Predators can facilitate herbivory in nutrient-limited marine ecosystems

Predators influence ecosystem functioning through consumptive and non-consumptive effects. Recent studies suggest that predators can also be an essential source of limiting nutrients in ecosystems such as coral reefs, potentially influencing prey ecology through nutrient input via their excreta. With rising commercial fishery, mesopredatory fishes are being selectively harvested from reefs. Yet, there is incomplete knowledge of the consequences of this extraction on essential ecosystem processes. Using field experiments and observations, we examined how mesopredatory fishes influence herbivory along a fishing-induced mesopredatory fish biomass gradient in the Lakshadweep Archipelago in the northern Indian Ocean. We found that mesopredatory fish excreta have greater proportion of phosphorus than nitrogen. Along the gradient, primary and secondary productivity increased, after accounting for pelagic nutrient subsidies. Further, herbivory rates increased with increasing mesopredator biomass, while prey anti-predator response remained unchanged. Our results suggest that mesopredator-induced alterations of nutrient stoichiometry stimulate primary and secondary productivity and enhance herbivory in phosphorus-limited coral reefs, particularly in systems experiencing mesopredator release following selective fishing of apex predators. Our study shifts focus from the traditional top-down role of predators, highlighting an overlooked bottom-up pathway by which mesopredators can influence ecosystem functioning. Global decline of predators could modify ecosystem processes in ways that are yet unknown, leaving them increasingly vulnerable to future disturbances.