The impacts of roads and road use on snakes

Snakes live in a wide range of habitats throughout the world and play an important role in ecosystems. No matter what the habitat, snakes can be affected in a variety of ways by both the presence and use of roads. Effects of roads on snakes include direct road mortality, habitat loss, fragmentation of habitat, and behavioral changes. If these impacts become significant
enough, they may result in population level effects, which could have ripple effects in the ecosystem. In addition, unlike nearly any other animal, snakes are at higher risk for road mortality for social reasons. Several studies have shown that some drivers will intentionally run over snakes due to their dislike of the reptiles (e.g., Langley et al. 1989, Row et al. 2007). However, mitigation measures are available to protect snakes near roads and ensure their persistence. In this article I review the impacts of roads and road use on snakes.

Direct Mortality
Many snakes are killed each year by being run over by cars. Snakes may cross roads because roads cut through their habitat and home range, thus forcing them across to access important parts of their habitat, or in search of mates. Snakes also use roads to increase their body temperature by basking in the sun — a common behavior of cold-blooded animals. Studies show a direct increase in snake mortality with a decrease in temperature (e.g., Shepard et al. 2008b), however, other studies have found an increase in snake mortality in areas where there is a high maximum temperature (e.g., Ciesiolkiewicz et al. 2006). Thus depending on the area, temperature can have a large effect on snake road mortality.

According to one study, juvenile snakes appear to be the most susceptible to road mortality, especially during hatching times when there are many juvenile snakes in the area.  However, it is unknown whether juveniles are more susceptible to becoming roadkill because of lack of experience with roads, or if it is simply that they make up a larger percentage of the population certain times of the year, and thus they make up a larger percentage of the roadkill (Ciesiolkiewicz et al. 2006). 

It is possible that roadkill alone can even lead to population level impacts for snakes in certain places. For example, one study on black ratsnakes (Elaphe obsoleta) in Ontario, Canada found that out of 115 snake crossings recorded, 3 snakes were killed (Row et al. 2007). The study area was mixed deciduous forest habitat with an 80 km/hour road cutting through. If 340 adult snakes cross the road during a season (which is predicted) then 9 would be directly killed by cars. If this level of mortality continued, the authors estimated there would be a 99% extinction rate within 500 years.

Once a snake is dead and lying on the road it may attract scavengers (Antworth et al. 2005). Thus road-killed snakes can cause additional roadkill. If the scavenger is significantly larger, then the problem can be worse than simply more roadkill, as the animal/vehicle collision may then also cause injury to the people in the car.

In addition, one issue that makes snakes differ from potentially all other animals is the likelihood of intentional road mortality. For example, one study placed snakelike objects on the road and then observed driver behavior on a Kansas highway. Eighty percent of drivers intentionally moved to hit the snakelike objects (Langley et al. 1989). Another study, focused on the effects of road mortality in black ratsnakes, found that a majority of the time drivers see snakes in advance and can decide whether or not to avoid a collision (Row et al. 2007).

Habitat Fragmentation and Other Effects
Not only do roads cause direct mortality, they also have other effects such as acting as barriers and fragmenting landscapes, which can make it difficult to find potential mates. The idea that snakes (especially those of smaller species) will avoid roads is supported by several studies (Shepard et al. 2008a, Row et al. 2007, Aresco 2005, Andrews et al. 2005). If  mitigation is not successful, then in time, animals could acclimate to avoid roads, which could lead to less diversity within the population and the eventual negative effects of inbreeding (Shepard et al. 2008a).

Off-road vehicles (ORVs) can also have impacts on snakes including direct mortality, destruction of habitat, and behavioral changes. Some snakes hibernate and/or dig their nests underground and when ORVs are driven in snake habitat, they can impact reproduction by destroying these underground nests and crushing snake eggs (Burger et al. 2007). Snakes themselves may also be intentionally or unintentionally run over by ORVs, causing direct mortality. One study on pine snakes (Pituophus melanoleucus) in New Jersey found that snake movement is slowed by off-road vehicle tracks, further increasing their vulnerability (Burger et al. 2007). In addition, off-road vehicles trample vegetation and over time change the vegetation community. These changes in vegetation can affect how snakes camouflage themselves which in turn, can result in higher snake mortality as they are preyed upon by hawks and other predators (Burger et al. 2007).

One recent study on restoration found a different and unexpected impact to snakes from restoration projects, and this same impact is also occurring adjacent to roads. The study found that numerous snakes were dying after becoming entangled in the mesh of rolled erosion control products (Barton and Kinkead 2005; see adjacent photo from roadside erosion control on US Hwy 93 in MT.) The weave of the matting is wide enough to trap snakes. While such products are important for erosion control, and are used in all sorts of projects,  including along roadsides, they can be deadly to snakes. In addition to recommending alternative erosion control measures, the authors recommend reducing the size of the mesh to decrease the likelihood that snakes will get trapped (Barton and Kinkead 2005).

Mitigation
Protective measures can help reduce snake road mortality and the barrier/habitat fragmentation effects of roads. One mitigation method is the use of fences to lead animals toward an overpass or underpass across a road. Fences that are not specifically engineered for snakes may not be effective, but species-specific fencing is used effectively in some areas. Nonetheless, few studies address the  types of fencing needed for snakes and other herpetofauna in association with wildlife crossings. For example, rails, pipes and even concrete walls  can help snakes get across roads (Griffen 2005). By ensuring that at least a small portion of snakes can successfully access and use the structures to cross the road, biodiversity in the area can be improved (Griffen 2005). Another method could be the use of animal/snake warning signs for drivers. However this hasn’t necessarily been proven effective. Such warning signs are only minimally successful in tests (Shepard et al. 2008b). In addition, for those drivers who intentionally hit snakes, signs could actually result in an increase in road mortality (Shepard et al. 2008b). Finally, aggressive management and reducing ORV use has been found to increase reproductive success of snakes (Burger et al. 2007) and should be considered as a mitigation strategy as well.

Conclusion
Snakes are measurably impacted by roads and off-road vehicles. Some specific issues with snakes and animal/vehicle collisions don’t apply to other animals because of both snakes’ size and people’s disposition towards them. More research in mitigation techniques for snakes would be helpful. In addition, reducing the use of ORVs in important snake habitat, and improving road management, design and placement can also help reduce the negative impacts of roads and off-road vehicles on snakes.

— Alexander Walder is a high school senior at South Plantation High School in Plantation, FL. He is a student of the magnet program on Everglades Restoration. He is also Executive Director Bethanie Walder’s nephew. He researched and wrote this paper for an internship with Wildlands CPR. He would like to thank Adam Switalski, Marcel Huijser and Bethanie Walder for their assistance on this paper.

Li terature Cited
Ament, R, A. Clevenger, O. Yu, and A. Hardy. 2008. An assessment of road impacts on wildlife populations in U.S. national parks. Environmental Management 42(3):480-496
Andrews, MK and JW Gibbons. 2005. How do highways influence snake movement? Behavioral responses to roads and vehicles. Copeia 2005(4):772-782
Antworth, LR, DA Pike, and EE Stevens. 2005 Hit and Run: Effects of scavenging on estimates of roadkilled vertebrates. Southeastern Naturalist. 4(4): 647-656
Aresco, MJ. 2005. Mitigation measures to reduce highway mortality of turtles and other herpetofauna at a North Florida lake. The Journal of Wildlife Management. 69(2):549-560
Barton, C. and K. Kinkead. 2005. Do erosion control and snakes mesh? Journal of Soil and Water Conservation. 60(2):33A- 35A
Burger, J, RT Zappalorti, M. Gochfeld, and E DeVito. 2007 Effects of off-road vehicles on reproductive success of pine snakes (Pituophus melanoleucus) in the New Jersey pinelands. Urban
Ecosystems 10:275-285
Ciesiolkiewicz, J, G. Orlowski, and A. Elzanowski. 2006. High juvenile mortality of grass snakes Natrix natrix (L.) on a suburban road. Polish Journal of Ecology. 54(3):465-472
Coelho, IP, A. Kindel, and AVP Coelho. 2008 Roadkills of vertebrate species on two highways through the Atlantic Biosphere Reserve, southern Brazil. European Journal of Wildlife Research. 54(4):689-699
Dodd Jr., CK, WJ Barichivich, and LL Smith. 2003. Effectiveness of a barrier wall and culverts in reducing wildlife mortality on a heavily traveled highway in Florida. Biological Conservation. 118: 619-631
Eigenbrod, F, SJ Hecnar, and L. Fahrig. 2007 Accessible habitat: an improved measure of the effects of habitat loss and roads on wildlife populations. Landscape Ecology 23:159-168
Enge, KM. 2005 Herpetofaunal drift-fence surveys of steephead ravines in the Florida Panhandle. Southeastern Naturalist. 4(4):657-678
Langley, WM, HW Lipps, and JF Theis. 1989. Responses of Kansas motorists to snake models on a rural highway. Transactions of the Kansas Academy of Science. 92(1/2):43-48
Row, JR, G. Blouin-Demers, and PJ Weatherhead PJ. 2007. Demographic effects of road mortality in black ratsnakes (Elaphe obsolete). Biological Conservation. 137:117-124
Shepard, DB, AR Kuhns, MJ Dreslik, and CA Phillips. 2008a. Roads as barriers to animal movement in fragmented landscapes. Animal Conservation. 11(4):288-296
Shepard, DB, MJ Dreslik, BC Jellen, and CA Phillips. 2008b. Reptile road mortality around an oasis in the Illinois Corn Desert with emphasis on the endangered Eastern Massasauga. Copeia 2008(2): 350-359
Shine R, M. Lemaster, M. Wall, T. Langkilde, and R. Mason. 2004 Why did the snake cross the road? Effects of roads on movement and location of mates by garter snakes (Thamnophis sirtalis parietalis). Ecology and Society. 9(1):9