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TRANSCRIPT
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.
Maui
BA
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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