechanisms of how that occurs,” he said. “We are identifying a new threat for migrating ungulates, which will likely worsen as climate change continues.”

Ensuring that there are movement corridors with sufficient food gives migrating animals more options in the face of global warming, Aikens added. For some species, such corridors could be the difference between survival and extinction as climate change reshapes regional landscapes by killing some forests, drying up streams or hastening the spread of destructive invasive species.

Wildlife Luring Hunters to Climate Concerns

Mule deer are an important part of today’s wildlife-associated recreation economy in the West, which generates $65 billion annually in 19 states, according to the Western Governors’ Association. And climate change is increasingly seen as the biggest threat to mule deer by some hunters.

“I don’t think there’s a bigger region-wide impact on the species,” said Todd Tanner, a Montana hunter who started an organization called Conservation Hawks that focuses on global warming threats to fish and wildlife. 

“There are certainly a ton of other issues that impact the species, from development on mule deer wintering range, to migratory corridor blockages, to diseases like chronic wasting disease,” he said. “But climate is the one over-arching threat that impacts mule deer from Arizona and New Mexico to Idaho and Montana.”

Along with direct impacts like those identified in the new study, Tanner said global warming is reshaping the region’s climate, often to the detriment of ecosystems. Snow comes later, winter temperatures are warmer, spring runoff comes sooner and fire season starts earlier. That leads to huge areas of dead forests, not to mention wildfire smoke that also affects human health. Wildfire danger has also led to long and widespread closures of public lands, he said.

“It gets so damn smoky that nobody wants to be out. That was never an issue earlier. I’ve lived here near Glacier National Park, for 16 years, and the guys who have been here longer than I have, they say they never thought about smoke in the summer.”

Tom Remington, a wildlife biologist with the Western Association of Fish and Wildlife Agencies, said wildlife managers in all western states are trying to boost mule deer herds because the populations are not as large as they need to be. Crowds of hunters in the fall spend their money at local outdoor shops and gas stations, and for outfitters and guides. And the sale of hunting licenses helps fund wildlife conservation programs in various states, including efforts to buffer some species from global warming impacts.

Environmental Justice

Changes to animal migration are also identified as an important issue in the context of environmental justice by the Fourth National Climate Assessment (2018) and related research. A 2016 report from the U.S. Forest Service states that because of climate change, “Tribes across the United States are experiencing reductions in access to culturally important habitats and species.”

Native American dependence on wildlife resources like mule deer predates European settlement by thousands of years, and historic adaptations to changed conditions included a high level of mobility by indigenous populations. In other words, if mule deer habitat and migrations changed, tribes would adapt by changing hunting areas or the timing of their hunting.

But under today’s tribal boundaries, that’s not always an option. The National Climate Assessment explained it like this: “When ecosystems or species’ habitats or migration routes shift due to changes in climate, tribes’ rights to gather, hunt, trap, and fish within recognized areas are constrained by reservation or other legally defined borders, making adaptation more challenging.”

Some of the mule deer migration routes examined in the new study pass through the 3,740-square-mile Wind River Indian Reservation in west-central Wyoming, home to about 10,000 Eastern Shoshone and Northern Arapaho tribal members. Kaufmann said mule deer are an important cultural resource on the reservation, and that researchers are working with the tribal fish and game department to track the movement of elk and deer.

Some of the research suggests that drought-induced changes to migration could affect the length of time mule deer stay on reservation lands. If they move away, “the question of whose deer they are becomes relevant,” he said.

Other Threats

The new study helps raise the important issue of wildlife migration corridors across the West, especially with regard to climate change. And the findings should be considered in the context of other impacts, including oil and gas drilling, invasive species and residential development, said Remington, who was not involved in the new study.

“Climate change will amplify the importance of migration for mule deer. That said, the direct impacts of climate change on wildlife pale in comparison to the indirect effects,” he said. 

Invasive and flammable cheatgrass is already displacing the native vegetation needed by mule deer and other migratory animals across Wyoming and Montana. “Those changes are occurring in a lot of areas in the Great Basin and are essentially irreversible short of massive infusions of money and effort to bring them back,” he said of the deer. 

Remington said understanding how mule deer surf the green wave is important in connection with energy development in the West, because other studies have shown how mule deer respond to oil and gas drilling.

“Part of surfing is, if you find a good wave, you stay with it. If mule deer find a good area to feed, they’ll stop or slow down, but if they encounter a new oil and gas field they don’t do that,” he said. It’s also important to look beyond narrow impacts to one type of animal to assess climate affects, he added.

“For the most part, when you look at individual species, you don’t see much. The key thing is impacts to habitats. As we see cheatgrass expand, and we have more intense, dry thunderstorms, with more lightning strikes, the fire danger really goes up,” he said, describing linked climate impacts that are worsening conditions for wildlife across much of the West

Fifteen years ago, climate change was still widely disregarded as a threat by wildlife managers.

“It was dismissed as trivial, and then people said, we can’t do anything about it anyway, but that has changed really dramatically,” Remington said. “I don’t think there’s a state wildlife director today who doesn’t think it’s a real problem.”