The mule deer (Odocoileus hemionus) is a species common to western North America. More limited in range than its closest relative, the white-tailed deer (Odocoileus viginianus), the mule deer is prevalent in mixed habitats, forest edges, mountains and foothills from the Southern Yukon and Mackenzie south through the western United States and as far east as Wisconsin and western Texas. Mule deer living in mountainous environs are well-known for their seasonal migrations to avoid heavy snows and find forage through long winters. Inherent in the fact that mule deer are migratory and have large home ranges which they frequently leave, is the fact that except in the most pristine wilderness, mule deer will inevitably encounter humans and the ubiquitous road.

Anyone who has driven almost certainly remembers a close encounter between their vehicle and a deer. The statistics are staggering. In a 1980 study, it was estimated that 200,000 deer were killed each year on highways. (Williamson 1980) By 1991 this number had more than doubled to 538,000. (Romin, Bissonette 1996a) As human development, highways and roads encroach on the landscape, traditional deer home ranges and migratory routes will continue to be bisected by roadways.

Anthropocentric Concerns

Although roads exert impacts on mule deer other than direct mortality, by fragmenting habitat and degrading habitat security, deer-vehicle collisions dominate the scientific literature. Deer-vehicle collisions cause bodily injury and loss of life, damage vehicles, and decimate the "wildlife resource." (Reed, Woodard, Pojar, 1975) Annual economic loss associated with deer-vehicle collisions was estimated at $7.8 million, a figure based on the estimated value of each deer killed and each vehicle damaged. (Romin and Bissonette 1996b)

Factors Contributing To Highway Mortality

To understand and predict patterns of highway mortality, the scientific literature has examined topographic and vegetative features, road design, temporal factors and characteristics of mule deer populations.

Mule deer customarily move along drainages and riparian areas when approaching a road. (Romin, Bissonette 1996b, Lehnert, Romin, Bissonette 1996) Another study cited highways situated along rivers and below deer winter ranges as the foci of vehicle induced mule deer mortality. (O'Gara, Harris 1988)

Deer-car collisions are influenced by vegetative cover along highway rights-of-way. In areas with high cover, mule deer are more likely to approach the right-of-way for forage, increasing their vulnerability. (Lehnert, Romin, Bissonette 1996) Sagebrush-grass communities, wet meadows, or agricultural areas are typically associated with low highway induced mortality; dense mountain brush habitats are associated with higher rates. (Romin, Bissonette 1996b)

Road location is a critical factor in highway mortality. Roads that bisect migratory routes and winter or spring ranges adversely impact deer movements and increase mortality. Traffic characteristics also have a strong influence, the primary factors being volume and speed. (Romin, Bissonette 1996b) Roads and Off Road Vehicles also reduce habitat security, and in the winter can cause deer to waste precious energy and become exhausted. (Parker, Robbins, Hanley 1984) Highway mortality peaks during breeding and hunting season (fall), and a second, smaller peak occurs in spring when deer forage along highway rights-of-way. (Romin, Bissonette 1996b)

The age, sex, and health of a mule deer may influence its susceptibility to highway mortality. The very young and old are more likely to fall prey to the automobile: one study showed that 67% of adults killed on the highway were less than 2.5 years old. (Romin, Bissonette 1996b) Another study compared the effects of automobiles and predators on mule deer mortality, finding that predators kill more deer 1-6 years old, while autos kill more fawns and mule deer older than seven years. (O'Gara, Harris 1988)

Does tend to have higher energy requirements than bucks because of gestation, parturition, and lactation; therefore they are more attracted to high-quality roadside vegetation. This explains why females have higher highway mortality rates than males. (Romin, Bissonette 1996b)

Physical condition is cited as another factor in highway mortality, and can be assessed by the composition of bone marrow: gelatinous marrow indicates a deer suffering from malnutrition. In one study 89% of mule deer killed by automobiles were malnourished. Sick and malnourished deer tend to travel downslope, utilizing highways where moving is easier. These same deer often do not have the energy to jump over roadside fences, and get trapped along the right of way. (O'Gara, Harris 1988) This explains why winter severity, specifically snow depth and temperature, is related to auto-induced mortality. (Reed 1981) During a severe winter, deer are more likely to be malnourished and sick.

Mitigation

Because roadways are prevalent in mule deer habitat, they impart a risk both to deer and humans. There is a desire to reduce deer-vehicle collisions, exemplified by numerous studies focused on keeping deer off roadways and away from vehicles. Attempts to reduce deer-vehicle collisions were initiated in the 1960's. (Reed 1981)

Wildlife underpasses, overpasses, and fencing structures have been constructed to prevent deer from entering the highway right-of-way. These structures have worked, but they are expensive and fallible. (Reed, Woodard Pojar 1975; Reed 1981; Reed, Pojar, Woodard 1974) Other mitigation techniques used alone or in combination with those above have shown mixed degrees of success. These techniques include: highway illumination (Reed 1981), intercept feeding (Woodard, Wolfe 1988), deer crosswalks (Lehnert, Romin, Bissonette 1996), deer-cattle guards (Reed, Pojar, Woodard 1974), ultrasonic wildlife warning whistles (Romin, Dalton 1992), and Swareflex reflectors (Reeve, Anderson 1993).

Conclusion

The impacts of roads on mule deer are evident and profound, though most efforts to prevent deer-vehicle collisions are undertaken because of the impact collisions have on humans. The obvious solution rather than keeping deer off highways, is to keep highways and automobiles out of deer habitat. Designing roads that avoid mule deer migration routes would lessen the problem removing roads would eliminate it altogether. But because most of the damage is already done, it is important to continue to mitigate highway impacts to mule deer. More effort must be made to facilitate natural deer movements, despite the omnipresence of roads and highways. And while mitigating the impacts of existing roadways, we can prevent new and expanded roadways from exacerbating the problem.

Dana Jensen was the information specialist for Wildlands CPR and will shortly be entering graduate school for environmental studies.

Bibliography

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