Extinction debt across three taxa in well-connected calcareous grasslands

The biodiversity in calcareous grasslands suffered from severe habitat loss due to land-use intensification and abandonment across Europe. Although these grasslands are now protected under the EU Habitats Directive, many species in the remaining habitat patches are still declining. Recent studies suggest that species across different taxa may become extinct with a substantial time delay, even without further habitat loss. Consequently, there might be an extinction debt, which poses a major challenge for conservation. Here, we analysed the response of plant, grasshopper and butterfly species richness in calcareous grasslands to habitat fragmentation over the last five decades. In this study, habitat area and connectivity have undergone a marked decline between 1970 and 1990 but have only slightly declined during the last three decades. Despite this, the current richness of specialist and generalist species among plants and butterflies was equally or better explained by past than present landscape conditions. This finding indicates the existence of an extinction debt in both taxa in the still well-connected grasslands of the study area. We conclude that increased conservation measures since the 1990s have favoured species persistence, despite severe habitat loss in the more distant past. By contrast, grasshopper diversity weakly responded to habitat area and connectivity; rather it is likely to depend mainly on habitat quality. To inhibit future extinctions, it is crucial to maintain large-scale patches by traditional land-use practices (i.e. rough grazing or mowing once a year), as well as restore former habitat to facilitate species dispersal in fragmented landscapes.