Vegetation encroachment drives changes in the composition of butterfly assemblages and species loss in Mediterranean ecosystems

Land abandonment and loss of grazing have been amongst the primary drivers of landscape change in the Mediterranean basin in the recent decades. As a consequence, forest cover has greatly expanded in detrimental of semi‐natural grasslands, areas of cultivation and pasture mosaics. Although predictably important, the impact that this phenomenon has on biodiversity has remained largely unexplored, partly because of lack of appropriate data. Here, we make use of an extensive citizen science program, the Catalan Butterfly Monitoring Scheme, to quantify the response of butterfly assemblages to vegetation encroachment in NE Spain. We first adapted an index to describe the preference of 147 butterfly species for open or closed habitats and found a strong association of most species for open habitats. We developed a community index to record changes in 54 long‐term monitored sites (10 years or more), where plant communities were also periodically monitored. Butterfly assemblages have undergone changes toward species preferring closed habitats in 72% of the studied sites, in parallel to a process of vegetation encroachment in the region. Community changes were linked to population trends, and could be locally predicted by the interaction of the preference of butterfly species for open or closed habitats and the magnitude of vegetation encroachment at each site. These changes were accompanied by frequent extinction events (4.53% of the studied populations), that were highly biased toward species preferring open habitats. Our study confirms and quantifies the threat that vegetation encroachment imposes on biodiversity in this highly diverse region. We have developed an index of preference for open/closed habitats for 147 butterfly species using BMS data. We found a high bias toward open habitat preference. Butterfly assemblages have undergone changes toward species preferring closed habitats in 72% of the studied sites, in parallel to a process of vegetation encroachment in the region. Population trends could be predicted by the interaction of the open/closed preference of species and local vegetation encroachment. Frequent extinction events were highly biased toward species preferring open habitats.