The Ecology of Overpopulation


One of the more contentious bun fights among environmental and ecological activists is over the role of overpopulation in the anthropogenic deterioration of the natural world. The debate coalesces loosely into two opposing camps: the overpopulation camp and the overconsumption camp.

The former insists that raw human numbers play a decisive role in the growing ecological damage.  As evidence, the adherents point to the parallel curves of energy consumption, waste generation, resource depletion and population growth.  They argue that aggregate human activity is responsible for much of the damage, and that a reduction in population would automatically result in both less ongoing damage and a greater opportunity for the Earth’s systems to heal themselves.

On the other side of the coin are those who locate the problem in our consumption habits.  They point to the relative consumption patterns of industrialized and developing nations (where an American consumes 30 times as much of the world’s resources as a Bangladeshi), and argue that restraint in consumption trumps restraint in population growth.

Those who argue that overconsumption is more damaging have an easier time making their case.  The evidence is intuitively obvious from the amount of resource use and waste generation in rich, industrialized countries.  Even when these countries have very low (or even negative) rates of population growth, their overall consumption and waste production tends to grow.  This situation supports the vehement insistence of low-consumption countries with higher levels of population growth that the rich nations must address their own consumption before insisting on population curbs.


There is a smaller third camp that treads a middle path.  Many of us who have been involved in the environmental debate for a while have run across the symbolic ecological equation I=PAT.  While not an actual mathematical equation, it’s a very handy illustrative tool.  “I” stands for the Impact we humans have on our planet.  This impact is affected by our population (“P”) multiplied by our activity level (“A”) and the level of technology ("T") used to achieve that activity.  For our purposes,  activity is well represented by our Consumption (“C”), so the ecological equation becomes I=PCT.  Our impact on the planet is the product of our population times our consumption times the level of technology.

This insight, important as it is, however, is still not terribly useful.  The ecological equation forces us to aggregate all consumption and all population into single terms.  When we do that, we lose the ability to discriminate the levels of damage caused by different kinds of consumption.  It also limits our chance of detecting those impacts that are strongly dependent on simple human numbers, regardless of whether they are high- or low-consumption populations.

In order to detect an impact that is strictly population-dependent, the ecological equation tells us that we need to reduce the terms C and T to constants.  In other words, for all populations the amount of activity and the level of technology used to accomplish it would have to be similar.

Is there such a thing – a significant realm of human activity whose impact is tied directly to our numbers, so that population is the main independent variable?  Such an activity would have to be relatively uniform across the human species, engaged in by all people to a similar extent regardless of their culture, material wealth or the region in which they live.  The supporting technology for the activity would also have to be fairly uniform between different countries or regions, so that a unit of activity would have a similar impact no matter where it was done.

Asking the question in this way points to an obvious example of such an activity: eating and food production.


Eating is as close to a constant activity across human cultures as we are likely to find.  Regardless of where we live or how rich we are, an adult human needs to eat between 2,000 and 2,800 calories a day.  Most of us do not need more (though some of us may consume somewhat more, to our long-term detriment) and we cannot survive for long on less. Compared to other human activities such as driving automobiles or working in factories, the amount we eat is influenced very little by either cultural or individual circumstances.  What we eat may change from place to place, but the amount we eat always stays in that narrow range of 2,000 to 2,800 calories per day. An Australian or a Finn may consume 50 times more energy than a Bangladeshi, but they all eat about the same amount of food.

A similar uniformity holds true for the technology of food production as well.  Whether we consider grain production, fishing or animal husbandry, the technology used is remarkably similar from place to place.  Similar amounts of arable land with similar inputs produce similar yields.  With the rise of industrial agriculture, the inputs (fertilizer, pesticides, machinery and irrigation water) have become largely standardized.  This standardization has been assisted by the global trade in food.  If a country needs more food than it can produce, there is little impediment, short of extreme poverty, to them importing the deficit.  The food production technology of one country is effectively at the disposal of any other country through the mechanism of trade.  The production of a calorie of food can therefore be considered to have a fairly constant ecological cost that is relatively evenly distributed across the overall production activity.

This reveals an important chain of logic.  If the production of an “average” calorie of food has a fairly constant ecological cost, then the aggregate, global impact of food production depends mainly on the number of calories produced.  And if the number of calories consumed by an “average” person is likewise fairly constant, then the total number of calories to be produced depends mainly on the number of people to be fed.

That line of reasoning leads us to the following insight.  Given that global levels of food production and consumption are balanced (so there is little overall shortage or surplus), the ecological impact of food production is directly proportional to the global population.

The Ecological Toll of Food Production

This whole argument would be moot, however, if the level of ecological damage from food production was insignificant. It’s obvious that this is not the case.  Consider the following laundry list of ecological damage related to food production:

  • The number of oceanic “dead zones” caused by eutrophication from fertilizer runoff has been doubling every ten years since the 1960s.
  • Predatory fish species (the ones we eat) have declined by 90% in the last 50 years.  This is due to our over-fishing the oceans for food.
  • The estimated extinction rate of plants and animals is at least 75 species per day. This is mainly the result of habitat loss due to human encroachment and the expansion of agriculture.
  • Over 75,000 square miles of arable land is lost each year to urbanization and desertification.
  • A billion people in over 110 countries are affected by desertification.  Agriculture was the main reason for the desertification that has reduced the cradle of civilization in the Middle East and North Africa from lush, fertile lands to the barren sands we see today.
  • On the American Great Plains, half the topsoil has been lost in the last hundred years, and the Ogallala aquifer is being drained up to 100 times faster than it is being refilled.
  • Indian farmers have drilled over 21 million water wells using oil-well technology. They take 200 billion tonnes of water out of the earth each year for irrigation.

Every one of these and similar impacts is directly proportional to the number of people we are trying to feed.

Of course there are ecological problems like climate change that are more dependent on consumption levels than on population numbers.  However, a significant subset of our ecological problems can be attributed to agriculture, and through that door those problems can be laid directly at the feet of our growing population numbers.


A species is said to be in overshoot if the resource requirements of its population exceed the carrying capacity of its environment — in other words, its needs exceed the ability of its environment to supply those needs sustainably over the long term.  Humanity is already in overshoot, by at least 25% and perhaps by as much as 100% or more.

Because so many of the problems pushing us into overshoot are directly related to our population level, only a reduction in population would help to redress the balance.  A lower population would reduce the pressure on the planetary ecosystems we depend on and give them a chance to recover.  Unfortunately, there is no sign that our population will even stabilize within the next 40 years, let alone start to decline.

Because our population is going to continue growing, the ecological insults we are visiting upon Mother Earth will continue and may even increase as the years go by.  For a species that is already in overshoot, this is a very ominous prediction.  As we run into resource limits such as Peak Oil, the underlying damage we have done will assume ever greater importance as our degradation of the world's carrying capacity is progressively revealed.


What can be done about this predicament?  Global population growth rates are declining, of course, and show every sign of continuing to do so.  As I have pointed out in other articles though, our growth rate will not decline fast enough to rescue our species from the ecological fires we have already started.  Programs of voluntary population reduction that might accelerate the necessary decline run head-on into the problems so elegantly described by the Prisoner’s Dilemma game – nobody wants to risk getting the sucker’s payoff, so nobody is prepared to be a front-runner in the race for less.

From a purely rhetorical perspective though, it remains a fact that there are aspects of our ecological difficulties that are strictly the result of our excessive numbers, and could be alleviated by reducing our population.  I hope that this article helps bring some clarity to the debate of consumption versus population as the underlying culprit – it’s transparently clear that population numbers play a major role.

September 23, 2008


© Copyright 2008, Paul Chefurka
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