The shape of the lesson
By the end of the lesson, learners can
- Chains, webs and the roles in them: In the food web every arrow points from the organism that is eaten to the organism that eats it, which is the direction energy flows.
- Pyramids of numbers and biomass: Each block is one trophic level with the producers at the bottom.
- Why so little gets through: Extended.
- Arrows show energy flow: from the organism eaten to the organism that eats it.
- Trophic levels: producer, primary, secondary, tertiary, quaternary consumer.
- Biomass beats numbers because it allows for size; (Extended) energy beats both because it allows for time.
- Extended: about 10% passes on; the rest is lost as heat from respiration, movement, faeces, urine and uneaten parts.
One picture every idea returns to
A food chain shows the transfer of energy from one organism to the next, beginning with a producer; a food web is a network of interconnected food chains. A trophic level is an organism's position in a food chain, food web or ecological pyramid: producers, then primary, secondary, tertiary and quaternary consumers. Pyramids of numbers and biomass compare the levels. Extended: only a small fraction of the energy at one trophic level reaches the next, so food chains usually have fewer than five trophic levels.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Who eats whom, and how much gets through.. A food chain follows energy from a producer to the animals that eat it. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Chains, webs and the roles in them | In the food web every arrow points from the organism that is eaten to the organism that eats it, which is the direction energy flows. Project tab 2 of the lab (A woodland food web) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Pyramids of numbers and biomass | Each block is one trophic level with the producers at the bottom. Project tab 3 of the lab (Ecological pyramids for two food chains) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Why so little gets through | Extended. Project tab 4 of the lab (Energy efficiency: crops versus livestock) and let a student drive it while the class predicts. | Think-Pair-Share |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 22 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Why so little gets through, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Think-Pair-Share
Pose the question, give thirty seconds of silent thinking, then a minute to compare with a partner before a few pairs share with the class. The silent minute is the part that cannot be cut.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Arrows in a food chain show the direction of energy flow, so they point towards the eater: grass → rabbit means the rabbit eats the grass. | Drawing them the other way loses the mark. |
| A pyramid of biomass is better than numbers, but it is still a snapshot at one moment. | A fast-growing producer that is eaten quickly (such as plankton) can have a small standing biomass, so even biomass pyramids are occasionally inverted. |
| Energy is not lost "in" the food chain because animals are wasteful: most of it leaves as heat from respiration, which every organism must carry out to stay alive. | Return to Why so little gets through: ask a pair to state the correct version and show it on the lab before moving on. |
Food chains, webs and pyramids: worksheet
6 questions, 22 marks. Written in the style of the papers; no past paper question is reproduced.
Define each term: (a) producer, (b) consumer, (c) herbivore, (d) decomposer.
In a pond: algae are eaten by water fleas and by snails; water fleas are eaten by small fish; snails are eaten by ducks; small fish are eaten by pike and by ducks. (a) Draw the food web. (b) Write the longest food chain. (c) Name the trophic level of the duck in each chain it belongs to.
A fish species that eats water fleas is introduced into the pond in the previous question. It has no predators there. Predict and explain the effects on the water fleas, the algae and the pike.
A rose bush supports 2000 aphids, which are eaten by 40 ladybirds. Sketch the pyramid of numbers and the pyramid of biomass for this chain and explain why they have different shapes.
Extended. In a grassland, producers fix 20 000 kJ per m2 per year, primary consumers receive 2000 kJ and secondary consumers 180 kJ. (a) Calculate the percentage efficiency of energy transfer from primary to secondary consumers. (b) Give two ways energy is lost between these levels.
Extended. Explain why it is more energy efficient for humans to eat crop plants than to eat livestock that have been fed on crop plants, and give one advantage of a pyramid of energy for showing this.
Answer key and marking notes
- (a) An organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis.
- (b) An organism that gets its energy by feeding on other organisms.
- (c) An animal that gets its energy by eating plants.
- (d) An organism that gets its energy from dead or waste organic material.
- (a) algae → water flea, algae → snail, water flea → small fish, snail → duck, small fish → pike, small fish → duck (arrows towards the eater).
- (b) algae → water flea → small fish → pike (or → duck): four trophic levels.
- (c) In algae → snail → duck the duck is a secondary consumer.
- In algae → water flea → small fish → duck it is a tertiary consumer.
- Water fleas decrease, because they are eaten by the new fish as well as the small fish.
- Algae may increase, because fewer water fleas are eating them.
- Small fish have less food (more competition for water fleas), so their numbers may fall.
- Pike, which eat small fish, may then decrease too.
- Pyramid of numbers: a narrow bar for 1 rose bush, a wide bar for 2000 aphids and a medium bar for 40 ladybirds (not a pyramid shape).
- Pyramid of biomass: a wide bar for the rose bush, narrower for aphids and narrowest for ladybirds (a true pyramid).
- Numbers ignore size: one large bush supports many small aphids; biomass takes the mass of each level into account.
- (a) 180 ÷ 2000 × 100 = 9%.
- (b) Heat released by respiration (including energy used for movement) in the primary consumers.
- Energy in faeces and urine, or in parts such as bones and fur that are not eaten (any two).
- Feeding crops to livestock adds an extra trophic level to the food chain.
- At each transfer only about 10% of the energy becomes biomass in the next level; most is lost as heat from respiration, movement, faeces and urine.
- So eating the crop directly gives humans about ten times as much energy from the same area of land.
- A pyramid of energy shows energy flow per m2 per year, is always pyramid shaped and allows a fair comparison of the levels.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
The diagram shows a seashore food web. How many food chains include the gull?
- A2
- B1
- C3
- D4
In the seashore food web, what is the trophic level of the seal?
- ATertiary consumer
- BSecondary consumer
- CPrimary consumer
- DQuaternary consumer
In the seashore food web, the small fish are overharvested by fishing boats. Which is the most likely immediate effect?
- AZooplankton increase and seals decrease
- BZooplankton decrease and seals increase
- CMussels decrease and crabs increase
- DPhytoplankton increase and gulls increase
Which organism is a decomposer?
- AA fungus feeding on a fallen log
- BA caterpillar eating a living leaf
- CA moss photosynthesising on a wall
- DA thrush eating snails from a hedge
Grey squirrels were introduced to Britain from North America. They compete with native red squirrels for nuts and carry a disease that kills red squirrels. What is the most likely effect?
- AThe red squirrel population falls
- BThe red squirrel population rises
- CThe number of nut-bearing trees falls to zero
- DGrey squirrels become producers in the web
Use the pyramid of energy for a pond. What is the percentage efficiency of energy transfer from water fleas to small fish?
- A11.4%
- B16.1%
- C5.6%
- D1.8%
What is an advantage of a pyramid of energy over a pyramid of biomass?
- AIt shows energy flow over time, so it is never inverted
- BIt counts the number of individuals at each trophic level
- CIt can be drawn from a single sample taken on one day
- DIt ignores how quickly organisms grow and reproduce
One hectare of wheat can feed far more people directly than if the wheat is fed to pigs and the people eat pork. Which explanation is correct?
- APigs lose most of the energy in respiration, movement and faeces, adding a level of loss
- BPigs convert wheat energy into pork with almost 100% efficiency
- CWheat contains no energy until it has been eaten by animals
- DPeople cannot digest wheat, so it must pass through pigs
Quiz key
Two chains reach the gull: phytoplankton → zooplankton → small fish → gull, and phytoplankton → mussels → crabs → gull. 1 misses one of these routes. 3 counts the chain ending at the seal, which does not include the gull. 4 counts some links twice.
Phytoplankton (producer) → zooplankton (primary) → small fish (secondary) → seal (tertiary consumer). Secondary consumer is the small fish's level. Primary consumers eat producers directly. A quaternary consumer would need one more step above the seal.
With fewer small fish eating them, zooplankton increase; seals lose their only food in this web, so they decrease. Zooplankton would not fall because their predator is removed, and seals cannot increase with less food. Mussels and crabs are not directly linked to the small fish. More zooplankton would eat more phytoplankton, and gulls lose one food source, so neither is likely to increase.
The fungus gets its energy from dead organic material (the log), so it is a decomposer. The caterpillar eats living plant tissue, so it is a primary consumer (herbivore). The moss makes its own food by photosynthesis, so it is a producer. The thrush eats other animals, so it is a carnivore.
Competition for the same food and a disease to which red squirrels have no resistance both reduce the red squirrel population. It would not rise with extra competition and disease. Trees are not wiped out just because a different squirrel eats the nuts. Squirrels are consumers; an introduced animal cannot become a producer.
1600 ÷ 14 000 × 100 = 11.4%. 16.1% is the algae to water fleas transfer (14 000 ÷ 87 000). 5.6% is the small fish to pike transfer (90 ÷ 1600). 1.8% divides the small fish by the algae (1600 ÷ 87 000), skipping a level.
A pyramid of energy shows the energy passing through each level per unit area per year, so it allows for fast-growing organisms and is always pyramid shaped. Counting individuals is a pyramid of numbers. A single-day sample describes a biomass snapshot, which is the weakness, not the strength. Taking account of growth and reproduction rate is exactly what the energy pyramid does, so ignoring it is wrong.
Only about 10% of the energy in the wheat becomes pork; the rest is lost as heat from respiration, in movement, faeces and urine, so eating the wheat directly supplies about ten times more energy. Transfers are never close to 100% efficient. Wheat stores chemical energy from photosynthesis. People can digest wheat products such as bread.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.