The Perils of Crossing the Road

In today’s world, there are numerous threats to worldwide amphibian and reptile diversity, ranging from habitat loss and fragmentation to human persecution and over-collection for commercial exploitation.  In many places the human population continues to expand while simultaneously pushing into previously unsettled or sparsely populated areas.  With this expansion generally comes an ever denser network of roads, linking cities and rural areas.  Roads can be a primary cause of habitat fragmentation, clearing long stretches of native landscape and creating a barrier to animal movement.  In highly populated areas like the southeastern United States, road networks are quite dense, and it is nearly impossible to find large expanses of natural landscape that are not at least partially bordered or bisected by roads.

For most species, crossing roads is a dangerous affair that often ends in encounters with vehicles and death, especially in areas with high traffic volume.  Road networks are often so dense that many individuals must cross roads to exploit resources on the opposite side.  This issue can be exacerbated in populations that make yearly migrations to breeding habitats (pond breeding amphibians) or that must search out appropriate nesting habitat (turtles).  Furthermore, research has shown that many drivers will go out of their way to hit snakes and turtles on the road, even if they are easily avoidable or completely on the shoulder.  Roads can also directly impact individual behavior, making it more or less likely that animals encounter vehicles.  Some species may prefer the open habitat along roadsides; Gopher Tortoises (Gopherus polyphemus) often move onto the sides of roads when their habitat becomes overgrown.  Snakes will often bask on the warm surface of paved roads, making them a target for most drivers.  However, some snakes have actually been shown to actively avoid crossing roads (ironically this may create a more effective barrier to movement than direct mortality).

Roads not only influence native wildlife but also aid the movement of invasive species by acting as dispersal corridors.  In Georgia, both Brown Anoles (Anolis sagrei) and Cuban Treefrogs (Osteopilus septentrionalis) commonly ride on cars and trucks to new locations.  In fact, at the northern extent of these species ranges, new records tend to follow major interstates and highways.  Last summer, we captured a single adult Cuban Treefrog at Little Ocmulgee State Park in Wheeler County, Georgia, which is close to the northernmost record for this species.  This individual was almost certainly transported to this location on or in a vehicle.  One of the most interesting examples of road aided dispersal of invasive species comes from Cane Toads (Rhinella marina) in Australia.  Cane Toads actually orient and move parallel to roads likely because their locomotor performance increases in sparely vegetated roadside when compared to thicker vegetation.  Invasive species are certainly not immune to negative impacts from roads, but the qualities that make them invasive also make then resistant to experiencing negative effects at a population level.

Road effects will continue to impact populations of amphibians and reptiles throughout the world as more roads are constructed and more roads are converted to pavement.  Identifying ways to mitigate the negative impacts from roads is currently a key issue in animal research.  Under or overpasses can increase habitat connectivity but the success of wildlife crossings has been mixed and this solution is often expensive.  In certain cases, vegetation management along roadsides may be able to deter animals from crossing or make roadside habitats less appealing.  Educating drivers and informing them via signs, especially during breeding migrations, can make them less likely to hit animals crossing the road.

While the overall impact of roads on herpetofauna is negative and finding solutions is often difficult and expensive, there are a few small benefits that come with the negative impacts discussed above.  Many snake species are notoriously difficult to study because of their low detection probabilities.  Road cruising is often the most efficient method for locating certain species (e.g., both Southern Hog-nosed Snakes [Heterodon simus] we caught last year were on the road).  Unfortunately, while driving all over the state, we find many reptiles and amphibians that are dead on the road (DOR).  This is never a welcome sight, but even DOR animals can be useful for research.  We deposit as many fresh specimens as possible into museum collections, making them available to other researchers and helping document the distributions of secretive species (even finding the occasional county record).  Once in collections, specimens can be used for numerous education and research projects.  For species of conservation concern, this is likely the only way for new specimens to enter collections.  In the past, DOR Eastern Indigo Snakes (Drymarchon couperi) have been used to assess internal parasite communities, providing valuable information that would otherwise be unavailable.

The challenges associated with road-wildlife conflicts are numerous and difficult to solve.  They also are not going away anytime in the near future as we are more dependent than ever on driving from place to place.  It will take many creative solutions to lessen the impacts of roads on reptiles and amphibians.