Back to the future: Climate change effects on habitat suitability of Parnassius apollo throughout the Quaternary glacial cycles

Alpine grasslands above the treeline are severely threatened by climate change, mainly due to forest expansion driven by warmer conditions. Analogous lowland grasslands experience severe reductions due to land-use abandonment and forest encroachment.To address how climate change impacted open-areas insects, we used Parnassius apollo as a model, a butterfly with wide Palearctic distribution inhabiting both alpine and low-altitude steppe grasslands. We modelled upper Pleistocene range changes from the Last Interglacial (130 Kya) to the present and future (2050/2070), using medium and high greenhouse gas emission rates for the latter.We combined bioclimatic variables (Worldclim, Paleoclim, Chelsa) with distribution records of P. apollo and two of its most often used larval host plants (Sedum album; Hylotelephium telephium) to formulate species distribution models (SDMs) via the Maximum entropy method.We estimated a substantial range expansion during cold periods (last glacial maximum, 22 Kya) and contractions in warmer periods. Including the host plants in the models brought reduced suitable areas estimate, possibly due to differences in climatic requirements of hosts and the butterfly. Future projections of the extent of suitable climates are surprisingly better than would be expected from a warming climate, likely because the current distribution, especially at lower elevations, is probably restricted by habitat loss due to land abandonment and afforestation.We recommend preventing afforestation in critical habitats across Europe and Asia, and increasing survey activities to perform more accurate SDMs.