Algal overgrowth on coral reefs threatens reef fish biodiversity worldwide, yet how this pressure varies among reefs of different intact statuses remains unclear. Using a global dataset spanning 2006–2023, we examined how cover by low-lying algae and macroalgae modulates the relationship between live hard coral cover and fish species richness. Our results demonstrated that overgrowth of both algal forms attenuated the positive association between coral cover and fish species richness, with context-dependent thresholds: the effects of low-lying algae were observed when coral cover was below 45%, whereas macroalgae effects persisted until 65% coral cover. Within these ranges, the per-unit reduction in fish richness due to algal overgrowth became more pronounced as coral cover increased, indicating that reefs with greater intactness are disproportionately susceptible to biodiversity losses when algal expansion occurs. Trait analyses showed that the attenuation strength varied among species according to water-column position, gregariousness, diet, and diel activity. Collectively, these results highlight that algal overgrowth can erode the biodiversity-supporting function of coral reefs even where live coral cover remains moderate to high, underscoring the importance of managing algal expansion alongside coral protection to sustain reef fish diversity.

coral reefs, algal overgrowth, fish biodiversity, functional trait