Contribution of rarity and commonness to patterns of species richness in biogeographic transitions regions: Woody plants of Uruguay

There is a growing body of evidence suggesting that widespread (i.e. common) rather than geographically restricted species (i.e. rare) shape the overall distribution patterns of species richness. This is a non-intuitive fact, given that local and regional assemblages are normally composed by numerous rare species and few common ones. We evaluated here the primacy of common species in a biogeographic transition zone, where rarity has frequently a higher incidence. We analysed the geographical variability of trees and shrubs in Uruguay, located in a transitional zone between prairie and forest biomes, to assess the relative contribution of rare and common species to the generation of richness patterns. The distribution of 301 species of the native woody assemblage of Uruguay was mapped over the national grid system (302 quadrants of approximately 22 × 30 km), using published data and herbarium records. The overall assemblage was segregated into four subassemblages in function of species distribution (quartiles). Species richness in the four quartiles was positively correlated with overall richness, but common species (quartile 3) showed the highest level of correlation. Then, we ranked species from the most widespread to the most restricted (common-to-rare) and from the most restricted to the most widespread (rare-to-common). Along each stage of the sequences we obtained a series of species richness patterns for increasing numbers of species. Correlating the species richness pattern for each subassemblage of both sequences with that of the full assemblage, we also found higher correlations in the common-to-rare sequence. We conclude the Uruguayan woody plants assemblage has a very large number of rare species as expected for a transitional biogeographical zone, but it was the common species that contributed most to the overall pattern of species richness. We propose the low contribution of rare species is explained by the most interspecific variability in ecological determinants within the assemblage of rare species. Therefore the spatial covariance among rare species is low, and so is the relationship with overall species richness.