Challenging the practice of biodiversity offsets: ecological restoration success evaluation of a large-scale railway project

Large-scale infrastructure development projects are discussed transdisciplinarily in several domains of society. Critics often claim that environmental impact assessments lack real influence on planning, management, and monitoring. We report herein the evaluation of ecological compensation via biodiversity offsets and technical constructions with a secondary compensation function for a new railway in Austria. We asked: (1) where can ecological restoration success be detected, and (2) is our new method of a composite biotope value calculation representative for all criteria we used? We conducted a vegetation inventory on reference areas and all types of measures that created new habitats. Together with a comprehensive, spatially dense habitat mapping, evaluation of six attributes of restoration success and testing of our new method were carried out. Current threats typical for intensive agriculture have been generally reduced. Ecological compensation measures had the highest connectivity but the lowest plant community diversity. Surprisingly, technical constructions provided significantly more plant communities and hosted most Red List species. The species assemblage characteristics of compensation measures, their biotope type diversity, and their mean biotope values were, although lower, comparable to the reference. Despite the poor performance of technical areas in the final biotope values, our results call for their reconsideration as potential tools for ecological compensation by greening grey infrastructure in the near future. The tested new method provided an overall statement for the ecological restoration evaluation and could also be used for nature evidence inventories and as a valuable decision support tool in landscape planning.