Carrying capacity is the maximum population size (i.e., number of individuals) of a species that a geographic area, region or ecosystem can sustain indefinitely without degrading essential resources, such as food, water, shelter and habitat.
It is determined by the availability of resources and competition for them. When a population exceeds its carrying capacity, resource depletion occurs, causing the population to decline until a balance is restored.
Several mechanisms reduce population sizes when species exceed their carrying capacities. These include increased resource depletion and competition, which lead to malnutrition, disease, starvation and reduced reproduction rates. Increased predation is another factor, as larger populations tend to attract more predators.
Disease outbreaks can become more severe at higher population densities because closer and more frequent among individuals facilitates pathogen transmission. In densely populated areas, the likelihood of an infected individual encountering a susceptible host increases, allowing diseases to spread more rapidly and extensively. Higher densities may also make individuals more vulnerable to diseases by reducing food availability.
A commonly cited example of carrying capacity involves North American deer in forests. Predators, such as wolves, kept the deer population in check, but after removal through hunting, the deer population grew rapidly and exceeded the environment's carrying capacity. As a result, leaves, shrubs, berries and other food sources became scarce, leading to malnutrition, starvation and a population decline.
Carrying capacity for humans differs from other species primarily because humans can alter their environment and their social behavior. Humans use technology to increase food production (e.g., agriculture, fertilizers, irrigation), modify their environment (e.g., building infrastructure), and change their consumption patterns. This adaptability makes population limits less fixed and more variable over time. Human migration also shifts local carrying capacities by redistributing populations in search of resources. However, technological advances often come with environmental costs, such as resource depletion and climate change, which, in turn, reduce the earth's overall carrying capacity.
Estimates for the earth's carrying capacity for humans, maintaining a quality of life typical of advanced industrial countries and with minimal adverse environmental effects, typically range from 1.5 to four billion. This contrasts with the current population of about eight billion, many of whom have far lower standards of living amid the substantial and ever-increasing environmental degradation.