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Carrying capacity transgressed two ways
by William R. Catton. Jr.

Biology is, as Hardin (1986) has reminded us, rich with insights that indicate a need for a "massive restructuring of popular opinions." In particular, the supposition that Earth is a cornucopia for mankind needs serious modification. Unfortunately, appropriate opinion restructuring is impeded by an inherent antagonism: although ecologists recognize there are limits to ecosystem sustainability, politicians are professionally compelled to remain deaf to suggestions that growth of human activities and elevation of consumption cannot be perpetual. The ecologists time horizons are based on evolution or succession; politicians' horizons are seldom more than two or four years away, because they get re-elected by encouraging electorates to expect them (at least in election years) to promote economic growth.

This article will offer suggestions for getting some fundamental ecological insights onto the public policy agenda. Specifically, I will try to go a step beyond Hardin and make the case for remarriage of sociology and biology as a means to this end. Although knowledge of both the ecological and sociological nature of the human species is politically necessary to forestall disaster, few national leaders yet recognize it. Carrying capacity needs to be understood as the maximum load an environment can permanently support (i.e., without reduction of its ability to support future generations), with load referring not just to the number of users of an environment but to the total demands they make upon it. For human societies, as for populations of other species, the relation of load to carrying capacity is crucial in shaping our future. Public comprehension of the concepts of carrying capacity and load is both vague and inadequate, and the need to correct these deficiencies is urgent.

Human ecology

For two reasons, Homo sapiens is a species especially likely to transgress an environment's sustainable carrying capacity. First, humans have an unusually long period of maturation. Therefore, sociologists commonly view learned culture (rather than biological instincts) as the explanatory mainspring in accounting for human behavior patterns. We must now also see that the modesty of an infant's demands upon an ecosystem obscures the immensity of the load each adult may later impose. Second, our cultural nature enables our wants vastly to exceed mere physiological appetites.

Ecology has long been described as the study of interrelationships among organisms and their environment. My task is to show what special turns in such study are required by the special nature of human organisms. When sociology shuns such biological concepts as carrying capacity (or distorts their meaning in embracing them), it ignores important determinants of human experience. But unless ecologists take the facts of human culture appropriately into account, they, just as truly, are being unrealistic. Various sociologists who have styled themselves human ecologists have purported to resolve the differences. Let us examine some of their efforts.

Aware of the central importance of the ecosystem concept, Duncan (1959, 1961) sought to adapt that concept for use in human ecology, taking into account two fundamental ways in which humans differ from other organisms in their ways of environmental interaction. Humans develop technology and humans organize into groups more elaborately and more variably than nonhuman populations do. So to Duncan the human ecological version of the ecosystem concept seemed to consist of what he called an ecological complex comprising four classes of interdependent variables--population, organization, environment, and technology (POET.)

Hawley (1973), reacting somewhat sternly to a paper by Odum (1969), also insisted on the special nature of human involvement with ecosystems. In the second part of Figure 1, I have used Duncan's POET notation to represent the insistence by Hawley that for humans the relation between a population and its environment is always mediated by the organization and technology employed by that population. To Hawley this mediation not only seemed to mitigate the specificity of environment as a finite (local) territory, it appeared also to abrogate environmental limits to social progress such as were presupposed by the author of the famous 1798 essay on population pressure, Robert Malthus (and seemingly accepted by Odum).

This diagrammatic representation of Hawley's idea (using Duncan's notation), with O and T enclosed between two arcs, resembles a lens. Thus it seems to reflect Hawley's view that organization and technology could magnify an environment's carrying capacity. However, as the symmetry of the diagram reveals, it is equally plausible to imagine looking through the lens from the E side, in which case O and T would magnify P. Indeed, organization and technology have enlarged the resource appetites and environmental impacts of various human populations.

Next I applied the Duncan notation to the conception of human ecology put forth by Park, the Chicago sociologist credited with first using the term human ecology more than 60 years ago. Park (1936) differentiated human ecology from plant and animal ecology by pointing out the need "to reckon with the fact that in human society [there is a] cultural superstructure [that] imposes itself as an instrument of direction and control upon the biotic substructure." Pursuant to that difference, Park spoke of a social complex comprising three elements--population, artifact (technological culture), and custom and beliefs (non-material culture). In the third part of Figure 1, these ideas are represented with the POET notation, and as a result the social complex is seen as an entity; interaction occurs between it and its environment, not just between each of its component variables and the environment, or between P and E mediated by O and T.

The third model of ecological reality is, I submit, superior to either of the first two described. For ecosocial theory purposes, this representation of Park enables us to think of O and T as modifications of P (Winner 1986), and I propose therefore to construe Park's work as recognition of some new (ecosocial) taxa, which, without waiting for official acceptance of the idea by systematists, can be referred to as Homo colossus. This article will demonstrate the appropriateness of this designation.

Prosthetic polymorphism

With different organizations and technologies, one population of humans can be a very different sort of ecological entity than another human aggregate. Accordingly, let us invoke the biological concept of polymorphism. It has been defined somewhat simplistically by Topoff (1981) for a colony of social insects as "the existence of individuals that differ in both size and structure," and defined more generally, yet more precisely, by Ford (1955) as "the occurrence together in the same habitat of two or more distinct forms of a species in such proportions that the rarest of them cannot be maintained merely by recurrent mutation."

Leaving aside the issues of genetics and natural selection implicit in Ford's definition, let us consider division of labor, a classic topic in sociology that was recognized as early as 1893 to be an extension of the biological phenomenon of organic specialization (Durkheim 1933). Division of labor arises even in very simple human societies, based at least on age and sex differences. In modern societies it becomes much more elaborate (Catton 1985).

For the task of modeling ecosystem processes when humans are involved, we need to broaden the concept of polymorphism. I propose that the possible differentiation of functions is limited when it has to depend either on biological polymorphism within a single species or on genetically based differences between the Various species cooperating in a biotic community. Human societies have transcended these limits, and, ecologically speaking, what is distinctive about our species is that we have substituted sociocultural differentiation and technology for biological polymorphism and interspecific differences.

This is the way biologists' and sociologists' views of the special nature of the human species ought to converge. Among the members of a human labor force, the polymorphism that makes possible a highly ramified division of functions is in the tools rather than in the hands that wield them. There is polymorphism in the socially instilled contents of the brains that control those hands and those tools, not in the biological structure of those brains.

Machines, tools, and other artifacts can be described as prosthetic organs--detachable extensions of the human body. The British Museum of Natural History in London has an eloquent exhibit on natural selection that includes a display comparing variations in an organ with variations among tools adapted for different tasks. It acquaints viewers with the ecological significance seen by Darwin in the assorted types of beaks on the several species of finches he observed exploiting different resources on the Galapagos Islands (British Museum of Natural History Staff 1981).

To advance the task of modeling ecosystem processes in which humans are involved, we should therefore broaden (in a sociological direction) our use of the term polymorphism. If we consider an industrial civilization's human labor force not just as a population of furless and bipedal mammals but instead as a population of social complexes (tool-users-modified-by their-respective-tools-and-organizational-roles), then it can be seen that our species is impressively polymorphic. Culture enables Homo colossus to be, in this sense, the world's most polymorphic species.

This important ecological implication of culture has new significance for reuniting sociology and biology. Substitution of human sociocultural polymorphism for the less diversified and much less flexible biological version has important consequences. Ecologists and sociologists should stress this fact to the public--who may then require such understanding among elected policy makers.

Sociocultural polymorphism had already been recognized (without the label) when Colinvaux (1973, p. 579) wrote, "Man alone can change his niche without speciating." I prefer to speak of quasispeciation, meaning the adaptation of various members of the one human species to different niches by cultural (i.e., technological and organizational) differentiation without recourse to genetic differentiation. It is by this means that humans have in the course of their evolution several times succeeded in usurping from other species portions of the planet's total life-supporting capacity. Each time, human numbers increased. Now it is essential to see the latest episodes of quasispeciation can lead to resource scarcity and environmental degradation. Colinvaux (1973, p. 579) recognized that "The time is already on us when…the carrying capacity of our living space is not enough to provide a broadened niche for all men who now exist."


Freese (1985) provides a clear definition of a serial trap, further elucidating issues raised by the now famous description of the commons dilemma by Hardin (1968). A serial trap exists when resources required by a user population are replaced over time at a more or less constant rate; replacement rate is exceeded by use rate; resources depletion cumulatively affects further availability, so that relative scarcity intensifies exponentially; and as time passes, system degradation becomes less and less reversible. As Freese notes, serial traps clearly do occur in natural ecosystems not under human domination. Various species in Various ecosystems have experienced the cycle or irruption and crash (e.g., birds, caribou; see Remmert 1980, Welty 1982). But he seeks to persuade sociologists that serial traps also occur in human-dominated ecosystems (Whittaker 1975), and that the dependence of industrial societies on nonrenewable resources must be seen as an example. (By definition, the replacement rate for nonrenewable resources must be effectively constant, i.e., zero, and any nonzero rate of use must exceed it.) Modern societies have consistently mistaken rates of discovery for rates of replacement (Pratt 1952, Simon and Kahn 1984), entrapment being the result of the illusion that all is well if use rates are just not yet in excess of recent discovery rates.

What political and economic decision makers and their constituents most need to learn from an ecosocial theory is the idea that cumulative effects of ecosystem use can make it progressively less feasible to retreat from an accustomed use pattern back to an earlier one after the newer pattern belatedly comes to be seen for the trap it is (Costanza 1987).

Quite recently, man-machine combinations enlarged the effective environment, but precariously so. Between 1930 and 1960, most draft animals on US farms were replaced by tractors. According to the Office of Technology Assessment (1985), this released some 20% of US cropland from raising feed for animals and made it available for growing crops for human consumption. Conventional wisdom accepts this as unmitigated progress. It is not seen as a trap. Ecologists may astutely ask, however, what is to happen after humans have expanded their numbers or their appetites in response to the 20% capacity increment? If the fossil fuels for tractors become depleted and too costly, some land may need again be devoted to producing biomass fuel (either for the tractors or as feed for a new generation of draft animals).

The OTA (1985, p. 19) went on to say, "The increased mechanization of farming permitted the amount of land cultivated per farm worker to increase fivefold from 1930 to 1980." For purposes of ecological modeling, it is as if farm workers (as PTO complexes, not just as P) had been enlarged by a factor of five; each can do five times as much farming as could his less colossal grandfather. How was this enlargement accomplished?

According to the OTA (1985, p. 19) report, "The amount of capital...used per worker increased more than 15 times in this period" and furthermore there is now heavy reliance "on the nonfarm sector for machinery, fuel, fertilizer, and other chemicals." Clearly, then, the farm labor force is not just P; O and T can sensibly be viewed as extensions of it. But again, we need to recognize as a trap this conversion of Homo sapiens into Homo colossus.

Carrying capacity and von Liebig's law

The concept of carrying capacity, if correctly understood, can spotlight traps. For any use of any environment by any population, there is a volume and intensity of use that can be exceed only by degrading that environment's future suitability for that use. Carrying capacity, the word for maximal sustainable use level, can be exceeded--but only temporarily. Ecologically, Malthus's main error was supposing that it was not possible for a population to increase beyond the level of available sustenance. It can and does happen, but always the overshoot will be temporary.

The comparably tragic error of Malthus's latter-day critics has been to mistake serial traps for progress, i.e., to construe technological change that facilitates temporary evasion of carrying capacity limits as permanent elevation (or repeal) of those limits. When load comes to exceed carrying capacity, the overload inexorably causes environmental damage; then the reduced carrying capacity leads to load reduction (i.e., a crash).

Ecologists have not made this situation clear enough. Too often they have embraced the logistic curve model f or population growth and have construed the upper asymptote as the best representation of carrying capacity (e.g., Emlen 1984). The logistic curve does not rise above that upper limit, and the limit is represented by a constant in the mathematical formula. But carrying capacities are not constant; they can and do change.

Political and economic leaders, and social scientists tend to exaggerate any recognition that carrying capacity is not constant into the supposition that it is infinite. The fact that carrying capacities can be difficult to measure cannot exempt populations from the consequences of exceeding their environments' power to sustain them. The human prospect would be brighter if somehow these points were to be central to the agenda for the next super power summit meeting--but of course they won't even be mentioned.

Recently there has appeared both a biological and sociological literature alleging an inescapably subjective element in so--called carrying capacity. It is said that carrying capacity ultimately depends on people's value judgements (McHale and McHale 1976, Shelby and Heberlein 1984, Wagar 1964). Some imply that, unless carrying capacity can be assigned a precise value, the concept has no significance. Politicians and industrialists grasp such straws all too eagerly.

The antidote to such thinking is provided by the relation between the carrying capacity concept and von Liebig's law of the minimum. Justus von Liebig ( 1842), an agricultural chemist, showed that it was the least abundantly available nutrient that limited the yield a farm could produce. It need not be difficult to see why this must be so ( and why it can be generalized to so many phenomena), given that any organism is a complex chemical structure. The number of specimens of any organism that can be constructed from a given assortment of chemical components will be limited by the scarcest component. (The principle also can be illustrated with a nonbiological example. Imagine a collection of a dozen flashlight batteries, five battery cases, six reflector and lens assemblies, and two three-volt bulbs. The availability of only two bulbs will limit to just two the number of working two-cell flashlights that can be assembled, even though there will be other parts left.)

With this in mind, let us see why it is so misleading to imply or assert that the concept of carrying capacity is based on value judgment. In Figure 2a, I have represented carrying capacity by a circle and load by a square (drawn so that its area is equal to the area of the circle). Think of the circle as the cross section of a pipeline through which there is a constant flow of some limiting resource. Quantitatively, an environment's carrying capacity for a particular life form is set (according to von Liebigs law) by the continual rate of flow of the least abundantly available necessary resource. The load is clearly the product of two dimensions: the number of users of that limiting resource multiplied by the mean per capita rate of use. The point is this: a sustainable load is a load not exceeding the sustained rate of supply.

Clearly, the load may have different shapes and still be compatible with carrying capacity. Instead of the schematic square we may substitute a vertical rectangle (Figure 2b), representing an increase in the number of users, and a commensurate reduction per capita mean use of the limiting resource. As long as the area of the rectangle remains no larger than the area of the circle, we have a representation of a sustainable load. Alternatively, we could have a horizontal rectangle (Figure 2c), where per capita use level has increased and the trade-off enabling the load to remain sustainable is a reduction of user numbers.

From these comparisons what is most vital to note is that the question of a load's sustainable magnitude is an objective ecological issue, not a value question. The question of which trade-off is preferable to its alternative is a value issue. Choosing whether to increase the user population at the cost of lowering its standard of living or to raise affluence at the cost of population reduction depends on a value judgment. But it is a serious mistake to suppose this denudes carrying capacity of any objective meaning.

The importance of avoiding such erroneous thinking becomes clear when we imagine increasing one dimension of load without the trade-off on the other dimension (Figure 3). If population growth continues so that a maximum sustainable load has an overload added to it, habitat damage takes a bite out of carrying capacity. Likewise, if per capita use rises beyond the level prevailing in an already maximal load, and there is no trade-off reduction of user numbers, the overload must again result in habitat damage and carrying capacity reduction.

The normative questions that have led some analysts to declare carrying capacity a useless concept have to do with questions of equity rather than of sustainability. Neither biologists nor sociologists should confuse the two. Political disagreements as to what constitutes equitable allocation of finite resources should not obscure the fact that nature exacts penalties when loads exceed carrying capacity, whether the excess comes on the vertical or the horizontal dimension.

Actually, the human load has been expanding on both dimensions. The number of humans on Earth has increased enormously since prehistoric times. Also there has been great technological progress over the millennia, especially in the last two centuries. We are not yet accustomed, however, to putting these two items of knowledge together and recognizing the two-dimensioned enlargement (or the enormity of that enlargement) of the human load, nor have we come to terms with its ecological implications.

Homo colossus

This two-way expansion of the human load is represented graphically in Figure 4. For many warm blooded species, to maintain life with no gain or loss of body substance an animal needs average daily food energy intake of

W3/4 70 Kcal where w = body weight in kilograms (Kleiber 1947). Applying this formula to size estimates for various cetacean species (Gaskin 1982, Minasian et al. 1984) and to estimates of human exosomatic energy use plus food intake (Catton 1986) enables us to select particular dolphin or whale types to represent various stages in the evolution of Homo sapiens into Homo colossus. (It is cultural evolution, not biological evolution, we thus represent.)

The human load to be supported by the ecosystems of the world has not just grown from 3 x 106 individuals in 35,000 B.C. to 5 x 109 equivalent individuals today. Each of those three million hunter-gatherers was the energy-using counterpart of a common dolphin (Delphinus delphis), whereas each of today's 232 million Americans matches the energy use of a sperm whale (Physeter macrocephalus) .

Projecting that all humans can someday be as industrialized as Americans have become (Kahn et al. 1976) is equivalent to imagining a world populated by five billion sperm whales. It reflects woeful ignorance of the ecological consequences of cultural polymorphism. Such is the folly implicit in declaring that "the term carrying capacity has by now no useful meaning" (Simon and Kahn 1984, p. 45).

We urgently need widespread dissemination of the fact that carrying capacity is not infinitely expandable. The time has come when "tragic choices" must be acknowledged (Calabresi and Bobbitt 1978); in the world as it is and is going to be, human loads can grow on one axis only by shrinking on the other axis. Otherwise our legacy to posterity will be reduced carrying capacity and the human suffering that it will entail. Anyone wishing for a more humane and happier future should strive to spread ecological literacy. Those who aspire to leadership positions should be required to demonstrate that they understand the load and carrying capacity concepts.

William R. Catton, Jr., is a professor emeritus at the Department of Sociology, Washington State University, Pullman, WA 99164. Among his research interests are division of labor and the ecological basis of revolutionary change.


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