A systematic review of global road ecology camera trap studies that monitored animals’ use of wildlife crossings in road-fragmented landscapes

Much research has emphasised the importance of incorporating wildlife crossing-structures in the design of road networks to facilitate connectivity of wildlife crossings in road-fragmented landscapes. Although camera traps have been effective in monitoring wildlife crossing structures, limited studies explore camera trap protocol to monitor wildlife use of crossing structures, particularly in Africa. Our study reviewed and assessed camera trap peer-reviewed research that monitored the use of crossing-structures by wildlife to navigate landscapes fragmented by roads. We found 70 camera trap peer-reviewed publications from 2001 to 2022 that monitored wildlife use of crossing-structures in landscapes intersected by roads, and these were from 22 countries and six continents. The included peer-reviewed studies varied significantly globally, with geographical trends indicating that most studies were conducted in North America. However, the methods used varied considerably between studies, especially in terms of camera trap placement protocol (placement height of camera trap, survey length, and camera multi-shot settings). This showed that camera trap usage for monitoring animal use of crossing structures is still an emerging area of research, and there is a potential for developing a standardised protocol for each type of crossing structure design and size. Future camera trap studies exploring wildlife use of crossing-structures should consider monitoring existing crossing structures (culverts, bridges, and tunnels) as this provides a less costly method of restoring landscape connectivity. We recommend that further research develop a standardised camera trap protocol for monitoring wildlife using crossing-structures to reduce the threats to biodiversity.

Methods
We conducted a  systematic literature search and review of peer-reviewed journal articles between 1 July and 1 September 2023 using Google Scholar (https://scholar.google.co.za/) and Semantic Scholar databases with the programme Publish or Perish (Harzing, 2022). The review comprised only peer-reviewed literature spanning the years 2001 to 2022 that examined the use of camera traps as a tool to assess the efficacy of wildlife crossing-structures as a mitigation strategy for restoring landscape connectivity and reducing wildlife-road-mortality. The search string used the following words to identify each publication: camera traps, wildlife crossings, cameras underneath underpasses, cameras under culverts, undercrossings, overcrossings, canopy bridges, crossing corridors, and connectivity. We used a Microsoft Excel© spreadsheet to collate the data from each publication.