Road Mortality

Reptile and amphibian road mortality is most significant in southern Canada where road density and traffic volume is highest. In fact, road mortality is the primary threat to many of Canada’s reptile species. Reptiles and amphibians are especially vulnerable to road mortality because they can be difficult to see, are slow-moving, often freeze in response to passing cars, and are intentionally run over by some people. Turtles are also attracted to gravel shoulders for nesting, while snakes use both road-edge habitats and road surfaces for thermoregulation.

Freshwater turtles exhibit delayed sexual maturity and low recruitment (survival of young to reproductive age), but have high adult survival and extremely long lifespans (50-120 years). Given these characteristics, increases in annual adult mortality of only a few individuals can cause populations to decline. Road mortality can also create male-biased sex ratios in turtles since females are more likely to be killed on roads during nesting migrations; loss of females further reduces the population’s capacity to cope with increased mortality.

Many of Canada’s snake species are also relatively long-lived (20-30 years) and only reproduce every two or three years. A study on Gray Ratsnakes in eastern Ontario found that annual mortality of only three adult females would be sufficient to cause the local population to decline, and current levels of road mortality throughout its range in Eastern Ontario likely meet or exceed this threshold. In October 2016, a major snake road mortality event at the Ojibway Prairie Complex near Windsor, Ontario, was reported in national media. Over 30 Butler’s Gartersnakes and 25 Eastern Foxsnakes were run over as they headed towards denning areas. This level of mortality poses significant risk to the survival of populations of these two endangered species.

Road mortality of amphibians can also be extremely high during mass-migrations to and from breeding habitats, where 10 per cent of an adult population may be lost each year. With relatively long-lived species like Spotted Salamanders, this level of mortality can result in population extinctions. Furthermore, newly metamorphosed amphibians emigrating en masse from ponds (e.g. toads) often suffer staggering losses. Several studies have also demonstrated that amphibian species richness and abundance is lower near roads, indicating that mortality and other road effects (e.g. environmental contamination from petrol, anti-freeze and salt runoff) can have significant long-term effects on amphibian populations.

Further Reading

Ashley, E. P., A. Kosloski and S. A. Petrie. 2007. Incidence of intentional vehicle-reptile collisions. Human Dimensions of Wildlife 12:137–143.

Ashley, E. P., J. T. Robinson. 1996. Road mortality of amphibians, reptiles, and other wildlife on the Long Point Causeway, Lake Erie, Ontario. The Canadian Field-Naturalist 110: 403–412.

Baxter-Gilbert J.H., J.L. Riley, D. Lesbarrères, J.D. Litzgus. 2015. Mitigating Reptile Road Mortality: Fence Failures Compromise Ecopassage Effectiveness. PLoS ONE 10(3): e0120537. doi:10.1371/journal.pone.0120537

Carr, L. W., and L. Fahrig, 2001. Effect of road traffic on two amphibian species of differing vagility. Conservation Biology 15: 1071–1078.

Eigenbrod, F., S. J. Hecnar, and L. Fahrig. 2008b. The relative effects of road traffic and forest cover on anuran populations. Biological conservation 141:35–46.

Gibbs, J. P. and W. G. Shriver. 2002. Estimating the effects of road mortality on turtle populations. Conservation Biology 16:1647–1652.

Gibbs, J. P. and W. G. Shriver. 2005. Can road mortality limit populations of pool-breeding amphibians? Wetlands Ecology and Management 13:281–289.

Hels, T. and E. Buchwald, 2001. The effect of road kills on amphibian populations. Biological Conservation 99: 331–340.

Row, J. R., G. Blouin-Demers, and P. J. Weatherhead. 2007. Demographic effects of road mortality in black ratsnakes (Elaphe obsoleta). Biological Conservation 137:117–124.