Alpine butterflies want to fly high: Species and communities shift upwards faster than their host plants

Despite sometimes strong co-dependencies of insect herbivores and plants, responses of individual taxa to accelerating climate change are typically studied in isolation. Thereby, biotic interactions that potentially limit species in tracking their preferred climatic niches are ignored. Here, we chose butterflies as a prominent representative of herbivorous insects to investigate the impacts of temperature changes and their larval host plant distributions along a 1.4 km elevational gradient in the German Alps. Following a sampling protocol of 2009, we re-visited 33 grassland plots in 2019 over an entire growing season. We quantified changes in butterfly abundance and richness by repeated transect walks on each plot and disentangled the direct and indirect effects of locally assessed temperature, site management, and larval and adult food resource availability on these patterns. Additionally, we determined elevational range shifts of butterflies and host plants at both the community and species level. Comparing the two sampled years (2009, 2019), we found a severe decline in butterfly abundance and a clear upward shift of butterflies along the elevational gradient. We detected shifts in the peak of species richness, community composition and at the species level, whereby mountainous species shifted particularly strongly. In contrast, host plants showed barely any change, neither concerning species richness, nor individual species shifts. Further, temperature and host plant richness were the main drivers of butterfly richness, with change in temperature explaining best the change of richness over time. We conclude that host plants are not yet hindering butterfly species and communities from shifting upwards. However, the mismatch between butterfly and host plant shifts might become a problem for this very close plant-herbivore relationship, especially towards higher elevations, if butterflies fail to adapt to new host plants. Further, our results support the value of conserving traditional extensive pasture use as a promoter of host plants and thereby butterfly richness.