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THINK ABOUT IT What happens to energy stored in body tissueswhen one organism eats another? That energy moves from the "eaten"to the "eater." You've learned that the flow of energy through an eco-system always begins with either photosynthetic or chemosyntheticprimary producers. Where it goes from there depends literaUy on whoeats whom!

Food Chains and Food Websf3 How does energy flow through ecosystems?In every ecosystem, primary producers and consumers are linkedthrough feeding relationships. Despite the great variety of feedingrelationships in different ecosystems, energy always flows in similarways. ^"Sp Energy flows through an ecosystem in a one-way stream,from primary producers to various consumers.

Food Chains You can think of energy as passing through an eco-system along a food chain. A food chain is a series of steps in whichorganisms transfer energy by eating and being eaten. Food chains canvary in length. For example, in a prairie ecosystem, a primary pro-ducer, such as grass, is eaten by an herbivore, such as a grazing ante-lope. A carnivore, such as a coyote, in turn feeds upon the antelope. Inthis two-step chain, the carnivore is just two steps removed from theprimary producer.

In some aquatic food chains, primary producers are a mbcture offloating algae called phytoplankton, and attached algae. As shown inFigure 3-7, these primary producers may be eaten by small fishes,such as flagfish. Larger fishes, like the largemouth bass, eat the smallfishes. The bass are preyed upon by large wading birds, such as theanhinga, which may ultimately be eaten by an alligator. There arefour steps in this food chain. The top carnivore is therefore four stepsremoved from the primary producer.

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Key Questionslr'1!? How does energy flowthrough ecosystems?

What do the three types ofecological pyramids illustrate?

Vocabularyfood chain • phytoplankton •food web • zooplankton •trophic level •ecological pyramid •biomass

Taking NotesPreview Visuals Before youread, look at Figure 3-7 andFigure 3-9. Note how they aresimilar and how they are ditfer-ent. Based on the figures, writedefinitions for food chain andfood web.

FIGURE 3-7 Food Chains Foodchains show the one-way flowof energy in an ecosystem.Apply Concepts What is theultimate source of energy for thisfood chain?

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plJHgVocabulory1 ACADEMIC WORDS The verbI convert means "to change fromone form to another." Decomposersconvert, or change, dead plantmatter into a form called detritus

that is eaten by detritivores.

::yj^a^L '^]Ma^fiFIGURE 3-8 Earth's RecyclingCenter Decomposers breakdown dead and decaying matterand release nutrients that can bereused by primary producers.

How are decomposerslike a city's recycling center?

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In most ecosystems, feeding relationships are muchmore complicated than the relationships described in a single, simplechain. One reason for this is that many animals eat more than onekind of food. For example, on Africa's Serengeti Plain, herbivores, suchas zebras, gazelles, and buffaloes, often graze upon several differentspecies of grasses. Several predators such as lions, hyenas, and leopards,in tun-i, often prey upon those herbivores! Ecologists call this networkof feeding interactions a food web.

Food Chains Within Food Webs The Everglades are a complexmarshland ecosystem in southern Florida. Here, aquatic and terrestrialorganisms interact in many overlapping feeding relationships thathave been simplified and represented in Figure 3-9. Starting with aprimary producer (algae or plants), see how many different routes youcan take to reach the alligator, vulture, or anhinga. One path, from thealgae to the alligator, is the same food chain you saw in Figure 3-7. Infact, each path you trace through the food web is a food chain. You canthink of a food web, therefore, as linking together all of the food chainsin an ecosystem. Realize, however, that this is a highly simplified repre-sentation of this food web, in which many species have been left out.Now, you can begin to appreciate how complicated food webs are!

Decomposers and Detritivores in Food Webs Decomposers anddetritivores are as important in most food webs as other consumers are.Look again at the Everglades web. Although white-tailed deer, moor-hens, raccoons, grass shrimp, crayfish, and flagfish feed at least partlyon primary producers, most producers die without being eaten. In thedetritus pathway, decomposers convert that dead material to detri-tus, which is eaten by detritivores, such as crayfish, grass shrimp, andworms. At the same time, the decomposition process releases nutrientsthat can be used by primary producers. Thus, decomposers recyclenutrients in food webs as seen in Figure 3-8. Without decomposers,nutrients would remain locked within dead organisins.

In Your Ho}€:fwcw Explain how food chains and food webs are related.

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FIGURE 3-9 Food Web in theEverglades This illustration of afood web shows some of the feedingrelationships within the FloridaEverglades. The orange-highlightedfood chain from Figure 3-7 is one ofmany that make up this food web.Interprst Visuals Describe three foodchains that are part of this food web.

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Researchers discoveredthat zooplankton inNarrugansett Bay nowgraze on floating algaemore actively throughthe winter than theyever did before. Whateffect do you thinkthis might have onthe annual tate-winter"bloom" of algaethat occurs inthe water? ^

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FIGURE 3-10 Antarctic FoodWeb All of the animals in this foodweb depend on one organism: krill.Disturbances to the krill's food source,marine algae, have the potentialto cause changes in all of thepopulations connected to thealgae through this food web.

^ •; ; What do ecologistsmean when they say that killerwhales indirectly depend on krill forsurvival?

;d.y\<< ^tftwflo^C'C Food webs are complex, so it i.s oftendifficult to predict exactly how they will respond to environmentalchange. Look again at Figure 3-9, and think about the questions anecologist might ask about the feeding relationships in it following adisturbance. What if an oil spill, for example, caused a serious declinein the number of the bacteria and fungi that break down detritus?What effect do you think that might have on populations of crayfish?How about the effects on the grass shrimp and the worms? Do youthink those populations would decline? If they did decline, how mightpig frogs change their feeding behavior? How might the change in frogbehavior then affect the other species on which the frog feeds?

Relationships in food webs are not simple, and, as you know, thefood web in Figure 3-9 has been simplified! So, you might expectthat answers to these questions would not be simple either, and you'dbe right. However, disturbances do happen, and their effects can bedramatic. Consider, for example, one of the most important food websin the southern oceans. All of the animals in this food web, shownin Figure 3-10, depend directly or indirectly on shrimplike animalscalled krill, which feed on marine algae. Krill are one example of adiverse group of small, swimming animals, called zooplankton(zoh oh PLANK tun), that feed on marine algae. Adult krill browseon algae offshore, while their larvae feed on algae that live beneathfloating sea ice. In recent years, krill populations have dropped sub-stantially. Over that same period, a large amount of sea ice aroundAntarctica has melted. With less sea ice remaining, there are fewer ofthe algae that grow beneath the ice. Given the structure of this foodweb, a drop in the krill population can cause drops in the populationsof all other members of the food web shown.

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Trophic Levels and Ecological PyramidsWhat do the three types of ecological pyramids illustrate?

Each step in a food chain or food web is called a trophic level.Primary producers always make up the first trophic level. Variousconsumers occupy every other level. One way to illustrate thetrophic levels in an ecosystem is with an ecological pyramid.Ecological pyramids show the relative amount of energy or mattercontained within each trophic level in a given food chain or food web.There are three different types of ecological pyramids: pyramids ofenergy, pyramids of biomass, and pyramids of numbers.

t^l In Your Notebook Make a two-column chart to compare the three^ fypes of ecological pyramids. .s

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Pyramids of Energy Theoretically, there is no limit to the number oftrophic levels in a food web or the number of organisms that live oneach level. But there is one catch. Only a small portion of the energythat passes through any given trophic level is ultimately stored in thebodies of organisms at the next level. This is because organisms expendmuch of the energy they acquire on life processes, such as respiration,movement, growth, and reproduction. Most of the remaining energyisreleased into the environment as heat—a byproduct of these activ-ities. ?^n3 Pyramids of energy show the relative amount of energyavailable at each trophic level of a food chain or food web.

The efficiency of energy transfer from one trophic level to anothervaries. On average, about 10 percent of the energy available withinone trophic level is transferred to the next trophic level, as shown inFigure 3-11. For instance, one tenth of the solar energy captured andstored in the leaves of grasses ends up stored in the tissues of cows andother grazers. One tenth of that energy—10 percent of 10 percent, or1 percent of the original amount—gets stored in the tissues of humanswho eat cows. Thus, the more levels that exist between a producer anda given consumer, the smaller the percentage of the original energyfrom producers that is available to that consumer. ^ Heat

Third-level / \ /FIGURE 3-11 Pyramid ofEnergy Pyramids of energyshow the relative amount ofenergy available at eachtrophic level. An ecosystemrequires a constant supply ofenergy from photosynthetic orchemosynthetic producers.Apply Concepts Explainhow the amount of energyavailable at each trophic/eve/ often limits the numberof organisms that each levelcan support.

The 10 Percent RuleAs shown in Figure 3-11, anenergy pyramid is a diagramthat illustrates the transferof energy through a foodchain or food web. In general,only 10 percent of the energyavailable in one level is storedin the level above. Look atFigure 3-11 and answer thequestions below.

1. Calculate If there are1000 units of energy avail-able at the producer level ofthe energy pyramid, approxi-mately how many units ofenergy are available to thethird-level consumer? S^&

2. Interpret Diagrams Whatis the original source of theenergy that flows throughmost ecosystems? Why mustthere be a continuous supplyof energy into the ecosystem?3. Infer Why are thereusually fewer organisms inthe top levels of an energypyramid? I.

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FIGURE 3-12 Pyramids of Biomass and NumbersIn most cases, pyramids of biomass and numbersfollow the same general pattern. In the field modeledhere, there are more individual primary producers thanfirst-level consumers. Likewise, the primary producerscollectively have more mass. The same patterns hold forthe second and third-level consumers. With each step toa higher trophic level, biomass and numbers decrease.

P^^6,o-^^^amount of living tissue within a given tropliic levelis called its biomass. Biomass is usually measured ingrams of organic matter per unit area. The amountofbiomass a given trophic level can support is deter-mined, in part, by the amount of energy available.'^-' A pyramid ofbiomass illustrates the relativeamount of living organic matter available at eachtrophic level in an ecosystem.

Ecologists interested in the number of organismsat each trophic level uses a pyramid of numbers.^" ' A pyramid of numbers shows the relativenumber of individual organisms at each trophiclevel in an ecosystem. In most ecosystems, the shapeof the pyramid of numbers is similar to the shape ofthe pyramid ofbiomass for the same ecosystem. Inthis shape, the numbers of individuals on each leveldecrease from the level below it. To understand thispoint more dearly, imagine that an ecologist markedoff several square meters in a field, and then weighedand counted every organism in that area. The resultmight look something like the pyramid in Figure 3-12.

In some cases, however, consumers are much lessmassive than organisms they feed upon. Thousandsof insects may graze on a single tree, for example, andcountless mosquitos can feed off a few deer. Both thetree and deer have a lot ofbiomass, but they each rep-resent only one organism. In such cases, the pyramidof numbers may be turned upside down, but the pyra-mid ofbiomass usually has the normal orientation.

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1. a. fie^'<>*/^Energy is said to flow in a "one-way stream" throughan ecosystem. In your own words, describe what that means.b. fvYt* *. ^ypt^to '* '. Explain what you think might happen tothe Everglades ecosystem shown in Figure 3-9 if there were asudden decrease in the number of crayfish.

2. a. Rfci/if.t/ On average, what proportion of the energy in anecosystem is transferred from one trophic level to the next?Where does the rest of the energy go?b. Cft-t^larft Draw an energy pyramid for a five-step food chain.If 100 percent of the energy is available at the first trophic level,what percentage of that energy is available at the highest trophiclevel? j/^.TrJ

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