Increasing mortality of European bison ( Bison bonasus ) on roads and railways

Traffic mortality can pose a serious risk to endangered species that occur in small populations, are mobile and occupy fragmented habitats. This is the case for the European bison ( Bison bonasus ) yet, how traffic mortality affects this species is unknown. Here, we assessed patterns and trends of European bison mortality on roads and railways in Poland, which harbors a large share of the global free-ranging population of this species. We identified 70 records of European bison mortality due to roads or railways during 2010–2021, which involved three free-ranging populations: Białowieska Forest, Knyszyńska Forest, and Zachodniopomorskie. Most reported mortality (73%) was from the Zachodniopomorskie population, likely an effect of the high traffic volumes on a national road passing through the core range of that population. Furthermore, our analysis revealed an increasing trend of European bison traffic fatalities, which is likely associated with increasing European bison numbers in these populations. We conclude that traffic accidents may pose a risk to both European bison and people, and reintroductions should therefore prioritize roadless areas and avoid areas with busy roads and railways. Although traffic mortality of European bison has been a negligible threat to the species as a whole, our study shows that this threat may be substantial locally. As European bison numbers grow, which is desirable given the still small size of many free-ranging populations, effective mitigation measures are needed to ensure the safety of European bison and people.