Resilience in the heights: Elevational filtering preserves taxonomic and functional beta diversity over time against biotic homogenization

Environmental heterogeneity generates taxonomic and functional beta diversity, but a key question regards whether heterogeneity can maintain functional diversity when global change causes range shifts, potentially causing biotic homogenization by favouring species sharing certain traits over others. We tested the capacity of environmental filtering to conserve taxonomic and functional beta diversity through time, using recent changes to butterfly communities across elevation gradients as a focal system. We sampled species in historic (1985–2005) and recent periods (2017–2022) in 163 discrete sites in four mountain ranges of the central Iberian Peninsula, a reservoir of genetic and taxonomic diversity during glacial periods. We used 35 binary traits to calculate functional diversity and partitioned pairwise Sørensen dissimilarity into turnover and nestedness for taxonomic beta diversity and functional replacement. In historical and recent periods, pairwise taxonomic and functional beta diversity were related to geographic distance and elevation. Distance had a stronger effect on taxonomic beta diversity than on functional replacement, suggesting different species played similar functional roles at equivalent elevations in different mountain ranges. However, the effects of distance on taxonomic and functional beta diversity decreased over time, whereas strong effects of elevation persisted in the recent period. Turnover was the main component of taxonomic and functional beta diversity, but the relative contribution of nestedness increased in the recent period, implying non‐random losses of rare species or gains of widespread species. Effects of mountain range and microclimate on taxonomic and functional differences at equivalent elevations also decreased over time. Variation over the elevation gradient in species richness per functional group generally persisted over time, although at some higher elevations, average richness per functional group increased. Despite some evidence of biotic and functional homogenization across four mountain ranges, effects of elevation on taxonomic and functional beta diversity persisted over time. These results emphasize the importance of elevational filtering to maintain taxonomic and functional differences related to ecosystem resilience, and support the role of environmental heterogeneity to sustain functional diversity and provide refugia from global change. Read the free Plain Language Summary for this article on the Journal blog.