Populations and Communities

[Pages:16]Name:

Block:

Populations and Communities

Populations

Chapter 21Lesson 1 Do all marked tasks/questions.

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

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.

Key Concepts

? What defines a population?

? What factors affect the size of a population?

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.

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.

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.

3TUDY#OACH

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.

1. Explain Why are

meerkats considered to be part of an ecosystem?

Reading Essentials

Populations and Communities 349

Key Concept Check

2. Describe What defines

a population?

Community The figure below shows a family group of 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.

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Biosphere

Ecosystem

Visual Check

3. Identify Name three

populations shown in the figure.

Community

Population

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.

350 Populations and Communities

Reading Essentials

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.

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.

Key Concept Check 4. Specify What factors affect the size of a population?

Reading Check 5. Describe two ways you can estimate population size.

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Reading Essentials

Populations and Communities 351

Make a half-book. Use it to take notes on the relationship between population size and carrying capacity.

Carrying Capacity

Reading Check 6. Define What is carrying capacity?

Key Concept Check 7. Summarize How can overpopulation affect a community?

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

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.

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.

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

352 Populations and Communities

Reading Essentials

Mini Glossary

biosphere (BI uh sfihr): the parts of Earth and the surrounding atmosphere where there is life

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

community: all the populations of different species that live together in the same area at the same time

competition: the demand for resources, such as food, water, and shelter, in short supply in a community

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 the amount of space available

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.

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.

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

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?

Reading Essentials

ConnectED

Log on to ConnectED.mcgraw- and access your textbook to find this lesson's resources.

END OF LESSON

Populations and Communities 353

Chapter 21 Lesson 2 Do all marked tasks/ questions.

Key Concepts

? How do populations change?

? Why do human populations change?

Populations and Communities

Changing Populations

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

After

3. Some populations decrease in numbers because of low birthrates.

4. An extinct species has only a few surviving individuals.

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

3TUDY#OACH

Organize Information Take notes as you read. Organize your notes into two columns. On the left, list a main idea about the material under each heading. On the right, list the details that support the main idea.

How Populations Change

Spiders lay hundreds of eggs. When these eggs hatch, the spider population suddenly becomes larger. But it does not stay that way for long. Many spiders die or are eaten before they reproduce. The size of the spider population increases when the eggs hatch. The population decreases as the spiders die.

A population change can be measured by the population's birthrate and death rate. A population's birthrate is the number of offspring produced over a given time period. The death rate is the number of individuals that die over the same time period. If the birthrate is higher than the death rate, the population increases. If the death rate is higher than the birthrate, the population decreases.

SCIENCE USE V. COMMON USE

exponential Science Use a mathematical expression that contains a constant raised to a power, such as 23 or x2

Common Use in great amounts

Exponential Growth

A population in ideal conditions with unlimited resources grows in a pattern called exponential growth. During exponential growth, the larger a population gets, the faster it grows. For example, it takes E. coli, a type of microscopic bacterial organism, only 10 hours to grow from one organism to more than 1 million organisms. Exponential growth cannot continue for long. Limiting factors stop the population growth.

354 Populations and Communities

Reading Essentials

Population Size Decrease

Population size can increase, but it also can decrease. Several factors cause a population size to decrease.

? Lack of resources A mouse population might decrease in size during the winter because less food is available. Some mice will starve. More mice will die than will be born. When food is plentiful, the population usually increases.

? Natural disasters Floods, fires, hurricanes, and volcanic eruptions affect population size. They destroy habitats and food sources for organisms. The populations decrease.

? Disease The spread of disease causes populations to decrease. In the mid-1900s, a disease destroyed thousands of elm trees in the United States. The size of the population of elm trees decreased.

? Predation The hunting of animals for food is predation. Cats that live in barns feed on mice and reduce the mouse population. Predation reduces population size.

Extinction Populations can decrease in numbers until they disappear. An extinct species is a species that has died out and no individuals are left. Extinctions can be caused by predation, natural disasters, or damage to the environment.

Scientists hypothesize that the extinction of the dinosaurs about 65 million years ago was caused by a meteorite crashing into Earth. The impact would have sent tons of dust into the atmosphere, blocking sunlight. Without sunlight, plants could not grow. Dinosaurs that ate plants probably starved.

Most extinctions involve fewer species. For example, about 700 years ago humans settled in New Zealand where a flightless bird called the giant moa lived. Humans hunted the moa for food. As the human population increased, the size of the moa population decreased. The species became extinct within 200 years.

Endangered Species Wild mountain gorillas are an endangered species. Just over 400 gorillas remain in the wild in Africa. An endangered species is a species whose population is at risk of extinction.

Threatened Species A threatened species is a species at risk, but not yet endangered. California sea otters are a threatened species. Worldwide, more than 4,000 species are classified as endangered or threatened.

Reading Check 1. Summarize What are four reasons that a population might decrease in size?

Reading Check 2. Contrast What is the difference between an endangered species and a threatened species?

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Reading Essentials

Populations and Communities 355

Visual Check 3. Locate Circle the northern area on the map where the whales migrate in summer.

Key Concept Check 4. Find the Main Idea List three ways that populations change.

Reading Check 5. Determine Cause and Effect Explain how human population growth affects other species.

Movement

Populations change when organisms move from place to place. When an animal population becomes overcrowded, some individuals might move to find food or living space.

Plant populations also move from place to place. Seeds might be carried by the wind. Animals also help spread seeds.

Canada

N

W E S

Migration Sometimes an entire population moves from one place to another. It later returns to its original location.

United States

ATLANTIC OCEAN

Migration is the instinctive seasonal movement of a population of

organisms from one place to another.

Gulf of Mexico

Bahamas

Cuba

Puerto Rico

Jamaica Caribbean Sea

Haiti

Dominican Republic

Ducks, geese, and monarch butterflies are examples of organisms that migrate annually. As shown by the arrows in the map, humpback whales mate and give birth in

warm ocean waters near the

Bahamas during the winter. In the summer, they migrate

north to find food.

Human Population Changes

Birthrates, death rates, and movement also affect human population size. But unlike other species, humans have

developed ways to increase the carrying capacity of their environment. Improved crop yields, domesticated farm

animals, and timely methods of transporting foods and other resources enable people to survive in all types of

environments.

The human population is growing quickly. Scientists estimate that there were about 300 million humans on Earth a thousand years ago. Today there are more than 6 billion humans on Earth. By 2050, there could be more than 9 billion. No one knows when the human population will reach Earth's carrying capacity. Some scientists estimate that Earth's carrying capacity is about 11 billion.

As the human population grows, people need more houses and roads. They clear more land for crops. This means less living space, food, and other resources are available for other species. Also, people use more energy to heat and cool homes; to fuel cars, airplanes, and other forms of transportation; and to produce electricity. This energy use adds pollution that affects other populations.

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

356 Populations and Communities

Reading Essentials

 ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download