Roads and the human activities associated with them have profound impacts on wildlife. Roads, however, are not the only human intrusion that cuts through natural landscapes. Many other linear barriers mar the natural landscape, including trails, electrical power lines, oil and gas pipelines, and railways. According to Lyon et al. (1985), the linear development itself typically does not cause a disturbance response; it’s the human presence on it that causes problems, therefore the level of use must be assessed and evaluated. Foreman (1995) determined that some linear features could be positive and some negative in terms of wildlife impacts: they can provide habitat, serve as conduits for travel or seriously impact wildlife by becoming barriers or sinks that negatively affect wildlife travel and mortality.
How do linear developments affect wildlife?
There have been a significant number of comprehensive studies conducted to assess the impacts of paved roads, but much less has been written about the impacts of dirt roads and trails on wildlife. There are even fewer studies that directly compare the diverse species-specific impacts that emerge along this spectrum of linear features. Jalkotzy et al. (1997) described possible wildlife disturbances within distinct categories to help assess potential impacts within the spectrum of linear barriers. These categories are as follows:
1. Individual Disruption
This refers to wildlife disruption that occurs in the immediate vicinity of a linear feature. Although all linear developments can cause problems, roads probably have the greatest impact on wildlife populations (Foreman et al. 2003). Larger-ranging, more sensitive species are most affected by roads in sometimes not so obvious ways. Wolves, for example, are sensitive to road activity near their natal dens and these disturbances can cause wolves to move pups to less-disturbed areas (Chapman 1977).
The effects of trails on wildlife can have similar negative impacts. In general, smaller linear barriers tend to be less disruptive because they usually have narrower rights-of-way, more curvilinearity and less intense human use (Foreman 1995). Large carnivores tend to be quite sensitive to human presence on trails. Both grizzly bears and black bears avoided trails and tended to maintain an average distance of 274m from trails. Avoidance distances by bears increased to 883m of trails in areas that were more heavily used (Kasworm 1990). Mountain lions were found to adopt a greater degree of nocturnal feeding behavior in order to avoid humans on trails (Jalkotzy et al. 1997).
2. Social Disruption
This is considered any change to the social structure of a population that results from a linear barrier. Changes can include displacement into habitat areas already occupied by other animals of the same species, changes in group structure, or mortality in different age classes as a result of the linear feature (Foreman 1997). Again, these impacts vary significantly for different species and can therefore be difficult to quantify in terms of general impacts on wildlife when comparing and contrasting roads and trails.
3. Habitat Avoidance
When wildlife avoid habitat because of linear barriers or the activities associated with them, habitat can be lost or not used to its full potential (Jalkotzy et al. 1997). Elk tend to avoid habitat close to roads, particularly in areas where they are hunted (Czech 1991). In northwest Montana, grizzlies avoided habitats within 274 m of trails (Kasworm 1990). Mace (1996) found that grizzly bears in the Swan Mountains of Montana appeared to move away from trails during spring, summer, and autumn. They concluded that grizzly bears had become negatively conditioned to human activities and avoided what would otherwise have been valuable habitat. In general, the impacts of trails depend on the types of activities and frequency of use (e.g., hiking, snowmobiling, biking). Studies revealed a broad variation among species in their response to human presence and specific recreational activities on trails.
4. Habitat Disruption or Enhancement
Linear developments can disrupt habitat by introducing exotic plants, and pollutants like dust, salt and vehicle emissions (Foreman 1995). Habitat can also be altered when vegetation is removed for road construction or when new plants or grasses are planted in highway right-of-ways. Whether these human activities disrupt or enhance a particular habitat can usually be linked to the width of the linear feature. The effects of habitat disruption on wildlife are probably small when compared with the effects of habitat avoidance (Jalkotzy et al. 1997).
5. Direct or Indirect Mortality
Direct mortality is a result of the road itself (e.g. wildlife-vehicle collisions or powerline collisions/electrocutions), while indirect mortality can result when roads or trails provide greater access for hunting, trapping, and poaching. A report compiled by Hellmund Associates cited research where aggressive bird species were observed following trails and displacing other sensitive species, resulting in increased predation on songbirds and other neotropical birds. Changes to the area surrounding a trail can have impacts that extend for hundreds to thousands of feet and are referred to as the trail distance effect (Hellmund Associates 1998).
6. Population effects
These are defined as wildlife populations suffering losses as a direct result of linear developments. Roads and vehicles can have a tremendous negative impact on terrestrial vertebrates, but seem to have less impact on overall population size (Foreman and Alexander 1998). There are many information gaps that make it difficult to assess the impacts of roads, but there is growing evidence that linear developments can affect the distribution, movements, and overall populations of wildlife species (Jakotzy et al. 1997). For example, there is concern that development within the petroleum and forest industries has impacted woodland caribou populations (James et al. 2000).
Also, studies found that desert bighorn sheep fail to utilize 20-35% of suitable habitat in Canyonlands National Park, Utah, as a result of human activity and this habitat avoidance has resulted in a decreased population (Papouchis et al. 2001). Another potential impact is the displacement of wildlife to areas with greater risk of predation (Papouchis et al. 2001). The presence of humans on trails and roads can also lead to increased stress on wildlife populations and disrupt wildlife behavior (Vohman 2002).
How do we measure the cumulative impacts of linear features?
Cumulative effects of roads, trails, and other linear barriers on wildlife should be measured through a cumulative effects assessment (CEA) and utilize geographic information systems (GIS). Fairbanks and Tullouse (2002) urged managers to incorporate research findings on wildlife into GIS mapping to help assess the impacts of proposed linear developments. The three main ways to measure the cumulative impacts of linear features are: 1) measure road densities, 2) assess road/trail design, and 3) conduct an ecological evaluation prior to development (Hellmund Associates 1998).
Existing systems for evaluating road/trail impacts on a landscape level
After reviewing existing literature to identify a viable system for comparatively evaluating and weighting the impacts of roads and trails, I discovered three systems (two of which have specific evaluative measures). The first included in the Arc Wildlife Services study presented a model developed by Weaver (1986) for assessing impacts on large carnivores or other sensitive species that tend to avoid linear developments. The second system provides a set of assessment tools to evaluate impacts on small mammals (Joslin et al. 1999). The third provides a weighted system based on flight distances to compare and contrast reactions of wildlife to a variety of trail uses (Hellmund Associates 1998). It would be ideal to have a system that includes all elements, species and factors in one, however, since species’ needs and reactions to linear development are often dramatically different, it may be more appropriate to use a series of weighted systems in order to best represent the big picture impacts of a proposed road or trail.
It seems safe to infer from the research that the larger and more significant the linear feature and the greater the density of developments within area, the greater the impacts on wildlife. It is also evident from the research that the cumulative effects of all linear intrusions (roads, trails, seismic lines, power lines, rail lines) should be considered collectively to truly grasp the overall effects on the diverse species that inhabit any given ecosystem.
Only when we consider the overall density of roads and trails, taking into account the zones of avoidance for multiple species, will we glimpse the ecological impacts of our choices. Perhaps by doing so in a meaningful way, decision-makers will choose to limit new linear features or realign human travel routes in a way that will provide meaningful ecological benefits to wildlife populations.
—Sharon Mader has been a life-long environmental activist and is currently an Environmental Studies graduate student at the University of Montana.
References
Chapman, R.C. 1977. The Effects of Human Disturbance on Wolves. M. Sc. Thesis. University of Alaska, Fairbanks, Alaska.
Czech, B. 1991. Elk behavior in response to human disturbance at Mount St. Helens National Volcanic Monument. Applied Animal Behavior, SCI. 29(1-4):269-77.
Fairbanks, S.W., R. Tullous. 2002. Distribution of Pronghorn on Antelope Island State Park, Utah, USA, Before and After Establishment of Recreational Trails. Natural Areas Journal 22: 277-282.
Forman, R.T., 1995. Land Mosaics: The ecology of landscapes and regions. Cambridge University Press, Cambridge, Mass.
Forman, R.T., L.E. Alexander. 1998. Roads and Their Major Ecological Effects, Annu. Rev. Ecol. Syst. 29: 207-31.
Hellmund Associates. 1998. Planning Trails with Wildlife in Mind: A Handbook for Trail Planners by Trails and Wildlife Task Force, Colorado State Parks. 50 pp.
Jalkotzy, M.G., P.I. Ross and M.D. Nasserden. 1997. The Effects of Linear Developments on Wildlife: A Review of Selected Scientific Literature. Prepared for Canadian Association of Petroleum Producers. Arc Wildlife Services, ltd., Calgary. 115 pp.
James, A. R., A.K. Stuart-Smith. 2000. Distribution of Caribou and Wolves in Relation to Linear Developments, Journal of Wildlife Management 64(1):154-159.
Joslin. G., and H. Youmans, coordinators. 1999. Effects of Recreation on Rocky Mountain Wildlife: A Review for Montana, Montana Chapter of The Wildlife Society. 307 pp.
Kasworm, W.F., T. L. Manley. 1990. Road and Trail Influences on Grizzly Bears and Black Bears in Northwest Montana, International Conference on Bear Research and Management 8:79-84.
Lyon, L.J., T.N. Lonner, J.P. Weigand, C.L. Marcum, W.D. Edge, J.D. Jones, D.W. McCleerey. 1985. Coordinating Elk and Timber Management: Final Report of the Montana Cooperative Elk-Logging Study 1970-1985. Montana Department of Fish, Wildlife and Parks, Helena. 53 pp.
Mace, R.D., J.S. Waller, T.L. Manley, L.J. Lyon, and H. Zurring. 1996. Relationships among grizzly bears, roads, and habitat in the Swan Mountains, Montana. Journal of Applied Ecology 33:1395-1404.
Papouchis, C.M., F.J. Singer, W.B. Sloan. 2001. Responses of Desert Bighorn Sheep to Increased Human Recreation, Journal of Wildlife Management 65(3):573-582.
Vohman, E.C., Effects of Roads and Trails on Wildlife and Ecosystems, Prepared for High Country Citizen’s Alliance.
Weaver, J., R. Escano, D. Mattson, T. Puchlerz, and D. Despain. 1986. A cumulative effects model for grizzly bear management in the Yellowstone Ecosystem. In:Glen P. Contreras and Keith E. Evans (compilers). Proc. Grizzly Bear Habitat Symposium. USDA Forest Service Gen. Tech. Rep. INT-207. P. 234-46.