Multidimensional β diversity reveals the interplay of dispersal limitation and ecological filtering in structuring land‐bridge island butterfly communities

The theory of island biogeography (TIB) explains species richness patterns, yet its extension to compositional dissimilarity (β diversity), particularly from multidimensional perspectives, remains limited. Using butterfly assemblages across 44 land‐bridge islands in the Zhoushan Archipelago, China, we applied an integrative framework to unravel how island geography and species traits shape multi‐faceted β diversity. We partitioned taxonomic, phylogenetic, and functional β diversity into turnover and nestedness components, quantified species and site contributions, and distinguished roles of widespread versus restricted species. Key findings reveal: 1) dimensionality‐specific geographic drivers: inter‐island isolation differences primarily influenced nestedness in taxonomic and phylogenetic dimensions, suggesting its role as a filter for ordered species loss. In contrast, island area differences consistently affected both turnover and nestedness across all dimensions, underscoring its role as a master filter of resources and habitat. Island geographic attributes (e.g. isolation) only explained local community uniqueness in the taxonomic dimension. 2) A trait‐mediated mechanism: species with weaker dispersal capacity (smaller thorax area) contributed more to β diversity (higher species contribution), providing direct trait‐based evidence for dispersal limitation. 3) Internal metacommunity structure: nestedness was predominantly structured by widespread species, whereas turnover was driven mainly by the idiosyncratic distributions of restricted species. Our study demonstrates that island community assembly emerges from deterministic filtering (area and isolation shaping the widespread species backbone) interacting with stochastic processes (governing restricted species). This integrates TIB with β diversity by showing that TIB's core predictors (area and isolation) differentially shape the components of β diversity and offers a multidimensional, mechanistic framework for island conservation that emphasizes protecting island networks, source islands, and unique species and habitats.