The limits of an exclusively ecological approach to the history of human society are quickly reached when considering the nature of demand. As we have already seen, the list of goods that might be considered essential in a given situation, the limited availability and uneven allocation of which might stimulate the development of systems of distribution, is long and varied. It is certainly not determined by nutritional requirements alone; except in famine situations, human diets reflect social and cultural preferences (Garnsey 1999). It can be useful to construct a model of the minimum volume of grain required for the basic sustenance of a given population, in order to estimate the carrying capacity of a region or to give an order of magnitude for the minimum level of redistribution required to keep people fed (Garnsey 1988; Hopkins 1983b).