Threatened grassland butterflies as indicators of microclimatic niches along an elevational gradient - Implications for conservation in times of climate change

Among the factors that determine habitat quality for butterflies, an adequate microclimate is of crucial importance, especially for the less mobile immature stages. Due to their narrow microclimatic preferences, stenotopic butterflies are potential indicators of specific microclimatic niches. Although the outstanding importance of the microclimate is widely acknowledged, the thermal and hygric requirements of butterflies are usually inferred from habitat structure or regional climate instead of being exactly measured. Here, we present the results of year-round measurements of temperature and relative air humidity at typical oviposition micro habitats of three threatened grassland butterflies (Erebia medusa, Melitaea aurelia and Satyrium spini) inhabiting different zones along a climatic gradient in the Diemel Valley (Central Germany). Furthermore, we analysed how the climate in the study area has changed since the middle of the 20th century. The interspecific differences in mean temperature and humidity at the oviposition sites roughly reflected the differences in overall distribution of the three species, but separate analyses of day- and nighttime values revealed that local habitat characteristics and radiative heating of the near-ground air layer have a strong modifying effect on the microclimate. Since the 1950s, the climate in the Diemel valley has become significantly warmer. The magnitude of the observed increase in mean temperature was similar or even greater than the interspecific differences recorded by the microclimatic measurements. This implies that thermophilous species may expand their ranges within the Diemel Valley if climate warming continues. Species living in the relatively cool Upper Diemel Valley such as E. medusa, however, may incur population declines because there are few grasslands available at higher elevations or at microclimatically cooler sites such as north-facing slopes.