Can we use body size and road characteristics to anticipate barrier effects of roads in mammals? A meta-analysis

Habitat fragmentation and loss caused by road development are recognized as major threats to biodiversity and challenges to reconcile the pursuit of economic growth with the protection of wildlife habitats. Assessment of potential environmental impacts is essential in planning and design of road projects. Behavioral responses such as road avoidance that causes barrier effects are critical in assessment of effects of roads on species persistence. In this study, we synthesized literature of barrier effects on mammals to identify road characteristics and species traits that might serve as management indicators to anticipate barrier effects. We conducted a meta-analysis with 118 statistics of road crossings by 45 species from 36 studies. We used logit-transformed proportion of individuals not crossing roads as the effect size of barrier effect. Overall, all types of roads, from major highways to narrow forest roads, can impede movements for certain species of mammals. For data collected by observational methods, body mass, road width, road surface and data collection methods explained 53% of variation among data. Barrier effect decreased as body mass increased, and was increased by greater road width. Paved roads posed stronger barriers compared to gravel dirt roads. Capture-recapture methods tended to detect a weaker barrier effect compared to methods that tracked individual movements. For data collected by experimental translocation, the probability of crossing following translocation was not affected by road width and body mass. We showed that interspecific variation of mammals in barrier effects can be explained by road characteristics and body size under natural condition, and can be useful to anticipate the species-specific magnitude of barrier effects of roads and aid in planning and design of road projects, as well as reassessment of existing roads.