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55 How Ecosystems WorkC H A P T E R 5

1 Energy Flow in Ecosystems

2 The Cycling of Materials

3 How Ecosystems Change

PRE-READING ACTIVITY

Double-Door FoldBefore youread this chap-

ter, create the FoldNoteentitled “Double-Door Fold”described in the Reading andStudy Skills section of theAppendix. Write “Energy flow in ecosystems” on oneflap of the double door and“Movement of materials inecosystems” on the otherflap. As you read the chapter,compare the two topics, and write characteristics ofeach on the inside of theappropri-ate flap.

This green frog gets the energy itneeds to survive by eating otherorganisms, such as dragonflies.

OverviewTell students that in this chapter,they will learn how the flow ofenergy, cycling of materials, andecological succession combine to affect how ecosystems work.Organisms need energy to stayalive. Some organisms, such asplants, can directly convert usableenergy from the sun. The cycling ofmaterials such as carbon, nitrogen,and phosphorus is essential to keepnutrients balanced in ecosystems.Human activities can affect thesecycles. Through ecological succes-sion, ecosystems can change overtime.

Using the Figure Frogs and dragonflies are both con-sumers that are high in an aquaticfood chain. Ask the students to dis-cuss how energy would continue to be transferred in this food chain.(Sample answer: a heron might gain energy by eating the green frog.When the heron dies, microbes mightgain energy by breaking down theheron.) LogicalLS

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124 Chapter 5 • How Ecosystems Work

For information about videosrelated to this chapter, go togo.hrw.com and type in the keyword HE8 ECOV.

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PRE-READING ACTIVITY

LS 1e Plant cells contain chloroplasts, the site of photosynthesis. Plants and many microorganisms use solar energy to combine molecules of carbondioxide and water into complex, energy rich organic compounds and releaseoxygen to the environment. This process of photosynthesis provides a vitalconnection between the sun and the energy needs of living systems. (Section 1)

LS 4b Energy flows through ecosystems in one direction, from photosyn-thetic organisms to herbivores to carnivores and decomposers. (Section 1)

LS 5f As matter and energy flows through different levels of organization ofliving systems—cells, organs, organisms, communities—and between livingsystems and the physical environment, chemical elements are recombined indifferent ways. Each recombination results in storage and dissipation of energyinto the environment as heat. Matter and energy are conserved in eachchange. (Section 1 and Section 2)

ES 2a The earth is a system containing essentially a fixed amount of each stable chemical atom or element. Each element can exist in several differentchemical reservoirs. Each element on earth moves among reservoirs in thesolid earth, oceans, atmosphere, and organisms as part of geochemical cycles.(Section 2)

ES 2b Movement of matter between reservoirs is driven by the earth’s inter-nal and external sources of energy. These movements are often accompaniedby a change in the physical and chemical properties of the matter. Carbon, forexample, occurs in carbonate rocks such as limestone, in the atmosphere ascarbon dioxide gas, in water as dissolved carbon dioxide, and in all organismsas complex molecules that control the chemistry of life. (Section 2)

SPSP 4b Materials from human societies affect both physical and chemi-cal cycles of the earth. (Section 2)

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Chapter 5 • How Ecosystems Work 125

Just as a car cannot run without fuel, an organism cannot survivewithout a constant supply of energy. Where does an organism’senergy come from? The answer to this question depends on theorganism, but the ultimate source of energy for almost all organ-isms is the sun.

Life Depends on the SunEnergy from the sun enters an ecosystem when a plant uses sunlightto make sugar molecules in a process calledDuring photosynthesis, plants, algae, and some bacteria capturesolar energy. Solar energy drives a series of chemical reactionsthat require carbon dioxide and water, as shown in Figure 1. Theresult of photosynthesis is the production of sugar molecules knownas carbohydrates. Carbohydrates are energy-rich molecules thatorganisms use to carry out daily activities. As organisms consumefood and use energy from carbohydrates, the energy travels fromone organism to another. Plants, such as the sunflowers in Figure 2,produce carbohydrates in their leaves. When an animal eats a plant,some energy is transferred from the plant to the animal. Organismsuse this energy to move, grow, and reproduce.

photosynthesis.

Objectives� Describe how energy is transferred

from the sun to producers andthen to consumers.

� Describe one way in which con-sumers depend on producers.

� List two types of consumers.

� Explain how energy transfer in afood web is more complex thanenergy transfer in a food chain.

� Explain why an energy pyramid isa representation of trophic levels.

Key Termsphotosynthesisproducerconsumerdecomposercellular respirationfood chainfood webtrophic level

S E C T I O N 1

Energy Flow in Ecosystems

Figure 2 � The cells in the leaves ofthese sunflowers contain a greenchemical called chlorophyll. Chloro-phyll helps plants trap energy fromthe sun to produce energy-richcarbohydrates.

Figure 1 � During photosynthesis,plants use carbon dioxide, water, andsolar energy to make carbohydratesand oxygen.

OverviewBefore beginning this section,review with your students theObjectives in the Student Edition.This section discusses the principlethat sunlight is the ultimate energysource for nearly all living things.The section describes energy trans-fer in ecosystems. Students will alsolearn about the roles of producers,consumers, and decomposers infood chains and food webs.

Ask students to write in theirEcoLog three plants or animalsand the animals that eat them.Also ask them to write down anyplants they know of that eat animals(Venus flytrap and pitcher plant aretwo examples). Tell them to be crea-tive, and to think about animalsand plants on different continents.

IdentifyingPreconceptions Energy: The Missing IngredientWrite the following ingredients on the board: 1/2 bathtub full of oxygen, 50 glasses of water, 1/2 cup of sugar, 1/2 cup of cal-cium, 1/10 thimbleful of salt, asmall pinch of assorted elementssuch as phosphorus, potassium,nitrogen, sulfur, magnesium, andiron, and a mystery ingredient.

Ask students to write downwhat these ingredients will make,and what they think the mysteryingredient is. Tell students, “Believeit or not, these are the basic ingre-dients that you are made of andenergy is the missing ingredient.”Then ask, “Where does that energycome from?” (the food we eat) Ask,“Where does the energy in foodcome from?” (ultimately, the sun)

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From Producers to Consumers When a rabbit eats a clover plant,the rabbit gets energy from the carbohydrates the clover plant madethrough photosynthesis. If a coyote eats the rabbit, some of theenergy is transferred from the rabbit to the coyote. In Figure 3, theclover is the producer. A is an organism that makes itsown food. Producers are also called autotrophs, or self-feeders. Therabbit and the coyote are organisms that get their energyby eating other organisms. Consumers are also called heterotrophs,or other-feeders. In Figure 3, the clover, rabbit, and coyote get theirenergy from the sun. Some producers get energy directly from thesun by absorbing it through their leaves. Consumers get energy indi-rectly from the sun by eating producers or other consumers.

An Exception to the Rule: Deep-Ocean Ecosystems The bottomof the ocean off the coast of Ecuador is teeming with life. Scientistsfound large communities of worms, clams, crabs, mussels, and bar-

nacles living near thermal vents in the ocean floor.These deep-ocean communities exist in total

darkness, where photosynthesis cannot occur.So where do these organisms get theirenergy? Bacteria, such as those pictured inFigure 4, live in some of these organisms anduse hydrogen sulfide to make their own food.

Hydrogen sulfide is present in the hot waterthat escapes from the cracks in the ocean floor.

Therefore, the bacteria are producers. The bacteriaare eaten by the other underwater organisms and thus

support a thriving ecosystem.

How do producers and consumers get energy fromthe sun? (See the Appendix for answers to Reading Checks.)��Reading Check

consumers,

producer

Figure 3 � Transfer of EnergyAlmost all organisms depend on thesun for energy. Plants like the clovershown above get energy from thesun. Animals like the rabbit andcoyote get their energy by eatingother organisms.

Figure 4 � The tube worms (above)depend on bacteria that live insidethem to survive. The bacteria (right)use energy from hydrogen sulfide tomake their own food.

Using the Figure Life Depends on the Sun Usethe photographs in Figure 3 to helpstudents visualize how life dependson the sun. Ask, “In this series ofphotographs, how does the coyotedepend on the sun?” (The coyotedepends on the sun because it gainsthe energy it needs to survive by eat-ing the rabbit. The rabbit gains itsenergy from the clover, which in turngains energy from the sun.) Visual

Group Activity Classroom Hydrothermal VentCommunity Bring in some piecesof poster board and photos oforganisms associated with deepocean hydrothermal vent commu-nities. Ask students to draw indi-vidual pictures of creatures thatdepend on the bacteria associatedwith hydrothermal vents. Ask stu-dents to sketch bacterial coloniesand the vents themselves. Tell stu-dents to clearly label each creaturewith its common and scientificname, if possible. Ask students tocut out their drawings and arrangethem into communities on thepieces of poster board. After eachcommunity is constructed, have students explain how all the crea-tures interact with each other. Ifstudents are unsure of the role ofsome of the organisms, ask them toresearch these roles before the nextclass period. Visual/Kinesthetic

Vocabulary The Greek suffix–troph means “feeder.” Therefore,trophic levels are “feeding” levels.

AAnnsswweerr ttoo RReeaaddiinngg CChheecckkA producer gets energy from the sunthrough photosynthesis. A consumergets energy from the sun indirectly:by eating other organisms.

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MISCONCEPTION ALERT

Thermal Vents Are Diverse With LifeStudents may think that hydrothermal ventsare hot, hostile places that support only bac-teria and a few other unique organisms.However, more than 300 species are asso-ciated with deep ocean thermal vents. Bacteriacolonize the vents, and serve as the food basefor other organisms such as mussels, shrimp,and crabs. Alternately, students may have

heard that the hydrothermal vents producevery hot water, and they may think that theorganisms living around the vents are livingthere to escape the frigid ocean conditions.Even though the water around the vents canreach up to 400 °C (752 °F), temperaturescan drop to just above freezing just outsidethe vents. Most of the organisms that feed on the vent bacteria live in the cold zone.

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Vocabulary To help students learn some of the new vocabulary in this section, directtheir attention to Table 1. Read the table outloud, discussing each portion. Then use maga-zine photos and other visual aids to providemore examples of each type of organism.Check student comprehension by holding upvarious photos and asking students whetherthe organism shown is a producer or a con-sumer. If it is a consumer, ask them whichtype of consumer it is. (herbivore, carnivore,omnivore, or decomposer)

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What Eats WhatTable 1 below classifies organisms by the source of their energy.Consumers that eat only producers are called herbivores, or planteaters. Rabbits are herbivores and so are cows, sheep, deer, grass-hoppers, and many other animals. Consumers, such as lions andhawks, that eat only other consumers are called carnivores, orflesh eaters. You already know that humans are consumers, butwhat kind of consumers are we? Because most humans eat bothplants and animals, we are called omnivores, or eaters of all.Bears, pigs, and cockroaches are other examples of omnivores.Some consumers get their food by breaking down dead organismsand are called Bacteria and fungi are examples ofdecomposers. The decomposers allow the nutrients in the rottingmaterial to return to the soil, water, and air.

decomposers.

What Eats What in an Ecosystem

Energy source Examples

Producer makes its own food grasses, ferns, cactuses,through photosynthesis flowering plants, trees, or chemical sources algae, and some bacteria

Consumer gets energy by eating mice, starfish, elephants, producers or other turtles, humans, and antsconsumers

Types of Consumers in an Ecosystem

Energy source Examples

Herbivore producers cows, sheep, deer, and grasshoppers

Carnivore other consumers lions, hawks, snakes, spiders, sharks, alligators, and whales

Omnivore both producers bears, pigs, gorillas, rats, and consumers raccoons, cockroaches,

some insects, andhumans

Decomposer breaks down dead fungi and bacteriaorganisms in an ecosystem and returns nutrients to the soil, water, and air

Table 1 �

Figure 5 � Bears, such as the grizzlybear below, are omnivores. Grizzlybears eat other consumers, such assalmon, but they also eat variousplants.

MATHPRACTICEA Meal Fit for a Grizzly Bear Grizzly bears are omnivores that can eat up to 15 percent of their body weightper day when eating salmon andup to 33 percent of their bodyweight when eating fruits andother vegetation. How manypounds of salmon can a 200 lbgrizzly bear eat in one day? Howmany pounds of fruits and othervegetation can the same bear eatin one day?

Group Activity Creating Food Chains and FoodWebs Pass out 1 or 2 blank indexcards to each student. Ask studentsto write an organism on each card(tell them to use large letters, sothat everyone can see the name ofeach organism at the back of theclassroom). Divide the middle of theboard into the following sections(from top to bottom): Carnivore,Herbivore, and Producer. WriteDecomposer to the right andOmnivore to the left of Herbivore.Have students stick their cards ontothe correct section of the boardwith tape. Then ask a couple of students to stay at the board and,using suggestions from the class,have them arrange the cards intospecific food chains/food webs(drawing lines between the con-nected elements). Many of the foodwebs will get messy, but that shouldhelp students to understand thatecological interactions in the realworld can be complicated. If noneof the students contribute anydecomposers to the board, havethem brainstorm some possibleorganisms that break down animalsor plants, and link them to the food chains/food webs.

Kinesthetic/Visual

Math Have students figure outhow many calories there are in 30 lbs of salmon if each pound ofsalmon � approx. 654 cal (30 lbs �654 cal � 19,620 cal). Have stu-dents calculate how many caloriesthere are in 66 lbs of blueberries if each pound of blueberries �approx. 261 cal (66 lbs � 261 cal �17,226 cal). LogicalLS

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English Language Learners

Answers15% of 200 lbs � 30 lbs; 33%of 200 lbs � 66 lbs

MATHPRACTICE

Transparencies

TT A Food ChainTT A Food Web

StrategiesStrategiesINCLUSIONINCLUSION

Ask students to draw a picture of an imagi-nary ecosystem. The picture should includeorganisms that are producers and con-sumers as defined in Table 1. The types ofconsumers and producers should be labeledclearly. The student may present theirecosystem to the class or small group toshow their understanding of the concept.

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Cellular Respiration: Burning the FuelSo far, you have learned how organisms get energy. But how dothey use the energy they get? To understand the process, useyourself as an example. Suppose you have just eaten a large meal.The food you ate contains a lot of energy. Your body gets theenergy out of the food by using the oxygen you breathe to breakdown the food. By breaking down the food, your body obtainsthe energy stored in the food.

The process of breaking down food to yield energy is calledwhich occurs inside the cells of most organ-

isms. This process is different from respiration, which is anothername for breathing. During cellular respiration, cells absorb oxy-gen and use it to release energy from food. As you can see inFigure 6, the chemical equation for cellular respiration is essen-tially the reverse of the equation for photosynthesis. During cellu-lar respiration, sugar and oxygen combine to yield carbondioxide, water, and, most importantly, energy.

cellular respiration,

Figure 6 � Through cellular respira-tion, cells use glucose and oxygento produce carbon dioxide, water,and energy.

ChemistryConnection to

Chemical Equations Chemicalreactions are represented bychemical equations. A chemicalequation is a shorthand descrip-tion of a chemical reaction usingchemical formulas and symbols.The starting materials in a reactionare called reactants, and the sub-stances formed from a reactionare called products. The number ofatoms of each element in the reac-tants equals the number of atomsof those elements in the productsto make a balanced equation.

� A high concentration of DDT decreases the thickness and the strength ofeggshells of many birds of prey.

DDT in an Aquatic Food Chain

In the 1950s and 1960s, something

strange was happening in the estu-

aries near Long Island Sound, near

New York and Connecticut. Birds of

prey, such as ospreys and eagles,

that fed on fish in the estuaries had

high concentrations of the pesticide

DDT in their bodies. But when the

water in the estuaries was tested, it

had low concentrations of DDT.

What accounted for the high

levels of DDT in the birds? Poisons

that dissolve in fat, such as DDT, can

become more concentrated as they

move up a food chain in a process

called biological magnification. When

the pesticide enters the water, algae

and bacteria take in the poison.

When fish eat the algae and bacteria,

the poison dissolves into the fat of

the fish rather than diffusing back

into the water. Each time a bird feeds

concentrations in fatty tissues of

organisms were magnified almost 10

million times from the bottom to the

on a fish, the bird accumulates more

DDT in its fatty tissues. In some estu-

aries on Long Island Sound, DDT

Demonstration Cellular Respiration Obtain the following materials: 50 mLErlenmeyer flask, 2.5 mL of yeast,40 mL of apple cider, and a latexballoon.

Mix the yeast and cider in theflask and place the balloon securelyon top of the flask. Yeast will respireaerobically and anaerobically, pro-ducing CO2, which will inflate theballoon in about 2–3 days. Duringthese days, encourage students toresearch how respiration works inyeast. Have students record theirresearch and daily observations intheir EcoLog. Visual/Kinesthetic

Silent Spring The book SilentSpring was written to educate thepublic on the potential harm to theenvironment from the overuse ofchemical pesticides, such as DDT.The author, Rachel Carson, wasa former U.S. Fish and WildlifeService employee who researchedher findings carefully before pub-lishing them. When Silent Springwas published, chemical companiesresponded with a campaign toutingthe benefits of pesticides andattacking Carson. This actuallyhelped to publicize the book. Herbook was responsible for changesto pesticide regulations in the U.S.Urge students to check out thisimportant book at their local library.

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Toxicologist Toxicologists study adverseeffects of chemicals on living organisms andecosystems. They look at relationshipsbetween chemicals and disease, and environ-mental risks associated with various chemi-cals. A toxicologist can be trained as abiologist or chemist. Toxicologists typicallywork in government, universities, chemical

industries, or consulting firms. Toxicologistsare concerned with the short and long-termsafety of chemicals. Many participate in set-ting guidelines for the regulation of certainchemicals. If your local university or collegehas a toxicology program, contact the depart-ment to invite a toxicology professor or grad-uate student to speak with the class.

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You use a part of the energy you obtain through cellular respiration to carry out your daily activities. Every time you walk,breathe, read a book, think, or play a sport, you use energy. Theenergy you obtain is also used to make more body tissues and tofight diseases so that you grow and stay healthy. Excess energy youobtain is stored as fat or sugar. All living things use cellular respira-tion to get the energy they need from food molecules. Even organ-isms that make their own food through photosynthesis use cellularrespiration to obtain energy from the carbohydrates they produce.

Energy TransferEach time one organism eats another organism, a transfer of energyoccurs. We can trace the transfer of energy as it travels throughan ecosystem by studying food chains, food webs, and trophiclevels. Food chains, food webs, and trophic levels can tell us howenergy is transferred, as well as how much energy is transferred,between organisms in an ecosystem. Studying the paths of energybetween organisms can also tell us which organisms in an ecosys-tem depend on other organisms to survive.

BiologyConnection to

Calories from Food The sub-stances your body needs to surviveand grow come from food. Carbo-hydrates, proteins, and fats aremajor sources of energy for thebody. The energy content of foodcan be found by burning a dryfood sample in a special calorim-eter. Both carbohydrates and pro-teins provide 4 Calories (Cal) ofenergy per gram, while fats pro-vide 9 Cal of energy per gram.

� Poisons such as DDT have the greatest affect on organisms at the topof food chains. For example, the osprey shown here would have a greaterconcentration of DDT in its body than the perch it’s about to eat.

top of the food chain. Large concen-

trations of DDT may kill an organism,

weaken its immune system, cause

deformities, or impair its ability to

reproduce. DDT can also weaken the

shells of bird eggs. When eggs break

nant and in 1972 banned its sale

except in emergencies. The aquatic

food chains immediately started to

recover, and the populations of

ospreys and eagles started to grow.

Food chains are still not free of

DDT. DDT is still legal in some coun-

tries, where it is used in large quanti-

ties to eliminate mosquitoes that

carry the disease malaria. As

a result, migratory birds may be

exposed to DDT while wintering in

locations outside the United States.

too soon, bird embryos die. There-

fore, the effects of these chemicals

cause a tremendous drop in the pop-

ulation of carnivorous bird species.

The U.S. government recognized

DDT as an environmental contami-

CRITICAL THINKING

1. Analyzing Processes DDTdoes not dissolve readily in water. Ifit did, how would the accumulationof the pesticide in organisms beaffected?

2. Evaluating Information Eventhough DDT is harmful to the envi-ronment, why is it still used in somecountries?

Debate The Use of DDT DDT is a cheapinsecticide that most effectivelyeliminates the mosquitoes thatcarry malaria. Malaria is a majorproblem in many tropical coun-tries. Malaria infects 400 millionpeople per year. Several millionpeople die from this disease per year. Many countries haveswitched to non-DDT insecticides,but some mosquitoes are nowbecoming resistant. Have studentsresearch the effects of DDT, and itsuses as an insecticide. Then breakstudents up into small groups andexplain that they are in Belize, aCentral American country with anecotourism industry and a malariaproblem. Assign students withineach group the roles of public healthprofessional, parent, ecologist, tribeleader, ecotourist, wildlife manager,and construction worker. Ask students to discuss the problem of DDT in the roles they wereassigned. Also ask them to comeup with a solution that all partiescan agree on. Will they use alterna-tive insecticides that cost more andtry to compensate for the cost intourism dollars? Or will they uselocalized spraying of DDT? Haveeach group present their solutionto the entire class. Interpersonal

Biomagnification Tell studentsthat the pesticide DDT is just oneexample of a substance that is sub-ject to biological magnification.Suggest that students research threeother substances that can accumu-late in food webs. PCBs, strontium-90, and mercury are possibleoptions. Encourage students toinclude their research in theirPortfolio. VerbalLS

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DDT in an Aquatic Food Chain Tell stu-dents that when DDT undergoes biomagni-fication in a food chain, its concentrationincreases by tenfold at each step in the foodchain. Ask students to apply this increase tothe organisms in the following food chain:bass, perch, minnows, invertebrates, andalgae. Tell students that the concentration of DDT in algae was measured at 1 part in 1,000,000. Students should arrive at the following answers: bass (1 part in 100),perch (1 part in 1,000), minnows (1 part in10,000), invertebrates (1 part in 100,000),

algae (1 part in 1,000,000). Be sure that allstudents understand that 1 part in 1,000,000is smaller than 1 part in 100. Logical

AAnnsswweerrss ttoo CCrriittiiccaall TThhiinnkkiinngg1. If DDT dissolved in water, it would be flushed

out of an animal’s system almost immediatelyand it would not become biomagnified.

2. DDT is a cheap and effective insecticide thatkills the mosquitoes that carry the deadlyparasite that causes malaria. Since malaria isa serious problem in many tropical countries,there is still a market for DDT.

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Figure 7 � Energy is transferredfrom one organism to another in afood chain, such as the one shownbelow. Algae are the producers inthis ocean food chain.

Food Chains and Food Webs A is a sequence in whichenergy is transferred from one organism to the next as each organ-ism eats another organism. Figure 7 shows a typical food chain in anocean ecosystem. Algae are eaten by krill, which are eaten by cod.The cod are eaten by leopard seals, which are eaten by killer whales.

Energy flow in an ecosystem is more complex than energyflow in a simple food chain. Ecosystems almost always containmany more species than a single food chain shows. In addition,most organisms, including humans, eat more than one kind offood. So a food web, such as the one shown in Figure 8, includesmore organisms and multiple food chains linked together. Afood web shows many feeding relationships that are possible inan ecosystem.

Trophic Levels Each step through which energy is transferred in afood chain is known as a In Figure 8, the algae are inthe bottom trophic level, the krill are in the next level, and so on.Each time energy is transferred from one organism to another,some of the energy is lost as heat and less energy is available toorganisms at the next trophic level. Some of the energy is lost dur-ing cellular respiration. Organisms use much of the remainingenergy to carry out the functions of living, such as producing newcells, regulating body temperature, and moving.

What is the difference between a food web and afood chain?��Reading Check

trophic level.

food chain

Figure 8 � This food web showshow the largest organisms, such as akiller whale, depend on the smallestorganisms, such as algae, in an oceanecosystem.

Group ActivityHuman Diets Divide the classinto Group A and Group B. Tellstudents in Group A that theirassignment is to determine the costper ounce of rice, dried beans, androlled oats. Tell students in Group Bthat their assignment is to researchand determine the cost per ounceof ground beef, chicken, and porkchops. Then, ask each group todetermine the number of caloriesper ounce and the cost of one calo-rie for each type of food. HaveGroup A and Group B comparetheir results. (Costs per calorie arelower for grains and other plantproducts than for meats.) Ask,“What is the significance of your calculations for humans?”(Humans could have more food formore people and spend less moneyby eating foods lower on the foodchain.) Logical

AAnnsswweerr ttoo RReeaaddiinngg CChheecckkA food chain shows the sequence inwhich energy is transferred from oneorganism to the next as each organ-ism eats another. A food web showsmany different feeding relationshipsand is made up of many intercon-nected food chains.

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Math Skills Write the following energy pyra-mid on the board, and ask students to figureout the percentage of energy loss at each level(from studies done at Cayuga Lake, New York):

1. Humans: 1.2 calories

2. Trout: ~6 calories

3. Smelt (a small fish): 30 calories

4. Small aquatic animals: 150 calories

5. Algae: 1000 calories (Energy loss from levels 5–4: 85%; 4–3: 80%; 3–2: 80%; 2–1: 80%) LogicalLS

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Chew on This Ask students to name a typicalmammalian carnivore (any wild cat), a herbivore(rabbit or deer), and an omnivore (humans arebest for this exercise). Have them describe theshape of each organism’s teeth (molars, canines,incisors). Bring pictures of skulls, or actual skullsin as a resource. Ask students to describe howthe shape of teeth relates to feeding groups.(Cats have sharp canines for grabbing and piercingflesh. Rabbits have incisors for cutting plantmaterial, and flat molars for grinding, and deerhave only molars. Humans have a mix of grindingand sharp teeth to eat all types of food.) VisualLS

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Ask students to draw a foodchain from a different ecosystemthan the one shown in Figure 7.The food chain should includeproducers and consumers.Students can cut and paste eachorganism onto a note card. Onthe back of the card, have stu-dents indicate the name of theorganism and whether it is a pro-ducer or consumer in the foodchain. Have students exchangenote cards with each other andreorder them as a study activity.

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AAnnsswweerrss ttoo SSeeccttiioonn RReevviieeww 1. Energy is transferred from producers to

consumers when consumers eat producers.Consumers also gain energy by eating otherconsumers. Decomposers gain energy by break-ing down dead producers and consumers.

2. Producers such as plants, algae, and some bac-teria capture and convert energy into a formthat is usable by other organisms.

3. An herbivore only eats producers. An omni-vore eats both producers and consumers.

4. Energy transfer in a food chain is linear andrepresents different trophic levels, from pro-ducers to consumers. Energy transfer in a foodweb is complex and contains many food chains.

5. The crabeater seal eats herring, which eatsboth krill and algae. The crabeater seal iseaten by the killer whale and leopard seal.

6. Yes. As you move up trophic levels, 90 percentof available energy is lost at each level. Thecrop stores 10 times more energy than herbi-vores in the same area, so there is more energyfor humans to use if they eat lower on thefood chain.


About 90 percent of the energy at each trophic level is used inthese ways. The remaining 10 percent of the energy becomes partof the organism’s body and is stored in its molecules. This 10 per-cent that is stored is all that is available to the next trophic levelwhen one organism consumes another organism.

Energy Pyramids One way to visualize the loss of energy fromone trophic level to the next trophic level is to draw an energypyramid like the one shown in Figure 9. Each layer in the energypyramid represents one trophic level. Producers form the base ofthe pyramid, the lowest trophic level, which contains the mostenergy. Herbivores contain less energy and make up the secondlevel. Carnivores that feed on herbivores form the next level, and carnivores that feed on other carnivores make up the top level. Organisms in the upper trophic levels store less energy than herbivores and producers. A pyramid isa good way to illustrate trophic levels because the pyramid becomes smaller toward the top, where less energy is available.

How Energy Loss Affects an Ecosystem The decreased amount of energy at each trophic level affects theorganization of an ecosystem. First, because so much energy islost at each level, there are fewer organisms at the higher trophiclevels. For example, zebras and other herbivores outnumber lionson the African savanna by about 1,000 to 1. In this example, theresimply are not enough herbivores to support more carnivores.

Second, the loss of energy from trophic level to trophic levellimits the number of trophic levels in an ecosystem. Ecosystemsrarely have more than four or five trophic levels because the eco-system does not have enough energy left to support higher levels.For example, a lion typically needs up to 250 km2 of land to huntfor food. Therefore, an animal that feeds on lions would have toexpend a lot of energy to harvest the small amount of energyavailable at the top trophic level. The organisms that do feed onorganisms at the top trophic level are usually small, such as para-sitic worms and fleas that require a very small amount of energy.

1. Describe how energy is transferred from one organ-ism to another.

2. Describe the role that producers play in an ecosystem.

3. Explain the difference between an herbivore and anomnivore.

4. Compare energy transfer in a food chain to energytransfer in a food web.

CRITICAL THINKING5. Interpreting Graphics Look at Figure 8. What

feeding relationships does the crabeater seal have?

6. Inferring Relationships Read the paragraphunder the heading “Trophic Levels” in this section.Could more people be supported by 20 acres of landif they ate only plants instead of both plants and ani-mals? Explain your answer. READING SKILLS

S E C T I O N 1 Review

Figure 9 � This energy pyramidshows how energy is lost from onetrophic level to the next. The grass atthe bottom level stores 1,000 timesmore energy than the hawk at thetop level.

www.scilinks.orgTopic: Foods Chains,Food Webs, and Trophic LevelsCode: HE80595

Reteaching Discussion Bring in a variety offoods and cooking ingredients,including both plant and animalproducts. Ask students to namethe source of each product. Listthe product and its source on theboard. Organize the class intosmall groups, and have themarrange the materials into a foodweb. Conclude by asking, “Inwhat ways is this human foodweb like the food webs discussedin this section?” Kinesthetic

Quiz1. Describe the chemical equation

for cellular respiration using asimple sentence. (Glucose andoxygen combine to produce car-bon dioxide, water, and energy.)

2. What is the difference betweena food chain and a food web?(Food chains are linear representa-tions of different trophic levels,from the producers to consumers.Food webs contain many foodchains, and show all trophic inter-actions in an ecosystem.)

3. Draw an energy pyramid for anecosystem that includes grass,antelope, and lions. If the grassrepresents 1000 units of energy,what do the antelope and lionsrepresent? [the pyramid shouldstart with grass at the base (1000units), antelope next (100 units)and lions on the top (10 units)]

Alternative Assessment Global Food Webs Divide theclass into five groups. Give eachgroup one of the following loca-tions, and tell them that their taskis to determine a food web thatexists there: a salt marsh inFlorida; a coral reef in theCaribbean; the Sonoran Desert in Arizona; the Kodiak NationalWildlife Refuge in Alaska; and theFt. Pierre National Grassland inSouth Dakota. Suggest that stu-dents report their findings in theform of posters, oral presentations,or video documentaries.

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GENERAL

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Objectives� Describe the short-term and long-

term process of the carbon cycle.� Identify one way that humans are

affecting the carbon cycle.� List the three stages of the nitro-

gen cycle.� Describe the role that nitrogen-

fixing bacteria play in the nitrogencycle.

� Explain how the excess use of fertilizer can affect the nitrogenand phosphorus cycles.

Key Termscarbon cyclenitrogen-fixing bacterianitrogen cyclephosphorus cycle

S E C T I O N 2

The Cycling of Materials

What will happen to the next ballpoint pen you buy? You willprobably use it until its ink supply runs out and then throw itaway. The plastic and steel the pen is made of will probably neverbe reused. By contrast, materials in ecosystems are constantlyreused. In this section, you will read about three cycles by whichmaterials are reused—the carbon cycle, the nitrogen cycle, andthe phosphorus cycle.

The Carbon CycleCarbon is an essential component of proteins, fats, and carbohy-drates, which make up all organisms. The is aprocess by which carbon is cycled between the atmosphere, land,water, and organisms. As shown in Figure 10, carbon enters ashort-term cycle in an ecosystem when producers, such as plants,convert carbon dioxide in the atmosphere into carbohydratesduring photosynthesis. When consumers eat producers, the con-sumers obtain carbon from the carbohydrates. As the consumersbreak down the food during cellular respiration, some of the car-bon is released back into the atmosphere as carbon dioxide.Organisms that make their own food through photosynthesis alsorelease carbon dioxide during cellular respiration.

Some carbon enters a long-term cycle. For example, carbonmay be converted into carbonates, which make up the hard partsof bones and shells. Bones and shells do not break down easily.

carbon cycle

Figure 10 � The Carbon CycleCarbon moves between air, water,soil, and organisms. What role do plants play in the carbon cycle?

EARTH SCIENCE CONNECTION

OverviewBefore beginning this section,review with your students theObjectives in the Student Edition.This section describes the carbon,nitrogen, and phosphorus cycles.The section also describes howhuman activities affect these cycles.

Ask students to list 3 products thatthey recycle. Ask, “Where do theproducts come from? Where willthe products go after they are recycled?” Discuss their answers in relation to natural cycles.

Intrapersonal

ActivityCarbon Cycle Stories Tell stu-dents that, on average, it takes acarbon atom about 300 years tocomplete one cycle. Based on thatinformation, have each studentwrite a short story describing whoor what might have used the samecarbon atom that he or she is usingnow.

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Biogeochemical Cycles The cycling ofmaterials is accomplished in huge cyclescalled biogeochemical cycles. Biogeo-chemical means “life-earth-chemical.”Different elements require different lengthsof time to go from the living part of anecosystem, such as an organism, to thenonliving part, such as soil, water, oratmosphere, and back to the body of a liv-ing organism again. The amount of matterprocessed through biogeochemical cycles is

enormous. For example, about 90 millionmetric tons of nitrogen are cycled per year—90 percent of it is cycled by bacteria and 10 percent of it is cycled by human-producedfertilizers. Although animals are usuallythought of as returning CO2 to the atmos-phere, both plants and decomposers returnmore CO2 than animals do—plants return42 percent, decomposers return 46 percent,and animals return 12 percent.

BRAIN FOOD

GENERAL

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Over millions of years, carbonate deposits have produced hugeformations of limestone rocks. Limestone is one of the largestcarbon sinks, or carbon reservoirs, on Earth.

Some carbohydrates in organisms are converted into fats, oils,and other molecules that store energy. The carbon in carbohydratesand these other molecules may be released into the soil or air afteran organism dies. These molecules can form deposits of coal, oil,and natural gas underground. The deposits are known as fossilfuels. Fossil fuels are made up of carbon compounds from the bodies of organisms that died millions of years ago.

How Humans Affect the Carbon Cycle When we burn fossil fuels,we release carbon into the atmosphere as carbon dioxide. Cars, fac-tories, and power plants rely on these fossil fuels to operate. In theyear 2000, vehicles, such as the truck in Figure 11, were the sourceof one-third of all carbon dioxide emitted in the United States. Eachyear, about 6 billion metric tons of carbon are released into theatmosphere as carbon dioxide by the burning of fossil fuels and the natural burning of wood in forest fires. About half of this car-bon dioxide remains in the atmosphere. As a result, the amount ofcarbon dioxide in the atmosphere has steadily increased.

Increased levels of carbon dioxide may contribute to globalwarming, which is an overall increase in the temperature of theEarth. What happens to the carbon dioxide that does not remainin the atmosphere? Scientists estimate that, each year, over a bil-lion metric tons of carbon dioxide dissolves into the ocean, whichis a carbon sink. Plants probably absorb the remaining carbondioxide.

How can driving a car affect the carbon cycle?��Reading Check

Figure 11 � This truck releases carbon into the atmosphere when itburns fuel to operate.

QuickLABMake EveryBreath CountProcedure

1. Pour 100 mL of water from agraduated cylinder into a250 mL beaker. Add severaldrops of bromthymol blue tothe beaker of water. Make sureyou add enough to make thesolution a dark blue color.

2. Exhale through a straw into thesolution until the solution turnsyellow. (CAUTION: Be sure notto inhale or ingest the solution.)

3. Pour the yellow solution into alarge test tube that contains asprig of Elodea.

4. Stopper the test tube, andplace it in a sunny location.

5. Observe the solution in the testtube after 15 minutes.

Analysis

1. What do you think happenedto the carbon dioxide that youexhaled into the solution?

2. What effect do plants, such as the Elodea, have on the carbon cycle?

BiologyConnection to

The Rise of Carbon DioxideThe concentration of carbondioxide today has increased 30percent since preindustrial times.If the present amount of carbondioxide emission continues, thisconcentration will double by2080. Many scientists speculatethat as a result, Earth’s tempera-ture may rise by 3OC.

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QuickLABSkills Acquired:• Predicting• InterpretingTeacher’s Notes: Bromthymolblue will stain clothes, so youmay want to have students wear aprons for this activity.

AAnnsswweerrss1. The carbon dioxide that was

exhaled into the solution wasused by the sprig of Elodea,which caused the color of thesolution to turn from yellowand back to blue.

2. Plants, such as the Elodea,convert atmospheric carbondioxide into carbohydrates during photosynthesis. Plantsalso release carbon dioxide into the atmosphere during cellular respiration.

CONNECTIONCONNECTIONEARTH SCIENCEEARTH SCIENCE

Carbon Sequestration The increased use offossil fuels has caused the amount of carbondioxide in the atmosphere to increase as well.In order to reduce the amount of atmosphericCO2, some scientists are researching carbonsequestration. Carbon sequestration involvescapturing atmospheric CO2 and storing it inthe terrestrial biosphere, underground, or inthe oceans. The U.S. Department of Energy iscurrently focusing on ways to enhance carbon

sequestration in the terrestrial biosphere byincreasing the storage of carbon in biomassand soils. They are also focusing on enhanc-ing the net oceanic uptake of CO2 from theatmosphere by fertilizing phytoplankton aswell as by injecting liquid CO2 to oceandepths greater than 1000 m. Scientists arealso investigating methods of pumping carbondioxide into abandoned mines and petroleumwells.

AAnnsswweerr ttoo RReeaaddiinngg CChheecckkCar engines burn fossil fuels, releas-ing carbon dioxide into the atmos-phere. Increased concentrations ofcarbon dioxide in the atmospheremay contribute to global warming.

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The Nitrogen CycleAll organisms need nitrogen to build proteins, which are used tobuild new cells. Nitrogen makes up 78 percent of the gases inthe atmosphere. However, most organisms cannot use atmo-spheric nitrogen. It must be altered, or fixed, before organismscan use it. The only organisms that can fix atmospheric nitrogeninto chemical compounds are a few species of bacteria known as

All other organisms depend upon thesebacteria to supply nitrogen. Nitrogen-fixing bacteria are a cru-cial part of the a process in which nitrogen iscycled between the atmosphere, bacteria, and other organisms.As shown in Figure 12, bacteria take nitrogen gas from the airand transform it into molecules that living things can use.

Nitrogen-fixing bacteria, shown in Figure 13, live in noduleson the roots of plants called legumes. Legumes include beans,peas, and clover. The bacteria use sugars provided by the legumesto produce nitrogen-containing compounds such as nitrates. Theexcess nitrogen fixed by the bacteria is released into the soil. Inaddition, some nitrogen-fixing bacteria live in the soil rather thaninside the roots of legumes. Plants that do not have nitrogen-fixing bacteria in their roots get nitrogen from the soil. Animalsget nitrogen by eating plants or other animals, both of which aresources of usable nitrogen.

Decomposers and the Nitrogen Cycle In the nitrogen cycle, nitrogen moves between the atmosphere and living things. Someof the nitrogen that cycles from the atmosphere to living things isreleased to the soil with the help of bacteria. These decomposersare essential to the nitrogen cycle because they break down wastes,such as urine, dung, leaves, and other decaying plants and animals

nitrogen cycle,

nitrogen-fixing bacteria.

Figure 13 � The swellings on theroots of this soybean plant are callednodules. Nitrogen-fixing bacteria,shown magnified at the top right, live inside the nodules of some plants.

Figure 12 � The Nitrogen CycleNitrogen could not be cycled in theatmosphere without nitrogen-fixingbacteria. What role do animals playin the nitrogen cycle?

Using the Figure The Nitrogen Cycle Refer stu-dents to Figure 12 and ask, “Ifonly bacteria can use nitrogen fromthe atmosphere, how do plants andanimals take part in the nitrogencycle?”(Nitrogen-fixing bacteria con-vert atmospheric nitrogen into com-pounds plants can use. Plants absorbthese compounds and use them tomake proteins. Herbivores obtainnitrogen by eating plants. Carnivoresget nitrogen by eating herbivores andother carnivores. Nitrogen is releasedin animal wastes and when decom-posers break down dead plant andanimal matter.) Logical

ActivityObserving Nitrogen FixingBacteria Obtain samples of freshsoybeans, alfalfa, or clover withtheir root nodules intact. Have stu-dents squash a nodule and thenswab the material on a microscopeslide. Have students stain the sam-ple with methylene blue. Then havestudents apply a cover slip and ex-amine the nitrogen-fixing bacteriathat live within the nodules. Suggestthat they draw what they see. Next,have students dry the plant samplesand mount them on poster boardalong with their drawings of thenitrogen-fixing bacteria. Caution:Methylene blue will stain skin andclothes. VisualLS

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GENERAL

How Humans Affect the Nitrogen CycleThe article Human Alteration of the GlobalNitrogen Cycle: Causes and Consequenceswas written by a distinguished group of ecolo-gists, which included Peter M. Vitousek, GeneE. Likens, David W. Schindler, and G. DavidTilman. According to their findings, humanshave doubled the natural rate of nitrogenentering the land-based nitrogen cycle,through the use of chemical fertilizers andfossil fuels. The increase in nitrogen has

increased water and atmospheric pollution,has led to the acidification of lakes, streamsand soils, and has contributed to greenhousegases. Some plants adapted to soils low innitrogen have been negatively affected orreplaced by nitrogen-loving plants. This hasaffected the animals that consume those plants.Encourage students to read this article, andwrite about it. The article can be found online,or in the journal Ecological Applications(Volume 7, August 1997).

CONNECTIONCONNECTIONECOLOGYECOLOGY

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“In nature there are neither rewards norpunishments—there are consequences.”

—R. G. IngersollAsk students, “What are some of theconsequences of humans affecting thecarbon, nitrogen, and phosphoruscycles?” (Sample answers: global climatechange, destruction of ecological systems,algal blooms, fish kills, acid rain, and soilerosion)


GeofactMinerals in Your MouthPhosphorus is the 11th most abun-dant element in the Earth’s crustand occurs naturally as phosphatein the mineral apatite. Apatite canexist in igneous, metamorphic, andsedimentary rocks as well as in yourteeth and bones.

and return the nitrogen that these wastes contain to the soil. Ifdecomposers did not exist, much of the nitrogen in ecosystemswould be stored forever in wastes, corpses, and other parts oforganisms. After decomposers return the nitrogen to the soil, bacteria transform a small amount of the nitrogen into nitrogengas, which then returns to the atmosphere. So, most of the nitro-gen that enters an ecosystem stays within the ecosystem. It cyclesbetween organisms and the soil, and is constantly reused.

The Phosphorus CycleThe element phosphorus is part of many molecules that make up the cells of living organisms. For example, phosphorus isneeded to form bones and teeth in animals. Plants get the phos-phorus they need from soil and water, while animals get theirphosphorus by eating plants or other animals that have eatenplants. The is the movement of phosphorusfrom the environment to organisms and then back to the environ-ment. This cycle is slow and does not normally include the atmosphere because phosphorus rarely occurs as a gas.

As shown in Figure 14, phosphorus may enter soil and waterin a few ways. When rocks erode, small amounts of phosphorusdissolve as phosphate in soil and water. Plants absorb phosphatesin the soil through their roots. In addition, phosphorus is addedto soil and water when excess phosphorus is excreted in wastefrom organisms and when organisms die and decompose. Somephosphorus also washes off the land and eventually ends up inthe ocean. Many phosphate salts are not soluble in water, so theysink to the bottom of the ocean and accumulate as sediment.

phosphorus cycle

Figure 14 � The PhosphorusCycle Phosphorus moves from phos-phate deposits in rock to the land,then to living organisms, and finallyto the ocean.

www.scilinks.orgTopic: Nitrogen Cycle

Code: HE81036Discussion Nutrient Cycling and EcosystemStructure Ask groups to discussthe following question: How doesthe structure of an ecosystem influ-ence nutrient cycling? (Structuredinterrelationships such as those thatexist among soil, water, nutrients,producers, consumers, and decom-posers allow for the transfer of nutri-ents from one thing to another in achain of events. If one link in thechain is missing, the transfer cannotbe completed.) Have each groupreport their answers to the wholeclass, and discuss their answers.

Logical/Interpersonal

Nitrogen Fertilizers Modernagriculture relies on the use ofnitrogen-rich fertilizers. Ask stu-dents to find out how these fertil-izers are produced and what impactthey have on the environment. Whywould some environmentalists saythat nitrogen fertilizers harm theenvironment twice? (First, energyis needed to produce nitrogen-richfertilizers. The energy needed to pro-duce fertilizers usually comes fromfossil fuels, which add carbon to theatmosphere. Second, when excessnitrogen compounds run off fieldsand into bodies of water, they cancause an overgrowth of algae andaquatic plants. When aquatic plantsdie and decay, bacteria use oxygen to decompose the plant remains.When there is an abundance of plantremains, decomposers can deplete thewater of oxygen which other aquaticorganisms need to survive.) Havestudents record their findings intheir EcoLog. LogicalLS

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Fertilizers and the Nitrogen and Phosphorus Cycles Peopleoften apply fertilizers to stimulate and maximize plant growth.Fertilizers contain both nitrogen and phosphorus. If excessiveamounts of fertilizer are used, the fertilizer can enter terrestrialand aquatic ecosystems through runoff. Excess nitrogen and phos-phorus in an aquatic ecosystem or nearby waterway can causerapid and overabundant growth of algae, which results in an algalbloom. An algal bloom, as shown in Figure 15, is a dense, visiblepatch of algae that occurs near the surface of water. Algal blooms,along with other plants and the bacteria that break down deadalgae, can deplete an aquatic ecosystem of important nutrientssuch as oxygen. Fish and other aquatic organisms need oxygen tosurvive.

Acid Precipitation We affect the nitrogen cycle when we burnfuel, because nitric oxide is released into the atmosphere. Nitricoxide can combine with oxygen and water vapor in the atmo-sphere to form nitric acid. Nitric acid can dissolve in rain andsnow, which contributes to acid precipitation.

How do algal blooms harm aquatic ecosystems?��Reading Check

Figure 15 � More than 30 per-cent of fertilizer may flow withrunoff from farmland intonearby waterways. Largeamounts of fertilizer in watercan cause an excessive growthof algae (below).

1. Describe the two processes of the carbon cycle.

2. Describe how the burning of fossil fuels affects thecarbon cycle.

3. Explain how the excessive use of fertilizer affects thenitrogen cycle and the phosphorus cycle.

4. Explain why the phosphorus cycle occurs more slowlythan both the carbon cycle and the nitrogen cycle.

CRITICAL THINKING

5. Making Comparisons Write a short paragraphthat describes the importance of bacteria in the car-bon, nitrogen, and phosphorus cycles. What role dobacteria play in each cycle?

6. Applying Ideas What is one way that a person canhelp to reduce the level of carbon dioxide in theatmosphere? Can you think of more than one way?

WRITING SKILLS


AAnnsswweerr ttoo RReeaaddiinngg CChheecckkAlgal blooms can harm aquaticecosystems by depleting their nutrients, such as oxygen.

ReteachingComparing the Cycles Have stu-dents study the carbon, nitrogen,and phosphorus cycles shown inFigures 10, 12, and 14. Ask them tolist the ways that the three cyclesare similar and different. (Thecycles are similar in that all movematerials between different compo-nents of the biosphere, have nobeginning or end, and conserve thematerials in the cycle. The cycles dif-fer in that the phosphorus cycle onlyremotely involves living organisms.)

Logical/Visual

Quiz1. What are the major sources of

phosphorus? (Weathered rocks,the soil, and decaying organismsare all sources of phosphorus.)

2. How are the following elementsused in organisms: carbon,nitrogen, and phosphorus?(Carbon is a component of pro-teins, fats and carbohydrates.Nitrogen builds proteins. Phos-phorus builds bones, teeth, andis essential to many molecules.)

AlternativeAssessment Connecting to the Carbon CycleAsk students to write an essay fortheir Portfolio, describing the rolethey play in the carbon cycle.

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AAnnsswweerrss ttoo SSeeccttiioonn RReevviieeww1. Producers convert carbon dioxide to carbohy-

drates during photosynthesis. Carbon is thenpassed on to consumers when they eat produc-ers and other consumers. Producers and con-sumers release carbon dioxide back into theatmosphere during cellular respiration.

2. Fossil fuels contain carbon from plants andanimals that died millions of years ago. Whenfossil fuels are burned, they release carbon intothe atmosphere.

3. Excess use of fertilizer containing nitrogen and phosphorus can enter aquatic ecosystemsin runoff, causing algal blooms which depletethem of oxygen.

4. Phosphorus is rarely cycled in the atmosphere,because it rarely occurs as a gas. Gases, such asatmospheric carbon and nitrogen, are easilyexchangeable.

5. Some bacteria take in carbon dioxide duringphotosynthesis and release carbon dioxide during cellular respiration in the carboncycle. Bacteria take in and fix nitrogen forother organisms to use in the nitrogen cycle.Decomposing bacteria also release carbon,nitrogen and phosphorus back into theenvironment.

6. Answers may vary.


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GENERAL

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Objectives� List two types of ecological

succession.

� Explain how a pioneer speciescontributes to ecological succession.

� Explain what happens during old-field succession.

� Describe how lichens contribute to primary succession.

Key Termsecological successionprimary successionsecondary successionpioneer speciesclimax community

S E C T I O N 3

How Ecosystems Change

Figure 16 � Taller beech trees com-pete with shorter, young beech treesfor sun and make it hard for theyounger trees to survive.

Ecosystems are constantly changing. A forest hundreds of yearsold may have been a shallow lake a thousand years ago. A deadtree falls and lets sunlight reach the forest floor. The sunlightcauses some seeds to germinate, and soon wildflowers and shrubscover the forest floor. Mosses, shrubs, and small trees cover theconcrete of a demolished city building. These are all examples ofan environmental change called ecological succession.

Ecological Successionis a gradual process of change and replace-

ment of some or all of the species in a community. Ecological succession may take hundreds or thousands of years. Each newcommunity that arises makes it harder for the previous commu-nity to survive. For example, the younger beech trees in Figure 16will have a hard time competing with older beech trees for sun-light. However, if a shade-loving species of tree began to grow inthe forest, the new species might replace the smaller beech trees.

is the type of succession that occurs on asurface where no ecosystem existed before, such as on rocks orsand dunes. the more common type of succession, occurs on a surface where an ecosystem has previouslyexisted. Secondary succession occurs in ecosystems that have beendisturbed or disrupted by humans or animals, or by naturalprocesses such as storms, floods, earthquakes, and volcanoes.

How is secondary succession different from primarysuccession?��Reading Check

Secondary succession,

Primary succession

Ecological succession

Chain-of-Events Chart

Create the Graphic Organizerentitled “Chain-of-Events Chart”described in theAppendix. Then, fill inthe chart with detailsabout each step of ecological succession.

OverviewBefore beginning this section,review with your students theObjectives in the Student Edition.This section introduces the conceptof ecological succession. The sectiondistinguishes between secondaryand primary succession and explainsthe importance of pioneer species.

Ask students to consider this ques-tion: “Is your school experiencingecological succession?” Then walkoutside with the students, and lookfor moss and lichens on the schoolbuilding. Look for cracks in theparking lot where weeds are break-ing through. Talk about what willhappen if this vegetation continuesto grow. (It would probably take along time, possibly hundreds of years,for lichens or moss to break down theschool building, but eventually theywould if they were not disturbed.)

DiscussionChange Over Time Ask the classto imagine that on graduation daytheir school is bulldozed and a tallfence is placed around the bareschool grounds. Then ask them to picture returning to the schoolgrounds in 1 month, 1 year, 5 years,and 100 years. Call on individualstudents to describe what the schoolgrounds might be like then. (Livingthings would return to the area, butthe types of living things wouldchange over the years. Weeds might bethe first to appear, then brush andsmall trees.) Explain that this processis called ecological succession.

Verbal

AAnnsswweerr ttoo RReeaaddiinngg CChheecckkPrimary succession occurs where noecosystem existed before, whilesecondary succession occurs wherean ecosystem previously existed.

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Chain-of-Events ChartYYoouu mmaayy wwaanntt ttoo uussee tthhiiss GGrraapphhiiccOOrrggaanniizzeerr ttoo aasssseessss ssttuuddeennttss’’ pprriioorr kknnoowwll--eeddggee bbeeffoorree bbeeggiinnnniinngg aa ddiissccuussssiioonn ooffeeaacchh sstteepp ooff eeccoollooggiiccaall ssuucccceessssiioonn.. YYoouummaayy aallssoo cchhoooossee ttoo uussee aa ssiimmiillaarr aaccttiivviittyyaass aa qquuiizz ttoo aasssseessss ssttuuddeennttss'' kknnoowwlleeddggeeooff eeccoollooggiiccaall ssuucccceessssiioonn oorr ttoo aasssseessss ssttuu--ddeennttss'' kknnoowwlleeddggee aafftteerr ssttuuddeennttss hhaavvee rreeaaddtthhee ddeessccrriippttiioonn ooff eeccoollooggiiccaall ssuucccceessssiioonn..

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Secondary Succession In 1980, the volcano MountSt. Helens erupted in Washington State. The eruptionat Mount St. Helens has been described as one of theworst volcanic disasters because more than 44,460 acresof forest were burned and flattened by the force of hotash and other volcanic debris, as shown in Figure 17.After the eruption, plants began to colonize the volcanicdebris. Such plants are called —thefirst organisms to colonize any newly available areaand begin the process of ecological succession. Over

time, pioneer species will make the new area habitable for otherspecies. If you visited Mount St. Helens today, you would findsecondary succession. Figure 18 shows how after 12 years, plantsand flowers had covered most of the lava and new trees andshrubs had started to grow. If these organisms at Mount St.Helens continue to grow, over time they will eventually form aclimax community. A is a final and stableclimax community

pioneer species

Figure 17 � When Mount St. Helenserupted in 1980, much of the forestaround the volcano was destroyed.

� Fireweed is one type of plantthat colonizes land after the landhas been burned by fire.

Communities Maintained by Fire

Fires set by lightning or human

activities occasionally sweep

through large areas. Burned

areas undergo secondary succes-

sion. In the forests of the Rocky

Mountains, for example, burned

areas are rapidly colonized by

fireweed, which clothes the

slopes with purple flowers. In

some places, fire determines

the nature of the climax commu-

nity. In the United States, eco-

logical communities that are

maintained by fire include the

chaparral of California, the tem-

perate grassland of the Midwest,

and many southern and western

pine forests.

Plants native to these commu-

nities are adapted to living with

fire. A wildfire that is not unusually

hot may not harm fire-adapted

pine trees, but it can kill decidu-

ous trees—those trees that lose

their leaves in winter. Seeds of

Longleaf pines have a strange

growth pattern. When they are

young, they have long needles

that reach down to the ground.

The trees remain only about a half

of a meter high for many years,

while they store nutrients. If a fire

occurs, it sweeps through the tops

of the tall trees that survived the

last fire. The young longleaf pines

near the ground may escape the

fire. Then, the young pines use

their stored food to grow very

rapidly. A young pine can grow

as much as 2 m each year. Soon

the young pines are tall enough so

that a fire near the ground would

not harm them.

If regular fires are prevented

in a fire-adapted community,

deciduous trees may invade

the area. These trees form a thick

barrier near the ground. In addi-

tion, their dead leaves and

branches pile up on the ground

some species will not germinate

until exposed to temperatures

of several hundred degrees.

When a fire sweeps through a

forest, the fire kills plants on the

ground and stimulates the seeds

to germinate.

Discussion Yellowstone Fire System Twelveyears after the Yellowstone fire,researchers Millspaugh, Whitlock,and Bartlein wrote: “The historictrend toward infrequent severefires, such as those in 1988, will be short-lived and in all likelihoodreplaced by a regime of many smallfires . . . The forests of centralYellowstone will change not somuch in composition as in stand-agedistribution. Thus the disturbanceregime will serve to perpetuate lodge-pole pine where it now grows andallow its expansion to higher eleva-tions.” Ask students to discuss thisquote, and what the researchers areimplying. (Now that fires are allowedto burn in national parks, natural firesuccession will be restored. Smallerfires will keep the fuel load down,and the lodgepole pine ecosystemwill expand and remain viable.)

Logical

DebateFires in National Parks Have stu-dents divide into two groups andask them to research and debate thepros and cons of the National ParkService’s fire policy. You mightwant to get the discussion movingby asking some questions such as:“Would controlled burns be a goodidea for maintaining ecosystems?Since national parks attract manyvisitors, would fires affect tourismand hurt the economic future ofthe park system? Is it dangerous tolet fires burn in areas that humansuse? Would smaller frequent fireshelp to maintain each park’s eco-system? Should people be allowedto build near the boundaries offire-maintained parks?” VerbalLS

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Secondary Succession Some animalsplay a part in preparing an ecosystem for secondary succession. For example,pocket gophers in old abandoned farmfields cycle fresh soil from underground,and deposit the soil in mounds outside of their burrows. These mounds attractseeds from pioneer plant species that arefloating through the air. Eventually, tallergrasses from the field shade out the pio-neer species, and these grasses take overthe mounds and the field.

BRAIN FOOD

GENERAL

GENERAL

Writing Ask students if the following state-ment is true or false: “An ecosystem has histori-cal aspects; the present is related to the past,and the future is related to the present.” (True.The Earth’s cycles and the predictable patterns ofchange, such as those evident in succession, con-nect an ecosystem’s past to its present.) Suggestthat students think about the statement andwrite an essay outlining their thoughts in theirPortfolio. VerbalLS

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Writing Ask students, “When clearing landfor ranching, agriculture, or other uses, why isit important to preserve large patches of theoriginal habitat?” (By preserving patches ofspecies, we are improving the chances for anecosystem to return to its original condition. Butif patches are not preserved, those species andtheir potential value to humans could be lost forever.) Have them write their ideas in theirEcoLog. Encourage them to research the func-tion of habitat patches in human-altered land-scapes, and to write about their research intheir Portfolio. Intrapersonal LS

BUILDERSKILL


� These young lodgepole pine trees have started growing after a devastating forest fire.

� This firefighter is helping to maintain a controlled firein South Dakota.

community. Even though a climax community contin-ues to change in small ways, this type of communitymay remain the same through time if it is not disturbed.

Fire and Secondary Succession Natural fires causedby lightning are a natural cause of secondary succes-sion in some communities, as discussed in the CaseStudy below. Some species of trees, such as the Jackpine, can release their seeds only after they have beenexposed to the intense heat of a fire. Minor forest firesremove accumulations of brush and deadwood thatwould otherwise contribute to major fires that burn outof control. Some animal species also depend on occa-sional fires because they feed on the vegetation that sprouts aftera fire has cleared the land. Therefore, foresters sometimes allownatural fires to burn unless the fires are a threat to human life orproperty.

Figure 18 � The photo above was taken 12 years after the eruptionof Mount St. Helens and shows secondary succession.

and form extra fuel for fires. When

a fire does occur, it is hotter and

more severe than usual. The fire

destroys not only the deciduous

trees but also the pines. It may end

up as a devastating wildfire.

Although it may seem odd,

frequent burning is essential to pre-

serve many plant communities and

the animals that depend on them.

This is the reason the U.S. National

Park Service adopted the policy of

letting fires in national parks burn if

CRITICAL THINKING

1. Understanding ProcessesExplain how a longleaf pine treemight be more likely to survive aforest fire than a deciduous tree,such as a maple or oak tree.

2. Understanding ConceptsWhy must controlled fires be set in some ecosystems? What are the advantages? What are the dis-advantages?

they do not endanger human life or

property.

This policy caused a public out-

cry when fires burned Yellowstone

National Park in 1988, because

people did not understand the

ecology of fire-adapted communi-

ties. The fires later became an

opportunity for visitors to learn

about the changes in an ecosystem

after a fire.

139

Communities Maintained byFire Ask a forest firefighter or a park ranger to come to yourclass. Have students prepare alist of questions in advance.Their questions might include:“Do any species benefit fromforest fires? How are forest firesfought? What is done to preventforest fires?” The NatureConservancy has chapters insome states perform controlledburning to preserve certain cli-max communities. If such achapter exists in your area, con-tact the organization to see ifyour class could witness or helpwith a controlled-burn projectand learn about succession ecol-ogy. If not, see if a representativefrom the organization couldcome to the class to speak aboutthe group’s work.

AAnnsswweerrss ttoo CCrriittiiccaall TThhiinnkkiinngg1. A young longleaf pine is low

to the ground, so it can survivefire that sweeps through thecanopy. Deciduous trees mayburn because they are not aslow to the ground.

2. Some communities, such as thetallgrass prairie, have evolvedin an environment of frequentfires. In these communities,controlled fires can kill anycompetitors that try to move intothe ecosystem. Disadvantagesto using controlled fire are therisk to human property, and thechance that the competitivespecies might not be destroyed.

CulturalAwarenessCulturalAwareness

Early Prescribed Burning It is likely thatNative Americans practiced burning to alterplant and animal communities for humanbenefit. Evidence of this comes partially fromthe fact that aspen is difficult to burn, and yetaspen burned frequently in the past. Only inearly spring and late fall is it vulnerable, andthese are times of very few lightning strikes.This suggests that native peoples started thefires. Have students research and report onthe use of fire by indigenous groups thatexist today.

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Old-field Succession Another example of secondary succession isold-field succession, which occurs when farmland is abandoned.When a farmer stops cultivating a field, grasses and weeds quicklygrow and cover the abandoned land. The pioneer grasses andweeds grow rapidly and produce many seeds to cover large areas.

Over time, taller plants, such as grasses, grow in the area. Theseplants shade the ground, keeping light from the shorter plants. Thelong roots of the taller plants also absorb most of the water in thesoil. The pioneer plants soon die from lack of sunlight and water. Assuccession continues, the taller plants are deprived of light and waterby growing trees. Finally, slower-growing trees, such as oaks, hicko-ries, beeches, and maples, take over the area and block sunlight tothe smaller trees. As shown in Figure 19, the area can eventuallyestablish a climax community dominated by a mature oak forest.

Primary Succession On new islands created by volcanic eruptions,in areas exposed when a glacier retreats, or on any other surfacethat has not previously supported life, primary succession canoccur. Primary succession is much slower than secondary successionbecause primary succession begins where there is no soil. It can takeseveral hundred to several thousand years to produce fertile soilnaturally. Imagine that a glacier melts and exposes an area of barerock. The first pioneer species to colonize the bare rock will proba-bly be bacteria and lichens, which can live without soil. Lichens, asshown in Figure 20, are important early pioneers in primarysuccession. They are the colorful, flaky patches that you see on treesand rocks. A lichen is a producer that is actually composed of two

Figure 19 � The illustration aboveshows what an abandoned farm areamight look like during old-field succes-sion. Why do you think young oaktrees begin to appear around year 20?

FIELD ACTIVITYFIELD ACTIVITY Investigating SuccessionExplore two or three blocks inyour neighborhood, and find evi-dence of succession. Make notesin your EcoLog about the loca-tion and the evidence of succes-sion that you observe. Payattention to sidewalks, curbs,streets, vacant lots, and build-ings, as well as parks, gardens,fields, and other open areas.Create a map from your data thatidentifies where succession is tak-ing place in your neighborhood.

ActivityMusical Succession Organizestudents into four groups, and giveone of the following objects to eachstudent in each group: a crumpledsheet of paper; a pencil; a hard-cover book; and a plastic jar filledwith beans. Tell students they aregoing to make a succession band.Ask group 1 to start crumplingtheir paper, as if playing an instru-ment. After a minute or so, askgroup 2 to tap their pencils quietly.Allow this to go on for a few min-utes. Then ask group 3 to startslamming the covers of their bookssoftly at first, but then with moreforce (but ask them not to damagethe books!). Groups 1 and 2 shouldbe told to fade out. Have Group 4come in, shaking their jars quietlyand slowly, and then more quicklyand loudly. Have students shakingthe jars try to drown out the bookslammers. Ask the paper crumplersand pencil tappers to try to drownout the fourth group (they shouldnot be able to). Bring students to ahalt. Ask students how this “band”represents succession. (Each groupof instruments succeeded the next.The sound from groups 3 and 4could drown out groups 1 and 2.The last group finally formed a cli-max community that was not subjectto competition from the first threegroups.) Auditory/KinestheticLS



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Transparencies

TT Secondary Succession: Old-FieldSuccession

Mammals and Succession This chapterfocuses on plant communities and how theychange over time. But how do the animalcommunities respond to that change? Havestudents think about the mammals that livein an old field. (Mice, voles and shrews live inmost old fields. Other mammals might includeground squirrels, gophers, and badgers.) Ask,“What happens when taller grass takes overthe field?” (There will be more cover, so moreof the same animals might move into the area,or reproduction might increase.) “If trees start

moving in, how will that change the com-munity?” (The grass might be shaded out, sovoles would be reduced; shrews that eat volesmight move elsewhere; other animals that livemainly in open areas would move out, such as ground squirrels, gophers, and badgers;species that use trees and open areas wouldstart to colonize (some types of mice); the for-est creatures would follow (deer, chipmunks,forest mice and voles.) Encourage students tofurther investigate the links between mam-mal communities and succession. Logical LS

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AAnnsswweerrss ttoo SSeeccttiioonn RReevviieeww1. Primary succession occurs on a surface where

no ecosystem previously existed. Secondarysuccession occurs on a surface where anecosystem previously existed.

2. Pioneer species are the first organisms to colonize any newly available area and beginthe process of ecological succession.

3. Some communities rely on fire to reproduce oreliminate invading species. Putting out forestfires in these communities will cause them tobe taken over by other species and change intoa different community.

4. Lichens can live without soil, so they colonizebare rock and begin to break it down throughphysical and chemical processes.

5. secondary succession; primary succession only occurs in areas where there was no previous ecosystem

6. Lichens and the first old-field succession plants are pioneers. Lichens, however, do not need soil to colonize an area.


different species, a fungus and an alga. The alga photosynthesizes,while the fungus absorbs nutrients from rocks and holds water.Together, they begin to break down the rock.

As the growth of the lichen breaks down the rock, water mayfreeze and thaw in cracks, which further breaks up the rock. Soilslowly accumulates as dust particles in the air are trapped in cracksin the rock. Dead remains of lichens and bacteria add to the soilin the cracks. Mosses may increase in number and break up therock even more. When the mosses die, they decay and add nutri-ents to the growing pile of soil. Thus, fertile soil forms from the broken rock, decayed organisms, water, and air. Primary successioncan also be seen in any city street, as shown in Figure 20. Mosses,lichens, and weeds can establish themselves in cracks in a sidewalkor building. As well, fungi and mosses can invade a roof thatneeds repair. Even New York City would eventually turn into acement-filled woodland if it were not constantly maintained.

What is an example of primary succession in a city?��Reading Check

1. Compare primary and secondary succession.

2. Describe what role a pioneer species plays duringthe process of ecological succession.

3. Explain why putting out forest fires may be damag-ing in the long run.

4. Describe the role lichens play in primarysuccession. Write a short paragraph to explain youranswer.

CRITICAL THINKING5. Analyzing Processes Over a period of 1,000 years,

a lake becomes a maple forest. Is this process primaryor secondary succession? Explain your answer.

6. Analyzing Relationships How are lichens similarto the pioneer species that colonize abandoned farmareas? How are they different?

WRITING SKILLS


Figure 20 � Lichens (left) are colo-nizing a boulder in Wyoming. Over a long period of time, lichens canbreak down rock into soil. Plants thatgrow through cracks in city sidewalks(below) can also be described as pio-neers of primary succession.

www.scilinks.orgTopic: Ecological

Succession

Code: HE80461

AAnnsswweerr ttoo RReeaaddiinngg CChheecckkSample answer: An example of primary succession in a city occurswhen weeds grow through cracks in sidewalks.

ReteachingSuccession Quizzes Have eachstudent come up with five quizquestions based on material in thissection. Have students pair off andexchange quizzes. Each studentshould answer a quiz and grade hisor her partner’s quiz. Discuss stu-dents answers with the class.

Verbal

Quiz1. What are some of the ways an

ecological community changesinto a different communitythrough secondary succession?(Answers may vary. Pioneerspecies can move in and colonizean area such as in an old field;taller perennial grasses can com-pete with shorter grasses for sunand water and eventually takeover; disturbances such as a fire or a volcanic eruption can occur.)

2. Would a newly-formed volcanicisland be a site of primary suc-cession or secondary succession?(primary succession)

Alternative AssessmentSuccession Animation Give students 50 index cards. On thecards, have them illustrate succes-sion in an ecosystem of theirchoice. The cards should showgradual changes in the same areaover time. Have them clip theindex cards together, and animatethe scene by flipping through thecards rapidly using their thumb. If students videotape their anima-tions, they can share them with the class. Kinesthetic/VisualLS

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GENERAL

GENERAL

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HighlightsCCHHAAPPTTEERR

55 HighlightsC H A P T E R 5

1 Energy Flow in Ecosystems

2 The Cycling of Materials

3 How Ecosystems Change

Key Termsphotosynthesis, 125producer, 126consumer, 126decomposer, 127cellular respiration,

128food chain, 130food web, 130trophic level, 130

Main Ideas� The majority of the Earth’s organismsdepend on the sun for energy. Producers har-ness the sun’s energy directly through photo-synthesis, while consumers use the sun’s energyindirectly by eating producers or other con-sumers.

� The paths of energy transfer can be followedthrough food chains, food webs, and trophiclevels.

� Only about 10 percent of the energy that anorganism consumes is transferred to the nexttrophic level when the organism is eaten.

carbon cycle, 132nitrogen-fixing

bacteria, 134nitrogen cycle, 134phosphorus cycle,

135

� Materials in ecosystems are recycled andreused by natural processes.

� Carbon, nitrogen, and phosphorus are essen-tial for life. Each of these elements follows arecognizable cycle.

� Humans can affect the cycling of materials inan ecosystem through activities such as burningfossil fuels and applying fertilizer to soil.

ecological succession,137

primary succession,137

secondary succes-sion, 137

pioneer species, 138climax community,

138

� After a disturbance, organisms in an environ-ment follow a pattern of change over time,known as ecological succession.

� Primary succession occurs on a surfacewhere no ecosystem existed before. Secondarysuccession occurs on a surface where an eco-system existed before.

� Climax communities are made up of organ-isms that take over an ecosystem and remainuntil the ecosystem is disturbed again.

AlternativeAssessmentSchoolyard Succession Havestudents observe the process of suc-cession during the school year. Earlyin the school year, get permissionto mark off a section of the schoolgrounds that is approximately 4 ft.� 4 ft. Post “Do Not Mow” signsin the marked-off area. Have stu-dents monitor the area by keepinga regular journal of the changesthat they observe over time. Theirjournal can be written or recordedon videotape. Students may alsowish to collect and preserve a fewspecimens of the plants and insectsthat occupy the area. Be sure stu-dents carefully label their notes andcollections with a complete dateand a detailed description of theirfindings. The project could be con-tinued from one school year to thenext, with current students build-ing on the documentation of previ-ous students. Kinesthetic/Visual

Succession Collage Bring insome posterboard, glue, and copiesof National Geographic, or othermagazines with pictures of naturalscenes and organisms. Organizestudents into small groups, andassign a type or stage of ecologicalsuccession to each group. Havestudents find pictures of landscapesand organisms that represent theirtype of community. Ask them tocut out the pictures and to arrangeand glue them into ecosystems onthe posterboard. Instruct studentsto label each component of theirsystem, and to provide a rationale as to why it was included in theircollage. VisualLS

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• Chapter Test• Chapter Test• Concept Review• Critical Thinking• Test Item Listing• Observation Lab• CBL™ Probeware Lab• Consumer Lab• Long-Term Project

GENERAL

GENERAL

GENERAL

GENERAL


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ReviewUsing Key TermsUse each of the following terms in a separatesentence.

1. photosynthesis2. trophic level3. carbon cycle4. nitrogen-fixing bacteria5. decomposers

For each pair of terms, explain how the meaningsof the terms differ.

6. producer and consumer7. primary succession and secondary succession8. nitrogen cycle and phosphorus cycle9. food chain and food web

Understanding Key Ideas10. Which of the following statements is not true

of consumers?a. They get energy indirectly from the sun.b. They are also called heterotrophs.c. They make their own food.d. They sometimes eat other consumers.

11. Which of the following is correctly arrangedfrom the lowest trophic level to the highesttrophic level?a. bacteria, frog, eagle, raccoonb. algae, deer, wolf, hawkc. grass, mouse, snake, eagled. grass, bass, minnow, snake

12. Communities of bacteria have been foundliving thousands of feet underwater. Whichof the following statements is a proper con-clusion to draw about these bacteria?a. Somehow they are conducting photosyn-

thesis.b. They are living on borrowed time.

c. They were somehow introduced by humanactivities.

d. They use an energy source other thansunlight.

13. Which of the following pairs of organismsprobably belong to the same trophic level?a. humans and bearsb. bears and deerc. humans and cowsd. both (a) and (c)

14. The energy lost between trophic levels a. can be captured only by parasitic organisms.b. cools the surrounding environment.c. is used in the course of normal living.d. evaporates in the atmosphere.

15. From producer to secondary consumer,about what percentage of energy is lost?a. 10 percentb. 90 percentc. 99 percentd. 100 percent

16. Which of the following statements about thenitrogen cycle is not true?a. Animals get nitrogen by eating plants or

other animals.b. Plants generate nitrogen in their roots.c. Nitrogen moves back and forth between

the atmosphere and living things.d. Decomposers break down waste to yield

ammonia.

17. Which of the following are most likely to be the pioneer organisms on an area of bare rock?a. treesb. shrubsc. lichensd. perennial grasses

18. Excessive use of fertilizer that contains nitro-gen and phosphorus a. affects the carbon cycle.b. may cause algal blooms in waterways.c. causes soil erosion.d. contributes to primary succession.

C H A P T E R 5

Taking Multiple-Choice Tests When you takemultiple-choice tests, be sure to read all of thechoices before you pick the correct answer. Bepatient, and eliminate choices that are obviouslyincorrect.

STUDY TIP

ANSWERS

UUssiinngg KKeeyy TTeerrmmss1. Sample answer: Photosynthesis

uses carbon dioxide, water, andenergy to produce sugars andoxygen.

2. Sample answer: In an aquaticecosystem, algae are in the bot-tom trophic level.

3. Sample answer: Organisms addcarbon to the carbon cycle dur-ing cellular respiration.

4. Sample answer: Nitrogen-fixingbacteria live inside the root nodules of legumes.

5. Sample answer: Decomposersbreak down dead organic matter.

6. A producer is an organism thatmakes its own food. A consumeris an organism that gets its energyby eating other organisms.

7. Primary succession is a type ofsuccession that occurs on a sur-face where no ecosystem existedbefore. Secondary successionoccurs on a surface where anecosystem has previously existed.

8. The nitrogen cycle is the move-ment of nitrogen between theatmosphere, bacteria, and otherorganisms. The phosphorus cycleis the movement of phosphorusfrom the environment to organ-isms and back.

9. A food chain is a sequence inwhich energy is transferred fromone organism to another. A foodweb is made up of many foodchains and shows the possiblefeeding relationships in anecosystem.

UUnnddeerrssttaannddiinngg KKeeyy IIddeeaass10. c11. c12. d13. a14. c15. c16. b17. c18. b


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ReviewCCHHAAPPTTEERR

55

Section Questions1 1, 2, 5–6, 8–15, 19–21, 24–28, 31–322 3–4, 16, 18, 22–23, 29, 34–373 7, 17, 30, 33

Assignment Guide


ReviewShort Answer19. Explain the relationship between cellular res-

piration and photosynthesis.20. Why is the number of trophic levels that can

exist limited?21. Why are decomposers an essential part of an

ecosystem?22. Write a short paragraph that explains why

the phosphorus cycle occurs slower than thecarbon and nitrogen cycles.

23. Describe what happens to carbon dioxide inthe carbon cycle.

Interpreting GraphicsUse the diagram to answer questions 24–26.24. How many organisms depend on the squid

as a source of food?25. If the population of Adélie penguins

decreased drastically, what effect would this have on elephant seals?

26. What role do algae play in this food web?

WRITING SKILLS

Concept Mapping27. Use the following terms to create a concept

map: algae, humans, solar energy, carnivores,consumers, producers, directly, herbivores,indirectly, and omnivores.

Critical Thinking28. Comparing Processes How are producers

and decomposers opposites of each other?29. Inferring Relationships Abandoned fields in

the southwestern part of the United Statesare often taken over by mesquite trees,which can grow in nutrient-poor soil. If theland is later cleared of mesquite, the soil isoften found to be enriched with nitrogen andis more suitable for crops. What might bethe reason for this phenomenon?

30. Understanding Concepts Read the descrip-tion under the head “What Eats What” inthis chapter, and explain why decomposersare considered to be consumers.

31. Drawing Conclusions Suppose that a plagueeliminates all the primary consumers in anecosystem. What will most likely happen toorganisms in other trophic levels in thisecosystem?

Cross-Disciplinary Connection32. Mathematics If a lake contains 600,000 kg

of plankton and the top consumers are apopulation of 40 pike, which each weigh anaverage of 15 kg, how many trophic levelsdoes the lake contain? Make a graph orpyramid that illustrates the trophic levels.

Portfolio Project33. Researching Local Succession Do a special

project on succession. Find areas in your community that have been cleared of vegeta-tion and left unattended at different times in the past. Ideally, you should find severalareas that were cleared at different times,including recently and decades ago. Photo-graph each area, and arrange the pictures to show how succession takes place in yourgeographic region.

READING SKILLS

C H A P T E R 5?

? ?SShhoorrtt AAnnsswweerr19. Answers may vary. Cellular

respiration provides the carbondioxide that fuels photosynthesis;photosynthesis provides the oxy-gen that fuels cellular respiration.

20. Answers may vary. A largeamount of energy is lost at eachlevel, so there has to be a limiton the number of trophic levels.For example, in an Africansavanna ecosystem, animals thatare larger than lions and that eatlions do not exist, because thereis not enough energy to supporta trophic level higher than lionson the African savanna.

21. Answers may vary. If decom-posers did not exist, dead organicmatter would not be brokendown. Also, carbon, nitrogen,and other nutrients essential tolife would stay in an unusableform and nutrients would not becycled in ecosystems.

22. Answers may vary. Phosphorusrarely occurs as a gas. And phos-phorus takes a long period oftime to weather from rock. Phos-phorus in water gets trapped inan unusable form called phos-phate, which sinks to the bottomof the ocean and lakes. Much of the phosphorus on Earth islocked in the form of phosphate-bearing rock.

23. Answers may vary. Carbon dioxide gas is released into theatmosphere by the process of cellular respiration and by com-bustion reactions. Plants use carbon dioxide from the atmo-sphere to make carbohydratesduring photosynthesis.


144

IInntteerrpprreettiinngg GGrraapphhiiccss24. four25. The elephant seal population might decrease

because the killer whale would have to huntmore of the seals to make up for the loss ofpenguins as prey.

26. Algae are the base food for the entire foodweb.

CCoonncceepptt MMaappppiinngg27. Answers to the concept mapping questions

are on pp. 715–720.

CCrriittiiccaall TThhiinnkkiinngg28. Producers transform chemicals into a form

usable by consumers; decomposers transformdead organisms back to chemicals that pro-ducers can use.

29. Mesquite is a legume and has nitrogen-fixingbacteria in its roots that enrich the soil.

30. Decomposers are considered to be consumersbecause they get their food by breaking downdead organisms. Therefore, they depend onother organisms to gain energy.

ReviewCCHHAAPPTTEERR

55


Read the passage below, and then answerthe questions that follow.

The Peruvian economy and many sea birdsdepend on normal atmospheric conditions.But sometimes, usually in December, the nor-mal east-to-west winds do not form over thePacific Ocean. Instead, winds push warmwater eastward toward the coast of SouthAmerica. The warm surface water cuts off the upwelling of nutrients. This event is calledEl Niño, which means “the child,” because ithappens near Christmas.

Because all convection cells are linked inthe atmosphere, the effects of El Niño extend beyond Peru. Under a strong El Niño, north-eastern Australia can suffer summer drought,which leads to reduced grain production. Thesoutheastern United States gets higher rainfallin El Niño years, which boosts agriculture anddecreases forest fires.

1. According to the passage, a possiblecause of reduced grain production inAustralia isa. a rate of convection that is higher than

the average rate.b. an amount of rainfall that is higher

than the average amount. c. a reduced fish population.d. a summer drought.

2. According to the passage, which of thefollowing statements is true?a. The effects of El Niño do not extend

beyond Peru.b. During El Niño years, the U.S. agricul-

tural industry suffers.c. El Niño is caused by winds that push

warm water eastward toward SouthAmerica.

d. Australia’s agricultural industry ben-efits the most from strong windsduring El Niño.

MATH SKILLS

Use the data in the table below to answer ques-tions 34–35.

34. Making Calculations If 137.25 million metric tons of fertilizer is used worldwideper year, how many million metric tons does Asia use?

35. Graphing Data Make a bar graph that com-pares the percentage of fertilizer use in dif-ferent regions worldwide per year.

WRITING SKILLS

36. Communicating Main Ideas Describe the importance of the carbon, nitrogen, andphosphorus cycles to humans.

37. Writing from Research Research informa-tion on how countries regulate carbon diox-ide emissions. Write an essay that describesthe laws regulating carbon dioxide emissionsand the solutions some countries havedevised to decrease the amount of carbondioxide emitted.

Percentage of Fertilizer Use per Year

Region of the World Percentage

North America 17

Asia 52

Africa 3

Europe 18

Latin America and the Caribbean 8

Oceania 2

READING SKILLS

31. Producers may flourish, becausethey are not being eaten, but sec-ondary consumers would dwin-dle and die or move out of thearea. Primary consumers from adifferent area may be attracted tothe area of high primary produc-tion, and their predators may fol-low. This type of an event wouldprobably change the ecosystemsignificantly.

CCrroossss--DDiisscciipplliinnaarryyCCoonnnneeccttiioonn32. The total mass for the pike

would be 600 kg. Since eachtrophic level loses 90% of itsenergy, this would be a systemwith 4 trophic levels (1st level �600,000 kg, 2nd level �60,000 kg, 3rd level � 6000 kg,4th level (pike) � 600 kg).

PPoorrttffoolliioo PPrroojjeecctt33. Answers may vary.

MMaatthh SSkkiillllss34. 137.25 � 0.52% � 71.37 million

metric tons35. The graph should be arranged

from lowest to highest use. Thecountries should be on the x-axisand percentage should be on they-axis.

WWrriittiinngg SSkkiillllss36. Answers may vary. Be sure that

students link the cycles to thefoods they eat and the wastematerials they excrete.

37. Students may want to review the Kyoto Protocol on the Web.The Kyoto Protocol is an inter-national agreement to reduceCO2 emissions.

RReeaaddiinngg SSkkiillllss1. d 2. c


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CC H A P T E R 5 Standardized Test PrepUnderstanding ConceptsDirections (1–4): For each question, write on aseparate sheet of paper the letter of the correctanswer.

1 How does energy move through mostecosystems on Earth? A. from the sun to consumers to producers B. from the sun to producers to consumers

to decomposersC. from the sun to decomposers to

producers to consumersD. from the sun to consumers to producers

back to consumers

2 Which of the following statements indicatesan understanding of the importance ofenergy to life on Earth? F. Many organisms on Earth require energy

for their life processes.G. All organisms on Earth require energy

for their life processes.H. Energy is required for the most impor-

tant life processes on Earth.I. The most important organisms on Earth

require energy for their life purposes.

3 What role do bacteria play during thenitrogen cycle? A. Bacteria store nitrogen in wastes.B. Bacteria turn nitrogen into phosphates.C. Bacteria convert nitrogen into water.D. Bacteria turn nitrogen gas into a form

that living things can use.

4 What is the process that breaks down foodto yield energy called? F. cellular digestionG. cellular respirationH. decompositionI. photosynthesis

Directions (5): Write a short response for thequestion.

5 Ecological succession is the gradual processof changes in a community. Describe thesuccession process that occurs when farm-land is abandoned.

Reading SkillsDirections (6–8): Read the passage below. Thenanswer the questions.

Carbon is an essential component of proteins, fats, and carbohydrates, which makeup all organisms. The carbon cycle is one ofthe processes by which materials are reused in an ecosystem. This process cycles carbonbetween the atmosphere, land, water, andorganisms. Carbon enters a short-term cycle inan ecosystem when producers convert carbondioxide into carbohydrates during photosyn-thesis. When consumers eat producers, theconsumers obtain carbon from the carbohy-drates. As the consumers break down the foodduring cellular respiration, some of the carbonis released back into the atmosphere.

Some carbon enters a long-term cycle. Carbon may be converted into carbonates,which make up the hard parts of bones andshells. Over millions of years, carbonatedeposits produce huge formations of limestonerock. Limestone is one of the largest carbonreservoirs on Earth.

6 During what process do producers convertcarbon dioxide into carbohydrates?A. Carbon dioxide is converted into

carbohydrates during the carbon cycle.B. Carbon dioxide is converted into

carbohydrates during cellular respiration.C. Carbon dioxide is converted into

carbohydrates during the eating process. D. Carbon dioxide is converted into

carbohydrates during photosynthesis.

7 Which of the following groups are producers?F. animalsG. decomposersH. herbivoresI. plants

8 How is carbon converted into limestonerock?

Interpreting Graphics

A.B.C.D.

F.G.H.I.

A.B.C.D.

146

Estimated TimeTo give students practice undermore realistic testing conditions,allow them 30 minutes to answerall of the questions in this practicetest.

146 Chapter 5 • Standardized Test Prep

Standardized Test Prep

9. B10. F11. C

Answers1. B2. G3. D4. G5. Answers will vary. See Test Doctor for detailed

scoring rubric.6. D7. I8. Answers will vary. See Test Doctor for detailed

scoring rubric.

Question 1 Answer B shows theproper sequence. Students choos-ing answers A or D may have con-fused producers and consumers.Consumers cannot use energydirectly from the sun. Answer Cshows decomposers using energydirectly from the sun which isincorrect. Decomposers get energyfrom breaking down dead organ-isms.

Question 5 Full-credit answersshould include the followingpoints:

• the first thing that happenswhen farmland is abandoned isthat native grasses and plantsbegin to replace the plantedcrops

• after these plants grow, the ani-mals that feed on them alsoreturn to the area

• large trees may also return tothe area if they grew therebefore

Question 8 Full-credit answersshould include the followingpoints:

• Carbon may be converted intocarbonates.

• Carbonates make up the hardparts of bones and shells.

• It takes millions of years for car-bonate deposits to form lime-stone rock.

hes08TE_c05_146-147_STP 1/24/07 6:15 PM Page 146

TestThe key helps youinterpret the map andget information aboutthe different types of regions.

Interpreting GraphicsDirections (9–11): For each question below, record the correct answer on aseparate sheet of paper.

The map below shows carbon dioxide output from the burning of fossilfuels in different regions of the world. Use this map to answer questions 9and 10.

Carbon Dioxide Output From Fossil Fuels

9 Which continent has the lowest percentage of carbon dioxide output?A. AsiaB. AustraliaC. EuropeD. North America

0 What regions are responsible for the highest percentage of carbondioxide output?F. developed regions in the western hemisphereG. developed regions in the eastern hemisphereH. developing regions in the western hemisphereI. developing regions in the eastern hemisphere

q Which of the following is an effect of the increased burning of fossilfuels on the carbon cycle?A. More carbonates remain in fossil fuels.B. More carbon dioxide is absorbed by organisms.C. More carbon dioxide is absorbed by the atmosphere.D. More carbohydrates remain buried deep in the ground.

U N I T E DS T A T E S30.3%

C A N A D A2.3%

J A P A N3.7%

SOUTH ANDCENTRALAMERICA

3.8%

EUROPE27.7%

SOUTHWESTASIA2.6%

CHINA, INDIA, AND DEVELOPING

ASIA 12.2%

A F R I C A2.5%

FORMER SOVIET UNION13.7%

AUSTRALIA1.2%

Percentage of Carbon Dioxide Output from Fossil Fuels (1990-1999)

Developed Region Developing Region

147

Standardized Test Prep

Chapter 5 • Standardized Test Prep 147

Question 10 Answer F is correct.Answer G incorrectly identifiesNorth America as being a part ofthe eastern hemisphere. Answers Hand I incorrectly identify NorthAmerica as a developing region.

Question 11 Answer C is correct.Students struggling with this typeof question may benefit from prac-ticing organizing information. Forexample, this type of cause-and-effect question can be solved bydrawing a flow chart. The majorevent is the increase in carbondioxide being released by the burn-ing of fossil fuels. This predictableeffect would cause more carbondioxide to be absorbed into theatmosphere. Another effect is lesscarbon being contained in fossilfuel reservoirs as it is released intothe atmosphere by the burning offossil fuels.

hes08TE_c05_146-147_STP 8/24/06 2:55 PM Page 147

CCHHAAPPTTEERR

55Dissecting Owl PelletsOwls are not known as finicky eaters. They prey on almost anyanimal that they can swallow whole. Like many other birds, owlshave an interesting adaptation—a special structure called a giz-zard. The gizzard acts as a filter and prevents the indigestibleparts of their prey, such as fur, feathers, and bones, from passinginto their intestines. These indigestible parts are passed to a stor-age pouch, where they accumulate. A few hours after consuminga meal, the owl coughs up the accumulated indigestible material,which has been compressed into a pellet. By examining such apellet, you can tell what the owl ate. In addition, by examiningthe remains of the owl’s prey found in the pellet, you can get agood idea of what the prey ate. Using this information, you canconstruct a food chain of the owl and its prey.

Procedure1. Work in groups of three or four. Place an owl pellet in the

dissecting pan, and remove it from its aluminum-foil casing.

2. Examine the owl pellet. Using the dissecting needle and for-ceps, carefully break apart the owl pellet. Separate the fur orfeathers from the bones. Be careful not to damage the smallbones. Place the bones onto a piece of white paper.

3. Identify the major components of the pellet.

4. If the pellet contains remains from more than one organism,determine as best as you can how many different animalsand species are present.

5. Attempt to group the remains by type of organism. Countthe number of skulls to find out how many prey were in thepellet. Decide which bones belong with which skulls. Thentry to assemble complete skeletons. Sample skeletal diagramsare shown below.

Objectives� Examine the remains of an owl’s

diet.� Construct

a food chain based on your obser-vations.

Materialsdisposable glovesdissecting needledissecting panegg cartonsforcepsowl pellet(s)piece of white papersmall animal identification

field guide that includes skull illustrations

USING SCIENTIFIC METHODS

Exploration Lab: OBSERVATIONC H A P T E R 5

� Types of Organisms Usethese drawings to help youdetermine if the organismyou put together is a bird,mammal, or reptile.

DISSECTING OWL PELLETS

Teacher’s Notes

Time Requiredone 45 minute class period

Lab Ratings

TEACHER PREPARATION

STUDENT SETUP

CONCEPT LEVEL

CLEANUP

Skills Acquired• Constructing Models• Classifying• Inferring• Organizing and Analyzing Data

The Scientific MethodIn this lab, students will:• Make Observations• Ask Questions• Analyze the Results• Draw Conclusions

MaterialsThe materials listed are enough fora group of 4 to 5 students. Youmay want to provide bleach to dis-infect the skulls. If skulls are disin-fected, students can take themhome. This would make the labmore memorable for students.

E A S Y H A R D

Safety CautionsBecause students will be dealing with materialassociated with a bird, there is a risk of expo-sure to Salmonella bacteria. Be sure that stu-dents use the gloves provided, and you mayalso want to provide masks for students. Alsorequire that students wash their hands thor-oughly after the exercise.

148


Exploration LabObservation

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6. Closely examine the skulls of each prey. Compare the skullsto the diagrams of skulls on this page. What purpose do theteeth or bills seem to have—tearing flesh, chewing plantparts, or grinding seeds? If you are able to identify the prey,find out their typical food sources.

7. On a separate piece of paper, construct a simple food chainbased on your findings.

8. Compare your findings with those of other groups of students.

Analysis1. Examining Data How many skeletons were you able to make

from your pellet? What kinds of animals did you identify inthe owl pellet?

2. Organizing Data Compare your findings with those of yourclassmates by using the following questions:

a. What animals were represented most often in the pellets?b. What common traits do these animals have?c. How many animals found in the pellets were herbivores?

How many were carnivores?

Conclusions3. Interpreting Information What biological relationships were you

able to determine from your examination of the owl pellet?4. Evaluating Data How many different trophic levels are repre-

sented by the animals you found in the pellet?5. Drawing Conclusions Most owls hunt at night and sleep dur-

ing the day. From that information, what can you infer abouttheir prey?

� Identify the Prey Use thesedrawings to identify the owl’s prey.

1. Research and Communications Research information on anowl species and the types of organisms found in its habitat.Make a poster of a food web, including the owl species. Besure to include producers, consumers, and decomposers.

Extension

AAnnsswweerrss ttoo AAnnaallyyssiiss1. Students are likely to find the

remains of a variety of small ani-mals such as mice, lizards, shrews,voles, young squirrels, or rabbits.

2. Answers may vary.

AAnnsswweerrss ttoo CCoonncclluussiioonnss3. Answers may vary. In many cases,

owls feed on different types of ani-mals that are either primary orsecondary consumers.

4. Answers may vary. There may be one or two trophic levelsrepresented.

5. Answers may vary. Most of theanimals that owls eat are active atnight but not always. If studentsfind a ground squirrel or chip-munk skull, the owl was probablyactive at dawn or dusk, or evenduring the day.

AAnnsswweerrss ttoo EExxtteennssiioonn1. Owl pellets are a great way to

learn about the diet of owls, andto learn about which animals arerepresented in a local ecosystem.

149

Richard P. FilsonEdison High SchoolStockton, California

CLASSROOM

TESTED

& APPROV

ED

• Datasheets for In-Text Labs• Lab Notes and Answers


Holt Lab Generator CD-ROM

Search for any lab by topic, standard, difficulty level, or time. Edit any lab to fit your needs, or create your own labs. Use the Lab Materials Quicklist software to customize your lab materials list.

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M A P S K I L L S

Use the Doppler radar images of bats and insectsin Central Texas to answer the questions below.

1. Analyzing Data At what time was the bat andinsect concentration the lowest? At what time wasthe bat and insect concentration the highest?

2. Using a Key Use the concentration key to deter-mine which area of Central Texas has the highestconcentration of bats and insects at 8:14 P.M.

3. Analyzing Data Approximately how many kilo-meters wide is the concentration of bats and insectsat 7:27 P.M.? at 8:14 P.M.?

4. Inferring Relationships Bracken Cave is home to20 million bats that eat millions of pounds of insectsnightly. Approximately how far is Bracken Cave fromthe city of San Antonio? If the bat population in thecave drastically decreased, what effect would thisdecrease have on the people living in San Antonioand Central Texas?

5. Identifying Trends These Doppler radar images ofbats and insects were taken in the beginning of thesummer season. How might these four images look inthe month of December?

� These images of bat and insect concentration in

Central Texas were cre-ated using Dopplerradar on the evening of May 19, 2002.Doppler radar cantrack the movementof objects in the airby bouncingelectromagneticenergy off of them.

DOPPLER RADAR TRACKING OF BATS AND INSECTS IN CENTRAL TEXAS

EARTH SCIENCE CONNECTION

150

AAnnsswweerrss ttoo MMaapp SSkkiillllss1. 7:27 P.M.; 8:14 P.M.2. Answers may vary. Bracken Cave would be

the most accurate area showing the highestconcentration.

3. Answers may vary. At 7:27 P.M., it wasapproximately 200 km. At 8:14 P.M., it was approximately 350 km.

4. Answers may vary. Bracken Cave is approxi-mately 50 km from San Antonio. If the bat

population drastically decreased, the insectpopulation in San Antonio would probablyincrease.

5. In December, these four images would probably look very different due to the colder weather. The concentration of bats and insects would probably be much lower.

DOPPLER RADAR TRACKINGOF BATS AND INSECTS INCENTRAL TEXAS

Scientists can use Doppler weatherradar to track the movement andconcentration of bats and insects inareas such as Central Texas. Thefour Doppler images show the con-centrations of Mexican free-tailedbats and the insects they eat, onthe evening of May 19, 2002. Batsmigrate to Central Texas duringthe warmer months to feed on bil-lions of insects. Bracken Cave ishome to a record number 20 mil-lion bats, which is the world’slargest colony. Have students findout more about radar entomologyand radar ornithology by research-ing on the Internet. The RadarOrnithology Lab at ClemsonUniversity offers a wealth of onlineinformation.

Transparencies

TT Doppler Radar Tracking of Batsand Insects in Central Texas

Maps in ActionMaps in Action

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AAnnsswweerrss ttoo WWhhaatt DDoo YYoouu TThhiinnkk??Answers may vary, depending on your area. Youmight record the types of fish seen at the nearestgrocery store, so that students know whichtrophic level is most represented in your area.


� A squid is an example of anorganism from a lower trophic levelthat was used for bait but is nowsold in restaurants.

� Overfishing of organisms fromhigher trophic levels has forced thecommercial fishing industry to har-vest organisms in lower trophiclevels in order to fulfill the demandfor fresh fish.

EATING THE BAIT

What Do You Think?The next time you go to a fishmarket or seafood restaurant,take note of the different typesof species for sale. Write downthe names of the species, andtry to assign each species to atrophic level. How many of thespecies for sale belong tolower trophic levels? Howmany belong to higher trophiclevels? How do prices differbetween the species for sale?

Most of the food we eat comesfrom agriculture and farming, butwe also rely heavily on the fishingindustry to provide us with freshfish. Because of a high demand forfish, however, many fish specieshave been overharvested. Manyorganisms depend on these fish-eries, places where fish are caught,to survive. The swordfish and codfisheries of the North Atlantic andthe salmon fishery off the north-western coast of the United Statesare examples of fisheries that havebecome depleted. These fisheriesnow contain so few fish that harv-esting the fish is not economical.

Fishing Down the Food ChainFish such as cod, tuna, and snapper are top carnivores in ocean food chains and food webs. As these fish have disappeared, species from lower trophic levels have begun to appear in fish markets. Fish that were onceswept back into the sea when theywere caught in nets by accident arenow being kept and sold. Organ-isms from lower trophic levels suchas mullet, squid, mackerel, andherring, which were typically usedas bait to catch larger fish, nowappear on restaurant menus.

According to data from theUnited Nations on worldwide fishharvests, the overall trophic levelat which most fish are caught hasdeclined since the 1950s. Over-fishing of organisms in lowertrophic levels disrupts food chainsand food webs. If the food websof ocean ecosystems collapse, thecommercial fishing industry willalso collapse. For example, in theNorth Atlantic cod fisheries, thecod began to disappear, so thefishers concentrated on the cods’prey, which is shrimp. Cod arehigher trophic level organisms,while shrimp are in the lowertrophic levels and feed on algaeand detritus. If the shrimp becomeoverfished, the cod and otherorganisms that depend on boththe shrimp and cod to survive areaffected.

Creating Sustainable FisheriesOne aim of environmental scienceis to determine how fisheries canbe managed so that they are sus-tainable or capable of supplyingthe same number of fish to be harvested each year. However,few, if any, countries manage their

fisheries in this way. Almost allcountries permit unsustainable,large harvests. One solution tooverfishing is to establish “no-take” zones. These are areas ofthe sea where no fishing is permit-ted. Studies have shown that fishpopulations grow rapidly in “no-take” zones. When a populationgrows in a “no-take zone,” thehigher trophic level organismsleave the zone and become avail-able to fishers. “No-take” zoneshelp populations recover andallow food chains and food websto remain intact.

EATING THE BAIT

BackgroundGreater demand and more efficientmethods of catching fish have ledto the depletion of predator fishpopulations, and subsequently, thedepletion or collapse of those fish-eries. Cod and salmon fisheries areexamples of fisheries that have collapsed in North America. Ashumans move down the trophiclevel to “eat the bait,” we competewith the depleted populations ofpredator fish, so their numbers do not rise. We also deplete the“bait” trophic level, which as aresult depletes two trophic levels.Other animals that consume eitherpredator or bait fish will also benegatively affected. Fish popula-tions have been shown to growquickly in “No-take” zones, sothese zones might promote localrecovery of populations.

Overfishing: A Global ProblemMany scientists are afraid that wemay overfish many species in theocean to the point of extinction.Some overfished areas of the oceanhave been compared to an Africansavanna where termites are at the top of the food chain due topoaching. Have students search forarticles on overfishing and factorytrawling on the Internet. Ask themto find out which fish are sufferingpopulation declines, and how thefish are used by humans. Manynews outlets have covered this storyin recent years, and students will getan idea of the severity of this prob-lem. After students have done someresearch, lead a discussion designedto pinpoint problems and potentialsolutions. Then ask students tomake a seafood guide that isdesigned to help people avoid eatingspecies that have been overfished.

151

Society & the EnvironmentSociety & the Environment

CONNECTIONCONNECTIONREAL-LIFEREAL-LIFE

GENERAL

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