Redundancy hypothesis implies function is maintained until:
The redundancy hypothesis groups species with overlapping ecological roles. Loss of one member may have little immediate effect because another can compensate, but function declines after losses remove enough members—or a uniquely important member—to cross the system's effective functional threshold. Biodiversity–function hypotheses differ in how they assign ecological importance among species. Complementarity emphasizes distinct contributions, redundancy emphasizes overlap and compensation, rivet theory emphasizes cumulative weakening, keystone theory emphasizes particular influential species
Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 20