HUMAN BIOLOGY

hUMaN iMpaCts oN the Biosphere 473

Why has the human population groWn

so fast?

• Advances in agriculture, industrialization, sanitation, and health
  care have allowed human population growth to surge in the last
  several centuries.


• Differences in population growth among countries correlate with
  economic development.


• The human population will soon reach a level that will severely
  strain Earth’s resources.

taKe-Home messaGe

Demographics help explain differences in
population growth in different countries
and regions

In any given area, a population’s demographics—
characteristics such as its size, density, and overall age
structure—strongly influence its growth and its impact on
ecosystems. “Size” is the number of individuals in the pop­
ulation’s gene pool. Population density is the number of
individuals in a given habitat, such as the number of people
who live within a given area of land. Another demographic
is the general pattern in which a population’s members are
distributed in their habitat. We humans tend to cluster in
towns and cities, where we interact socially, find jobs, and
access other resources.
Age structure is the relative number of individuals of
each age in a particular population. Often, the population
is divided into prereproductive, reproductive, and post­
reproductive age categories. In theory, people in the first
category will be able to produce offspring when they are

Figure 25.2 Demographic information provides insight into
the future growth of a population. The graph at right shows
key demographic indicators for three countries, mainly in 2006.
The United States (gold bar) is highly developed, Brazil (brown
bar) is moderately developed, and Nigeria (ivory bar) is less
developed. The photograph shows just a few of the 1.3 billion
people estimated to live in India in 2015. (© Cengage Learning)

Population

in 2006

Population

in 2025

(projected)

Infant

mortality

rate

Life

expectancy

Total fertility

rate (TFR)

Population

under

age 15

Population

above

age 65

Per capita

income

298 million

$43,740

188 million

132 million

349 million

211 million

206 million

6 per 1,000 live births

29 per 1,000 live births

97 per 1,000 live births

78 years

72 years

47 years

2.1

1.9

5.5

20%

26%

42%

13%

6%

3%

$3,460

$560

Figure 25.3 An age structure diagram provides a snapshot of the number of people in a population who are of

reproductive age. These diagrams show age structure of the populations of the United States and India in 1997. (© Cengage Learning)

12 10 8 46 2 20 4 86 10 12

UNITED STATES

60 50 203040 10 0 10 20 30 40 50 60

INDIA

© Darko Sikman/Shutterstock.com

sexually mature. Along with the people in the reproductive

category, they help make up the population’s reproductive

base. The population of the United States has a narrow base

and so grows slowly. This pattern contrasts with the age

structure in rapidly growing populations, which have a

broad reproductive base (Figure 25.3).

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