Population Growth

Population growth is the change in a population over time, and can be quantified as the change in the number of individuals of any species in a population using "per unit time" for measurement. In biology, the term population growth is likely to refer to any known organism, but this article deals mostly with the application of the term to human populations in demography.

In demography, population growth is used informally for the more specific term population growth rate (see below), and is often used to refer specifically to the growth of the human population of the world.



Estimated size of human population from 10,000 BCE–2000 CE.

Population growth is the change in a population over time, and can be quantified as the change in the number of


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:

\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 above formula can be expanded to: growth rate = crude birth rate - crude death rate + net immigration rate, or ∆P/P = (B/P) - (D/P) + (I/P) - (E/P), where P is the total population, B is the number of births, D is the number of deaths, I is the number of immigrants, and E is the number of emigrants.

This formula allows for the identification of the source of population growth, whether due to natural increase or an increase in the net immigration rate. Natural increase is an increase in the native-born population, stemming from either a higher birth rate, a lower death rate, or a combination of the two. Net immigration rate is the difference between the number of immigrants and the number of emigrants.

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 indicates the population is decreasing. 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.
[edit] Human population growth rate
Annual population growth rate in percent, as listed in the CIA World Factbook (2006 estimate).[2]
Growth rate of world population (1950-2000)
Population of the world from 10,000 BCE to 2000 CE (logarithmic scale)