Plant species richness is key for restoring grassland butterfly assemblages

Palaearctic grasslands are heavily threatened by land‐use changes that lead to degradation and habitat loss. Thus, successful grassland restoration for vegetation and wildlife is urgently needed. Butterflies, as sensitive indicators, could provide a holistic view of restoration outcomes and landscape effects.

We surveyed 121 restored, 33 positive (ancient, species‐rich) and 33 negative (ancient, degraded) reference sites in northern, central, and southern Germany to analyse how recovery of butterflies is affected by site and landscape variables. Restored grasslands cover various restoration methods, that is, seeding of cultivar or regional seed mixtures, transfer of directly harvested seeds from suitable donor sites, or management adaptation without plant species introduction. Landscape effects were analysed at different scales (300, 600, 1200 m). We defined target butterflies as forb‐feeding grassland specialists with low dispersal abilities, supplemented by threatened grass feeders and burnet moths.

Total butterfly species richness, total abundance, and target abundance on restored sites were similar to positive, but higher than on negative references. However, target species richness and Hill–Simpson diversity on restored sites did not reach the level of positive references. Of all the restoration methods, sowing of cultivar seed mixtures was least successful and the failure was more pronounced for target species. Re‐establishing the characteristic plant species richness was most important for butterflies, whereas landscape variables played a subordinate role. However, total butterfly abundance was promoted by higher grassland cover within 1200 m, and total Hill–Simpson diversity increased with grassland plant species richness. Target butterfly recovery was lower in older restored grasslands than in younger ones.

Synthesis and applications . The results indicate that successful restoration of grassland vegetation and butterflies is strongly linked. To fully restore target butterfly assemblages, future grassland restoration should aim to re‐establish and maintain plant species‐rich habitats by implementing high‐quality restoration methods. Successful grassland restoration promotes butterflies in different landscapes. However, further efforts are needed to restore butterfly assemblages close to the levels of favourable reference sites, for example by increasing the cover of high‐quality grasslands in the surroundings. Although restoration is successful, preservation of species‐rich ancient grasslands should have the highest priority.