Day 1
Tell students that in today’s lesson, they will be looking at how energy is transferred as it passes through the plants and animals in ecosystems. Explain the concept of trophic levels, recalling a food chain from the previous lesson, such as:
Sun → maple leaves → caterpillar → bluebird → garter snake → raccoon
Sun → producers → primary consumers → secondary consumers → tertiary consumers
Remind students that decomposers, such as bacteria and fungi, break down food into nutrients at every trophic level in a food chain.
Call four volunteers to the front of the class. Tell the class to imagine that the volunteers are the Sun, foxes, rabbits, and grass in a Pennsylvania ecosystem. Have the class re-arrange the volunteers so that they are standing in the form a food chain. Give the “grass” the cup with 100 beads in it, and the “rabbit” and “fox” the empty cups. Tell the class that the beads represent energy. Ask the class,
- “Where did the grass get its energy?” (from the Sun)
- “What kind of energy is found in the grass?” (stored energy, or chemical energy)
Tell the class the grass has 100 units of stored energy, and when the rabbits eat the grass, only about 10 percent of that energy is available to the rabbits. Ask,
- “What is 1/10 of 100?” (10)
- “What is another way to say 1/10?” (10 percent)
- “What is 10 percent of 100?” (10)
Have the “grass” give 10 energy beads to the “rabbit.” Then, tell the class that the rabbit can only pass on up to 10 percent of the energy it received from the grass. Then ask, “What is 10 percent of 10?” (one) Have the “rabbit” give the “fox” one bead. Have the student volunteers return to their seats.
Tell students that only about ten percent of the energy available in one trophic level is available to organisms in the next trophic level. (Note: This is sometimes called the ten percent rule. Studies have shown that the energy efficiency in food chains ranges from 1–20 percent, but the ten percent rule is still used as a general model to show energy transfer in ecosystems.) Ask students to explain how this concept applies to the students’ demonstration with the beads.
Show students the food chain below:
grass → cows → humans
Ask students, “What percentage of the energy captured by grass during photosynthesis ends up in the humans that eat the cows?” Elicit that humans only receive one percent of the energy, and ask students to explain what happened to the rest of the energy. Explain that at every level of a food chain, some energy is used for life processes, some is stored, and some is given off as heat energy.
Explain that the amount of energy available in food is measured in kilocalories, which are expressed as Calories, or kcal. Ask students, “If the producers in a food pyramid contain 500,000 kcal of food energy, how much energy will be available to the first-level consumers?” (50,000 kcal will be available, because that is ten percent of 500,000.) Model how to solve this problem step-by-step and have students write the solution in their notes.
Show students the Energy Pyramid resource (S-5-5-2_Energy Pyramid.docx). Cover up all levels except for the producers. Ask, “The producers in this food chain have 10,000 kcal of stored energy. If ten percent of that energy is available to the primary consumers, how much energy is that?” (1,000 kcal) Continue moving up through the trophic levels, explaining that the energy pyramid is another way to model energy flow in a food chain.
Give students the Energy Flow Through a Farm Ecosystem worksheet (S-5-5-2_Energy Flow and KEY.docx). Tell them they’ll be watching a short presentation on energy flow on a farm and read over the questions with them.
Use a projector to show students the Energy Flow Flash interactive at www.teachersdomain.org/resource/tdc02.sci.life.oate.energyflow/. It gives an example that explains the transfer of energy among organisms in an ecosystem. Allow time for students to answer the questions on the worksheet as they watch the presentation. If you do not have a projector, you can read it aloud to the class as a story instead.
Have students complete the Energy Flow Through a Farm Ecosystem worksheet. Review answers with the class.
Day 2
Have students recall the organisms that they decided were at the top of Pennsylvania’s food chain at the end of Lesson 1. Ask them if those animals only eat one other kind of animal. Choose one organism and write a food chain for it on the board. Tell students that each organism belongs to many food chains, and you can connect them all together in a food web. Guide students in creating a simple food web with several organisms, based on that food chain. Emphasize that the arrows show the direction of energy flow, and have students tell you which way the arrow should point for each relationship.
If you have a computer with a projector, it may be helpful to show students an online simulation of food chains connecting to build a food web at:
www.ngflcymru.org.uk/vtc/Phase3delivery/Wales/Science/Keystage4/Livingthingsand/Feedingrelation/Introduction/act2.swf.
Divide students into the same small groups as in Lesson 1 and have them sit at computer stations. Have each group create a food web online at:
http://teacher.scholastic.com/ACTIVITIES/explorer/ecosystems/be_an_explorer/map/foodweb_play.htm. This activity should take about 10 minutes.
When students are finished, give each group a sheet of chart paper, ten index cards, a marker, and tape or glue. Give students a choice of whether they want to create a food web for a Pennsylvania aquatic ecosystem or field ecosystem. Give each group a list of ten organisms to write on the index cards and then arrange in a food web on the chart paper. Suggestions for organisms:
- Aquatic: algae, clams, human, muskrat, sunfish, largemouth bass, salamander, frog, mosquito, Elodea (aquatic plant)
- Field: white-tailed deer, corn, Eastern bluebird, black snake, red fox, mouse, earthworm, Queen Anne’s lace (wildflower), crow, grasshopper
Circulate around the room and help students with feeding relationships for the organisms if needed. If time permits, have each group share its food web with the class.
Weaving a Food Web
Hand out an index card and marker to each student. Assign each student an organism and have him/her make a nametag with just the organism’s name on it. Have the student tape the nametag to his/her shirt.
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Suggested organisms for an aquatic food web:
- Producers: algae, water lilies, Elodea, aquatic plants
- Primary consumers (herbivores): snails, mayflies, weevils, leaf beetles, catfish, tadpoles
- Secondary consumers (carnivores): frogs, toads, snapping turtles, crayfish, diving beetles, dragonflies, damselflies, minnows, snakes, salamanders, newts, ducks
- Decomposers: worms, bacteria, water mold
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Go outside or clear a large space in the classroom and have students make a fairly large circle. The larger the circle, the easier it is to represent a web pattern. Give the ball of string to the designated first organism and have the student say what s/he is and name one organism in the circle that s/he eats or is eaten by. Once the web is created, show what happens when an organism is removed from the web. Have that student drop his/her segment of string to represent that the web’s tightly woven structure will change when other organisms gently tug on the string. Discuss the interdependent relationships in the food web.
Point out the terms producer, consumer, and decomposer, using examples from the activity. Review the concept of trophic levels in food chains and webs. Ask,
- “If a rabbit (or other organism) was removed from our game’s food web, how are two other organisms in the ecosystem affected?” Answers will vary, but should reflect an understanding of organism interactions.
- “What is the original source of energy for all of the organisms in the food web?” (the Sun)
- “Why do we need producers in every food web?” (Producers capture the Sun’s energy and change it into food energy.)
Have students work alone to complete the Pennsylvania Food Web worksheet (S-5-5-2_PA Food Web Worksheet and KEY.docx). This worksheet assesses concepts from both parts of this lesson.
Extension:
- For students who may need opportunities for additional learning, model how to begin building the food web on the chart paper and provide information about feeding relationships among the organisms by giving them several food chains to connect.
- Challenge students who might be going beyond the standards to create a food web using their food chain from Lesson 1.
