Limiting Factors


Tom Easton


In my last essay, I discussed the question of whether there are limits to growth. I noted that endless growth—in population, in fossil fuel use, and even in the global economy–is not possible and that those who insist that it is are guilty of crimes against humanity.

Now I would like to introduce the ecological concept of limiting factors. Simply put, a population can grow only until it uses up the resource in least supply. That resource is the limiting factor.

Consider a car factory. It has fifty car bodies, three dozen chassis, eighty engines, and twelve wheels, with tires. How many cars can it make?

Three, if each one has four wheels. With a little reengineering, you could get four three-wheeled cars. All the bodies, chassis, and engines beyond that are wasted. Wheels are the limiting factor.

The same line of thought applies to populations of humans, other animals, and plants. Since we are chiefly concerned with humans, we can ignore other living things (though they must obey the same rules).

Before we go further, what are the human equivalents of car bodies, chassis, engines, and wheels? We need living space, food, water, and air. Beyond those basics, there is a long list of things we would miss so badly if civilization collapsed (how about toilet paper?) that we might consider them needs as well.

But back to the basics. Living space is not a limiting factor. As is often pointed out by people opposed to idea that there is overpopulation, the entire global stock of humans could be put in Texas, with room left over. Of course, that doesn’t count the farms needed to feed the crowd, or the forests needed to supply the TP.

Air—or the oxygen in it—isn’t a limiting factor either. There is much more than we need, at least as long as we don’t kill off the green plants that replenish the oxygen for us.

Food, on the other hand, is definitely a limiting factor. Russell Hopfenberg points out that “Human Carrying Capacity Is Determined by Food Availability,” Population & Environment (November 2003). That is, you can’t grow more people than you can feed.

Components of food such as vitamins and minerals are also important, but shortages of these can be made up, at least as long as we can make vitamin pills.

Water—fresh water–comes in because although we need some for drinking and washing, we need a great deal more for irrigating food crops. If we don’t have enough water, we don’t have enough food. And unfortunately, global climate change (or warming) is projected to increase the frequency of droughts and heat waves, as well as to diminish snow in mountains such as the Sierra Nevadas and even the Himalayas, as well as to make that snow melt earlier in the spring, which will mean less water available during the growing season.

But surely if less water runs down the rivers, farmers can just pump more out of the ground? The trouble is that they’ve been doing that for decades already. Water tables are dropping, so that it is harder to pump water to the surface. Some aquifers are nearing exhaustion. Try these links for the straight dope:

See also David Pimentel, et al., “Will Limited Land, Water, and Energy Control Human Population Numbers in the Future?” Human Ecology (August 2010).

It seems reasonable to say that the ultimate limiting factor for human beings is water. There is, of course, a huge supply of salt water in the oceans. It’s possible to remove the salt from, or desalinate, sea water, but that is a very energy-intensive process. If we become dependent on that, energy supply may be the ultimate limiting factor. There is also a lot of fresh water locked up in glaciers and especially in the ice that covers Antarctica. There have been proposals, and pilot projects, to tow icebergs to places that need water and let them melt. The latest such proposal is described by Avery Thompson, “Cape Town Wants to Tow an Iceberg to Solve Its Water Shortage,” Popular Mechanics (July 6, 2018) ( But that possibility is only good until global warming melts all the ice. That day is centuries off, we think, but…








“Carrying capacity” is another crucial ecological concept. It is defined very simply as the size of the population that the environment can support, or “carry,” indefinitely, through both good years and bad.  It is not the size of the population that can prosper in good times alone, for such a large population must suffer catastrophically when droughts, floods, or blights arrive or the climate warms or cools.  Of course, really bad times do happen, and both local populations and species can go extinct as a result. But you want to put that time off as long as possible.

Carrying capacity is a long-term concept, where “long-term” means not decades or generations, nor even centuries, but millennia or more. It is thus in alarming contrast with what human society’s decision-makers think of as long-term—at most, five or ten years.

What is Earth’s carrying capacity for human beings?  It is surely impossible to set a precise figure on the number of human beings the world can support for the long run.  As Joel E. Cohen discusses in How Many People Can the Earth Support? (W. W. Norton, 1996), estimates of Earth’s carrying capacity range from under a billion to over a trillion.  The precise number depends on our choices of diet, standard of living, level of technology, willingness to share with others at home and abroad, and desire for an intact physical, chemical, and biological environment (including wildlife and natural environments), as well as on whether or not our morality permits restraint in reproduction and our political or religious ideology permits educating and empowering women.  The key, Cohen stresses, is human choice.

It is worth stressing that we get the largest carrying capacity only with minimal per capita use of resources (or minimal standard of living). Think of living in a grass shack and burning dung to cook your daily rice or beans. No doctors or hospitals or supermarkets or cars or… And then we are subject to disease and famine as we have not been for the last couple of centuries.

If you want a higher standard of living—something like what you are used to today, if you’re reading this online—you will need a much smaller population. If you want that standard of living for everyone, you may need a world population of less than a billion.

Getting there is the problem. I doubt very much that we can do it voluntarily. But running down our fresh water supplies might do the job for us.


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