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Populations and Communities
Populations
Key Concepts
What do you think? Read the two statements below and decide
whether you agree or disagree with them. Place an A in the Before column
if you agree with the statement or a D if you disagree. After you’ve read
this lesson, reread the statements to see if you have changed your mind.
Before
Statement
• What defines a population?
• What factors affect the size
of a population?
After
1. Some life exists in the ice caps of the North
Pole and the South Pole.
2. A community includes all organisms of one
species that live in the same area.
3TUDY#OACH
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
The Biosphere and Ecological Systems
Earth’s biosphere (BI uh sfihr) is the parts of Earth and the
surrounding atmosphere where there is life. The biosphere includes
all the land of the continents and islands. It also includes all
of Earth’s oceans, lakes, and streams. It includes the ice caps
at the North Pole and the South Pole. Parts of the biosphere
with large numbers of plants or algae often contain many
other organisms.
Make Flash Cards Think
of a quiz question for each
paragraph. Write the question
on one side of a flash card.
Write the answer on the other
side. Work with a partner to
quiz each other using the
flash cards.
What is a population?
The Kalahari Desert in Africa is a part of Earth’s biosphere.
Several groups of meerkats live there in a wildlife refuge.
Meerkats are small mammals that live in family groups and
help each other care for their young. Meerkats interact with
each other for survival. They sleep underground in burrows.
They hunt for food during the day. They stand upright to
watch for danger and call out warnings to others.
1. Explain Why are
meerkats considered to be
part of an ecosystem?
Meerkats are part of an ecosystem. An ecosystem is a group
of organisms that live together in an area at one time. It also
includes the climate, soil, water, and other nonliving parts of
the environment. The Kalahari Desert is one of many
ecosystems that makes up Earth’s biosphere. The study of all
ecosystems on Earth is ecology.
Reading Essentials
Populations and Communities
349
Community The figure below shows a family group of
Key Concept Check
2. Describe What defines
a population?
meerkats on the bottom right. Many species besides meerkats
also live in a wildlife refuge in the Kalahari Desert. They
include scorpions, spiders, insects, snakes, birds, zebras,
giraffes, lions, shrubs, grasses, small trees, and melon vines.
All these plants and animals form a community. A community
is all the populations of different species that live together in the same
area at the same time.
Population All the family groups of meerkats that live in
this refuge form a population. A population is all the organisms
of the same species that live in the same area at the same time. A
species is a group of organisms that have similar traits and
are able to produce fertile offspring.
Ecosystem
Biosphere
Population
Visual Check
3. Identify Name three
populations shown in the
figure.
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Populations and Communities
Competition
At times, there is not enough food for every organism in
a community. Members of a population must compete with
other populations and each other for enough food to survive.
Competition is the demand for resources, such as food, water, and
shelter, in short supply in a community. When there are not
enough resources available to survive, there is more
competition in a community. In the Kalahari Desert, where
water is scarce, the meerkats compete with other animals for
resources such as food and water.
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Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Community
Population Sizes
When there is less food available, a population of meerkats
gets smaller. Female meerkats cannot raise as many young.
Some meerkats might leave the area to find food elsewhere.
If there is plenty of food, the size of a population of
meerkats grows larger. More meerkats survive to adulthood
and live longer. Changes in environmental factors can result
in changes to the size of a population.
Limiting Factors
Environmental factors, such as available food, water,
shelter, sunlight, and temperature, are possible limiting
factors for a population. A limiting factor is anything that
restricts the size of a population. If there is not enough sunlight,
green plants cannot make food by photosynthesis. A lack of
green plants affects organisms that eat green plants.
Temperature is a limiting factor for some organisms.
When the temperature drops below freezing, many
organisms die because it is too cold for them to survive.
Disease and predators—animals that eat other animals—can
be limiting factors for organisms. Natural disasters such as
fires and floods also limit the size of populations.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Measuring Population Size
Measuring the size of a population can be difficult.
Biologists often use the capture-mark-and-release method to
count and observe animal populations. A population of lynx
in Poland is counted and monitored using this method. To
use the capture-mark-and-release method, biologists capture
several animals of a species. They sedate the animals and put
a radio collar on each one. Then they release the animals
back into the wild. The radio collars help biologists estimate
the size of the population and track the animals’
movements.
Population density is the size of a population compared to the
amount of space available. Biologists estimate population density
by a sample count. Suppose you want to know how closely
together Cumberland azaleas (uh ZAYL yuhz), a type of flower,
grow in the Great Smoky Mountains National Park. Rather
than counting every azalea shrub, you would count only the
azalea shrubs in an area, such as 1 km2. By multiplying the
number of square kilometers in the park by the number of
azaleas in 1 km2, you would find the estimated population
density of azalea shrubs in the entire park.
Reading Essentials
Key Concept Check
4. Specify What factors
affect the size of a
population?
Reading Check
5. Describe two ways you
can estimate population size.
Populations and Communities
351
Biotic Potential
Imagine that a population of raccoons has plenty of food,
water, and den space. The population has no disease and is
not in danger from other animals. The only limit to the size
of this population is the number of offspring the raccoons
can produce. Biotic potential is the potential growth of a
population if it could grow in perfect conditions with no limiting factors.
No population on Earth ever reaches its biotic potential
because no ecosystem has an unlimited supply of natural
resources.
Carrying Capacity
Make a half-book. Use it to
take notes on the relationship
between population size and
carrying capacity.
Carrying Capacity
Reading Check
capacity?
Key Concept Check
7. Summarize
How can overpopulation
affect a community?
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Populations and Communities
The carrying capacity of an environment does not
stay the same. It increases and decreases as the amount of
available resources increases and decreases. At times, a
population can briefly grow beyond the carrying capacity of
an environment.
Overpopulation
Populations can grow so large that they cause problems
for other organisms in the community. Overpopulation
occurs when a population becomes larger than the carrying
capacity of its ecosystem. For example, meerkats eat spiders.
An overpopulation of meerkats causes a decrease in the size
of the spider population in that community. Populations of
birds and other animals that eat spiders also decrease when
the number of spiders decreases.
Elephants in Africa’s wild game parks present another
example of overpopulation. Elephant herds searching for
food can cause tree damage. They push over trees to feed on
treetops. Other animals that use those trees for food and
shelter must compete with the elephants. Also, the loss of
trees can damage the soil. This might prevent other trees
and plants from growing in that area.
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Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
6. Define What is carrying
What happens when a population reaches its biotic
potential? It stops growing when the available resources in
the ecosystem are used up. The largest number of individuals of
one species that an environment can support is the carrying capacity.
A population grows until it reaches the carrying capacity of
an environment. Disease, space, food, and predators are
some of the factors that limit the carrying capacity of an
ecosystem.
Mini Glossary
biosphere (BI uh sfihr): the parts of Earth and the
surrounding atmosphere where there is life
competition: the demand for resources, such as food, water,
and shelter, in short supply in a community
biotic potential: the potential growth of a population if it
could grow in perfect conditions with no limiting factors
carrying capacity: the largest number of individuals of one
species that an environment can support
limiting factor: anything that restricts the size of a population
population: all the organisms of the same species that live in
the same area at the same time
population density: the size of a population compared to
community: all the populations of different species that live
the amount of space available
together in the same area at the same time
1. Review the terms and their definitions in the Mini Glossary. Write a sentence that
explains how limiting factors and biotic potential are related.
2. Identify each example in the flowchart as a population, an ecosystem, or a community.
Write the correct term in the box with its example.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
a. the Amazon Rain Forest
of Brazil
b. all species of monkeys,
parrots, frogs, and plants
that live in the Amazon
Rain Forest
c. all poison dart frogs that
live in the Amazon Rain
Forest
3. List two facts about population that you learned from your partner’s flash cards.
What do you think
Reread the statements at the beginning of the
lesson. Fill in the After column with an A if you
agree with the statement or a D if you disagree.
Did you change your mind?
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END OF
LESSON
Populations and Communities
353