Modelling road mortality risks to persistence of a Western Toad (Anaxyrus boreas) population in British Columbia

Road mortality may be a significant factor in the global decline of amphibian populations, yet rigorous assessments of its effect on long-term population persistence are lacking. Here, we investigate population persistence through a field study and mathematical model of a western toad (Anaxyrus Boreas (Baird and Girard, 1852)) population within a highway corridor in the Selkirk Mountains of British Columbia. The analysis shows traffic levels strongly correlate with toad mortality, with each additional vehicle causing a 3.1% ± 1.3% (p = 0.020) increase in toad deaths. Although the current risk of the population becoming threatened or endangered is low, it rises to 50% if baseline road mortality increases from 10% to 30%. Gravid female mortality is higher than the baseline mortality and can increase the probability of endangerment by nearly two-fold at higher baseline mortality levels. We make the case that a small increase in vehicle traffic resulting from future development and recreational pressures could destabilize this apparently healthy toad population. The high sensitivity to traffic levels and rapid transition from healthy to endangered raises concerns for similar populations worldwide. Compensatory structures such as amphibian underpasses (toad tunnels) should be given high priority.