Vegetation change threatens the sustainability of cultural keystone species and traditional ecological knowledge in a social-ecological system.

Traditional ecological knowledge (TEK) and cultural practices are often closely linked to local biodiversity through keystone species. However, the long-term ecological dynamics affecting the utility of such species remain underexplored. This study combines TEK with 40 years of vegetation monitoring in a Japanese wetland to examine how ecological change affects culturally important thatch. Interviews and sample observations categorized species into four ranks, from essential to undesirable for thatching. This index was then applied to analyze long-term vegetation monitoring data. Conventional indices such as species richness and the Shannon-Weaver index showed no clear trends, while NMDS detected shifts in species composition. However, these shifts were not directly linked to cultural usefulness. In contrast, the usefulness-based assessment revealed a substantial decline in species essential for high-quality thatch and an increase in undesirable species. This shift suggests a decline in thatch quality and a potential threat to the sustainability of the social-ecological system reliant on thatch use. Our findings demonstrate the value of integrating TEK with ecological monitoring to detect ecologically and culturally significant change. This interdisciplinary approach yields insights into social-ecological interactions that would be missed by standard metrics, and can inform context-sensitive management strategies to support system resilience.