Chapter 5 Populations

Three important characteristics of a population are geographic
distribution, density, and growth rate.
• Geographic distribution, or range, is the area in which a
population lives.
• Population density is the number of individuals per
unit area. An example is the number of people per square
kilometer.
• Growth rate is how quickly a population increases or
decreases in size.

Three factors can affect population size: the number of births,
the number of deaths, and the number of individuals that enter
or leave the population. Populations increase through births and
immigration. Immigration is the movement of individuals into an
area. Populations get smaller through deaths and emigration.
Emigration is the movement of individuals out of an area.

Exponential growth occurs when members of a population
reproduce at a constant rate. This growth pattern is shown
by a J-shaped curve. As the population grows, the number of
reproducing members keeps rising. Thus the population grows
faster and faster. Under ideal conditions with unlimited
resources, a population will grow exponentially. In nature,
exponential growth does not go on for long. Resources are used
up in time. This causes population growth to slow or stop.
Predators and disease also slow exponential growth.

As resources become less available, the growth of a population
slows or stops. This is called logistic growth. Logistic
growth is shown by an S-shaped curve. Logistic growth occurs
when a population’s growth slows or stops following a period
of exponential growth. For example, if food resources start to
run out or space becomes limited, a population that was growing
exponentially may start to exhibit logistic growth. The population
size when growth stops is called carrying capacity. Carrying
capacity is the number of individuals of a specific species a given
environment can support.

5–2 Limits to Growth
A limiting factor is anything that slows population growth.
There are two kinds of limiting factors.

• Density-dependent limiting factors rely on population
size. They include competition, predation, parasitism,
and disease. Competition occurs when organisms are using
the same ecological resource at the same time. Predation
occurs when one organism captures and feeds on another
organism. The organism that gets eaten is the prey. The
organism that eats the prey is the predator. Predator-prey
relationships can affect the size of both the predator
population and the prey population. For example, a decrease
in the prey population will be followed, sooner or later, by a
decrease in the predator population.

• Density-independent limiting factors do not rely on
population size. They include natural disasters and human
activities such as damming rivers. When such factors occur,
many species show a rapid drop in population size.

5–3 Human Population Growth
The size of the human population tends to increase with time. For
most of human existence, the population grew slowly. About 500
years ago the population began growing faster. Agriculture and
later industry made food more available. Improved sanitation and
medicine reduced death rates. However, birthrates stayed high in
most places. This led to exponential growth. Today, the human
population continues to grow exponentially.

Demography is the scientific study of human populations.
Demographers try to predict how human populations will change
over time. Birthrates, death rates, and the age structure of a
population help predict why some countries have high growth
rates while other countries grow more slowly.
Over the past century, population growth in the United States,
Japan, and much of Europe slowed greatly. Demographers call
this shift a demographic transition. The transition began as
death rates fell, causing a brief rise in population growth. Then
birthrates fell, slowing population growth. Most people live in
countries that still have high population growth.

Demographers use age-structure diagrams to help predict
population growth. An age-structure diagram is a bar graph.
It shows how many people of each gender are in each age group
in the population. To predict how world population will change,
demographers need to think about the age structure of each
country as well as the number of people with fatal diseases,
including AIDS.

© Pearson Education, Inc., publishing as Pearson Prentice Hall.
44