Determining the relative roles of species replacement and species richness differences in generating beta-diversity patterns

Aim To determine the relative contribution of species replacement and species richness differences to the emergence of beta-diversity patterns. Innovation A novel method that disentangles all compositional differences (beta cc, overall beta diversity) in its two components, species replacement (beta-3) and species richness differences (beta rich) is proposed. The performance of the method was studied with ternary plots, which allow visualization of the influence of the relative proportions of shared and unique species of two sites over each metric. The method was also tested in different hypothetical gradients and with real datasets. The novel method was compared with a previous proposal based on the partitioning of overall compositional differences (beta sor) in replacement (beta sim) and nestedness (beta nes). The linear response of beta cc contrasts with the curvilinear response of beta sor to linear gradients of dissimilarity. When two sites did not share any species, beta sim was always 1 and beta-3 only reached 1 when the number of exclusive species of both sites was equal. beta-3 remained constant along gradients of richness differences with constant replacement, while beta sim decreased. beta rich had a linear response to a linear gradient of richness differences with constant species replacement, whereas beta nes exhibited a hump-shaped response. Moreover, beta sim > beta nes when clearly almost all species of one site were lost, whereas beta-3 < beta rich in the same circumstances. Main conclusions The behaviour of the partition of beta cc into beta-3 and beta rich is consistent with the variation of replacement and richness differences. The partitioning of beta sor into beta sim and beta nes overestimates the replacement component and underestimates richness differences. The novel methodology allows the discrimination of different causes of beta-diversity patterns along latitudinal, biogeographic or ecological gradients, by estimating correctly the relative contributions of replacement and richness differences.