WildEarth Guardians works to protect and restore wildlife, wild places and wild rivers in the American West.

Off-Road Vehicle Impacts on Wildlife

Forests are home to hundreds of species of fish and wildlife, providing recreational opportunities for hunters, anglers, and wildlife enthusiasts.  Millions of hunters and fishermen enjoy pursuing their sport while increasing numbers of birders and photographers enjoy simply catching a glimpse of the diversity of forest life.  In addition to recreational benefits, diverse wildlife are a sign of overall ecosystem health and integrity.  While there are many threats to preserving wildlife ranging from global warming to development, the negative impacts from off-road vehicles (ORVs) on wildlife have been well documented in the scientific literature.  ORVs can impact wildlife through direct mortality, increased legal and illegal harvest, disturbance, and habitat loss.   

Direct Mortality

One of the most apparent impacts of ORVs on wildlife is collisions and direct mortality.  Direct impact will kill most species, but amphibians, reptiles, small mammals and ground nesting birds are most vulnerable (Wilkins 1982, Rei and Seitz 1990, Fahrig et al. 1995, Ashley and Robinson 1996, Gibbs 1998, DeMaynadier and Hunter 2000).  With millions of ORVs traversing the landscape at high speeds (up to 60 mph), the number of animals being killed can be significant.

Habitat Security

Several studies have found that large animals such as elk, wolves, and bears are negatively impacted by the loss of habitat security resulting from increased motorized access.  Depending on the species, some wildlife are more sensitive to disturbance during critical times of year, such as winter habitat for ungulates or areas important for grizzly bear food sources during spring (USDI 1987).  

Elk have been the most extensively studied animal in relation to motorized access.  While recent studies have made a direct connection between ORVs and impacts to elk (Vieira 2000, Wisdom et al. 2004, Wisdom 2007, Grigg 2007), most studies have looked more broadly at the impacts of motorized travel and roads on elk.  It can be assumed that these impacts would be similar on ORV routes.  Many studies have found that increased motorized access results in decreased elk habitat and security (Lyon 1983; Figure 3), and increased elk mortality from hunter harvest both legal and illegal (Hershey and Leege 1982, Hayes et al. 2002, McCorquodale et al. 2003, see Rowland et al. 2005 for review).  

Closing or decommissioning roads has been found to decrease hunter induced mortality (Leptich and Zager 1991), increase elk survivorship (Cole et al. 1997), increase the number of bulls (Leptich and Zager 1991), extend the age structure (Leptich and Zager 1991), increase hunter success (Gratson and Whitman 2000), and allow elk to remain in preferred habitat longer (Irwin and Peek 1979).  Studies have also recommended closing entire areas to motorized use— as opposed to individual roads— to best promote healthy elk populations (Hurley 1994, Burcham et al. 1998, Rowland et al. 2005).

ORVs can also allow access for illegal harvest of wildlife in areas that are difficult for game wardens to patrol.  Weaver (1993) reported that increased ORV access increases the trapping vulnerability of American marten, fisher, and wolverine.  For wolves, one study found that 21 of 25 human caused mortalities in the U.S. northern Rockies occurred within 650 ft. of a motorized route (Boyd and Pletscher 1999).  Wolves often travel on roads and off-road vehicle routes where they risk increased poaching pressure.  Several studies have found that wolf persistence is reduced when road density exceeds approximately 1 mi./mi.2 (Table 1). Lynx are also thought to be sensitive to road density, but to a lesser extent than wolves (Singleton et al. 2001, 2002).  Grizzly bears are at risk from poaching and have been found to be negatively affected by roads and to avoid open roads (Elgmork 1978, Zager et al. 1983, Archibald et al. 1987, Mattson et al. 1987, McLellan and Shackleton 1988, Kasworm and Manley 1990, Mace et al. 1996).  

Disturbance

Probably the most widespread impact of ORVs is disturbance to wildlife.  Within individual species, a number of factors can influence the degree of ORV impact, including the animal’s breeding status, its size, and the size of the group it is with (Burger et al. 1995).  Studies have shown a variety of disturbance is possible from ORVs.  While these impacts are difficult to measure, repeated harassment of wildlife can result in increased energy expenditure and reduced reproduction.  Noise and disturbance from ORVs can result in a range of impacts including increased stress (Nash et al. 1970, Millspaugh et al. 2001), loss of hearing (Brattstrom and Bondello 1979), altered movement patterns (e.g., Wisdom et al. 2004, Preisler et al. 2006), avoidance of high-use areas or routes (Janis and Clark 2002, Wisdom 2007), and disrupted nesting activities (e.g., Strauss 1990).

Again, elk are one of the most studied species in regards to disturbance by mechanized use.  Vieira (2000) found that elk moved twice as far from ORV disturbance than they did from pedestrian disturbance, and Wisdom et al. (2004) found that elk moved when ORVs passed within 2,000 yards but tolerated hikers within 500 ft.  Recently, Wisdom (2007) reported preliminary results suggesting that ORVs are causing a shift in the spatial distribution of elk that could increase energy expenditures and decrease foraging opportunities for the herd.  Elk have been found to readily avoid and be displaced from roaded areas (Irwin and Peek 1979, Hershey and Leege 1982, Millspaugh 1995, Weber 1996).  Additional concomitant effects can thus occur, such as major declines in survival of elk calves due to repeated displacement of elk during the calving season (Phillips 1998).  Alternatively, closing or decommissioning roads has been found to decrease elk disturbance (Cole et al. 1997, Millspaugh et al. 2000, Rowland et al. 2005).  

Disruption of breeding and nesting birds is a particularly well documented problem.  Several species are sensitive to human disturbance with the potential disruption of courtship activities, over-exposure of eggs or young birds to weather, and premature fledging of juveniles (Hamann et al. 1999).  Repeated disturbance can eventually lead to nest abandonment.  These short-term disturbances can lead to long-term bird community changes (Anderson et al. 1990).  Several authors have recommended spatial nest buffer zones from motorized recreation for raptors (Table 2).   On the Loa Ranger District of the Fishlake National Forest in southern Utah, successful goshawk nests occur in areas where the localized road density is at or below 2-3 mi./mi.2 (USDA 2005).

Loss of Habitat

The cumulative effect of loss of habitat security, soil erosion, vegetation loss, introduction of non-native invasive species, and forest fragmentation results in the loss of functional wildlife habitat that supports healthy individuals and populations of wildlife.  Animals may be impacted directly and/or indirectly.  A direct impact may be an ORV that collapses a small mammal burrow or runs an animal over.  An indirect impact would be reduced habitat for cavity-nesting species caused by increased access for firewood collection (Bury 1980).  Any additional habitat loss for sensitive, threatened, and endangered species is also of concern. Wilcove et al. (1998) reported that as many as 13 percent of endangered species are impacted by ORVs.

The indirect impacts of ORVs can have cascading effects throughout the ecosystem.  For example, on an intensively used ORV route in Idaho, native shrubs, bunch grasses, and microbiotic crust were greatly reduced close to the route and replaced with non-native cheat grass (Bromus tectorum) and the native shrub, rabbitbrush (Chrysothamnus sp.; Munger et al. 2003). Because of these habitat changes, fewer reptiles were found alongside the route than were found 325 ft. away.  

— Adam is Science Coordinator for Wildlands CPR and Allison is Conservation Biologist for the Wild Utah Project.

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