Lesson

ESSENTIAL QUESTION

What determines

a population’s

size?

By the end of this lesson, you

should be able to explain how

population size changes in

response to environmental

factors and interactions between

organisms.

Population Dynamics

By looking like a snake, this caterpillar may scare off predators. However, the effectiveness of this defense depends on population size.If there are few real snakes, predators won’t be fooled for long.

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Vocabulary Terms3 Synthesize You can often define an unknown

word if you know the meaning of its word parts.

Use the word parts and sentences below to

make an educated guess about the meaning of

the words immigrate and emigrate.

Word part Meaning

im- into

e- out

-migrate move

Example sentenceMany deer will immigrate to the new park.

immigrate:

Active Reading

1 Predict Check T or F to show whether you think

each statement is true or false.

T FPlants compete for resources.

Populations of organisms never

stop growing.

Animals never help other

animals survive.

Living things need the nonliving

parts of an environment to

survive.

2 Explain When a chameleon eats a butterfly,

what happens to the number of butterflies in

the population? How could a sudden decrease

in butterflies affect chameleons?

Engage Your Brain

4 Identify This list contains the vocabulary

terms you’ll learn in this lesson. As you

read, circle the definition of each term.

Example sentenceBirds will emigrate from the crowded island.

emigrate:

Quick Labs

Exploration Lab

Lesson 3 Population Dynamics 105

Movin’ OutHow can a population grow or get smaller?

Active Reading 5 Identify As you read, underline the processes that

can cause a population to grow or to get smaller.

A population is a group of organisms of one species that lives in

the same area at the same time. If new individuals are added to the

population, it grows. The population gets smaller if individuals

are removed from it. The population stays at about the same size if

the number of individuals that are added is close to the number of

individuals that are removed.

Visualize It!6 Apply Label the arrow that shows

emigration from Maui and the arrow that

shows immigration to Maui.

If fruit flies move to a new island, that island’s population increases.

Fruit fly population sizes change as individuals move between islands.

By Immigration and EmigrationPopulations change in size when individuals move to

new locations. Immigration occurs when individuals

join a population. For example, fruit flies may

travel on fruit to a new island. The population of

fruit flies on the new island grows as fruit flies

immigrate. Emigration occurs when individuals leave

a population. The population of fruit flies on the

original island decreases when fruit flies emigrate.

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By Birth and DeathPopulations increase as individuals are born. For example, consider

a population of 100 deer in a forest. The population will increase

if 20 fawns are born that year. But what if 12 deer are killed by

predators or disease that year? Populations decrease as individuals

die. If 20 deer are added and 12 are lost, the population will have

an overall increase. At the end of the year, there will be 108 deer.

The number of births compared to the number of deaths helps to

determine if a population is increasing or decreasing.

7 Apply Use the terms birth, death, and

immigration to label each way that

this population is changing.

A

B

C

An individual being carried off by a predator

A wandering male joins the population

A mother with nursing babies

Visualize It!

Lesson 3 Population Dynamics 107

Visualize It!

Know Your Limits What environmental factors influence population size?A tropical rain forest can support large

populations of trees. A desert, however,

will probably support few or no trees. Each

environment has different amounts of the

resources that living things need, such as food,

water, and space.

Resource AvailabilityThe amount of resources in an area influences

the size of a population. If important resources

are lost from the environment, a population may

shrink. The population may grow if the amount

of resources in the environment is increased.

But if the population continues to grow, the

individuals would eventually run out of resources.

The carrying capacity is the maximum number of

individuals of one species that the environment

can support. For example, the carrying capacity,

or the number of owls that a forest can

support, depends on how many mice

are available to eat and how

many trees are available for

the owls to live in.

Animals use plants as food and shelter. Plants depend on sunlight and water as resources.

Deforestation causes a sudden change in resource availability.

8 Identify Make a list of each population

in the image that would be affected

by drought.

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Think Outside the Book

Changes in the EnvironmentThe carrying capacity can change when the environment changes.

For example, after a rainy season, plants may produce a large

crop of leaves and seeds. This large amount of food may allow an

herbivore population to grow. But what if important resources

are destroyed? A population crash occurs when the carrying

capacity of the environment suddenly drops. Natural disasters,

such as forest fires, and harsh weather, such as droughts, can cause

population crashes. The carrying capacity can also be reduced

when new competitors enter an area and outcompete existing

populations for resources. This would cause existing populations

to become smaller or crash.

Active Reading 9 Describe What are two ways in which the

environment can influence population size?

Drought slowly reduces the amount of water available as a resource to different populations.

10 Apply With a classmate,

discuss how the immigration of

new herbivores might affect the

carrying capacity of the local

zebra population.

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Visualize It!

Abiotic FactorsThe nonliving parts of an environment are called abiotic factors. Abiotic factors include water, nutrients, soil,

sunlight, temperature, and living space. Organisms need

these resources to survive. For example, plants use sunlight,

water, and carbon dioxide to make food. If there are few

rocks in a desert, lizard populations that use rocks for

shelter will not become very large.

Biotic FactorsRelationships among organisms affect each one’s growth

and survival. A biotic factor is an interaction between living

things. For example, zebras interact with many organisms.

Zebras eat grass, and they compete with antelope for this

food. Lions prey on zebras. Each of these interactions is a

biotic factor that affects the population of zebras.

What factors can limit population size? A part of the environment that keeps a population’s size

at a level below its full potential is called a limiting factor. Limiting factors can be living or nonliving things in an

environment.

Maximum Capacity 12 Identify Label each of the

following factors that limit

plant population growth as

abiotic or biotic.

11 Apply Think about how people limit the populations of pests

such as insects and mice. List one abiotic factor and one biotic

factor that humans use to limit these pest populations.

Abiotic Biotic

Inquiry

This plant grows between the rocks.

This plant has a disease.

A

B

C

Herbivores are eating this leaf.

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A Fungus Among Us!

Why It Matters

In many parts of the world, frog populations are shrinking. We now know that many of these frogs have died because of a fungal infection.

Meet the fungus Chytrid fungi [KY trid jy] live in water. They are important decomposers. One of them, called Bd, infects frogs.

Inquiry

13 Describe How does Bd fungus harm frogs?

14 Recommend Imagine that an endangered frog lives

near an area where Bd was just found. How could

you help protect that frog species?

15 Apply Design an experiment to test whether using

soap or using bleach is the better way to clean

boots to prevent Bd contamination. What are the

independent and dependent variables? Remember

to include a control in designing your experiment.

Extend

Stop the Spread Bd is found in wet mud. If you go hiking in muddy places, washing and drying your boots can help stop Bd from spreading.

Deadly DiseaseFrogs take in oxygen and water through their skin. Bd interferes with this process. The fungus also affects an infected frog’s nervous system.

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Teamwork

What interactions between organisms can influence population size? As living things try to gather the resources

they need, they often interact with each other.

Sometimes interactions help one individual

and harm another. At other times, all of the

organisms benefit by working together.

CompetitionWhen two or more individuals or populations

try to use the same limited resource, such as

food, water, shelter, space, or sunlight, it is called

competition. Competition can happen among

individuals within a population. The elk in a

forest compete with each other for the same

food plants. This competition increases in winter

when many plants die. Competition also happens

among populations. For example, different

species of trees in a forest compete with each

other for sunlight and space.

Visualize It!

16 Predict The image above shows individuals from

two populations competing for access to water.

What would happen to the size of the lion

population if elephants usually won this

competition?

What would happen to each population if lions

usually won this competition?

Animals compete for access to water.

Unit 2 Interactions of Living Things112

Active Reading 17 Identify As you read,

underline how cooperation can influence population

dynamics.

CooperationCooperation occurs when individuals work

together. Some animals, such as killer whales,

hunt in groups. Emperor penguins in Antarctica

stay close together to stay warm. Some

populations have a structured social order that

determines how the individuals work with each

other. For example, ants live in colonies in which

the members have different jobs. Some ants find

food, others defend the colony, and others take

care of the young. Cooperation helps individuals

get resources, which can make populations grow.

These ants cooperate to protect aphids that produce a substance that ants eat.

18 Compare Make an analogy between an ant colony

and a sports team. How does each group work

together to achieve a goal?

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22 Synthesize Describe how a change in the environment

could lead to increased immigration or emigration.

Populations grow due to birth and immigration and get smaller due to death and emigration.

The carrying capacity is the maximum number of individuals of one species an environment can support.

Both populations and individuals can compete or cooperate.

21 Some birds warn other birds when predators are close. This type of interaction is called

Population Dynamics

To complete this summary, fill in the blanks with the correct word or phrase. Then use the key below to check your answers. You can use this page to review the main concepts of the lesson.

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Visual Summary

Answers: 19 grow; 20 decrease; 21 cooperation

19 If more individuals are born in a population than die or emigrate, the population will

20 If the amount of resources in an environment decreases, the carrying capacity for a population will probably

Unit 2 Interactions of Living Things114

Population A

Population B

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LessonLesson ReviewVocabularyCircle the term that best completes the following sentences.

1 Individuals joining a population is an example of emigration / immigration.

2 A part of the environment that prevents a population from growing too large is a(n) abiotic / limiting / biotic factor.

3 Individuals cooperate / compete when they work together to obtain resources.

Key Concepts4 Identify What is a limiting factor?

5 Describe How do limiting factors affect the carrying capacity of an environment?

6 Explain Give one example of how cooperation can help organisms survive.

7 Provide Name two factors that increase population size and two factors that decrease population size.

Critical ThinkingUse the illustration to answer the following questions.

8 Infer What might cause birds in Population A to immigrate to the island?

9 Predict How will the level of competition among birds in Population B change if many birds from Population A join Population B?

10 Conclude Explain how a change in the environment could cause a population crash.

11 Relate How does population size relate to resource availability in an environment?

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Lesson 3 Population Dynamics 115