Occurrence of an endangered grassland butterfly is mainly driven by habitat heterogeneity, food availability, and microclimate

The Marsh Fritillary (Euphydryas aurinia) was once widespread in large parts of Central Europe. However, in the course of the last century, populations of the butterfly largely collapsed. Here, we surveyed patch and microhabitat occupancy and its drivers in one of the last vital populations in calcareous grasslands. Our study revealed that environmental conditions at the landscape and habitat level determined the occurrence of E. aurinia in a montane agricultural landscape with low land-use intensity. Patch occupancy increased with the cover of Devil's-bit Scabious (Succisa pratensis) grasslands in the surroundings of the patches, habitat heterogeneity and host-plant cover. Microhabitat occupancy was driven by a warm microclimate and high availability of host plants. In the well-connected landscape of nutrient-poor grasslands, patch occupancy of E. aurinia was driven by parameters defining a high habitat quality. Habitat heterogeneity very likely buffers E. aurinia populations against environmental stochasticity and, hence, enhances long-term viability. For the gregariously feeding caterpillars of E. aurinia, host-plant biomass is essential. Due to their more luxuriant growth, S. pratensis plants were clearly preferred, although the Glossy Scabious (Scabiosa lucida) was also widespread. Additionally, the growth of large Succisa plants was favored by soil humidity and grassland abandonment. To cope with the adverse macro- and mesoclimatic conditions of the study area, females of the butterfly selected host plants growing in extraordinarily warm microhabitats for oviposition. To secure long-term viability of E. aurinia populations, we recommend creating mosaics of traditionally managed grasslands and early stages of abandonment within the patches.