Mule Deer Distribution

Mule Deer Distribution

Mule deer are distributed throughout western North America from the coastal islands of Alaska, down the West Coast to southern Baja Mexico and from the northern border of the Mexican state of Zacatecas, up through the Great Plains to the Canadian provinces of Saskatchewan, Alberta, British Columbia and the southern Yukon Territory. With this wide latitudinal and geographic range comes a great diversity of different climatic regimes and vegetation associations. Amongst those vegetation associations some of the largest concentrations of mule deer occur in the sagebrush biome (Figure 1).

Within the broad geographic range of mule deer the WAFWA Mule Deer Working Group has designated 7 separate ecoregions: 1) California Woodland Chaparral, 2) Colorado Plateau Shrubland and Forest, 3) Coastal Rain Forest, 4) Great Plains, 5) Intermountain West, 6) Northern Forest, and 7) Southwest Deserts. The greatest degree of overlap with the sagebrush biome occurs in the Intermountain West ecoregion(Figure 2).

Mule Deer and Sagebrush Habitat Relationships

Mule deer are primarily browsers, with a majority of their diet comprised of forbs and browse (leaves and twigs of woody shrubs). Deer digestive tracts differ from cattle (Bostaurus) and elk (Cervuselaphus) in that they have a smaller rumen in relation to their body size and so they must be more selective in their feeding. Instead of eating large quantities of low-quality feed like grass, deer must select the most nutritious plants and parts of plants. Because of this, deer have more specific forage requirements than larger ruminants.

The presence and condition of the shrub component is an underlying issue found throughout different ecoregions and is important to many factors affecting mule deer populations. Shrubs occur mostly in early successional habitats; that is, those recently disturbed and going through the natural processes of maturing to a climax state. This means disturbance is a key element to maintaining high quality deer habitat. In the past, different fire cycles and human disturbance, such as logging, resulted in higher deer abundance than we see today. Although weather patterns, especially precipitation, drive deer populations in the short-term, only landscape-scale habitat improvement will make long-term gains in mule deer abundance in many areas.

Sagebrush can be an important component of mule deer diets, especially during winter. In the Bridger Mountains of Montana, sagebrush ranked first in mule deer diets during December, January, and February (Peterson 1995). Near Gardner, mean use by mule deer and elk on winter ranges was 59% for mountain big sagebrush, 42% for Wyoming big sagebrush, 32% for basin big sagebrush and 16% for black sagebrush (A. nova) (Peterson 1995). Sagebrush is highly palatable and selected by deer and other wildlife such as pronghorn (Antilocapraamericana). The U.S.Forest Service(unpublished data) ranked sagebrush according to preference and included 3 rankings: (1) high preference – mountain big sagebrush (Artemisia tridentatavaseyana), low sagebrush (A. arbuscula), and silver sagebrush (A. cana); (2) intermediate preference – Wyoming big sagebrush (A. t. wyomingensis), and alkali sagebrush (A. longiloba); and (3) low preference – basin big sagebrush (A.t. tridentata)and black sagebrush(A. nova).

Impacts to Sagebrush Habitats

Various changes and impacts have occurred in sagebrush and associated mountain shrub communities over the past 80-100 years, primarily due to human intervention. These changes have taken many forms, including: (1) actual removal of habitat due to the construction of housing developments, mining, oil-mineral development, and road building, (2) effects of domestic livestock grazing, especially sheep and cattle, (3) manipulation of these communities for other forms of production (e.g., grass production increase, farming, irrigation), and (4) fire suppression. While not all-inclusive, these are the major impacts to sagebrush and mountain shrub communities.

Perhaps the greatest impact to mule deer range results from the combination of livestock grazing and fire suppression. This conclusion seems to coincide with past mule deer irruptions and declines in the Intermountain West. Four associated causes of such declines and irruptions were cited by (Gruell1986): (1) Succession of rangelands from dominance by grasses to dominance by woody plants that constitute superior mule deer habitat (Julander 1962, Leopold 1950, Longhurst et al. 1976); (2) conversion of forests to shrubs by wildfire and logging, which generally resulted in improved deer habitat, particularly availability of browse (Lyon 1969); (3) conservation and predator control dramatically reduced deer mortality (Leopold et al. 1947, Rasmussen and Gaufin 1949); and (4) reduction in numbers of livestock on the open range increased the amount of forage available to mule deer (Rasmussen and Gaufin 1949).

These past effects, coupled with continued high deer numbers, livestock use, and fire suppression, have led to a wide range of shrub community conditions in sagebrush-grasslands and mixed mountain shrub communities, which continue to become less productive (Anderson 1958, Bennett 1999). Anderson (1958:Pgs. 26-27) reported browsing of shrub communities in Wyoming was heavy, and stated: “It is very alarming … to note that each of the areas examined and reported on here exhibit definite signs of range deterioration. This vegetation deterioration is in the incipient stages in some areas; in other areas is much more serious and has progressed to the point where recovery will be a long, slow process.” Anderson (1958:28) continued by saying: “It is suggested the ultimate goal be to maintain game herds at a level where average winter mortality is kept at a minimum, average annual forage utilization falls within the proper limits, and vegetative trends are stabilized or are upward.” Nonetheless, based on others’ examination of these sites at later dates (1960s and 1970s), even further declines in shrub conditions had taken place.

Both vegetation and mule deer populations can respond positively to disturbance. Shepherd (1971), concluded at moderate removal rates (20-30%) of current annual growth, browsing was invigorating and decreased leader die-off. He also found serviceberry, antelope bitterbrush, big sagebrush (Artemisia tridentata), and mountain mahogany could sustain even higher removal rates. But depending on the amount of timely moisture, plant age, and cumulative years of browsing, consistent removal rates >40% greatly diminish the plant’s ability to set seed and restrict recruitment of young plants.

As vegetative productivity and vigor declines, the availability of food or browse for mule deer consumption also declines. Competition between free-ranging and domestic ungulates occurs in this situation, for the remaining vegetative components. Specific effects on mule deer herds are difficult to discern due to many other factors which may affect vegetative production and deer populations. Yearly vegetative production variations and winter severity can substantially change population levels between years. Consequently, long-term wildlife population trends will reflect the changes in vegetative community structure and age.

These population trends and vegetation changes have been noted for mule deer by various researchers. On the Kaibab Plateau, where relatively good records exist on population trends, Mitchell and Freeman (1993) hypothesized the decline of the deer herd was due to multiple factors, including: (1) extreme overgrazing, primarily due to a rapidly increasing deer herd; (2) drought conditions in 1924 and 1925 that aggravated the die back of the deer herd; (3) interruption of the regular ground fire cycle in the latter nineteenth century, in response to overgrazing by livestock and horses, which accelerated loss of available browse and habitat.

Key management issues for mule deer habitat include loss of shrubland (sagebrush and mountain brush species) integrity, conversion of native vegetation to agriculture lands and residential developments, and cumulative habitat degradation from overgrazing. Loss of lands and fragmentation of habitats caused by urbanization and recreation use are major threats. Pinyon (Pinus spp.)-juniper (Juniperusspp.) encroachment is also a major problem because thousands of acres of valuable mule deer range, primarily shrublands, are being taken over by pinyon-juniper (P-J) each year (Miller et al. 2008).

Sagebrush Management for Mule Deer

Manage for early successional stages to provide forage. —Deer populations do not respond demographically to each habitat or habitat alteration consistently, and interactions occur along soil and climatological gradients (Dasmann and Dasmann 1963). However, early successional stages tend to be favored by mule deer for foraging. Numerous tools are available for converting vegetative associations to an earlier successional stage, including fire, grazing, or mechanically or chemically induced changes. Removing climax vegetation and providing early successional communities favors mule deer in forest and chaparral habitats by increasing the amount and quality of forage (Brown 1961, Dasmann and Dasmann 1963, Taylor and Johnson 1976, Krueger 1981, Thill et al. 1990, Kucera and Mayer 1999). However, early succession forage plants may be higher in digestion-inhibiting secondary plant compounds (Happe et al. 1990). The benefits of fire on herbaceous plants are generally short term, about 6–11 months, but the beneficial effects on browse species can be longer lasting (Carlson et al. 1993). Fire can improve winter forages, as young forbs, grasses, and shrubs have elevated concentrations of protein and superior in vitro digestible organic matter in winter diets of mule deer (Hobbs and Spowart 1984). However, large, intense wildfires can eliminate or reduce shrubs from winter ranges, decreasing their value to deer and even eliminating deer use. Within sagebrush communities, artificial disturbances must be planned carefully (Dasmann and Dasmann 1963). Sagebrush can be eliminated from communities inadvertently through poorly planned activities, which may be detrimental to deer populations that rely on it. Sagebrush should be retained in the disturbed community, but forbs and grasses can be stimulated. If big sagebrush exceeds 30% of the diet, tannin concentrations can be detrimental to deer rumen activity and function (Carpenter et al. 1979). Herbicidal treatments of live oak in dense areas may improve grass and forb yield but may not result in improved body condition of deer (Fulbright and Garza 1991).

Manage for a diversity of key plants, including forbs. —Targeting the desired forage component when considering habitat improvements is important. With the exception of moisture, which is generally higher in more preferred species, tissue components may not show any consistent relationship to preference in deer diets (Radwan and Crouch 1974). Bryant et al. (1981) found that white-tailed deer feed more on grass and forbs on excellent condition range, whereas they fed more on browse on poor condition range. Crude protein and phosphorous were higher in pastures in excellent condition, while digestible energy (DE) levels were higher on excellent condition range in every season except winter, when browse DE was superior to that of herbaceous material (Bryant et al. 1981). Forage quantity is generally not a problem during winter, but forage quality may be at unacceptable levels for maintaining mule deer populations (Bartmann 1983). Even with varying degrees of forage quality, mule deer managed to select a diet that maintained consistent quality throughout late winter (Bartmann 1983). Even on excellent condition rangeland, crude protein may be low during times of the year, especially winter (Bryant et al. 1980). Energy seems to be more limiting to animal production than protein (Bryant et al. 1980). Providing a diversity of forage composition across a landscape provides the greatest opportunity for mule deer to meet their year-round nutritional requirements.

Impacts of sage-grouse oriented management actions

Research and published findings on the role of sage grouse management actions to benefit ungulate species including mule deer has been relatively scant. However, Copeland et al (2014) found that conservation measures to protect sage-grouse in Wyoming overlapped with 66-70% of migration corridors, 74-75% of stopover habitat, and 52-91% of wintering areas for mule deer. Bergman et al (2014) found that habitat treatments using a combination of mechanical (hydro-axe and roller-chop) and chemical application, increased winter survival for mule deer fawns in pinyon-juniper woodlands. Finally, Bombaci and Pejchar (2016) reviewed literature pertaining to impacts of reducing pinyon-juniper woodlands on sagebrush obligates and sagebrush associated species and found that an equal proportion of studies had either negative or non-significant results on ungulate populations. Those authors recommended that additional research is needed to address long-term and landscape scale effects of pinyon and juniper woodland reductions to benefit wildlife species.

Science needs

• Prediction of climate change scenarios on fire risk and mule deer distribution
• Thermal cover dynamics of mule deer in arid environments
• Density dependent population responses on sagebrush winter ranges
• Multiple species interactions such as predator-prey dynamics in shrubland ecosystems
• Empirical data on impacts of pinyon/juniper removal to benefit mule deer

Literature Cited

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Bartmann, R. M. 1983. Composition and quality of mule deer diets on pinyon–juniper winter range, Colorado. Journal of Range Management 36:534–541.

Bergman, E. J., C. J. Bishop, D. J.Freddy,G. C. White, and P. F. Doherty. 2014. Habitat management influences overwinter survival of mule deer fawns in Colorado.Journal of Wildlife Management78:448-455.

Bombaci, S., and L. Pejchar. 2016. Consequences of pinyon and juniper woodland reduction for wildlife in North America.Forest Ecology and Management365:34-50.

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Carlson, P. C., G. W. Tanner, J. M. Wood, and S. R. Humphrey. 1993. Fire in key deer habitat improves browse, prevents succession, and preserves endemic herbs. Journal of Wildlife Management 57:914–928.

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Julander, O. 1962. Range management in relation to mule deer habitat and herd productivity. Journal of Range Management 15:278-281.

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Miller, R. F., R. J. Tausch, D. E. McArthur, D. D. Johnson, and C. Stewart. 2008.Age structure and expansion of piñon-juniper woodlands: a regional perspective in the Intermountain West. U.S. Forest Service Research Paper Report RMRS-RP-69. Fort Collins, Colorado, USA.

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Figure 1. Range-wide distribution of black-tailed and mule deer in relation to the sagebrush biome by Floristic Province. Data source:

Figure 2. Range-wide distribution of black-tailed and mule deer by ecoregion in North America. Data source: