Factors explaining community contrast of Trichoptera assemblages at insular Western Carpathian spring fens to the adjacent headwaters

The specific biota of isolated habitats is determined by habitat properties (e.g., age, size, environmental conditions), their spatial isolation and the characteristics of the surrounding landscape matrix. In this study, we examine the contrast in Trichoptera assemblages between isolated island-like Western Carpathian spring fens and nearby headwater streams. We evaluate the potential of streams and their mesohabitats to serve as stepping-stone habitats for fen species, and identify factors determining the dissimilarity of the Trichoptera between the fens and streams. Of 24 studied fen-stream pairs, 80 Trichoptera taxa, mainly species, were identified. Out of these, 17 and 34 species were exclusive for fens and streams, respectively. Although the remaining 29 species were in common, a clear preference of each species to one of these two habitat types was observed, which resulted in highly contrasting assemblages in terms of species composition. A clear difference was found also at the level of species traits, as cold-adapted crenobiont and crenophilous species of small body size, limited dispersal ability, and long larval development prevailed at the fens. The fen-stream dissimilarity of Trichoptera was not associated either with fen habitat properties (age, size and environmental conditions) or with the distance between the habitats. Only stream characteristics reducing the fen-stream environmental contrast were important. Abundance of fen species at streams was negatively related with substrate roughness and stream size, suggesting that small streams with a fine substrate can serve preferably as stepping-stone habitats for the fen species. Our results show the high conservation value of spring-fen Trichoptera assemblages due to the prevalence of habitat specialists as well as cold stenothermic and pollution-sensitive species. It also seems that the isolation of fens surrounded only by unsuitable streams is additionally increased by a lack of stepping-stone habitats in the surrounding matrix.