Citizen science reveals silent habitat loss and expansion in 1,864 native vascular plant species in Kanagawa, Japan

This study examines distributional changes in 1,864 native vascular plant taxa in Kanagawa Prefecture, Japan, using presence–absence data from three citizen science surveys conducted between 1979 and 2017. We find that 835 taxa (44.8%) declined in presence, particularly those inhabiting grasslands, forest edges, upland fields and wastelands—habitats strongly affected by land-use change. In contrast, 688 taxa (36.9%) increased, especially in evergreen forests. Taxa associated with these increases include mycoheterotrophic orchids and drought-tolerant ferns, potentially linked to the abandonment of coppice forest management and climate warming. By estimating disappearance rates of habitat, we detected a deceleration in recent decades; however, disappearance rates remain non-zero. Based on these estimates, we identified 66 potentially threatened taxa not designated as threatened species in Red Data Book, many linked to underprotected habitats. These findings highlight the urgency of designating such habitats as Other Effective area-based Conservation Measures (OECMs) and restoring degraded ecosystems. More broadly, our study underscores the value of long-term, site-consistent monitoring data, as promoted by global frameworks such as the Global Biodiversity Observing System (GBiOS). It also demonstrates the pivotal role of citizen scientists in generating data critical to large-scale biodiversity conservation.