Food Webs and Energy Flow
Every living thing needs energy. But energy does not appear from nowhere — it travels through ecosystems in predictable patterns. This lesson explains how food chains, food webs and energy pyramids show where energy goes and why most of it never reaches the top.
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Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
Q1: Where does the energy in a kangaroo's muscles come from? Trace it back as far as you can.
Many students think energy comes from the animal that was eaten, but that is only part of the story. Write your best chain of sources before reading on.
Q2: Why do ecosystems need more grass than lions?
Think about how much food a lion needs compared to how much grass grows. This prepares you for the energy pyramid later in the lesson.
Know
- food chains show the flow of energy from one organism to another
- food webs are made of many connected food chains
- energy pyramids show that energy is lost at each level
Understand
- the arrow in a food chain shows the direction of energy flow
- most energy is lost as heat, movement and waste at each level
- food webs are more realistic than food chains because organisms eat many things
Do
- construct a simple food chain and food web
- interpret an energy pyramid
- explain why there are fewer top predators than producers
Food chains show who eats whom
A food chain is a simple way to trace energy from its source through an ecosystem by following who eats whom.
A food chain always starts with a producer, usually a plant or algae, because producers capture energy from sunlight. The next organism is a primary consumer that eats the producer. The next is a secondary consumer that eats the primary consumer. The chain can keep going with tertiary consumers at the top.
The arrow in a food chain points from the organism that is eaten to the organism that eats it. This shows the direction of energy flow. For example, in grass → grasshopper → lizard → hawk, the arrow points from grass to grasshopper because energy moves from the grass into the grasshopper when it is eaten.
Food webs are interconnected food chains
Real ecosystems are more complex than a single food chain. A food web shows the many feeding relationships that exist in the same place.
A grasshopper does not only eat grass. It might eat leaves from several plants. A lizard does not only eat grasshoppers. It might also eat caterpillars or spiders. A hawk might eat lizards, small birds or mice. When you draw all these connections together, you get a food web.
Food webs are more useful than food chains because they show what happens when one species changes. If a drought kills off the grasshoppers, a lizard that also eats caterpillars might survive. In a simple food chain, the lizard would have no other food source and would die out. Food webs show that ecosystems have some backup connections.
Energy pyramids show energy loss at each level
Energy does not cycle through a food chain. It flows in one direction, and most of it is lost before it reaches the next level.
When a grasshopper eats grass, it does not turn all the grass into grasshopper body mass. Some energy is used for movement, some is lost as heat, and some passes out as waste. On average, only about 10 percent of the energy from one level transfers to the next. This means producers must capture a huge amount of energy to support just a few top predators.
An energy pyramid shows this visually. The bottom level is wide because producers capture the most energy. Each level above is narrower because less energy is available. This is why there is more grass than grasshoppers, more grasshoppers than lizards, and more lizards than hawks.
Misconceptions to Fix
Wrong: "Energy is recycled in a food chain."
Right: Energy flows through an ecosystem and is lost as heat at each level. Matter is recycled, but energy is not.
Wrong: "The arrow points from predator to prey."
Right: The arrow shows energy flow from prey to predator. It points to the organism that receives the energy.
Right: Producers are at the bottom of the energy pyramid. Top predators are at the top because they receive the least energy.
Right: Only about 10 percent of energy transfers to the next level. The rest is lost as heat, movement and waste.
Diagram 2: Energy Pyramid
Stacked pyramid showing energy levels from producers at the wide base to top predators at the narrow peak, with approximate percentages labelled at each level.
Food chains
Show who eats whom. Arrows point from prey to predator, showing energy flow.
Food webs
Networks of connected food chains. More realistic than a single chain.
Energy pyramids
Show energy loss at each level. Only about 10 percent transfers to the next level.
Common error
Drawing arrows the wrong way. Remember: arrow points to who gets the energy.
Activities
Activity 1: Build a Simple Australian Food Web
Use the organisms below to build a simple Australian food web. Draw arrows showing energy flow and label each organism with its trophic level.
Organisms: grass, grasshopper, lizard, hawk, caterpillar, magpie, nectar, bee
Activity 2: Evaluate the Claim
A student says: "If there are more kangaroos, there will automatically be more dingoes." Use the Claim-Evidence-Reasoning frame below to evaluate this claim.
Claim-Evidence-Reasoning Frame
Claim: State whether the student is correct or incorrect.
Evidence: Use facts from the lesson about food webs and energy flow.
Reasoning: Explain how the evidence supports your claim.