Dissociating several forms of commonness in birds sheds new light on biotic homogenization

AimWe disentangle three facets of species commonness (local abundance, geographical range size, degree of habitat generalization) to identify how species segregate along these axes and how each of these facets determines the relative functional originality of each species (i.e. the mean trait distance of a species with others). At the community level we test whether changes in the relative abundances of species with different levels of commonness contribute to the spatial and temporal dynamics of species assemblages and to the local trend in functional homogenization. LocationFrance. MethodsData from the French Breeding Bird Survey from 2001 to 2012 were used to represent the yearly change in species abundances of 198 species over 2057 plots. Functional originality and three commonness indices were calculated for each species related to their geographical range, habitat generalization and specific local abundances. These species-specific indices were then used to compute community-weighted means of commonness and functional originality. Hierarchical partitioning was used to investigate the relationships between each of the three facets of commonness and functional originality. Generalized additive models were used to estimate the spatial and temporal trends of each community index. ResultsSpecies that are generalist and abundant but with a restricted range had higher functional originality. From 2001 to 2012, assemblages were increasingly composed of species with large ranges and of habitat generalists. The functional simplification of communities was mainly linked to the increase in species with large ranges and, to lesser extent, to a decrease in species with high local abundances. Main conclusionsThe decomposition of commonness into complementary facets is useful for describing the causes and consequences of biotic homogenization at large spatial scales. Functional simplification can occur during a short time period and is driven mainly by the greater success of species with large ranges.