ABSTRACT
The scientific study of the size, composition, and rate of change of human populations (demography) shows that they have the potential to grow like all other populations in naturenamely, exponentially-and are subject to all the ecological principles discussed in Chapter 8. Change in population size over a given time and space in a closed system, such as the world, is the product of its current population size and the rate of change, or growth rate (the birth rate minus the death rate). It is generally expressed as the number of births and deaths per 1,000 people per year:
Future population size
= present population size
× (birth rate – death rate)
Growth rate (r) of the human population at the global scale (usually expressed as percent per year) can undergo a natural increase or a natural decrease, or it can remain stable, a
condition known as zero population growth, where the birth rates equal the death rates:
Annual growth rate (%)
= [(birth rate – death rate)/1,000 persons] × 100
When population dynamics (changes over time and space) are considered at the local, regional, and international levels, migration (immigration and emigration) is included as a factor in the estimation of population change. Immigration and emigration are the migration of people into or out of a population from another area. With the inclusion of the migration factors, population change can be estimated as follows:
Annual growth rate (%)
= [(births + immigration)
– (deaths + emigration)]/1,000 persons
Exponential growth is a system behavior that is exhibited when a component of a system feeds upon itself by a positive feedback loop. As an example, consider the world’s human population, in which the birth of humans increases the initial human population size, which further increases the number of humans who are born (Figure 11.1). The
larger the human population gets, the faster is its growth rate (see Box 11.1 for power of exponential growth and Box 11.2 for doubling times in human population growth). When the numbers of people on Earth are plotted over time, the form of exponential growth takes a characteristic “J” shape (Figure 11.1). A useful measure of population growth rate is the doubling time (Td) for the population size, which is calculated from the annual percent growth rate (r) as follows2:
Td = 70/r
The world population of ca. 500 million in 1650 increased to ca. 4 billion (109) by doubling in 1800, 1930, and 1975 (Anderson 1981;
BOX 11.1 The Power of Exponential Growth: The Story of a King and a Mathematician (Bartlett 1978)
The growth in numbers may be illustrated by the doubling of numbers associated with the board used in the game of chess. Having performed a service for the king, the mathematician, the story goes, was asked how he could be rewarded. The mathematician told the king that he would like to be paid a wage in grains of wheat. He asked that the king place a grain of wheat on the first square of a chessboard and double the number of grains thereafter on each subsequent square. Note the number of grains the mathematician would win when he reached the 64th square (see following table). How much wheat is 264 grains, 1 grain doubled 63 times? Simple arithmetic calculation shows that it is approximately 500 times the 1976 annual worldwide wheat harvest! This quantity is probably larger than all the wheat that has been harvested by humans in the history of the Earth.
