Are roads and highways killing snakes because of habitat fragmentation?

(Photo source: http://www.oriannesociety.org/blog/umass)

Roads and highways pose huge impacts on snake populations in Ontario; the main reasons being habitat fragmentation and habitat loss as well as an increase incidence of roadkill. Researchers showed that the movement of eastern hognose snakes was affected by the type of road substrate in the Long Point region of Ontario, Canada (Robson and Blouin-Demers 2013). The study found that this particular species avoided paved roads, which could potentially lead to genetic isolation and decline of population size (Robson and Blouin-Demers 2013). Other studies have shown that snakes, such as the northern watersnakes and the threatened gray ratsnakes (previously known as black ratsnakes, and by the scientific name of Elaphe obsolete and Elaphe spiloides), had relatively high rate of road mortality (Garrah et al. 2015).

What is habitat fragmentation?

Figure 1. The decrease of interior habitat and species and increase of edge habitat and species as consequences of road construction fragmenting a habitat patch (European Environment Agency Coperhagen 2011).

Human activities, such as the land conversion of rural areas into urban cities or highways, could potentially alter the habitats of many wildlife creatures. The construction of roads and highways are advantageous to humans traveling from one town to another. However, such infrastructures could fragment the original habitats of many species, leading to small populations and eventually facilitating extinctions.

Habitat fragmentation poses huge threats to biodiversity. For humans, roads serve as connections between towns and cities. However, roads could be potential barriers to wild species movement because some will experience difficulties moving between these fragmented habitats. Animals either intentionally avoid roads or simply can not cross roads due to mortality during crossing attempts. Many natural habitats are isolated and greatly reduced in total area because of road development. Even though some of these areas would be preserved, most of them would not serve the same purpose to these native species once the damage has been done.

Consequences of habitat fragmentation

Habitat alteration could harm species suffering from habitat loss and habitat degradation, leading to reduction in fitness. Smaller habitats might be inadequate for large species, like the gray ratsnakes and eastern hog-nosed snakes, which require large area to inhabit and hunt. Some would then become a small population isolated in patches. These isolated patches also reduce the potential of species dispersal.  Edge effects are common in fragmented habitats, where there would be increase predation and parasitism (Groom et al. 2006). Small isolated populations are particularly susceptible to regional extinction (Groom et al. 2006). Fragmented small populations show increased sensitivity to inbreeding, that is the likelihood of breeding between close relatives (Shepard et al. 2008). This ultimately reduces gene flow, the exchange of genetic material, between populations (Shepard et al. 2008).

Figure 2. Time leg effect on wild species for highway effects (Ontario Road Ecology Group and Toronto Zoo 2011).

Clark et al. (2010) has shown a significant difference in genetic diversity between an isolated population and a contiguous populations of timber rattlesnakes in New York. Shine et al. (2004) demonstrated that garter snake males exhibited lower capability searching for pheromonal trails of mates on gravel roads than on grassland. Habitat alteration could also lead to change in nest site availability because of differences in sun penetration in the altered sites (Shine et al. 2004). These top predators might also experience a reduction in prey availability due to the narrowed home range and that fact that their prey could also be affected by the roads (Reading et al. 2010). 

Although research studies indicated that some species were more influenced by the roads than others (Andrews and Gibbons 2005), inter-individual variation should still be taken into consideration that individuals might have different behavioural responses. Webb and Shine (1997) indicated the importance of large home ranges to broad-headed snakes (Hoplocephalus bungaroides) considering that adult snakes demonstrated frequent movements and strong site fidelity. Most snakes species have to move from their hibernation sites to mating sites to breed during the summer. The barrier effects of roads and highways have tremendous genetic and demographic consequences which could ultimately cause vanishing of some local species.

References:

Andrews, K. M., and J. W. Gibbons. (2005). How do highways influence snake movement. Behavioral responses to roads and vehicles. Copeia 4: 772-782.

Clark, R. W., W. S. Brown., R. Stechert., and K. R. Zamudio. (2010). Roads, interrupted dispersal, and genetic diversity in timber rattlesnakes. Conservation Biology 24(4): 1059-1069.

European Environmental Agency Copenhagen. (2011). Landscape fragmentation in Europe. Joint EEA-FOEN Report.

Garrah, E., R. K. Danby., E. Eberhardt., G. M. Cunnington., and S. Mitchell. (2015). Hot spots and hot times: wildlife road mortality in a regional conservation corridor. Environmental Management 56: 874-889.

Groom, M. J., G. K. Meffe., and C. R. Carroll. Principles of Conservation Biology, Third Edition. Sunderland, Massachusetts: Sinauer Associates, Inc, 2006. Print.

Ontario Road Ecology Group and Toronto Zoo (2010). A guide to road ecology in Ontario. https://www.rom.on.ca/sites/default/files/imce/oreg_final.pdf

Reading, C. J., L. M. Luiselli, G. C. Akani, X. Bonnet, G. Amori, J. M. Ballouard, E. Filippi, G. Naulleau, D. Pearson and L. Rugiero. (2010). Are snake populations in widespread decline? Biology Letters doi:10.1098.

Robson, L. E., and G. Blouin-Demers. (2013). Eastern Hognose Snakes (Herodon platirhinos) avoid crossing paved roads, but not unpaved roads. Copeia 3: 507-511.

Shepard, D. B., A. R. Kuhns., M. J. Dreslik., and C. A. Phillips. (2008). Roads as barriers to animal movement in fragmented landscapes. Animal Conservation 11: 288-296.

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.

Webb, J. K., and R. Shine. (1997). A field study of spatial ecology and movements of a threatened snake species, Hoplocephalus bungaroides. Biological Conservation 82: 203-217.