Algal overgrowth disproportionately undermines fish species richness in coral-rich reefs

Algal overgrowth on coral reefs threatens reef fish species richness worldwide, yet the variation in this pressure across gradients of coral cover 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 groups attenuated the positive contribution of coral cover to fish species richness, with context-dependent thresholds: the effects of low-lying algae were observed when coral cover was below 45%, whereas the effects of macroalgae persisted until 65% coral cover. Notably, within these ranges, the negative impact of algal overgrowth on fish richness was linearly amplified as coral cover increased. Trait analyses showed that the attenuation strength varied among fish species according to their diet and body size. By synthesizing findings on specific coral-cover thresholds for algal impacts and fish trait-mediated vulnerabilities, our study provides a strategic basis for algal management designed to prevent rapid declines in fish species richness and protect highly sensitive functional guilds, thereby ensuring the stability of coral reef ecosystems.