Latitudinal patterns of species richness in the European freshwater fauna: dispersal matters

In Europe, the major mountain ranges are oriented in east-west direction. During the glaciations and climatic fluctuations of the Pleistocene these mountain ranges were barriers for species retreating to the Mediterranean refugia. After the last glacial maximum, recolonization processes were also affected by the mountain ranges. Therefore, we expect that diversity patterns in central and northern Europe depend on the dispersal ability of organisms. We analyzed patterns of species richness for the European freshwater fauna (14,020 species across 25 regions). In a multiple regression analysis across all species, we found a hump-shaped relationship between species richness and latitude. Area per se had little power to explain the variation in species richness and the mid-domain effect failed to explain the pattern as well. However, comparing different taxa and habitats, we found a monotonous latitudinal decrease of species richness for taxa and habitats harboring species with a low propensity for dispersal (e.g. fish, mollusks; lotic habitats) and a hump-shaped relationship for those with a high propensity for dispersal (e.g. insects; lentic habitats). These effects of dispersal on geographical patterns of species richness are underlined by our recent studies on the range sizes and distributions of lentic and lotic odonates and on the genetic differentiation of lentic and lotic freshwater invertebrates. Furthermore, our comparisons between groups of species with different ecological traits (dispersal ability) showed that the monotonous latitudinal decline of species richness in certain groups of European freshwater animals, although expected from factors like area or energy, is due to the fact that these lineages have not reached an equilibrium.