The 39 land cover categories on this map were lumped into 13 habitat types (Appendix Panobinostat nmr 1, Table 5). For each 5 × 5 km grid square we calculated the area occupied by

the different habitat types. In addition we calculated the Shannon index expressing the land cover heterogeneity in each grid square: $$ H^\prime = – \Upsigma p_i \ln p_i $$where p i (>0) is the proportion of area of the i-th habitat type in a grid square. Climate data were obtained from the Royal Netherlands Meteorological Institute (KNMI 2002). Relative humidity in spring, duration of sunshine, amount of radiation, temperature and precipitation surplus are given as the mean annual values measured over the period 1971–2000. Elevation was derived from the Dutch national digital elevation model (2002, Rijkswaterstaat). Soil types were abstracted from the Dutch soil type map (Steur and Heijink 1992). Average groundwater level in spring was derived from the map of groundwater classes (Hinsbergen et al. 2001). For data on nitrogen deposition (1995–1997 means) we used

the results of the STONE model (Overbeek et al. 2002). Data on pH (1991–1997 means), available Daporinad datasheet nitrogen (1991–1997 means), and salinity (1970–1997 means) were all obtained from Bio et al. (1999). A map depicting the age of the Dutch landscape, based on the last major shift in land cover, was constructed using literature and topographical maps dating from ca. 1850 to 2002 (Cormont et al. 2004). Data analysis We followed a five-step procedure to define the hotspots of characteristic species. First, TWINSPAN was used to cluster grid squares according to similarity in species composition for find more each individual taxonomic group. Due to large differences in the number of species in the taxonomic groups (Table 1), we analyzed the groups separately instead of combining them from the start. Then we identified characteristic species for each cluster. Subsequently we identified corresponding clusters among the different taxonomic groups and selected regions containing characteristic species for at least two of the taxonomic groups. These regions were then defined as hotspots of characteristic species. Finally,

we assessed the environmental differences between these regions. Identifying regions for individual taxonomic groups Species composition of each 5 × 5 km grid square was analyzed for each taxonomic group individually, using two-way indicator species analysis (TWINSPAN), a hierarchical divisive numerical classification technique (Hill 1979). We used the adjusted TWINSPAN version as described in Oksanen and Minchin (1997). Highly common species (distributed across the entire country and in >40% of the squares) were omitted from the analysis to prevent the formation of separate clusters with a low sampling intensity, as unevenness in sampling intensity is a common problem in the kind of databases used in studies such as this (e.g.