Evaluating the effectiveness of an innovative road mortality mitigation design for at-risk reptiles in eastern Georgian Bay, Ontario

Abstract

Road mortality poses a serious threat to reptiles, yet the effectiveness of mitigation strategies is often compromised by poor durability and designs that fail to account for species-specific behaviours and morphology. I evaluated the effectiveness of a 1-m-tall, concave, High-Density Polyethylene (HDPE) fence paired with ecopassages (3 aquatic, 1 terrestrial) installed along a residential road bisecting a wetland in eastern Georgian Bay. Using a 4-year Before-After- Control-Impact (BACI) design, I found a 26% decrease in reptile observations post-mitigation and a marginally significant interaction between Site (Control, Impact) and Period (Before, After), providing some support that the mitigation was effective. However, opportunistic observations indicated that the fence was not preventing reptiles from accessing the road. Camera traps installed in the ecopassages recorded 2371 crossings by 13 reptile species, and turtle species composition in the ecopassages was not significantly different than that in the road-adjacent wetland based on a 2-year mark-recapture study. Finally, in situ behavioural assays using Eastern Foxsnakes (Pantherophis vulpinus) showed that the concave fence successfully prevented climbing in 23 assays with 18 individuals, although one large snake (>120 cm snout-vent-length) used the supporting rebar to surmount the fence. Additionally, 4 individuals exhibited increased climbing attempts of the fence and supporting rebar in repeated trials, suggesting potential for learned behaviour. Overall, I concluded that the concave fence design and ecopassages were mostly effective, but with the important caveat that reptiles could still access the road due to the short length of the fence. To improve the efficacy of this design, I recommended extending the fence length past the wetland edge, modifying the fence overhang to prevent snakes from climbing, utilizing pre-existing culverts as ecopassages, and installing ecopassages near fence ends.