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.
