Even though there is no reason to believe in a unique mechanism that would explain rarity, recognizing common patterns might aid in identifying effective conservation strategies. This study therefore approached the problem of rarity in spring habitats both at the level of multiple taxonomic groups (molluscs, oligochaeta, water mites, copepods, ostracods, chironomids, stoneflies, caddisflies, diatoms, and vascular plants) and, in more detail, at the level of a single group (bryophytes). The aim was to evaluate whether the proportion of rare species was associated with uncommon environmental conditions, while for the bryophytes an additional aim was to test whether common and rare species differed concerning their niche parameters (niche breadth and niche position), their biological traits (involved in dispersal processes), and their habitat preferences. Overall, 4 major results may be highlighted. 1) The significant concordance between the rarity of virtually all the taxonomic groups inhabiting spring habitats and their environmental conditions suggested that most of the rare species at community level might be explained by their uncommon resource requirements. 2) Rare bryophyte species not only had high niche positions, as did all the taxonomic groups, but also had narrow niche breadths. This result suggests an interesting distinction between resource use and resource availability. 3) Because the species traits linked to dispersal ability did not differ between common and rare species, the hypothesis of an important role being played by these traits in determining species distribution was not supported. 4) Among the rare bryophyte species, an undefined number were casual. Because they were less hygrophilous and less influenced by the environment, this result suggests that they might recruit essentially by chance from the surrounding habitats.
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