Landscape planning and defragmentation measures: an assessment of costs and critical issues

•Wildlife crossing structures, typically underpasses, allow for landscape defragmentation.•Landscape defragmentation is assessed as both decrease of fragmentation and increase of connectivity.•Denser wildlife crossing structures trigger more intense landscape defragmentation.•The specific average cost of the decrease of fragmentation is always higher if computed with reference to IFI than to CI.•Practical barriers hinder an efficient realization of a number of underpasses implied in the hypothetical scenarios.