I’m often frustrated by the poor quality of discussion of the problem of overpopulation (if indeed it is a problem). It seems to me that almost all participants to the discussion have missed one of the most important insights of evolutionary theory, an insight attributable to Malthus.
The population of any species in any closed habitat would rise geometrically, but it cannot, because it always hits a “ceiling”. This ceiling is mostly set by supply. I mean to construe “supply” in the most general terms – usually what matters is the availability of such necessities for living as food, water, and light. But it can include more, such as the availability of ornaments used by bower birds in sexual selection.
Where weeds can grow, weeds do grow. The weed population expands, and only stops expanding when overcrowding prevents further expansion. Where bower birds can build ornate bowers, bower birds do build ornate bowers. The bower bird population expands, and only stops expanding when the quality of the poorest-quality bowers is too low for their builders to have realistic hopes of getting chosen by female bower birds, and hence to reproduce. In both cases, the population bounces along the ceiling like a helium balloon that has slipped out of a child’s grasp. The ceiling is set by supply, although what needs to be supplied differs sharply from one species to the next.
Of course there is an attrition rate: some members of the population are picked off by predators. But that rate is set by the population of the predators, which in turn is set by their food supply – in other words, by the replacement rate of the population they prey upon, which is exactly where we started.
How much each individual consumes of the supply decides how many individuals there are. For example, a given field that can sustain a population of 100 rabbits might only be able to sustain 10 sheep, or 50 rabbits and one fox.
So two components determine the number of individuals: the supply and the rate of consumption. (Perhaps I mean “demand” here, but I know nothing about economics, and I don’t want to suggest that I am talking about anything other than biology.)
The human population rose dramatically in recent centuries, not because humans decided they wanted to have more children, or because they became more sexually promiscuous, or because many generations have passed since The Great Flood, or even much because advances in sanitation and medicine lowered the attrition rate. It was mostly because food became easier to procure, thanks to cheaper energy and advances in agricultural technology.
So although there may be a human overpopulation problem, an increase in the population is a sign of good things happening, or at least of good things having happened. Although there may be trouble ahead, the trouble will not be that the expanding population finally “hits a wall” of the Earth’s “carrying capacity”. That “wall” is better understood as a ceiling, and the population has always been already at that ceiling. It is hardly ever acknowledged that the normal condition of the Earth is to be at “carrying capacity”, and that at all times some places enjoy a surplus while others suffer a famine, with the same statistical inevitability as floods and droughts. The trouble is not that we hit a wall or a ceiling but that the ceiling might start to get lower. This could happen if energy to produce food became significantly more expensive. The reality of a lowering ceiling is famine.
There are two obvious ways to lower the population, if indeed that is a good thing to do. The first is to artificially lower the ceiling by limiting the supply. The second is to increase consumption of each individual so that the same habitat can sustain fewer individuals. Suppose we are again considering grazing animals in a given field of grass. In effect, the first solution is to lower the number of rabbits by having less grass. The second solution is to lower the number of grazers by turning the rabbits into sheep.
In the case of rabbits and sheep, the “supply” is of grass. In the case of humans, the “supply” is of more costly and abstract items, things more like the ornaments of bower birds. All humans need an education, for example, although giving a child a good education generally entails having fewer children. In increasingly affluent countries, humans get increasingly ambitious about their need for houses, and cars, and expensive clothes, and foreign holidays, and memberships to golf clubs. Although we may disapprove of the levels of consumption here, we should remind ourselves that such levels are a good way of keeping the population down. People who have high expectations for their children have fewer of them, and invest more in each. This explains why as societies become more affluent, life becomes less cheap, and the birth rate generally drops.