CHAPTER 16 Ecosystems Opening Activity · succession. Key Terms ecology habitat community ecosystem...

Chapter 16 • Ecosystems 339

Opening Activity Ecological Spheres Tell studentsthat scientists have developedsealed glass spheres in which a self-sustaining ecosystem exists.Although there is no input of mate-rials, ask students whether there isan input of anything else into theecosystem. (Yes; there is a constantinput of energy in the form of light.)

IdentifyingMisconceptionsStudents may believe that anecosystem must be an area that isdelineated by specific boundaries—like the walls of an aquarium or theedge of a pond. Point out that thelimits of an ecosystem are definedby the observer. Thus, an ecosystemcould be in a jar of pond water, orit could be the entire pond itself.

Answers1. Autotrophs are able to make

food by using available energyand materials; heterotrophsmust obtain their energy fromthe food produced byautotrophs.

2. In photosynthesis, organismsuse light energy to rearrangethe atoms in carbon dioxideand water to make carbohy-drates. Oxygen is given off tothe environment or used by theorganism itself.

3. In cellular respiration, a carbo-hydrate is combined with oxygen, rearranging its atomsinto carbon dioxide and water.Some of the energy releasedduring this process is availablefor use by the cell.

4. In photosynthesis, an input ofenergy is required, and theenergy is stored in carbohydratemolecules. In cellular respira-tion, this stored energy isreleased for the cell to use.

Answers

1. Agree; as consumers use the plants for food, some of the energy is lost to theenvironment.

2. Agree; some species, called keystonespecies, may be critical links in the foodweb.

Quick Review

Reading Activity

Looking AheadQuick ReviewAnswer the following without referring to

earlier sections of your book.

1. Contrast autotrophs with heterotrophs.

(Chapter 5, Section 1)

2. Summarize the process of photosynthesis.


3. Describe the process of cellular respiration.


4. Compare the energy flow in photosynthesis

with the energy flow in cellular respiration.

(Chapter 5, Sections 2 and 3)

Did you have difficulty? For help, review the

sections indicated.

Section 1

What Is an Ecosystem?Interactions of Organisms and Their

Environment

Diverse Communities in Ecosystems

Change of Ecosystems over Time

Section 2

Energy Flow in EcosystemsMovement of Energy Through Ecosystems

Loss of Energy in a Food Chain

Section 3

Cycling of Materials in Ecosystems

Biogeochemical Cycles

The Water Cycle

The Carbon Cycle

The Phosphorus and Nitrogen Cycles

www.scilinks.orgNational Science Teachers Association sciLINKS Internet

resources are located throughout this chapter.

Reading ActivityCopy the following statements on a piece of

paper or in your notebook, leaving a few blank

lines after each.

1. In an ecosystem, more energy is stored in

plants than in consumers.

2. The extinction of one species in an ecosystem

can have an impact on all other species.

Before you read the chapter, write down whether

you agree or disagree with each statement. After

you have finished reading the chapter, decide

whether or not you still agree with your first

response.

Materials and energy cycle continuously through the

components of this coral reef. The complex relation-

ship of organisms and their physical environment

makes up an ecological system, or ecosystem.

EcosystemsCHAPTER

16

339

Copyright © by Holt, Rinehart and Winston. All rights reserved.

OverviewBefore beginning this sectionreview with your students theobjectives listed in the StudentEdition. This section introducesstudents to the differences betweencommunities and ecosystems.Students will also explore the con-cept of biodiversity, and how onespecies may gradually be replacedby another.

Point to your aquarium (or show apicture of an aquarium with a vari-ety of organisms—different speciesof fish, snails, plants, etc.) and askyour students to list all of theorganisms they see. Tell them thatwhen they finish, they are to makea list of all the factors that affectthe survival of the organisms in theaquarium. (They should include suchthings as water, food, temperaturerange, light, pH, oxygen, and so on.)

Activity Species Count If possible, takethe students on a walk-aroundtour of your school. Point outdifferent species of plants and ani-mals that you observe. Have onestudent keep a count of the num-ber of species identified on thecampus. When finished, mentionthat biologists sampling a tropicalrain forest in Ecuador obtainedsamples suggesting there are asmany as 24,000 different insectspecies alone per acre.

MotivateMotivate

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Section 1

340 Chapter 16 • Ecosystems

GENERAL

CulturalAwarenessCulturalAwareness

Isolated Ecosystem The Yanomami are atribe living in remote rain-forest jungles ofVenezuela and Brazil. The Yanomami inVenezuela have had very little contact withoutsiders and are one of the few culturesremaining in the world in which people arestill an integral part of an intact naturalecosystem. The Yanomami and their land in

Venezuela are protected as an internationalbiosphere reserve. Outsiders must get writtenpermission to visit, but even these few visitshave given the Yanomami a taste of the out-side world. Many observers question howlong the Yanomami hunting and gatheringculture can last.

Section 1 What Is an Ecosystem?

Interactions of Organisms and Their EnvironmentIt is easy to think of the environment as being around but not part

of us—something we always use, sometimes enjoy, and sometimes

damage. But in fact, we are part of the environment along with all

of Earth’s other organisms. All of Earth’s inhabitants are interwoven

in a complex web of relationships, such as the one illustrated in

Figure 1. To understand how the interactions of the parts can affect

a whole system, think about how a computer operates. Removing

one circuit from a computer can change or limit the interactions of

the computer’s many components in ways that influence the com-

puter’s overall operation. In a similar way, removing one species

from our environment can have many consequences, not all of them

easily predictable.

In 1866, the German biologist Ernst Haeckel gave a name to the

study of how organisms fit into their environment. He called this

study ecology, which comes from the Greek words oikos, meaning

“house,” or “place where one lives,” and logos, meaning “study of.”

is the study of the interactions of living organisms with one

another and with their physical environment (soil, water, climate,

and so on). The place where a particular population of a species

lives is its . The many different species that live together in a

habitat are called a . An , or ecological system,

consists of a community and all the physical aspects of its habitat,

such as the soil, water, and weather. The physical aspects of a habi-

tat are called (ay bie AHT ihk) , and the organisms in

a habitat are called .biotic factors

factorsabiotic

ecosystemcommunity

habitat

Ecology

Objectives

● Distinguish an ecosystem

from a community.

● Describe the diversity

of a representative

ecosystem.

● Sequence the process of

succession.

Key Terms

ecology

habitat

community

ecosystem

abiotic factor

biotic factor

biodiversity

pioneer species

succession

primary succession

secondary succession

Figure 1 Organisms

interact within an

ecosystem. Organisms within

an ecosystem continually

change and adjust. This plant

species is dependent on the

bat for its reproduction, and

the bat uses part of the

flower for food.

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Group ActivityBiodiversity Before class, find anarea at your school with a goodamount of biodiversity. Organizethe students into groups, and haveeach group create a chart with thefollowing headings: Name,Diagram, Approximate AverageSize, and Approximate Number.Take your students to the area andhave them use their charts to makean inventory of the organisms foundthere. Students may not know thenames of the plants or animals, butthey can make a small diagram ofeach organism, and for the timebeing, make up a fictitious name foreach organism they draw. Havethem save this information to usefor a food web in the next section.

Reading Hint Point out to stu-dents the importance of studyingthe figures in each section. Stronglyencourage them to stop after eachsection and read the captions foreach figure. Prepare questions thatcorrespond to each figure. Forexample, in reference to Figure 1,you might ask students to hypothe-size as to what would happen if theplant species in the photographwere eradicated from the area.

SKILL

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GENERAL

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Chapter Resource File

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CareerCareerWildlife Biologists are hired by federal, state,and sometimes local government agencies tostudy and manage wildlife. Common game andnongame species, and sometimes rare andendangered species are studied and/or managedby wildlife biologists. Knowledge of eachspecies’ role in the ecosystem and its relation toother species is critical. Starting salary varieswith agency and geographic region.

• Unit 7—Ecosystem Dynamics

This engaging tutorial introducesstudents to Ecosystem Dynamics.

BIOLOGYBIOLOGY

Transparencies

TR Bellringer

TR E25 Ecological Succession at Glacier Bay

Diverse Communities in EcosystemsThe variety of organisms, their genetic differ-

ences, and the communities and ecosystems in

which they occur is termed .

Consider a pine forest in the southeastern

United States, such as the one shown in Figure

2. If you could fence in a square kilometer

(0.4 mi2) of this forest and then collect every

organism, what would you expect to get?

Which of the six kingdoms of organisms

would be represented in your collection?

Ecosystem InhabitantsLarge animals in the forest might include a bear or a white-tailed

deer. The woods also contain smaller mammals—raccoons, foxes,

squirrels, rabbits, and chipmunks. Snakes and toads often remain

hidden among the leaves. Many birds can be found, including

hawks, warblers, and sparrows. If the square kilometer included a

lake, you might find catfish, bass, perch, a variety of turtles, and

perhaps an alligator.

There are pine trees, a variety of smaller trees, and shrubs. Beneath

the trees, grasses and many kinds of flowers grow on the forest floor.

The soil contains an immense number of worms. Hidden under

the bark of trees and beneath the leaves covering the ground are

many different species of insects and spiders, such as those shown

in Figure 3.

Many of the life-forms in the soil and water of a pine forest are

too small to be seen without a microscope. Protists, which include

algae and related microscopic eukaryotes, thrive in water. There

may be billions of bacteria in a handful of soil.

biodiversity

Jumping spider Male stag beetle

The jumping spider is found in sunny, dry parts of the forest. The larvae of the

stag beetle live in and eat decaying wood and bark.

Figure 3 Forest spider and insect

Figure 2 Pine forest.

Pine forests like this one are

common in the southeastern

United States.

www.scilinks.org

Topic: Biodiversity

Keyword: HX4020

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Teaching Tip Detritivores Bacteria and fungibelong to a group of organismsknown as detritivores, which live ondetritus, the dead remains of plantsand animals. Ask students to thinkof other organisms that would bedetritivores. (Maggots, grubs, andearthworms would be examples.)

Teach, continuedTeach, continued

Introduced Species Introducing species tonew areas can wreak havoc on an ecosystem.In the 1870s Lord George Bennet founded thesmall coastal town of Bandon, Oregon. Bennetis also the one believed by many to haveimported a plant called gorse (Ulex europaeus)from his native Ireland. Gorse spreads out rap-idly, leaving a dead, dry center. Its nasty thornsmake it difficult to remove, and its naturallyhigh oil content makes it very flammable and

causes it to reach high temperatures when itburns. Having no natural enemies in Oregon,gorse spread quickly and could not be con-trolled. On Sept. 26, 1936, a spark from anearby fire ignited a gorse thicket whicheventually spread to burn 287,000 acres,destroying all but 16 of Bandon’s nearly 500buildings and killing 14 people. In 1994 thegorse spider mite was released near Bandon tohelp control this noxious weed.


GENERAL

Evaluating Biodiversity

Skills AcquiredObserving, drawingconclusions

Teacher’s Notes

Have each student use an areaof equal dimensions, at least50 3 50 m, to make the taskmore manageable and to allowcomparisons between differentecosystems. Encourage studentsto take their time; they willobserve much more if they do.

Answers to Analysis1. Answers will vary.

2. Answers will vary. Example:3 robins/29 total organisms 50.103 5 10%

3. Answers will vary.

4. Answers will vary. In general,the abiotic factors in anecosystem provide organisms(biotic factors) with a physicalplace to live, energy, nutrients,and water. The organisms alterand recycle some of theseabiotic factors, changing thelandscape in the process.

You might find many kinds of fungi growing on fallen trees and

spreading as fine threads through the decaying material on the forest

floor, as illustrated in Figure 4. Other fungi are found on the surface

of trees or rocks as lichens. Lichens are associations between fungi

and algae or cyanobacteria.

If you were to remove every organism from your square kilometer,

the nonliving surroundings that remain make up the abiotic factor.

This would include the minerals, organic compounds, water, wind

that blows over the Earth, rain, and sunlight.

Ecosystem Boundaries The physical boundaries of an ecosystem are not always obvious,

and they depend on how the ecosystem is being studied. For exam-

ple, a scientist might consider a single rotting log on the forest floor

to be an ecosystem if he or she is interested only in the fungi and

insects living in the log. Often individual fields, forests, or lakes are

studied as isolated ecosystems. Of course, no location is ever totally

isolated. Even oceanic islands get occasional migrant visitors, such

as birds blown off course.

Mushrooms are often found on moist

forest floors.

Shelf fungi grow on and digest trees.

These fungi digest plants and

other materials they find in the

forest.

Figure 4 Forest fungi

Evaluating BiodiversityBy making simple observations, you can draw

some conclusions about biodiversity in an ecosystem.

Materials

note pad, pencil

Procedure

1. CAUTION: Do

not approach

or touch any wild animals.

Do not disturb plants.

Prepare a list of biotic and

abiotic factors that you

observe around your home

or in a nearby park.

Analysis

1. Identify the habitat and

community that you observed.

2. Calculate the number of dif-

ferent species as a percentage

of the total number of organ-

isms that you saw.

3. Rank the importance of

biotic factors within the

ecosystem you observed.

4. Infer what the relationships

are between biotic factors

and abiotic factors in the

observed ecosystem.

342


water. When a gopher emerged, the moundthey made would catch seeds blowing acrossthe moon-like landscape, helping plants to getstarted. In another surprise to scientists, thegophers helped to save amphibians, whichused the tunnels as cool underground areas inwhich to avoid the hot, dry landscape.

DemonstrationBring to class a rock covered withlichens or mosses. Ask students whythese organisms are known as “pio-neer species.” (Like pioneer settlers,they are the first to inhabit a newarea.) Point out that lichen canabsorb nutrients from the bare rock.

Teaching TipSuccession Have each studentdraw an illustrated timeline repre-senting the succession that occursafter a forest fire has burned all ofthe vegetation in an area. Startingat time zero, the land should bebarren, followed by the appearanceof grasses and weeds, then smallbushes, and finally trees. Visual

Using the FigureHave students examine the topphoto in Figure 5. Ask them tothink of other events that mightoccur that would lead to primarysuccession. (Landslides and volcan-ism would be examples.) Tell stu-dents that primary succession canbe a very slow process. Scientistsestimate that the primary succes-sion from sand dunes to the beech-maple forest along the shores ofLake Michigan took about 1,000years. In contrast, secondary suc-cession may take less than 100years. Ask students why the hem-lock and spruce trees don’t comebefore the grasses and shrubs. (Thegrasses and shrubs often provide amicrohabitat that makes it possiblefor the seeds of the trees to survive.)

GENERAL

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GENERAL


did you know?

Mount St. Helens After the eruption ofMount St. Helens on May 18, 1980, succes-sion was aided by an unlikely source—pocketgophers. Pocket gophers that survived theblast dug miles of tunnels under the barrenpumice fields. The digging helped mix the soil,which improved its quality and ability to hold

Change of Ecosystems over TimeWhen a volcano forms a new island, a glacier recedes and exposes

bare rock, or a fire burns all of the vegetation in an area, a new

habitat is created. This change sets off a process of colonization

and ecosystem development. The first organisms to live in a new

habitat where soil is present tend to be small, fast-growing plants,

called . They may make the ground more hos-

pitable for other species. Later waves of plant immigrants may

then outcompete and replace the pioneer species.

SuccessionA somewhat regular progression of species replacement is

called . Succession that occurs where life has

not existed before is called . Succession

that occurs in areas where there has been previous growth,

such as in abandoned fields or forest clearings, is called

. It was once thought that the stages of

succession were predictable and that succession always led to

the same final community of organisms within any particular

ecosystem. Ecologists now recognize that initial conditions

and chance play roles in the process of succession. For exam-

ple, if two species are in competition, a sudden change in the

climate may favor the success of one species over the other.

For this reason, no two successions are alike.

Glacier Bay: an Example of SuccessionA good example of primary succession is a receding glacier

because land is continually being exposed as the face of the

glacier moves back. The glacier that composes much of the

head of Glacier Bay, Alaska, has receded some 100 km

(62 mi) over the last 200 years. Figure 5 shows the kinds of

changes that have taken place as time passed.

The most recently exposed areas are piles of rock and

gravel that lack the usable nitrogen essential to plant and ani-

mal life. The seeds and spores of pioneer species are carried

in by the wind. These include lichens, mosses, fireweed, wil-

lows, cottonwood, and Dryas, a sturdy plant with clumps

about 30 cm (1 ft) across. At first all of these plants grow

close to the ground, severely stunted by mineral deficiency,

but Dryas eventually crowds out the other plants.

After about 10 years, alder seeds blown in from distant

sites take root. Alder roots have nitrogen-fixing nodules, so

they are able to grow more rapidly than Dryas. Dead leaves

and fallen branches from the alder trees add more usable

nitrogen to the soil. The added nitrogen allows willows and

cottonwoods to invade and grow with vigor. After about 30

years, dense thickets of alder, willow, and cottonwood shade

and eventually kill the Dryas.

secondary succession

primary succession

succession

pioneer species

Recently exposed land has few nutrients.

Alders, grasses, and shrubs later take over

from pioneer plants.

As the amount of soil increases, spruce and

hemlock trees become plentiful.

A receding glacier makes primary

succession possible.

Figure 5 Glacier Bay

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ReteachingShow students a picture of a forestecosystem. Have them describe theevents that would take place aftera fire.

Quiz1. What is the difference between

communities and ecosystems?(An ecosystem is a community andits abiotic factors.)

2.How are the boundaries of anecosystem set? (in whatever waymakes it easier to study)

GENERAL

CloseClose

Answers to Section Review

1. the abiotic environment

2. Because the land is kept in a constant state ofdisturbance, secondary succession in gardensand farms does not usually get beyond the firststage of fast-growing weeds and grasses.

3. Primary succession occurs on land where therehas not been any previous plant growth, suchas recently exposed land that has few nutrients.Secondary succession occurs in areas wherethere has been previous plant growth, such asin abandoned fields or forest clearings.

4. Answers may vary, but students should citefactors such as climate, geology, and humanintervention.

5. A. Correct. B. Incorrect. They do requireminerals. C. Incorrect. Alders do have roots.D. Incorrect. Alders are not shade tolerant.


ModelingSuccession

Skills AcquiredObserving, inferring

Teacher’s NotesMake sure students do notscrew the lid down on the jarstoo tightly because the growingculture will suffocate, or the jarmay explode from anaerobi-cally produced gas.

Answers to Analysis1. The pH dropped as the envi-

ronment became more acidic.

2. By-products from the microor-ganisms change the pH. Thennew organisms that are betteradapted to the changed pHbegin to thrive.

3. Like the Glacier Bay model,organisms colonize and slowlychange a new environmentsuch that it becomes moresuitable for other organisms.


About 80 years after the glacier first exposes the land, Sitka

spruce invades the thickets. Spruce trees use the nitrogen released

by the alders and eventually form a dense forest. The spruce blocks

the sunlight from the alders, and the alders then die, just as the

Dryas did before them. After the spruce forest is established, hem-

lock trees begin to grow. Hemlocks are very shade tolerant and have

a root system that competes well against spruce for soil nitrogen.

Hemlock trees soon become dominant in the forest. This commu-

nity of spruce and hemlock proves to be a very stable ecosystem

from the perspective of human time scales, but it is not permanent.

As local climates change, this forest ecosystem may change too.

Modeling SuccessionYou can create a small ecosystem and measure

how organisms modify their environment.

Materials

1 qt glass jar with a lid, one-half quart of pasteurized

milk, pH strips

Procedure

1. Prepare a table like the one

below.

2. Half fill a quart jar with

pasteurized milk, and cover

the jar loosely with a lid.

Measure and record the

pH. Place the jar in a 37°C

incubator.

3. Check and record the pH of

the milk with pH strips every

day for seven days. As milk

spoils, its pH changes.

Different populations of

microorganisms become

established, alter substances

in the milk, and then die off

when conditions no longer

favor their survival.

4. Record any visible changes

in the milk each day.

Analysis

1. Identify what happened to

the pH of the milk as time

passed.

2. Infer what the change in

pH means about the popu-

lations of microorganisms in

the milk.

3. Critical Thinking

Evaluating Results How

does this model confirm the

model of succession in

Glacier Bay?

DATA TABLE

Day pH Appearance

1

2

3

Identify what components of an ecosystem arenot part of a community.

Relate how gardening or agriculture affectssuccession.

Differentiate primary succession fromsecondary succession.

Critical Thinking Applying Information

Why do some ecosystems remain stable forcenturies, while others undergo succession?

In the succession thatoccurs as a glacier recedes, alders can growrelatively rapidly because alders have

A nitrogen-fixing nodules. C no roots.

B no need for minerals. D shade tolerance.

Standardized Test PrepStandardized Test Prep

Section 1 Review

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Section 2

Overview

Before beginning this sectionreview with your students theobjectives listed in the StudentEdition. This section discusses howproducers and consumers facilitatethe flow of energy through ecosys-tems in the form of food webs andfood chains. Students will then dis-cover that a depletion in theamount of available energy limitsthe number of steps that can occurin a food chain.

Tell students to think about theirlocal area and make a diagram of afood chain that would be typicalfor your area. Ask them to try toput 6 organisms into the foodchain. (It is very difficult to do—askstudents why this is so, and lead intoa discussion of trophic levels andenergy loss.)

Demonstration

List the following organisms thatcan be found in an open field: robin,hawk, snake, frog, grasshopper,mouse, and rabbit. Have studentsdraw arrows to show what eatswhat in this field ecosystem.Students should see the complexityof even this simple food web inwhich each predator can take morethan one type of prey and each typeof prey could be exploited by sev-eral different species of predators.

GENERAL

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• Occupational Application WorksheetWildlife Biologist

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Transparencies

TR Bellringer

TR E4 Trophic Levels

TR E5 Food Chain in an AntarcticEcosystem

TR E6 Food Web in an AntarcticEcosystem

Movement of Energy Through EcosystemsEverything that organisms do in ecosystems—running, breathing,

burrowing, growing—requires energy. The flow of energy is the

most important factor that controls what kinds of organisms live in

an ecosystem and how many organisms the ecosystem can support.

In this section you will learn where organisms get their energy.

Primary Energy Source Most life on Earth depends on photosynthetic organisms, which cap-

ture some of the sun’s light energy and store it as chemical energy in

organic molecules. These organic compounds are what we call food.

The rate at which organic material is produced by photosynthetic

organisms in an ecosystem is called . Primary

productivity determines the amount of energy available in an ecosys-

tem. Most organisms in an ecosystem can be thought of as chemical

machines driven by the energy captured in photosynthesis.

Organisms that first capture energy, the , include plants,

some kinds of bacteria, and algae. Producers make energy-storing

molecules. All other organisms in an ecosystem are consumers.

are those organisms that consume plants or other organ-

isms to obtain the energy necessary to build their molecules.

Trophic LevelsEcologists study how energy moves through an ecosystem by

assigning organisms in that ecosystem to a specific level, called a

(TROHF ihk) , in a graphic organizer based on the

organism’s source of energy. Energy moves from one trophic level to

another, as illustrated in Figure 6.

leveltrophic

Consumers

producers

primary productivity

Energy Flow in Ecosystems

Section 2

Objectives

● Distinguish between

producers and

consumers.

● Compare food webs with

food chains.

● Describe why food chains

are rarely longer than three

or four links.

Key Terms

primary productivity

producer

consumer

trophic level

food chain

herbivore

carnivore

omnivore

detritivore

decomposer

food web

energy pyramid

biomass

The sun is the ultimate source of energy for producers and all consumers.

Figure 6 Trophic levels

ProducerSun Consumer Consumer

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Interactive Reading AssignChapter 16 of the Holt BiologyGuided Audio CD Program to helpstudents achieve greater success inreading the chapter.

Teaching TipTrophic Levels Ask a volunteerfrom the class to describe his or herdinner last night. Then have stu-dents describe the trophic level ofeach different food item in themeal. What was the trophic level ofthe student as he or she ate eachitem? (Answers will vary.) Verbal

Using the FigureAsk students to look at the foodchain in Figure 7. Ask students ifthis picture shows a lot of diversity.(no) Ask them what would happento this food chain if leopard sealsate only cod, killer whales ate onlyleopard seals, and a disease wipedout all of the cod. (Leopard sealsand killer whales would starve.)

LS

GENERAL

SKILL

BUILDER

READINGREADING

TeachTeach


Answer

These bacteria are chemosynthetic(as opposed to photosynthetic)producers. They are at the bot-tom of their food chain.

Real Life

Integrating Physics and Chemistry

Reiterate that digestion involves physical and chemicalchanges. Whereas physical processes of digestionchange large pieces of food into smaller ones primarilyby means of chewing and muscle movements of thestomach and the intestines, chemical digestion takesplace when various enzymes are added to the ingestedfood. These enzymes chemically break down foodparticles into molecules that are small enough to beabsorbed and used by an organism’s cells.

First Level The path of energy through the trophic levels of an

ecosystem is called a . An example is shown in Figure 7.

The lowest trophic level of any ecosystem is occupied by the pro-

ducers, such as plants, algae, and bacteria. Producers use the energy

of the sun to build energy-rich carbohydrates. Many producers also

absorb nitrogen gas and other key substances from the environment

and incorporate them into their biological molecules.

Second Level At the second trophic level are (HUHR beh

vohrz), animals that eat plants or other primary producers. They are

the primary consumers. Cows and horses are herbivores, as are cater-

pillars and some ducks. A herbivore must be able to break down a

plant’s molecules into usable compounds. However, the ability to

digest cellulose is a chemical feat that only a few organisms have

evolved. As you will recall, cellulose is a complex carbohydrate found

in plants. Most herbivores rely on microorganisms, such as bacteria

and protists, in their gut to help digest cellulose. Humans cannot

digest cellulose because we lack these particular microorganisms.

Third Level At the third trophic level are secondary consumers, ani-

mals that eat other animals. These animals are called .

Tigers, wolves, and snakes are carnivores. Some animals, such as

bears, are both herbivores and carnivores; they are called

(AHM nih vohrz). They use the simple sugars and starches stored in

plants as food, but they cannot digest cellulose.

In every ecosystem there is a special class of consumers called detri-

tivores, which include worms and fungal and bacterial decomposers.

(deh TRIH tih vohrz) are organisms that obtain their

energy from the organic wastes and dead bodies that are produced at

Detritivores

omnivores

carnivores

herbivores

food chainReal Life

Not all producers are

photosynthetic.

At the bottom of oceans

near volcanic

vents live

bacteria that

harvest

energy from

the reduced

sulfur compounds ejected

by these volcanic vents.

Applying Information

Where in their food

chains do these

bacteria lie?

Algae

Krill

Cod

Leopard seal

This food chain shows one path of energy flow in an Antarctic ecosystem.

Figure 7 Aquatic food chain

Killer whale

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MISCONCEPTION

ALERT

Food Chains vs. Food Webs Emphasizethat food chains, while being useful toolsfor showing the flow of energy through anecosystem, can be misleading to studentsbecause they imply that each organism eatsONLY the organism below it in the foodchain. We know, of course, that mostorganisms eat a wide variety of otherorganisms, and for this reason the term“food web” gives a more accurate pictureof what actually happens in an ecosystem.


Teaching Tip Understanding the Flow ofEnergy Have students draw aGraphic Organizer that summarizesthe flow of energy from producersto herbivores, omnivores,carnivores, and detritivores. Acompleted graphic organizer isshow at the bottom of this page.

Group ActivityFood Webs & BiodiversityOrganize students into groups of 4.Obtain sheets of butcher paper(each about 2–3 feet on a side).Have one student in each groupbegin by writing the name of anorganism anywhere on the sheet inlarge letters. After the name theyshould write one of these letters:P (producer), C (carnivore), H (herbivore), O (omnivore), andD (detritivore). Moving clockwise,each student writes the name ofanother organism randomly on thesheet. Each student draws an arrowfrom this organism to anything thateats it, and an arrow to this organ-ism from organisms it would eat.Continue until the food web getsvery messy. Stop the activity andhave each group hold up theirpaper and have the class “vote” onwhich food web shows the greatestbiodiversity. Discuss what wouldhappen to the “winning” food webif just one organism becameextinct. (In most cases there wouldbe no effect if there were a lot of bio-diversity, but if the extinct organismwas the only producer, it could havea profound effect.) Co-op Learning

GENERAL


Algae

Small animals

and protists

Krill

Cod Squid

Leopard

seal

Killer whale

Adelie

penguin

Crabeater seal

Elephant seal

all trophic levels. Bacteria and fungi are known as

because they cause decay. Decomposition of bodies and wastes releases

nutrients back into the environment to be recycled by other organisms.

Many ecosystems contain a fourth trophic level composed of

those carnivores that consume other carnivores. They are called ter-

tiary consumers, or top carnivores. A hawk that eats a snake is a

tertiary consumer. Very rarely do ecosystems contain more than

four trophic levels.

In most ecosystems, energy does not follow simple straight paths

because individual animals often feed at several trophic levels. This

creates a complicated, interconnected group of food chains called a

, as illustrated in Figure 8. food web

decomposers

www.scilinks.org

Topic: Food Chains and Webs

Keyword: HX4085

This food web shows a more complete picture of the feeding relationships in an

Antarctic ecosystem.

Figure 8 Aquatic food web

347

Use this graphic organizer with

Teaching Tip on this page.

Graphic Organizer

Detritivores: consume producers,

herbivores, carnivores, and omnivores

Producers: make

energy-storing molecules

Herbivores:

consume producers

Carnivores:

consume herbivores

Omnivores: consume

producers and herbivores


Teaching TipPhysics Remind students that thefirst law of thermodynamics statesthat energy cannot be created ordestroyed but only changed in form.Have students give examples ofenergy changes from one form intoanother. (Examples: in a light bulb,electrical energy is changed to lightenergy and heat; batteries changechemical energy into electrical energy)The second law states that energychange between forms is never 100percent efficient. Ask students tothink of an example that shows theinefficiency of energy conversions.(In an automobile engine, gasoline—chemical energy—is changed tokinetic energy to turn the wheels;however, much of the energy is con-verted to heat, which is unavailable topower the automobile.) Logical

Interpreting Visuals Have stu-dents examine the energy pyramidshown in Figure 9. Ask studentshow many trophic levels there arein the diagram. (4) Have them notethat the top trophic level is verysmall. Ask them how the size of theupper level relates to the fact thatthere are a small number of trophiclevels. (There is not enough energy tosupport another level of consumers.)

BUILDERSKILL

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GENERAL



Transparencies

TR E8 Energy Transfer ThroughTrophic Levels

TR E11 Energy Efficiency in Food Consumption

MISCONCEPTION

ALERT

Lost Energy It is quite common for teachersand students to refer to energy being “lost.”Remind students that the first law of thermo-dynamics says energy cannot be “lost”—itcan only be converted from one form intoanother. What people mean when they say

“lost” is that the energy is made unavailable—usually in the form of heat. For example,when an automobile engine heats up, the heatenergizes nearby air molecules as it leaves theengine, but this heat cannot realistically beused to move the automobile.

www.scilinks.org

Topic: Energy Pyramids

Keyword: HX4069

Producers

Herbivores

Carnivore

Top carnivore

Loss of Energy in a Food ChainA deer browsing on leaves is acquiring energy. Potential energy is

stored in the chemical bonds within the molecules of the leaves.

Some of this energy is transformed to other forms of potential

energy, such as fat. Some of it aids the deer in running and breath-

ing, and in fueling cellular processes. But much of the energy is

dispersed into the environment as heat.

Energy TransferDuring every transfer of energy within an ecosystem, energy is lost as

heat. Although heat can be used to do work (as in a steam engine), it

is generally not a useful source of energy in biological systems. Thus,

the amount of useful energy available to do work decreases as energy

passes through an ecosystem. The loss of useful energy limits the

number of trophic levels an ecosystem can support. When a plant

harvests energy from sunlight, photosynthesis captures only about 1

percent of the energy available to the leaves. When a herbivore uses

plant molecules to make its own molecules, only about 10 percent of

the energy in the plant ends up in the herbivore’s molecules. And

when a carnivore eats the herbivore, about 90 percent of the energy

is lost in making carnivore molecules. At each trophic level, the

energy stored by the organisms in a level is about one-tenth of that

stored by the organisms in the level below.

The Pyramid of EnergyEcologists often illustrate the flow of energy through ecosystems

with an energy pyramid. An is a diagram in which

each trophic level is represented by a block, and the blocks are

stacked on top of one another, with the lowest trophic level on the

bottom. The width of each block is determined by the amount of

energy stored in the organisms at that trophic level. Because the

energy stored by the organisms at each trophic level is about one-

tenth the energy stored by the organisms in the level below, the

diagram takes the shape of a pyramid, as shown in Figure 9.

energy pyramid

The word ecosystem is

from the Greek words

oikos, meaning “house,”

and systematos, meaning

“to place together.”

Knowing this information

makes it easier to

remember that an

ecosystem includes a

community of living things

as well as all physical

aspects of its environment.

In this simple ecosystem, each

trophic level contains about 90

percent less energy than the

level below it.

Figure 9 Trophic levels of

a terrestrial ecosystem

348



1. Producers use energy (usually from the sun) toassemble food molecules, hence “producingfood.” Consumers must take in (or consume)these food molecules to obtain their energy.

2. As energy moves through the food chain, about90 percent is “lost” at each level. So, 1,000kilocalories in grass eaten by a rabbit wouldresult in only about 100 available to the foxthat eats the rabbit. The eagle that eats the foxwould only have 10 kilocalories of the original1,000 kilocalories available.

3. Answers will vary. Students should be able to explain the interactions between all theorganisms.

4. Because so much energy is “lost” at eachtrophic level, it is difficult to exceed four links, or trophic levels.

5. Plants are the producers that make food.Without them, animals would starve.

6. A. Incorrect. Algae is the producer.B. Incorrect. Cod is not a producer.C. Incorrect. A leopard seal is not a producer.D. Correct. Algae is the producer, and mustcome first in the food chain.

Activity Energy in Trophic Levels Tell stu-dents that if an average of 1,500kilocalories of light energy per dayfalls on a square meter of land sur-face covered by plants, only about15–30 kilocalories become incorpo-rated into chemical compoundsthrough photosynthesis. Howmuch of this energy could end upin a person who eats these plants?(1.5–3.0 kilocalories) How much ofthis energy could end up in a per-son who eats a steak from a steerthat ate the plants? (0.15–0.30kilocalories) Logical

ReteachingHave students construct a food weband energy pyramid for the school-yard. Ask students to identifyproducers, herbivores, omnivores,carnivores, and detritivores. Whatspecies would they expect to find ifthe area had been a natural park.

Quiz1. What do all consumers rely on

for their food? (producers)

2. Give an example of consumersoutnumbering the producers thatthey are feeding upon. (One treemight have thousands of insectsfeeding on it.)

AlternativeAssessmentHave students draw a food chain, afood web, and an energy pyramidfor any ecosystem. They mustchoose enough organisms for severaltrophic levels and feeding pathways.

GENERAL

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GENERAL

Limitations of Trophic LevelsMost terrestrial ecosystems involve only

three or, on rare instances, four levels. Too

much energy is lost at each level to allow

more levels. For example, a large human

population could not survive by eating

lions captured on the Serengeti Plain of

Africa because there are too few lions to

make this possible. The amount of grass in

that ecosystem cannot support enough

zebras to maintain a large enough popula-

tion of lions to feed lion-eating humans. In

other words, the number of trophic levels

that can be maintained in a community is

limited by the dispersal of potential energy.

Humans are omnivores, and unlike lions,

we can choose to eat either meat or plants.

As illustrated in Figure 10, about 10 kg

(22 lb) of grain are needed to build about

1 kg (2.2 lb) of human tissue if the grain is

directly ingested by a human. If a cow eats

the grain and a human eats the cow, then

about 100 kg (220 lb) of grain are needed to

build about 1 kg (2.2 lb) of human tissue.

Also, the number of individuals in a

trophic level may not be an accurate indica-

tor of the amount of energy in that level.

Some organisms are much bigger than oth-

ers and therefore use more energy. Because

of this, the number of organisms often does not form a pyramid when

one compares different trophic levels. For instance, caterpillars and

other insect herbivores greatly outnumber the trees they feed on. To

better determine the amount of energy present in trophic levels, ecol-

ogists measure biomass. is the dry weight of tissue and other

organic matter found in a specific ecosystem. Each higher level on the

pyramid contains only 10 percent of the biomass found in the trophic

level below it.

Biomass

It takes 10 times

more grain

to feed one cow

to make enough beef

to provide one

person with the same

amount of energy.

It takes a certain

amount of grain

to produce

enough bread

to provide one

person with a certain

amount of energy.

Adding a trophic level to a food chain increases the

energy demand of consumers by a factor of about 10.

Figure 10 Energy efficiency in food consumption

Section 2 Review

Explain how producers differ from consumers.

Analyze the flow of energy through a food chainthat contains four tropic levels, one of which is acarnivore.

Construct a food web, and explain the inter-actions of the organisms that compose it.

List the reasons why food chains do not tend toexceed four links.

Critical Thinking Justifying an Argument

Explain why scientists believe that most animalswould become extinct if all plants died.

Which series shows a cor-rect path of energy flow in a marine food chain?

A krill → cod → algae

B cod → leopard seal → krill

C leopard seal → algae → krill

D algae → krill → cod


349


Overview

Before beginning this sectionreview with your students theobjectives listed in the StudentEdition. This section explains howimportant materials necessary forsurvival, such as water, carbon,phosphorus, and nitrogen, rotatethrough natural systems, constantlyre-supplying organisms. Studentswill also learn that bacteria play animportant role in the recycling ofmany elements.

Have students make a list of any-thing they have used in the pastweek that had previously beenrecycled or is made of recycledmaterials. After they have madetheir lists, have students volunteeritems and discuss what the item isrecycled from, if known, and thepluses and minuses (if any) of recy-cling each item.

Discussion

Make the following statement toyour class: “Someone once saidthat if a person dumped a glass ofwater on the ground, one year later,no matter where they were onEarth, any glass of water they pourwould contain at least one mole-cule of water from the originalglass.” Ask students if they thinkthis is true, and if so, how couldthis occur? (Probably not, but itmakes for a great discussion aboutthe water cycle!)

GENERAL

MotivateMotivate

Bellringer

FocusFocus

Section 3



• Active Reading GENERAL




• Supplemental Reading Guide Silent Spring

Planner CD-ROM

Transparencies

TR Bellringer

TR Graphic Organizer

TR Concept Map

TR E12 Water Cycle

TR E13 Carbon Cycle

TR E14 Nitrogen Cycle

Section 3 Cycling of Materials in Ecosystems

Biogeochemical CyclesHumans throw away tons of garbage every year as unwanted,

unneeded, and unusable. Nature, however, does not throw anything

away. Most energy flows through the Earth’s ecosystems from the sun

to producers to consumers. The physical parts of the ecosystems,

however, cycle constantly. Carbon atoms, for example, are passed

from one organism to another in a great circle of use. Producers are

eaten by herbivores, herbivores are eaten by carnivores, and carni-

vores are eaten by top carnivores. Eventually the top carnivores die

and decay; their carbon atoms then become part of the soil to feed the

producers in a long and complex cycle that reuses this important ele-

ment. Carbon is not the only element that is constantly recycled in

this way. Other recycled elements include many of the inorganic (non-

carbon) substances that make up the soil, water, and air, such as

nitrogen, sulfur, calcium, and phosphorus.

All materials that cycle through living organisms are important in

maintaining the health of ecosystems, but four substances are partic-

ularly important: water, carbon, nitrogen, and phosphorus. All

organisms require carbon, hydrogen, oxygen, nitrogen, phosphorus,

and sulfur in relatively large quantities. They require other elements,

such as magnesium, sodium, calcium, and iron, in smaller amounts.

Some elements, such as cobalt and man-

ganese, are required in trace amounts.

The paths of water, carbon, nitrogen, and

phosphorus pass from the nonliving environ-

ment to living organisms, such as the trees in

Figure 11, and then back to the nonliving

environment. These paths form closed cir-

cles, or cycles, called biogeochemical (bie

oh jee oh KEHM ih kuhl) cycles. In each

, a pathway forms when

a substance enters living organisms such as

trees from the atmosphere, water, or soil;

stays for a time in the living organism; then

returns to the nonliving environment.

Ecologists refer to such substances as cycling

within an ecosystem between a living reser-

voir (an organism that lives in the ecosystem)

and a nonliving reservoir. In almost all bio-

geochemical cycles, there is much less of the

substance in the living reservoir than in the

nonliving reservoir.

biogeochemical cycle

Objectives

● Summarize the role of

plants in the water cycle.

● Analyze the flow of energy

through the carbon cycle.

● Identify the role of bacteria in

the nitrogen cycle.

Key Terms

biogeochemical cycle

ground water

transpiration

nitrogen fixation

Figure 11 Trees and the carbon cycle. Approximately

500 million tons of carbon were taken up as a result of

forest regrowth in the Northern Hemisphere between

1980 and 1989.

350


water molecules and bonding with them.When they reach 0º C they have formed a uni-form latticework of water molecules called ice.Because ice is less dense than the warmer wateraround it, it floats. This property is critical forliving organisms, because the floating ice insu-lates the water beneath it. This prevents thewater under the ice from freezing solid, killingthe organisms in it.

Paired Reading Pair each studentwith a partner. You may want topair ELL students with nativeEnglish speakers. Have each studentread about the water cycle silentlywhile making a question mark on asticky note next to the passages thatthey find confusing. After they fin-ish reading, ask one student tosummarize and the second to addanything omitted. Both readersshould then help each other withany passages that either (or both)did not understand. Have themcreate a list of questions to ask theclass. Students should repeat thisprocess for the carbon and nitrogencycles. Interpersonal

Using the FigureHave each student look atFigure 12 and notice the twoarrows labeled “evaporation” andthe one arrow labeled “transpira-tion.” Ask them what is needed forevaporation or transpiration tooccur. (an input of energy) Next askstudents if this input of energy thatcauses evaporation and transpira-tion can be harvested by humans,and if so, how? (As the clouds thathold the evaporated and transpiredwater move across the land, precipi-tation may occur, and the waterrunoff may eventually enter a river. Ifthe river has a hydroelectric dam onit, the water will run through the tur-bines, spinning them and convertingthe energy of the water into electricalenergy, to be used by man.)

GENERAL

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SKILL

BUILDER

READINGREADING

TeachTeach


Changes in State of Water Water is aunique liquid because, unlike other liquids, it isnot at its densest point when it freezes. Picturea pond in early winter. As the water cools inthe pond, its molecules pack in tighter andtighter until the water reaches 4º C, at whichpoint it is at its densest. As the water continuesto cool, the molecules now begin moving apartfrom each other as they swing outward—like adoor on a hinge—being attracted to other

CHEMISTRYCHEMISTRYCONNECTIONCONNECTION

Transpiration

Precipitation

Water vapor(clouds)

EvaporationEvaporation

Runoff

LakeOcean

Groundwater Percolation

into soil

The Water Cycle Of all the nonliving components of an ecosystem, water has the great-

est influence on the ecosystem’s inhabitants. In the nonliving portion

of the water cycle, water vapor in the atmosphere condenses and falls

to the Earth’s surface as rain or snow. Some of this water seeps into

the soil and becomes part of the , which is water

retained beneath the surface of the Earth. Most of the remaining

water that falls to the Earth does not remain at the surface. Instead,

heated by the sun, it reenters the atmosphere by evaporation. The

path of water within an ecosystem is shown in Figure 12.

In the living portion of the water cycle, much water is taken up by

the roots of plants. After passing through a plant, the water moves

into the atmosphere by evaporating from the leaves, a process called

. Transpiration is also a sun-driven process. The sun

heats the Earth’s atmosphere, creating wind currents that draw

moisture from the tiny openings in the leaves of plants.

In aquatic ecosystems (lakes, rivers, and oceans), the nonliving

portion of the water cycle is the most important. In terrestrial ecosys-

tems, the nonliving and living parts of the water cycle both play

important roles. In thickly vegetated ecosystems, such as tropical

rain forests, more than 90 percent of the moisture in the ecosystem

passes through plants and is transpired from their leaves. In a very

real sense, plants in rain forests create their own rain. Moisture trav-

els from plants to the atmosphere and falls back to the Earth as rain.

transpiration

ground water

www.scilinks.org

Topic: Water Cycle

Keyword: HX4188

This diagram shows the major steps in the water cycle.

Figure 12 Water cycle

351


Teaching Tip Atmospheric Carbon DioxideHave each student make a list ofactivities that contribute to risinglevels of carbon dioxide. (Burninggasoline in a car, propane in a stove,charcoal in a barbecue, etc.) Askthem how electrical power is gener-ated in your area. If fossil fuels areburned, then their use of electricityalso contributes (indirectly) to car-bon dioxide levels. Have studentsidentify ways to reduce the amountof carbon dioxide they contribute.Afterwards, encourage interestedstudents to use the InternetConnect box to learn more aboutthe carbon cycle. Intrapersonal

Using the FigureHave students observe theprocesses of the carbon cycle asdepicted in Figure 13. Point outthat an increase in “activity” in onepart of the cycle will affect otherparts of the cycle. Ask studentswhich part of the carbon cycle haschanged the most drastically overthe last 200 years. (the amount ofcombustion due to the IndustrialRevolution) VisualLS

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“greenhouse effect.” Many scientists believethat a buildup of carbon dioxide in the atmos-phere may act like the glass in a car, absorbingthe heat, causing the atmosphere to slowlyheat up. This could have disastrous effects. Ifthe greenhouse effect causes an increase in themelting of the polar ice caps, coastal areascould experience serious flooding.


GENERAL

did you know?

Greenhouse Effect As light energy enters acar windshield in the heat of summer, some ofthe energy is lost, and the high energy, short-wavelength light is changed to lower energy,long-wavelength heat. The longer wavelengthcannot escape through the glass, and theinside of the car heats up. This process alsooccurs in greenhouses and is known as the

www.scilinks.org

Topic: Carbon Cycle

Keyword: HX4031

The Carbon Cycle Carbon also cycles between the nonliving environment and living

organisms. You can follow the carbon cycle in Figure 13. Carbon

dioxide in the air or dissolved in water is used by photosynthesizing

plants, algae, and bacteria as a raw material to build organic mol-

ecules. Carbon atoms may return to the pool of carbon dioxide in

the air and water in three ways.

1. Respiration. Nearly all living organisms, including plants,

engage in cellular respiration. They use oxygen to oxidize organic

molecules during cellular respiration, and carbon dioxide is a

byproduct of this reaction.

2. Combustion. Carbon also returns to the atmosphere through

combustion, or burning. The carbon contained in wood may stay

there for many years, returning to the atmosphere only when the

wood is burned. Sometimes carbon can be locked away beneath

the Earth for thousands or even millions of years. The remains of

organisms that become buried in sediments may be gradually

transformed by heat and pressure into fossil fuels—coal, oil, and

natural gas. The carbon is released when the fossil fuels are burned.

3. Erosion. Marine organisms use carbon dioxide dissolved in sea

water to make calcium carbonate shells. Over millions of years,

the shells of the dead organisms form sediments, which form

limestone. As the limestone becomes exposed and erodes, the

carbon becomes available to other organisms.

Cellularrespiration

Combustion

Photosynthesis

Death anddecomposition

Fossil fuels

Limestone

Marineplanktonremains

Carbon dioxide (CO2)in atmosphere

Dissolved

CO2

in

water

This diagram shows the major

steps of the carbon cycle.

Figure 13 Carbon cycle

352


Trends in AgricultureCrop Rotation Farmers often rotate a nonleguminous crop, such as corn, with a legu-minous one, such as alfalfa. The alfalfa will fixnitrogen and release some of it into the soil. Ifa crop of alfalfa is plowed back into the soil, itmay add as much as 350 kg (770 lb.) of nitro-gen per hectare (2.5 acres) of soil, enough togrow a crop of nonleguminous plants withoutthe need for additional fertilizer.

DemonstrationTell students that beans are legumi-nous plants, the roots of whichhave nodules containing nitrogen-fixing bacteria. Then tell them thatother such plants include clover,peas, alfalfa, lupines, and locustand alder trees. If any of theseplants are available nearby, see ifyou can dig up an example to showthe class any nodules that may bepresent. Visual

Teaching TipGene Splicing Only some typesof plants have the symbiotic rela-tionship with nitrogen-fixing bacteria that enables them to con-vert nitrogen gas into a usable formof nitrogen. Scientists are workingto isolate and remove the “nitro-gen-fixing gene” in hopes they cansplice it into other types of plantsand make them nitrogen fixers aswell. Have students discuss thepros and cons of doing this. (pro—reduce amount of fertilizerneeded, tremendous economic impact;this could help feed starving peoplesin countries with poor agriculturalresults; con—no one knows for surewhat will happen with a new,genetically-engineered organism)

Using the FigureHave students study Figure 14.Point out the difference betweennitrogen fixation (nitrogengas➞ammonia) and nitrification(ammonia➞nitrates). Tell studentsthat lightning also changes nitrogengas to ammonia, but such atmos-pheric action amounts to less than10 percent of that carried out byorganisms through nitrogen fixation.Finally, have students recognize thatdenitrification returns nitrates to the atmosphere as nitrogen gas.

LogicalLS

GENERAL

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The Phosphorus and Nitrogen CyclesOrganisms need nitrogen and phosphorus to build proteins and

nucleic acids. Phosphorus is an essential part of both ATP and DNA.

Phosphorus is usually present in soil and rock as calcium phosphate,

which dissolves in water to form phosphate ions, PO43-. This phos-

phate is absorbed by the roots of plants and used to build organic

molecules. Animals that eat the plants reuse the organic phosphorus.

The atmosphere is about 78 percent nitrogen gas, N2. However,

most organisms are unable to use it in this form. The two nitrogen

atoms in a molecule of nitrogen gas are connected by a strong triple

covalent bond that is very difficult to break. However, a few bacteria

have enzymes that can break it, and they bind nitrogen atoms to

hydrogen to form ammonia, NH3. The process of combining nitro-

gen with hydrogen to form ammonia is called .

Nitrogen-fixing bacteria live in the soil and are also found within

swellings, or nodules, on the roots of beans, alder trees, and a few

other kinds of plants.

The nitrogen cycle, diagramed in Figure 14, is a complex process

with four important stages.

1. Assimilation is the absorption and incorporation of nitrogen

into organic compounds by plants.

2. Ammonification is the production of ammonia by bacteria dur-

ing the decay of organic matter.

3. Nitrification is the production of nitrate from ammonia.

4. Denitrification is the conversion of nitrate to nitrogen gas.

nitrogen fixation

Reviewing Information

Using your own words, write

four sentences, each one

describing one of the four

biogeochemical cycles.

Denitrification

AssimilationNitrogenfixation

Nitrification Nitrogenfixation

Ammonification

Nitrogen-fixingbacteria inplant roots

Nitrogen-fixingbacteria in soil

Nitrifyingbacteria

Denitrifyingbacteria

Atmosphericnitrogen (N2)

Animals

Death Death

Plants

Ammonia (NH3)

Nitrates(NO3)

Waste(urine and feces)

Decomposers

Bacteria carry out many of

the important steps in the

nitrogen cycle, including the

conversion of atmospheric

nitrogen into a usable form,

ammonia.

Figure 14 Nitrogen cycle

353


ReteachingAsk students to write a short essaydescribing what the world wouldlook like without fungi or bacteria.

QuizTrue or False:

1. Of all the abiotic components ofan ecosystem, water has thegreatest influence on the ecosys-tem’s inhabitants. (true)

2.Carbon is necessary forphotosynthesis. (true)

AlternativeAssessmentHave students select a biogeochemi-cal cycle (except the water cycle).Being specific and writing in essayform, they should explain howhumans would be affected if theirchosen element did NOT cycle.

GENERAL

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1. The sun’s heating causes wind currents, whichdraws moisture from plant leaves.

2. Carbon and energy both move through ecosystems. Light energy is captured by photo-synthesizers and used to make organic moleculesusing carbon dioxide. Energy flows out ofecosystems mainly as heat during cellular respi-ration and combustion. These processes alsorelease carbon, but it does not “leave” theecosystem; photosynthesizers recycle it.

3. Nitrogen-fixing bacteria convert nitrogen gasto ammonia. Nitrifying bacteria convertammonia to nitrates, also usable by some

plants. Denitrifying bacteria turn the nitratesinto nitrogen gas.

4. Nutrients can cycle because they are in a formusable by at least some organisms, which keepsthe nutrient moving through (but still within)the ecosystem. Most energy, on the other hand,only flows in one direction. By the end of thefood chain, nearly all the original energy hasbeen “lost” as unusable heat.

5. A. Incorrect. Combustion puts CO2 into theatmosphere. B. Incorrect. This process putsCO2 into the atmosphere. C. Incorrect. Erosionputs CO2 into the atmosphere. D. Correct.


Sustainable AgricultureOrganic farming is a form ofsustainable agriculture that doesnot use chemical fertilizers orpesticides. Ask students if theythink this would lead to biggerprofits for farmers. (It may beless profitable in the short term,but in the long run, they maycome out ahead because they willhave healthy soil and will not beusing costly fertilizers.)


The growth of plants in ecosystems is often limited by the

availability of nitrate and ammonia in the soil. Today most of the

ammonia and nitrate that farmers add to soil is produced chemi-

cally in factories, rather than by bacterial nitrogen fixation. Genetic

engineers are trying to place nitrogen-fixing genes from bacteria

into the chromosomes of crop plants. If these attempts are successful,

the plants themselves will be able to fix nitrogen, thus eliminating the

need for nitrogen-supplying fertilizers. Some farmers adjust their

farming methods to increase natural recycling of nitrogen.

Sustainable Agriculture

In an ecosystem, decomposers

return mineral nutrients to the

soil. However, when the plants are

harvested and shipped away,

there is a net loss of nutrients from

the soil where the plants were

growing. The amount of organic

matter in the soil also decreases,

making the soil less able to hold

water and more likely to erode.

What is SustainableAgriculture?

Sustainable agriculture refers to

farming that remains productive

and profitable through practices

that help replenish the soil’s

nutrients, reduce erosion, and

control weeds and insect pests.

Use of Cover Crops

After harvest, farmers can plant

cover crops, such as rye, clover,

or vetch, instead of letting the

ground lie bare. Cover crops

keep the soil from compacting

and washing away, and they help

the soil absorb water. They also

provide a habitat for beneficial

insects, slow the growth of

weeds, and keep the ground

from overheating. When cover

crops are plowed under, as illus-

trated in the figure at right, they

return nutrients to the soil.

Rotational Grazing

Farmers who raise cattle and

sheep can divide their pastures

into several grazing areas. By

rotating their livestock from one

area to another, they can prevent

the animals from overgrazing the

pasture. This allows the plants

on which the animals feed to

live longer and be more produc-

tive. Water quality improves as

the pasture vegetation becomes

denser. Animals distribute manure

more evenly with rotational

grazing than they do in feed lots

or unmanaged pastures.

There are many other methods

used in sustainable agriculture.

Farmers must determine which

methods work best for their crops,

soil conditions, and climate.

www.scilinks.org

Topic: Sustainable Agriculture

Keyword: HX4170

Identify the role of energy in the part of thewater cycle in which plants transfer water to theatmosphere.

Analyze the carbon cycle’s relationship to theflow of energy.

Describe how bacteria participate in thenitrogen cycle.

Critical Thinking Defend the argument thatnutrients can cycle but energy cannot.

Which component of thecarbon cycle removes carbon dioxide from theatmosphere?

A combustion C erosion

B cellular respiration D photosynthesis


Section 3 Review

354


Alternative Assessment Have students choose an ecosystemfrom anywhere in the world. (Forexample, students could choose theCosta Rican rain forest, SonoranDesert, Kalahari Plain, or FloridaEverglades.) Have them researchand report on the biotic and abioticcomponents. Tell them to includedescriptions of four producers,three herbivores, two small carni-vores, one top carnivore, and onedetritivore. Have them draw a foodchain, food web, and energy pyra-mid for this ecosystem.

Answer to Concept Map

The following is one of several possible answers to Performance Zone item 15.


GENERAL

• Science Skills Worksheet

• Critical Thinking Worksheet

• Test Prep Pretest

• Chapter Test GENERAL

GENERAL

GENERAL


contains several

composed of

which contain

are

areare prey for

feedon

Food web

food chains

trophic levels

producers

carnivores consumers

herbivores detritivores

Key Concepts

Study CHAPTER HIGHLIGHTS

ZONE

Key Terms

Section 1

ecology (340)

habitat (340)

community (340)

ecosystem (340)

abiotic factor (340)

biotic factor (340)

biodiversity (341)

pioneer species (343)

succession (343)

primary succession (343)

secondary succession (343)

Section 2

primary productivity (345)

producer (345)

consumer (345)

trophic level (345)

food chain (346)

herbivore (346)

carnivore (346)

omnivore (346)

detritivore (346)

decomposer (347)

food web (347)

energy pyramid (348)

biomass (349)

Section 3

biogeochemical cycle (350)

ground water (351)

transpiration (351)

nitrogen fixation (353)

BIOLOGYBIOLOGY

Unit 7—Ecosystem Dynamics

Use Topics 1, 3–6 in this unit to review the

key concepts and terms in this chapter.

What Is an Ecosystem?

● Ecology is the study of how organisms interact with each

other and with their environment.

● A community of organisms and their nonliving environment

constitute an ecosystem.

● Ecosystems contain diverse organisms.

● Ecosystems change through the process of succession.

● Succession on a newly formed habitat is primary succession.

● Secondary succession occurs on a habitat that has previously

supported growth.

Energy Flow in Ecosystems

● Energy moves through communities in food chains, passing

from photosynthesizers (producers) to herbivores (consumers)

to carnivores (consumers), creating a food web.

● Energy transfers between trophic levels transfer only 10

percent of the energy in a trophic level to the next level.

● Most terrestrial communities have only three or four trophic

levels because energy transfers between trophic levels are

inefficient.

Cycling of Materials in Ecosystems

● Minerals and other materials cycle within ecosystems among

organisms and between organisms and the physical environment.

● In the water cycle, water falls as precipitation and either

evaporates from bodies of water, is stored in ground water,

or cycles through plants and then evaporates.

● Carbon enters the living portion of the carbon cycle through

photosynthesis. Organisms release carbon through cellular

respiration. Carbon trapped in rocks and fossil fuels is

released by erosion and burning.

● Bacteria fix atmospheric nitrogen, thus making ammonia

available to other organisms.

3

2

1

355


ANSWERS

Understanding Key Ideas

1. b

2. b

3. c

4. d

5. b

6. c

7. Greater flexibility in diet allowsomnivores to eat whatever foodis available.

8. Plowing the corn into the fieldwould return organic matter andnutrients to the soil—a more sus-tainable approach.

9. Photosynthesis enables plants to incorporate nitrogen intoproteins, making it available toconsumers. When consumersexcrete waste or decompose,plants (with the aid of bacteria)may take up the nitrogen again,completing the cycle.

10. One possible answer to the con-cept map is found at the bottomof the Study Zone page.

Critical Thinking

11. Each trophic level contains about90% less energy than the levelbelow it. If 1,000 kilocalories ofseeds were eaten by a mouse, itwould result in about 100 kilo-calories for the snake that eatsthe mouse. And there would onlybe about 10 kilocalories availablefor the hawk that eats the snake.

12. Many pioneer organisms, such aslichens, have the ability to fixnitrogen, but all pioneer plantsare able to fix carbon duringphotosynthesis. Therefore, nitro-gen cycling is probably more

important to a pioneer species during primarysuccession.

13. Dead organisms and wastes would not decayand nutrients would not be recycled back intothe ecosystem.

Alternative Assessment

14. Answers will vary depending on the organ-isms pictured.


CHAPTER 16

Section Questions

1 1, 3, 11, 12, 14

2 2, 5, 6, 7, 10, 13

3 4, 8, 9, 12

Assignment Guide

Understanding Key Ideas

1. Ecosystems differ from communities inthat ecosystems usually containa. several climates. b. several communities. c. only one habitat. d. only one food web.

2. What critical role is played by fungi andbacteria in any ecosystem? a. primary productionb. decompositionc. boundary settingd. physical weathering

3. Which sequence shows the correct order ofsuccession at Glacier Bay, Alaska?a. alder, Dryas, hemlockb. Dryas, hemlock, alderc. Dryas, alder, Sitka spruced. mosses, hemlock, Sitka spruce

4. Which role is not performed by bacteria inthe nitrogen cycle? a. fixing nitrogenb. changing urea to ammoniac. turning nitrates into nitrogen gasd. changing nitrates to ammonia

5. How would the food web below be affectedif the plants were eliminated?

a. Herbivores would become carnivores. b. The food web would collapse. c. The herbivores would change trophic levels.

d. Nothing would happen.

6. How much energy is available at the third trophic level of an energy pyramid if1,000 kcal is available in the first level? a. 1,000 kcal c. 10 kcalb. 100 kcal d. 1 kcal

7. Humans, raccoons, and bears areomnivores. What adaptive advantage might this feeding strategy provide?

8. After harvesting, a farmercould either plow the remaining cornstalksinto the field or burn them. Which optionis best for sustainable agriculture? Explainyour answer.

9. Relate photosynthesis to the nitrogen cycle.(Hint: See Chapter 5, Section 2.)

10. Concept Mapping Make a conceptmap that describes the flow of energythrough an ecosystem. Try to include thefollowing terms in your map: trophic level,food web, food chain, producer, consumer,carnivore, detritivore, and herbivore.

Critical Thinking

11. Inferring Relationships Analyze the flow ofenergy between an ecosystem and one of itstop carnivores, such as a hawk.

12. Applying Information Is nitrogen cycling orcarbon cycling more important to a pio-neer species during primary succession?Explain your answer.

13. Predicting Results Describe the probableeffects on an ecosystem if all decomposerswere to die.

Alternative Assessment

14. Identifying Functions Obtain photocopiesof nature paintings by American painterssuch as John James Audubon or EdwardHicks. Choose three animals, and write areport that compares the animals, theecosystems in which they live, their roles inbiogeochemical cycles, and the trophiclevel they occupy.

PerformanceZONE

CHAPTER REVIEW

356


Question 5 Answer D is the cor-rect choice. Transpiration involveswater evaporating from a plant’sleaves and entering the atmos-phere. Answer A is incorrectbecause assimilation involvesnitrogen being absorbed by plantsto make organic compounds anddoes not return water directly tothe atmosphere. Answer B is incor-rect because nitrification is theproduction of nitrates from ammo-nia and does not return waterdirectly to the atmosphere. AnswerC is incorrect because successioninvolves the cycling of entireorganisms in an ecosystem, notjust water.

Question 6 Changes in climate,large-scale disturbances such asfire or volcanic eruption, and evenchanges in biotic factors such asan insect pest outbreak can changethe conditions of an ecosystem.

Question 7 Answer H is the cor-rect choice. Answer F is incorrectbecause algae do not necessarilyharm meadow grasses. Answer Gis incorrect because the presence ofdecaying organic matter is benefi-cial to most plants, as it providesnutrients for the soil. Answer I isincorrect because, as the ecosystemmatures, meadow grasses eventu-ally crowd out the marsh plants.

Question 8 Answer C is the cor-rect choice. The lowest level ofcarbon dioxide indicates the great-est rate of photosynthesis. AnswerA is incorrect because the carbondioxide level is highest in January.Answer B is incorrect becauseMarch shows a higher carbondioxide level than May. Answer Dis incorrect because Septembershows a higher carbon dioxidelevel than May.

Answers

1. C

2. I

3. B

4. F

5. D

6. Ecosystems are dynamic by nature.

7. H

8. C


Standardized Test Prep

Understanding ConceptsDirections (1–5): For each question, write ona separate sheet of paper the letter of thecorrect answer.

1 Which of the following situationsdescribes a carnivore and an herbivore?A. A horse eats an apple.B. A rabbit eats a dandelion.C. A mountain lion eats a rabbit.D. A fungus breaks down a dead oak tree.

2 What term applies to most humans?F. carnivore H. herbivoreG. detrivore I. omnivore

3 What is an organism that obtains energyfrom organic wastes and dead bodiescalled?A. carnivore C. herbivoreB. detrivore D. omnivore

4 What is the process by which materialspass between the nonliving environmentand living organisms?F. biogeochemical cycleG. energy pyramidH. food webI. primary succession

5 Through what process do plants returnwater to the atmosphere?A. assimilationB. nitrificationC. successionD. transpiration

Directions (6): For the following question,write a short response.

6 Ecologists once referred to stable ecosys-tems as a final or climax community. Nowmost ecologists say that no ecosystem cantruly have a final end point. Analyze whyecologists have changed their viewpoint.

Reading SkillsDirections (7): Read the passage below.Then answer the question.

Artificial ecosystems used in the treatment of waste water and pollutants can demonstrate succession. Artificial wastewater-treatment ecosystems tend to undergo eutrophication, just as natural wetlands do. However, the high nutrient levels in waste water promote rapid algae growth. If the systems are not manipulated, they will eventually fill with algae and decay-ing organic matter, providing nutrients for other species. The system can then form a marsh and eventually a meadow.

7 Why don’t meadow grasses populate thenew ecosystem before the marsh plantsand algae begin to grow there?F. The presence of algae is harmful tomeadow grasses.

G. The presence of decaying organic matter is harmful to meadow grasses.

H. Meadow grasses require that pioneerspecies first make nutrient-rich soil.

I. Meadow grasses cannot compete withmarsh plants in established ecosystems.

Interpreting GraphicsDirections (8): Base your answer to question8 on the graph below.

Atmospheric Carbon Dioxide Variation

8 During which of the following months isthe rate of photosynthesis greatest?A. January C. MayB. March D. September

Carbon dioxide concentration

(pa

rts p

er

mill

ion

)

Month

March May July Sept.Jan. Nov.

348

350

352

354

356

358

Test

For multiple-choice questions, try to eliminate any

answer choices that are obviously incorrect, and

then consider the remaining answer choices.

357

Standardized Test Prep


Exploration Lab

MODELING ECOSYSTEM

CHANGE OVER TIME

Teacher’s Notes

Time Required About20 minutes on day 1 and about10 minutes each day thereafter over a period of a few weeks.

Ratings

TEACHER PREPARATION

STUDENT SETUP

CONCEPT LEVEL

CLEANUP

Skills Acquired• Collecting Data• Constructing Models• Organizing and Analyzing Data

Scientific MethodsIn this lab, students will:• Make Observations• Test the Hypothesis• Analyze the Results

Materials and EquipmentMaterials for this lab can beordered from WARD’S. See MasterMaterials List at the front of thisbook for catalog numbers. Havestudents bring clear plastic 2- or 3-L soda bottles from home. Soilcan be collected from around theschool, brought from home, or pur-chased from a garden center. Youmay also be able to find earth-worms and crickets in the localenvironment or at bait shops.

Safety CautionsReview all safety symbols with stu-dents before beginning the lab.Warn students to take care whenhandling insects and other smallanimals. Small animals are easilyharmed, and some are capable ofbiting when disturbed.

E A S Y H A R D

Tips and TricksRemind students that the ecosystems aredependent on humans for care. They shouldnot be permitted to be overheated or becometoo cold. If water evaporates from the ecosys-tem, it should be replenished. Recording thenumber of organisms may be tricky in somecases and estimates may be required.

Answers to Before You Begin1. ecosystem—an ecological system encompassing

a community and its abiotic factors; food web—a network of feeding relationshipsin an ecosystem; closed ecosystem—an ecosys-tem that does not exchange materials outsideof itself; producer—organisms that first cap-ture energy; decomposer—organisms thatdecompose dead organic material; consumer—organisms that consume producers;herbivore—organisms that eat plants or otherprimary producers; carnivore—organisms thatare secondary consumers; trophic level—ecosystem level based on the organism’s sourceof energy.


Before You Begin

Organisms in an interact with

each other and with their environment. One

of the interactions that occurs among the

organisms in an ecosystem is feeding. A

describes the feeding relationships

among the organisms in an ecosystem. In

this lab, you will model a natural ecosystem

by building a in a bottle

or a jar. You will then observe the interac-

tions of the organisms in the ecosystem and

note any changes that occur over time.

1. Write a definition for each boldface term in

the paragraph above and for each of the

following terms: producer, decomposer,

consumer, herbivore, carnivore, trophic

level.

2. Based on the objectives for this lab, write a

question you would like to explore about

ecosystems.

Procedure

PART A: Building an Ecosystem in a Jar

1. Place 2 in. of sand or pea gravel in

the bottom of a large, clean glass jar

with a lid. CAUTION: Glassware is fragile.

Notify your teacher promptly of any

broken glass or cuts. Do not clean up

broken glass or spills with broken glass

unless your teacher tells you to do so.

Cover the gravel with 2 in. of soil.

2. Sprinkle the seeds of two or three types of

small plants, such as grasses and clovers,

on the surface of the soil. Put a lid on the

jar, and place it in indirect sunlight. Let the

jar remain undisturbed for a week.

3. After one week, place a handful of rolled

oats into the jar. Place the mealworms in

the oats, and then place the other animals

into the jar and replace the lid. Place the

lid on the jar loosely to enable air entry.

You ChooseAs you design your experiment, decide the following:

a. what question you will explore

b. what hypothesis you will test

c. how you will plant the seeds

d. where you will place the ecosystem for oneweek so that it remains undisturbed and inindirect sunlight

e. how often you will add water to the ecosys-tem after the first week

f. how many of each organism you will use

g. what data you will record in your data table

closed ecosystem

web

food

ecosystem

Exploration Lab

Modeling Ecosystem Change over Time

SKILLS

• Using scientific methods

• Modeling

• Observing

OBJECTIVES

• Construct a model

ecosystem.

• Observe the interactions of

organisms in a model

ecosystem.

• Predict how the number

of each species in a model

ecosystem will change

over time.

• Compare a model

ecosystem with a natural

ecosystem.

MATERIALS

• coarse sand or pea gravel

• large glass jar with a lid or terrarium

• soil

• pinch of grass seeds

• pinch of clover seeds

• rolled oats

• mung bean seeds

• earthworms

• isopods (pill bugs)

• mealworms (beetle larva)

• crickets

358


Answers to Do You Know?1. Biosphere 2 is a large artificial ecosystem near

Tucson, Arizona. The 204,000-m3 glass andsteel structure contains seven ecosystems. Thestructure has been used for research on thebiosphere, the sum of all of Earth’s ecosystems.

2. Answers will vary. Students should list severalexamples of problems, including food short-ages, oxygen shortages, and populationexplosions of microorganisms, ants, and cockroaches.

2. Answers will vary. For example,What are the effects of continuousexposure to bright light on theecosystem?

Part B: Sample Procedure1. Place the jar to the side of a win-

dow so that it receives indirectsunlight throughout the day.

2. After one week, add three earth-worms, five isopods, threemealworms, and three crickets tothe jar. Using a mister, add foursquirts of water per square decime-ter of soil surface every other day.

3. Record population data everyother day for two weeks.

Answer to Analyze andConclude1. Answers will vary. Students should

make one graph for each speciesobserved or use different colors toindicate each species.


3. Answer will vary. All plants areproducers (primary trophic level);earthworms feed on dead plantmaterial in the soil; crickets feedon plants; mealworms (beetle lar-vae) feed on plants; isopods (pillbugs) eat wood.

4. Yes and no. Natural ecosystemsand the model ecosystem bothcontain organisms at severaltrophic levels, have living and non-living components, and depend onthe sun for energy. However, themodel ecosystem is less diverse,much younger, and has more defi-nite boundaries than a naturalecosystem.


6. No. Strengths are that the organ-isms in the model ecosystem didnot leave the ecosystem and thatother organisms could not enterfrom the outside. Weaknesses arethat water and air probably had tobe added to maintain a healthyecosystem.

7. Answers will vary. For example:What are the effects of certain abi-otic factors, such as temperature,light, and moisture, on the organ-isms in an ecosystem?


PART B: Design an Experiment

4. Work with the members of your lab group

to explore one of the questions written for

step 2 of Before You Begin. To explore the

question, design an experiment that uses

the materials listed for this lab.

5. Write a procedure for your experiment.

Make a list of all the safety precautions you

will take. Have your teacher approve your

procedure and safety precautions before

you begin the experiment.

6. Set up your group’s experiment. Conduct

your experiment for at least 14 days.

PART C: Cleanup and Disposal

7. Dispose of solutions, broken glass,

and other materials in the designated

waste containers. Do not put lab materials

in the trash unless your teacher tells you to

do so.

8. Clean up your work area and all lab

equipment. Return lab equipment to

its proper place. Wash your hands thor-

oughly before you leave the lab and after

you finish all work.

Analyze and Conclude

1. Summarizing Results Make graphs

showing how the number of individuals of

each species in your ecosystem changed

over time. Plot time on the x-axis and the

number of organisms on the y-axis.

2. Analyzing Results How did your results

compare with your hypothesis? Explain

any differences.

3. Inferring Conclusions Construct a food

web for the ecosystem you observed.

4. Recognizing Relationships Does your

model ecosystem resemble a natural

ecosystem? Explain.

5. Analyzing Methods How might you have

built your model ecosystem differently to

better represent a natural ecosystem?

6. Evaluating Methods Was your model

ecosystem truly a “closed ecosystem”? List

your model’s strengths and weaknesses as a

closed ecosystem.

7. Further Inquiry Write a new question

about ecosystems that you could explore

with another investigation.

www.scilinks.org

Topic: Ecosystems

Keyword: HX4066

Do You Know?

Do research in the library or media center

to answer these questions:

1. What is Biosphere 2?

2. What problems were encountered by

the Biosphere 2 crew during the

1991–1993 project?

Use the following Internet resources

to explore your own questions about

ecosystems.

359


CHAPTER 16 Ecosystems Opening Activity · succession. Key Terms ecology habitat community ecosystem...

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