Understanding unexpected reintroduction outcomes: Why aren't European bison colonizing suitable habitat in the Carpathians?

Reintroductions are an important tool for re-establishing or reinforcing populations of threatened species, and thus to restore ecosystems. However, predicting how reintroduced populations will spread is difficult, and past reintroductions often lacked a thorough assessment of habitat availability and connectivity. Using the case of reintroduced European bison (Bison bonasus L.) in the northern Carpathians, we show how habitat suitability models in combination with connectivity assessments based on circuit theory can remedy such shortcomings, and identify potential habitat patches and corridors between these. European bison were reintroduced in our study area in the 1960s, and against prior expectation, have not spread along the Carpathian ridge, but instead expanded their range towards human settlements. Our analyses provide an explanation for this pattern. Although we identified a network of suitable habitat patches along the Carpathian ridge, the functional connections between them were limited due to a number of major barriers to movement. To avoid future conflicts between European bison and people, and to achieve the long-term goal of a viable bison metapopulation in this region, conservation action should focus on establishing connectivity between habitat patches through the creation and legal protection of corridors and wildlife passages, which would benefit Carpathian wildlife in general. Our study emphasizes the importance of landscape-scale connectivity analyses to guide restoration efforts, and of adaptive management to ensure the success of reintroduction projects.