Beyond the SLOSS debate: patch area shapes turnover and nestedness of multi-taxa β diversity in fragmented landscapes

Habitat fragmentation reshapes biodiversity not only by reducing species richness, but also by altering patterns of community differentiation across space. Although island biogeography theory predicts strong effects of habitat area and isolation on species distributions, it remains insufficiently resolved how these spatial attributes structure β diversity and its turnover and nestedness components, across multiple taxa within a shared fragmented landscape. Here, we ask whether patch area and isolation regulate β diversity through consistent mechanisms across taxa, and whether species’ dispersal ability modulates the balance between turnover- and nestedness-driven β diversity in fragmented landscapes. Using a land-bridge island system formed by reservoir inundation in eastern China, we quantified β diversity patterns for woody plants, insects, spiders, and breeding birds. We focused exclusively on spatial attributes derived from island biogeography theory, patch area and isolation, and partitioned overall β diversity into turnover and nestedness components to evaluate their relative contributions across taxa and dispersal groups. Maximum patch area emerged as the dominant predictor of β diversity in mobile taxa, whereas woody plants were more strongly influenced by maximum isolation and minimum patch area. Across all taxa, increasing patch area consistently shifted β diversity from turnover-dominated to nestedness-dominated patterns. Within taxa, dispersal ability further modulated these patterns: high-dispersal groups exhibited higher turnover contributions, whereas low-dispersal groups were characterized by a stronger reliance on nestedness-driven β diversity. Our results show that pacth area and isolation regulate regional biodiversity primarily by redistributing the relative importance of turnover and nestedness, rather than by uniformly increasing compositional dissimilarity. By integrating β diversity mechanisms into island biogeography and fragmentation theory, this study reveals how large and small patches play complementary, process-dependent roles in sustaining biodiversity, thereby extending the SLOSS debate beyond species richness toward a mechanistic, multi-taxon framework.