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5.1 How Populations Grow
Lesson Objectives
List the characteristics used to describe a population.
Identify factors that affect population growth.
Describe exponential growth.
Describe logistic growth.
Lesson Summary
Describing Populations Researchers study five important characteristics of a population:
Geographic range is the area in which a population lives.
Population density is the number of individuals per unit area.
Population distribution is how individuals are spaced out in their range.
Growth rate determines whether a population grows, shrinks, or stays the same size .
Age structure is the number of males and females of each age in a population.
Population Growth Populations can grow, shrink, or stay the same size.
Factors that increase population size include births and immigration, which is the movement
of individuals into an area.
Factors that decrease population size include deaths and emigration, which is the movement
of individuals out of an area.
Exponential Growth When conditions are ideal, the larger a population gets, the faster it
grows. When a population’s numbers grow larger with each generation, exponential growth is
occurring. Ideal conditions include unlimited resources and absence of predation and disease.
Logistic Growth Resources become less available as a population grows.
Logistic growth occurs when population growth slows and then stops after a period of
exponential growth has occurred.
Population size stabilizes at the carrying capacity, the maximum number of individuals of a
given species that an environment can support.
Describing Populations
For Questions 1–5 complete each statement by writing the correct word or words.
1. The
is the area in which a population lives.
2. Population density is the
of individuals per unit area.
3. How the individuals are spaced in their range is a population’s
4. Growth rate is how quickly a population
5. To find the
of each age.
in size.
of a population, count the number of males and females
Population Growth
For Questions 6–10, write True if the statement is true. If the statement is false, change the
underlined word or words to make the statement true
6. If the death rate is less than the birthrate, the population is likely to shrink.
7. Immigration increases population size.
8. Young animals may immigrate from the place where they were born to establish
new territories.
9. A high birthrate and immigration decrease population size.
10. Populations grow if more individuals are born than die in a period of time.
Exponential Growth
age structure
exponential growth
carrying capacity
immigration
emigration
logistic growth
Number of individuals per area
Population density
Population Growth
is described by
Number of males and females
of certain ages
is affected by
with unlimited
resources is
with limited resources is
Population grows
until reaching
12. Complete the graph by drawing the characteristic shape of exponential population growth.
13. What letter is used to refer to the characteristic shape of an exponential growth curve?
Logistic Growth
14. Complete the graph by drawing the characteristic shape of logistic population growth.
15. What letter is used to refer to the characteristic shape of the logistic growth curve?
16. When real-world populations of plants and animals are analyzed, why do they most often
have the logistic growth curve?
17. What does the term carrying capacity refer to?
18. Complete the table to name and explain three phases of logistic growth. Use the terms growth
rate, population size, and carrying capacity in your explanations.
Phases of Logistic Growth
Phase
Phase name
1
2
3
Vocabulary
Term
Age structure
Carrying capacity
Emigration
Exponential growth
Immigration
Logistic growth
Definition
Explanation
5.2 Limits to Growth
Lesson Objectives
Identify factors that determine carrying capacity.
Identify the limiting factors that depend on population density.
Identify the limiting factors that do not depend on population density.
Lesson Summary
Limiting Factors A limiting factor is a factor that controls the growth of a population.
Some factors depend on the density of the population. Others do not.
Acting separately or together, limiting factors determine an environment’s carrying capacity.
Limiting factors produce the pressures of natural selection.
Density-Dependent Limiting Factors
Density-dependent limiting factors operate strongly when the number of individuals per
unit area reaches a certain point.
Examples include:
 competition
 predation and herbivory
 parasitism and disease
 stress from overcrowding
Density-Independent Limiting Factors Some limiting factors do not necessarily
depend on population size.
Density-independent limiting factors depend on population density, or the number of
organisms per unit area.
Examples include severe weather, natural disasters, and human activities.
Some of these factors may have more severe effects when population density is high .
Limiting Factors
For Questions 1–6, write True if the statement is true. If the statement is false, change the
underlined word to make the statement true
1. Limiting factors determine the immigration capacity of a population.
2. A limiting factor controls the growth of a population.
3. Limiting factors operate when growth is exponential.
4. Populations grow too large in the absence of limiting factors.
5. Competition is an example of a limiting factor.
6. Population size can be limited by factors such as predation.
Density-Dependent Limiting Factors
7. When do density-dependent factors operate most strongly?
8. What are four density-dependent limiting factors?
Density-Independent Limiting Factors
9.
What term describes a limiting factor that affects all populations in similar ways,
regardless of population size?
10. What is the usual response in the population size of many species to a densityindependent limiting factor?
11. Complete the graphic organizer with examples of density-independent limiting factors.
Population
Size
can be limited by
density-independent
factors such as
Vocabulary
Term
Density-dependent limiting
factor
Density-independent
limiting factor
Limiting factor
Definition
5.3 Human Population Growth
Lesson Objectives
Discuss the trend of human population growth.
Explain why population growth rates differ in countries throughout the world.
Lesson Summary
Historical Overview The size of the human population has increased over time.
For most of human existence, limiting factors such as the scarcity of food kept death rates
high.
As civilization advanced, agriculture, industry, improved nutrition, sanitation, and
medicine reduced death rates. Birthrates stayed high in most places. This led to
exponential growth.
Today, the human population continues to grow exponentially, although the doubling
time has slowed.
Patterns of Human Population Growth Demography is the scientific study of
human populations. Demographers try to predict how human populations will change over
time.
Over the past century, population growth in developed countries slowed. As death rates
dropped, birthrates dropped also. Demographers call this shift the demographic
transition. Most people live in countries that have not undergone the demographic
transition.
An age-structure graph shows how many people of each gender are in each age group in a
population. Demographers use such graphs to predict how a population will change. More
people of reproductive age usually means faster growth.
Many factors, including disease, will affect human population growth in the twenty-first
century. Current data suggest the human population will grow more slowly over the next
50 years than it did for the last 50 years.
Historical Overview
For Questions 1–5, write True if the statement is true. If the statement is false,
change the underlined word or words to make the statement true.
1. Over the last 1000 years, the size of the human population has
decreased.
2. Since the 1800s, human population growth has been logistic.
3. The human population has increased because birthrates have dropped.
4. The combination of low death rates and high birthrates led to
exponential growth.
5. Charles Darwin suggested that human populations are regulated by war,
famine, and disease.
6. Complete the table below to explain how each factor affected the size and growth rate of
the human population over the last 10,000 years.
Factors That Affected Human Population Growth
Cause
Effect
Agriculture
Improved health care and medicine
Improved sanitation
Bubonic plague
Industrial Revolution
Vocabulary
Term
Demographic
transition
Demography
Definition
Chapter Vocabulary Review
Crossword Puzzle Complete the puzzle by entering the term that matches each
numbered description. For two-word answers, leave a blank space between words. For
an answer with a hyphen, include the hyphen.
Across
Down
1. a limiting factor that affects populations
no matter what their size
2. the larger a population gets, the faster it
grows
4. the number of males and females of each
age in a population
7. moving out of the population’s range
3. a type of limiting factor that does not
affect small, scattered populations very
much
8. a growth pattern in which population size
stabilizes at a maximum limit
5. the maximum number of individuals of a
species that an environment can support
10. moving into a population’s range
11. a shift from high birthrates and death
rates to low birthrates and death rates
12. the number of individuals per unit area
6. a factor that controls the growth of a
population
9. the study of human population