Is species richness of small spring fens influenced by the spatial mass effect?

Ecological theory predicts that the species composition of island habitats is, among others, shaped by the mass effect of nearby ill-adapted populations, but this has rarely been tested. We investigated small calcareous spring fens scattered within species-rich grasslands in a region in which particular locations differ in climatic and edaphic conditions. Fens in colder, moister and less calcareous north-eastern (NE) locations harboured more plant species from surrounding grasslands and displayed higher similarity with the surroundings than the fens in warmer, drier and more calcareous south-western (SW) locations. Fen species richness correlated significantly with surrounding species richness in the NE locations, whereas no correlation was found for the SW locations. Fens in SW location are characterized by both intense tufa formation, which creates extreme ecological conditions, and the contrasting environmental conditions compared to surrounding grasslands. We demonstrated that the mass effect does not always significantly shape the species composition of island habitats and that the landscape species pool may not influence extreme habitats. This finding accords with the Intermediate difference hypothesis, which predicts a unimodal relationship between the strength of the spatial mass effect and the ecological similarity between neighbouring plots, and could be utilized in the selection of model habitats for exploring large-scale ecological phenomena without the possibly confounding spatial mass effects.