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Drivers of mollusc assemblage diversity in a system of lowland lentic habitats

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LORENCOVÁ Erika HORSÁK Michal

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Popis Freshwater mollusc diversity was repeatedly found to peak in lowland shallow stagnant waters, which are highly exposed to human-made degradation, pollution, and spreading of non-native species. Despite significant human-made changes of these habitats, still little is known about main predictors of their diversity patterns, which are also rely to their passive mode of dispersion. Therefore, understanding the processes shaping mollusc assemblages, richness and composition changes between and also within sites are important. Among main aims of PhD research is to identify principal diversity patterns by comparing results from two climatically contrasting regions; to explore diversity patterns at large spatial scale along a north-south gradient across temperate and Mediterranean realms and possibly to tract target species’ dispersion based on genetic data. We already studied aquatic mollusc assemblages in 62 lowland water bodies in the Dyje River floodplain (south-eastern Czechia), located in a flat agricultural floodplain landscape, covering ca 50 km2. The environmental heterogeneity among the sites was expressed mainly by a high variation of vegetation cover and the nutrient content. For the analysis, we established 17 environmental variables divided into two groups: plot-specific variables and site-specific variables. Recorded species richness and abundances were analysed by GLM and GEE; species compositional variance by NMDS and db-RDA ordinations. In total, 32 species (24 gastropods and 8 bivalves) were recorded at 59 sites. Mollusc species richness, ranging between 0 and 15 species per site, and abundance sharply decreased towards high trophy sites, with chlorophyll-a concentration and water pH highly correlated with the species counts. Species composition variation was driven mainly by water trophy and quality of organic detritus. The plot specific and site specific variables explained all together 18.5% of the variation in mollusc species composition data, with only 3.4% of the variation shared. Site heterogeneity turned out to significantly control local assemblages of molluscs, while the presence of individual species seemed to be filtered by the site trophy and mineral richness. We conclude that to maintain a high regional diversity of aquatic molluscs in lowland agricultural landscapes, the presence of lower-trophy sites seems to be essential.
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