Historical and recent fragmentation of temperate floodplain grasslands: Do patch size and distance affect the richness of characteristic wet meadow plant species?

Patch size and isolation are thought to have a large influence on the extinction risk of specialist plant species in grassland fragments in the modern agricultural landscape. We combined a re-sampling study in semi-permanent floodplain grassland plots with a GIS-based analysis of historical (1950s/1960s) and recent landscape patterns. Based on historical and recent vegetation maps and relev,s from six study areas (plus a protected reference area) covering 50-60 years of vegetation change following agricultural intensification, we aimed at analysing the importance of fragmentation on the diversity of potentially sensitive specialist species of wet floodplain meadows in northern Germany. On the plot scale, we found 30-66 % reductions in species richness of these characteristic wet meadow plant species over time and an associated increase in the fragmentation of grassland habitats. Distance to the nearest suitable habitat had a modest negative effect on modern plot-scale richness, while the other tested landscape metrics (total meadow area, mean patch size and landscape proximity index distribution) had no significant influence. There was also no evidence for a legacy of historical landscape structure on current richness of specialist species. Instead, management intensity and its change over the past decades, as indicated by altered Ellenberg indicator values for nutrients and moisture, had a strong influence on plot-scale diversity. The results suggest that fragmentation is not the proximate cause of impoverishment and point to habitat deterioration as a main driver. We conclude that conservation measures in Central-European floodplain meadows should not only focus on large continuous grassland areas, but should also consider small meadow patches if they remained species-rich.