The term carrying capacity refers to the sustainable relationship between an organism and its environment.  It describes a population level of an organism that can be supported, given the quantity of food, habitat, water, and other life infrastructure present.  Since food availability, population size, and environmental factors all vary inconsistently over time, the Earth's carrying capacity is not an absolute number, but is better described by a differential equation.

            As the Earth's human population continues to grow and environmental degradation becomes more widespread, some people have begun to wonder what the Earth's maximum carrying capacity is and whether we are below or above that level (or are likely to be in the future).  Doomsayers like Paul Ehrlich (author of The Population Bomb) and Garrett Hardin (author of "The Tragedy of the Commons") believe that the Earth has already exceeded its carrying capacity.  Their critics, such as Julian Simon (author of The Ultimate Resource) are proponents of the cornucopian belief in endless resources and unlimited population growth empowered by technological progress.  Others, such as demographer Joel Cohen, think that the real problem is with the concept of carrying capacity itself – "on examination, none of the existing concepts of carrying capacity in basic or applied ecology turn out to be adequate for the human population" (Cohen 1995, p. 237).  I tend to agree with Cohen, that the concept of a carrying capacity is very real, but that it is too complex for us to be able to identify outright.  In this essay I will highlight some of these complexities to explain why the carrying capacity concept has very limited applications to human beings.

            First of all, the term "carrying capacity" has several different definitions.  In basic ecology it is defined as K in a formula called the logistical curve as the "number of individuals in a population that the resource of a habitat can support".  More specifically, some ecologists define it as the point at which the birth rate equals the death rate.  Still others describe it as "the average size of a population that is neither increasing nor decreasing".  Alternatively it can be difined in terms of Liebig's law of the minimum which "asserts that, under steady-state conditions, the population size of a species is constrained by whatever resource is in shortest supply" (Cohen 1995, p. 238-241). 

            The branch of applied ecology uses even more varied definitions.  Cohen cites five distinct definitions: population size at which the standing stock of animals is maximal; population size at which the steady yield of animals is maximal; animal population size for maximal plants; the size of a harvested population that belongs to a sole owner; and the population size of an open-access resource (Cohen 1995, pp.249-51).

            Ecologists have made arguments on both sides of these definitions of carrying capacity, either supporting or critiquing their usefulness.  In this discussion, though, it is relevant to keep in mind that all of these definitions were deduced from non-human behavior and may or may not even be applicable to human beings.  We may not be similar enough to non-human species for these definitions to allow us to make useful predictions.  It is possible that some feature of human beings, such as our intelligence and self-awareness, make it difficult to apply the carrying capacity concept to our species.

            Environmentalist Bill McKibben sums up the dilemma of applying conclusions made from studying species, such as deer, to human problems:

"Consider the simplest difficulties. Human beings, unlike deer, can eat almost anything and live at almost any level they choose: hunter-gatherers use 2,500 calories of energy a day, while modern Americans use seventy-five times as much. Human beings, unlike deer, can import what they need from thousands of miles away. And human beings, unlike deer, can figure out new ways to do old things. If we need to browse on hemlock trees to survive, we could crossbreed lush new strains, chop down competing trees, irrigate forests, spray a thousand chemicals, freeze or dry the tender buds at the peak of harvest, genetically engineer new strains -- and advertise the merits of pine branches til everyone was ready to switch. The variables are so enormous that professional demographers barely bother even trying to figure out carrying capacity" (McKibben, 1998, p.73).

Cohen concludes that the basic ecological definitions of carrying capacity fail to be applicable to human populations.  The logistical curve has been horrendously wrong at prediction human population trends; we cannot calculate an average population size of a human population that is neither growing nor declining because the population has yet to stop growing; and Liebig's law of minimum fails to take into account human beings' ability to substitute resources when one becomes scarce (Cohen 1995, pp.256-7).

            Likewise, the definitions from applied ecology concepts also fail to be applicable.  We are clearly not trying to maximize the human stock since we have set aside much land for wilderness all over the world; no one seems to be cropping human beings; no one seems to be maximizing the plant life either, considering the destruction of flora; and no one owns or harvests human beings (Cohen 1995, p.257).

            Cohen concedes that one definition does have some bearing on human beings and that is the idea of a population size in an open-access resource.  This is same sort of idea that Garrett Hardin illustrates in his article "The Tragedy of the Commons".  Hardin describes the conflict over resources between individual interests and the common good by explaining how every action has both a positive and a negative component.  As applied to population growth, the parents benefit from having each additional child but the larger community suffers slightly from the degradation of resources that result from each additional child.  Crucially, the division of these components is unequal: the individual parent gains all of the advantage, but the disadvantage is shared between all members of the community.  Consequently, the weighing up of these utilities makes it rational for each individual to choose to have additional children.  The overpopulation and environmental degradation that result are externalities.

            However, these externalities are difficult to quantify precisely and so it may be possible for economists, demographers, and others to take opposite positions about the net external benefits or net external costs of each additional birth.  In fact, this quantification may be impossible because it is not based on the objective analysis of facts.  Rather, at its core this quantification revolves around political and moral judgments and so the answer is closely tied to what the individual observer values.  For example, the sufficient level and distribution of material well-being and the anticipated sophistication of technology necessary for subsistence, are factors that would influence an individuals evaluation of the Earth's carrying capacity.

Sources:

Cohen, Joel E. How many people can the earth support? New York: WW Norton and Company, 1995.

McKibben, Bill. Maybe one: a personal and environmental argument for single-child families. New York: Simon and Schuster, 1998.