Sustainability is a complex and sometimes fragile condition, as shown by this case study from Zambia's North-Western Province. The long-term evolution of a cassava-based agricultural system provided enough staple food for a rapidly growing population of immigrants and refugees, but a mealybug invasion in 1985-1989 destroyed much of the cassava and triggered a famine for many people. The famine, caused by the interaction of ecological, political, and economic factors, demonstrated that locally self-sufficient sustainability was an illusion. Affected villagers tried to cope by growing maize and buying maize imported from other regions. But if they used only local resources, many people could not protect themselves, and they suffered severe food shortages. By 1989, the mealybug had apparently been controlled by a biological control program (as much political as technical) that coordinated institutions on three continents. This case demonstrates that: (1) resourceful villagers are an essential dynamic element in evolving food-production systems and in coping with famine; (2) localities are ecologically, politically, and economically incorporated and vulnerable, rather than being isolated and self-sufficient; and (3) biodiversity is essential in a world of not-yet-recognized pests (and diseases).

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