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Population growth |
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.
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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:
(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:
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.
When population growth can exceed the carrying capacity of an area or environment the results end with overpopulation. Spikes in human population can cause problems such as pollution and traffic congestion, though these can be addressed by technological and economic changes. Oddly enough, many people consider mass starvation to be slightly more important than traffic congestion and even pollution. Conversely, such areas may be considered "underpopulated" if the population is not large enough to maintain an economic system (see population decline).
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.
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