“For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled.”

Richard Feynman


Human Carrying Capacity Defined

It was thus becoming apparent that nature must, in the not far distant future, institute bankruptcy proceedings against industrial civilization, and perhaps against the standing crop of human flesh, just as nature had done many times to other detritus-consuming species following their exuberant expansion in response to the savings deposits their ecosystems had accumulated before they got the opportunity to begin the drawdown… Having become a species of superdetritovores, mankind was destined not merely for succession, but for crash. —Catton,


published by The International Society for Ecological Economics 1994. Buy here.


“Ecologists define ‘carrying capacity’ as the population of a given species that be supported indefinitely in a defined habitat without permanently damaging the ecosystem upon which it is dependent. However, because of our culturally variable technology, different consumption patterns, and trade, a simple territorially-bounded head-count cannot apply to human beings. Human carrying capacity must be interpreted as the maximum rate of resource consumption and waste discharge that can be sustained indefinitely without progressively impairing the functional integrity and productivity of relevant ecosystems wherever the latter may be. The corresponding human population is a function of per capita rates of material consumption and waste output or net productivity divided by per capita demand (Rees 1990). This formulation is a simple restatement of Hardin’s (1991) ‘Third Law of Human Ecology’:

(Total human impact on the ecosphere) = (Population) x (Per capita impact).

“Early versions of this law date from Ehrlich and Holdren who also recognized that human impact is a product of population, affluence (consumption), and technology: I = PAT (Ehrlich and Holdren 1971; Holdren and Ehrlich 1974). The important point here is that a given rate of resource throughput can support fewer people well or greater numbers at subsistence levels.

“Now the inverse of traditional carrying capacity provides an estimate of natural capital requirements in terms of productive landscape. Rather than asking what population a particular region can support sustainably, the question becomes: How much productive land and water area in various ecosystems is required to support the region’s population indefinitely at current consumption levels?

“Our preliminary data for developed regions suggest that per capita primary consumption of food, wood products, fuel, and waste-processing capacity co-opts on a continuous basis up to several hectares of productive ecosystem—the exact amount depends on the average levels of consumption (i.e., material throughput). This average per capita ‘personal planetoid’ can be used to estimate the total area required to maintain any given population. W call this aggregate area the relevant community’s total ‘ecological footprint’ (see Figure 20.2) on the earth (Rees 1992).

“This approach reveals that the land ‘consumed’ by urban regions is typically at least an order of magnitude greater than that contained within the usual political boundaries or the associated built-up area. However brilliant its economic star, every city is an entropic black hole drawing on the concentrated material resources and low-entropy production of a vast and scattered hinterland many times the size of the city itself. Borrowing from Vitousek et al. (1986), we say that high density settlements ‘appropriate’ carrying capacity from all over the globe, as well as from the past and the future (Wackernagel 1991).

“The Vancouver-Lower Fraser Valley Region of British Columbia, Canada, serves as an example. For simplicity’s sake consider the region’s ecological use of forested and arable land for domestic food, forest products, and fossil energy consumption alone: assuming an average Canadian diet and current management practices, 1.1 ha of land per capita is required for food production, 0.5 ha for forest products, and 3.5 ha would be required to produce the biomass energy (ethanol) equivalent of current per capita fossil energy consumption. (Alternatively, a comparable area of temperate forest is required exclusively to assimilate current per capita C02 emissions (see ‘Calculating the Ecological Footprint’). Thus, to support just their food and fossil fuel consumption, the region’s 1.7 million people require, conservatively, 8.7 million ha of land in continuous production. The valley, however, is only about 400,000 ha. Our regional population therefore ‘imports’ the productive capacity of at least 22 times as much land to support its consumer lifestyles as it actually occupies (see Figure 20.3). At about 425 people/km2 the population density of the valley is comparable to that of the Netherlands (442 people/km2)” [p.p. 369-371]

“Even with generally lower per capita consumption, European countries live far beyond their ecological means. For example, the Netherlands’ population (see Figure 20,4) consumes the output of at least 14 times as much productive land as is contained within its own political boundaries (approximately 110,000 km2 for food and forestry products and 360,00 km2 for energy)(basic data from WRI 1992).” [p. 374]

OUR ECOLOGICAL FOOTPRINT: Reducing Human Impact on the Earth (New Catalyst Bioregional Series) (Paperback) 

by Wackernagel and Rees

The Ecological Footprint is a measure of the “load” imposed by a given population on nature. It represents the land area necessary to sustain current levels of resource consumption and waste discharge by that population.


Some years ago, I read of a species of tiny woodland wasp that lives on mushrooms. It seems that when a wandering female wasp chances upon the right kind of mushroom in the forest, she deposits her eggs within it. Almost immediately, the eggs hatch and the tiny grubs begin literally to eat themselves out of house and home. The little maggots grow rapidly, but soon something very odd happens. The eggs in the larvaes’ own ovaries hatch while still inside their immature mothers. This second generation of parthenogenic grubs quickly consumes its parents from within, then breaks out of the empty shells to continue feeding on the mushroom. This seemingly gruesome process may repeat itself for another generation. It doesn’t take long before the entire mushroom is over-filled by squirming maggots and fouled by their bodily wastes. The exploding population of juvenile wasps consumes virtually its entire habitat which is the signal for the largest and most mature of the larvae to pupate. The few individuals that manage to emerge as mature adults then abandon their mouldering birthplace, flying off to begin the whole process over again.

We wrote this book in the belief that the bizarre life-cycle of the mushroom wasps may offer a lesson to humankind. The tiny wasps’ weird reproductive strategy has apparently evolved under extreme competitive pressure. Good mushrooms—like good planets—are hard to find. Natural selection therefore favored those individual wasps and reproductive traits that were most successful in appropriating the available supply of essential resources (the mushroom) before the competition had arrived or became established.

No doubt human beings also have a competitive side and both natural and sociocultural selection have historically favored those individuals and cultures that have been most successful in commandeering resources and exploiting the bounty of nature. There is also plenty of archeological and historic evidence that, like the over-crowded mushroom, many whole cultures have collapsed from the weight of their own success. Human societies as temporally and spatially far-flung as the Mesopotamians, Mayans, and Easter Islanders likely came to ruin by expanding beyond the capacity of their environments to sustain them. Like the forest wasps, they depleted their local habitats. Humanity as a whole survived, however, because there were always other figurative “mushrooms” elsewhere on Earth capable of supporting people.

Today, of course, humankind has become a global culture, one increasingly driven by a philosophy of competitive expansionism, one which is subduing and consuming the Earth. The problem is that, unlike the wasp, even the fattest and richest among us have no means to abandon the withered hulk of our habitat once consumed and there is no evidence yet of other Earth-like “mushrooms” in our galactic forest.

The good news is that—also unlike the wasp—humans are gifted by the potential for self-awareness and intelligent choice, and knowing our circumstances is an invitation to change.

The first step toward reducing our ecological impact is to recognize that the environmental crisis” is less an environmental and technical problem than it is a behavioral and social one. It can therefore be resolved only with the help of behavioral and social solutions. On a finite planet, at human carrying capacity, a society driven mainly by selfish individualism has all the potential for sustainability of a collection of angry scorpions in a bottle. Certainly human beings are competitive organisms but they are also cooperative social beings. Indeed, it is no small irony (but one that seems to have escaped many policy advisors today) that some of the most economically and competitively successful societies have been the most internally cooperative—those with the greatest stocks of cultural and social capital.

Our primary objective with this book is to make the case that we humans have no choice but to reduce our “Ecological Footprint.” We hope that it also conveys our essential confidence in the resourcefulness of the human spirit. People have great untapped potential to meet this greatest of challenges to our collective security. As William Catton stated in his 1980 classic, Overshoot: “If, having overshot carrying capacity, we cannot avoid crash, perhaps with ecological understanding of its real causes we can remain human in circumstances that could otherwise tempt us to turn beastly.” Indeed, we believe that confronting together the reality of ecological overshoot will force us to discover and exercise those special qualities that distinguish humans from other sentient species, to become truly human. In this sense, global ecological change may well represent our last great opportunity to prove that there really is intelligent life on Earth.

William Rees
Gabriola Island
Summer 1995


BALTIMORE (Feb 9, 1996)—If humans can’t control the explosive population growth in the coming century, disease and starvation will do it, Cornell University ecologists have concluded from an analysis of Earth’s dwindling resources.

A grim future—without enough arable land, water and energy to grow food for 12 billion people—is all but inevitable and all too soon, a worried David Pimentel today (Feb. 9) told an American Association for the Advancement of Science (AAAS) session on “How Many People Can the Earth Support?” “Environmentally sound agricultural technologies will not be sufficient to ensure adequate food supplies for future generations unless the growth of human population is simultaneously curtailed,” the Cornell professor of ecology said, speaking for researchers who produced the report, “Impact of Population Growth on Food Supplies and Environment.”

The “optimum population” that the Earth can support with a comfortable standard of living is less than 2 billion, including fewer than 200 million people in the United States, the Cornell scientist noted. But if the world population reaches 12 billion, as it is predicted to in 50 years, as many as 3 billion people will be malnourished and vulnerable to disease, the Cornell analysis of resources determined. The planet’s agricultural future—with declining productivity of cropland—can be seen in China today, Pimentel suggested.

China now has only 0.08 hectare (ha) of cropland per capita, compared to the worldwide average of 0.27 ha per capita and the 0.5 ha per capita considered minimal for the diverse diet currently available to residents of the United States and Europe. Nearly one-third of the world’s cropland has been abandoned during the past 40 years because erosion makes it unproductive, he said.

Competition for dwindling supplies of clean water is intensifying, too, the Cornell ecologists concluded. Agricultural production consumes more fresh water than any other human activity—about 87 percent—and 40 percent of the world’s people live in regions that directly compete for water that is being consumed faster than it is replenished. Further, water shortages exacerbate disease problems, the ecologists’ analysis pointed out. About 90 percent of the diseases in developing countries result from a lack of clean water. Worldwide, about 4 billion cases of disease are contracted from water each year and approximately 6 million people die from water-borne disease, Pimentel said. “When people are sick with diarrhea, malaria or other serious disease, anywhere from 5 to 20 percent of their food intake is lost to stress of the disease,” he said.

Prices of fossil fuels will rise as the world’s supplies are depleted. While the United States can afford to import more petroleum when its reserves are exhausted in the next 15 to 20 years, developing countries cannot, Pimentel said. “Already, the high price of imported fossil fuel makes it difficult, if not impossible, for poor farmers to power irrigation and provide for fertilizers and pesticides,” he said. The analysis was conducted by Pimentel, professor of entomology and of ecology in the College of Agriculture and Life Sciences at Cornell; Xuewen Huang, a visiting scholar in the agriculture college; Ana Cordova, a graduate student in the agriculture college; and Marcia Pimentel, a researcher in Cornell’s Division of Nutritional Sciences.

The ecologists pointed to two alarming trends: At the same time that world population is growing geometrically, the per capita availability of grains, which make up 80 percent of the world’s food, has been declining for the past 15 years. Food exports from the few countries that now have resources to produce surpluses will cease when every morsel is needed to feed their growing populations, the ecologists predicted. That will cause economic discomfort for the United States, which counts on food exports to help its balance of payments. But the real pain will wrack nations that can’t grow enough, Pimentel said. “When global biological and physical limits to domestic food production are reached, food importation will no longer be a viable option for any country,” he said. “At that point, food importation for the rich can only be sustained by starvation of the powerless poor.”

EDITORS: David Pimentel can be reached at (607) 255-2212.
Cornell University News Service, 840 Hanshaw Road, Ithaca, NY 14850
Phone: 607-255-4206; FAX: 607-257-6397