• Assess student understanding of the Ten Percent Rule as you ask the class questions about the flow of energy through the trophic levels of organisms in food pyramids.

• Collect and check the Energy Pyramid worksheet.

• Circulate around the classroom and check students’ understanding of conservation of energy and matter by discussing their Venn diagrams.

Ask students, “We live in a world with a growing human population. If more people eat from the top of the food chain, will we able to feed more or fewer people altogether?” Take a vote for more versus fewer, then ask a few students from each group to share their thoughts on the question.

Explain that the reason for this is that only part of the energy that is stored in one trophic level is passed on to the next level. That is because the organisms use most of the energy that they take in for life processes. These include: movement, growth, respiration, and reproduction. Some of the remaining energy is released into the environment as heat.

Draw an energy pyramid like the one below on the board or overhead projector (S-B-2-3_Energy Pyramid.doc and S-B-2-3_ Energy Pyramid Worksheet KEY.docx).

Energy Pyramid

Tell students that only about 10 percent of the energy available in one trophic level is available to organisms in the next trophic level. This is sometimes called the Ten Percent Rule. Note: Studies have shown that the energy efficiency in food chains ranges from 1 percent to 20 percent, but the Ten Percent Rule is still used as a general model to show energy transfer in ecosystems.

Show students the food chain below:

Grass → Cows → Humans

Have them draw a three-level energy pyramid in their notes, with grass at the bottom, cows on the next level, and humans on top. Ask students, “What percentage of the energy captured by grasses during photosynthesis ends up in the humans who eat the cows?” Elicit that humans have access to as much as one percent of the energy, and ask students to explain what happened to the rest of the energy. Explain that the more levels there are in a food chain, the less energy is available for the top-level consumers. Many producers are required in order to support fewer herbivores, and even fewer carnivores.

Have students draw an energy pyramid for a five-step food chain. Ask students, “If 100 percent of the energy is available to the first trophic level, what percent of the energy is available at the highest trophic level?” At the fifth level, only 0.01 percent of the energy is available to organisms.

Explain that the amount of energy available in food is measured in kilocalories, which are expressed as Calories with the letter C. Ask students, “If the producers in a food pyramid contain 500,000 C of food energy, how much energy will be available to the first-level consumers?” 50,000 C will be available, because that is 10 percent of 500,000.

Distribute the Energy Pyramid Worksheet (S-B-2-3_ Energy Pyramid Worksheet.docx) and have students work independently to complete the questions.

Small Group Activity: Energy Pyramid

Divide students into groups of three and give each group one piece of graph paper (S-B-2-3_Graph Paper.doc) and a pair of scissors. Assign students the roles of producer, primary consumer, and secondary consumer. Have the producer cut a 10 by 10 block of squares from the graph paper. This block represents the total amount of food energy stored in the producer. Have the producer cut a row of 10 squares from the block and hand it to the primary consumer. The 10 squares represent the amount of energy that is transferred up the food chain to the primary consumer. Have the primary consumer cut 1 square from the row and pass it to the secondary consumer. Ask students what percentage of the original energy stored in the producer reaches the second level consumer (i.e., only 1 out of 100 squares, or 1 percent).

Explain that the law of conservation of energy states that energy cannot be created or destroyed; it can only transform into different forms of energy. For example, chemical energy from food can become kinetic (i.e., movement) energy in an organism.

Then, provide each group of students with diagrams of matter cycles (i.e., nitrogen cycle, carbon cycle, and water cycle) (S-B-2-3_Matter Cycles.docx). Have students examine the diagrams to trace the flow of matter through the ecosystem. Have each group create a Venn diagram comparing the flow of energy through ecosystems with the cycling of matter.

Students should infer that matter is recycled through an ecosystem, while energy flows through a system and changes form as it moves. Also, energy flows through feeding relationships of organisms, while matter cycles through living and nonliving parts of an ecosystem. The similarity students should find is that both energy and matter are conserved in an ecosystem.

Define the term biogeochemical cycles. Explain that while energy does not cycle through an ecosystem, chemicals are recycled. Chemical elements, such as nitrogen, carbon, and compounds, such as water, cycle through the organisms and enter into the atmosphere, the oceans, and even rocks. Since these chemicals cycle through both the biological and the geological world, we call the cycles biogeochemical cycles.

To conclude the lesson, hold a whole-class discussion about how conservation of energy and conservation of matter affect each of us.

Extension:

• At the beginning of the lesson, students who need opportunities for additional learning may benefit from using one of the food webs from Lesson 1 as they try to construct a long food chain.

• Also, provide students who may need opportunities for additional learning with a diagram of the water cycle (which is less complex and should be the most familiar) during the Venn diagram activity. Ask students to describe to you the various stages of the cycling of water in complete sentences.

• Have students performing above and beyond the standards read about biomagnifications in ecosystems and give a short oral presentation to the class (see the “Biomagnification: How DDT becomes concentrated as it passes through a food chain” Web site in Related Resources).