Habitat Structural Complexity Mediates Coral Reef Resilience and Biodiversity Loss Across Anthropogenic Disturbance Gradients

<title>Abstract</title> <p>Coral reef ecosystems are increasingly threatened by the combined effects of climate change and local anthropogenic pressures, yet the mechanisms underpinning their resilience remain incompletely resolved. In particular, the extent to which habitat structural complexity mediates ecological stability across disturbance gradients has not been fully quantified. Here, we provide an integrative assessment of the interactions among structural complexity, coral cover dynamics, and fish assemblages across a well-defined gradient of anthropogenic disturbance in a tropical reef system. We combined underwater visual censuses, benthic photo-quadrat analyses, and in situ rugosity measurements across 24 reef sites, complemented by environmental datasets including sea surface temperature anomalies, nutrient concentrations, and proximity to coastal settlements. Long-term coral cover trajectories were further evaluated using a 15-year dataset. Reefs exposed to high anthropogenic pressure exhibited substantial declines in live coral cover (&lt; 38%), shifts toward macroalgal dominance, and marked reductions in fish richness and biomass. In contrast, structurally complex reefs consistently supported higher biodiversity and displayed greater resistance to thermal stress events. Structural complexity emerged as a strong predictor of both coral persistence and fish diversity, highlighting its role as a key ecological buffer. Multivariate analyses identified nutrient enrichment and thermal anomalies as the primary drivers of reef degradation, with local stressors amplifying climate-induced impacts. Importantly, structurally complex reefs retained higher functional integrity and recovery potential even under elevated environmental stress. These findings advance current understanding of coral reef resilience by explicitly linking habitat complexity to ecological responses across spatial and temporal scales. We demonstrate that effective conservation strategies must integrate the protection of structural complexity with the mitigation of local stressors to enhance reef persistence under accelerating climate change.</p>