Functional Redundancy in Coral Reef Systems

Abstract

Functional redundancy -- the number of species performing similar ecological roles -- is a critical determinant of ecosystem resilience, with high redundancy buffering communities against species loss and maintaining ecosystem function following disturbance. Coral reef fish communities provide an ideal system for testing functional redundancy theory because their species diversity is exceptional, functional roles are well-characterised, and reef systems are experiencing accelerating disturbance from bleaching, storms, and fishing pressure. This study quantified functional redundancy across 42 Indo-Pacific coral reef sites spanning a bleaching impact gradient (three bleaching events 2016, 2020, 2022; coral cover range 8-68%) and a fishing pressure gradient (unfished marine reserves to heavily fished), using underwater visual census data (2,840 species-abundance observations from 284 transects) and 12-dimensional trait data for 184 reef fish species. Mean functional redundancy (species per functional entity) was 3.84 +/- 0.84 at unfished, high-coral-cover sites and declined to 1.24 +/- 0.42 at heavily fished, bleaching-degraded sites -- a 67.7% reduction. Functional vulnerability -- the proportion of functional entities represented by a single species -- increased from 18.4 +/ 4.8% at reference sites to 52.4 +/- 8.4% at degraded sites, identifying a large fraction of functional roles as extinction-vulnerable in degraded reefs. Bleaching impact and fishing pressure showed independent, additive effects on functional redundancy (two-way ANOVA, both p < 0.001; no significant interaction p = 0.42), with fishing pressure having a 1.84-fold larger effect per unit standardised predictor than bleaching. Recovery trajectory analysis showed that functional redundancy recovered 84.4% of reference values within 8 years at unfished, fully protected sites but only 28.4% at sites with ongoing fishing pressure. These results demonstrate that controlling fishing pressure is the single most impactful management intervention for maintaining functional redundancy and reef resilience under climate-driven bleaching.