The carrying capacity of a biological species in an environment is the population size of the species that the environment can sustain indefinitely, given the food, habitat, water and other necessities available in the environment. 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 (Sayre, 2007).
It is possible for a species to exceed its carrying capacity temporarily. Population variance occurs as part of the natural selection process but may occur more dramatically in some instances. Due to a variety of factors, one determinant of carrying capacity may lag behind another. For example, a waste product of a species may build up to toxic levels slower than the food supply is exhausted. The result is a fluctuation in the population around the equilibrium point which is statistically significant. These fluctuations are increases or decreases in the population until either the population returns to the original equilibrium point, or a new equilibrium is established. These fluctuations may be more devastating for an ecosystem than are gradual population corrections, because, if they produce drastic decreases or increases, those may affect other species in the ecosystem and they may begin to move with statistical significance around their own equilibrium points. The fear is of a domino-like effect, where the final consequences are unknown and may lead to collapses of species or even whole ecosystems.
One of the world's best-studied predator-prey relationships is the moose and wolf population of Isle Royale National Park  in Lake Superior. Without the wolves, the moose would overgraze the island's vegetation.. Without the moose, the wolves would die. The first scientists who studied the issue thought that the wolves would eventually overpopulate and kill all the moose calves, then die from famine. This has not occurred as inbreeding, disease and environmental factors have limited the wolf population naturally.
Easter Island has been cited as an example of a human population crash. When fewer than 100 humans first arrived, the island was covered with trees with a large variety of food types. In 1722, the island was visited by Jacob Roggeveen, who estimated a population of 2000 to 3000 inhabitants with very few trees, "a rich soil, good climate" and "all the county was under cultivation". Half a century later, it was described as "a poor land" and "largely uncultivated". The ecological collapse which followed has been variously attributed to overpopulation, slave traders, European diseases (including a smallpox epidemic which killed so many so quickly, the dead were left unburied and a tuberculosis epidemic which wiped out a quarter of the population), social upheaval and invasive species (such as the Polynesian rats which may have wiped out the ground nesting birds and eaten the palm tree seeds). Whatever the combination of factors, only 111 inhabitants were left on the island in 1877. For whatever reasons (whether Moai worship, survival, status or sheer ignorance), the question of how many humans the island could realistically support never seems to have been answered. This example, and others, are discussed at length in Jared Diamond's Collapse and subsequent literature (see Easter Island article for a thorough discussion).
Both herds are managed differently. The National Park Service owns and manages the Maryland herd while the Chincoteague Volunteer Fire Company owns and manages the Virginia herd. The Virginia herd, referred to as the "Chincoteague" ponies, is allowed to graze on Chincoteague National Wildlife Refuge, through a special use permit issued by the U.S. Fish and Wildlife Service. The size of both herds is restricted to approximately 150 adult animals each in order to protect the other natural resources of the wildlife refuge.
A further example is the Island of Tarawa, where the finite amount of space is evident, especially since landfills cannot be dug to dispose of solid waste, due to constraints in the subsurface rock and lack of topographic elevations. With colonial influence and an abundance of food (relative to life before the year 1850), the population has expanded to the extent that overpopulation is transparently present.
The Lotka-Volterra equations are simple mathematical model of population dynamics which show how in a closed system, like that of the wolves and moose on Isle Royale, limited prey will cause the predator population to decline rapidly. An extended example can be used where multiple species are competing for the same resources, or single species feed on multiple prey.
In the words of one researcher: "Over the past three decades, many scholars have offered detailed critiques of carrying capacity—particularly its formal application—by pointing out that the term does not successfully capture the multi-layered processes of the human-environment link, and that it often has a blame-the-victim framework. These scholars most often cite the fluidity and non-equilibrium nature of this relationship, and the role of external forces in influencing environmental change, as key problems with the term."
In other words, the relationship of humans to their environment may be more complex than is the relationship of other species to theirs. Humans can alter the type and degree of their impact on their environment by, for instance, increasing the productivity of land through more intensive farming techniques, leaving a defined local area, or scaling back their consumption; of course, humans may also irreversibly decrease the productivity of the environment or increase consumption (see Overconsumption).
Supporters of the concept argue that humans, like every species, have a finite carrying capacity. Animal population size, living standards, and resource depletion vary, but the concept of carrying capacity still applies. The World3 model of Donella Meadows deals with carrying capacity at its core.
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 or 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), 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 IPAT equation
This is another way of stating the carrying capacity equation for humans which substitutes impact for resource depletion, adding the technology term to cover different living standards. As can be seen from the equation, money affects carrying capacity - but it is too general a term for accurate carrying capacity calculation.
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 historically 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. This overshoot can be maintained for some time since stocks can be liquidated (e.g., overfishing, deforestation) and sinks can be filled up (e.g., carbon accumulation in the atmosphere). Countries can have Footprints that are higher than their biocapacity for three reasons: they can a) net-import, b) net-emit CO2 to other countries, and c) overshoot their own ecosystems. But for the world as a whole only overshoot is available, since there is no trade with other planets.
Carrying capacity debates focusing on the possibility or impossibility of future technological break-throughs for substituting resources raise the question of whether or not it is possible to define a measure of carrying capacity or sustainability which does not already contain implicit assumptions about solutions to the problems of resource over-exploitation and environmental degradation.
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.
The process of defining Tourism Carrying Capacity (TCC) is composed of two parts. It follows (in principle) the conceptual framework for TCC as described by Shelby and Heberlein (1986), and these parts are described as follows:
Descriptive part (A): Describes how the system (tourist destination) under study works, including physical, ecological, social, political and economic aspects of tourist development. Within this context of particular importance is the identification of:
Evaluative part (B): Describes how an area should be managed and the level of acceptable environmental impacts. This part of the process starts with the identification (if it does not already exist) of the desirable condition or preferable type of development. Within this context, goals and management objectives need to be defined, alternative fields of actions evaluated and a strategy for tourist development formulated. On the basis of this, Tourism Carrying Capacity can be defined. Within this context, of particular importance is the identification of:
First of all, the carrying capacity can be the motivation to attract tourists visit the destination. The tourism industry, especially in national parks and protected areas, is subject to the concept of carrying capacity so as to determine the scale of tourist activities which can be sustained at specific times in different places. Various scholar over the years have developed several arguments developed about the definition of carrying capacity. Middleton and Hawkins defined carrying capacity as a measure of the tolerance of a site or building which is open to tourist activities, and the limit beyond which an area may suffer from the adverse impacts of tourism (Middleton & Hawkins, 1998). Chamberlain defined it as the level of human activity which an area can accommodate without either it deteriorating, the resident community being adversely affected or the quality of visitors' experience declining (Chamberlain, 1997). Clark defined carrying capacity as a certain threshold (level) of tourism activity, beyond which there will be damage to the environment and its natural inhabitants (Clark, 1997).
The World Tourism Organisation argues that carrying capacity is the maximum number of people who may visit a tourist destination at the same time, without causing destruction of the physical, economic and socio-cultural environment and/or an unacceptable decrease in the quality of visitors' satisfaction (http://ec.europa.eu/environment/iczm/pdf/tcca_material.pdf. Date assessed 08/03/07). In the publication, ‘Agenda 21 for the Travel and Tourism Venture: towards environmentally sustainable development’, the Secretary-General of the World Tourism Organization.
The definitions of carrying capacity need to be considered as processes within a planning process for tourism development which involves:
“Carrying capacity is not just a scientific concept or formula of obtaining a number beyond which development should cease, but a process where the eventual limits must be considered as guidance. They should be carefully assessed and monitored, complemented with other standards, etc. Carrying capacity is not fixed. It develops with time and the growth of tourism and can be affected by management techniques and controls” (Saveriades, 2000).
The reason for considering carrying capacity as a process, rather than a means of protection of various areas is in spite of the fact that carrying capacity was once a guiding concept in recreation and tourism management literature. Because of its conceptual elusiveness, lack of management utility and inconsistent effectiveness in minimising visitors' impacts, carrying capacity has been largely re-conceptualized into management by objectives approaches, namely: the limits of acceptable change (LAC), and the visitor experience and resource protection (VERP) as the two planning and management decision-making processes based on the new understanding of carrying capacity (Lindberg and McCool, 1998). These two have been deemed more appropriate in the tourism planning processes of protected areas, especially in the United States, and have over the years been adapted and modified for use in sustainable tourism and ecotourism contexts (Wallace, 1993; McCool, 1994; Harroun and Boo, 1995).