Impacts of Road Mortality on the Western Rattlesnake (Crotalus oreganus) in British Columbia

Winton, Stephanie Environmental Science Program Thompson Rivers University Kamloops, British Columbia, Canada

Bishop, Christine Science and Technology Branch Environment and Climate Change Canada Delta, British Columbia, Canada

Larsen, Karl

Department of Natural Resource Sciences Thompson Rivers University Kamloops, British Columbia, Canada

Mortality due to wildlife-vehicle collisions has emerged as a major threat to wildlife. Roadkill may be particularly adverse for populations at the periphery of their range, where existing natural constraints already limit population growth. Thus, conservation assessment and planning for many peripheral species-at-risk will benefit from a fundamental understanding of the impacts of road mortality, yet these can be difficult to isolate due to the interaction of numerous factors. Using population viability analysis (PVA) we evaluated the persistence of a Western Rattlesnake (Crotalus oreganus) population impacted by road mortality in a protected area of the dry interior of British Columbia, Canada with relatively low traffic volumes (mean = 350 vehicles/day). We quantified roadkill through methodical road surveys and concurrent assessments of scavenging rates and observer detection probability. Additionally, we conducted intensive mark-recapture and radio-telemetry to establish a detailed database on population demography and functional use of the landscape. After accounting for sources of error, our modelling showed that the estimated number of rattlesnake deaths was 2.7× the number of carcasses detected through unadjusted surveys and incidental observations. Our analysis indicated that, under the current road mortality rate (6.6% of population/year), persistence was likely over the next 100 years (extinction probability <0.01), but with it a continual and substantial decline in population size (stochastic growth rate -0.032, 96% decrease); any increases in road mortality rates would greatly hasten extirpation. Our study adds to the growing body of evidence that low levels of road mortality may still exert a significant effect on healthy populations of long-lived species, and in many cases population recovery will only be possible with reductions in road mortality.

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