Butterfly and plant specialists suffer from reduced connectivity in fragmented landscapes

Summary 1. Calcareous grasslands are diversity hotspots for plant and butterfly species in Europe, but connectivity of these grasslands has been reduced by habitat loss and fragmentation in recent decades. Reduced habitat area leads to biodiversity loss, but the significance of habitat connectivity independent of patch size and habitat quality is unclear. 2. Here, we focus on the impact of habitat connectivity on both butterfly and plant species richness and compare (i) a connectivity index, (ii) percentage habitat cover and (iii) distance to the next suitable habitat patch as alternative measures of connectivity. Species were recorded in 2007 in northern Bavaria (Germany) in paired large and small study sites in 31 independent landscapes. 3. We found that total loss of grassland connectivity would reduce species richness of specialist butterflies (38–69%) and specialist plants (24–37%). A connectivity index combining patch size and distance in the surrounding landscape was a better measure of connectivity than percentage habitat cover or distance to the next suitable habitat. 4. Species richness, species density and abundance of habitat‐specialist butterflies and plants were significantly higher in large compared with small study sites. 5. Synthesis and applications. We conclude that connectivity is highly relevant for conservation of butterfly and plant species with specialized habitat requirements, but the decision as to which connectivity measure is most appropriate depends on patch configuration, landscape context and study design. We suggest that management strategies should aim to increase connectivity by active restoration of former calcareous grasslands to ensure long‐term survival of habitat‐specialist species.