Restoration of abandoned calcareous grasslands promotes habitat diversity and overall wild bee beta diversity

Land-use change has led to a widespread loss of biodiversity across Central Europe, with calcareous grasslands being particularly affected, mainly due to the shift from traditional management to intensive agriculture or complete abandonment. To counteract the negative impacts of management cessation, shrub removal followed by grazing can serve as an effective restoration measure. In this study, we used wild bees as bioindicators to assess the ecological value of restored calcareous grasslands for biodiversity conservation. Across 21 restored grasslands (RestGras) and 21 regularly managed calcareous grasslands (Control) within the largest connected calcareous grassland area in northern Germany, we investigated environmental conditions and wild bee communities for all, threatened, belowground-nesting and snail shell-nesting species. Despite differences in habitat structure between the two grassland types, wild bee species richness and abundance did not differ. However, species composition varied to a certain extent, with some species, particularly those reliant on specific nesting structures like deadwood, plant stems and snail shells, favoring restored grasslands. Both direct incident radiation (DIR) and flower cover were found to have a positive impact on species richness and abundance of all and belowground-nesting wild bees. However, threatened species were positively influenced only by DIR. Furthermore, a higher cover of shrubs proved to be a crucial factor positively influencing snail shell-nesting wild bees. Our results show that restoration measures create structures that are not present in the control plots, thereby increasing heterogeneity at the regional scale. This, in turn, supports diverse pollinator communities in grassland ecosystems, highlighting the importance of restoration efforts for biodiversity conservation.