Rarity of microalgae in macro, meso, and microhabitats

Climate change and human-induced habitat degradations result in loss of species diversity in natural ecosystems. While the extinction of macroscopic organisms has been well documented in the scientific literature and public media, we have only limited knowledge on the loss of microscopic elements of the ecosystems. Because rarity coincides with the increased risk of extinction, we investigated the commonness and rarity of microalgae in the Pannonian ecoregion. We reviewed the published literature of microalgal research in Hungary over the last 140 years and created a species-by-site matrix containing 2489 algae species and 1145 localities. Analysing this dataset, we found that although the core-satellite hypothesis suggests a bimodal site occupancy distribution, microalgae displayed a unimodal pattern with a high number of rarely occurring species. We also demonstrated that the well-known negative relationship between body size of organisms and number of occupied habitats holds for microalgae. Rarity values of taxa have a phylogenetic signal indicating that in terms of rarity, closely related species (desmids, dinoflagellates, euglenophytes) show considerable similarities. The various habitat types showed differences in the number of rare taxa. Small- and medium-sized habitats (bog lakes, streams, oxbows) hosted the majority of rare species. These results highlight the conservation importance of small habitats in preserving microbial diversity.