Salinity represents a major structuring factor in aquatic habitats which strongly affects species richness. We studied the relationships among species richness, density and phylogenetic diversity of zooplankton communities along a natural salinity gradient in astatic soda pans in the Carpathian Basin (Hungary, Austria and Serbia). Diversity and density showed opposing trends along the salinity gradient. The most saline habitats had communities of one or two species only, with maximum densities well above 1000 ind l−1. Similarity of communities increased with salinity, with most of the highly saline communities being dominated by one highly tolerant calanoid copepod, Arctodiaptomus spinosus, which was at the same time the only soda-water specialist. Salinity obviously constrained species composition and resulted in communities of low complexity, where few tolerant species ensure high biomass production in the absence of antagonistic interactions. The pattern suggests that environmental stress may result in highly constrained systems which exhibit high rates of functioning due to these key species, in spite of the very limited species pool.