Mule Deer Decline

A treasured icon of the West, the mule deer nevertheless is a species in crisis

  • Chris Madson
  • Conservation
  • Nov 24, 2014
FOR MORE THAN A CENTURY, the mule deer has been the most abundant big-game animal in the West, serving as food for such predators as mountain lions and grizzly bears. It is also a financial mainstay—through the sale of hunting licenses and permits—for state wildlife agencies, which is why biologists and conservationists who have been monitoring mule deer populations across the West are viewing the species with growing concern.

Wyoming was home to about 578,000 deer in 1991, but by late 2012 the number had dropped 36 percent to 369,000, says Daryl Lutz, a wildlife management coordinator for the Wyoming Game and Fish Department and member of a team the department formed to analyze the status of the state’s mule deer. Similarly, Colorado’s highest population estimate occurred in 1983, when deer numbers reached an estimated 625,000, says Andy Holland, a biologist with the Colorado Division of Parks and Wildlife. “Our current population estimate is 391,000,” he says. Some of the variation is due to changes in methods that biologists use to estimate the mule deer population, Holland cautions, but, he adds, “we all know and agree that it’s declined a lot.”

In 1998, wildlife agencies from 17 western states and four Canadian provinces formed a committee of biologists to find solutions to their common mule deer management problems. In 2002, the committee confirmed that mule deer numbers and distribution “have been declining throughout the West since the latter third of the 20th century.” The causes are a complex interaction of factors ranging from housing and energy development to the behavior of deer themselves.

On the Move
Mule deer occur in North America from central Mexico to Canada’s Yukon and as far east as Kansas and Nebraska. Some spend their lives in a small home range, but most migrate between summer haunts and winter refuges. Researchers in western Wyoming recently used radio collars to follow a group of mule deer that move from the high desert where they spend winter to the mountain meadows and aspen groves where they summer, a distance of 150 miles (below). “This was the longest mule deer migration ever recorded and among the longest land-mammal migrations in the lower 48 states,” says Hall Sawyer, the Western Ecosystems Technology biologist who led the study.

Increasingly, scientists are realizing that mule deer need a lot of room. At the same time, important mule deer foraging areas are shrinking across the species’ range as housing projects sprawl on the fringes of major cities and in particularly scenic rural areas. A team of researchers led by Andrew Hansen of Montana State University writes that from 1990 to 1998, the human population “in rural areas grew faster than in urban areas in over 60 percent of the counties in the Rocky Mountain states.” Such development eliminates shrubs that provide important forage for mule deer, which also try to avoid people and their pets.

Growing Threats to Mule Deer
The energy boom in Colorado and Wyoming also is disturbing once-isolated sagebrush and mountain-shrub habitats. According to the U.S. Energy Information Administration, Wyoming had 2,431 natural gas wells in 1989; in 2012, it had more than 22,000. Colorado grew from 5,000 to 32,000 gas wells in the same period. More trucks on more roads driving to more oil and gas wells mark a burgeoning threat for mule deer.

In addition, a century-long campaign to eliminate wildfires has allowed the growth of plants that crowd aspen out of mountain forests, damaging habitat that was nearly ideal for deer. As the occurrence of wildfires declined in foothills and basins, important deer foods such as mountain mahogany, bitterbrush and big sage grew older, becoming much less productive. Meanwhile, dead timber accumulated, stockpiling amounts of fuel that had seldom, if ever, been seen and that contribute to the massive wildfires that have inflamed the West during the past decade.

“The fire regimes we had historically are going to be different than what we’ve got now for two reasons,” says Steve Kilpatrick, a wildlife biologist with more than 30 years of experience managing western deer habitat and now executive director of the Wyoming Wildlife Federation, an NWF affiliate. “We’ve got a lot more fuels on the landscape, so we’ve got a lot more that’s going to burn. You couple that with the fact that we are having some climate change, and the fires are, as the experts predict, going to be hotter and larger. That’s not going to be particularly good for mule deer.”

Then there’s cheatgrass, an invasive species introduced from Europe and  Asia that provides little food for mule deer and that aggressively changes plant communities on a landscape scale. Barry Perryman and Tamzen Stringham, rangeland ecologists at the University of Nevada–Reno, report that in the West “there are as much as 25 million acres dominated by cheatgrass, and about 60 million acres are either infested or susceptible to invasion.”

Many parts of the West also may be suffering the consequences of climate change. “At least since 2001, with the exception of a couple of years in the middle, we’ve been in drought,” Holland says. Lutz has tracked the impact of drought on mule deer for more than 20 years. “It’s all about fawns,” he says. “Fawn productivity started going way down in 1989, and that was a classic response to the start of the drought.”

Stabilizing a Population
The current decline in mule deer numbers has to be viewed against the longer history of deer in the region. In a 1986 report, U.S. Forest Service biologist George Gruell wrote, “Mule deer populations began increasing regionally in the 1930s. In Utah the mule deer population was estimated to have increased from 8,500 in 1916 to a peak of 375,000 in the 1945-50 period. Idaho officials estimated that their deer population (including white-tailed deer) increased from 45,000 in 1923-24 to 315,000 in 1963.”

The primary cause of the increase was a change in vegetation. “Succession of rangelands from grass dominance to dominance by woody plants created vast expanses of optimum deer habitat,” Gruell says. “Conversion of coniferous forests to shrub fields by logging and wildfire improved deer habitat. Reductions in numbers of livestock on the open range increased the amount of forage available to mule deer.”

Even as the deer population crested, biologists recognized it might be too much of a good thing. Holland points out that in Colorado during the 1960s, “We had a lot of concern that [the deer] were overbrowsing our range and that we had too many animals out there.” There’s reason to believe that the deer herds of that era did long-lasting damage to important forage plants on heavily used winter ranges, reducing the land’s ability to produce mule deer, especially in areas plagued by drought.

Biologists see no way to return to the deer populations of the 1950s and 1960s, even if that goal were advisable. “There are very few things going on in our landscape and society to the benefit of mule deer,” Holland says. “The deck is stacked right now, and it’s against deer.”

But Lutz adds a dash of optimism: “It’s my hope that we can stabilize mule deer populations. We’ve got to try to do that first before we even think about the possibility, if there is one, of increasing deer populations. It’s all about getting money on the ground in a focused way to see if we can affect mule deer through habitat.”

Action for Wildlife: Protecting Western Species

NWF and its state affiliates have a long history of cooperating on western wildlife issues on behalf of such species as mule deer, pronghorn and elk. The organizations have worked for habitat protection throughout the West, including wise energy development in Wyoming and Colorado, where public lands have been subject to intensive drilling for natural gas. NWF recently released a fact sheet on Colorado’s mule deer decline. For additional information, visit and

Chris Madson is the former editor of Wyoming Wildlife magazine.

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