After you have finished reading this chapter, you should be able … · 2014-01-19 · After you have finished reading this chapter, you should be able to: Relate energy flow to

After you have finished reading this chapter, you should be able to:

Relate energy flow to trophic levels of organisms.

Define biological magnification and explain why it is important.

Explain the importance of biodiversity to human life.

We must protect the forests for our children, grandchildren, and children notyet born. We must protect the forests for those who can’t speak for themselvessuch as the birds, animals, fish and trees.

Qwatsinas (Hereditary Chief Edward Moody), Nuxalk Nation

IntroductionPopulations interact with and depend on each other. They also interactwith the physical environment around them. Interaction and interde-pendence are two important relationships. Because of these relationships,living organisms and their physical surroundings are often studiedtogether, as parts of a single unit called an ecosystem. The flow of energyand matter through ecosystems will now be the focus of our attention.

To begin, let’s imagine ourselves in one of the most impressive ofEarth’s ecosystems—a tropical rain forest. (See Figure 26-1.) Charles Dar-win described his feelings as he first set foot in a South American rain for-est in 1832:

The day has passed delightfully. Delight itself, however, is a weakterm to express the feelings of a naturalist who for the first time, haswandered by himself in a Brazilian forest. The elegance of the grasses,the novelty of the parasitical plants, the beauty of the flowers, theglossy green of the foliage, but above all the general luxuriance of thevegetation, filled me with admiration. A most paradoxical mixture

558

Ecosystems26

of sound and silence pervades the shady parts of the wood. The noisefrom the insects is so loud that it may be heard even in a vesselanchored several hundred yards from the shore; yet within therecesses of the forest a universal silence appears to reign. To a personfond of natural history, such a day as this brings with it a deeperpleasure than he can ever hope to experience again.

nn THE BASIC CHARACTERISTICS OF ECOSYSTEMS

An ecosystem is made up of living (biotic) and nonliving (abiotic) fac-tors. In other words, biotic factors, such as living organisms, and abioticfactors, such as water, air, light, or temperature, function together in anecosystem. For living organisms to survive, there must be a source ofenergy. The energy flowing between organisms and their environment isa basic characteristic of an ecosystem. Organisms are made up of matter.The flow of matter between organisms and their environment is anothermain characteristic of an ecosystem.

What is the source of energy for a tropical rain forest? As with almostall ecosystems on Earth, it is the sun. While energy is constantly reach-

Chapter 26 / Ecosystems 559

Figure 26-1 A tropicalrain forest in Guatemala.

ing Earth from the sun, matter is not. The amount of matter on Earthremains constant. However, matter moves back and forth between organ-isms and the environment. As with energy, the cycling of matter can beseen in all ecosystems.

nn ENERGY FLOW THROUGH ECOSYSTEMS

In most ecosystems, energy arrives as sunlight. Some organisms are ableto use the energy from sunlight directly. Other organisms use this energyindirectly. They get their energy by eating other organisms. Scientistsdescribe and group organisms in an ecosystem based on whether anorganism can make its own food or must eat food made by otherorganisms.

Organisms are grouped in a system of trophic levels. Trophic means“feeding.” On the first level are organisms that use energy directly fromthe environment, such as the energy in sunlight. These first-level organ-isms are called producers. Plants are producers because they use theprocess of photosynthesis to make their own food with water, carbondioxide, and the energy from sunlight. Both the grass in a field and themighty oak trees in a forest are producers. Organisms that feed on pro-ducers are in the next trophic level. These organisms are called primaryconsumers. A caterpillar that eats oak leaves is a primary consumer. In thenext trophic level are the organisms that eat primary consumers. Theseorganisms are called secondary consumers. A bird that eats the caterpil-lar is a secondary consumer. A large hawk or a cat that eats the bird is atertiary consumer. Each of these steps is called a trophic level because itdescribes the source of the organisms’ food. We can describe the flow ofenergy in an ecosystem by using trophic levels. (See Figure 26-2.)

Energy enters an ecosystem at the producer level. Energy flowsthrough an ecosystem by being passed along from an organism in onetrophic level to an organism in a higher trophic level. This transfer offood energy from one organism to the next is called a food chain. Oaktree leaf to caterpillar to small bird to hawk is a food chain. In a realecosystem, a single, simple food chain like the one described is neverfound. Obviously, caterpillars are not the only animals that eat oakleaves. Other insects, deer, and perhaps rabbits may also eat oak leaves.Caterpillars are eaten not only by birds but also by frogs and other smallanimals. Food chains are interconnected. In reality, food chains make upa complex pattern in which food energy may be passed in many differ-ent directions and to many different organisms. This complex pattern

560 Interaction and Interdependence

formed by food chains in an ecosystem is known as a food web. (SeeFigure 26-3 on page 562.)

However, no matter how complex a food web is, energy always movesin one direction—from a lower to a higher trophic level. Energy does notget recycled. As energy moves through each trophic level, some of it isused and some of it is lost when it is changed to heat during energy trans-fer and conversion. The greatest amount of energy is present at the low-est trophic level (the producers); the least is present at the highest level(upper-level consumers). For this reason, additional energy must con-stantly enter an ecosystem. In other words, for life on Earth to continue,the sun must continue to shine.

Ecologists use a pyramid to describe the flow of energy through anecosystem. The wide base of the pyramid represents the amount of usableenergy in all of the producers. The energy gathered from the sun andstored in all of the plants in the forest is represented by this level. Thenext step up in the pyramid shows the energy that primary consumers get


Secondary consumer

Primary consumer

Tertiary consumer

Producer

LIVING ENVIRONMENT BIOLOGY, 2e/fig. 26-2 s/s

Figure 26-2 The trophiclevels in an ecosystemdescribe the energy flow.

from producers. This layer is smaller than the layer that represented theenergy present in the producers. The passing of energy from one level tothe next is actually not a very efficient process. Only about 10 percent ofall energy gets passed from one level to the next. This is true as we moveup the pyramid from primary consumers to secondary consumers andthen to the top level. (See Figure 26-4.)

A pyramid of energy can provide an important lesson in how to feedthe ever-increasing human population. Leaves such as spinach, and seedssuch as rice and beans, come from plants, which are producers. Cattle, asource of meat for human consumption, are consumers. Throughout theworld, much more food energy is present at the producer level than atthe consumer level. Which type of food is more abundant and availablefor everyone? Which type of food makes a more efficient use of energysources—spinach with rice and beans or hamburgers?

Scientists also show the flow of energy through an ecosystem in twoother types of pyramids. Usually there are more individual organisms atthe producer level. At each higher trophic level, the number of organismsdecreases. Therefore, a pyramid of numbers can be made. A pyramid of



Algae Raccoon

Producers Herbivores

Tadpole

Worm

Frog

SnakeGrasshopper

Decomposers

Bacteria Molds

Carnivores

Grass

Figure 26-3 The interconnection of food chains forms a food web.

numbers shows that fewer organisms are supported at each higher trophiclevel in an ecosystem. Also, if the total mass of all organisms at eachtrophic level is measured, a pyramid of biomass can be drawn. Onceagain, the lowest trophic levels usually have the largest biomass; the high-est levels have the smallest biomass. (See Figure 26-5.)

nn A HIDDEN DANGER IN A FOOD CHAIN

It is a wonderful thing to go for a walk in the country and perhaps dis-cover a small stream. The water sparkles like diamonds as sunlight playson its surface. The stream makes gurgling sounds as it flows over andaround rocks. In water as clear as glass you can even see some small fishas they dart along the stream’s rocky bottom. The following food chainmight be found in this stream: The stream’s microorganisms are eaten by


Tertiary consumers

Energy from sun

100%Producers

10%Primaryconsumers

1%Secondaryconsumers

.1%


Figure 26-4Pyramid of energyflow.

Directi

onof

decr

ease

Producer Grass

Grasshopper

Frog

Snake

Hawk

Herbivore

Carnivore

Carnivore

Carnivore

Figure 26-5 Pyramid of decreasingnumbers and mass.

insects. The insects are eaten by small fish; these small fish are eaten bylarger fish. Finally, fish-eating birds such as eagles or osprey eat the largerfish. This is a typical food chain.

Now suppose this stream runs next to a farmer’s field. In the past, U.S.farmers used the chemical DDT to protect crops from insect damage.When it rained, some of the DDT washed off the fields and entered thestream. In the water, the DDT entered the microorganisms, which becamefood for the insects. As organisms in each trophic level fed on organismsin the previous level, DDT was passed on. In addition, the DDT becamemore concentrated. You can think of it in this way. Each microorganismcontained a tiny bit of DDT, but the insects eat a great many microor-ganisms, so the tiny bit of DDT in each became a more concentrated levelof DDT when it was stored in the insects. (See Figure 26-6.)

This is true for organisms at each trophic level. The level of DDT ineach organism increases as the DDT is moved along the food chain. Thelittle fish contain more DDT, the larger fish even more, and finally, thebirds the most. This process is known as biological magnification. Yearsago, when this happened to eagles and osprey, the high levels of DDTinterfered with the proper buildup of calcium in the shells of their eggs.Egg shells contaminated with DDT are very fragile. The eggs usually brokebefore the developing birds hatched. Although they still laid eggs, fewbirds were able successfully to produce young. This was especially true ofbald eagles and osprey, birds whose diet consists mostly of fish. In partsof the United States, populations of these birds began to diminish quickly.It is only since the use of DDT has been banned in this country that thesemagnificent fish-eating birds have reestablished their populations. Thebald eagle has recently been removed from the endangered species list—a great success story in wildlife management. Although no longer used inthe United States, DDT is still used in some parts of the world, where itcontinues to enter various natural food chains.

nn THE RECYCLING OF MATERIALS IN ECOSYSTEMS

In many parts of the United States, people are now required to recyclecertain consumer wastes. Paper, glass, metal, and plastic are often recycledinstead of being discarded. Recycling, although a new idea for people, is

Check Your Understanding

Why is a food web more accurate than a food chain in portraying therelationships that exist among organisms in an ecosystem?


not a new idea in nature. (See Figure 26-7 on page 566.) Natural ecosys-tems have recycled materials since life began on Earth. Life would notcontinue without this recycling of materials. Why is this so?

All substances are made up of chemical elements. There are about 90chemical elements that occur in nature. Of these, only a relatively smallnumber of elements are found in significant amounts in organisms. These


DDT in water 0.003 ppb

DDT in zooplankton 0.04 ppm

DDT in small fish (minnows) 0.5 ppm

DDT in large fish (pike) 2 ppm

DDT in fish-eating birds 25 ppm

LIVING ENVIRONMENT BIOLOGY, 2e/fig. 26-6 s/s (rev.10/22/03)

Figure 26-6 The level of DDT in each organism increases throughbiomagnification as the chemical moves along the food chain.

include carbon, hydrogen, oxygen, phosphorus, sulfur, and nitrogen. Theamount of these elements on Earth today is approximately the same aswhen the planet formed. Because they are needed by living things, andtheir supply does not increase, these elements need to be reused, or recy-cled, again and again. The atoms of these elements that are present todayin your body may have been present in other organisms before. You mayhave atoms in you that were once part of a tree that grew in an ancient for-est, or in a dinosaur that walked through that forest. (See table.)

SOME ELEMENTS IN LIVING MATTER

Percentage of Percentage ofAtomic Earth’s Crust Human Body

Symbol Element Number by Weight by Weight

Ca Calcium 20 3.6 1.5C Carbon 6 0.03 18.5Cl Chlorine 17 0.01 0.2H Hydrogen 1 0.14 9.5Mg Magnesium 12 2.1 0.1N Nitrogen 7 Trace 3.3O Oxygen 8 46.6 65.0P Phosphorus 15 0.07 1.0K Potassium 19 2.6 0.4Na Sodium 11 2.8 0.2S Sulfur 16 0.03 0.3

The Carbon Cycle. How do these elements get recycled in the nat-ural world? Let’s look at the element carbon. All organisms are made ofmolecules that contain carbon. This carbon is obtained from carbon diox-ide in the air. Producers such as grasses, trees, and other plants take in


Figure 26-7 Theseteenagers are helping torecycle newspapers.

carbon dioxide from the air during photosynthesis. They use the carbonfrom the CO2 gas to build carbohydrates—sugars and starches. Con-sumers, including humans, obtain carbon from producers and sometimesfrom other consumers that serve as food. To complete carbon’s recycling,plants and animals return carbon to the atmosphere. This occurs throughrespiration as we breathe out carbon dioxide.

Recycling of carbon also occurs after an animal or plant dies. Anothervery important part of the recycling process occurs through the actionsof decomposers. Decomposers are heterotrophs—organisms that areunable to make their own food. They get their food by feeding on dead

organisms. The most common decomposers are bacteria and fungi. Asthey carry out their life processes, they too release carbon as CO2 into theatmosphere. (See Figure 26-8.)

The Oxygen Cycle. Oxygen, another element, also moves betweenliving organisms. All animals need oxygen for respiration. Respiration isthe process that releases the chemical energy stored in food. Land ani-mals obtain oxygen for respiration from the air they breathe. Fully aquaticanimals like fish get the oxygen they need from the water they live in.(Oxygen can dissolve in water, and this is the oxygen that fish use.)

Almost all the oxygen in Earth’s atmosphere originally came from themetabolic activities of plants. During the process of photosynthesis,plants give off oxygen as a waste product. So we are breathing in a wastegas given off by plants. This is natural recycling.


Respiration

Decay

Burning

Green plants

Photosynthesis

Oxygen and food

Carbon dioxide (in air

or water)

Animals and fungi


Figure 26-8 Carbon is recycled viaphotosynthesis, respiration, decay (ordecomposition), and burning.

The Nitrogen Cycle. Nitrogen is an element that is used by organ-isms when they make proteins. Nitrogen is the most abundant gas in theair; approximately 78 percent of the atmosphere is nitrogen. So it wouldseem that there is plenty of it around. The problem is that most livingthings cannot use free nitrogen, the nitrogen present in air.

How do plants obtain the nitrogen they need? Plants can take in and usenitrogen in the form of nitrates. Nitrate compounds are combinations ofnitrogen, oxygen, and some other element. One way that nitrates areformed naturally is during lightning storms. These nitrates fall to theground and enter the soil. Plants get their nitrates from the soil by absorb-ing them through their roots. The nitrogen in plants is passed on to pri-mary, secondary, and tertiary consumers when these consumers eat plantsor organisms that eat plants. Each of these organisms releases nitrogen backto the environment in the form of nitrogenous wastes. Decomposers alsorecycle nitrogen when they digest proteins into amino acids, then ammo-nia, and even into another form, nitrites. Finally, nitrites are converted intonitrates, which can be used by plants. Special kinds of bacteria are involvedin each of the steps of this conversion process. (See Figure 26-9.)

One remarkable step in the recycling of nitrogen takes place in theroots of a special group of plants. These plants are called legumes.Legumes include peas, beans, peanuts, alfalfa, and clover. A special typeof bacteria lives inside nodules on the roots of legumes. These bacteria areable to take free nitrogen from the air and change it into nitrates. The



Fertilizer production

Denitrification (via denitrifying bacteria)

Animals

CropsPlants

Decomposers

Ammonia, nitrates, nitrites

Nitrogen-fixing bacteria

Atmospheric nitrogen fixation

Figure 26-9 The nitrogen cycle.

process is called nitrogen fixation. The nitrates are used by plants togrow, and the plants provide nutrients for the bacteria. This is a won-derful example of mutualism. It is also very important to life on land. Soilin which legumes live becomes richer because the bacteria add morenitrates to it. In turn, this natural fertilizer helps other plants grow. (SeeFigure 26-10.)

Long ago, farmers realized that by sometimes growing legumes in afield, they could improve the growth of other crops in other years. Forexample, corn is an important food crop. However, because corn needs agreat deal of nutrients, it can be grown in a field for only a few years. Byplanting clover one year, farmers made the soil ready to grow more cornthe following year. Farmers were allowing the nitrogen-fixing bacteria torecycle necessary nitrates into the soil from the air. This kind of crop rota-tion allows nature to replace soil nutrients and is much less expensivethan the continuous application of fertilizers that would be needed toproduce corn year after year in the same field.

nn CHANGE AND STABILITY IN POPULATIONS ANDCOMMUNITIES: THE IMPORTANCE OF BIODIVERSITY

Biologists have long wondered what causes populations to change ratherthan remain the same. For example, why do rabbits in England get largerover a long period of time, while rabbits in the United States remain thesame size? Remember that all the characteristics that are passed from onegeneration to the next in a population are determined by genes. Biolo-gists wondered under what conditions the genes in a population wouldremain the same from generation to generation. A mathematician, God-frey Harold Hardy, and a physician, Wilhelm Weinberg, answered this


Roots

Nodules formed by bacteria


Figure 26-10 Nitrogen fixation inthe nodules on the roots of legumes isa good example of mutualism.

question. According to the Hardy-Weinberg Law, five conditions mustoccur for a population not to change:

u There must be no mutations.

u There must be no arriving (immigration) or leaving (emigration) ofindividuals to or from the population.

u The population must be large.

u All individuals must have the same chance of surviving.

u The matings of individual organisms must be random; no mates canhave preferences.

Once these conditions were described by Hardy and Weinberg, evolu-tion made more sense. It was clear that most of the time, for most popu-lations, some of these conditions are not met. As a result, populationschange. Size of individuals, shape, structure, coloring, behavior, and anyother inherited characteristic may change from generation to generation.

The Everglades is a vast, wide freshwater marsh that covers much of thesouthern part of Florida. The Everglades begins at the northern edge ofLake Okeechobee, with the overflow of rainwater out of the lake, andextends all the way to the southern tip of the state just before the FloridaKeys. Although some cypresses, mangroves, palms, and oaks grow inscattered clumps, the vast majority of the Everglades is covered by a densegrowth of saw grass. A very slow, steady flow of water moves through thesaw grass from north to south. To your eyes, the Everglades looks like ahuge swampy area of tall, sharp-edged grass; but in many ways it is really awide river because of the water that flows through it. The Everglades istherefore called a “River of Grass.”

One hundred years ago, the Everglades was considered a wasteland. In1906, construction projects built drainage canals that altered the flow ofthis water that had moved unchallenged for centuries. After the land wasdrained, huge areas were turned over to agriculture. By the 1980s,continued growth of sugarcane farms, housing developments, and highwayconstruction had reduced the Everglades ecosystem of grass marshes toabout one-half its original size.

Voices of alarm were raised for many years. The survival of many plant andanimal species was threatened. Finally, it was noticed that the quantity andquality of the entire underground freshwater supply on which humans insouthern Florida depend were being placed at risk.

In 1996, the federal government endorsed the Everglades restorationproject. The project will be one of the largest ecological restoration efforts

a


Restoring the River of Grass

anywhere in the world. Hundreds of millions of dollars will be spent onprotecting the fragile Everglades ecosystem. Included in the plan is theremoval from sugarcane production of 100,000 acres of farmland inecologically sensitive areas.

Much of the water that flows through the Everglades has becomecontaminated by pesticides and fertilizers that are used to increase cropyields on farms in the area. One of the main goals of the restoration projectis to let large areas of land act as natural water filters to remove some of the waterborne contaminants. Today, there are too many farms and toofew natural areas in the Everglades. The restoration project will help restorethe balance. Six large wetlands are being constructed between LakeOkeechobee and the Everglades. One of these projects, for example, wasbegun in 2001 and is scheduled for completion in 2009. These areas, called storm-water treatment areas, will use naturally occurring biologicalprocesses to reduce the levels of phosphorus carried by the water thatmoves through the Everglades.

Another important part of the Everglades project will restore the naturalnorth-south flow of water. The natural pattern of water flow through theEverglades was disrupted by the canals, pumping stations, and water-control structures that were built to create flood-control and water-supplysystems for southern Florida. In fact, these unnatural attempts to control theEverglades’ water flow have been harmful to the entire ecosystem. Today,planning is under way to find alternatives that can meet flood-control andwater-supply needs while ensuring the long-term health of the Everglades.

This is evolution in action. The Hardy-Weinberg Law shows, by lookingat the genes of a total population, how the environment interacts with apopulation to produce evolutionary change.

Do entire communities in an ecosystem stay the same? What causes aparticular community to change? Is the number of species that make upa community critical? These important questions are now being studiedby ecologists. The amount of variety in a community is called speciesdiversity or biodiversity. A community with only a few species of plantsand animals has low biodiversity. The Great Salt Lake, because of its highsalinity, has few species that are adapted to living there. A communitywith many species has great biodiversity. A tropical rain forest communitymay have the greatest biodiversity of any community on Earth.

Biodiversity is one major concern of ecologists today. In fact, at the1992 Earth Summit held in Rio de Janeiro, Brazil, it was hoped that allcountries attending the conference would sign The Biodiversity Treaty.Although 167 nations have signed the treaty since 1992, some large


nations, including the United States, have not. Why is biodiversity sucha great concern to scientists? One in five known species present on Earthwhen you were born is already extinct. For the most part, human actionsare the cause of these extinctions. As species disappear, biodiversitydecreases. Scientists are concerned about the effects of diminished biodi-versity on ecosystems. They have learned that there is a great deal of inter-action and interdependence in ecosystems. Does a community in anecosystem need a certain number of species interacting with each otherto remain viable? How many species can a community afford to lose with-out being harmed? The ability of an ecosystem to continue and to remainhealthy is called its stability. If all the insects in a forest died, would theforest survive? Would the plants that the insects ate grow too quickly?Would these plants interfere with the growth of trees? Would bird popu-lations suffer with no insects to eat?

Many studies that investigate biodiversity and stability in specific com-munities are currently being conducted. In the 1960s, the ecologist RobertPaine studied the animals that live along a stretch of rocky seashore. Inthis community, there were 15 species of small animals, including bar-nacles, clams, and one large predator species, a sea star. In one experi-ment, Paine removed all the sea stars. After a time, the biodiversitydecreased greatly. Instead of the original 15 species present before the seastar was removed, only eight species of smaller animals remained. Sevenspecies had disappeared. The population of one type of mussel hadincreased dramatically. The community had changed a great deal. Stabil-ity no longer existed. Paine realized that by removing the sea star—a pred-ator—the interactions among the smaller animals had changed. The seastar had kept the density of other populations low. Without the predators,competition among the other animals increased for the limited space.Only a few species survived and increased their numbers.

How much loss of biodiversity can occur before Earth’s ecosystemsstop functioning properly? This is a very serious concern of many people.It is also an important concern for all species that depend on Earth’secosystems. We are one of those species.

nn HABITAT DESTRUCTION

There is one main reason why biodiversity is decreasing. Many speciesare disappearing because of habitat loss. Humans are using and changingmany places where organisms formerly lived. For example, in the Mid-west, many fields contained low-lying areas. These low places remainedfilled with water all year long. Many birds, such as ducks and geese, found


food in these bodies of water. Although the birds did not live in the pondsall year, they visited these same places during their migrations each springand fall. The birds were flying between their winter feeding grounds andsummer breeding grounds and were able to rest and find food here ontheir long journeys.

However, the farmers could not grow crops in these wet places. Wheatand corn need drier land. As a result, most of these wet places were filledin. This was a critical loss of habitat for the migrating water birds. Overtime, the populations of geese and ducks decreased. Some species evenbecame extinct. Biodiversity was reduced. This is only one example of theloss of a habitat affecting biodiversity. In many other places, habitat losshas also occurred. Most of the forests in the eastern United States are gone,replaced by farms and cities. Habitat loss occurs on a river when a dam isbuilt. Fish that can survive only in moving water die in the still water ofthe lake that forms behind a dam. Today, the greatest habitat destructionis occurring in the world’s tropical rain forests. It is estimated that 70 to90 percent of Earth’s biodiversity will be lost if the rain forests aredestroyed. Sadly, this is happening while scientists are trying to identifyand classify the many organisms still being discovered in these forests. Inaddition, many fear that species containing substances that could proveto be extremely valuable medicines are being lost forever before evenbeing discovered. (See Figure 26-11.)


Figure 26-11 This rain-forest habitat has been destroyed to make room for abanana plantation.


INTRODUCTION

Mushrooms and molds are members of the fungus kingdom. Organismsin this kingdom lack chloroplasts and so are unable to carry on photo-synthesis. Fungi, like animals, are heterotrophs—organisms that cannotmake their own food. They are thus dependent on other organisms fortheir energy. Unlike animals, which digest their food by using enzymesthey produce in their digestive system, fungi secrete enzymes into thefood on which they grow. Fungi then absorb the nutrients.

In ecosystems, fungi play a very important role. As decomposers, fungireturn nutrients from dead plants and animals to the soil. This investi-gation focuses on a particular decomposer available in a grocery store,the edible mushroom.

MATERIALS

Edible mushrooms, cardboard, scalpels, spore prints prepared in advance,compound and dissecting microscopes, glass slides, water, coverslips

PROCEDURE

1. Observe a mushroom. Draw the structures of the mushroom that youcan see with your unaided eyes. Label as many of the structures as youcan.

2. Place the mushroom on a piece of cardboard so that it rests on its flatside. Use a scalpel to slice the mushroom in half. Make another draw-ing of the internal structures that are now exposed. Label the struc-tures you observe.

3. Observe a spore print. Compare this print to the structures of themushroom you have just observed.

4. Place the mushroom under the dissecting microscope and record anynew structures that become visible under magnification.

LABORATORY INVESTIGATION 26What Role Do Mushrooms Play in Ecosystems?


5. Use the scalpel to prepare thin sections of the mushroom from variousparts. Try to make slices from the stem, cap, and gills. Place a thin sliceon a glass slide. Add a drop of water and a coverslip. Place the slide onyour compound microscope and examine it using the low-powerobjective. Draw what you observe. Make notes of any similarities anddifferences you observe in the various structures.

6. Prepare a wet mount of spores from the gills. Observe the spores underthe high-power objective. Draw what you observe.

INTERPRETIVE QUESTIONS

1. Study the structures and life cycle of a mushroom from reference mate-rials. Describe the function of the structures of the mushroom youobserved. List the parts of the mushroom you found in the referencematerials that you were not able to observe in your specimen.

2. What parts of the mushroom are involved in obtaining nutrients? Howdo these parts obtain nutrients?

3. What parts of the mushroom are involved with reproduction? Howdoes a mushroom reproduce?

4. Why is reproduction by spores considered to be an example of asex-ual reproduction?

5. Why are the actions of fungi, and other decomposers, essential for thecontinuation of life on Earth?

nn CHAPTER 26 REVIEW

Answer these questions on a separate sheet of paper.

VOCABULARY

The following list contains all of the boldfaced terms in this chapter. Defineeach of these terms in your own words.

biodiversity, biological magnification, decomposers, ecosystem, foodchain, food web, legumes, nitrogen fixation, primary consumers,producers, secondary consumers, tertiary consumers, trophic levels

PART A—MULTIPLE CHOICE

Choose the response that best completes the sentence or answers the question.

1. The source of energy for almost all ecosystems on Earth isa. the sun b. geothermal forces c. photosynthesisd. cellular respiration.

2. The organisms on the first trophic level are a. primary consumersb. tertiary consumers c. heterotrophs d. producers.

3. During the process of nitrogen fixation, a. animals releasenitrogen back to the environment in the form of nitrogenouswastes b. the nitrogen in nitrates is used to make plant proteinsc. bacteria take free nitrogen from the air and change it to nitratesd. lightning breaks down nitrates to form nitrogen gas.

4. A clam that feeds on the phytoplankton it filters from the water is eaten by a walrus, which is in turn eaten by a polar bear. In this example, the primary consumer is the a. clamb. photoplankton c. walrus d. polar bear.

5. Which of these communities has the highest biodiversity?a. the Great Salt Lake in Utah b. a deciduous forest in upstateNew York c. the open ocean off the shore of Californiad. a rain forest in Costa Rica.

6. The series of events in which food energy is transferred from an organism at one trophic level to an organism at a higher trophic level, and from that organism to the next, formsa. a feeding pyramid b. a food web c. a food chaind. an ecological succession.

7. The interacting biotic and abiotic factors in an area make upa. a trophic level b. an ecosystem c. biodiversityd. a food web.


8. During the oxygen cycle, oxygen is released into the atmosphere bya. producers during the process of photosynthesisb. decomposers during the process of fermentationc. consumers during the process of respirationd. bacteria during the process of nitrogen fixation.

9. Which condition is necessary for a population not to change?a. mutations b. movement of individuals into and out of thepopulation c. individuals having different chances of survivingd. random matings.

10. In an ecological pyramid, the most food energy is founda. at the top of the pyramid b. in the middle of the pyramidc. at the bottom of the pyramid d. none of these.

11. Scientists have found very few fossils of Tyrannosaurus rex andcomparatively large numbers of the dinosaurs on which T. rexfed. The best explanation for this fact is that a. energy is lostwith each successive link in the food chain b. there are fewerindividual organisms at each successive level of an ecologicalpyramid c. consumers do not form fossils as readily as producers d. scientists have not been searching in the best places to find T. rex fossils.

12. The complex pattern of interlocking food chains in an ecosystem forms a a. biomass pyramid b. food chainc. biogeochemical cycle d. food web.

13. The main reason for loss of biodiversity is a. diseaseb. the limited number of crop and livestock varieties used inmodern agriculture c. habitat loss d. hunting and fishing.

14. Biological magnification involves a. an increase in the number of individuals at successive levels of an ecological pyramidb. the increasing concentration of harmful chemicals at eachsuccessive step of a food chain c. the viewing of microscopic life-forms with special equipment d. the increasing amount ofenergy represented by a unit of biomass at each successive level ofan ecological pyramid.

15. The organisms that break down dead plants and animals, therebyreleasing CO2 into the atmosphere, are a. decomposersb. autotrophs c. producers d. consumers.


PART B—CONSTRUCTED RESPONSE

Use the information in the chapter to respond to these items.

16. The events in the diagram show that materials are cycled betweena. living things only b. heterotrophs only c. the living andnonliving parts of the environment d. the nonliving parts of theenvironment through evaporation, condensation, andprecipitation.

17. What process does this diagram illustrate? Identify the type ofnitrogen compounds found at the points lettered A, B, C, D, and E.

18. Explain why a farmer may plant a field with alfalfa rather thancorn every few years.

19. Why is it ecologically wise to eat “lower on the food chain”?20. What is the Hardy-Weinberg Law? How does it tie ecology to

evolution?

PART C—READING COMPREHENSION

Base your answers to questions 21 through 23 on the information below andon your knowledge of biology. Source: Science News (October 26, 2002): vol.162, p. 269.

Insects, Pollen, [and] Seeds Travel Wildlife Corridors

In an unusual test of a conservation strategy called wildlife corridors,strips of habitat boosted insect movement, plant pollination, and seeddispersal among patches of the same ecosystem.


Soil organisms Soil

Dead plants

Animal waste

Air

Corn plants

Legumes

A

B C

D

E

LIVING ENVIRONMENT BIOLOGY, 2e/fig. 26-Q16 s/s

21. Explain how (at all eight locations) three of the test plots weremade different from the central and fourth plots.

22. State the difference that was observed in the number of butterflies(released from the central plot) that showed up in the connectedplots as opposed to those that showed up in the isolated plots. Howwould you explain this difference?

23. Why did researchers study the droppings of birds in these testplots, and what was their conclusion from this study?

Theory predicts that adding such corridors enhances the benefits ofotherwise isolated preserves, says Joshua Tewksbury of the University ofWashington in Seattle. He and his colleagues tested that strategy inSouth Carolina pine forests.

At eight locations, the researchers cleared mature vegetation and cre-ated open habitat on five 1-hectare plots—arranged as a central plotwith four satellites. In each case, a 150-meter-long corridor connectedthe central plot to one outlier, while the others remained isolated. Theunlinked patches had dead-end corridors or additional area so theymatched the habitat area of another patch and its connecting corridor.Thus, scientists could distinguish between effects of biological entities’ease of movement and of extra habitat.

Butterflies, pollen, and seeds all moved most often between thecorridor-connected patches, the researchers report in an upcoming Pro-ceedings of the National Academy of Sciences. Variegated fritillary andcommon buckeye butterflies that the researchers captured, marked, andreleased in the central patch proved two to four times as likely to showup in connected patches as in unconnected ones.

When researchers placed male holly plants in the center patches,females in connected patches showed an average increase in seed pro-duction of nearly 70 percent, compared with that of female hollies inunconnected patches. Also, bird droppings in connected patches har-bored more berries from shrubs in the center patches than did drop-pings in patches not connected to the central patch.

This is the first test of a corridor’s effect on plant-animal interactions,says Tewksbury.


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After you have finished reading this chapter, you should be able … · 2014-01-19 · After you have finished reading this chapter, you should be able to: Relate energy flow to