Carrying Capacity

Carrying capacity is the ability of an environment to provide the resources for life-forms to survive and reproduce indefinitely. Every species or organism has needs that must be met for it to survive, but if any population gets too large in relation to the environment's ability to provide for those needs, the ecosystem becomes overloaded and cannot provide basic needs to every organism. Human beings, for example, need space, clear air and water, food, and other essentials to survive and maintain a certain quality of existence, but if the human population gets too large relative to its environment and resource availability, the carrying capacity of the ecosystem may be overtaxed with adverse effects on human welfare. An ecosystem does have limits relative to the size of various populations it can support, whether one is talking about human beings or animal populations.

Below the carrying capacity, populations will tend to increase, while they will decrease above the carrying capacity. Population size decreases above the carrying capacity due to either reduced survivorship because of insufficient space or food or reduced reproductive success because of insufficient food or behavioral interactions. The carrying capacity of an ecosystem will vary for different species in different habitats and can change over time due to a variety of factors including trends in food availability, environmental conditions, and space. The field of population ecology, which deals with the dynamics of species populations and how these populations interact with the environment, attempts to predict the long-term probability of a species persisting in a given habitat.

William Rees (1996) has defined human carrying capacity “as the maximum rates of resource usage and waste generation that can be sustained indefinitely without progressively impairing the productivity and functional integrity of relevant ecosystems wherever the latter may be located.” The size of the corresponding population that can be maintained is a function of the technology employed and the per capita material standard of living. Regardless of the state of technology, however, humankind depends on a variety of ecological goods and services provided by nature. For sustainability, these goods and services must be available in increasing quantities from somewhere on the planet as per capita resource consumption and population increase.

Humans have developed technologies to grow more food and dispose of the wastes that we create. These technologies have extended the carrying capacity of earth. However, there is still a limit to the human population that the earth can support. This carrying capacity is a function of the number of people, the amount of resources each person consumes, and the ability of the earth to process all the wastes produced. Sustainability is about finding the right balance point among population, consumption, and waste assimilation at any point in time given the existing technologies.

This same concept of carrying capacity applies to certain elements of the environment such as air and water. Every such medium has a certain ability to absorb waste material without serious harm done to the quality of that medium. Thus, air, for example, can absorb a certain amount of waste material without serious harm being done to its quality. But if the carrying capacity of the air is exceeded, the air starts to become fouled by certain pollutants and the quality of the air is affected. Its natural dilutive capacity is violated and human health is affected as a result of exposure to harmful pollutants.

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