Citizen science reveals patterns of distributional decline and expansion across 1864 native vascular plant species in Kanagawa, Japan

Rapid land-use change and climate change increase the need for quantitative, region-wide assessments of current status across many plant species. Long-term citizen-led distribution surveys help meet this need, and European examples in particular have been widely recognized internationally. In contrast, comparable efforts in non-English-speaking regions remain underrepresented due to language barriers. We present a citizen-science project in Kanagawa Prefecture, Japan, and analyze presence–absence data from three distribution surveys (1979–1987, 1996–2000, 2013–2017) conducted under this project to quantify changes in the number of presence-recorded survey districts for 1864 native vascular plant species. Presence-recorded districts decreased for 835 species (44.8%), especially grassland, forest-edge, upland field, and wasteland species. By contrast, 688 species (36.9%) increased, particularly evergreen forest species. Within this group, mycoheterotrophic orchids and ferns typical of dark, humid understories expanded rapidly, suggesting effects of coppice-forest abandonment. Although the annual disappearance rate of presence-recorded districts declined over 1979–2017, the recent disappearance rate remained significantly greater than zero, indicating ongoing habitat loss. Using disappearance rates, we identified 53 non-threatened species with disappearance rates comparable to or exceeding those of threatened species, and define them as “potentially threatened species.” These results underscore the value of long-term, systematic, citizen-led surveys for evidence-based conservation planning.