Ecological specialization matters: long‐term trends in butterfly species richness and assemblage composition depend on multiple functional traits

AIM: To quantify spatio‐temporal changes in species richness and assemblage composition of Danish butterflies over more than 100 years and to assess whether different functional groups of butterflies show different temporal trajectories. LOCATION: Denmark, Europe. METHODS: We applied species accumulation curves to estimate relative species richness changes from unequally sampled occurrence records, collected over more than a century (1900–2012), at local (i.e. 10 km× 10 km) and regional (country‐wide) spatial scales. Furthermore, we calculated similarity in species composition between 10 km× 10 km grid cells to evaluate changes in assemblage composition (i.e. biotic homogenization) of butterflies through space and time. Trends were evaluated for all butterflies as well as for three functional groups, using hierarchical clustering and species‐specific values of eight ecological traits. RESULTS: We document severe declines in butterfly diversity over the last century, with substantial regional‐scale extinctions leading to a net loss of 10% of all Danish butterfly species. Segregating species into functional groups showed that the highest rate of regional extinction occurred among sedentary habitat specialists overwintering in the egg stage, while the most severe local‐scale declines occurred among sedentary host plant specialists overwintering in the larval stage. In contrast, mobile generalist species with mature overwintering stages showed stable richness trends throughout the century. However, a pervasive spatio‐temporal biotic homogenization of butterfly assemblages was revealed for all functional groups. MAIN CONCLUSIONS: Our results suggest that observed changes in species richness were driven by a gradual replacement of ecological specialists by broadly adapted ecological generalists. In addition to drawing attention to a severe long‐term impoverishment of the Danish butterfly fauna, our results suggest that trajectories of butterfly species richness and assemblage composition over the last century are related to multiple functional traits, highlighting the need to consider different aspects of ecological specialization when assessing extinction risk.