Amphibian road mortality in response to nightly variations in traffic intensity

Pond-breeding amphibians frequently encounter roads during their movements across the landscape to reach their breeding, summering, or hibernation sites. Through night driving surveys conducted each summer from 1995-2002 on a 20-km stretch of secondary road within a national park of eastern Canada, I evaluated whether road traffic had cumulative effects on amphibian abundance over this period. I also investigated the effect of nightly variations in traffic intensity on the number of amphibians killed on the road. I recorded a total of 4643 amphibian crossing events during the 37 surveys. I did not detect any decreasing trend in abundance for amphibian roadside populations over the 8 years. The number of dead American toads (Bufo americanus) increased with increasing traffic intensity. The number of ranid frogs (Rana clamitans, R. pipiens, and R. sylvatica) dead on the road was greatest when many individuals were moving on the road and at moderate traffic intensities (approximately 10-18 vehicles/h). In contrast, spring peepers (Pseudacris crucifer) dead on the road increased with decreasing traffic intensity. The number of ambystomatid salamanders (Ambystoma laterale and A. maculatum) dead on the road, on the other hand, did not respond to traffic intensity. Nonetheless, results indicate that subtle variations in traffic intensity as those observed in this study (i.e., 5-26 vehicles/h) can increase mortality on the road for certain amphibian species. Future studies in landscape ecology should routinely consider measures associated with roads, such as the proximity and density of roads, when investigating amphibian abundance patterns in wetlands.