Ecological drivers of rarity and commonness in freshwater benthic diatoms

Natural communities are generally composed of many rare species constituting the major part of species richness. Despite the ecological importance and vulnerability of rare species, our knowledge about them is very limited, particularly concerning the environmental factors influencing them. This gap is largely due to their exclusion from ecological analyses. Benthic diatoms have crucial importance in aquatic ecosystems making it vital to enhance our understanding of their ecological drivers. In the present research, the niche- and neutral-based ecological processes governing common and rare benthic diatoms were analysed. For this purpose, a total of 100 sampling sites in the Danube catchment (Hungary and Croatia) were investigated in spring and autumn 2022. The study (i) analysed the extent of environmental and spatial processes contributing to the distribution of rare (small population size, habitat specialist, geographical restricted, and absolute rare encompassing all the three types) and common species; moreover (ii) identified the constraining variables. Our findings revealed that the distribution patterns of rare species characterized by small population sizes are generally more distinct from other types of rarities, and are similar to that of the common species. All types of rare species exhibited marginal environmental optima and narrow tolerances, yet both environmental and spatial processes determine their distributions at similar extents. The explanatory power of the background variables was lower for rare species, and notable differences were found in the key parameters affecting them (fine to large spatial scales, temperature, conductivity, N and P forms) compared to common species (large spatial scale, temperature, conductivity, silica). The results of the present research highlight the need to define and investigate rarity in different ways, to consider spatial and environmental processes together, and to develop new statistical approaches. • Niche- and neutral-based processes are equally important for rare and common species. • Common diatom species have broad tolerance and central ecological preference. • Rare diatom species have narrow tolerance and marginal ecological preference. • Different explanatory variables determine the common and rare diatom species. • The ecological drivers of rare diatom species differ according to their types.