Off-road vehicles (ORVs) have had a substantial negative impact upon the ecological systems of Sierra Nevada national forests. This motorized recreation, if not effectively managed will lead to severe long-term damage to the natural environment by placing soil, vegetation, and air and water quality at risk. In addition to threatening these important environmental attributes, Sierra Nevada wildlife and habitat are at risk through other affects of ORVs such as pollution, erosion, sedimentation of streams, and habitat fragmentation and disturbance. Motorized vehicle use in sensitive habitats also alters the remote and wild character of the backcountry, denying other users the quiet, pristine, backcountry experience they seek and presenting safety and health threats to other forest user groups.
The use of ORVs on forestlands of the Sierra has exploded in the past twenty years. On a nation level, surveys have shown that usage on national forests doubled to approximately 55 million users in 2005. That same year the U.S. Forest Service developed a rule in an attempt to better manage the designation of trails that could be utilized by ORVs. In general, federal agencies have been very slow to respond to the massive increase in ORV use and have largely failed to adequately combat the ecological impact that unmanaged use is having on Sierra forests.
Plant and wildlife species that occupy a small geographical area or are themselves small is size are particularly vulnerable by ORVs. Though the Forest Service has at times insisted that if ORV use is adequately dispersed the impact on species will be negligible, impacts on sensitive and imperiled species can be severe and irreparable.
Impacts on Soils
ORVs cause substantial erosion of the soils on which they travel. This problem is exacerbated on steep slopes and in areas with fragile soils. Eroded soil has several impacts on forest ecology besides the obvious impact of the loss of valuable nutrients and organic matter which are vital to the core functions of a forest system. The eroded soils also find their way into local streams and rivers which increases the both the sedimentation and turbidity of that waterway. This aquatic impact is seriously detrimental to the organisms living in the aquatic environment including fish which depend upon clear water during spawning.
ORVs also impact soils due to their weight. The compaction of soils diminishes the ability of the roots of plants to penetrate into the earth and establish themselves. As the soil is compacted by the vehicle time and time again it becomes less porous, causing a decrease in its permeability to both water and air, thus significantly disrupting the natural nutrient cycle and hydrologic function that soil plays in the forest ecosystem.
Areas in which ORVs operate in large numbers typically experience severe air pollution problems. Most of the air pollution generated by ORVs ends up settling on the soil and in the water of the areas surrounding active trails. This pollution harms soils, soil dependent organisms, vegetation, aquatic habitat, and wildlife. ORVs also directly impact the areas that they visit due to leaking fuel, oil, antifreeze, and other dangerous chemicals which are toxic to wildlife and can persist in area waterways for some time. In a time when California is implementing programs to reduce regional air and water pollution, off-road vehicles are allowed to pollute the air, water, and soil, greatly impacting the health of Sierra Nevada forests.
Damage to Vegetation
ORVs harm vegetation directly through trampling, soil compaction, and pollution. All of these impacts can be devastating to an area of the forest by resulting in fewer and less vigorous plants, reduced plant cover, lower plant diversity, adverse changes in plant species composition, and disruptions to natural plant succession and nutrient cycling processes. The loss of vegetation results in increased soil temperatures, with negative impacts to soil fauna, soil fertility, nutrient cycling, and hydroponic processes. Vegetation loss also increases the likelihood of erosion. The erosion of soil caused by ORVs impacts vegetation by exposing roots to potential damage and by covering vegetation on the downslope side of the eroded area.
The soil and vegetation disturbances caused by ORVs also lead to another rapidly growing threat to forest communities; the invasion of Sierra Nevada forests by non-native plant species. ORV use disturbs the natural conditions of soils and native vegetation aiding the establishment of these invasive plants.
Affects on Wildlife
ORVs are responsible for four primary impacts upon wildlife; direct mortality, general disturbance, noise impacts, and habitat degradation. In the case of direct mortality, ORVs have an immediate impact on the individual that comes into contact with the vehicle either through road kill or illegal poaching. ORV presence in a forest also impacts wildlife by causing many species to flee areas of high vehicle use. These are may in fact be important breeding, feeding, or denning grounds but the accumulated stress of noise, pollution and increased activity causes them to search our other areas suitable as habitat. This behavior change due to the presence of this disturbance can lead to a change in home range and dispersal patterns which will impact the entire forest ecosystem due to the interconnectedness of all species.
The onset of winter causes increased stress on all species and the effects of ORVs on the sensitive behavior which has evolved over time can lead to direct conflict and even death. Snowmobiles in particular can significantly impact small animals by compacting the snow and the thus the pathways and corridors which have been created under the surface, and by simply crushing them under the weight of the vehicle. Compacted snow is also far more difficult to dig through for animals in search of rare and precious food sources in the winter months. Large animals, such as bears, can abandon their dens when located in areas that receive heavy snowmobile use in winter, thus threatening the survival of that individual.
As stated above, ORVs can have a tremendous impact on vegetation. By reducing the abundance and diversity of many plant species and by changing species composition, the habitat characteristics on which various wildlife species rely can be dramatically degraded. As wildlife is forced to depend on less valuable habitat or is forced to use energy in search of better habitat, reproductive success decrease and mortality increases. ORV use also fragments wildlife habitat by creating the same type of disturbance that road construction would. These routes impede the dispersal and migration of wildlife species and in some cases completely eliminates the ability of some species to traverse this impediment.
Aquatic habitat is also significantly impacted through increase sedimentation, destruction of important aspects of aquatic systems through direct contact, and decreased hydrologic health through the effects of pollution. Such disruptions to hydrological processes also increase the risk of additional erosion, flooding, and landslides.