Biology 12

510 MHR • Unit 5 Population Dynamics

available in subsequent years. In other words, the

age structure of a population is also very important

in determining its rate of growth.

Figure 15.4The medium projection on this graph depends

on all regions of the world reaching the replacement fertility

rate by 2050. The high and low projections assume that the

fertility rates are somewhere within the ranges shown on the

legend. Projections were calculated in 1998 by staff of the

United Nations Population Division.

As a country or region passes through a

demographic transition, its age structure changes.

This can be seen by looking at a series of age

pyramids for a nation like Sweden, as shown in

Figure 15.5. Sweden’s population was growing in

1900 (r=0.01), whereas in 2000 its rvalue was

zero and its fertility rate was 1.5. Notice that in

Figure 15.5(B), a large number of children under

the age of five are shown in the bar at the base of

the diagram, which had already started to become

less pyramid-shaped. In most countries involved in

World War II, birth rates increased substantially in

the years following the war (from about 1948 to

1960 in many countries). This period was referred

to as the baby boom. In many countries, when this

group of children reached reproductive age (roughly

25 to 30 years later), they produced their own baby

boom (referred to as the echo). Because fertility rates

had already fallen in most of these countries, the

number of births resulting from the echo tended to

be smaller than the original boom. In general, as a

country completes its demographic transition, its age

structure becomes less pyramidal and, in some cases,

starts to taper at the bottom as well as at the top.

Figure 15.6 on page 511 shows the contrast

between the age pyramids of three countries, drawn

using population statistics for the year 2000. The

Democratic Republic of Congo was still showing

rapid growth (r=3.1), whereas the United States

displayed slow growth (r=0.6) and Germany

negative growth (r=−0.1). Some countries with age

pyramids currently shaped like the Democratic

1950197019902010203020502070209021102130

5

10

15

20

25

30

2150

Population size (in billions)

Year

low

medium

high

0200 200 200 0 200 200 0 200 200 0 200

Population size (in thousands)

Post-

reproductive

Pre-

reproductive

Age group

Reproductive

100 – 104

90 – 94

80 – 84

70 – 74

60 – 64

50 – 54

40 – 44

30 – 34

20 – 24

10 – 14

0 – 4

baby

boom

Figure 15.5When a country is in the early stages of

demographic transition, a large proportion of its population

is young, and therefore the population grows quickly. As the

population passes through Stages 2 and 3, the shape

of the age pyramid changes to become less pyramidal.

A Sweden 1900 B Sweden 1950 C Sweden 1977 D Sweden 2025