Famine isn't unique to humans. All species are subject to it everywhere in the world. When the population of any species outstrips its food resources, that population declines until it's once again in balance with its resources. Mother Culture says that humans should be exempt from that process, so when she finds a population that has outstripped its resources, she rushes in food from the outside, thus making it a certainty that there will be even more of them to starve in the next generation. Because the population is never allowed to decline to the point at which it can be supported by its own resources, famine becomes a chronic feature of their lives.