Many environmentalists and conservation biologists enjoy riding mountain bikes. Mountain biking is often perceived as a low-impact pursuit, more like hiking, backpacking, and paddling than motorized four-wheeling or dirt biking. However, there actually isn't much evidence to support this intuition.

There is currently a lack of scientific literature comparing the effects of mountain biking and other recreational uses on natural systems (Thurston et. al., 2001). While the effects of recreation generally have been well studied, the extent to which mountain biking affects natural systems relative to other forms of recreation has been studied only superficially (Knight, pers. comm.).

In recent years, the sport of mountain biking has boomed in popularity. The BLM reports that "an estimated 13.5 million mountain bicyclists visit public lands each year to enjoy the variety of trails." (BLM, 2002). Important questions about mountain bikes remain for conservationists and land managers in adopting appropriate land management policies: Do mountain bikes impact ecosystems differently than hiking? What kinds of impacts do mountain bikes have? Should special impacts by mountain bikers be considered when devising management strategies?

The impacts of non-motorized recreational use on ecological systems can be divided into three categories: "trampling," mechanical destruction of ground level vegetation; "erosion," the mechanical mobilization of sediment; and "wildlife disturbance," disruption of animal ecosystems by human presence. This article reviews the current literature on the impact of mountain biking recreation on ecosystems.

Trampling

Trampling studies examine the impact of recreational use on vegetation through the mechanical application of force on plants. Terence Yorks of Utah State University has developed a general model for understanding the varying impact of different modes of travel on vegetation:

Land Impact = ((weight + output acceleration) x swath)).

The amount of damage inflicted on vegetation can be understood as a function of the energy released. In the above equation, "output acceleration" is defined as vehicle power divided by its mass. "Swath" is the width of the vehicle's track (tire, foot, or track) multiplied by its length of travel (Yorks, 2000). This methodology provides an analytic framework for examining the amount of energy transmitted to plant structures by various modes of travel. Using this comparative analysis, mountain bicyclists have more impact than hikers, but are comparable to hikers in impact when compared to motorized vehicles (Yorks, 2000). Much of the higher impact of mountain bikes relative to hikers comes from the longer distance typically traveled, and subsequently larger "swath."

There are some problems with this method of quantifying impact, particularly when evaluating a specific user type in a specific area. For example, it has been shown that motorcycles actually widen trails less than horses when going downhill (Weaver et. al., 1978), though Yorks's framework would demonstrate that motorcycles have more impact. This is explained by the fact that walkers (human or animal) must check their speed as they proceed downhill by generating friction with the ground surface. Wheel-driven vehicles can check their speed by using brakes, without applying a shearing force to the ground surface.

Despite some limitations in modeling local effects, Yorks's framework is supported overall by his 1997 meta-analysis of the 400 extant citations examining the effects of foot and vehicle impacts on vegetation. According to Yorks (2000), "the weight, power, and swath equation that was presented here is consistent with long term observations of vegetation, soil, and pavement changes following land use."

The only published direct comparison of hiking and mountain biking's trampling effects on untrammeled vegetation was conducted by Thurston and Reader (2001) in Boyne Valley Provincial Park, Ontario. Applying experimental treatment passes of varying intensity to a series of test plots in a deciduous forest, they concluded that mountain bikes and hikers do not do significantly differ in their effect on plant stem density, species richness, or soil exposure. Cole and Bayfield (1993), however, assert greater standardization of study design and terminology is needed.

Erosion

Studies of erosion tend to focus on the effects of recreational use on existing trails, including trail widening through destruction of vegetation. Since many mountain bike riders stay on the trail, such studies may be of relevance.

Past studies have found that different user groups do indeed affect trails and adjacent vegetation differently. Weaver and Dale (1978) showed that hikers, motorcycles, and horses have different impacts on trails through meadow and pine forest. In general, they found that hikers have the least impact followed by motorcycles, then horses. However, motorcycles tend to deepen trails more than horses when traveling uphill and on grasslands (Weaver and Dale, 1978).

Wilson and Seney (1994) applied experimental passes to various sites on an existing trail system in the Gallatin National Forest of Montana and, simulating rainfall, measured sediment erosion. In the only study that specifically includes mountain bikes in such comparisons, they found that horseback riders mobilize more sediment than do mountain bikers, motorcycles, or hikers. However, they also found a slightly greater mobilization of sediment caused by hikers compared to off-road bicycles, suggesting the pounding action of feet or hooves may have a more erosive effect than a wheeled form of travel.

The available literature suggests that different user groups cause different levels of trail erosion (Weaver et al, 1978). Wilson et al (1994) reported that mountain bikes have no greater impact on trails than do hikers and motorcyclists and have less impact than horses. However, given the little available information, further study seems warranted.

Wildlife Disturbance

The presence of human recreation in wildlife habitat, whether motorized or non-motorized, has an effect on animals living in that area. Through flushing, increased stress, and disruption of breeding and feeding cycles, humans can diminish the health of animal populations (Knight 1995). While a great deal of literature exists on the effect of human recreation on animals, few researchers have examined the effects of mountain bikers and other user classes (Knight, pers. comm.). Taylor (2001) examined the differential effects of hikers and mountain bikers on several species, including bison, mule deer and pronghorn antelope, on Utah's Antelope Island in the Great Salt Lake.

Taylor (2001) observed a negative relationship between animal body size and flushing response, but found no difference in likelihood of flushing caused by mountain bikers and hikers. Regular flushing and harassment imposes energetic burdens on animals, disrupts feeding and breeding, increases mortality, and reduces population health. However, because short-term behavioral changes are more easily studied, there is less data on the effect of recreation on long-term population health. (Knight, 1995).

Stake (2002) looked at the impacts of mountain biking activity on golden-cheeked warblers at Fort Hood, Texas, a military training area. This study was able to examine a wildlife population before and after the opening of a mountain biking park. They reported no impacts from mountain biking on warbler territory density, return rates, or age structure (Stake, 2000).

Another effect mountain bikes can have on animals is direct mortality caused by collision. There is no literature on this topic, however, anecdotal evidence suggests that small mammals and reptiles are vulnerable to impact and not uncommonly killed. (A. Switalski, pers. comm.).

Conclusions and Future

Research Needs As a non-motorized recreational use, mountain biking has so far received little scrutiny by conservationists and biologists when compared to motorized use. While the studies discussed above suggest that mountain biking does not have a greater impact on some aspects of ecological health than other non-motorized user groups, to date there is not a sufficient body of data to support this conclusion. However, much anecdotal evidence suggests that there may be negative impacts including erosion and wildlife disturbance.

There are some problems inherent in applying all of these study designs to management. For example, while Taylor (2001) concluded that pronghorn antelope do not have a greater likelihood of flushing with each single encounter, mountain bikers typically travel much farther per trip, therefore exposing wildlife to more total stress per recreational visit. Experimental trampling studies currently do not take into account speed, braking technique, and other elements of mountain bicyclist behavior. Unless such factors are taken into account, these experiments will be limited in their applicability to real-world mountain biking situations.

As mountain biking increases in popularity and improved equipment and technique enable trail bicyclists to extend their presence further into wildlife habitat, the potential for negative impact on trails and wildlife will increase. Proper management of wildland systems, and ensuring their ecological health, will require more data and a clearer scientific understanding of the impacts of mountain biking.

--- Jason Lathrop is a graduate student in environmental studies at the University of Montana and a recovering journalist. He has written for Outside, Mungo Park, ABCNews.com, and The New York Times Sunday Magazine.

References

Bureau of Land Management 2002. Letter inviting involvement in the development of a new National Mountain Bicycling Action Plan.

Cole D.N. and N.G. Bayfield. 1993. Recreational trampling of vegetation: standard experimental procedures. Biological Conservation 63: 209215.

Fairbanks, W.S. Distribution of pronghorn antelope (Antilocapra americana Ord) on Antelope Island State Park, USA, before and after establishment of recreational trails. Nature Areas Journal 22(4):277-282.

Knight, R.L. and K.J. Gutzwiller (eds.). 1995. Wildlife and Recreationists: Co-existence Through Management and Research. Island Press, Washington D.C.

Stake, M. M. 2000. Impacts of Mountain Biking Activity on Golden Cheeked Warblers at Fort Hood, Texas. In: Endangered Species Monitoring and Management at Fort Hood Texas: 2000 Annual Report, Fort Hood Project, The Nature Conservancy of Texas, Fort Hood Texas, USA.

Taylor, A. 2002. Wildlife Responses to Recreation and Associated Visitor Perceptions at Antelope Island State Park Utah, Master's Thesis, Colorado State University Department of Fishery and Wildlife Biology.

Thurston, E. and R.J. Reader. 2001. Impacts of experimentally applied mountain biking and hiking on vegetation and soil of deciduous forest. Environmental Management. 27(3): 397-409.

Weaver, T. and D. Dale. 1978. Trampling effects of hikers, motorcycles, and horses in meadows and forests. Journal of Applied Ecology 15:451-457.

Wilson, J.P. and J.P. Seney. 1994. Erosional impact of hikers, motorcycles, and off-road bicycles on mountain trails in Montana. Mountain Research and Development. 14(1): 77-88.

Yorks, T.P. et. al. 1997. Toleration of traffic by vegetation: life form conclusions and summary extracts from a comprehensive database. Environmental Management. 21(1): 121-131

Yorks, T.P. 2000. Should People or Machines Have Equal Rights, an automated Web presentation available at http://cc.usu.edu/~olorin/ [1] vehicles/index.htm