Population growth is the change in population over time, and can be quantified as the change in the number of individuals in a population using "per unit time" for measurement. The term population growth can technically refer to any species, but almost always refers to humans, and it is often used informally for the more specific demographic term population growth rate (see below), and is often used to refer specifically to the growth of the population of the world.

Simple models of population growth include the Malthusian Growth Model and the logistic model.

Population growth rate

In demographics and ecology, Population growth rate (PGR) is the fractional rate at which the number of individuals in a population increases. Specifically, PGR ordinarily refers to the change in population over a unit time period, often expressed as a percentage of the number of individuals in the population at the beginning of that period. This can be written as the formula: P=Poe^{kt}

\mathrm{Growth\ rate} = \frac{(\mathrm{population\ at\ end\ of\ period}\ -\ \mathrm{population\ at\ beginning\ of\ period})} {\mathrm{population\ at\ beginning\ of\ period}}

(In the limit of a sufficiently small time period.)

The most common way to express population growth is as a ratio, not as a rate. The change in population over a unit time period is expressed as a percentage of the population at the beginning of the time period. That is:

\mathrm{Growth\ ratio} = \mathrm{Growth\ rate} \times 100%.

A positive growth ratio (or rate) indicates that the population is increasing, while a negative growth ratio (or rate) indicates population decline. A growth ratio of zero indicates that there were the same number of people at the two times -- net difference between births, deaths and migration is zero. However, a growth rate may be zero even when there are significant changes in the birth rates, death rates, immigration rates, and age distribution between the two times. ^[1] Equivalently, percent death rate = the average number of deaths in a year for every 100 people in the total population.

A related measure is the net reproduction rate. In the absence of migration, a net reproduction rate of more than one indicates that the population of women is increasing, while a net reproduction rate less than one (sub-replacement fertility) indicates that the population of women is decreasing.

Human population growth rate

Annual population growth rate in percent, as listed in the CIA World Factbook (2006 estimate).^[2]

Globally, the growth rate of the human population has slowed down a little since its peak in the 1980s (see External Links), although the last one hundred years have seen a rapid increase in population due to medical advances and massive increase in agricultural productivity^[3] made by the Green Revolution.^[4][5][6]

The actual annual growth in the number of humans fell from its peak of 87 million per annum in the late 1980s, to a low of 75 million per annum in 2002, at which it stabilised and has started to slowly rise again to 77 million per annum in 2007. Growth remains high in the Middle East, South Asia, Southeast Asia, Latin America, and primarily in Sub-Saharan Africa.^[7]

In some countries there is negative population growth (ie. net decrease in population over time), especially in Central and Eastern Europe (mainly due to low fertility rates) and Southern Africa (due to the high number of HIV-related deaths). Within the next decade, Japan and some countries in Western Europe are also expected to encounter negative population growth due to sub-replacement fertility rates.

See also

Past and projected population growth on different continents. The vertical axis is logarithmic and its scale is millions of people.

Estimated population growth from 10000 BCE–2000 CE.

• Carrying capacity
• Demographic transition
• Density dependent inhibition
• Exponential growth
  • Compound annual growth rate
  • Doubling time
• Family planning
• Green Revolution
• Natalism and Antinatalism
• Overpopulation
• Population pyramid
• Population decline
• Immigration
• World population
• World population estimates
• World development
• Zero population growth
• Logistic function - concept related to logistic model
• Ronald Fisher - who referred to the population growth rate as the Malthusian Parameter
• List of countries by fertility rate
• List of countries by population growth rate

References

External links

• World population growth rates 1950-2050
• UN University annual "State of the Future" report, including updates on Millennium Project goals including balancing global population growth & resources
• BBC News - Birth rate 'harms poverty goals' - 08/12/06
• Trend of growth rate with total global population

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