Biological invasions are reshaping communities worldwide, yet their mechanistic links to their functional organization have rarely been quantified at broad spatial scales. Classical theories invoke biotic resistance and biotic acceptance. However, empirical understandings that integrate functional structure and invasion dynamics across spatial scales still lacks a unified framework thereby hindering biodiversity conservation strategies. Here, we integrate multi-gene eDNA metabarcoding data from 139 watersheds across the western United States with a harmonized trait database for fishes, amphibians, mammals, and invertebrates to quantify how functional organization of communities governs invasibility by nonnative species. We characterized multiple dimensions of functional diversity including richness, redundancy, evenness, originality, and trait-space vacancy of communities, and related them to watershed-level invasion indices. Across watersheds, the proportion of unoccupied functional trait space i.e., functional vacancy, emerged as the strongest predictor of invasibility (R² = 0.51, P < 0.01). Communities exhibiting high functional redundancy and compact trait distributions resisted invasion, whereas those with greater vacancy and reduced overlap among species were more susceptible. Environmental gradients, particularly stream temperature, stream gradient, and habitat complexity, indirectly shaped invasion risk by altering community functional architecture that in turn affected the opportunity for nonnative species to establish and persist. Together, these results unify classical invasion hypotheses by mechanistically linking functional trait-space structure to invasion opportunity within a quantitative, trait-based framework from eDNA data, showing that invasion opportunity arises from measurable gaps in community trait space.
Keywords: Functional diversity, trait-space vacancy, biological invasions, eDNA, watershed, ecosystems, community invasibility