The importance of species replacement and richness differences in small-scale diversity patterns of aquatic macroinvertebrates in spring fens

This study provides new information on beta diversity (variation in species composition across space) of three invertebrate groups (Clitellata, Plecoptera, and Chironomidae) on a small, within-site spatial scale. It was conducted at small-sized spring fens where two contrasting mesohabitats, a flowing-water and a standing-water, can be clearly distinguished. Besides flow conditions, these habitats differed in the amounts of inorganic substratum and dissolved oxygen.Factors related to flow conditions (dissolved oxygen and water temperature) influenced the composition of the studied assemblages at both mesohabitats, while substratum features were important at the standing-water habitat. The water chemistry was important for Clitellata at both mesohabitats and for Chironomidae at the flowing-water.Using a new approach of disentangling beta diversity into two components (species replacement and species richness differences), clear difference in beta diversity patterns among the three assemblages were found, despite their roughly equal total beta diversities. Variation in species composition of those assemblages strongly limited by flow conditions (Plecoptera and partly Clitellata) was induced by differences in species richness, while replacement governed the variation in species rich assemblages that were able to utilise a wide range of conditions (Chironomidae and partly Clitellata). These results indicate that the size of species pool and the level of environmental filtering play an important role in forming beta diversity patterns, and caution that the same amount of beta diversity can be promoted by essentially different mechanisms, even at a very fine spatial scale with no involvement of dispersal limitations.