People on the island of Gaua in northern Vanuatu have witnessed a series of demographic transformations over the past one hundred years, from Anglican mission-driven resettlement to more recent regional migrations from nearby islands. The distinctions which organized social life in Gaua's past have given way to a new division between indigenous landowning families and various communities of non-indigenous "renters," people from elsewhere permanently settledin the island's east. Kastom, the category of worldviews and associated practices regarded as autochthonous, has provided indigenous Gauans with a sense of cultural-historical continuity in the face of such extensive change. This dissertation examines two interrelated fields of action and experience--ethics and human-environment interaction--to address questions of how kastom provides continuity and how it is challenged by the exigencies of contemporary life in a small subsistence society in insular Melanesia. Indigenous Gauans describe a set of ethical capacities and responsibilities which are particular to them as kastom persons--that is, as "persons of the place" who trace local tribal connections to land and to each other. Such persons perceive shared human- ontological traits of situational risk and frailty as catalysts for ethical responsibility, fulfilled through expressions of care made possible through productive subsistence work. They understand situational vulnerability as moments of possibility for creating and maintaining social relations, producing meaningful futures for self and others, and affirming identities as autochthonous persons with unique moral attributes. Recent changes to subsistence regimes, owing to such factors as NGO-led conservation initiatives, shifting climatological patterns, and the introduction of new fishing technologies, have motivated concerns about another type of vulnerability--that of local ecologies to anthropogenic disturbance. Discourses of ecological vulnerability, disseminated by external agencies and locally transformed, tend to reduce Gaua's residents to uniform statuses of "stakeholder" or "vulnerable subject." These discourses and their associated practices erase ethically relevant distinctions among persons with respect to the productive capacities and responsibilities comprehended as kastom. For indigenous Gauans, kastom emerges as the precarious space for possibility--the locus of struggle for cultural- historical continuity i a changing social and ecological landscape.