[I]ntensification in agriculture and resource production follows a pattern of declining marginal productivity [...] rationally-acting human populations will first exploit those resources that yield the best return per unit of effort, and still meet the needs of the population. If this is so, then it follows that any change in resource extraction must be in the direction of using resources that are more costly to obtain, process, distribute, and/or market, so that the marginal product of labor and other inputs declines. Thus, hunters and gatherers first exploit foods that are higher in nutritional value, and easier to obtain and process, than resources that are less favorable for these characteristics [...] In other spheres of resource extraction, minerals and energy forms can be ranked in terms of their ease of discovery, extraction, processing, and use. Resources that rank higher in these dimensions will be used before resources that don't, and when these are no longer sufficient, secondary resources will be employed. [...] Among whatever set of resources a population obtains, for whatever reasons, the law of diminishing returns is likely to apply. As demand for a commodity grows, increased production will at some point mean depletion or insuffciency of the least costly sources. At that point, more costly sources must be used, with declining marginal returns.