Drivers of bird species turnover in the largest archipelago of China: Insights from zeta (ζ) diversity and multi‐site generalized dissimilarity modelling ( MS ‐ GDM )

Species turnover, the spatial dissimilarity in species composition, is a central concept in island biogeography and community ecology. Traditional pairwise beta diversity metrics are limited to two-site comparisons and cannot fully resolve the mechanisms driving turnover across multiple communities. Zeta (ζ) diversity overcomes this limitation by quantifying multi-site species sharing and capturing how both rare and widespread species contribute to species turnover along a continuum. However, existing applications of zeta diversity to birds have focused mainly on continental, urban or land-use gradients, and have not examined island systems while simultaneously comparing phenological periods and migratory groups, nor have they separated richness differences from species replacement. As a typical land-bridge archipelago containing multiple bird groups, the Zhoushan Archipelago of China provides an ideal system for addressing these research gaps. We surveyed forest bird assemblages across 40 islands in this archipelago. We combined zeta diversity, multi-site generalized dissimilarity modelling (MS-GDM) and generalized additive models, using both Sørensen and Simpson normalizations to distinguish the contribution of richness differences from that of species replacement. We classified forest birds by season (breeding vs. wintering) and by migratory strategy (resident vs. migrant), yielding five ecological groups: all forest birds, breeding birds, wintering birds, residents and migrants. We then tested whether the relative importance of island characteristics varied among these five bird groups. Zeta decay followed a pure power-law form in all five bird groups, indicating a scale-invariant pattern of species loss across sites. Comparing MS-GDM results based on Sørensen-normalized and Simpson-normalized zeta diversity revealed a fundamental duality: island area primarily drove richness differences through selective species loss on small islands, whereas distance to the nearest mainland consistently governed true species replacement across all bird groups. Overall, our results demonstrate that deterministic assembly operates through two distinct pathways, that is area-driven environmental filtering and isolation-driven dispersal limitation, which jointly structure multi-site species turnover. The richness difference and replacement duality identified here can be generalizable to other land-bridge archipelagos and fragmented landscapes where area and isolation jointly shape community assembly, and provides a mechanistic framework for interpreting multi-site species turnover beyond simple richness gradients.