Niche adaptation mechanisms of coral reef fishes across disturbance gradients: a case study of Ctenochaetus striatus from the Xisha Islands

Human disturbance alters benthic resource availability in coral reef ecosystems, yet its effects on trophic niche structure at the island scale remain unclear. We quantified spatial variation in the trophic structure of the herbivorous reef fish Ctenochaetus striatus across eight reefs in the Xisha Islands spanning a gradient of anthropogenic disturbance. Stable carbon and nitrogen isotope analyses revealed systematic variation in δ 13 C and δ 15 N values among reefs, with consistent directional shifts along carbon and nitrogen gradients. Reefs with intensive human inhabitation and fishing showed elevated δ 15 N values, suggesting local enrichment of benthic nitrogen baselines by anthropogenic nutrient inputs. By contrast, δ 13 C values increased along the Anthropogenic Disturbance Index gradient, indicating a broader shift in basal carbon sources toward algal- and detritus-based pathways in more degraded habitats. Low-disturbance reefs supported broader trophic niches and more diverse resource use, whereas highly disturbed reefs exhibited niche contraction and convergence. Niche overlap analyses revealed a nested “narrow-within-wide” structure, with trophic niches from degraded reefs largely embedded within those of less disturbed reefs. These findings suggest that spatial variation in human disturbance is associated with consistent restructuring of trophic niche differentiation in coral reef fishes. • Stable isotopes revealed clear spatial shifts in carbon and nitrogen use by Ctenochaetus striatus across Xisha reefs. • A distance-based disturbance index provided a preliminary proxy for relative human disturbance among reef sites. • Reefs with long-term human habitation and fishing showed elevated δ¹⁵N values and enriched benthic nitrogen baselines. • Increasing disturbance was associated with higher δ¹³C values and a shift toward algal- and detritus-based carbon pathways. • Low-disturbance reefs supported broader niches, whereas disturbed reefs showed contraction, convergence, and nested overlap.