population growth
population growth, in population ecology, a change in the number of members of a certain plant or animal species in a particular location during a particular time period. Factors affecting population growth include fertility, mortality, and, in animals, migration—i.e., immigration to or emigration from a particular location. The average change in a population over time is referred to as the population growth rate. A positive growth rate indicates a population increase, and a negative growth rate indicates a population decrease. The upper limit of a population in a given environment, referred to as the environment’s carrying capacity, is determined by the amount and availability of resources that are life-sustaining for that population.
Calculating population growth
Population growth rates in a given location and time period can be calculated by subtracting population loss, or the combined rates of mortality and emigration, from population gain, or the combined rates of fertility and immigration. Fertility is the number of offspring produced on average by an individual species member under certain environmental conditions over a period of time. (Fertility is not to be confused with fecundity—the theoretical maximum number of offspring that can be produced by a species member in a given time period. The sex ratio and age structure of a population affect fertility rates, because they constrain the number of individuals capable of reproducing.
Population dynamics
The natural environment plays a crucial role in population growth through its carrying capacity. The life-sustaining resources in an environment are limited and can be reduced for individual species members by greater population density and competition from other species, among other factors. Moreover, fertility, mortality, and migration are all affected by food availability, mate availability, and environmental stress factors such as pollution and natural disasters.
Population growth dynamics may be graphically depicted in an S-shaped curve, known as a logistic curve, as depicted in Figure 1. The logistic curve (right) represents an initial lag in growth, a burst of exponential growth, and finally a decline in population growth. When population density is high, mortality tends to increase because of competition for resources, predation, or increased disease transmission, resulting in the plateau in growth at the end of the curve. Population growth rates may also fluctuate in correlation with seasonal variations in the environment. For example, in deep lakes, a spring thaw causes colder, deeper waters to rise to the lake’s surface, releasing nutrients that then cause bursts in the growth of plankton, including algae, bacteria, and protozoans (see water bloom).
The population growth of different species may also be determined by predator-prey relationships—for example, following an increase in the population of zooplankton (minute aquatic animals), the population of their algae prey may decrease or grow at a slower rate. This results in an oscillating population pattern, such as the cyclical fluctuation of the snowshoe hare and lynx populations, as depicted in Figure 2.
Human population growth
The world’s human population experienced exponential growth from the 18th century, but growth rates have been declining since the second half of the 20th century. Although population growth rates vary significantly between countries and some regions continue to experience increasing growth rates, the overall rate of growth is decreasing.
The world’s human population reached 8 billion in November 2022 and is predicted to peak at 10.4 billion by 2080 and to remain at that level until the end of the century. Although human mortality rates have been decreasing on average, the main reason for the decreasing population growth rate is lower fertility. Fertility varies based on reproductive behaviour patterns, which in turn depend upon factors such as cultural traditions, socioeconomic conditions, access to contraception, and ecological variables, such as population density.
Decreasing mortality among humans in recent decades has changed the age structure of the population. It has been predicted by the Population Division of the United Nations Department of Economic and Social Affairs that by 2050 the world’s population of people age 65 or older will be twice as large as the population of children under age 5. That means that proportionately fewer people will be of reproductive age in the coming decades, which will contribute to a decline in population growth.