Cryptic diversity, niche displacement and our poor understanding of taxonomy and ecology of aquatic microorganisms

The analysis of ecological niche is an important task to correctly identify the role exerted by species within ecosystems, to assess their vulnerability, to plan effective measures addressed at fulfilling the postulates of biological conservation, and ultimately to prevent biodiversity loss. However, for the majority of organisms our knowledge about the actual extent of their ecological niche is quite limited. This is especially true for microscopic organisms. Evidence exists that in different geographical areas allegedly conspecific populations can show different, if not antithetical, ecological requirements and not-overlapping ecological niches. This opinion paper discusses whether this "niche displacement" effectively occurs between conspecific populations or if the pattern is rather due to an inaccurate identification of species and/or to an insufficient knowledge about population and community ecology. Hence, some subjectively selected study cases when alleged phenomena of niche displacements take place are presented, and shortfalls in the correct assessment of the identity and ecological niches of microscopic aquatic eukaryotes, namely phytoplankton, zooplankton, and meiofauna are shown.