Limnetica 38

An extensive data set of the occurrence of 55 cladoceran species in 432 Alpine lakes was used to document patterns of species richness, assemblage composition and distribution across the Alpine ecoregion in Italy, determining the spatial and macroecological parameters explaining the observed patterns. Moreover, the influence of dispersal, environmental filtering and life cycle strategies in shaping cladoceran alpha and beta-diversity patterns was examined. Only three species where widely distributed in the Alpine lakes, with more than 80 % of the species present in less than 10 % of the lakes. Alpha-diversity ranged from 1 to 13 species, while beta-diversity was almost exclusively due to spatial turnover. Our data suggest that the excess of rarity in these environments was better explained by post-glacial random recolonization patterns, high dispersal ability, peculiar life strategies including the production of diapausing eggs, and compliance of cladocerans to the monopolization hypothesis. Spatial determinants (studied using Moran’s Eigenvector Maps) were important for large scale species distribution patterns, suggesting that dispersal was not a limiting factor, although altitudinal heterogeneity shaped beta-diversity patterns. The altitudinal gradient (together with the associated macroclimatic factors) was the second main determinant of species distribution, followed by the geological setting of the basins. Local factors played only a minor role in explaining cladoceran distribution patterns at the ecoregional scale.