When protected areas are not enough: low-traffic roads projected to cause a decline in a northern viper population

Abstract

Animal mortality resulting from collisions with vehicles has emerged as a major human-caused threat to wildlife. While direct mortality of wildlife from vehicles has been well documented, fewer studies have focussed on the population-level effects of road mortality, particularly due to low-traffic volume roads. We conducted a population viability analysis (PVA) on western rattlesnakes Crotalus oreganus occupying a protected area with low road density and an average traffic volume of ≈350 vehicles d-1, near the northern periphery of the species’ range. We used the program Vortex with a field-derived database on road mortality, population demography, and extent of occurrence. The model showed that although the population had a high likelihood of persistence over the next 100 yr (extinction probability <0.01), a substantial decline was projected (stochastic growth rate -0.035, 97% decrease in mean population size, from 2131 to 72) under the current road mortality rate (6.6% of population yr-1); any increases in road mortality rates were projected to cause extirpation in under 100 yr. Our study provides strong evidence that road mortality is and will continue to be a significant contributor to the decline of this threatened species, even without higher traffic volumes and other significant anthropogenic impacts.