Butterfly population trends track vegetation encroachment but not climate warming in a Mediterranean mountain

Climate and land cover changes are known to exert strong pressure on mountain biodiversity, but the potential interactions between these drivers remain unknown. Research concerning how changes in climate and land cover impact montane species is often based on single-year and short-term studies, which can limit the conclusions about population trends. We used meteorological and vegetation structure data, butterfly counts for 57 species over 14 years, and species’ ecological characteristics to examine how changes in climate and land cover affect trends in butterfly populations in a Mediterranean mountain range. Temperature, forest area, shrub cover and vegetation height increased over time, at similar rates across the elevational gradient. While 35% of the butterfly species showed stability in temporal abundance, the proportions that decreased and increased in abundance were similar (24–28%), with comparable rates across the elevational gradient. Species associated with open habitats, and with larval feeding on herbs and grasses, tended to decrease in abundance more than those associated with closed habitats, and with larval feeding on woody plants, thus indicating that population changes were mainly related to vegetation encroachment, but not climate warming. Our findings support the idea that mountains can act as climate refugia relative to more disturbed flat lowlands, which are usually inhabited by butterfly populations tending to decrease in abundance. They also suggest that possible effects of interactions between vegetation encroachment and climate change may counteract negative warming impacts on populations by microclimatic cooling, thus enabling potential adaptation to climate change and protection of biodiversity.