Role of rare species in benthic diatom communities: patterns, processes and networks

Biodiversity loss in freshwater is five times higher compared to terrestrial ecosystems and especially threatens rare species. However, even basic knowledge about rarity is missing in aquatic algal ecology, which could be the base of their conservation strategies. Therefore, we investigated rare species in benthic diatom communities of the River Danube's network system using different rarity concepts. We found that rare and common species are of similar quantities on a regional scale. Genera with fewer taxa provided more rare taxa. The majority of the rare diatom species had small population size among which 45-47% were habitat specialists and geographically restricted, locating streams and river sections whose protection would be outstandingly important. Rare diatom species provided 25-50% higher variability of the communities than common ones. Beta-diversity was also primarily determined by rare species and was mostly driven by species replacement especially in case of specialized and localized rare taxa. Benthic diatom communities established complex community structures. In this network organization, rare species have a core species role with a large impact on the structure and function of the ecological community. Study of rarity is not evident and a number of factors distract it, therefore, we have to carefully consider these uncertainties when dealing with rarity.