Objectives

1. Explain the characteristics used to describe a population.

2. Explain factors that affect population size.

3. Explain exponential and logistic growth.

4. Define the following:

A. Population density

B. Immigration

C. Emigration

D. Carrying capacity

Characteristics of Populations

1. Geographic distribution – the area inhabited by a population

2. Density – number of individuals per unit area

3. Growth rate

4. Age structure

Math Connections

Population density = Number of Individuals/Unit area

Suppose there are 150 bullfrogs living in a pond that covers an area of 3 square kilometers. What is the density of the bullfrog population?

150 divided by 3 = 50 bullfrogs per square kilometer

Factors that Affect Population Size

1. Number of births

2. Number of deaths

3. Number of individuals that enter or leave the population

A. Immigration – movement of individuals into an area

B. Emigration – movement of individuals out of a population

Population Growth

Exponential Growth

A. Occurs under ideal conditions with unlimited resources

(Abundance of food, water, and shelter; absence of predation and

disease)

B. Population reproduces at a constant rate

Logistic Growth

A. Occurs when a population’s growth slows or stops following a period

of exponential growth

B. Resources become less available

Carrying Capacity

Carrying capacity is the largest number of individuals that a given environment can support.

What is the growth rate if a particular species reaches its carrying capacity in a given environment?

PopulationGrowth

can be

represented bycharacterized by characterized by represented by

which cause a

Exponentialgrowth

Logisticgrowth

Fallinggrowth rate

S-shapedcurve

Limits ongrowth

No limits on growth

J-shapedcurve

Constantgrowth rate

Unlimitedresources

Section 5-1

Concept Map

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Nu m

ber

of Y

east

Cel

l s

Time (hours)

Carrying capacity

Section 5-1

Figure 5-4 Logistic Growth of Yeast Population

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Name That Resource!

A situation that causes the growth of a population to decrease is called a limiting factor. Some limiting factors depend on the size of the population. Other limiting factors affect all populations in similar ways, regardless of the population size.

Section 5-2

Interest Grabber

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1. Imagine a small island that has a population of five rabbits. How might

each of the following factors affect the rabbit population?

a. climate

b. food supply

c. predation

2. Now imagine another small island that has a population of 500 rabbits.

How would the same factors affect this population?

3. Which of the factors depend on population size? Which factors do not depend on population size?

Section 5-2

Interest Grabber continued

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Name that Resource!1. All of these factors could limit this population. The food supply would have to dwindle substantially in order for it to affect only five rabbits. One predator could wipe out the entire population of five rabbits.

2. Predation and the availability of food will have more of an effect on this population than the smaller one because there are more rabbits to be preyed upon and to compete for the food.

3. The factors that depend on population size include the competition for food and predation. The factor that does not depend on population size is climate.

Objectives

1. Explain factors that limit population growth.

2. Define the following:

A. Limiting factor

B. Density-dependent limiting factor

C. Predator-prey relationship

D. Density-independent limiting factor

Limits to Growth

Density-Dependent Factors – depend on population size

1. Competition – Crowded; organisms struggle with one another for

food, water, space, sunlight, and other essentials of life

Note: Competition can also occur between members of

different species. This type of competition is a major

force behind evolutionary change.

2. Predation (Examples: relationships between sea otters and sea

urchins and sea otters and killer whales)

3. Parasitism and Disease

Density-Independent Factors – unusual weather, natural disasters, seasonal cycles, certain human activities

Section 5-2

Section Outline

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Growth of Aphids

Exponential growth

Steady populationsize

Peak populationsize

Rapid decline

Steady populationsize

Section 5-2

A Density-Dependent Limiting Factor

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60

50

40

30

20

10

01955 1960 1965 1970 1975 1980 1985 1990 1995

2000

1600

1200

800

400

0

2400

Moose Wolves

Section 5-2

Figure 5-7 Wolf and Moose Populations on Isle Royale

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Wolf and Moose

1. What density-dependent factors may have caused the crash in the wolf population?

Decline in the moose population 2. What density-independent factors may have caused the crash in the

wolf population? Unusually deep winter snows could have made it difficult for the wolf to

hunt, parasites or disease could have weakened or killed the wolves3. How are the increases and decreases in the moose population related

to changes in the wolf population? As the moose population increased, the wolf population increased. Decreases in the moose population were followed by decreases in the

wolf population.

How Fast Are We Growing?

Until about 500 years ago, the world’s human population remained fairly stable. Then, as advances in medicine, agriculture, and technology occurred, the human population began growing very rapidly. Today, the world’s human population is greater than 6 billion people, and it continues to grow, but at a slower rate.

Section 5-3

Interest Grabber

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Objectives

1. Describe how the size of the human population has changed over time.

2. Explain why population growth rates differ in countries throughout the world.

3. Define the following:

A. Demography

B. Demographic Transition

C. Age-structure diagram

Demography – the scientific study of human populations

Demographic transition – change in a population from high birth and death rates to low birth and death rates

Age-structure diagram – graphs that represent the numbers of people in different age groups in a population

Agriculturebegins

Plowingand irrigation

Bubonicplague

IndustrialRevolutionbegins

Section 5-3

Human Population Growth

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Important People:

Dr. Leila Denmark – oldest practicing physician in the United States;

helped invent the whooping cough vaccine in 1936

Connections to Economics:

Two centuries ago, English economist Thomas Malthus observed that human populations were growing rapidly. He predicted that such growth would not continue indefinitely. According to Malthus, famine and disease would limit human population growth.

Today, scientists have identified a variety of other social and economic factors that can affect human populations.

U.S. Population Rwandan PopulationMales MalesFemales Females

Section 5-3

Figure 5-13 Age Distribution

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How do the United States and Rwanda differ in the percentages of 10-14 year olds in the population?

In the United States, 10-14 year olds make up about 6.5% of the population, and in Rwanda they make up about 14% of the population. The difference is about 7.5% - about 3.5 percentage points for males and 4 percentage points for females.