Intensive grazing by green turtles depletes seagrass meadows and constrains turtle growth in Kume Island, Japan.

Conservation efforts have led to substantial recovery of green turtle (Chelonia mydas) populations globally, yet over- or intensive grazing by these megaherbivores is now degrading seagrass ecosystems. We assessed the impacts of turtle-driven grazing and its feedback on turtle nutritional status and growth at Kume Island, Japan, through a comprehensive study conducted between autumn 2023 and spring 2025. Our approach combined seagrass surveys, drone-based density estimation, capture-recapture studies, and stomach content analysis. The surveys revealed a 50.6% reduction in seagrass area since the 1990s, alongside a species shift from mid-sized, long-lived taxa (Thalassia hemprichii) to small, fast-growing species (Halophila ovalis). This species shift was consistent with changes observed in the turtles' diet composition. The foraging aggregation was dominated by immature turtles, with turtle densities reaching 452.6 individuals per km2, among the highest reported. Body condition indices from 342 captured turtles revealed temporal fluctuations, with severe emaciation in autumn 2023 and spring 2025. The mean growth rate in straight carapace length was only 0.31 ± 1.57 cm/year, substantially lower than other foraging populations. Collectively, our findings demonstrate that intensive grazing by green turtles has fundamentally altered the structure of seagrass habitats, with direct negative feedbacks on turtle nutritional status and growth performance.