Road Ecology through the Lens of Snakes in the Columbia Basin: Patterns of Mortality, Occurrence, and Activity

Adrian Slade
Tyler Larsen
Topic Area
Terrestrial Wildlife and Ecosystem Interactions with Transportation

Vertebrate wildlife populations are increasingly imperiled by roads: from both direct mortality and from loss of genetic connectivity due to habitat fragmentation. Snakes have an array of life history characteristics that make them particularly vulnerable to roads and are poorly represented in vertebrate road mortality studies. The mid-Columbia basin is home to nine species of snakes, each with unique ecological roles and niches. As of 2018, all 3 transects are confirmed localities for the desert striped whipsnake, Coluber taeniatus taeniatus, a candidate endangered species in Washington state. From May 2017 through October 2017, we investigated the crepuscular and nocturnal activity patterns of four sympatric snake species (Crotalus oreganus, Northern pacific rattlesnake; Charina bottae, Northern rubber boa; Hypsiglena chlorophaea deserticola, Northern desert nightsnake; and Pituophis catenifer deserticola, Great basin gopher snake) in the mid-Columbia basin of central Washington. Snakes were sampled by nightly road transect surveys, allowing for analysis of road ecology and mortality for 3 roads (totaling 44 km) and 4 snake species. We analyzed anthropogenic and seasonal factors associated with snake movement and road mortality. We characterized habitat features adjacent to roads to explore how they influence distribution of snake species along the roadway. We also walked areas along the roadways to assess snake activity when not crossing roads and discern specific affinity for road-bank habitats. The vast majority of our sample (n=919) were gopher snakes (n=435) and rattlesnakes (n=391), both of which showed a peak in movements on roads during the latter half of June. Gopher snakes were more abundant than rattlesnakes in the spring and fall, while rattlesnakes dominated the sample during the hottest part of the summer. Our data also reveal the spatial distribution of each species along the road transects, habitat preferences, and areas of high crossing density that might serve as locations for potential wildlife underpasses. Night snakes and rubber boas inhabit separate regions on transect 1, suggesting that further investigation into habitat preferences and life history characteristics of these two species may reveal new insight into future management practices. Adult gopher snakes and rattlesnakes are at highest risk of mortality. There was a notable disparity in snake mortality among transects, which is likely a result of differing traffic volumes and ecological features of each respective transect, particularly transect 2. Any snake on the deadliest road (transect 2, SR 243) had a 70% chance of mortality when attempting to cross the roadway.

Abstract Keywords
Road mortality
snakes
shrub-steppe
natural history
wildlife corridor