Nation-wide agrarian depopulation threatens semi-natural grassland species in Japan: Sub-national application of the Red List Index

To integrate monitoring and conservation of biodiversity at multiple spatial scales, biodiversity indicators measured at coarse spatial scales should be linked with anthropogenic pressures reported at finer scales. Applying the Red List Index (RLI) at the regional level, we investigated the relationship between Japanese semi-natural grassland species and proxies of threats caused by human activities over the past 50 years. We calculated and compared RLI values at the national and prefectural levels for all grassland species, plant functional groups (PFGs), and resource use types. Analyses of prefectural RLIs revealed trends that were not visible at the national level: annuals and wind-dispersed, tall and short perennials were more extinction prone (i.e., had lower RLI values) than tall and short perennials with unassisted and other types of dispersal mechanisms. Species with horticultural use also had higher risk of extinction than those without. Prefectural RLIs were further analyzed for their relationship to social factors (i.e., demographic and land use statistics). As a result, the conservation status of grassland plants was more deteriorated in prefectures with larger decrease in commercial and subsistence agrarian population density in the past (during 1960–1980) than the recent (1980–2000). This supports the perception that the “underuse” of agricultural landscapes, indicated by agrarian depopulation, has had a large impact on semi-natural grassland biodiversity in Japan. Our approach of relating regional RLIs to proxies of anthropogenic pressures demonstrated that human-induced drivers of species loss can be readily interpreted from spatial differences in RLI values in order to derive effective conservation strategies across regions.