--- title: "Where does the energy go?" subtitle: "Year 7 Biology · Food webs and ecosystems · Lesson 5" --- # Where does the energy go? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** A field of grass can feed many grasshoppers, which feed fewer lizards, which feed a couple of kookaburras, which feed perhaps one eagle. Why the shrinking numbers? :::teacher-note **In one sentence:** Students trace energy through at least three trophic levels, explain that energy is lost at each level (about 90%), and distinguish one-way energy flow from matter cycling (AC9S7U02, node 2, a hinge node). **Students should be able to say:** I can trace the direction of energy through at least three levels of a food web. I can explain that energy is lost at each level. I can explain the difference between energy flow and matter cycling. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **Grass captures 10 000 kJ of energy from sunlight. A grasshopper eats grass, a lizard eats grasshoppers, a kookaburra eats lizards. About how much of that energy reaches the kookaburra?** ☐ Almost all of it, 10 000 kJ ☐ About 10 kJ ☐ It goes round and comes back to the grass ☐ About half My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** About 10 kJ. Roughly 90% is used or lost as heat at each step, so only about 10% passes on: 10 000 → 1 000 → 100 → 10. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: flow, loss and cycling :::keep-together ### Down the levels **A lizard gets 100 kJ from the grasshoppers it eats. About how much reaches a kookaburra that eats the lizard, in kJ?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 10 About 10% passes on: 100 × 0.1 = 10 kJ. ::: :::keep-together ### Where it goes **Where does the other 90% go at each step?** Groups: Not passed to the next level, Passed to the next level - Heat given off by the animal's body: {{field-15}} - Energy used for moving, breathing and keeping warm: {{field-15}} - Parts not eaten (bones, fur, roots): {{field-15}} - Energy stored in the eaten animal's body: {{field-15}} ::: :::teacher-note **Answer:** Heat given off by the animal's body → Not passed to the next level; Energy used for moving, breathing and keeping warm → Not passed to the next level; Parts not eaten (bones, fur, roots) → Not passed to the next level; Energy stored in the eaten animal's body → Passed to the next level ::: :::keep-together ### Energy and matter **Which statement is true?** ☐ Energy is recycled by decomposers; matter is lost ☐ Energy and matter are both recycled ☐ Energy flows one way through a web and is lost as heat; matter (the atoms) is recycled by decomposers ::: :::teacher-note **Answer:** Energy flows one way through a web and is lost as heat; matter (the atoms) is recycled by decomposers That's why every food web needs a constant supply of sunlight, but no new atoms. ::: ## Mix it up :::keep-together **Looking back at food webs: in the chain grass → grasshopper → lizard → kookaburra, which is the secondary consumer?** ☐ The grasshopper ☐ The grass ☐ The kookaburra ☐ The lizard ::: :::teacher-note **Answer:** The lizard A secondary consumer eats a primary consumer. The lizard eats the grasshopper, which eats the grass. ::: :::typical ## Elaborate: why so few eagles? :::keep-together **Why are there far fewer wedge-tailed eagles than grasshoppers in an ecosystem?** ☐ Eagles have fewer babies ☐ So little energy reaches the top of the food chain that it can only support a few large predators ☐ Grasshoppers are easier to see ::: :::teacher-note **Answer:** So little energy reaches the top of the food chain that it can only support a few large predators Each level up gets about a tenth of the energy, so it can support far fewer organisms. ::: ::: :::wider ## Elaborate: a desert and a rainforest :::keep-together **A rainforest's plants capture far more energy each year than a desert's. What would you predict about their food webs?** ☐ They would be the same size ☐ The rainforest can support more organisms and more levels of consumers ☐ The desert would have more top predators ::: :::teacher-note **Answer:** The rainforest can support more organisms and more levels of consumers Energy captured by producers sets the size of everything above them. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - **Energy** enters as sunlight captured by producers and **flows one way** along the arrows. - At each step only about **10%** is passed on; about **90%** is used for living or lost as **heat**. - That's why there are **few top predators**. - **Matter cycles**: decomposers return atoms to the soil and air, and producers use them again. Energy does not cycle. ## Exit ticket :::keep-together **Why does a food chain rarely have more than four or five levels?** {{lines-3}} ::: :::teacher-note **Answer:** Only about 10% of the energy passes to each level, so after four or five steps there is too little left to support another population. :::