Introducing European bison in Iberia: Road safety concerns. A comment to Nores et al., 2024

This Comment builds on Nores et al. by highlighting an overlooked dimension of the proposed European bison introduction in the Iberian Peninsula: the risks posed by roads to both wildlife and human safety. Drawing on evidence from Central and Eastern Europe, we show that traffic-related mortality can be significant even in well-managed bison populations. We argue that any rewilding initiative in Iberia must explicitly account for these risks, which challenge both the ecological and social feasibility of such projects. Nores et al. (2024) questioned the ecological, biogeographical, ethical, and legal justification for introducing the European bison Bison bonasus in Spain. This species, the largest extant terrestrial mammal in Europe, has become a symbol of successful recovery and rewilding within its native range in Central and Eastern Europe (Olech & Perzanowski, 2022; Plumb et al., 2020). After its extinction in the wild in the early 20th century, populations have been progressively restored, and today over 8200 free-ranging individuals are distributed among more than 50 populations across this region (Plumb et al., 2020; Raczyński & Bołbot, 2023), where ongoing conservation efforts continue to promote range expansion and improve ecological connectivity (Olech & Perzanowski, 2022; Yanuta et al., 2021). In the Iberian Peninsula, however, recent initiatives to rewild landscapes with the European bison have sparked considerable debate and public controversy. Nores et al. (2024) concluded that the Mediterranean ecosystems of Spain are unsuitable for this species under both current and projected climatic conditions, and that there is no conclusive evidence that B. bonasus ever inhabited this region, where only the extinct steppe bison B. priscus occurred in the past. Nevertheless, despite these well-founded arguments, the discussion on the potential introduction of European bison in the Iberian Peninsula remains active, both in scientific and public arenas. Beyond these existing concerns, we highlight another dimension that deserves attention: the additional risk that the presence of European bison would pose to road safety, affecting both animals and people. Large herbivores are among the most hazardous species involved in wildlife–vehicle collisions, often causing severe vehicle damage and posing serious risks to human safety (Ascensão et al., 2021; Conover et al., 1995). The European bison, due to its massive body size (often exceeding 500 kg) and gregariousness, presents a particularly serious challenge in this regard (Klich et al., 2023; Tryjanowski et al., 2021). Despite being seldom addressed in rewilding debates, traffic collisions have become an increasingly significant source of mortality for European bison populations in Central and Eastern Europe (Balčiauskas et al., 2024; Klich et al., 2023). These patterns are particularly relevant when evaluating the feasibility and long-term sustainability of rewilding projects in densely roaded regions, such as some areas of the Iberian Peninsula. Evidence from Poland, Belarus, and Lithuania reveals that roads and railways not only fragment habitats and restrict dispersal (Kuemmerle et al., 2018; Ziółkowska et al., 2016) but also directly threaten population viability through collision-related mortality (Balčiauskas et al., 2024; Klich et al., 2023). While still relatively rare in absolute terms, traffic collisions involving European bison are increasing and can represent an important source of local mortality (Klich et al., 2023; Kučas & Balčiauskas, 2021). Between 2010 and 2021, Klich et al. (2023) compiled 70 confirmed cases of bison killed by vehicles or trains across three Polish populations: Białowieża Forest, Knyszyńska Forest, and the West Pomeranian Voivodeship. Most fatalities (73%) occurred in the West Pomeranian population, where a major national road crosses the core bison range. In this population, road mortality represented 3.3% of individuals in 2020, a figure comparable to the species' estimated natural mortality (~3% following Krasińska & Krasiński, 2007). Earlier data already indicated high traffic mortality, accounting for up to 35% of all recorded deaths in the West Pomeranian population (Lizoń, 2014) and 8% in Białowieża Forest (Krasińska et al., 2017). Although these losses have not prevented the continued growth of the West Pomeranian population, partly due to recurrent translocations from breeding centers, they demonstrate that traffic mortality can reach levels of demographic significance (Klich et al., 2023). Fatalities have also shown consistent upward trends across all studied populations in recent years (Klich et al., 2023). Importantly, given the large size of this species, collisions involving European bison can be catastrophic for humans (Balčiauskas et al., 2024). In Poland, European bison are considered among the most hazardous species involved in wildlife–vehicle collisions (Tryjanowski et al., 2021). Similarly, in Lithuania, collisions involving this species result in the highest rate of human injuries among all wildlife–vehicle accidents (Balčiauskas et al., 2024). This growing evidence supports the conclusion, consistent with previous studies, that bison rewilding initiatives must account not only for ecological feasibility but also for human safety, prioritizing roadless landscapes and avoiding areas with heavily trafficked roads and railways (Bluhm et al., 2023; Dănilă et al., 2022; Klich et al., 2023; Lord et al., 2020; Yanuta et al., 2021). The evidence summarized above provides an additional argument, beyond those raised by Nores et al. (2024), against the introduction of the European bison in the Iberian Peninsula. Road-related risks add a further layer of complexity to an initiative that is already ecologically, legally, and ethically questionable. In countries where releases have occurred, even well-managed populations face significant mortality from traffic collisions and generate concerns for public safety (Balčiauskas et al., 2024; Klich et al., 2023). Given that, the Iberian Peninsula has one of the most extensive road networks in Europe, overlapping and fragmenting several potential rewilding areas (Ibisch et al., 2016; Meijer et al., 2018), similar or even greater challenges can reasonably be expected if such releases were to be pursued. Introducing the European bison in the Iberian Peninsula would therefore not only lack ecological justification but also pose tangible risks to human safety. If, despite these warnings, bison introductions were ever to proceed, they should be recognized as a collective societal decision that assumes full responsibility for the ecological, economic, and road-safety consequences involved. Overall, introducing this species in the Iberian Peninsula appears neither ecologically sound nor socially responsible, reinforcing that rewilding decisions must remain grounded in both scientific evidence and social accountability. Marcello D'Amico has been supported by the project “Plan Complementario de I+D+i en el área de Biodiversidad (PCBIO)”, funded by the European Union within the framework of the Recovery, Transformation and Resilience Plan—NextGenerationEU and by the Regional Government of Andalusia. Fernando Ascensão was funded through the 2020-2021 Biodiversa+ and Water JPI joint call for research projects under the BiodivRestore ERA-NET Cofund (GA No. 101003777), with the particular support of Fundação para a Ciência e a Tecnologia—FCT in Portugal; DOIs: 10.54499/UIDB/00329/2020, 10.54499/DivRestore/0014/2020, LA/P/0121/2020, UID/00329/2025. No new data were generated or analyzed in this study. All data cited are available in the published literature referenced in the manuscript.