The carrying capacity of a biological species in an environment is the maximum population size of the species that the environment can sustain indefinitely, given the food, habitat, water and other necessities available in the environment. In population biology, carrying capacity is defined as the environment's maximal load, which is different from the concept of population equilibrium.
For the human population, more complex variables such as sanitation and medical care are sometimes considered as part of the necessary establishment. As population density increases, birth rate often decreases and death rate typically increases. The difference between the birth rate and the death rate is the "natural increase". The carrying capacity could support a positive natural increase, or could require a negative natural increase. Thus, the carrying capacity is the number of individuals an environment can support without significant negative impacts to the given organism and its environment. Below carrying capacity, populations typically increase, while above, they typically decrease. A factor that keeps population size at equilibrium is known as a regulating factor. Population size decreases above carrying capacity due to a range of factors depending on the species concerned, but can include insufficient space, food supply, or sunlight. The carrying capacity of an environment may vary for different species and may change over time due to a variety of factors, including: food availability, water supply, environmental conditions and living space.
The origins of the term carrying capacity are uncertain with researchers variously stating that it was used "in the context of international shipping" or that it was first used during 19th Century laboratory experiments with micro-organisms. A recent review finds the first use of the term in an 1845 report by the US Secretary of State to the Senate.
Several estimates of the carrying capacity have been made with a wide range of population numbers. A 2001 UN report said that two-thirds of the estimates fall in the range of 4 billion to 16 billion (with unspecified standard errors), with a median of about 10 billion. More recent estimates are much lower, particularly if resource depletion and increased consumption are considered.
The application of the concept of carrying capacity for the human population has been criticized for not successfully capturing the multi-layered processes between humans and the environment, which have a nature of fluidity and non-equilibrium, and that it often has a blame-the-victim framework.
Supporters of the concept argue that the idea of a finite carrying capacity is just as valid when applied to humans as when applied to any other species. Animal population size, living standards, and resource depletion vary, but the concept of carrying capacity still applies. The carrying capacity of Earth has been studied by computer simulation models like World3.
Food supply and consumption 
Carrying capacity, at its most basic level, is about organisms and food supply, where "X" amount of humans need "Y" amount of food to survive. If the humans neither gain nor lose weight in the long run, the calculation is fairly accurate. If the quantity of food is invariably equal to the "Y" amount, carrying capacity has been reached. Humans, with the need to enhance their reproductive success (see Richard Dawkins' The Selfish Gene[verification needed]), understand that food supply can vary and also that other factors in the environment can alter humans' need for food. A house, for example, might mean that one does not need to eat as much to stay warm as one otherwise would. Over time, monetary transactions have replaced barter and local production, and consequently modified local human carrying capacity. However, purchases also impact regions thousands of miles away. For example, carbon dioxide from an automobile travels to the upper atmosphere. This led Paul R. Ehrlich to develop the I = PAT equation
- I = P ∙ A ∙ T
- I is the impact on the environment resulting from consumption
- P is the population number
- A is the consumption per capita (affluence)
- T is the technology factor
Technology is an important factor in the dynamics of carrying capacity. For example, the Neolithic revolution increased the carrying capacity of the world relative to humans through the invention of agriculture. Currently, the use of fossil fuels has artificially increased the carrying capacity of the world by the use of stored sunlight, albeit at many other expenses. Other technological advances that have increased the carrying capacity of the world relative to humans are: polders, fertilizer, composting, greenhouses, land reclamation, and fish farming.
Agricultural capability on Earth expanded in the last quarter of the 20th century. But now there are many projections of a continuation of the decline in world agricultural capability (and hence carrying capacity) which began in the 1990s. Most conspicuously, China's food production is forecast to decline by 37% by the last half of the 21st century, placing a strain on the entire carrying capacity of the world, as China's population could expand to about 1.5 billion people by the year 2050. This reduction in China's agricultural capability (as in other world regions) is largely due to the world water crisis and especially due to mining groundwater beyond sustainable yield, which has been happening in China since the mid-20th century.
Ecological footprint 
One way to estimate human demand compared to ecosystem's carrying capacity is "Ecological footprint" accounting. Rather than speculating about future possibilities and limitations imposed by carrying capacity constraints, Ecological Footprint accounting provides empirical, non-speculative assessments of the past. It compares historic regeneration rates (biocapacity) against historical human demand (Ecological Footprint) in the same year. One result shows that humanity's demand for 1999 exceeded the planet's biocapacity for 1999 by over 20 percent.
See also 
- Arable land
- Asymmetry Principle
- Ecological footprint
- Environmental space
- Limit load
- List of countries by fertility rate
- Overpopulation in wild animals
- Overshoot (ecology)
- Population ecology
- Population growth
- Principles of Intelligent Urbanism
- r/K selection theory
- Simon–Ehrlich wager
- Thomas Malthus
- Toxic capacity
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