Marine Debris Undermines Herbivory in Coral-Algal Competition

Coral reefs are facing an emerging risk of coral-to-algae phase shift at global scale. Herbivory is known as the primary biological mechanism preventing phase shift. In contrast, marine debris has been recognized as an increasing threat to coral reef health. This paper examines whether these two factors are independent or synergistic from three aspects: marine debris impacts, the role of herbivory, and limited but growing evidence exploring interactions between them. The impact of marine debris on coral includes physical abrasion, chemical toxicity, and pathogen introduction. Microplastic ingestion has also been documented in fishes from seemingly unaffected reefs in remote areas. Herbivory has been found to influence the outcome of coral-algal competition through the timing of grazing relative to coral establishment and substrate availability, but this effect is highly variable depending on the spatial and temporal context of grazing activity and differs greatly among herbivore functional groups. Critically, herbivory itself can be disturbed by marine debris. These harms can take the form of both direct ingestion by scraping and browsing fish and behavioral avoidance of debris-covered substrate. This suggests that the effects of debris and herbivory loss may not be independent but instead potentiate. This paper proposes that they may produce a combined effect on coral-algal competition that may exceed what either factor would produce alone, although this compounded pathway has not yet been directly tested. These findings are crucial for reef management, as it may change conservation priorities from debris removal or herbivore protection alone to a combined effort.