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4.1: Energy Flow in a Community

Difficulty Level: At Grade Created by: CK-12
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How does energy flow through a biological community?

Now that you can trace the path energy takes from your breakfast to your muscles, let's think about how energy flows on a larger scale. This section will introduce you to the movement of energy throughout a biological community. You will explore the different ways organisms get their energy.

“At the junction of every pair of threads in this net of life there is a crystal bead, and each crystal bead is a living thing, shining forth with its own glow, its own radiance into space. And the glow of every crystal bead in the net of life reflects the glow of every other bead.

Steve Van Matre

The Earth Speaks

Energy moves through biological communities, keeping the organisms alive and functioning. But keep this in mind: An energy pathway isn't 100 percent efficient. Little bits of energy are lost along the way.

Single food chains, such as the ones described in Food Chains: How Energy Gets to You, show only one type of path that energy can take from the sun to an organism. There are many different types of paths energy can follow. Think about the food chain shown in Figure 2.11. You eat many other things besides eggs for breakfast, lunch, and dinner. And free-range chickens eat more than just the occasional grasshopper that jumps into the barnyard. You get energy from many different sources, and so does the chicken. By putting food chains together, you create a food web. A food web shows the important links between connected food chains. Food webs show a more accurate picture of how energy is passed around in the real world than one food chain does.

Draw Your Community List the ten people you consider to be the most important members of your community. Explain why you think each is important and how they interact within your community. Draw a picture or create a collage showing how these people interact to support your community.


What does the word community mean to you? Many people think of a community as all the people they see on a daily or weekly basis. These people might be family, neighbors, friends, teachers, mail carriers, police officers, and other people who live or work in a certain area. As a matter of fact, a human community does consist of all of the people who live around you and help you live where you do.

Figure 3.1 Who eats whom in this pond community?

Draw the Community of a Largemouth Bass Think of ten things that belong in the community of a largemouth bass. Draw a picture of the community showing each part of the food web.

Imagine you are an organism that lives in the pond. You can be the largemouth bass, the algae on the surface of the pond, the crawfish, or any organism you decide to be. When you decide which organism you want to pretend to be, write a story or poem about a day in your life in the community.

However, ecologists think of a community in a slightly different way. They recognize that each organism lives in a community that includes all of the other organisms with which it interacts. For example, a frog's community includes most of the organisms that live and grow in and around the frog's pond. The frog's community includes the algae that grow on the bottom of the pond, because algae are eaten by the snails and insects that the frog eats. The frog's community might include a lily pad on which the frog rests. It might even include a great blue heron, which could eat the frog if it's not careful. The frog's community would also include all of the other organisms that the snail, insects, lily pad, and blue heron need to stay alive.

Food Webs Can Be Complicated

A food web describes how energy flows between members of a community. Tracing the specific path of one food chain can be pretty simple. For example, the person-chicken-grasshopper-grass-sun food chain is very easy to follow. However, tracing the many paths through a food web is a little more complicated. The food web connects and interconnects all of the possible food chains in a community. For example, think carefully about the chicken in our food chain example. In a food web, me chicken will eat more than an occasional grasshopper. The chicken will eat feed corn and maybe a worm brought up by a rainstorm. A fox, which has been circling the barnyard, might break in and eat the chicken. Remember the grasshopper? What else do you think it might eat besides the grass described in our simple food chain? The grasshopper might eat some of the feed corn, also eaten by the chicken, or some of the wheat grain spread in the nesting area. So as you can see, food webs can get complicated. Figure 3.2 illustrates just the start of a barnyard food web.

Figure 3.2 This shows a very simple barnyard food web. Try to think of some other organisms that can be added to fill out this food web.

What Can You Add to the Web? Try to make the food web in Figure 3.2 more realistic. Think of other organisms you can add to the barnyard food web started in Figure 3.2. List the organisms you can think of. Then draw the food web in Figure 3.2. Draw the other organisms you listed on your food web, and link the organisms together to show how they interact.

Food webs in the wild are usually more complicated than our barnyard example. An ecologist named R. D. Bird described a real food web of a willow forest that he studied in central Canada. Even though this food web was fairly simple, it still included several different kinds of willow trees, six different kinds of birds, various spiders, many insects, frogs, snails, and garter snakes. Figure 3.3 is a simplified version of the web that he described. The arrows show the direction that energy flows within the system.

Figure 3.3 This illustration shows the organisms R. D. Bird observed and linked in the willow forest food web he studied in Central Canada.

Look at Figure 3.3. The energy in this picture moves in the direction the arrows are pointing. What organisms get their energy directly from the willows? Notice the word directly in that question before you answer it. The arrow at the top center of the willow forest food web starts at the willows and ends at the beetles. What other organisms get their energy directly from the willows? Again, notice the word directly. Also, notice that there are several kinds of willows.

Did You Know?

People who eat no meat, but do consume animal products such as milk and eggs are usually known as vegetarians. People whose diets consist only of plants with no animal products are called vegans.

The beetles and insects eat willow leaves in this willow forest. Now locate the frogs. Where do the frogs get their energy? What do they eat? Which organism gets energy from the frogs? After studying this food web for a little while, it sure looks like a “snake-eats-frog world”! One source of energy not shown in this food web is where the willow trees get their energy. You probably remember that the source of energy for almost all plants, including willow trees is-the sun!

Participants in a Food Web

In general, ecologists can divide organisms into two main groups on the basis of how they obtain energy. Ecologists call organisms that get their energy from the sun producers because they produce sugars that other organisms can eat to move, grow, and reproduce. Most producers are green plants because they capture the sun's energy through photosynthesis.

Would you ever consider being a vegetarian? Why or why not? Would you ever consider being a vegan? Why or why not?

Organisms that get their energy by eating other organisms are called consumers. They consume the energy that is produced by other living things. Consumers can also be divided into several groups. Consumers, such as cows and rabbits, which eat only plants, are called herbivores (HURB-ih-vors). Some humans choose to be herbivores. Consumers, such as bald eagles or wildcats, which eat only other animals, are called carnivores (KARN-ih-vors). Consumers, such as blue jays and most humans, which eat both plants and animals, are called omnivores (OM-nih-vors). Decomposers are the other major group of consumers. Decomposers get the energy they need by eating the remains of organisms that are already dead. Decomposers include such organisms as worms, some snails, mushrooms, dung beetles, and vultures.

List 5 foods that might be included in a vegetarian diet that would not be included in a vegan diet.

What Do You Think?

if you had your choice, would you rather be a producer, consumer, or decomposer? Why?

Energy Pyramids

As energy flows through a community, it changes form. Whenever energy changes form, some energy is lost. For example, when a light bulb changes electricity into light, the light bulb gets hot. The energy converted to heat is not converted to useful light, so it is considered lost. In the same way, a car engine changes gasoline into motion, but some of the energy in the gasoline is lost as heat. Even you lose energy. Your body changes a bowl of cereal into energy you can use to study or play. However, part of the energy in the cereal is used to keep you warm, to help you digest, and to do a lot of other chores your body does to maintain itself. Even though digesting and staying warm are important, scientists call this a loss of energy.

It's important to scientists to observe energy flow in biological communities. So ecologists study how energy flows and is lost in biological communities. In one study, a group of ecologists counted all of the producers, herbivores, and carnivores in a field of bluegrass. They found that 5,842,424 weeds and blades of grass fed 708,624, herbivores such as grasshoppers. These herbivores fed 354,904 carnivores such as spiders, ants, and beetles. The 354,904 carnivores fed the three top carnivores, such as birds and moles. The food web these ecologists studied and described forms an ecological pyramid. What's an ecological pyramid? First, what's a pyramid? It's a structure in which each level is made up of fewer stones than the level below. In a similar way, each layer in an ecological pyramid has fewer organisms than the level below it. In an ecological pyramid, many producers feed a few herbivores, which feed even fewer carnivores. This is shown in the bluegrass field food web. Each step up the bluegrass field food web supports many fewer individuals than the level below it.

Why do you think there are only three top carnivores in the bluegrass field and 5,842,424 grass plants?

What do you think is meant by the phrase “eating low on the food chain”? Could you feed more or fewer people from the same amount of land if everyone was a herbivore or if everyone was a carnivore?

Figure 3.4 This ecological pyramid was prepared by ecologists who counted the numbers of organisms at each feeding level in a bluegrass field.

Figure 3.5 This energy pyramid explains the flow of energy in Cayuga Lake.

Energy is lost as it is transferred from one level of the community to the next. The herbivores use up nearly ninety percent of the energy they get from eating the grass. They use it up by running around, digesting, and reproducing. Only about ten percent of the energy from the grass is changed into energy that can be used by the consumers, which eat the herbivores. So only ten percent of the energy that was in the grass is available for a carnivore to eat. Generally, only about ten percent of the energy in each level of the food chain transfers to the next level. For that reason, a community usually has many more producers than herbivores, and many more herbivores than carnivores.

Counting the number of individuals in a community and observing how they fit into different food chains is one way ecologists determine energy flow in a biological community. Another method scientists use to determine the amount of energy is to measure the amount of energy passed along at each step. One way to measure energy is in units called calories (KAL-or-rees). One calorie is the amount of energy it takes to raise the temperature of one gram of water one degree Celsius.

Lamont Cole, a scientist at Cornell University, decided to measure the calories passed along a food chain in Cayuga Lake. He found that for every 1,000 calories produced by the algae in the lake, only 150 calories were transferred to the little herbivores called zooplankton, which ate the algae. Of the 150 calories stored in the zooplankton, only 30 calories made it into a small fish called smelt, which ate the zooplankton. Of the 30 calories that made it into the smelt, only 6 calories made it to the humans who ate the smelt. In other words, of the 1,000 calories originally found in the algae, only 6 calories reached the humans as useful energy. Figure 3.5 shows an energy pyramid that summarizes the transferred energy in the Cayuga Lake food web Dr. Cole studied.

Look at the energy pyramid in Figure 3.5. Suppose a trout eats a smelt and then a human eats the trout. About how many of the original 1000 calories contained in the algae reach the human? How does this compare to a situation in which the human ate the smelt directly?

Activity 3-1: Classifying the Players in a Willow Forest


To really understand the flow of energy through a food web, it is important to classify the major players and their sources of energy. In this activity you examine the roles of organisms in the web and analyze how they relate to one another.


  • Colored marking pens, pencils, or crayons
  • Resource
  • Activity Report


Step 1 Look at the food web of the willow forest illustrated on your Resource. On your Activity Report, classify all of the “players” in the system as producers or consumers, using two different colored pens or different symbols. (We've left out the decomposers to make it a bit simpler.)

Step 2 Now using three other colored pens, classify the consumers as herbivores, carnivores, or omnivores. Make a key to show which color or symbol represents each classification.

Step 3 Imagine you live in the willow forest and eat only beetles and red-winged blackbirds. How would you be classified? Using the appropriate color or symbol put yourself in your classification system. Then draw yourself on your Willow Food Web Diagram.

Step 4 On your Activity Report, explain what would happen to the rest of the web if all frogs were removed.

Step 5 What do you think would happen to the web if one kind of willow tree such as the Salix petiolaris were removed? Explain your answer on your Activity Report.

Review Questions

  1. What is a producer?
  2. What is a consumer?
  3. What happens to useful energy when it is transformed from one form to another?
  4. What happens to the energy in an organism when that organism is eaten by another organism?
  5. Draw an ecological pyramid using producers, herbivores, and carnivores.

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