Effectiveness of Crossing Structures for Wildlife on Two Roads Associated with Biological Corridors in Costa Rica

After implementing mitigation measures on road projects, it is critical to assess whether they effectively address the targeted impactsnamely, restrict wildlife movement and mortality from wildlife-vehicle collisions. In this study, we used a control-impact design to answer two key questions: (1) Is the probability of use by terrestrial and arboreal animals similar in crossing structures compared to surrounding forest sites? (2) Is wildlife mortality lower on road sections with mitigation measures than on those without? Our study was conducted on two roads in Costa Rica National Route 160 and National Route 1 focusing on underpasses and canopy bridges. To address the first question, we applied single-season occupancy models to camera trap data for ground-dwelling and arboreal mammals and reptiles, collected both at crossing structures and in surrounding forest. To address the second question, we compared roadkill data collected through vehicle surveys between road segments with and without crossing structures. Our results show that multiple taxa, including ground-dwelling and arboreal species, used structures such as underpasses, box culverts, bridges adapted with dry ledges, and canopy bridges. The probability of use suggests that some species are relatively well adapted to these structures, although confidence intervals remain broad. However, certain species were never recorded using any crossing structures, and, on Route 1, the amount of roadkill was higher for road segments with crossing structures. In conclusion, while wildlife crossing structures show promise in facilitating animal movement across roads, they are insufficient on their own to prevent wildlife mortality, showing that the mitigation system needs improvement. Based on existing best practices, we recommend extending and upgrading the exclusion fence for the underpasses, retrofitting box culverts with dry ledges and exclusion fence and maintenance activities to improve the effectiveness of this mitigation system.