In population ecology, the carrying capacity is the population size at which the population growth rate equals zero. Population size is constrained by food availability, competition with other species, and interactions with predators and diseases. When the population size is smaller than the carrying capacity, the population growth rate is positive so populations increase in size and when population size is larger than the carrying capacity, the population growth rate is negative so that populations decrease in size. Eventually, these populations will either increase or decrease in size until the population size equals the carrying capacity at which time the growth of the population will stop. The carrying capacity represents a stable equilibrium of population size.
Populations are expected to reach a carrying capacity (rather than continuing to grow exponentially) because birth rates and death rates are density dependent (i.e., depend on population size). For example, as population sizes increase the population birth rates (the number of births/individual/time) will decline, because of increased competition for resources. As population sizes increase the per capita death rates (the number of deaths/individual/time) will increase, because of competition for resources and the influence of predators and disease. The carrying capacity is reached at the population size at which the per capita birth rate equals the per capita death rate.
Carrying capacity holds a slightly different meaning when applied to human population growth. When discussing human populations, the carrying capacity often refers to the number of number of individuals that the Earth could hold at different standards of living and levels of resource consumption. Thus, Earth’s carrying capacity is smaller if everyone is to achieve the average standard of living of people in the United States than if everyone is to achieve the average standard of living of people in developing countries. Carrying capacity has also been difficult to apply to human populations because humans can change environmental constraints through technological innovation. Ecological Footprint Analysis is an attempt to compute human carrying capacity on a global basis.