population

A population is all the organisms that both belong to the same species and live in the same geographical area. The area that is used to define the population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals from other areas. Normally breeding is substantially more common within the area than across the border

The twentieth century has been marked by a profound historical development: an unwitting evolution of the power to seriously impair human life-support systems. Nuclear weapons represent one source of this power. Yet, even the complexities of global arms control are dwarfed by those inherent in restraining runaway growth of the scale of the human enterprise, the second source of possible disaster. Diminishing the nuclear threat involves relatively few parties, well-established international protocols, alternate strategies that carry easily assessed costs and benefits, short -- and long-term incentives that are largely congruent, and widespread recognition of the severity of the threat. In contrast, just the opposite applies to curbing the increasingly devastating impact of the human population. In particular, the most personal life decisions of every inhabitant of the planet are involved and these are controlled by socioeconomic systems in which the incentives for sacrificing the future for the present are often overwhelming.

This article provides a framework for estimating the population sizes and lifestyles that could be sustained without undermining the potential of the planet to support future generations. We also investigate how human activity may increase or reduce Earth's carrying capacity for Homo sapiens. We first describe the current demographic situation and then examine various biophysical and social dimensions of carrying capacity.

Our analysis is necessarily preliminary and relatively simple; we anticipate that it will undergo revision. Nonetheless, it provides ample basis for policy formulation. Uncertainty about the exact dimensions of future carrying capacity should not constitute an excuse to postpone action. Consider the costs being incurred today of doing so little to halt the population explosion, whose basic dimensions were understood decades ago.

The current population situation

The human population is now so large and growing so rapidly that even popular magazines are referring to the possibility of a "demographic winter" (Time 1991). The current population of 5.5 billion, growing at an annual rate of 1.7%, will add approximately 93 million people this year, equivalent to more than the population of Mexico (unless otherwise noted, demographic statistics are from, or projected from, PRB 1991).

Growth rates vary greatly from region to region. The combined population of less-developed nations (excluding China) is growing at approximately 2.4% annually and will double in 30 years if no changes in fertility or mortality rates occur. The average annual rate of increase in more-developed nations is 0.5%, with an associated doubling time of 137 years. Many of those countries have slowed their population growth to a near halt or have stopped growing altogether.

The regional contrast in age structures is even more striking. The mean fraction of the population under 15 years of age in more-developed countries is 21%. In less-developed countries (excluding China) it is 39%; in Kenya it is fully 50%. Age structures so heavily skewed toward young people generate tremendous demographic momentum. For example, suppose the total fertility rate (average completed family size) of India plummets over the next 33 years from 3.9 to 2.2 children (replacement fertility). Under that optimistic scenario (assuming no rise in death rates), India's population, today some 870 million, would continue to grow until near the end of the next century, topping out at approximately 2 billion people.

The slow progress in reducing fertility in recent years is reflected in the repeated upward revisions of United Nations projections (UNFPA 1991). The current estimate for the 2025 population is 8.5 billion, with growth eventually leveling off at approximately 11.6 billion around 2150. These projections are based on optimistic assumptions of continued declines in population growth rates.

Despite the tremendous uncertainty inherent in any population projections, it is clear that in the next century Earth will be faced with having to support at least twice its current human population. Whether the life support systems of the planet can sustain the impact of so many people is not at all certain.

Environmental impact

One measure of the impact of the global population is the fraction of the terrestrial net primary productivity (the basic energy supply of all terrestrial animals ) directly consumed, co-opted, or eliminated by human activity. This figure has reached approximately 40% (Vitousek et al. 1986). Projected increases in population alone could double this level of exploitation, causing the demise of many ecosystems on whose services human beings depend.

The impact (I) of any population can be expressed as a product of three characteristics: the population's size (P), its affluence or per-capita consumption (A), and the environmental damage (T) inflicted by the technologies used to supply each unit of consumption (Ehrlich and Ehrlich 1990, Ehrlich and Holdren 1971, Holdren and Ehrlich 1974).

I = PAT

These factors are not independent. For example, T varies as a nonlinear function of P, A, and rates of change in both of these. This dependence is evident in the influence of population density and economic activity on the choice of local and regional energy supply technologies (Holdren 1991a) and on land management practices. Per-capita impact is generally higher in very poor as well as in affluent societies.

Demographic statistics give a misleading impression of the population problem because of the vast regional differences in impact. Although less developed nations contain almost four fifths of the world's population and are growing very rapidly, high per capita rates of consumption and the large-scale use of environmentally damaging technologies greatly magnify the impact of industrialized countries.

Because of the difficulty in estimating the A and T factors in isolation, per-capita energy use is sometimes employed as an imperfect surrogate for their product. Using that crude measure, and dividing the rich and poor nations at a per-capita gross national product of $4000 (1990 dollars), each inhabitant of the former does roughly 7.5 times more damage to Earth's life-support systems than does an inhabitant of the latter (Holdren 1991 a ). At the extremes, the impact of a typical person in a desperately poor country is roughly a thirtieth that of an average citizen of the United States. The US population has a larger impact than that of any other nation in the world (Ehrlich and Ehrlich 1991, Holdren 1991a,b).

The population projections and estimates of total and relative impact bring into sharp focus a question that should be the concern of every biologist, if not every human being: how many people can the planet support in the long run?

The concept of carrying capacity

Ecologists define carrying capacity as the maximal population size of a given species that an area can support without reducing its ability to support the same species in the future. Specifically, it is "a measure of the amount of renewable resources in the environment in units of the number of organisms these resources can support" (Roughgarden 1979, p. 305) and is specified as K in the biological literature.

Carrying capacity is a function of characteristics of both the area and the organism. A larger or richer area will, ceteris paribus, have a higher carrying capacity. Similarly, a given area will be able to support a larger population of a species with relatively low energetic requirements (e.g., lizards) than one at the same trophic level with high energetic requirements (e.g., birds of the same individual body mass as the lizards). The carrying capacity of an area with constant size and richness would be expected to change only as fast as organisms evolve different resource requirements. Though the concept is clear, carrying capacity is usually difficult to estimate.

For human beings, the matter is complicated by two factors: substantial individual differences in types and quantities of resources consumed and rapid cultural (including technological) evolution of the types and quantities of resources supplying each unit of consumption. Thus, carrying capacity varies markedly with culture and level of economic development.

We therefore distinguish between biophysical carrying capacity, the maximal population size that could be sustained biophysically under given technological capabilities, and social carrying capacities, the maxima that could be sustained under various social systems (and, especially, the associated patterns of resource consumption). At any level of technological development, social carrying capacities are necessarily less than biophysical carrying capacity, because the latter implies a human factory-farm lifestyle that would be not only universally undesirable but also unattainable because of inefficiencies inherent in social resource distribution systems (Hardin 1986). Human ingenuity has enabled dramatic increases in both biophysical and social carrying capacities for H. sapiens, and potential exists for further increases.

Carrying capacity today. Given current technologies, levels of consumption, and socioeconomic organization, has ingenuity made today's population sustainable? The answer to this question is clearly no, by a simple standard. The current population of 5.5 billion is being maintained only through the exhaustion and dispersion of a one-time inheritance of natural capital (Ehrlich and Ehrlich 1990), including topsoil, groundwater, and biodiversity. The rapid depletion of these essential resources, coupled with a worldwide degradation of land (Jacobs 1991, Myers 1984, Postel 1989) and atmospheric quality (Jones and Wigley 1989, Schneider 1990), indicate that the human enterprise has not only exceeded its current social carrying capacity, but it is actually reducing future potential biophysical carrying capacities by depleting essential nautral capital stocks. [1]

The usual consequence for an animal population that exceeds its local biophysical carrying capacity is a population decline, brought about by a combination of increased mortality, reduced fecundity, and emigration where possible (Klein 1968, Mech 1966, Scheffer 1951). A classic example is that of 29 reindeer introduced to St. Matthew Island, which propagated to 6000, destroyed their resource base, and declined to fewer than 50 individuals (Klein 1968). Can human beings lower their per-capita impact at a rate sufficiently high to counterbalance their explosive increases in population?

Carrying capacity for saints. Two general assertions could support a claim that today's overshoot of social carrying capacity is temporary. The first is that people will alter their lifestyles (lower consumption, A in the I = PAT equation) and thereby reduce their impact. Although we strongly encourage such changes in lifestyle, we believe the development of policies to bring the population to (or below) social carrying capacity requires defining human beings as the animals now in existence. Planning a world for highly cooperative, antimaterialistic, ecologically sensitive vegetarians would be of little value in correcting today's situation. Indeed, a statement by demographer Nathan Keyfitz (1991) puts into perspective the view that behavioral changes will keep H. sapiens below social carrying capacity:

"If we have one point of empirically backed knowledge, it is that bad policies are widespread and persistent. Social science has to take account of them/" [our emphasis]

In short, it seems prudent to evaluate the problem of sustainability for selfish, myopic people who are poorly organized politically, socially, and economically.

Technological optimism. The second assertion is that technological advances will sufficiently lower per capita impacts through reductions in T that no major changes in lifestyle will be necessary. This assertion rep resents a level of optimism held primarily by nonscientists. (A 1992 joint statement by the US National Academy of Sciences and the British Royal Society expresses a distinct lack of such optimism). Technical progress will undoubtedly lead to efficiency improvements, resource substitutions, and other innovations that are currently unimaginable. Different estimates of future rates of technical progress are the crux of much of the disagreement between ecologists and economists regarding the state of the world. Nonetheless, the costs of planning development under incorrect assumptions are much higher with overestimates of such rates than with underestimates (Costanza 1989).

A few simple calculations show why we believe it imprudent to count on technological innovation to reduce the scale of future human activities to remain within carrying capacity. Employing energy use as an imperfect surrogate for per-capita impact, in 1990 1.2 billion rich people were using an average of 7.5 kilowatts (kW) per person, for a total energy use of 9.0 terawatts (TOO; 10 12 watts). In contrast, 4.1 billion poor people were using 1 kW per person, and 4.1 TW in aggregate (Holdren 1991a). The total environmental impact was thus 13.1 TW.

Suppose that human population growth were eventually halted at 12 billion people and that development succeeded in raising global per capita energy use to 7.5 kW (approximately 4 kW below current US use). Then, total impact would be 90 TW. Because there is mounting evidence that 13.1 TW usage is too large for Earth to sustain, one needs little imagination to picture the environmental results of energy expenditures some sevenfold greater. Neither physicists nor ecologists are sanguine about improving technological performance sevenfold in the time available.

There is, indeed, little justification for counting on technological miracles to accomodate the billions more people soon to crowd the planet when the vast majority of the current population subsists under conditions that no one reading this article would voluntarily accept. Past expectations of the rate of development and penetration of improved technologies have not

been fulfilled. In the 1960s, for example, it was widely claimed that technological advances, such as nuclear agroindustrial complexes (e.g., ORNL 1968), would provide 5.5 billion people with food, health care, education, and opportunity. Although the Green Revolution did increase food production more rapidly than some pessimists (e.g., Paddock and Paddock 1967) predicted, the gains were not generally made on a sustainable basis and are thus unlikely to continue (Ehrlich et al. 1992). At present, approximately a billion people do not obtain enough dietary energy to carry out normal work activities.

Furthermore, as many nonscientists fail to grasp, technological achievements cannot make biophysical carrying capacity infinite. Consider food production, for example. Soil can be made more productive by adding nutrients and irrigation; yields could possibly be increased further if it were economically feasible to grow crops hydroponically and sunlight were supplemented by artificial light. However, biophysical limits would be reached by the maximal possible photosynthetic efficiency. Even if a method were found to manufacture carbohydrates that was more efficient than photosynthesis, that efficiency, too, would have a maximum. The bottom line is that the laws of thermodynamics inevitably limit biophysical carrying capacity (Fremlin 1964) if shortages of inputs or ecological collapse do not intervene first