Declining habitat directive butterflies within a major stronghold: Challenges for conservation

Abstract Biodiversity is globally declining, with specialists being more strongly affected than generalists. Myrmecophilous Phengaris nausithous and P. teleius are highly specialized grassland butterflies and flagship species for European nature conservation. Ongoing declines despite much research effort suggest implementation deficits or hitherto disregarded threats, for example imposed by climate change. We investigated population structure changes and habitat preferences of both species in the Westerwald mountain range, western Germany, to settle this issue. Since 1994, patch occupancy declined by 64% in P. teleius and by 51% in P. nausithous in the study area, and since 2006 by 56% in P. teleius and by 3% in P. nausithous in the core of the study area. Declines were mainly driven by inappropriate timing of mowing, grassland intensification and habitat conversion. The occurrence of both species was strongly related to host plant abundance and vegetation height, with higher vegetation indicating a longer time since mowing and a concomitantly higher abundance of flower heads for egg‐laying. Both species are suitable indicators for butterfly communities of moist meadows. In the future, climate change might impose additional threats, as indicated by spatial shifts of the principal host ant of P. nausithous , M. rubra , from open meadows to moister and shadier fringe habitats. Future conservation measures should therefore include water retention, establishing old grass strips, and planting solitary trees in addition to appropriate grassland management.