Carrying Capacity

Carrying capacity refers to the maximum abundance of a species that can be sustained within a given area of habitat. When an ideal population is at equilibrium with the carrying capacity of its environment, the birth and death rates are equal, and size of the population does not change. Populations larger than the carrying capacity are not sustainable, and will degrade their habitat. In nature, however, neither carrying capacity or populations are ideal—both vary over time for reasons that may be complex, and in ways that may be difficult to predict. Nevertheless, the notion of carrying capacity is very useful because it highlights the ecological fact that, for all species, there are environmental limitations to the sizes of populations that can be sustained.

Carrying capacity is never static. It varies over time in response to gradual environmental changes, perhaps associated with climatic change or the successional development of ecosystems. More rapid changes in carrying capacity may be caused by disturbances of the habitat occurring because of a fire or windstorm, or because of a human influence such as timber harvesting, pollution, or the introduction of a non-native competitor, predator, or disease. Carrying capacity can also be damaged by overpopulation, which leads to excessive exploitation of resources and a degradation of the habitat's ability to support the species. Of course, birth and death rates of a species must respond to changes in carrying capacity along with changes in other factors, such as the intensities of disease or predation.

Humans, like all organisms, can only sustain themselves and their populations by having access to the products and services of their environment, including those of other species and ecosystems. However, humans are clever at developing and using technologies; as a result they have an unparalleled ability to manipulate the carrying capacity of the environment in support of their own activities. When prehistoric humans first discovered that crude tools and weapons allowed greater effectiveness in gathering wild foods and hunting animals, they effectively increased the carrying capacity of the environment for their species. The subsequent development and improvement of agricultural systems has had a similar effect, as have discoveries in medicine and industrial technology.

Clearly, the cultural evolution of human socio-technological systems has allowed enormous increases to be achieved in carrying capacity for our species. This increased effectiveness of environmental exploitation has allowed a tremendous multiplying of the human population to occur. In prehistoric times (that is, more than 10,000 years ago) all humans were engaged in a primitive hunting and gathering lifestyle, and their global population probably amounted to several million individuals. In the year 2000, because humans have been so adept at increasing the carrying capacity of their environment, more than six billion individuals were sustained, and the global population is still increasing.

Humans have also increased the carrying capacity of the environment for a few other species, including those with which we live in a mutually beneficial symbiosis. Those companion species include more than about 20 billion domestic animals such as cows, horses, pigs, sheep, goats, dogs, cats, and chickens, as well as certain plants such as wheat, rice, barley, maize, tomato, and cabbage. Clearly, humans and their selected companions have benefited greatly through active management of Earth's carrying capacity.

However, an enormously greater number of Earth's species have not fared as well, having been displaced or made extinct as a consequence of ecological changes associated with the use and management of the environment by humans, especially through loss of their habitat and over harvesting. In general, any increase in the carrying capacity of the environment for one species will negatively affect other species.

In addition, there are increasingly powerful indications that the intensity of environmental exploitation required to sustain the large populations of humans and our symbionts is causing important degradations of carrying capacity. Symptoms of this environmental deterioration include the extinction crisis, decreased soil fertility, desertification, deforestation, fishery declines, pollution, and increased competition among nations for scarce resources. Many reputable scientists believe that the sustainable limits of Earth's carrying capacity for the human enterprise may already have been exceeded. This is a worrisome circumstance, especially because it is predicted that there will be additional large increases in the global population of humans. The degradation of Earth's carrying capacity for humans is associated with two integrated factors: (1) overpopulation and (2) the intensity of resource use and pollution. In recent decades human populations have been growing most quickly in poorer countries, but the most intense lifestyles occur in the richest countries.

If it is true that the human enterprise has exceeded Earth's carrying capacity for our species, then compensatory adjustments will either have to be made by the human economy, or they will occur naturally. Those managed or catastrophic changes will involve a combination of decreased per-capita use of environmental resources, decreased birth rates, and possibly, increased death rates.