Carrying Capacity

Mark Nathan Cohen

How many are too many?

The earth’s ability to support human life is usually described as the potential productivity of the world’s food supply enhanced by human technological invention. The term most often applied is carrying capacity, which refers to the maximum size of any population (including people) that available resources can support without themselves being overconsumed to the point of extinction. The prevailing human model comes from Thomas Malthus, who argued that the ability of new technology stimulated by fortuitous invention to increase the human carrying capacity must eventually fall behind the geometrical expansion of the human population, resulting in misery, mass hunger, and starvation.1 This is essentially an application of supply-side economics.

But the Malthusian argument ignores many other issues, including the importance of economic demand, the human history of population growth and technological change, and modern politics and culture. It also ignores the possibility that the human population may ultimately be limited by some resource other than food: perhaps water, epidemic disease organisms, or even limits on personal space with consequent dangerous psychological and biological results.

A reading of the archeological record suggests that during approximately the first 90 percent of human history (approximately ninety thousand years), human populations and their food needs played a major role in expanding the human food niche. Increasing economic demand forced increased labor, broadening of diets, exploitation of new environments, and the adoption of new “inventions.”

Agriculture: Invention or Response?

Through most of our history, human population grew very slowly. Groups could respond to population growth by broadening their geographic range and exploiting a wider range of foods. This enlargement of the resource base may have been necessary because past overhunting had exceeded the carrying capacity of large game animals, essentially destroying this resource and making the exploitation of other resources necessary.2

Perhaps fifteen to twenty thousand years ago, the potential for territorial expansion was largely exhausted. Thereafter, population growth took place in tightly bounded territories. There is clear evidence, both in the organic remains and in the range of tools found, that human groups began in this period to exploit a wider range of resources. Small projectile points, mortars and pestles, and grindstones demonstrate an increasing focus on smaller fauna, coarse vegetation needing to be pounded, and tiny seeds needing to be ground. That this is a function of demand, not “progress,” is apparent from contemporary observations of living populations. Their sequence of exploitation of resources, from big game and preferred vegetable resources through small animals to small seeds, is a sequence of diminishing returns for labor.

The new food resources were not adopted simply because they had been “discovered,” but because the population had no choice but to exploit them. In fact, the food-resource changes associated with agriculture probably brought about a decline in nutritional quality; grains like cereals, which can be found (and later farmed) in high density to feed dense populations, are not particularly nutritious. Human stature and tooth size vary primarily in response to individual nutrition and growth more than to genetic heritage. In many parts of the world, stature and tooth size declined as the range of resources exploited expanded and then as farming was adopted. Other markers of biological stress increased through the same sequence of transitions, further evidence that the quality of human diets probably declined over the period.3

What was once considered the “invention” of farming (occurring in various regions beginning ten to twelve thousand years ago) is now more often interpreted as a result of increased demand for resources. This increase results from either denser human populations, social structures that imposed demands for extra output, or climate changes that may have reduced the supply of preferred resources. This new technology was presumably forced, not invented.

Several lines of evidence support the hypothesis that the adoption of agriculture was driven by demand. First, most of the techniques that, in combination, make up “farming” (managing food supplies, planting seeds, fertilizing, soil preparation, fire clearance, even irrigation) were and are in fact used individually by various foraging cultures that did not practice agriculture. Second, agriculture actually seems to have been harder work than hunting or even gathering small resources. Agriculture involves a good deal of work in advance; seasonal agriculture additionally involves storage and preparation for storage, as well as intensive processing of cereals to convert them to food.4 Third, farming may have increased the risk of food shortage. Agricultural populations could not easily move in the face of crop failure, while stored grain is subject to destruction by rot and pests. Stored food and people who cannot move are vulnerable to conquest and expropriation of crops by other groups. Domesticated crops reshaped for human preference are often less hardy in the face of environmental stresses than their wild counterparts.5 The adoption of sedentary agriculture also seems to have resulted in increased levels of infection. Taken together, these would probably have furnished a perceptible disincentive to sedentary farming.6

Traditional theories view later improvements in farming (the plow, the hoe, fertilization, and irrigation) as inventions that further expanded carrying capacity. An alternative theory holds that we really are witnessing the effects of increasing demand forcing each plot of land to be used more often (i.e., increasing the crop-to-fallow ratio). This in turn creates the need for more intensive labor and importation of fertilizer; it also necessitates new tools and techniques (the hoe, the plow) to cope with increasingly intractable and nutrient-depleted soil.7

Emergence of the Political

Relatively small agricultural societies commonly are led by chiefs who have authority but no power. Under such circumstances, all individuals have the right to get their own food because they share communal ownership and each individual is a unit of labor and demand. One effect of sedentary farming is that populations require increasingly complex political organization in order to manage larger communities. Such communities are also very susceptible to conquest. For these or other reasons, beginning three to five thousand years ago, sedentary farming was followed by the emergence of “civilization” and “the state,” which anthropologists define in political terms as the power of kings and elites to rule societies and maintain social stratification by the use of physical force.8

States divide society into the class of those who own resources (“haves”) from those who do not (“have nots”). It is at this point in history that a Malthusian model of economic “advance” clearly becomes the rule—because demand is rendered ineffective as an impetus to economic and technological change. Demand involves not only need but also the ability to pay or earn, which can be and often is denied to the poor. When society stratifies, then, the poor no longer exert economic demand for food. They thus provide little incentive for technology to improve, so that technological innovation becomes a variable independent of population size. The well-to-do can provide new technologies to help the poor and increase food supplies overall, as with the Green Revolution of the 1970s or the development of genetically altered crops today. But the wealthy can also control or even withhold new technologies. Or they can manipulate outcomes so that new technologies do the poor little good, as when the poor are forced to grow cash crops rather than to farm for subsistence.

Today, as long ago, the problem of food is one of demand, not supply. It is obvious that the world has adequate potential to grow food. Despite world hunger, American farmers are going out of business or being paid not to grow food. Grain rots in storage, often literally within sight of the hungry. The problem is the lack of demand: If the poor had money to buy food, markets would respond, and technology would change or be applied more fully. Today’s problem is not about ecology or technology as much as it is about political economics.9

Population Growth and Carrying Capacity

How fast will human populations continue to grow? For the last two or three hundred years, world population has been growing at an extraordinary rate—many times the growth rates of previous eras. It is important to note that contemporary population growth accelerated in an era of unprecedented, worldwide epidemics and often well before Western medicine had any power to improve survivorship.10 Much of it seems to be related to political and economic conditions set by the modern world system relating to relaxed fertility controls and the (partly new) preference for large families as sources of wage labor. These factors must be addressed, if we really want to limit population growth.

Knowing the planet’s ultimate carrying capacity for human populations would be very helpful in deciding how much importance to attach to such work. Estimates of the earth’s ultimate human carrying capacity range from ten to seventy billion people. It is noteworthy, however, that these estimates have historically increased faster than the population is actually growing.11 That is, the estimated gap between contemporary demand and the earth’s ultimate carrying capacity is actually increasing. In any case, the actual maximum supportable human population depends on more than biological carrying capacity and new technology. It also depends on a series of human decisions:

•How much more to work.

•What to eat.

•Whether the poor will be allowed to provide economic demand stimulating new technology.

•Whether regions of the world will combine to produce and disseminate needed resources.

•How much decline in our own standard of living those in power are prepared to tolerate

These factors can interact in complex ways: For example, enhanced economic demand by the poor would probably increase real demand for grain as food. Food grain production might eventually outcompete the less spatially efficient growth of cattle—but the wealthy like beef. Enhanced economic demand by the poor might also displace production of some luxury cash crops.

A very new problem, of course, is the widely recognized potential for shifting and loss of farmland resulting from global warming.

There is a final consideration: the limiting resource may not be food at all. At this time epidemic diseases—which thrive on high population density and rapid intercity transportation of people and goods—may pose the greatest danger. Epidemiologists generally agree that our increasingly urbanized world is ripe for new epidemics, perhaps more so than ever before in human history. Germs are evolving faster than we can invent techniques to stop them.12 Or the limiting factor may be water. Freshwater supplies are already limited and their distribution contentious; they are also far less susceptible to technological augmentation than the food supply. Or the limit may be imposed simply by the limited ability of human brings to tolerate limited personal space and increasing environmental and personal “noise.” Inability to tolerate such conditions can result in a series of biological problems ranging from increased mortality, decreased fertility, and reduced immunity to the infectious diseases that we know in any case are coming.

Leibig’s Law of the Minimum says that whichever of these resources or conditions is most limited, or “kicks in” at the smallest population size, will determine the ultimate limit to the size of the human population.

Notes

1. Thomas Malthus, An Essay on the Principle of Population (New York: Norton, 1798 [1976]).

2. Mark N. Cohen, The Food Crisis in Prehistory (New Haven: Yale University Press, 1977).

3. See Mark N. Cohen, Health and the Rise of Civilization (New Haven: Yale University Press, 1989); Mark N. Cohen and George J. Armelagos, eds. Paleopathology at the Origins of Agriculture (Orlando, Fla.: Academic Press: 1984); Clark S. Larsen, “Biological Changes in Human Populations with Agriculture,” Ann. Rev. Anthropology 24 (1995): 185–213.

4. Kenneth W. Russell, After Eden (Oxford: U.K.: BAR International Series 391, 1988).

5. Cohen, The Food Crisis in Prehistory.

6. This pattern can be reconstructed in theory by knowing the natural distribution of nutrients and the life cycles of human parasites as they are affected by human behavior, and also from knowledge of the foods exploited. It can be observed in contrasts between historic and modern mobile foragers and their sedentary neighbors, and can also be seen in comparisons of prehistoric skeletons from before and after the transition. See Cohen, Health and the Rise of Civilization; Cohen, “Introduction to the Symposium,” In M. Cohen and G. Crane Kramer, eds., Ancient Health (Gainesville: University Press of Florida, [in press] 2006); Larsen, “Biological Changes in Human Populations with Agriculture.”

7. Ester Boserup, The Conditions of Agricultural Growth (Chicago: Aldine, 1966).

8. Morton Fried, The Evolution of Political Society (New York: Random House, 1967); Richard H. Robbins, Cultural Anthropology: A Problem Based Approach, 3rd Edition (Itaska, Ill.: Peacock Publishers, in press [2006]).

9. Robbins, Global Problems and the Culture of Capitalism (Boston: Allyn and Bacon, 2001); Robins, Cultural Anthropology.

10. Cohen, Health and the Rise of Civilization; Robbins, Global Problems and the Culture of Capitalism; Robbins, Cultural Anthropology.

11. For a summary see Joel E. Cohen, How Many People Can the Earth Support? (New York: W.W. Norton, 1995).

12. See Michael Shnayerson and Mark J. Plotkin, The Killers Within (Boston: Little, Brown, 2002) and Laurie Garrett, The Coming Plague (New York: Penguin, 1995).


Mark Nathan Cohen is University Distinguished Professor of Anthropology at the State University of New York. He is author of The Food Crisis in Prehistory (Yale University Press, 1977) and other books and senior editor of Paleopathology at the Origins of Agriculture (Academic Press, 1984).

Mark Nathan Cohen

Mark Nathan Cohen is University Distinguished Professor of Anthropology at the State University of New York. He is author of The Food Crisis in Prehistory (Yale University Press, 1977) and other books and senior editor of Paleopathology at the Origins of Agriculture (Academic Press, 1984).


How many are too many? The earth’s ability to support human life is usually described as the potential productivity of the world’s food supply enhanced by human technological invention. The term most often applied is carrying capacity, which refers to the maximum size of any population (including people) that available resources can support without themselves …

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