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The growth of a population in a given environment is theoretically limited by the availability of resources and susceptibility to disease or disaster, thus the maximum number or density of organisms an area can support is called the carrying capacity. The threshold for humans is unknown, because they respond to scarcity by moving to new areas, adopting new resources, or inventing technologies to increase capacity.

Carrying capacity is the theoretical limit on the size of a population of any organism that a given environment of defined size can support indefinitely. The limit is usually stated in terms of the food supply, but density-dependent diseases have probably been the most important factor in limiting many animal and human populations through most of history. Other limiting factors may have included specific nutrients or water, or even physiological responses to the psychological stresses of high population density. The “law of the minimum” suggests that the limit will be set by whichever resource is in shortest supply or is activated at the lowest population density.

If food is the limiting resource, as is usually assumed, any population of food consumers is limited by the regenerative capacity of its food supply. If the consuming animal eats only the amount regenerated, (if, for example, wolves eat only as many sheep as are born each year, or the sheep eat grass only as fast as it can regenerate), the consumers and the food supply can theoretically coexist forever in equilibrium (unless the environment itself changes). The equilibrium may be static or may involve interrelated fluctuations in the size of the two populations, as when, for example, wolves deplete the sheep population and then decline in numbers themselves, which then permits the sheep to regenerate. If the consumer continues to eat more of its prey than the prey can regenerate, the population of prey and therefore of consumers will be reduced conceivably to extinction. Exceeding carrying capacity may result either in excessive mortality among consumers, in reduced fertility from malnutrition or disease, or in both. Density-related social dynamics among the consumers may also affect both mortality and fertility. The same principles apply to specific isolated human populations subsisting on limited resources (as in the Arctic or extreme deserts) where there is nowhere else to go, no alternative resources, limited ability to move food, and limited ability of the human group to enhance the growth of its resources.

Carrying Capacity Applied to Human Beings

How important carrying capacity has been in human history is a matter of debate. A Malthusian perspective implies the importance of carrying capacity by arguing that human populations are—and by inference have been—limited by food-related technology that expands only through fortuitous human invention. Thomas Malthus (1766–1834) argued that the human population cannot expand indefinitely because it will inevitably outstrip its food supply. Humans will eat beyond Earth’s technologically enhanced carrying capacity, with dire consequences.

Although Malthus may be right about the long-term future, he was clearly wrong about much of human history to date and for the near future. The carrying capacity concept clearly has not applied overall and in the long run to the growth of our species as a whole (although it has almost certainly applied locally and in the short run).

People are omnivores who eat an extraordinarily wide (and still expanding) range of foods. We respond to shortage by expanding the breadth of our diets. (Few human populations consume all of the edible resources within range until forced to do so by hunger.) We also can increase the range of environments we occupy, and we can move food from place to place. Most important, we have demonstrated an enormous capacity to increase food supplies by investing additional effort in obtaining and processing them.

A controversy exists about the relative importance of supply and demand as determinants of human food supplies in human history. Many scholars believe that Malthus profoundly underestimated the ability of growing populations themselves to force changes in food choices, technology, and related behaviors. Throughout history, the human population has made adjustments that have ultimately resulted in the adoption (more than invention) of new technology. If demand can push supply, the meaning of carrying capacity as a fixed limit must be questioned.

Evidence for the importance of economic demand in human history comes in many forms. New foods eaten have often been less palatable, less nutritious, and harder to obtain that those they supplemented— and therefore unlikely to have been adopted just because they had been discovered or new technology invented. Many new environments colonized were clearly not preferred (deserts, tropical rain forests, and the Arctic) and would not have been colonized voluntarily. Many new technologies or steps taken to improve food supplies resulted in declining quality of food or declining efficiency of food-getting techniques. Much of the history of evolving economies seems to have involved diminishing returns, particularly in the quality of human health and nutrition.

Carrying Capacity for Foraging Peoples

Between 100,000 and about 12,000 years ago the human population consisted of foragers, small populations of whom survived into recent centuries and even into the present in forms modified by outside contact. These groups are mobile, live at low population density, and eat fresh wild foods. Malnutrition is quite rare among modern foragers and appears to have been rare among foragers in the past, but becomes increasingly important as populations grow and “progress” to new food resources and new technologies. Ancient and historic foragers also have been relatively disease-free, because low population densities and periodic movement prevent many infectious diseases from spreading or minimize their impact. The disease burden has clearly increased with increasing population density through human history. Major epidemic diseases such as smallpox are clearly of relatively modern origin. Modern peasants and the poor enjoy nowhere near foragers’ standard of nutrition and health.

Various studies show that large game is among the highest quality and most easily exploited of resources when available. But large game occupies large territories and is depleted easily. Early human foragers’ diets apparently included a relatively high proportion of large game; human hunters may have hunted many large mammals to extinction before falling back on secondary resources.

Early in prehistory population growth was extremely slow, absorbed mostly by territorial expansion. The slow growth probably resulted mostly from low fertility or birth control (since the life expectancy of such groups equaled that of later populations that grew much faster). As population densities increased, the role of infectious disease as a limit to populations increased. Malthusian constraints resulting from food shortages may also have been operating in specific times and places, but hunger and starvation may actually have increased in frequency and severity in later, non-foraging populations. Slow growth might also have been the result of population mechanisms geared not so much to the ultimate carrying capacity of natural resources as to the “carrying capacity” of choices defined by preferred labor inputs and food choices, or even of personal space.

As populations grew increasingly in the last 20,000 years, the territory available to each group declined, and large mammals became scarce, to the point where groups were ultimately forced to broaden their diets to include a wider range of more prolific resources (a wider range of vegetable foods, birds, small mammals, fish, and shellfish), resulting in an increase in the carrying capacity of each unit of land. But the new resources were apparently less desirable foods, less nutritious, and more difficult to exploit, thus commonly consumed only as preferred resources were exhausted. Our ancestors probably adopted, rather than invented, new technologies such as fish hooks, grindstones, and arrows suitable for exploiting smaller game that were obvious or long-known but unused until needed rather than being confined by the limits of fortuitous economic invention independent of need.

Wild small seeds, including cereals, were apparently very low on the list of preferred foods. They raise the carrying capacity of each unit of land but are not particularly nutritious and are very difficult to convert to food. (Even today, cereal grains and tubers are staples primarily for the poor, because they are plentiful and cheap.)

Carrying Capacity after the Adoption of Agriculture

Agriculture and the domestication of plants, first adopted about 10,000 years ago, are usually considered major inventions that further raised the productivity of land and permitted groups to become sedentary. They apparently resulted in a slight increase in the rate of population growth, probably not from increased life expectancy but from greater fertility and or altered birth control choices. But they, too, may have been a concession to increased population density. They clearly further lowered the quality of nutrition and may have increased labor demands.

On the one hand, sedentism and the ability to store food may have helped smooth out seasonal fluctuation and bottlenecks in the food supply; on the other, storage anchored populations to the vicinity of their stores, perhaps making those populations more vulnerable to crop failure, especially as domesticated crops modified for human use are often less suited to natural survival than their wild ancestors and hence more vulnerable to disease and pests. The concept of storing foods restricted diets to storable resources, but stored foods lose nutrients during storage, and actual physical losses of stored food (due, for example, to spoilage) threatened the reliability of the whole enterprise. Reliance on stored resources also increased people’s vulnerability to expropriation of stored resources by others, while sedentism also exposed populations to increased infection risks.

Common reconstructions hold that among established farmers, the invention of new tools or exploitation of new technologies—the hoe, the plow, draft animals, fertilizers, and irrigation—increased the carrying capacity of land and made labor more efficient. A more controversial thesis holds that methods of farming involving plentiful supplies of land and low population densities may have been more efficient than the more intensive methods associated with increased population density. Denser populations require increasing the productivity of land by shortening the periods of time that the land lies fallow, which in turn may necessitate the adoption of new tools. It is possible, therefore, that both before and after the adoption of agriculture, demand and labor investment, not technological “progress,” may have been the engine of economic growth. The idea of a fixed ceiling on resources modifiable only by fortuitous invention independent of need would then have little explanatory power.

Carrying Capacity in Recent Centuries

Artificial Malthusian constraints on population clearly became important with the emergence of civilization, because ruling classes (a defining feature of civilization) can withhold food from the lower classes and prevent them from generating demand for food. (Demand implies both desire or need and the ability to produce, pay for, or otherwise command food.) Hunger in the modern world among bountiful food supplies results from the inability of the poor to pay for food. Many argue that solutions to world hunger at present relate to the distribution of wealth, not natural Malthusian limits.

Moreover, in recent centuries, the growth of the world population has accelerated markedly. Nor can that growth be attributed to modern medicine’s reducing human mortality, as it began well before the advent of modern medicine. Some argue that high growth rates are not a property of so-called primitive groups—that is, groups that depend on a high birth rate to assure their survival—but are a conscious response to the demands of colonial or world systems. If or when new efforts and new technology become necessary to feed the world’s population, the implementation of those new technologies will depend on the degree to which the rich concern themselves with the poor. If that is the case, then increases in food technology are indeed independent of population and demand.

Ultimate Human Carrying Capacity of Earth

In the long run, demographers generally estimate the ultimate carrying capacity of Earth at 10–70 billion people (although some estimates are much higher.) The modern world population as of 2010 is approximately 6.8 billion. The variation in those estimates is due in part to different assumptions about the ability and willingness of human populations to exert more effort, employ new technologies, eat new foods, and accept lower standards of living.

But the human carrying capacity of Earth may well ultimately be measured not by food resources but by the limited supply of some other necessary resource. Fresh water is already in short supply, and that supply can be enhanced only at great cost. Carrying capacity might also be defined ultimately by the highest density that a human population can reach before it triggers an unstoppable epidemic of infectious disease—probably the most important limit on population for the foreseeable future. And it may well be defined by the limits of the capacity of social organization to mitigate social and psychological stresses brought on by declining personal space.

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