The Rapoport effect is detected in a river system and is based on nested organization

Aim To test two hypotheses: (i) the Rapoport effect is valid for a river continuum, particularly regarding the altitudinal distribution of mayflies over the large-scale continuum from alpine stream to medium-sized lowland river, and (ii) the observed Rapoport effect is based on high nestedness of the meta-assemblage, implying the presence of a common species pool and species-thinned nested subsets. Location Geometric centre of Eurasia, south-western Siberia, Russia. Methods Regression analysis of breadth and midpoints of species altitudinal ranges, species richness and altitude. Comparison of the observed patterns with the null model predicted by the mid-domain effect and conventional null hypothesis. Nestedness analysis. Results The Rapoport effect was explicitly demonstrated: species altitudinal ranges expanded with increase in the ranges' midpoints and average per-altitude ranges increased with increase in altitude and concurrent decrease in species richness. The significance of each trend was confirmed with respect to both the null models applied. Nestedness analysis has revealed that the observed Rapoport effect is based on nested organization of the meta-assemblage over the river altitudinal/longitudinal gradient: a common species pool at low (but not the lowest) altitudes and nested subsets of this pool at higher altitudes. Main conclusions This study for the first time credibly shows the altitudinal Rapoport effect in freshwaters. This finding demonstrates cross-habitat-type consistency of the Rapoport effect, but does not suggest that the Rapoport effect is an obligatory attribute of river systems or lotic organisms. High nestedness underlying the observed Rapoport effect together with some previous studies indicate that the Rapoport effect in altitudinal and bathymetric gradients in general may be based on nested organization. The study highlights a deficiency of cross-habitat-type analyses of macroecological patterns (i. e. over terrestrial, freshwater and marine habitats).