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. Its effect on population dynamics may be approximated in a logistic model, although this simplification ignores the possibility of overshoot which real systems may exhibit.
Carrying capacity was originally used to determine the number of animals that could graze on a segment of land without destroying it. Later, the idea was expanded to more complex populations, like humans. 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 US 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 non-renewable 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 for sometimes being employed in a blame-the-victim framework.
Supporters of the concept argue that the idea of a limited carrying capacity is just as valid 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 number of people is not the only factor in the carrying capacity of Earth. Waste and over-consumption, especially by wealthy people and nations, is putting more strain on the environment than overpopulation. This has been studied by computer simulation models like World3.
Some aspects of a system's carrying capacity may involve matters such as available supplies of food, water, raw materials, and/or other similar resources. In addition, there are other factors that govern carrying capacity which may be less instinctive or less intuitive in nature, such as ever-increasing and/or ever-accumulating levels of wastes, damage, and/or eradication of essential components of any complex functioning system. Eradication of, for example, large or critical portions of any complex system (envision a space vehicle, for instance, or an airplane, or an automobile, or computer code, or the body components of a living vertebrate) can interrupt essential processes and dynamics in ways that induce systems failures or unexpected collapse. (As an example of these latter factors, the "carrying capacity" of a complex system such an airplane is more than a matter of available food, or water, or available seating, but also reflects total weight carried and presumes that its passengers do not damage, destroy, or eradicate parts, doors, windows, wings, engine parts, fuel, and oil, and so forth.) Thus, on a global scale, food and similar resources may affect planetary carrying capacity to some extent so long as Earth's human passengers do not dismantle, eradicate, or otherwise destroy critical biospheric life-support capacities for essential processes of self-maintenance, self-perpetuation, and self-repair.
Thus, carrying capacity interpretations that focus solely on resource limitations alone (such as food) may neglect wider functional factors. If the humans neither gain nor lose weight in the long-term, 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), 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.
An important model related to carrying capacity (K), is the logistic growth curve. The logistic growth curve depicts a more realistic version of how population growth rate, available resources, and the carrying capacity are inter-connected. As illustrated in the logistic growth curve model, when the population size is small and there are many resources available, population over-time increases and so does the growth rate. However, as population size nears the carrying capacity and resources become limited, the growth rate decreases and population starts to level out at K. This model is based on the assumption that carrying capacity does not change. One thing to keep in mind, however, is that carrying capacity of a population can increase or decrease and there are various factors that affect it. For instance, an increase in the population growth can lead to over-exploitation of necessary natural resources and therefore decrease the overall carrying capacity of that environment.
Technology can play a role in the dynamics of carrying capacity and while this can sometimes be positive, in other cases its influence can be problematic. For example, it has been suggested that in the past that the Neolithic revolution increased the carrying capacity of the world relative to humans through the invention of agriculture. In a similar way, viewed from the perspective of foods, the use of fossil fuels has been alleged to artificially increase the carrying capacity of the world by the use of stored sunlight, even though that food production does not guarantee the capacity of the Earth's climatic and biospheric life-support systems to withstand the damage and wastes arising from such fossil fuels. However, such interpretations presume the continued and uninterrupted functioning of all other critical components of the global system. It has also been suggested that 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. In an adverse way, however, many technologies enable economic entities and individual humans to inflict far more damage and eradication, far more quickly and efficiently on a wider-scale than ever. Examples include machine guns, chainsaws, earth-movers, and the capacity of industrialized fishing fleets to capture and harvest targeted fish species faster than the fish themselves can reproduce are examples of such problematic outcomes of technology.
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.
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 footprint in 1999 exceeded the planet's bio-capacity by >20%. However, this measurement does not take into account the depletion of the actual fossil fuels, "which would result in a carbon Footprint many hundreds of times higher than the current calculation."
There is also concern of the ability of countries around to globe to decrease and maintain their ecological footprints. Holden and Linnerud, scholars working to provide a better framework that adequately judge sustainability development and maintenance in policy making, have generated a diagram that measures the global position of different countries around the world, which shows a linear relation between GDP PPP and ecological foot print in 2007. Possible answers to the question of where we are as individual countries attempting to reach sustainability and development methods to reduce ecological foot print. According to the Figure 1 diagram, the United States had the largest ecological foot print per capita along with Norway, Sweden, and Austria, in comparison to Cuba, Bangladesh, and Korea. 
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