Decade-Long Bird Trends in China: Stable Species Richness but Increasing Biotic Homogenization

While China harbors rich biodiversity, it is facing concurrent challenges from rapid socio-economic growth and severe biodiversity loss. Additionally, critical gaps in knowledge about the country's biodiversity trends have persisted for a long time because of a lack of a national-scale biodiversity monitoring program. Using a decade (2011-2020) of avian monitoring data from 142 sites and 864 species under the China Biodiversity Observation Network, we evaluated temporal trends in alpha and beta diversity, examining both the taxonomic and functional facets of biodiversity, and assessed their drivers. We found that species richness remained stable over 10 years, with no directional trend at 84.5% of sites and a small mean effect size (Zr = 0.102, 95% CI = [-0.012; 0.216]). In contrast, functional richness increased significantly (Zr = 0.122, 95% CI = [0.016; 0.227]). Taxonomic multiple-site beta diversity decreased over time (p < 0.01), indicating a trend towards biotic homogenization. Bird communities in regions with pronounced precipitation seasonality and lower elevation exhibited larger increases in species and functional richness, whereas those at higher elevations and topographic heterogeneity showed greater biotic turnover through time. Our results reveal asynchronous changes in taxonomic and functional diversity, and decoupled trends in alpha and beta diversity, implying that stable species richness at the local scale may mask broader-scale trends in functional diversity and biotic homogenization. Therefore, it is crucial for long-term monitoring to track multi-dimensional biodiversity trends to better develop targeted conservation practices in the Anthropocene.