This paper looks at (1) the abilities of artisanal fishers in Roviana Lagoon, Solomon Islands to monitor long-term ecological change occurring to seagrass meadows near their communities; and (2) their understandings of the dynamic drivers of change. When local resource users detect, understand, and respond to environmental change they can more effectively manage environmental resources. In a comparison of two villages, it documents local resource users’ abilities to monitor long term ecological change occurring to seagrass meadows near their communities, their understandings of the drivers of change, and their conceptualizations of seagrass ecology. Local community leaders in this region exercise governance and management over the use of and access to natural resources in the lagoons and the adjacent coastal areas within their respective customary land and sea estates. Despite this system of indigenous land and sea tenure, population growth and growing development pressures have begun to overwhelm local governance controls and undermine sustainable resource use. Increasingly, the lagoon ecology and the social and political stability of the region are under threat. Local observations of ecological change are compared with historical aerial photography and IKONOS satellite images that show 56 years of actual changes in seagrass meadows from 1947 to 2003. Results suggest that villagers detect long-term changes in the spatial cover of rapidly expanding seagrass meadows. However, for seagrass meadows that showed no long-term expansion or contraction in spatial cover over one-third of respondents incorrectly assumed changes had occurred. Examples from a community-based management initiative designed around indigenous ecological knowledge and customary sea tenure governance show how local observations of ecological change shape marine resource use and practices which, in turn, can increase the management adaptability of indigenous or hybrid governance systems.