Urban Filter vs. Natural Refuge: Divergent Diptera Community Assembly Mechanisms-Evidence from Beijing, China.

Urbanization can act as a powerful ecological filter, restructuring biodiversity through species loss, replacement, and altered resource pathways. While urban green spaces (UGS) are recognized as potential biodiversity refuges, the effectiveness and mechanisms for conserving insect diversity across the urban-to-natural gradient remain poorly understood. Here, we combine full-season Malaise trapping (April-November) with MinION-based DNA barcoding to test two predictions about how urbanization reshapes Diptera communities across five sites in Haidian District, Beijing, ranging from residential areas and urban parks to a nearby shallow mountain reserve (BWM). Based on 5528 barcoded individuals, we identified 686 putative species from 39 families. As predicted, β-diversity between urban and mountain sites was overwhelmingly driven by species turnover rather than nestedness, demonstrating that cities do not simply receive subsets of the surrounding fauna but actively reassemble communities. This filtering effect was, however, trophic-guild specific. Detritivores showed the highest replacement, consistent with a shift from natural to anthropogenic resource subsidies, while predators/parasitoids exhibited significant nested loss, aligning with their hypothesized sensitivity at higher trophic levels. Vegetation structure further clarified these patterns: vegetation density, not plant species richness, was the primary bottom-up driver for herbivore and predator/parasitoid diversity, whereas detritivores were decoupled from living plant biomass. These findings demonstrate that urban and near-natural habitats maintain distinct species pools via guild-specific assembly pathways, highlighting the need for guild-specific conservation strategies for urban biodiversity conservation. Extending beyond compositional analysis, we propose a temporal-abundance framework, classifying species by persistence and abundance, as a diagnostic tool for assessing ecological integrity and guiding conservation in urbanizing landscapes.