chapter 16 ecosystems - welcome to mr. walker's class …€¦ · · 2016-03-29chapter 16...

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.

Answers1. 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 toearlier sections of your book. 1. Contrast autotrophs with heterotrophs.

(Chapter 5, Section 1)2. Summarize the process of photosynthesis.

(Chapter 5, Section 2)3. Describe the process of cellular respiration.

(Chapter 5, Section 3)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 thesections indicated.

Section 1What Is an Ecosystem?

Interactions of Organisms and Their Environment

Diverse Communities in EcosystemsChange of Ecosystems over Time

Section 2Energy Flow in Ecosystems

Movement of Energy Through EcosystemsLoss of Energy in a Food Chain

Section 3Cycling of Materials in Ecosystems

Biogeochemical CyclesThe Water CycleThe Carbon CycleThe 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 ofpaper or in your notebook, leaving a few blanklines after each.

1. In an ecosystem, more energy is stored inplants than in consumers.

2. The extinction of one species in an ecosystemcan have an impact on all other species.

Before you read the chapter, write down whetheryou agree or disagree with each statement. Afteryou have finished reading the chapter, decidewhether or not you still agree with your firstresponse.

Materials and energy cycle continuously through thecomponents of this coral reef. The complex relation-ship of organisms and their physical environmentmakes 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

Bellringer

FocusFocus

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 partof us—something we always use, sometimes enjoy, and sometimesdamage. But in fact, we are part of the environment along with allof Earth’s other organisms. All of Earth’s inhabitants are interwovenin a complex web of relationships, such as the one illustrated inFigure 1. To understand how the interactions of the parts can affecta whole system, think about how a computer operates. Removingone circuit from a computer can change or limit the interactions ofthe computer’s many components in ways that influence the com-puter’s overall operation. In a similar way, removing one speciesfrom our environment can have many consequences, not all of themeasily predictable.

In 1866, the German biologist Ernst Haeckel gave a name to thestudy of how organisms fit into their environment. He called thisstudy 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 oneanother and with their physical environment (soil, water, climate,and so on). The place where a particular population of a specieslives is its . The many different species that live together in ahabitat 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 ina habitat are called .biotic factors

factorsabiotic

ecosystemcommunityhabitat

Ecology

Objectives● Distinguish an ecosystem

from a community.

● Describe the diversity of a representative ecosystem.

● Sequence the process ofsuccession.

Key Terms

ecologyhabitatcommunityecosystemabiotic factorbiotic factorbiodiversitypioneer speciessuccessionprimary successionsecondary succession

Figure 1 Organisms interact within anecosystem. Organisms withinan ecosystem continuallychange and adjust. This plantspecies is dependent on thebat for its reproduction, andthe bat uses part of the flower for food.

340


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.

SKILLBUILDER

READINGREADING

GENERAL

TeachTeach


• Directed Reading• Active Reading• Datasheet for Quick Lab GENERAL

GENERAL

Chapter Resource File

• Reading Organizers• Reading Strategies

Planner CD-ROM

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 DynamicsThis engaging tutorial introducesstudents to Ecosystem Dynamics.

BIOLOGYBIOLOGY

Transparencies

TR BellringerTR E25 Ecological Succession at

Glacier Bay

Diverse Communities in EcosystemsThe variety of organisms, their genetic differ-ences, and the communities and ecosystems inwhich they occur is termed .Consider a pine forest in the southeasternUnited States, such as the one shown in Figure2. If you could fence in a square kilometer (0.4 mi2) of this forest and then collect everyorganism, what would you expect to get?Which of the six kingdoms of organismswould be represented in your collection?

Ecosystem InhabitantsLarge animals in the forest might include a bear or a white-taileddeer. The woods also contain smaller mammals—raccoons, foxes,squirrels, rabbits, and chipmunks. Snakes and toads often remainhidden among the leaves. Many birds can be found, includinghawks, warblers, and sparrows. If the square kilometer included alake, you might find catfish, bass, perch, a variety of turtles, andperhaps an alligator.

There are pine trees, a variety of smaller trees, and shrubs. Beneaththe trees, grasses and many kinds of flowers grow on the forest floor.

The soil contains an immense number of worms. Hidden underthe bark of trees and beneath the leaves covering the ground aremany different species of insects and spiders, such as those shownin Figure 3.

Many of the life-forms in the soil and water of a pine forest aretoo small to be seen without a microscope. Protists, which includealgae and related microscopic eukaryotes, thrive in water. Theremay 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 thestag beetle live in and eat decaying wood and bark.

Figure 3 Forest spider and insect

Figure 2 Pine forest.Pine forests like this one arecommon in the southeasternUnited States.

www.scilinks.orgTopic: BiodiversityKeyword: HX4020

341


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 NotesHave each student use an areaof equal dimensions, at least50 ! 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 "0.103 " 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 andspreading as fine threads through the decaying material on the forestfloor, as illustrated in Figure 4. Other fungi are found on the surfaceof trees or rocks as lichens. Lichens are associations between fungiand 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, windthat 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 floorto be an ecosystem if he or she is interested only in the fungi andinsects living in the log. Often individual fields, forests, or lakes arestudied as isolated ecosystems. Of course, no location is ever totallyisolated. Even oceanic islands get occasional migrant visitors, suchas birds blown off course.

Mushrooms are often found on moist forest floors.

Shelf fungi grow on and digest trees.

These fungi digest plants andother materials they find in theforest.

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: Donot approach

or touch any wild animals.Do not disturb plants.Prepare a list of biotic andabiotic factors that youobserve around your home or in a nearby park.

Analysis

1. Identify the habitat andcommunity that you observed.

2. Calculate the number of dif-ferent species as a percentageof the total number of organ-isms that you saw.

3. Rank the importance ofbiotic factors within theecosystem you observed.

4. Infer what the relationshipsare between biotic factorsand abiotic factors in theobserved 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

LS

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 exposesbare rock, or a fire burns all of the vegetation in an area, a newhabitat is created. This change sets off a process of colonizationand ecosystem development. The first organisms to live in a newhabitat 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 maythen outcompete and replace the pioneer species.

SuccessionA somewhat regular progression of species replacement iscalled . Succession that occurs where life has not existed before is called . Successionthat 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 ofsuccession were predictable and that succession always led tothe same final community of organisms within any particularecosystem. Ecologists now recognize that initial conditionsand chance play roles in the process of succession. For exam-ple, if two species are in competition, a sudden change in theclimate 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 glacierbecause land is continually being exposed as the face of theglacier moves back. The glacier that composes much of thehead of Glacier Bay, Alaska, has receded some 100 km (62 mi) over the last 200 years. Figure 5 shows the kinds ofchanges that have taken place as time passed.

The most recently exposed areas are piles of rock andgravel that lack the usable nitrogen essential to plant and ani-mal life. The seeds and spores of pioneer species are carriedin by the wind. These include lichens, mosses, fireweed, wil-lows, cottonwood, and Dryas, a sturdy plant with clumpsabout 30 cm (1 ft) across. At first all of these plants growclose 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 distantsites take root. Alder roots have nitrogen-fixing nodules, sothey are able to grow more rapidly than Dryas. Dead leavesand fallen branches from the alder trees add more usablenitrogen to the soil. The added nitrogen allows willows andcottonwoods to invade and grow with vigor. After about 30years, dense thickets of alder, willow, and cottonwood shadeand eventually kill the Dryas.

secondary succession

primary successionsuccession

pioneer species

Recently exposed land has few nutrients.

Alders, grasses, and shrubs later take overfrom pioneer plants.

As the amount of soil increases, spruce andhemlock trees become plentiful.

A receding glacier makes primary succession possible.

Figure 5 Glacier Bay

343


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 Review1. the abiotic environment2. Because the land is kept in a constant state of

disturbance, 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.


ModelingSuccessionSkills 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, Sitkaspruce invades the thickets. Spruce trees use the nitrogen releasedby the alders and eventually form a dense forest. The spruce blocksthe sunlight from the alders, and the alders then die, just as theDryas did before them. After the spruce forest is established, hem-lock trees begin to grow. Hemlocks are very shade tolerant and havea 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 ecosystemfrom 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 measurehow organisms modify their environment.

Materials

1 qt glass jar with a lid, one-half quart of pasteurizedmilk, pH strips

Procedure

1. Prepare a table like the onebelow.

2. Half fill a quart jar withpasteurized milk, and coverthe jar loosely with a lid.Measure and record the pH. Place the jar in a 37°Cincubator.

3. Check and record the pH ofthe milk with pH strips everyday for seven days. As milkspoils, its pH changes.Different populations ofmicroorganisms becomeestablished, alter substancesin the milk, and then die off

when conditions no longerfavor their survival.

4. Record any visible changesin the milk each day.

Analysis

1. Identify what happened tothe pH of the milk as timepassed.

2. Infer what the change in pH means about the popu-lations of microorganisms inthe milk.

3. Critical ThinkingEvaluating Results Howdoes this model confirm themodel of succession inGlacier Bay?

DATA TABLEDay pH Appearance

123

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 InformationWhy 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

344


Section 2

OverviewBefore 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.)

DemonstrationList 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

MotivateMotivate

Bellringer

FocusFocus


• Directed Reading• Active Reading GENERAL


• Reading Organizers• Reading Strategies • Occupational Application Worksheet

Wildlife Biologist

Planner CD-ROM

Transparencies

TR BellringerTR E4 Trophic LevelsTR E5 Food Chain in an Antarctic

EcosystemTR E6 Food Web in an Antarctic

Ecosystem

Movement of Energy Through EcosystemsEverything that organisms do in ecosystems—running, breathing,burrowing, growing—requires energy. The flow of energy is themost important factor that controls what kinds of organisms live inan 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 inorganic molecules. These organic compounds are what we call food.The rate at which organic material is produced by photosyntheticorganisms in an ecosystem is called . Primaryproductivity determines the amount of energy available in an ecosys-tem. Most organisms in an ecosystem can be thought of as chemicalmachines driven by the energy captured in photosynthesis.Organisms that first capture energy, the , include plants,some kinds of bacteria, and algae. Producers make energy-storingmolecules. 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 byassigning organisms in that ecosystem to a specific level, called a

(TROHF ihk) , in a graphic organizer based on theorganism’s source of energy. Energy moves from one trophic level toanother, 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 withfood chains.

● Describe why food chainsare rarely longer than threeor four links.

Key Terms

primary productivityproducerconsumertrophic levelfood chainherbivorecarnivoreomnivoredetritivoredecomposerfood webenergy pyramidbiomass

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

Figure 6 Trophic levels

ProducerSun Consumer Consumer

345


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

SKILLBUILDER

READINGREADING

TeachTeach


AnswerThese 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 anecosystem 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 energyof the sun to build energy-rich carbohydrates. Many producers alsoabsorb nitrogen gas and other key substances from the environmentand incorporate them into their biological molecules.

Second Level At the second trophic level are (HUHR behvohrz), animals that eat plants or other primary producers. They arethe primary consumers. Cows and horses are herbivores, as are cater-pillars and some ducks. A herbivore must be able to break down aplant’s molecules into usable compounds. However, the ability todigest cellulose is a chemical feat that only a few organisms haveevolved. As you will recall, cellulose is a complex carbohydrate foundin plants. Most herbivores rely on microorganisms, such as bacteriaand protists, in their gut to help digest cellulose. Humans cannotdigest 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 asbears, are both herbivores and carnivores; they are called (AHM nih vohrz). They use the simple sugars and starches stored inplants 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 theirenergy from the organic wastes and dead bodies that are produced atDetritivores

omnivores

carnivores

herbivores

food chainReal LifeNot all producers arephotosynthetic. At the bottom of oceansnear volcanicvents livebacteria thatharvestenergy fromthe reducedsulfur compounds ejectedby these volcanic vents. Applying Information Where in their foodchains do these bacteria lie?

Algae

Krill

CodLeopard seal

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

Figure 7 Aquatic food chain

Killer whale

346

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 animalsand protists

Krill

Cod Squid

Leopardseal

Killer whale

Adeliepenguin

Crabeater seal

Elephant seal

all trophic levels. Bacteria and fungi are known as because they cause decay. Decomposition of bodies and wastes releasesnutrients back into the environment to be recycled by other organisms.

Many ecosystems contain a fourth trophic level composed ofthose carnivores that consume other carnivores. They are called ter-tiary consumers, or top carnivores. A hawk that eats a snake is atertiary consumer. Very rarely do ecosystems contain more thanfour trophic levels.

In most ecosystems, energy does not follow simple straight pathsbecause individual animals often feed at several trophic levels. Thiscreates a complicated, interconnected group of food chains called a

, as illustrated in Figure 8. food web

decomposers

www.scilinks.orgTopic: Food Chains

and WebsKeyword: HX4085

This food web shows a more complete picture of the feeding relationships in anAntarctic ecosystem.

Figure 8 Aquatic food web

347

Use this graphic organizer withTeaching Tip on this page.

Graphic Organizer

Detritivores: consume producers,herbivores, carnivores, and omnivores

Producers: makeenergy-storing molecules

Herbivores: consume producers

Carnivores: consume herbivores

Omnivores: consumeproducers 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

LS

GENERAL



Transparencies

TR E8 Energy Transfer ThroughTrophic Levels

TR E11 Energy Efficiency in Food Consumption

MISCONCEPTIONALERT

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.orgTopic: Energy PyramidsKeyword: HX4069

Producers

Herbivores

Carnivore

Top carnivore

Loss of Energy in a Food ChainA deer browsing on leaves is acquiring energy. Potential energy isstored in the chemical bonds within the molecules of the leaves.Some of this energy is transformed to other forms of potentialenergy, such as fat. Some of it aids the deer in running and breath-ing, and in fueling cellular processes. But much of the energy isdispersed into the environment as heat.

Energy TransferDuring every transfer of energy within an ecosystem, energy is lost asheat. Although heat can be used to do work (as in a steam engine), itis generally not a useful source of energy in biological systems. Thus,the amount of useful energy available to do work decreases as energypasses through an ecosystem. The loss of useful energy limits thenumber of trophic levels an ecosystem can support. When a plantharvests energy from sunlight, photosynthesis captures only about 1percent of the energy available to the leaves. When a herbivore usesplant molecules to make its own molecules, only about 10 percent ofthe energy in the plant ends up in the herbivore’s molecules. Andwhen a carnivore eats the herbivore, about 90 percent of the energyis lost in making carnivore molecules. At each trophic level, theenergy stored by the organisms in a level is about one-tenth of thatstored by the organisms in the level below.

The Pyramid of EnergyEcologists often illustrate the flow of energy through ecosystemswith an energy pyramid. An is a diagram in whicheach trophic level is represented by a block, and the blocks arestacked on top of one another, with the lowest trophic level on thebottom. The width of each block is determined by the amount ofenergy stored in the organisms at that trophic level. Because theenergy stored by the organisms at each trophic level is about one-tenth the energy stored by the organisms in the level below, thediagram takes the shape of a pyramid, as shown in Figure 9.

energy pyramid

The word ecosystem isfrom the Greek wordsoikos, meaning “house,”and systematos, meaning“to place together.”Knowing this informationmakes it easier toremember that anecosystem includes acommunity of living thingsas well as all physicalaspects of its environment.

In this simple ecosystem, eachtrophic level contains about 90percent less energy than thelevel below it.

Figure 9 Trophic levels ofa terrestrial ecosystem

348


Answers to Section Review1. Producers use energy (usually from the sun) to

assemble 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 consumers

outnumbering 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

CloseClose

LS


GENERAL

Limitations of Trophic LevelsMost terrestrial ecosystems involve onlythree or, on rare instances, four levels. Toomuch energy is lost at each level to allowmore levels. For example, a large humanpopulation could not survive by eatinglions captured on the Serengeti Plain ofAfrica because there are too few lions tomake this possible. The amount of grass inthat ecosystem cannot support enoughzebras to maintain a large enough popula-tion of lions to feed lion-eating humans. Inother words, the number of trophic levelsthat can be maintained in a community islimited 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 isdirectly ingested by a human. If a cow eatsthe grain and a human eats the cow, thenabout 100 kg (220 lb) of grain are needed tobuild about 1 kg (2.2 lb) of human tissue.

Also, the number of individuals in atrophic 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. Becauseof this, the number of organisms often does not form a pyramid whenone compares different trophic levels. For instance, caterpillars andother insect herbivores greatly outnumber the trees they feed on. Tobetter determine the amount of energy present in trophic levels, ecol-ogists measure biomass. is the dry weight of tissue and otherorganic matter found in a specific ecosystem. Each higher level on thepyramid contains only 10 percent of the biomass found in the trophiclevel below it.

Biomass

It takes 10 timesmore grain

to feed one cow

to make enough beef

to provide oneperson with the same

amount of energy.

It takes a certainamount of grain

to produceenough bread

to provide oneperson with a certain

amount of energy.

Adding a trophic level to a food chain increases theenergy 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 ArgumentExplain 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 → algaeB cod → leopard seal → krillC leopard seal → algae → krillD algae → krill → cod


349


OverviewBefore 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.

DiscussionMake 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


• Directed Reading• Active Reading GENERAL


• Reading Organizers• Reading Strategies • Supplemental Reading Guide

Silent Spring

Planner CD-ROM

Transparencies

TR BellringerTR Graphic OrganizerTR Concept MapTR E12 Water CycleTR E13 Carbon CycleTR 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 anythingaway. Most energy flows through the Earth’s ecosystems from the sunto producers to consumers. The physical parts of the ecosystems,however, cycle constantly. Carbon atoms, for example, are passedfrom one organism to another in a great circle of use. Producers areeaten by herbivores, herbivores are eaten by carnivores, and carni-vores are eaten by top carnivores. Eventually the top carnivores dieand decay; their carbon atoms then become part of the soil to feed theproducers in a long and complex cycle that reuses this important ele-ment. Carbon is not the only element that is constantly recycled inthis way. Other recycled elements include many of the inorganic (non-carbon) substances that make up the soil, water, and air, such asnitrogen, sulfur, calcium, and phosphorus.

All materials that cycle through living organisms are important inmaintaining the health of ecosystems, but four substances are partic-ularly important: water, carbon, nitrogen, and phosphorus. Allorganisms 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, andphosphorus pass from the nonliving environ-ment to living organisms, such as the trees inFigure 11, and then back to the nonlivingenvironment. These paths form closed cir-cles, or cycles, called biogeochemical (bieoh jee oh KEHM ih kuhl) cycles. In each

, a pathway forms whena substance enters living organisms such astrees from the atmosphere, water, or soil;stays for a time in the living organism; thenreturns to the nonliving environment.Ecologists refer to such substances as cyclingwithin 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 thesubstance in the living reservoir than in thenonliving reservoir.

biogeochemical cycle

Objectives● Summarize the role of

plants in the water cycle.

● Analyze the flow of energythrough the carbon cycle.

● Identify the role of bacteria inthe nitrogen cycle.

Key Terms

biogeochemical cycleground watertranspirationnitrogen fixation

Figure 11 Trees and the carbon cycle. Approximately500 million tons of carbon were taken up as a result offorest 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

LS

SKILLBUILDER

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 portionof the water cycle, water vapor in the atmosphere condenses and fallsto the Earth’s surface as rain or snow. Some of this water seeps intothe soil and becomes part of the , which is waterretained beneath the surface of the Earth. Most of the remainingwater that falls to the Earth does not remain at the surface. Instead,heated by the sun, it reenters the atmosphere by evaporation. Thepath of water within an ecosystem is shown in Figure 12.

In the living portion of the water cycle, much water is taken up bythe roots of plants. After passing through a plant, the water movesinto the atmosphere by evaporating from the leaves, a process called

. Transpiration is also a sun-driven process. The sunheats the Earth’s atmosphere, creating wind currents that drawmoisture from the tiny openings in the leaves of plants.

In aquatic ecosystems (lakes, rivers, and oceans), the nonlivingportion of the water cycle is the most important. In terrestrial ecosys-tems, the nonliving and living parts of the water cycle both playimportant roles. In thickly vegetated ecosystems, such as tropicalrain forests, more than 90 percent of the moisture in the ecosystempasses through plants and is transpired from their leaves. In a veryreal 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.orgTopic: Water CycleKeyword: 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

LS


“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.orgTopic: Carbon CycleKeyword: HX4031

The Carbon Cycle Carbon also cycles between the nonliving environment and livingorganisms. You can follow the carbon cycle in Figure 13. Carbondioxide in the air or dissolved in water is used by photosynthesizingplants, algae, and bacteria as a raw material to build organic mol-ecules. Carbon atoms may return to the pool of carbon dioxide inthe air and water in three ways.

1. Respiration. Nearly all living organisms, including plants,engage in cellular respiration. They use oxygen to oxidize organicmolecules during cellular respiration, and carbon dioxide is abyproduct of this reaction.

2. Combustion. Carbon also returns to the atmosphere throughcombustion, or burning. The carbon contained in wood may staythere for many years, returning to the atmosphere only when thewood is burned. Sometimes carbon can be locked away beneaththe Earth for thousands or even millions of years. The remains oforganisms that become buried in sediments may be graduallytransformed by heat and pressure into fossil fuels—coal, oil, andnatural gas. The carbon is released when the fossil fuels are burned.

3. Erosion. Marine organisms use carbon dioxide dissolved in seawater to make calcium carbonate shells. Over millions of years,the shells of the dead organisms form sediments, which formlimestone. As the limestone becomes exposed and erodes, thecarbon becomes available to other organisms.

Cellularrespiration

Combustion

Photosynthesis

Death anddecomposition

Fossil fuels

Limestone

Marineplanktonremains

Carbon dioxide (CO2)in atmosphere

DissolvedCO2 inwater

This diagram shows the majorsteps 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

GENERAL

LS


The Phosphorus and Nitrogen CyclesOrganisms need nitrogen and phosphorus to build proteins andnucleic 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, PO4

3-. This phos-phate is absorbed by the roots of plants and used to build organicmolecules. 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 nitrogenatoms in a molecule of nitrogen gas are connected by a strong triplecovalent bond that is very difficult to break. However, a few bacteriahave enzymes that can break it, and they bind nitrogen atoms tohydrogen 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 withinswellings, or nodules, on the roots of beans, alder trees, and a fewother kinds of plants.

The nitrogen cycle, diagramed in Figure 14, is a complex processwith four important stages.

1. Assimilation is the absorption and incorporation of nitrogeninto 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 InformationUsing your own words, writefour sentences, each onedescribing one of the fourbiogeochemical cycles.

Denitrification

Assimilation Nitrogenfixation

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 ofthe important steps in thenitrogen cycle, including theconversion of atmosphericnitrogen 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 of

an 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

CloseClose

Answers to Section Review1. The sun’s heating causes wind currents, which

draws 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 theavailability of nitrate and ammonia in the soil. Today most of theammonia and nitrate that farmers add to soil is produced chemi-cally in factories, rather than by bacterial nitrogen fixation. Geneticengineers are trying to place nitrogen-fixing genes from bacteriainto the chromosomes of crop plants. If these attempts are successful,the plants themselves will be able to fix nitrogen, thus eliminating theneed for nitrogen-supplying fertilizers. Some farmers adjust theirfarming methods to increase natural recycling of nitrogen.

Sustainable Agriculture

In an ecosystem, decomposersreturn mineral nutrients to the

soil. However, when the plants areharvested and shipped away,there is a net loss of nutrients fromthe soil where the plants weregrowing. The amount of organicmatter in the soil also decreases,making the soil less able to holdwater and more likely to erode.

What is SustainableAgriculture?Sustainable agriculture refers tofarming that remains productiveand profitable through practicesthat help replenish the soil’snutrients, reduce erosion, andcontrol weeds and insect pests.

Use of Cover Crops After harvest, farmers can plantcover crops, such as rye, clover,or vetch, instead of letting theground lie bare. Cover crops

keep the soil from compactingand washing away, and they helpthe soil absorb water. They alsoprovide a habitat for beneficialinsects, slow the growth ofweeds, and keep the groundfrom overheating. When covercrops are plowed under, as illus-trated in the figure at right, theyreturn nutrients to the soil.

Rotational GrazingFarmers who raise cattle andsheep can divide their pasturesinto several grazing areas. Byrotating their livestock from onearea to another, they can preventthe animals from overgrazing thepasture. This allows the plants on which the animals feed to live longer and be more produc-tive. Water quality improves as the pasture vegetation becomesdenser. Animals distribute manuremore evenly with rotational

grazing than they do in feed lotsor unmanaged pastures.

There are many other methodsused in sustainable agriculture.Farmers must determine whichmethods work best for their crops,soil conditions, and climate.

www.scilinks.orgTopic: Sustainable AgricultureKeyword: 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 erosionB 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 MapThe 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

ZONEKey Terms

Section 1ecology (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 2primary 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 3biogeochemical cycle (350)ground water (351)transpiration (351)nitrogen fixation (353)

BIOLOGYBIOLOGYUnit 7—Ecosystem DynamicsUse 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 10percent of the energy in a trophic level to the next level.

● Most terrestrial communities have only three or four trophiclevels because energy transfers between trophic levels areinefficient.

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 throughphotosynthesis. Organisms release carbon through cellularrespiration. Carbon trapped in rocks and fossil fuels isreleased by erosion and burning.

● Bacteria fix atmospheric nitrogen, thus making ammoniaavailable to other organisms.

3

2

1

355


ANSWERS

Understanding Key Ideas1. b2. b3. c4. d5. b6. c7. Greater flexibility in diet allows

omnivores 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 Thinking11. Each trophic level contains about

90% 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 Assessment14. Answers will vary depending on the organ-

isms pictured.


CHAPTER 16

Section Questions1 1, 3, 11, 12, 142 2, 5, 6, 7, 10, 133 4, 8, 9, 12

Assignment Guide

Understanding Key Ideas1. Ecosystems differ from communities in

that 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 Thinking11. Inferring Relationships Analyze the flow of

energy 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 Assessment14. Identifying Functions Obtain photocopies

of 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.

Answers1. C2. I3. B4. F5. D6. Ecosystems are dynamic by nature.7. H8. 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 to

meadow grasses.G. The presence of decaying organic

matter is harmful to meadow grasses.H. Meadow grasses require that pioneer

species first make nutrient-rich soil.I. Meadow grasses cannot compete with

marsh 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

Car

bon

dio

xid

e co

nce

ntr

atio

n(p

arts

per

mill

ion)

Month

March May July Sept.Jan. Nov.348

350

352

354

356

358

TestFor multiple-choice questions, try to eliminate anyanswer choices that are obviously incorrect, andthen consider the remaining answer choices.

357

Standardized Test Prep


Exploration LabMODELING ECOSYSTEMCHANGE 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 BeginOrganisms in an interact witheach other and with their environment. Oneof the interactions that occurs among theorganisms in an ecosystem is feeding. A

describes the feeding relationshipsamong the organisms in an ecosystem. Inthis lab, you will model a natural ecosystemby building a in a bottleor a jar. You will then observe the interac-tions of the organisms in the ecosystem andnote any changes that occur over time.

1. Write a definition for each boldface term inthe paragraph above and for each of thefollowing terms: producer, decomposer,consumer, herbivore, carnivore, trophiclevel.

2. Based on the objectives for this lab, write aquestion you would like to explore aboutecosystems.

ProcedurePART A: Building an Ecosystem in

a Jar1. Place 2 in. of sand or pea gravel in

the bottom of a large, clean glass jarwith a lid. CAUTION: Glassware is fragile.

Notify your teacher promptly of anybroken glass or cuts. Do not clean upbroken glass or spills with broken glassunless your teacher tells you to do so.Cover the gravel with 2 in. of soil.

2. Sprinkle the seeds of two or three types ofsmall plants, such as grasses and clovers,on the surface of the soil. Put a lid on thejar, and place it in indirect sunlight. Let thejar remain undisturbed for a week.

3. After one week, place a handful of rolledoats into the jar. Place the mealworms inthe oats, and then place the other animalsinto the jar and replace the lid. Place thelid on the jar loosely to enable air entry.

You ChooseAs you design your experiment, decide the following:a. what question you will exploreb. what hypothesis you will testc. how you will plant the seedsd. where you will place the ecosystem for one

week 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 useg. what data you will record in your data table

closed ecosystem

webfood

ecosystem

Exploration LabModeling Ecosystem Change over Time

SKILLS• Using scientific methods

• Modeling

• Observing

OBJECTIVES• Construct a model

ecosystem.

• Observe the interactions oforganisms in a modelecosystem.

• Predict how the number of each species in a modelecosystem will change over time.

• Compare a modelecosystem with a naturalecosystem.

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.

2. Answers will vary.3. Answer will vary. All plants are

producers (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.

5. Answers will vary.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 Experiment4. Work with the members of your lab group

to explore one of the questions written forstep 2 of Before You Begin. To explore thequestion, design an experiment that usesthe materials listed for this lab.

5. Write a procedure for your experiment.Make a list of all the safety precautions youwill take. Have your teacher approve yourprocedure and safety precautions beforeyou begin the experiment.

6. Set up your group’s experiment. Conductyour experiment for at least 14 days.

PART C: Cleanup and Disposal7. Dispose of solutions, broken glass,

and other materials in the designatedwaste containers. Do not put lab materialsin the trash unless your teacher tells you todo so.

8. Clean up your work area and all labequipment. Return lab equipment to

its proper place. Wash your hands thor-oughly before you leave the lab and afteryou finish all work.

Analyze and Conclude1. Summarizing Results Make graphs

showing how the number of individuals ofeach species in your ecosystem changedover time. Plot time on the x-axis and thenumber of organisms on the y-axis.

2. Analyzing Results How did your resultscompare with your hypothesis? Explainany differences.

3. Inferring Conclusions Construct a foodweb for the ecosystem you observed.

4. Recognizing Relationships Does yourmodel ecosystem resemble a naturalecosystem? Explain.

5. Analyzing Methods How might you havebuilt your model ecosystem differently tobetter represent a natural ecosystem?

6. Evaluating Methods Was your modelecosystem truly a “closed ecosystem”? Listyour model’s strengths and weaknesses as aclosed ecosystem.

7. Further Inquiry Write a new questionabout ecosystems that you could explorewith another investigation.

www.scilinks.orgTopic: EcosystemsKeyword: HX4066

Do You Know?Do research in the library or media centerto answer these questions:

1. What is Biosphere 2?

2. What problems were encountered bythe Biosphere 2 crew during the1991–1993 project?

Use the following Internet resources to explore your own questions aboutecosystems.

359


chapter 16 ecosystems - welcome to mr. walker's class …€¦ · · 2016-03-29chapter 16...

Documents