Nestedness-resultant community disassembly process of extinction debt in a highly fragmented semi-natural grassland

There are two major processes of species disassembly after landscape changes: non-random loss of species resulting in nested assemblages and species replacement resulting in spatial species turnover. Although time-lagged responses of species to landscape change have been widely recognized, few studies have empirically evaluated which of these two processes is more closely related to extinction debt (i.e., postponed species extinction following habitat loss). This study aimed to understand the underlying processes of extinction debt by partitioning β-diversity into components of species nestedness and species turnover. We measured grassland species richness at three spatial extents in a highly fragmented semi-natural grassland landscape in Japan. Dissimilarity-based β-diversity was partitioned into two components (i.e., nestedness-resultant dissimilarity β_sne] and turnover-resultant dissimilarity β_sim]), which were further analyzed using principal coordinates analyses (PCoA). The relationships between the variability of PCoA axis 1 scores and the current and past habitat proportions were evaluated. A significant positive relationship between current grassland species richness and past (i.e., the 1910s) grassland proportion was found at the largest spatial extent. The first axis of PCoA based on β_sne showed significant correlation with past habitat proportions, whereas the PCoA axis based on β_sim showed no significant correlation with either the current or past habitat proportions. A non-random loss of grassland species represented by nestedness underlay the extinction debt found at the landscape level. There is a chance of predicting the loss of species from the nested ranks of species which likely reflects the gradient of species vulnerability to historical landscape changes.