Grade by Grade
B. Life Systems · Interactions in the Environment

Energy Flow in Food Chains

B2.4 A1.5 A3.2About 50 minutes
Learning goals
  • I can describe how energy is transferred from the Sun through producers and consumers in a food chain.
  • I can explain why only about 10% of the energy at one trophic level passes to the next.
  • I can draw and read an energy pyramid and calculate the energy at each level.
  • I can explain why food chains are short and why there are fewer top predators.

Words to know

energy transfer
The movement of energy from one organism to another when one eats the other.
food chain
A single path showing who eats whom, with arrows pointing in the direction energy moves.
energy pyramid
A diagram that shows how much energy is at each trophic level. It is widest at the bottom (producers).
10% rule
A rough guide: only about 10% of the energy at one trophic level is passed on to the next level.
kilojoule (kJ)
A unit for measuring energy. Food labels in Canada show energy in Calories, and 1 Calorie is about 4.2 kJ.
top predator
A carnivore at the top of a food chain that is not usually eaten by other animals, such as a wolf.

Where does your energy come from?

Think about your breakfast. Toast came from wheat. Milk came from a cow that ate grass and hay. Eggs came from a hen that ate grain. Follow any food back far enough, and you always reach a plant, and the plant got its energy from the Sun.

You are running on sunlight! Today you will follow energy as it moves through a food chain, and discover why most of it never makes it to the top.

Following the arrows

In a food chain, the arrows point in the direction energy moves: from the food to the eater.

Sun → clover → snowshoe hare → Canada lynx

Read the arrow as "is eaten by" or "gives energy to". The clover gives energy to the hare, and the hare gives energy to the lynx.

  1. Producers capture light energy and store it as chemical energy in sugar.
  2. When a primary consumer eats the producer, it takes in some of that stored energy.
  3. When a secondary consumer eats the primary consumer, some of the energy moves up again.

Energy flows one way: from the Sun, up the chain. It does not cycle back to the Sun.

A brown snowshoe hare eating white clover flowers in a forest clearing.
Energy flows from the Sun to the clover to this hare. Only a small part of the energy in the clover ends up stored in the hare's body. AI-generated image

Where does the energy go?

A hare eats a lot of clover. But only a small part of that energy ends up stored in the hare's body, where a lynx could get it. The rest:

  • is used for life processes: moving, breathing, keeping warm, growing, healing. Cellular respiration releases this energy, and most of it ends up as heat that escapes into the environment.
  • is not eaten: the hare doesn't eat roots, and some clover is never found.
  • is not digested: some passes out in droppings (where decomposers get it).

Scientists use a rough guide called the 10% rule: only about 10% of the energy at one level is passed to the next level. The real number varies (it can be a bit lower or higher), but 10% is a good estimate.

grass: 10 000 kJgrasshoppers: 1 000 kJfrogs: 100 kJsnakes: 10 kJenergy pyramid (not to scale)
An energy pyramid. Each level holds about 10% of the energy of the level below it. Bars are drawn to show the idea, not exact sizes.
Worked example

Let's calculate energy up a food chain

In a meadow, the grass stores 10 000 kJ of energy. Use the 10% rule to find the energy that reaches grasshoppers, frogs, snakes and hawks in this chain: grass → grasshoppers → frogs → snakes → hawks.

  1. Grasshoppers: 10% of 10 000 kJ = 10 000 ÷ 10 = 1 000 kJ.
  2. Frogs: 10% of 1 000 kJ = 100 kJ.
  3. Snakes: 10% of 100 kJ = 10 kJ.
  4. Hawks: 10% of 10 kJ = 1 kJ.
  5. Check the loss: from grass to hawks, 10 000 kJ became 1 kJ. That means 9 999 kJ was used, lost as heat, not eaten or not digested along the way.

Grasshoppers 1 000 kJ, frogs 100 kJ, snakes 10 kJ, hawks 1 kJ. Each step keeps only about one tenth of the energy.

Just for fun

Funny but true: a hawk at the top of the meadow chain gets only 1 kJ for every 10 000 kJ the grass stored. If hawks could vote, the 10% rule would never win an election.

What the energy pyramid explains

  • Food chains are short. After 4 or 5 levels, there is too little energy left to support another level. That is why there is no animal that regularly hunts wolves as its main food.
  • There are fewer top predators. It takes a lot of grass to feed many hares, and many hares to feed one lynx. A forest might have thousands of hares but only a few lynx.
  • Top predators need large areas. A wolf pack may need hundreds of square kilometres to find enough prey.
  • Eating lower on the food chain uses less energy overall. Food from plants (like beans, grains and vegetables) usually takes less land and energy to produce than the same amount of food energy from meat, because there is one less step where energy is lost. People make food choices for many reasons, including culture, health and where they live, so this is one factor among many.
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A pyramid of numbers: in this model, 30 plants feed 6 rabbits, which feed 1 fox. (Real numbers are much bigger.)

Math connection

Finding 10% is the same as dividing by 10: move each digit one place to the right. 10% of 4 500 kJ = 450 kJ. To go down the pyramid (find the level below), multiply by 10: if hawks get 3 kJ, the snakes below had about 30 kJ.

Hands-on activity

The 10% energy relay

You need: a kitchen scale, about 1 kg of dry rice or dry pasta, 4 bowls, sticky notes, a pencil.

  1. Label the bowls: producers, primary consumers, secondary consumers, tertiary consumers.
  2. Put 1 000 g of rice in the producers bowl. Each gram stands for 10 kJ of energy (so this bowl holds 10 000 kJ).
  3. Pass 10% of the rice (100 g) to the primary consumers bowl. Set the other 900 g aside in a pile labelled used for life processes / lost as heat / not eaten.
  4. From the primary consumers bowl, pass 10% (10 g) to the secondary consumers. Add the other 90 g to the lost pile.
  5. Pass 10% of that (1 g) to the tertiary consumers. How much rice is that? Compare it with the lost pile.
  6. Record the grams and the kJ at each level in a table, then draw your own energy pyramid.

Questions: Could you add a fifth level? How much rice would it get? What does this tell you about why food chains are short?

What to look for: a correct table (1 000 g, 100 g, 10 g, 1 g; that is 10 000 kJ, 1 000 kJ, 100 kJ, 10 kJ) and an explanation that a fifth level would get only 0.1 g, which is too little to support a population.

Your turn

1

In a food chain, which way do the arrows point?

2

Where does almost all the energy in a food chain come from?

3

Give three reasons why a hare does not pass all the energy it eats on to a lynx.

4

If producers store 20 000 kJ, about how much energy reaches primary consumers?

5

Continue: how much reaches secondary consumers? tertiary consumers?

6

Why is an energy pyramid widest at the bottom?

7

Why are there fewer lynx than hares in a forest?

More practice: warm-up

1

What is 10% of 5 000 kJ?

2

What is 10% of 80 kJ?

3

Fill in: Sun → ______ → herbivore → carnivore.

4

Does energy cycle back to the Sun?

5

Most energy lost at each level ends up as ______.

More practice: on your own

1

Algae in a lake store 50 000 kJ. Chain: algae → zooplankton → minnows → walleye. How much energy reaches the walleye?

2

A hawk at the fourth level gets 6 kJ. About how much energy was in the producers at the first level?

3

Explain why a food chain rarely has more than 4 or 5 levels.

4

From grass with 10 000 kJ to hawks with 1 kJ, what percent of the original energy reached the hawks?

5

Explain why more people can be fed from a field of grain if they eat the grain than if the grain is fed to cattle and the people eat the beef.

Challenge

1

A biologist estimates that the producers in a meadow store 400 000 kJ per year. Each fox needs about 400 kJ per year from this meadow (in this model). Foxes are third-level (secondary) consumers here. How many foxes can the meadow support?

2

Energy flows, but matter cycles. Use the energy pyramid to explain what "energy flows" means.

3

A student says, "Bigger animals are always at the top of the pyramid." Give a counterexample and explain.

Check your understanding

Quiz

1.

In the food chain clover → hare → lynx, the arrows show...

2.

About how much energy passes from one trophic level to the next?

3.

Most of the energy that is not passed on is...

4.

Producers store 30 000 kJ. About how much reaches secondary consumers?

5.

Why are there usually only a few top predators in an ecosystem?

6.

Which level of an energy pyramid has the most energy?

For parents and teachers

Big idea: energy flows one way through a food chain, from the Sun to producers to consumers. Only about 10% passes to each next level; the rest is used for life and lost as heat. This explains why food chains are short and top predators are few.

How to help at home:

  • Do the rice relay together and let your child do the dividing.
  • At meals, trace foods back to the Sun: cheese ← milk ← cow ← grass ← Sun.
  • Practise 10% mentally: 10% of 350? of 4 200? of 75?

Common difficulties:

  • Drawing food-chain arrows backwards (pointing to the food instead of to the eater).
  • Thinking energy is destroyed. It is changed, mostly into heat, which living things can't use again.
  • Assuming big animals are always top predators.
  • Treating 10% as exact. It is an estimate; real values vary.