Much of the existing literature examining the effects of roads on wildlife communities has tended to focus on ungulates or large carnivores, perhaps because they tend to cause the most damage when involved in vehicular collisions. Roads and off-road vehicles (ORVs) also have a profound impact on other species, however, including reptiles. Roads and ORVs result in direct mortality, reduced habitat and fragmentation, and behavioral and physiological changes in reptiles. This paper reviews recent literature examining road and ORV impacts on reptiles and assesses proposed methods of mitigating negative impacts.

Direct Mortality
Roads and ORVs are directly responsible for the death of many reptile species (Garber and Burger 1995, Gibbs and Shriver 2002, Luchenbach and Bury 1983, Maxell and Hokit 1999, Spellerberg 1998, Rudolph 2000.) Reptiles on roads obviously risk being struck by cars (Spellerberg 1998, Maxell and Hokit 1999), but ORVs also contribute directly to reptile mortality. For example, in a 1983 study of ORV effects on the biota of Algodones Dunes in Imperial County California, Luchenbach and Bury (1983) determined that there were frequent encounters between ORVs and reptiles that resulted in the animal’s death. Also, although most incidences of roadkill are accidental, it should be noted that one researcher “demonstrated that people will often turn their vehicles towards . . . turtles to hit them intentionally.” (D. Sheppard, pers. comm., in Boarman et al. 1997)

Effects on Habitat
ORVs can also reduce the quality and quantity of habitat. A 2003 study found a direct connection between disruption of native vegetation caused by ORV use and decline in reptile populations on the Owyhee Front (Munger et al. 2003). Heavily used ORV routes disrupted the native vegetation on which reptiles depend and caused a subsequent increase in dense nonnative species like cheatgrass that are known to favor disturbed habitats. In addition, the dense nature of the supplanting cheatgrass hindered reptile movement, making foraging, escaping from predators, and moving through habitats more difficult. ORV use can also destroy the preferred food sources of reptiles like the tortoise that are highly selective foragers (Jennings 1997).

Roads, too, can have undesirable consequences for reptile habitat. A recent study on turtle nesting habitat found that roads created barriers through previously contiguous habitats, fragmenting them and separating female turtles from their preferred nesting habitats (Baldwin et al. 2004). Roads also help spread other dangers, such as exotic vegetation, raven predation, and collection by humans (Boaman et al. 1997).

In addition to the fragmentation of habitat and destruction of food sources, roads and ORVs contribute to a general degradation of conditions that affect all species in an ecosystem, including reptiles (Spellerburg 1998, Maxell and Hokit 1999). Soil disruption from road construction, runoff, and ORV use causes increased sedimentation in adjoining aquatic habitats, impacting water quality (Maxell and Hokit 1999). Cars and ORVs can also contribute to chemical contamination of the ecosystem (Maxell and Hokit 1999).

Effects on Populations
When habitats become fragmented, populations become isolated and face a higher risk of local extinction. Shine et al. (2004) found that if reptile species actively avoid roads (which they may do because of the dangers roads pose to individual survival) highway mortality may be minimized, but fragmentation of local populations will occur. A study on the efficacy of mitigation efforts on desert roads similarly pointed to studies that had determined roads were barriers to tortoise populations (Boarman and Sazaki 1996). Other studies have also noted the dangers of inbreeding depression for populations isolated by roads and road activity (Boarman et al. 1997, Spellerberg 1998.) Roads may also be more dangerous to breeding females because of higher rates of movement associated with their nesting migrations (Steen and Gibbs 2004).

Other Effects
A number of other indirect impacts have also been documented. Luchenbach and Bury (1983), for instance, found an increase in the frequency of tail loss among lizards following an increase in ORV activity in the area. Tail loss is an escape mechanism usually correlated to predator density. This impact is significant because females without tales produce fewer eggs than those with tails. Tail loss could likely lead to reduced survivorship and fecundity (Luchenbach and Bury 1983).

Maxell and Hokit (1999) also point to the noise generated by ORVs as a potential problem. This noise may induce fear in many reptiles, whose instinctual response is to freeze, thus increasing the likelihood they will be run over. Constant exposure to loud noises may also result in hearing loss, making the animals less fit to survive in the wild (Maxell and Hokit 1999).

Roads can also alter reptile behavior. Roads attract reptiles because the cold-blooded animals are attracted to the heat of road surfaces (Rosen and Lowe 1994). In desert ecosystems, roads are known to be favored by reptiles (especially snakes) as thermoregulation sites (Rudolph 2000). This attraction to roads obviously puts reptiles at risk for vehicle mortality. Other studies have demonstrated that male iguanas are attracted to roads as display sites (Rodda 1990) and that female turtles may be attracted to them for nesting purposes (Wood and Herlands 1997, Marchand and Livatis 2004). However, although some of these effects could be argued to be beneficial, Rudolph (2000) points out that: “they also have the potential of increasing road related mortality and exposure to pollutants to the detriment of populations.”

Strategies for Mitigation
There have been a number of studies in recent years assessing the viability of strategies to mitigate the effects of roads. At present, there seem to be few or no studies suggesting mitigating strategies for ORVs; presumably then, at present the only way to mitigate ORV effects is to limit ORV use in reptile habitats.

In general, the two methods of mitigation most discussed in the literature are using fencing to prevent reptiles from getting onto roads and using culverts and underpasses to allow them to move from one side to the other without being exposed to roadways. Barrier fencing can reduce the number of roadkills by preventing reptiles from getting onto roads, but must be properly designed and maintained to be effective (Boarman and Sazaki 1996, Dodd et al 2003.)

Using barrier fencing alone might mitigate the direct mortality of roadkill, but would only serve to enhance the population-fragmenting effects of roads. It is therefore important that tunnels and culverts be used in tandem with fencing to allow immigration and emigration. These, too, must be carefully designed, considering such factors as size, shape, light needs or preferences, temperature, noise, depth, approaches, fencing, human disturbance, and interaction with other species (Jackson and Griffin 2000; Hartman 2002). However, more limited movement and a lower capacity for learning may make underpasses less useful for reptiles than for other small animals (Rudolph 2000).

In Conclusion
Roads, road construction, and off-road vehicle use all pose significant dangers to wildlife. Though most studies have focused on mammals, these forces also have consequences for reptiles. The best proposal for protecting reptiles, if preventing road construction or removing roads are not options, seems to be a combination of barrier fencing to prevent reptiles from getting on to roads, and underpasses and/or culverts to allow individuals free movement between habitats and populations. This, coupled with mitigation methods for other species, seems to represent the best way to protect wildlife from the detrimental effects of roads. However, further study is necessary to determine the real efficacy of these methods.

— Allison Clark is a graduate student in Environmental Studies at the University of Montana.


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