Carrying Capacity Essay ⋆ Environment Essay Examples ⋆ EssayEmpire

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The concept of carrying capacity has a complex history; the only definition that would capture all of its meanings would be “conveyed or supported by some encompassing context, site, or resource.” However, one can distinguish four major types of carrying capacity concepts since the term was coined circa 1845, and it is the variety of more specific meanings the different Xs and Ys and their mutual influences over time that has given the concept both its power and its flaws.

Mechanical Capacity

At its origins, carrying capacity referred to mechanical or engineered attributes of manufactured objects or systems. It arose first in the context of shipping. In 1845, a “tonnage duty” imposed by the Republic of Texas was described as applying to steamboats “according to their carrying capacity only,” as distinct from the overall volume of the boats. In the late 19th century, the term was used in this sense in discussions of steam ships, Native American canoes, the British merchant fleet, and as a way of expressing the volume of the world’s glaciers. Parallel applications included the carrying capacity of the electric commuter rail system in Paris, measured in people transported per hour; the capacity of lightning rods and transmission lines to carry electricity; and the capacity of irrigation ditches and pipelines to carry water.

Beginning in the 1870s, carrying capacity was applied to living organisms and natural systems, while retaining its literal sense of conveying or transporting something. In an 1873 monograph, The Topography and Geology of Santo Domingo, carrying capacity referred to how much weight the inhabitants’ pack animals could haul. In the Botanical Gazette of 1887, the legs of certain bees were said to have a carrying capacity for the pollen of specific flowers. An 1888 article in Science referred to the carrying capacity for floodwaters of the main branch of the Atchafalaya bayou in Louisiana. The American Naturalist of 1896 used carrying capacity in relation to the cells through which water moved in cucumber plants.

Livestock and Wildlife

The second type of carrying capacity concept emerged in the context of livestock in the late 1880s, when the implied subject the animals that carried freight became instead the things being “carried” by the land where they lived. Two articles in 1886 referred to “the stock carrying capacity” of New Zealand; in 1889, in Science, carrying capacity was explicitly (and without the qualifier “stock”) used as a measure of rangeland productivity, in units of sheep per square mile. In essence, the meaning of “carrying” had changed from a literal to a much more figurative sense. This new concept gained momentum due to widespread and severe overgrazing in the American West at the time, and by the turn of the century it was sufficiently well established.

During the 1920s and 1930s, early wildlife managers applied this concept of carrying capacity to game animals and their habitats. Aldo Leopold encountered the term and the concept in 1914-15, when he briefly worked in the Forest Service’s Office of Grazing. According to C. Meine, “the discovery would reverberate through his work for the rest of his life,” beginning with the infamous collapse of the deer population on the Kaibab plateau in the mid-1920s. The episode, which recurred later in Wisconsin and elsewhere, introduced an additional variable not considered in the livestock context: predators. Leopold’s pioneering 1933 textbook, Game Management, defined carrying capacity as “a property of a unit of range” that varied in space and time and that could be exceeded during cyclic or irruptive increases in a species’s population, resulting in high mortality: “In hoofed animals there is so far no visible evidence of any density limit except the carrying capacity of the food.”

New Postwar Definitions

After World War II, two additional types of carrying capacity concepts emerged concurrently, with overlapping points of origin but widely divergent audiences and applications. One retained flora and fauna as its object but transformed the epistemological basis of carrying capacity from inductive and applied to deductive and theoretical. The other shifted the object of the concept to humans and expanded its scale to continents and the entire globe, giving rise to the neo-Malthusian sense of carrying capacity that pervades general use of the term today.

In his 1953 textbook, Fundamentals of Ecology, Eugene P. Odum observed that populations “characteristically increase in size in a sigmoid or Sshaped fashion … regardless of whether one is dealing with fruit flies in a milk bottle or with fish in a new pond.” He defined carrying capacity as the asymptote to which the sigmoid curve converged, an “upper limit” K, where “a more or less equilibrium level is reached.” The apparent universality of the sigmoid curve was derived not from field measurements-which Odum conceded were “few, incomplete, and hard to come by”but from laboratory experiments (with “fruit flies, flour beetles, or other convenient organisms”) in artificially optimized environmental conditions of temperature, food, space, and so on. Such conditions were said to reveal the “intrinsic rate of natural increase” of different organisms; differential equations could then be used to infer “the environmental resistance created by the growing population itself, which brings about an increasing reduction in the potential reproduction rate as population size approaches the carrying capacity.” Paradoxically, “ideal” environmental conditions allowed carrying capacity to appear as a property of organisms abstracted from any environment. Models could then be developed and tested for single or multiple species.

Odum recognized that his concept of carrying capacity could be applied to humans as well; indeed, the logistic equation at the basis of his model had its origins in the work of Belgian mathematician Pierre-Francois Verhulst (1804-49) to model human population growth. Yet the final type of carrying capacity concept differs fundamentally in scale, audience, and application. Ecologist-ornithologist William Vogt published Road to Survival in 1948, in the shadow of the horrors of World War II. Defining carrying capacity as the ratio of biotic potential to environmental resistance (“C = B: E”), Vogt conceded that “the equation finds complicated expression in terms of civilized existence.” But he insisted on applying it to continental and global scales: The equation is, perhaps, oversimplified, but it expresses certain relationships-almost universally ignored-that every minute of every day touch the life of every man, woman and child on the face of the globe. Until an understanding of these relationships on a world scale enters into the thinking of free men everywhere, and into the thinking of rulers of men who are not free, there is no possibility of any considerable improvement of the lot of the human race. Indeed, if we continue to ignore these relationships, there is little probability that mankind can long escape the searing downpour of war’s death from the skies.

It is important to recognize this as a new concept of carrying capacity, even though the idea it expressed was older than the term itself. As early as 1820, William Godwin had attempted to calculate the number of humans the world could support. In his polemical response to Malthus, Of Population, Godwin took China as demonstrating the maxima of possible cultivation and population density, which he then applied to the earth’s habitable area, arriving at a figure of 9 billion people. Although his estimate may now appear prescient, Godwin was in fact mocking the idea of a determinate number, and neither he nor Malthus ever used the term carrying capacity.

Bibliography:

H.J. Behnke, I. Scoones, et al., eds. Range Ecology at Disequilibrium: New Models of Natural Variability and Pastoral Adaptation in African Savannas (Overseas Development Institute, 1993);
R.Y. Edwards and D. Fowle, The Concept of Carrying Capacity, Twentieth North American Wildlife Conference, 1955;
Paul R. Ehrlich, The Population Bomb (Ballantine Books, 1968);
Garrett Hardin, “The Tragedy of the Commons,” Science (v.162/3859, 1968);
Aldo Leopold, Game Management (Charles Scribner’s Sons, 1933);
C. Meine, Aldo Leopold: His Life and Work (University of Wisconsin Press, 1988);
Eugene P. Odum, Fundamentals of Ecology (W.B. Saunders, 1953);
A. Paulsen Jr. and F.N. Ares, “Trends in Carrying Capacity and Vegetation on an Arid Southwestern Range,” Journal of Range Management (v.14/2, 1961);
William Vogt, Road to Survival (William Sloane Associates, 1948);
M. Woodbury, A. M. “Ecology and the Population Problem,” Science (v.122, 1955);
S. Zimmerer, “Human Geography and the ‘New Ecology’: The Prospect and Promise of Integration,” Annals of the Association of American Geographers (v.84/1, 1994).