Relaxation of management intensity promotes butterfly communities in mountain grasslands

Semi-natural mountain grasslands are currently threatened by either land abandonment or management intensification, which calls for evidence-based recommendations for sustainable and biodiversity-friendly farming of these valuable biodiversity hotspots in the long run. We investigated the effects of the relaxation of the management intensity on their plant and butterfly communities, via an experimental reduction of fertiliser inputs. In 2019, 13 study sites were selected in the SW Swiss Alps, each including three meadows that had been intensively managed for at least the previous 20 years. Adopting a randomised block design, two out of three meadows per study site were restored by shifting their mode of exploitation towards low-intensity (1/3 of the fertiliser dose applied beforehand) or extensive management (cessation of fertilisation), respectively, while the third continued to be managed intensively, serving as control. After four years (2023), plant species richness had increased by 9.3 % in extensively managed meadows compared to baseline. Yet, butterfly abundance was more than twice greater (112 %) and species richness 81 % higher in extensively managed meadows compared to controls. A higher butterfly abundance and species richness following management relaxation was mostly driven by a decrease in vegetation density and an increase in forb cover. Extensively managed meadows also exhibited higher conservation values, harbouring comparatively more threatened butterfly species, whilst no change was observed in low-intensity meadows. Altogether these results indicate that the butterfly communities of mountain hay meadow respond rapidly and massively to a relaxation of management intensity.