Abandonment of traditional land use and climate change threaten the survival of an endangered relict butterfly species

The decline of butterflies exceeds those of many other animal taxa due to their high sensitivity to habitat alterations driven by land-use change. Moreover, cold-adapted species frequently suffer severe range retractions due to rising temperatures at their trailing-edge range margins. In this study, we aim to identify drivers of occupancy of the post-glacial relict species Lycaena helle at three spatial scales - (i) landscape, (ii) habitat, and (iii) microhabitat - in one of its last refuges in central Europe. In our study in the Eifel low mountain range (western Germany), the occurrence of L. helle was mainly driven by the (i) isolation, (ii) size, and (iii) quality of habitat patches. Lycaena helle formed metapopulations that were dependent on networks of interconnected but often small habitat patches. Habitat quality within the semi-natural grasslands was determined by (i) macro- and mesoclimate, (ii) host-plant abundance, and (iii) vegetation structure, which was interrelated with microclimate. Lycaena helle preferred moist, nutrient-poor grasslands in deep, narrow valleys at the highest elevations of the study area, which were characterised by (i) cold winters, (ii) high abundance of the host plant, and (iii) short and sparse swards providing a warm microclimate in summer. According to these findings, abandonment of traditional land use and climate change are considered the most severe threats for long-term survival of the species. Hence, conservation measures should aim at maintaining and restoring networks of large and well-connected habitat patches of high quality, preferably in cold-air depressions within mountain systems.