Key Stage 3 science · Biology – Interactions and interdependencies
KS3 food webs and ecosystems: a guide for parents
An ecosystem is a community of living things and the environment they share, and the organisms in it depend on each other in many ways. In Key Stage 3 children use food webs to show how feeding links connect species, learn why insect pollination matters for the crops we eat, and find out how organisms change their environment and are changed by it, including how poisons can build up along a food chain.
Food webs and ecosystems, explained
Children have drawn food chains since primary school, but at Key Stage 3 they join several chains into a food web and use it to reason. A food web shows that most animals eat more than one thing and are eaten by more than one thing. The arrows point from the food to the feeder, showing the direction in which energy moves. Producers such as plants and algae make food by photosynthesis, and consumers eat them. Primary consumers eat producers, secondary consumers eat primary consumers and so on. Decomposers such as fungi and bacteria break down dead material and return nutrients to the soil.
Because the species are linked, a change in one affects others. If a food source declines, the animals that eat it may starve or switch to something else, which puts pressure on a different species. If a predator is removed, its prey may increase and eat more of the producers. Pupils practise predicting these knock-on effects from a diagram and explaining them in full sentences, which is a good test of their scientific reasoning.
Insect pollination is a key example of interdependence. Many flowering plants, including a large share of the fruit, vegetables and oil crops people eat, are pollinated by insects such as bees, hoverflies and butterflies. Apples, strawberries, beans and oilseed rape are common examples. Cereals such as wheat, rice and maize are pollinated by the wind or by their own pollen, so they do not depend on insects in the same way. Children discuss food security, meaning the ability to produce enough food reliably, and see why a fall in pollinator numbers, caused for example by habitat loss or some pesticides, could affect harvests and food prices.
The third idea is that organisms affect, and are affected by, their environment. Plants change the soil and shade the ground, animals disperse seeds, and humans change habitats on a large scale. Pupils also study accumulation of toxic materials. A poison that is not broken down or excreted, such as some pesticides and heavy metals, is taken in by small organisms at the start of the food chain. Each animal that eats many contaminated organisms collects the poison in its body, so the concentration builds up and is highest in animals at the top. This happened with some insecticides in the mid-twentieth century, which built up in birds of prey and harmed their breeding, and it is one reason why chemicals are tested and controlled today.
Many schools add fieldwork to this topic, for example sampling a school field with a square frame called a quadrat, or a pond with a net, to see which species live where and how conditions such as light and moisture shape the community.
Words your child will hear
- Ecosystem
- All the living things in an area and the non-living environment they interact with.
- Food web
- A diagram of the connected food chains in an ecosystem, with arrows pointing from food to feeder.
- Producer
- An organism that makes its own food by photosynthesis, such as grass or algae.
- Consumer
- An organism that gets its energy by eating other organisms.
- Decomposer
- Fungi and bacteria that break down dead material and return nutrients to the soil.
- Interdependence
- Organisms relying on each other, for example for food, pollination or shelter.
- Food security
- Having a reliable supply of enough food for everyone.
What the National Curriculum says
The programme of study for Key Stage 3 says:
- Pupils should be taught about the interdependence of organisms in an ecosystem, including food webs and insect pollinated crops
- Pupils should be taught about the importance of plant reproduction through insect pollination in human food security
- Pupils should be taught about how organisms affect, and are affected by, their environment, including the accumulation of toxic materials
Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.
Try it: explaining a build-up of poison
In a pond, algae are eaten by water fleas, water fleas are eaten by small fish and small fish are eaten by herons. A chemical that does not break down is washed into the pond. Explain why the herons are most at risk.
- Write the food chain with the arrows pointing from food to feeder: algae → water fleas → small fish → heron.
- Say how the poison enters. The algae take in the chemical from the water, and because it does not break down it stays inside them.
- Follow it along the chain. A water flea eats a very large number of algae, so it collects the chemical from all of them. Imagine each water flea eats 100 algae, each small fish eats 100 water fleas and a heron eats many fish.
- Explain the build-up. Each animal keeps the chemical it takes in, because it cannot get rid of it, so the amount in each body grows up the chain. With the imagined numbers, if one alga holds 1 unit of chemical, a water flea holds 100 × 1 = 100 units and a small fish holds 100 × 100 = 10,000 units. A heron that eats many fish holds more still.
- Link to the heron. The heron is at the top of the chain and receives the largest dose, so it is the most likely to be poisoned or to have fewer chicks.
The chemical is not broken down, so it builds up at every step of the food chain. Herons eat many contaminated fish, so they collect the highest amount and are most at risk.
Common mistakes and how to help
Drawing arrows from the eater to the food
It feels natural to point at the thing doing the eating: “the rabbit eats the grass” becomes an arrow from rabbit to grass.
What helps: The arrow shows where the energy goes. Energy moves from the grass to the rabbit, so the arrow points from grass to rabbit. Read each arrow as “is eaten by” or “gives energy to”.
Believing that removing one species only affects that species
Food chains drawn in a line make it look as though each link stands alone.
What helps: Use a web. If the caterpillars disappear, the birds that eat them lose food, and the plants that the caterpillars ate may grow better. Ask your child to trace at least two knock-on effects.
Thinking the top predator is the most numerous or the most important
Predators are the best known animals, so they appear to be at the centre of the story.
What helps: Numbers usually fall along the chain, as there is much less energy at each level, so predators are far fewer than their prey. The producers at the bottom, plants and algae, support almost everything else.
Believing that a toxic material gets weaker as it passes up the food chain
Children expect a poison to be diluted when it is passed from one creature to another.
What helps: If the animal cannot break the chemical down or get rid of it, each predator collects the amounts from all the prey it eats. The effect is concentration, not dilution.
How to help at home
- Build a food web of your garden or park. Spend 15 minutes looking under logs and stones or on plants, taking care not to harm anything, and note what you find. Then draw as many feeding links as you can, with the arrows pointing from food to feeder. Put creatures back where you found them.
- Spot the pollinators. On a warm day, watch a flowering plant for five minutes and count the insects that visit. Compare plants with different flower shapes and colours. Ask why the insect is doing this and what the plant gets in return.
- Sort the food shop. Look at the fruit and vegetables in a shop or at home. Which ones, like apples, strawberries and runner beans, usually need insect pollination? Which foods, like wheat, rice and maize, do not? Talk about what a bad year for bees could mean.
- Make space for wildlife. Leave a corner of the garden to grow wild, put out a shallow dish of water for insects with a stone for landing on, or plant flowers that bees like. Talk about who else benefits.
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