Essentials of Ecology

CONCEPT 6-3 131

years. In developing countries, the percentage is even

higher: 30% on average (41% in Africa) compared
with 17% in developed countries (20% in the United

States and 16% in Europe). These differences in popu-
lation age structure between developed and developing

countries are dramatic, as Figure 6-9 reveals. This fig-

ure also shows why almost all of future human popula-
tion growth will be in developing countries.

We Can Use Age-Structure

Information to Make Population

and Economic Projections

Changes in the distribution of a country’s age groups

have long-lasting economic and social impacts. Be-

tween 1946 and 1964, the United States had a baby
boom, which added 79 million people to its population.

Over time, this group looks like a bulge moving up
through the country’s age structure, as shown in Fig-

ure 6-10, p. 132.

Baby boomers now make up almost half of all adult
Americans. As a result, they dominate the population’s

demand for goods and services and play increasingly
important roles in deciding who gets elected and what

laws are passed. Baby boomers who created the youth

market in their teens and twenties are now creating the
50-something market and will soon move on to create

a 60-something market. After 2011, when the first baby
boomers will turn 65, the number of Americans older

than age 65 will grow sharply through 2029 in what

Expanding Rapidly

Guatemala

Nigeria

Saudi Arabia

Expanding Slowly

United States

Australia

China

Stable

Japan

Italy

Greece

Declining

Germany

Bulgaria

Russia

Prereproductive ages 0–14 Reproductive ages 15–44 Postreproductive ages 45–85+

Male Female Male Female Male Female Male Female

Active Figure 6-8 Generalized population
age structure diagrams for countries with rapid (1.5–3%), slow
(0.3–1.4%), zero (0–0.2%), and negative (declining) population
growth rates. A population with a large proportion of its people
in the prereproductive age group (far left) has a large potential for
rapid population growth. See an animation based on this figure
at CengageNOW. Question: Which of these figures best repre-
sents the country where you live? (Data from Population Reference
Bureau)

85+

80–85

75–79

70–74

65–69

60–64

55–59

45–49

50–54

35–39

30–34

25–29

20–24

15–19

10–14

5–9

0–4

40–44

85+

80–85

75–79

70–74

65–69

60–64

55–59

45–49

50–54

35–39

30–34

25–29

20–24

15–19

10–14

5–9

0–4

40–44

Male Female

300 200 100 0 100 200 300

Developed Countries

Population (millions)

Age

Age

Male Female

Developing Countries

Population (millions)

300200 100 0 100 200 300

Figure 6-9 Global outlook: population structure by age and

sex in developing countries and developed countries, 2006.

Question: If all girls under 15 had only one child during their life-

times, how do you think these structures would change over time?

(Data from United Nations Population Division and Population

Reference Bureau)