The illusion of functional stability: functional redundancy does not equal functional insurance in a natural-engineered aquatic continuum.

Functional redundancy-the capacity of multiple species to perform similar ecological functions-is widely regarded as a buffer that stabilizes ecosystem processes. However, whether functional redundancy truly translates into functional insurance remains unverified in microbial communities. Here, by integrating metagenomics and metatranscriptomics from 52 samples across a natural-engineered aquatic continuum in the Yanjin River watershed, China, results show that functional redundancy and functional insurance are fundamentally decoupled. We constructed a dual-layer analytical framework linking genomic functional overlap (effect trait) with transcriptional response heterogeneity (response trait) and developed a functional insurance index (FII) that quantifies whether redundant species respond differently to environmental variation. Our results reveal a striking paradox: engineered systems harbor the highest genomic functional redundancy yet the lowest functional insurance, because their functionally redundant species respond synchronously to perturbations-creating an illusion of stability. In contrast, natural communities exhibit lower absolute redundancy but significantly higher insurance, driven by diversified transcriptional responses among functionally similar species. Critically, only FII-not redundancy, response diversity, or asynchrony alone-significantly predicted CO