Due to ongoing urban development, vascular plants inhabiting Kanagawa Prefecture, located adjacent to the megacity of Tokyo, are expected to have undergone a continuous decline in their localities.
To quantify this decline, we utilized a database constructed through presence-absence surveys conducted during three survey periods, Phase 1 (1979-1989), Phase 2 (1996-2000), and Phase 3 (2013-2017), for 3,180 vascular plant species across 111 localities in Kanagawa. This database enables us to monitor transitions from presence (1) to absence (0) or from absence (0) to presence (1) at each locality between phases. Subsequently, we calculated per-year increase and decrease rates of species i's localities, denoted as b_i and d_i, between phases. Consequently, we obtained four rates for each species, b_(i,12), b_(i,23), d_(i,12), and d_(i,23), where subscripts 12 and 23 indicate the transition from phase 1 to 2 and phase 2 to 3, respectively. We then calculated the medians of increase and decrease rates, denoted as M_(b,12), M_(b,23), M_(d,12), and M_(d,23), and conducted statistical tests on M_(b,12) and M_(b,23), M_(d,12) and M_(d,23).
As a result, M_(b,12) is significantly lower than M_(b,23), and M_(d,12) is significantly higher than M_(d,23). These results suggest that an acceleration in new locality generation and a deceleration in existing locality loss. Furthermore, to determine if the number of localities is still decreasing, we calculated medians of net changes, b_(i,12) - d_(i,12) and b_(i,23) - d_(i,23). Then it is shown that b_(i,12) - d_(i,12) is significantly lower than b_(i,23) - d_(i,23), and both values are negative. This result indicates that the loss of localities exceeds the generation of new localities in Kanagawa.
Notably, both b_(i,12) - d_(i,12) and b_(i,23) - d_(i,23) are negative for 1,057 species of vascular plants, respectively, suggesting the potential extinction of these species in future. This calculation introduces a new indicator to quantify the threat of extinction in vascular plants.