<P> 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 . </P> <Dl> <Dd> I = P ∙ A ∙ T </Dd> </Dl> <Dd> I = P ∙ A ∙ T </Dd> <Dl> <Dd> I is the impact on the environment resulting from consumption </Dd> <Dd> P is the population number </Dd> <Dd> A is the consumption per capita (affluence) </Dd> <Dd> T is the technology factor </Dd> </Dl>

Which term is used to describe the maximum human population the earth can sustain