Factors affecting usage of crossing structures by wildlife – A systematic review and meta-analysis

Abstract

Wildlife crossing structures (WCS) are widely used to allow for safe animal movement across roads, promoting both human safety and wildlife conservation. These structures are expensive to build and maintain, and therefore cost-effective design is essential. Although there has been much research to date on the factors affecting the usage of WCS by wildlife, no attempt has been made to synthesize these results and assess the current state of knowledge. We conducted a systematic review of the scientific and professional literature to assess the state of empirical evidence on WCS and a meta-analysis to explore the factors that influence their effectiveness. We identified a total of 270 studies that reported empirical results spanning four decades of research. Most studies (161) measured the number of crossing events without monitoring approaches to the structure, thus limiting the ability to assess WCS effectiveness. Only 77 studies measured the proportion of successful crossings to approaches, which was the type of data used for meta-analysis. Our meta-analysis results show that viaducts are the most effective type of WCS for large mammals. For example, the odds of ungulates crossing through a viaduct are 2.9 times that of an overpass, and 3.6 times that of an underpass. WCS built specifically for wildlife are used significantly more than those built for dual use by humans and wildlife. For large carnivores, the odds of using a dedicated WCS are 15.9 times that of a structure used concurrently by humans. We additionally found that natural materials and round shapes are preferable in the design of effective WCS. Altogether, these results highlight the importance of adopting large-scale monitoring of wildlife crossing structures. More broadly, we conclude that further research focusing on under-studied species and structure characteristics is needed to facilitate cost-effective mitigation efforts that reduce wildlife-vehicle collisions and promote wildlife conservation. • We reviewed 270 empirical papers on the effectiveness of wildlife crossing structures. • Only 28% of global studies reported the proportion of successful crossings to overall approaches. • Viaducts are the most effective type of crossing structure for large mammals. • Natural materials and round shapes are preferable in the design of crossing structures. • Structures designed for dual use are less effective than those designed exclusively for wildlife.