An example of a regulating factor would be food supply. If the population increases to a certain size, there will be less food for each organism. This will lead to fewer births (a decrease in fecundity) and more deaths, making a negative growth rate. As there are now fewer animals, the amount of food for each organism will increase, meaning the growth rate will become positive. This would lead to a large population size again, and the cycle would start over. Therefore, food is a regulating factor in this scenario, as food supply keeps the population at relative equilibrium.
All regulating factors are density-dependent, meaning they keep populations at equilibrium by counteracting fluctuations in population size per unit area (or per unit volume for species living within three dimensional environments, such as water). Other regulating factors of the human population at present are drinking water supply, amount of arable land (obviously a more fundamental term for food), air pollution and prevalence of communicable disease. The major regulating factor for the human population in current times is inadequacy of safe drinking water, since waterborne disease is the principal environmental cause of mortality.
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