Road-effect mitigation promotes connectivity and reduces mortality at the population-level

Abstract

Roads are one of the most prevalent threats to wildlife because they fragment landscapes and increase mortality. In response to the threat of roads to population persistence, road-effect mitigation strategies are increasingly common, typically as a combination of exclusion fencing to reduce mortality and crossing structures to enhance connectivity. To evaluate the success of mitigation structures at reducing road mortality of amphibians and reptiles, we conducted a six year paired Before-After-Control-Impact (BACI) experiment. We used road surveys to estimate the effectiveness of exclusion fencing, and a combination of hoop trapping, passive integrated transponder tag scanners and camera traps to evaluate connectivity structure effectiveness. Exclusion fencing reduced the number of turtles and amphibians on the road but had no effect for snakes. Cameras recorded 54 turtles, 72 snakes, 615 frogs, and 271 salamanders using tunnels. PIT tag scanners detected similar usage by turtles (55 uses; 24 unique individuals) and snakes (69 uses; 15 unique individuals). Tunnel usage was neither spatially nor demographically biased, indicating that the tunnels were likely to be broadly used by the local species assemblage. Crossing rates of tagged individuals suggest ~5–15% of turtles and 44% of snakes are using crossing structures. We contextualized crossing rates with population size estimates for painted and snapping turtles in the surrounding habitats. Our study is one of the first of its kind to incorporate both a robust BACI design and the context of population-level effects to demonstrate the efficacy of road-effect mitigation for protecting wildlife, especially for reptiles and amphibians.