Beyond observed diversity: Community completeness negatively affects ecosystem invasibility across scales

Charles Elton proposed that species‐rich communities are more resistant to invasion, yet a well‐known paradox persists: native diversity suppresses invasion at small scales while correlating positively with non‐native species diversity at broad scales. Moving beyond native observed diversity alone to community completeness, defined as the ratio of native observed diversity to its specific species pool, may offer a potential framework to address this scale dependency. This is because the completeness index reflects the degree to which a habitat saturates its available niche space, shaped jointly by abiotic and biotic filtering in an origin‐blind manner. At small spatial scales, native observed diversity and dark diversity (suitable yet absent native species) are negatively correlated. This implies that high native observed diversity indicates high community completeness, leaving limited vacant niche opportunities for both un‐arrived native and non‐native species and thus reducing invasibility. In contrast, at large spatial scales, greater environmental heterogeneity can yield a vastly expanded regional species pool where dark diversity also accumulates more, potentially resulting in lower community completeness and heightened susceptibility. It should be noted that native community completeness, based on the extant species pool, fails to capture the niche spaces left by historical species losses—a gap tracked by temporal dark diversity dynamics—yet these hypervolume holes can instead be successfully exploited and predicted by non‐native community completeness. Following on this logic, I propose a Completeness‐Based Invasion Theory, which hypothesizes that community invasibility is inversely related to community completeness across spatial scales. While further empirical testing across diverse systems is required, measuring completeness alongside observed diversity may provide a novel, scale‐independent perspective to advance our understanding and management of biological invasions. Combining both native community completeness and non‐native community completeness to predict non‐native species invasions may currently represent the optimal approach. Read the free Plain Language Summary for this article on the Journal blog.