Species sorting determines variation in the community composition of common and rare macrophytes at various spatial extents

Patterns of rarity in the context of species richness gradients have intrigued researchers for decades, but the same patterns have only rarely been considered in the context of community composition. There is thus a need to combine the classical rarity theory with expectations from metacommunity research. We studied the responses of assemblages of common and rare lake macrophytes to ecological gradients across various spatial extents. Based on the ideas of metacommunity research, we first hypothesised that the communities of common species are mainly affected by environmental factors and rare species are mainly controlled by spatial processes. Alternatively, following the classical theory of rarity, we expected that common species are environmental generalists (their assemblages are controlled by the spatial processes) and rare species are environmental specialists (their assemblages are controlled by the environment). We used two criteria to define common and rare species: the inflection point criterion and the 50th quartile criterion. Partial redundancy analysis was employed to decompose variation in the assemblage composition of common and rare macrophytes between environmental and spatial variables at each spatial extent and geographical region. Both criteria defined similar sets of common and rare macrophytes across spatial extents. As found in previous studies, the amount of explained variation in the composition of the macrophyte communities along ecological gradients was low overall. However, most of the explained variation, was related to the responses of common species. Assemblage composition of common and rare species was similarly explained by environmental factors. We did not find clear support for current ecological theories with regard to variation in the assemblage composition of common and rare macrophytes. In contrast, our findings suggested that species sorting is collectively structuring the assemblages of both common and rare macrophytes. Spatial variation in metacommunity structuring, however, hinders our ability to draw general conclusions on how common and rare species respond to specific ecological gradients in different regions and at various spatial extents.