Functional Redundancy and Ecosystem Stability

Abstract

Functional redundancy -- the degree to which multiple species in a community perform the same ecological role, such that the loss of any single species does not eliminate that function from the community -- has been proposed as a primary mechanism by which biodiversity buffers ecosystem function against environmental perturbations, yet the quantitative relationship between redundancy and ecosystem stability remains incompletely characterised across real-world ecological systems subjected to natural and anthropogenic disturbances. This study quantified functional redundancy (FR) using the redundancy index (RI = species richness - functional richness / species richness) and three complementary metrics across 42 grassland ecosystems in Europe (Switzerland, Sweden, France; 2,840 species abundance records; 284 plant and 48 invertebrate species; 12-trait functional trait matrices; 2018-2022), and correlated FR with four ecosystem stability metrics (temporal variability of biomass production, resistance to drought disturbance, resilience post-disturbance, and productivity under fire perturbation). FR was significantly positively correlated with all four stability metrics (r = +0.74 to +0.84; all p < 0.001), confirming the insurance hypothesis: communities with higher functional redundancy showed lower temporal biomass variability (CV of ANPP: r = -0.84 with FR; p < 0.001) and faster post-drought recovery (r = +0.74; p < 0.001). Structural equation modelling confirmed that FR mediated 68.4% of the positive effect of species richness on stability -- confirming that the diversity-stability relationship operates primarily through the redundancy mechanism rather than directly through richness. A Functional Redundancy-Stability Index (FRSI) integrating RI, stability temporal CV, and recovery rate predicted ecosystem service provisioning reliability with AUC = 0.864.