Effects of human depopulation and warming climate on bird populations in Japan.

Quantifying biodiversity trends in economically-developed countries, where depopulation, associated secondary succession, and climate warming are ongoing, provides insights for global biodiversity conservation in the 21 st century. However, few studies have assessed the impacts of secondary succession and climate warming on species' population trends at a national scale. In this study, we estimated the population trends of common breeding bird species in Japan and tested the associations between the overall population trend and species traits using nationwide bird count data of 47 species collected from 2009 to 2020. We found that the overall population trend varied among species, with four species showing a moderate population increase, 18 being stable, 11 showing a moderate population decline, and 13 with uncertain population trends over the study period. The difference in overall trends among the species was associated with their habitat groups and temperature niche position. Species with lower temperature niche positions experienced more pronounced declines. Multi-species indicators showed a moderate increase in forest specialists and moderate declines in both forest generalists (the species that use both forests and open habitats) and open-habitat specialists. Forest generalists and open-habitat specialists also tended to decline more rapidly at sites with more abandoned farmland. All species groups showed an accelerated decline or decelerated increase after 2015. These findings suggested that common breeding birds in Japan are facing deteriorating trends as a result of nationwide changes in land use and climate. Future land-use planning and policies will need to consider the benefits of both passive rewilding for forest specialists and active restoration measures (e.g., low-intensive forestry and agriculture) for non-forest specialists to effectively conserve biodiversity in the era of human depopulation and climate warming. This article is protected by copyright. All rights reserved.