Ungulate substrate use in fauna passages

Fauna passages are increasingly constructed at major roads and railways to mitigate the negative effects of infrastructure and traffic on wildlife. The function of such passages depends on design, including the construction materials, soil, and vegetation. Providing ‘naturalness' in fauna passages may entail significant costs, yet its benefits are unclear. By using camera trap data, we examined ungulate (moose Alces alces, roe deer Capreolus capreolus, reindeer Rangifer tarandus tarandus, and white‐tailed deer Odocoileus virginianus) substrate use in seven passages serving both fauna and local roads (overpasses) and fauna and watercourses (underpasses) in boreal Sweden and Finland. While all substrates were used, during snow‐free periods, the general pattern was that ungulates used smoother surfaces (fine‐grained topsoil, grass, artificial fiber mat, and dirt road) more than expected based on their availability. Coarser surface (stony/rocky ground), shrub, and water were used less than expected. The results for road and water were, however, inconsistent between passages; in one, the overpass road was instead used less than expected by moose, and in one underpass the water section was used particularly during winter when the watercourse was frozen, but also by moose wading or swimming through in summer and autumn. Despite limitations, our study offers valuable insights for planning and constructing fauna passages. To our knowledge, this study was the first of its kind describing how ungulates use different substrates in fauna passages. We suggest that further research is conducted on this issue, for example by more detailed study of animal trajectories through passages and by experimental modification of substrates.