Rare butterfly species vitally depend on soil disturbance by an ecosystem engineer in abandoned calcareous grasslands

Ecosystem engineers like wild boar (Sus scrofa) can reset successional processes in grasslands, creating open swards rich in bare ground and exhibiting a warm microclimate. However, studies on the effects of soil disturbance by wild boar (Sus scrofa) on butterfly species in calcareous grasslands are missing. Here, we analysed the effects of wild boar rooting in abandoned calcareous grasslands of the Hainich National Park, Germany, on two rare butterflies: marsh fritillary (Euphydryas aurinia) and Nickerl's fritillary (Melitaea aurelia). We sampled different environmental parameters as well as butterfly frequency and abundance in wild boar rootings and undisturbed calcareous grassland vegetation. Our study demonstrated that wild boar play a vital role as soil disturbing ecosystem engineers for the persistence of the butterfly species. They created early seral stages rich in bare ground, hence a warm microclimate, and host plants. The two fritillary species used almost exclusively (E. aurinia) or even solely (M. aurelia) rooting patches for reproduction. Overall, occurrence and abundance of the butterfly species were best explained by a (i) high host plant abundance/biomass and (ii) warm microclimate. Based on our study, high densities of wild boar and additionally wild herbivorous ungulates, which slow down the expansion of woody plants by browsing, should generally be promoted in the calcareous grasslands of the national park. However, overall, they cannot halt shrub encroachment and thus the continuous loss of calcareous grasslands. Accordingly, we recommend active grassland management, e.g. exhaustive shrub removal followed by large-scale grazing by cattle and horses with low stocking rates.