The economy and ecology of Easter Island were relatively simple, but obtaining a tractable mathematical representation still requires significant abstraction and approximation. At a minimum, it is necessary to model the behavior of two ‘‘stocks’’ or ‘‘state variables’’: the forest stock and the population. In fact, it is an oversimplification to focus just on the forest stock. Even in the absence of the forest stock, grass, shrubs, and certain vegetables could be grown and this biomass was sufficient to support a diminished but significant population. Rather than introduce a third ‘‘stock’’ representing other biomass, it is simpler to aggregate the renewable resource base growing on the land into a single resource stock, denoted as S. For simplicity, sometimes just the term ‘‘forest’’ is used, but it should be kept in mind that other components of the flora biomass are also relevant.