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

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Aim. We disentangle three facets of species commonness (local abundance, geographicalrange size, degree of habitat generalization) to identify how species segregatealong these axes and how each of these facets determines the relativefunctional originality of each species (i.e. the mean trait distance of a species withothers). At the community level we test whether changes in the relative abundancesof species with different levels of commonness contribute to the spatial andtemporal dynamics of species assemblages and to the local trend in functionalhomogenization.Location. France.Methods. Data from the French Breeding Bird Survey from 2001 to 2012 wereused to represent the yearly change in species abundances of 198 species over 2057plots. Functional originality and three commonness indices were calculated foreach species related to their geographical range, habitat generalization and specificlocal abundances. These species-specific indices were then used to computecommunity-weighted means of commonness and functional originality.Hierarchicalpartitioning was used to investigate the relationships between each of the threefacets of commonness and functional originality. Generalized additive models wereused to estimate the spatial and temporal trends of each community index.Results. Species that are generalist and abundant but with a restricted range hadhigher functional originality. From 2001 to 2012, assemblages were increasinglycomposed of species with large ranges and of habitat generalists. The functionalsimplification of communities was mainly linked to the increase in species withlarge ranges and, to lesser extent, to a decrease in species with high localabundances.Main conclusions. The decomposition of commonness into complementaryfacets is useful for describing the causes and consequences of biotic homogenizationat large spatial scales. Functional simplification can occur during a short timeperiod and is driven mainly by the greater success of species with large ranges.