Effectiveness of road mortality mitigation in a northern community of snakes

Roads and vehicular traffic are one of the most wide-spread threats to wildlife globally. They are particularly detrimental to long-lived species with lower population growth rates that are less able to naturally compensate for increases in anthropogenic mortality. Road crossing structures and fencing often are implemented to mitigate the impacts of road mortality on wildlife. However, follow-up studies assessing their efficacy are limited, especially for reptiles who are particularly vulnerable to road effects. Although numerical (population) responses to mitigation structures are strong indicators of the overall effectiveness of the mitigation structures, they require intensive and/or long-term data. However, functional (behavioural) responses also may provide good indicators of the response to mitigative efforts; thus, functional and numerical responses used in tandem may provide a more robust assessment of the impact that the mitigation structures, including those designed to reduce road impacts, are having on populations. I conducted a detailed assessment on the short-term, immediate impact of recently installed ecopassages and directional fencing on a threatened Western Rattlesnake (Crotalus oreganus) population south-central British Columbia, Canada. Using road surveys, traffic monitoring, and mark-recapture methods, I analyzed trends in roadkill rates and population size during a 6-year period (2015-2020) that encompassed the periods before, during, and immediately after (two years) the installment of ecopassages under a highway. Roadkill rates appeared to decrease after mitigation installation (0.06 ± 0.03 SE deaths/km/day before, 0.03 ± 0.01 SE deaths/km/day after) despite an increase in traffic (302 vehicles/day before, 454 vehicles/day after), yet population trends for adults did not indicate a clear trajectory towards recovery. Wildlife cameras were installed in the ecopassages, and I used photograph data to quantify spatial and temporal usage patterns and assess the immediate post-installation effectiveness of drift fencing for the Western Rattlesnake (Crotalus oreganus) and two other threatened species in the same community: the Great Basin Gophersnake (Pituophis catenifer deserticola), and Western Yellow-bellied Racer (Coluber constrictor mormon). I quantified “appearances” (any time an individual appeared on camera), and “passages” (where a snake iii was documented travelling through an ecopassage within a 30 minute time span). Across two years (2019 and 2020) appearances and passages were, respectively, 115 and 12 for rattlesnakes, 183 and 29 for gophersnakes, and 3061 and 748 for racers. Although racers clearly used the ecopassages significantly more than the other two species, both appearances and passages increased for rattlesnakes, gophersnakes, and racers from 2019 to 2020. The proportion of entrances with fences showed increased usage from 2019 to 2020, however these changes only were significant for rattlesnakes. Temporal and spatial use patterns differed among species, and I postulate that this was due to differences in movement patterns and habitat preferences. This study highlights the short-term yet complex response of snake communities to the effects of roadway mitigation, as the animals presumably encounter, adjust, and respond to the new structures in their environment. Despite the three snake species inhabiting the same environment, different responses were demonstrated towards the new mitigation structures. Short-term assessments of the response to mitigation efforts, such as applied in this study, likely illustrate a ‘shock phase’ in the wildlife community, and should be coupled with longer term monitoring to gauge the full effect of the conservation actions.