Spread of common species results in local-scale floristic homogenization in grassland of Switzerland

Aim We assess changes in plant species richness and changes in species dissimilarity at local scale in Swiss grassland between the time periods 20012004 and 2006-2009. Further, we provide an ecological interpretation of the observed taxonomic homogenization of vascular plants. Location Switzerland. Methods Changes in species richness and changes in Simpson dissimilarity index of vascular plants in grassland (meadows and pastures) were examined. The analyses were based on species lists recorded on 339 10-m(2) sample plots from a systematic sample covering the entire Switzerland. Each sample plot had been surveyed once in 2001-2004 and once in 2006-2009 with 5 years between the first and the second survey. Changes in species dissimilarity were interpreted by comparing the relative contribution of several indicator species groups. Results Mean species richness of vascular plants in grassland increased during the study period. In contrast, species dissimilarity of plants decreased, suggesting local-scale floristic homogenization of grassland in Switzerland. It was mostly because of the spread of common species, namely the species that are tolerant to high nutrient levels, the species of low conservation value and the species adapted to moderate temperature levels that led to taxonomic homogenization. Target species for conservation did only marginally affect taxonomic homogenization. In contrast to the predictions from studies of taxonomic homogenization on larger scales, the taxonomic homogenization of grassland at local scale was not explained by the spread of neophytic species. Main conclusions The biotic diversity of grassland in Switzerland changed considerably between 2001-2004 and 2006-2009. The observed taxonomic homogenization was merely because of the spread of common species. Local-scale changes in land use regimes implemented by agri-environmental schemes and other conservation efforts on parts of the entire grassland area were, apparently, not enough to prevent the total grassland from recent taxonomic homogenization.