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mule deer on the wyoming range

The Wyoming Range Mule Deer herd has been widely renowned as the greatest Mule Deer herd in the West and has one of the largest and most diverse groups of deer in the country. At one time this herd was home to nearly 60,000 Mule Deer and has seen countless record book bucks taken by hunters.

Every year, these mule deer leave the top of the Wyoming Range and partake in the longest mule deer migration anywhere in the world. In some instances, these deer travel over 100 miles to winter in the sage flats of the Green River Basin in Western Wyoming. The winter range is essential for these deer to escape the high elevation snow pack and gives them access to critical vegetation during the rough winter months. These wintering mule deer rely almost exclusively on sagebrush and small shrubs to make it through the winter.

Occasionally, extremely harsh winters cause large die-offs in the population because the lack of feed and the deep snowpacks make it nearly impossible for these deer to get the nutrition they need. Deep snow not only exhausts these deer as they attempt to trample through feet of ice, but it also covers up the precious sagebrush and other shrubs, preventing these deer from accessing their only food source.

Historically we have seen these winters take extreme tolls on herd numbers. The winter of ‘92-’93 proved to be one of the most severe we have ever seen for the Wyoming Range herd. The population sat at an all time high in 1991 with an estimated 60,000 head. The ‘92-‘93 winter was extraordinarily harsh on these animals and wiped out over half the population, bringing the total number of deer to around 25,000 (Wyoming Wildlife, Sam Lockwood, WGFD).

How Do Harsh Winters Affect Mule Deer?

Nutrition is one of the single biggest factors that allow these deer to survive the snowy conditions of Western Wyoming every winter. Summer range vegetation provides necessary nutritional value to these mule deer and is of the utmost importance to their winter survival. Mule deer lose between 20 and 25 percent of their weight on the winter range(Montana State University), meaning that adding fat in the summer months is essential to survival.

Although the summer vegetation is critical for these deer, that's not to say winter range habitat isn’t just as important. In the harshest of winters, snow depth can be measured by the feet, even on the valley floor. When snow gets too deep, deer have a much harder time being able to access the few food sources that are available.

This year was an especially rough year for winter feed. We saw a historically harsh winter which followed previous years of drought. These years of drought that preceded this winter stunted the growth of sagebrush and mahogany, meaning there was even less food available beneath the deep snow (Gary Fralick, Billings Gazette).

How Did This Winter Affect The Wyoming Range Herd?

When a large herd of mule deer, like the Wyoming Range herd, is stuck to rely on very minimal food for a winter, it unfortunately results in a high likelihood for mass casualties. Professor Kevin Monteith runs a research lab within the University of Wyoming, dubbed the Monteith Shop, which has monitored the Wyoming Range herd for years. As of early June, data collected by the Monteith Shop reported that 70% of collared does, 60% of collared bucks, and 100% of collared fawns (born in 2022) had died this winter (EcoTour Adventures, Monteith Shop). For comparison, the percentage of collared does that perished in the severe winter of ‘16-’17 was a mere 30% (WyoFile, Mike Koshmrl). Going into the winter, the herd’s population was estimated at around 28,000 and although exact numbers can’t be determined, it is estimated that the population may have dropped to around 10,000 (EcoTour Adventures, Kelsey Wellington/Jackson Hole Conservation Alliance).

Although this winter saw a massive percentage of the population perish, it could have been even worse. The average body fat percentage of monitored deer going into this winter was 12% compared to 8% reported before the also harsh winter of ‘16-’17. This increase in average body fat meant many of these deer were in much healthier shape going into this winter than they have been in other given years (EcoTour Adventures, Monteith Shop).

Even though these deer were “prepared for winter”, according to Prof. Kevin Monteith (Wyofile), nothing could prepare them enough for the extreme snowfall they would face in the ‘22-’23 winter season. Low elevation snowpack was nearly twice the average in some areas of the winter ranger this year (Jackson Hole Conservation Alliance). The upper green river basin experienced only 108% of average snowfall compared to nearly 200% in the winter of ‘16-’17, but in ‘16-’17 most of the snowpack responsible for the staggering numbers was at high elevations. This meant the deep snow wasn’t as deadly for the wintering deer as it has been this winter with deep snow at low elevations (Gary Fralick, Mike Koshmrl, Wyofile).

In these life or death conditions, habitat and vegetation are the lifeline of the mule deer that look to survive the winter. The Winter Range foundation is working on multiple projects to improve winter range forage in Western Wyoming through restoration and nurseries. They are focusing on creating a wildlife nursery to house the essential plants that wintering ungulates like mule deer and antelope rely on. The Winter Range foundation will work on using this nursery to provide seedlings and plants including big sagebrush, mountain mahogany, and serviceberry to wildlife organizations in order to optimize winter range habitat. These efforts work to ensure these deer will be thoroughly nourished regardless of winter conditions.

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