CARRYING CAPACITY

Carrying capacity is the maximum population size that a species can maintain indefinitely in a given area–that is, without diminishing the capacity of the area to sustain the same population size in the future. Carrying capacity is thus a function of both the resource requirements of the organism and the size and resource richness of the area. The carrying capacity of an area with constant size and richness would be expected to change only as fast as organisms evolve different resource requirements.

Measuring Carrying Capacity

The concept is simple, but it is notoriously difficult to measure. The identity and dynamics of resources critical to a species, and the complex of other factors that regulate its population size, are typically poorly known. Moreover, as usage of the term has spread beyond its original context–to do with sustainable stocking levels of domestic livestock on rangeland–into disciplines such as ecology, carrying capacity has taken on subtle but substantive differences in meaning. For instance, in theoretical population ecology, carrying capacity is defined by the parameter K (the equilibrium density of a species) within the logistic equation of population growth. These variations in meaning make it difficult to apply the term consistently across disciplines. Despite these limitations, most authors consider at least the broad concept (as defined above) to be an important heuristic tool.

Carrying Capacity and Human Beings

Applied to human beings, the carrying capacity concept is further complicated by the unique role that culture, broadly interpreted, plays in our species. Three culturally linked factors stand out as critical: individual differences in types and quantities of resources consumed; rapid evolution in patterns of resource consumption; and technological and other cultural change. To take the case of energy consumption, which can be considered a proxy for over-all resource use, in 1990 an average person in a developed nation used about 7.1 kilowatts of energy per year, while the average person in the developing world used just 0.9 kilowatts per year. (These averages of course mask large variation at the individual level.) Moreover, economic, social, and technological development bring vast changes in patterns of energy consumption: Global energy consumption has increased more than 20 fold since 1850, and there have been dramatic changes in the composition of energy sources and technologies.

Carrying capacity for human beings is thus highly variable across space and time, depending on levels and styles of living and their supporting technologies and social systems. Ten thousand years ago, at the dawn of agriculture, the world's human population was somewhere between 2 and 20 million, perhaps an indication of global carrying capacity under those conditions. The cultural (including technological) advances associated with the development of agriculture allowed human populations to expand far beyond the levels possible under the resource demands of a hunter-gatherer lifestyle. A few populations have retained that preagrarian pattern of resource use.

Biophysical and Social Carrying Capacity

Biologists distinguish between biophysical carrying capacity–the maximum population size that could be sustained biophysically under given technological capabilities–and social carrying capacity–the maximum that could be sustained under a specified social system and its associated pattern of resource consumption.

At any level of technological development, social carrying capacities are necessarily lower than biophysical carrying capacity, because the latter implies a factory-farm lifestyle that would be both universally undesirable and also unobtainable because of inefficiencies inherent in social systems. For example, what is considered to be food by a society is largely culturally determined. A population that eats large quantities of grain-fed meat requires four to five times more grain than a population sustained by a solely vegetarian diet. A vegetarian diet is more efficient from a caloric point of view than a meat-oriented one, but is not widely acceptable in many societies. Further inefficiencies, from a biophysical standpoint, result from unequal resource distribution–at local, national, and international scales. The higher the level of inequality, the smaller the population that can be sustained. Many other aspects of culture play significant roles in determining carrying capacity, ranging from patterns of investment in education and social development to frequency and severity of warfare.

Sustainability

A sustainable condition, process, or activity is one that can be maintained without interruption, weakening, or loss of valued qualities. Sustainability is thus a necessary and sufficient condition for a population to be at or below carrying capacity. The wide appeal of sustainability as a societal condition or goal reflects the moral conviction that the current generation should pass on its inheritance of natural wealth–not unchanged but undiminished in potential–to support future generations.

Are the collective activities of today's human population sustainable? The answer is clearly no. Many essential activities, notably food and energy production, are maintained only through the exhaustion and dispersion of a one-time inheritance of natural capital. Maintenance of the world's present human population, and accommodation of its anticipated growth, requires safeguarding the critical resources and services that are provided by this natural capital. A partial list of these includes: generation and renewal of fertile agricultural soil; provision of fresh water, energy, construction materials, and minerals; purification of air and water; mitigation of flood and drought; waste treatment and nutrient cycling; seed dispersal; generation and maintenance of biodiversity; protection from ultraviolet radiation; stabilization and moderation of climate; and crop pollination. In many parts of the world, the natural capital stocks providing this stream of goods and services are being severely degraded and depleted.

The Ecological Footprint

Ecological footprint analysis is a heuristic tool that turns the carrying capacity issue around, asking what productive land area would be required to sustain a given population's activities. It is calculated that the productive land of about two and a half more planet Earths would be required to support a global population of 6 billion people at a consumption level comparable to that of the present-day inhabitants of Vancouver, Canada, the home base of the originator of the concept.

See also: Ecological Perspectives on Population; Land Use; Limits to Growth; Sustainable Development; Water and Population.

bibliography

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Jai Ranganathan

Gretchen C. Daily