Monitoring reintroduction success: a decade of restoring European bison in the southwestern Carpathians

Animal reintroductions are a key conservation strategy and central to trophic rewilding efforts, yet many reintroduction projects fail, with scarce evidence on why. This knowledge gap largely stems from a lack of clear and practical guidelines for how to define reintroduction success and thus what to monitor. Here, we evaluate early outcomes of a European bison reintroduction in the southwestern Romanian Carpathians by assessing whether released individuals establish a population, whether this population increases, and what financial investment was required to monitor these indicators. Specifically, we assessed (i) establishment through survival, (ii) displacement from the release site and spatial expansion, and (iii) proxies of population increase for the period 2014–2024. Our results provide clear evidence of early reintroduction success: released animals had a higher survival rate after improved animal selection, GPS-tracked European bison individuals showed low displacement, progressively expanded their range to cover a total of 127.5 km 2 , and genetic capture-recapture and population estimates showed that the European bison population increased substantially, from 91 surviving reintroduced individuals to 211 (95% CI: 209–290) in 2024, retaining genetic diversity. However, we also show that tracking reintroduction progress required substantial and sustained monitoring investment (€800,000, field personnel making up 75% of these costs), stressing the importance of clearly defining indicators and budgeting associated costs. More broadly, we highlight that monitoring the early phases of post-release establishment and population growth is critical to enable adaptive management of reintroduced megafauna, to proactively mitigate human-megafauna conflict, and to ultimately restore the ecological roles of these species.