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

Abstract

Among the factors that determine habitat quality for butterflies, an adequate microclimate has crucial importance, especially for the less mobile immature stages. Therefore, stenotopic butterflies are precise indicators of specific microclimatic niches. However, 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 microhabitats of three threatened grassland butterflies (Erebia medusa, Melitaea aurelia and Satyrium spini) inhabiting different zones along a climatic gradient in the Diemel Valley (Germany, Central Europe). Furthermore, we analysed how the climate in the study area has changed since the middle of the 20th century.

The interspecific differences in annual and seasonal averages of temperature and humidity at the oviposition sites roughly reflected the overall distribution of the three species along the climatic gradient in the Diemel Valley. Interestingly, the differences in mean spring, summer and autumn temperatures were mainly due to differences in nighttime temperatures. During the day, radiative heating of the near-ground air layer apparently compensated for the mesoclimatic differences in the study area. In contrast, the interspecific differences in relative humidity were stronger during the day when the air at the oviposition microhabitats of E. medusa remained more humid than in the case of M. aurelia and S. spini, probably due to the higher vegetation and the deeper soils in the larval habitats of E. medusa. 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.