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Home , Carrying capacity, Overpopulation, Population control, Population ecology

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Carrying Capacity

Carrying capacity has been used to assess the limits of into a single defi nition probably would be “the maximum a wide variety of things, environments, and systems to or optimal amount of a substance or organism (X ) that convey or sustain other things, organisms, or popula- can or should be conveyed or supported by some encom- tions. Four major types of carrying capacity can be dis- passing thing or environment (Y ).” But the extraordinary tinguished; all but one have proved empirically and breadth of the concept so defi ned renders it extremely theoretically fl awed because the embedded assump- vague. As the repetitive use of the word or suggests, car- tions of carrying capacity limit its usefulness to rying capacity can be applied to almost any relationship, bounded, relatively small-scale systems with high at almost any scale; it can be a maximum or an optimum, degrees of human control. a normative or a positive concept, inductively or deduc- tively derived. Better, then, to examine its historical ori- gins and various uses, which can be organized into four he concept of carrying capacity predates and in many principal types: (1) shipping and engineering, beginning T ways prefi gures the concept of . It has in the 1840s; (2) and game management, begin- been used in a wide variety of disciplines and applica- ning in the 1870s; (3) , beginning in tions, although it is now most strongly associated with the 1950s; and (4) debates about human population and issues of global human population. Th e idea that Earth “,” also beginning in the 1950s. Carrying has a fi nite ability to support humans, and that exceeding capacity continues to be used in all these senses, but in all that limit will result in or other cataclysms, is at except the fi rst, it has been forcefully criticized and least three hundred years old (Cohen 1996). British polit- largely discredited among scholars, often after a lengthy ical philosopher William Godwin’s estimate of 9 billion, period of enthusiastic use in both research and policy published in 1820, may seem prescient today. Th e term making. Its widespread popular use and continuing trac- carrying capacity was not coined until the middle of the tion in public debates stand in sharp contrast to these nineteenth century, however, and it was not originally critiques. conceived in relation to population at all. Rather, it emerged in the context of international shipping and subsequently was applied in a series of other fi elds— Shipping and Engineering including engineering, range and , and anthropology, and fi nally biology— Th e earliest use of carrying capacity is the most literal, before neo-Malthusians took it up in the second half of and it has been partially supplanted by other terms such the twentieth century. An understanding of this history as payload . It referred fi rst to the amount of cargo that a sheds valuable light on the limits of carrying capacity as ship could carry, measured in volume. Th is measurement a tool for evaluating and managing humanity’s impacts served a specifi c purpose in the context of international on Earth. trade in the 1840s, when steam propulsion was overtak- Intuitively, carrying capacity is a simple relation or ing the older, wind-powered technology of sailing ves- ratio: the quantity of some X that a given (amount of) Y sels. Previously, tariff s and duties had been imposed on can “carry.” Th e myriad uses of carrying capacity distilled cargo ships in terms of their “tonnage,” a measure of

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CARRYING CAPACITY • 55

volume descended from casks of wine known as tuns. A and object. Livestock, previously a Y that carried an X , ship’s hull was measured to compute its overall volume, became instead an X “carried” in the sense of “supported crews’ quarters were deducted, and the resulting fi gure or sustained by” a new Y : pastures or . Scientists in was used to assess levies on all of that ship’s voyages, Australia and New Zealand appear to have been the fi rst regardless of the amount of cargo it carried on any to use carrying capacity in this way, as they struggled to particular trip. determine how many and cattle these British pos- Although somewhat imprecise, this method was a sessions could reliably produce on their recently settled reasonably accurate way of calculating the volume of frontiers. Carrying capacity helped administrators allo- cargo a sailing ship could transport, because the hull was cate rangelands to as many settlers as possible while wholly available for cargo. With the rise of steamships, simultaneously avoiding overstocking. Th e idea quickly however, the tonnage system appeared faulty, at least to caught on in the United States, which experienced those whose interests lay in the newer technology— calamitous episodes of rangeland degradation in the notably the British, whose steam-powered merchant 1890s, especially on the unclaimed public domain and in marine fl eet led the world. Steamships had to devote areas prone to . Between 1905 and 1946, the gov- much of their “tonnage” to coal and fresh water (to gen- ernment implemented a system of leases for the vast areas erate steam), and to the huge boilers and engines that of land the Forest Service and the Bureau of Land propelled them and gave them decisive advantages over Management held, in which carrying capacity served the sailing ships (e.g., speed, power, and independence from key role of measuring the number of stock and the the vagaries of the wind). It seemed unfair to pay levies amount of time they could be grazed each year in fenced on this portion of a ship’s volume, as it could not be used areas known as allotments. Th ese measurements were to transport cargo. Carrying capacity was invented to averages calculated over periods of years, often extrapo- capture this distinction and provide an alternative basis lated from study sites to much larger areas of similar for tariff s and duties. climate, soils, and vegetation. Around 1880, carrying capacity began to be used to Th e US conservationist Aldo Leopold, who worked measure other human constructions, including canals, for the Forest Service’s Offi ce of in 1914–1915, railroads, pipelines, irrigation systems, hot air balloons, extended this use of carrying capacity from livestock to lightning rods, and electrical transmission lines (Sayre game animals. He formalized the concept in his famous 2008). No longer limited to shipping, it served the practi- 1933 textbook, Game Management , the founding work of cal need of engineers and public planners to know how the discipline now known as wildlife management. much X a particular Y was designed to carry without Leopold understood carrying capacity as an attribute of exceeding its tolerances. As in the case of shipping, it was a piece of land (rather than a particular animal ) possible to determine such limits with reasonable preci- and as a of multiple variables—including vege- sion and accuracy; they were static, fi xed by the design tation, weather, , , and disease— and materials used; and they were ideal—that is, they that together determined the size of a local wildlife referred not to the amount of X actually carried by Y at a population by aff ecting and survival. By given point in time, but the amount that could or should identifying the limiting or defi cient variable and manipu- be carried. Th ese features—numerical expression, stasis, lating it to improve the carrying capacity, the game man- and idealism—gave carrying capacity its analytical power ager could achieve conservation and optimize game and have persisted in subsequent uses of the term (Sayre for human uses such as and fi shing. 2008). Leopold’s ideas infl uenced wildlife management in the United States and abroad for most of the twentieth cen- tury, resulting in many notable successes in sustaining Livestock and Game Management popular species of game and fi sh, but also many outcomes that are now regretted by conservation , such as Carrying capacity was transferred to the measurement of the introduction of non-native species and the loss of living organisms and natural systems beginning in the (Botkin 1990). 1870s: how much X a human or a pack animal could In both range and wildlife management, scholars in carry; the amount of pollen carried on the legs of bees; the second half of the twentieth century began to critique the moisture carried by prevailing winds; the fl oodwaters carrying capacity, due primarily to practical shortcom- that a river channel could carry. Th ese were not engineer- ings and on-the-ground failures. International develop- ing questions, but they shared the literal sense of some- ment projects aimed at replicating the US model of range thing “carrying” another thing from one place to another. leases, fences, and carrying capacities in and other Th e second type of carrying capacity emerged from a developing world areas routinely failed, in part because more fi gurative notion that transposed the earlier subject fi xed carrying capacities, based on averages of rainfall or

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56 • THE BERKSHIRE ENCYCLOPEDIA OF SUSTAINABILITY: MANAGEMENT AND SUSTAINABILITY

forage production, overlooked the large year-to-year vari- and starlings in the United States. Th e pattern similarity ability of many rangelands (Behnke, Scoones, and helped validate the logistic curve empirically, while the Kerven 1993). Th e same problem occurred in wildlife diff erence between values of K in the lab and the fi eld management: if actual conditions varied from suggested that actual environments imposed restrictions place to place and year to year, and wildlife populations on , which Odum termed “environ- both responded and contributed to these changes, then mental resistance.” Second, by expressing population carrying capacity was merely an ephemeral or local growth as an equation, the logistic curve allowed scien- descriptor rather than a predictive or prescriptive tool for tists to develop mathematical models of organism- management. In shifting from engineered to natural sys- environment interactions for single or multiple species. tems, carrying capacity lost its static and ideal qualities Th ey could test the models in lab experiments or compare and therefore much of its coherence and usefulness. them to fi eld data, informing both management and research. As in range and wildlife management, carrying capac- ity in population biology eventually proved faulty. Although the sigmoid curve could indeed be found in Th e third type of carrying capacity emerged from labora- fi eld settings, the actual value of K varied over time and tory experiments in which scientists observed population . Odum had conceded that “one should not use the growth in carefully controlled environments. Provided sigmoid curve to predict the maximum size of future with optimal conditions of temperature, food, and so populations of man or organisms unless one is sure that forth, populations of fl our beetles and fruit fl ies grew the carrying capacity of the environment will remain slowly at fi rst, then accelerated, and then slowed in largely unchanged during the interval” (1953, 125). Th is asymptotic fashion toward a stable upper limit at which condition is rarely if ever met outside of the lab, however, births and deaths balanced each other. When graphed, except over very short time periods and in small or clearly the line had a sigmoid shape, like a stretched-out S . Th ese bounded settings such as ponds or islands. It follows that experiments took place in the 1920s, and the US models built on the logistic curve are unlikely to yield , who helped pioneer the research, also robust predictions of actual . As the rediscovered the forgotten work of the nineteenth- US ecologist Daniel Botkin noted, “[L]ogistic growth century Belgian mathematician Pierre-François Verhulst, has never been observed in nature” (1990, 40), and fi fty who had found a similar pattern in human population years of research has found little or no empirical support statistics and had quantifi ed it as “the logistic curve” for the concept of carrying capacity (see also Hutchinson (Hutchinson 1978). 1978). As population biology grew into a new scientifi c fi eld, the logistic curve provided scientists with a way to rede- fi ne carrying capacity as a core concept that linked Neo- research, theory, and application. In his famous textbook, Fundamentals of , the US ecologist Eugene Odum Th e fourth type of carrying capacity emerged concur- (1953) called Pearl’s and Verhulst’s asymptote “carrying rently with the third, and it drew on many of the same capacity” or, in mathematical language, K . Because sci- scientifi c developments. It applied the concept to human entists observed K under ideal environmental conditions, populations, however, and at much larger scales— they took it as the maximum possible population of an countries, continents, and the world as a whole—with a organism, independent of the environment. Odum thus view to infl uencing not scholars but policy makers and reversed Leopold’s view that carrying capacity was an the public at large. Buttressed by the scientifi c authority attribute of particular places or , defi ning it of ecology, this fi nal kind of carrying capacity helped to instead as a fi xed attribute of species themselves. In a revive the arguments made famous in T. Robert Malthus’s fi xed, ideal environment, one could observe fi xed, ideal Essay on the Principle of Population (1798). carrying capacities. Carrying capacity had been applied to human popula- Th is new carrying capacity enabled major advances in tions before. In addition to his scientifi c work, Raymond applied and theoretical population biology for two rea- Pearl had been active in debates in the 1920s and 1930s sons. First, it provided a benchmark or baseline against about , , and the specter of over- which to evaluate population dynamics in fi eld settings. population—although he had not employed the term Odum noticed that the logistic curve also approximated carrying capacity . And in the 1940s, the British colonial patterns observed when a new species arrived (or was administration of Northern Rhodesia (now Zambia) used introduced) in previously unoccupied habitats: sheep in soils maps, agricultural data, and population statistics to Tasmania, pheasants on Protection Island, Washington, calculate the carrying capacities of diff erent portions of

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CARRYING CAPACITY • 57

the colony for various forms of native farming. It relo- neo-Malthusians such as Garrett Hardin (1968, 1986) cated some fi fty thousand native Africans on the basis of and Paul and Anne Ehrlich (1990). the results (Allan 1949). Th e work went on to inform Th e concept of carrying capacity originated in con- research by anthropologists studying native agricultural texts in which human control can be eff ectively wielded practices elsewhere in Africa and beyond. over discrete objects and bounded systems at small to Ecologists enlarged and popularized carrying capacity medium scales such as a ship, a city, or a transportation as a tool for promoting beginning with system. In such settings, a quantifi ed, static, and ideal the US ecologist William Vogt’s popular 1948 book, Road measurement of limits was both desirable and achiev- to Survival . Vogt was an ornithologist who spent World able. As carrying capacity spread to other applications, War II doing rural reconnaissance for the US government however, these conditions were diffi cult or impossible to in South and Central America. His book sounded a plea meet, except in laboratory experiments. Scholars in a for conservation and development to improve the lives of wide range of social and environmental sciences con- poor people throughout the world, both for their own cluded long ago that it is fundamentally fl awed. Th e US sake and as a means to support the United States in the anthropologist Stephen Brush (1975, 806) summarized global struggle against communism. He built his argu- the problem: “the principal empirical weakness of the ments around what he called a “bio-equation”: C 5 B : E , concept of carrying capacity lies in in which C stood for carrying capacity, the fact that the theory of homeo- B for biotic potential, and E for envi- stasis inherent to the concept is ronmental resistance (Sayre 2008). neither testable nor refutable.” He applied the equation to the Similarly, the famous Anglo- world’s continents and concluded American zoologist G. Evelyn that all but North America and Hutchinson (1978, 21) off ered Antarctica had already exceeded this judgment in 1978: “When their carrying capacities, as evi- the possible value of K is con- denced by poverty, malnutrition, stantly increasing, Verhulst’s soil erosion, and other forms equation loses its value.” of environmental degradation. Th e limits of carrying capacity as Humanity faced a stark choice: a concept have direct relevance to raise the carrying capacity by debates about sustainability today. reducing environmental resistance Given its fl aws, the question that must through conservation and agricul- be asked is why the concept of carrying tural modernization or risk “the sear- capacity has persisted. Th is is due in ing downpour of war’s death from part, no doubt, to the concept’s intui- the skies” (Vogt 1948, 16). tive obviousness: everyone can under- Vogt’s concept of carrying stand the idea that a ship can carry capacity contained the same only so much cargo, or that a pasture fl aws as its predecessors’ ideas can support only so many livestock, and had. As in Odum’s case, the so forth. Also important is the authority idea of environmental resistance various advocates gave carrying capacity along was tautological, because it purported the way, before empirical evidence caught up with early to explain something that arose necessarily from an enthusiasm. Finally, agencies of the state embraced and ideal, fi xed concept of carrying capacity: namely, the promoted most of the uses of carrying capacity as they disparity between that ideal and actual empirical cases. sought to measure, regulate, tax, plan, allocate, or other- Th e very fact that humans could change their environ- wise control people, commerce, land, wildlife, and natu- ment, and thereby raise (or lower) the carrying capacity, ral of various kinds. But such control is elusive meant that Vogt’s bio-equation could really produce when sought over large, complex, and unbounded sys- only ephemeral and local inductive conclusions, just as tems that are poorly understood and diffi cult or impos- with wildlife. As the US geographer Nathan Sayre sible to control. Th e history of the concept of carrying (2008, 131) concludes, “If carrying capacity is conceived capacity teaches us that ideal, static, quantitative limits as static, it is theoretically elegant but empirically vacu- are extremely unlikely to exist in such cases; the same is ous; but if it is conceived as variable, it is theoretically probably true for sustainability. incoherent or at best question-begging.” Th ese weak- nesses did not prevent Vogt’s arguments from recurring, Nathan F. SAYRE in remarkable detail, in the works of subsequent University of California, Berkeley

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58 • THE BERKSHIRE ENCYCLOPEDIA OF SUSTAINABILITY: ECOSYSTEM MANAGEMENT AND SUSTAINABILITY

See also Ecology; Complexity Theory; Cohen, Joel E. (1996). How many people can the Earth support? New Ecosystem Services; Management; Global York: W. W. Norton. Ehrlich, Paul R., & Ehrlich, Anne H. (1990). Th e population explosion. ; Extreme Episodic Events; Human New York: Simon and Schuster. Ecology; ; Population Dynamics Hardin, Garrett. (1968). Th e tragedy of the commons. Science , 162 , 1243–1248. Hardin, Garrett. (1986). Cultural carrying capacity. Retrieved July 16, 2007 from http://www.garretthardinsociety.org/articles/ F URTHER R EADING art_cultural_carrying_capacity.html. Allan, William. (1949). Studies in African land usage in Northern Hutchinson, G. Evelyn. (1978). An introduction to . Rhodesia . Rhodes-Livingstone Papers no. 15. London: Oxford New Haven, CT: Yale University Press. University Press. Leopold, Aldo. (1933). Game management. New York: Charles Behnke, Roy; Scoones, Ian; & Kerven, Carol. (Eds.). (1993). Range Scribner’s Sons. ecology at disequilibrium: New models of natural variability and pasto- Odum, Eugene P. (1953). Fundamentals of ecology (1st ed.). New York: ral in African . London: Overseas Development W. B. Saunders Co. Institute. Sayre, Nathan F. (2008). Th e genesis, history, and limits of carrying Botkin, Daniel B. (1990). Discordant harmonies: A new ecology for the capacity. Annals of the Association of American Geographers , 98 , twenty-fi rst century. New York: Oxford University Press. 120–134. Brush, Stephen B. (1975). Th e concept of carrying capacity for systems Vogt, William. (1948). Road to survival. New York: William Sloane of shifting cultivation. American Anthropologist (new series), 77 , Associates. 799–811.

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