Species-area relationships and nestedness of four taxonomic groups in fragmented wetlands

Habitat fragmentation of semi-natural habitats is a serious problem in nature conservation because it leads to a decrease in habitat area and an increase of the isolation of habitat islands. In this paper, we examined the effects of habitat fragmentation on species richness and assemblages of four taxonomic groups. First, we used species-area relationships to assess the effect of habitat area on species richness per habitat island. Second, cumulative species-area curves were used to assess the question whether large habitat islands contain more species than several small islands of the same total area. Third, we examined to what extent species assemblages of species-poor habitat islands are nested subsets of those of species-rich ones. We analysed presence-absence data of vascular plants and bryophytes in 36, and of day-active butterflies and grasshoppers in a subset of 23 habitat islands (montane calcareous fens). The species-area relationship of each group was positive but was highly significant only for vascular plants. The slopes of the relationship on the log-log-scale were rather low (z-values: 0.06-0.11). The cumulative species-area curves of all groups showed that a set of small habitat islands contained more species than a set of a few large habitat islands with the same total area. The results of our nestedness analyses depended on the underlying null models of the test statistics. Applying the method of Wright & Reeves (1992), all groups were significantly nested within the archipelago of habitat islands. Yet, using the method of Brualdi & Sanderson (1999), none of the groups showed non-random patterns. Our study showed that species richness increased with the area of habitat islands, but overlap among them was so low that even small habitat islands contributed to overall species richness. Therefore, a mosaic of several wetland islands, including small ones, is necessary to maintain species diversity at the landscape level.