When coastal habitats fail: Transitioning from adaptation to vulnerability under habitat degradation

Coastal habitats provide critical protection by buffering natural hazards, sustaining biodiversity, and supporting local livelihoods. However, these functions are increasingly compromised by anthropogenic pressures that accelerate ecological degradation, amplifying the vulnerability of coastal environment. The research seeks to break the cycle of coastal habitat degradation and environmental vulnerability by evaluating the contribution of existing coastal habitats to mitigation of natural stressors, assessing the impact of anthropogenic stressors on these habitats, and quantifying scenarios of habitat loss on the combined risk posed by both natural and anthropogenic stressors. The CAVE (Coastal habitat Adaptation and Vulnerability under Emerging stressors) was established as an integrated framework to link habitat degradation with environmental vulnerability and to evaluate adaptation strategies for coastal resilience. The InVEST (Integrated Valuation of Ecosystem Services and Tradeoffs) coastal vulnerability and habitat risk assessment models were integrated with bivariate local indicators of spatial association analysis within the CAVE framework. It was used to evaluate both natural and anthropogenic stressors on Lyudao, Taiwan, under three simulated habitat condition scenarios. Results show that natural habitats significantly reduce exposure, with 58.1% of the coastline experiencing lower hazard levels due to their buffering effects. Anthropogenic stressors, particularly wastewater discharge and fisheries, overlap with natural hazards to create concentrated risk hotspots along the northwestern coast. The scenario results demonstrate that reductions in both habitat area and quality substantially increase vulnerability, with the combined share of high and very high exposure averaging 41.2% under degraded scenarios. These findings highlight the reinforcing cycle between habitat degradation and environmental vulnerability. To break this cycle, the study identifies pathways through ecosystem-based coastal defense, emphasizing habitat conservation, targeted regulation, hybrid ecological-engineered strategies, and stakeholder engagement for sustainable coastal governance and long-term adaptation. By explicitly linking habitat degradation to shifts from adaptation to vulnerability, this study provides decision-relevant evidence for small-island managers and policymakers to prioritize ecosystem-based coastal defense, targeted regulation of key anthropogenic pressures, and hybrid adaptation strategies.