The question of definition is unavoidable since the criteria of overpopulation are arranged along an elastic continuum. The most common theme is the "carrying capacity" of environments in relation to species (Ehrlich is essentially a macro-biologist and that is where the dominant conceptual frameworks originate.) This may make sense in relation to (say) the elephant population of the Kruger National Park, but it seems fairly obvious that you could never really know the carrying capacity of an environment in relation to a species capable of trade and technical innovation. An alternative is to define carrying capacity in relation to the effect on other species, but this is obviously problematic since at least 95 per cent of all species which have ever existed are extinct and extinction is a crucial mechanism of evolution; in any case, homo sapiens has a great, unique and increasing capacity to create other species. The weakest definition — though, perhaps, the most interesting — is when Ehrlich defines overpopulation as occurring when "human numbers are incompatible with human values". Naturally, I would prefer a world with only a couple of billion people (and 30 million in the UK), but I wouldn't give up my reproductive rights for that. And it does raise the logical possibility that any solution to the problem would be more damaging to "human values" than the problem unsolved.

For the most famous exponent of population theory, Thomas Malthus, the consequences were clear: a rather unpleasant, though thoroughly familiar, homeostatic mechanism. As the population increases beyond the capacity to feed it starvation and malnutrition will restore equilibrium. Not nice, but not new. We should remember that in the lifetime of Malthus (born in 1766) it could reasonably be argued that neither the standard of living nor the level of population were greater than they had been 1500 years earlier when the Roman Empire was at its height. When Malthus was born agriculture and transport, for example, were no better than Roman practices, though by the time of his death, in 1834, the story was very different. Our contemporary theorists, however, considering overpopulation in the context of much larger numbers, but also indefinite technical progress, tend to see the consequences in terms of some kind of breakdown or apocalypse. Ehrlich has actually referred to "the collapse of civilisation", a form of imagery better suited to the cinema than to scholarship. The nearest thing we know of to a "collapse of civilisation", the end of the Roman Empire in the West, must have looked like homeostasis to some and apocalypse to others, depending on who you were and from what distance you were perceiving events.