Functional diversity thresholds provide earlier signals of urbanization impact than species richness in mammal communities

Human activities and urban expansion can strongly affect mammal communities and biodiversity. Although these impacts have traditionally been assessed through species diversity monitoring, functional structure is likely altered before losses in species diversity become detectable. Using nationwide terrestrial mammal occurrence data from a long-term, repeated biodiversity monitoring program, we examined species-specific responses to urbanization, threshold responses of diversity metrics along urbanization gradients, and temporal relationships between changes in urban proportion and diversity metrics across administrative units in the Republic of Korea. We quantified species richness and multiple functional diversity indices, including functional richness, functional dispersion, and functional divergence, based on species ecological traits. Species-specific analyses revealed substantial heterogeneity in responses to urbanization, with several habitat-dependent or conservation-priority species showing strong negative associations with increasing urban proportion, whereas widespread generalist species exhibited weak or neutral responses. At the community level, species richness did not display a directional response to urbanization, while functional richness and functional divergence showed statistically supported negative changes in slope at relatively low levels of urbanization. However, temporal analyses did not detect consistent associations between changes in urban proportion and concurrent changes in diversity metrics. Together, the results suggest that functional richness and functional divergence may provide earlier signals of community change under urbanization than species richness, highlighting the potential value of functional diversity indices in urbanizing landscapes.