A rocky solution: evaluating the use of common construction materials as road-effect mitigation for turtle communities in a rock barren landscape

Abstract

Roads are pervasive linear features that bisect landscapes, altering how female turtles use and move between critical habitats during nesting migrations. While turtle population viability depends on the survivorship of reproductive females, few cost-effective mitigation strategies directly address their vulnerability to roads. The objective of my study was to evaluate a new cost-effective mitigation strategy that used common construction materials to reduce road threats for turtle communities in eastern Georgian Bay, Ontario. The mitigation design aimed to deter females from nesting in roadside habitats by replacing ~ 300 m of exposed gravel at 5 wetland crossings with rock rip-rap and paved road shoulders (using tar-and-chip), while the rest of the road shoulders remained unchanged. The success of the mitigation strategy was assessed by whether it successfully prevented females from using the road as nesting habitat. First, I used a Before-During-After comparison of nesting observations and nest hot spots on the road. I found a 15% decrease in the number of females nesting at Mitigated sites in the After period; however, females continued to nest in the nearest available Unmitigated roadside habitat, including in semi-compact tar-and-chip road shoulders at Mitigated sites. In addition, nesting hot spots remained at Mitigated sites in the After period. Second, I investigated the availability and suitability of natural nesting habitats in the surrounding rock barren landscape relative to nesting habitats used by female turtles on road shoulders. I conducted systematic habitat surveys (in 10800 1m x 1m plots) to quantify the availability of suitable nesting habitats on open rock barrens based on soil depth and canopy openness requirements for Species at Risk (SAR) turtles. I found road shoulders met nesting habitat requirements for three local turtle species, whereas only 1% of rock barrens in the 231-ha study area were suitable for turtle nesting. Overall, I found the availability of suitable nesting habitats was limited across the natural landscape, which may contribute to females’ selection of roadside nesting habitats. My findings suggest that the mitigation strategy was unsuccessful at deterring female turtles from nesting on roads and should not be applied without further research, especially in areas where natural nesting habitat may be limited. I identified additional recovery actions, such as mortality mitigation (i.e., fenceunderpass mitigation) and nesting habitat restoration, that may be required to reduce road effects for the turtle community. Overall, my project contributes to studies evaluating road-effect mitigation and highlights several important findings that can be incorporated into Best Management Practices for turtles during road development.