Example 1: a food chain
Write a food chain for: wheat is eaten by mice; mice are eaten by owls.
- Start with the producer: wheat.
- Arrows point to the eater.
- wheat → mouse → owl.
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IB DemystifiedMYP Sciences
Ecosystems, food chains and food webs
An owl hunting in a wheat field depends on mice, which depend on wheat, which depends on sunlight. Every living thing is linked to others through what it eats and what eats it, so a change to one species can ripple through a whole ecosystem.
Recommended for MYP 1 · About 3 lessons · Criteria A, B, C and D
On this page
- Learning objectives
- Before you start
- Key vocabulary
- Understanding the ideas
- Reading a food web
- Food webs in the real world
- Worked examples
- Assessment tips
- Check your understanding
- Practice questions
- Investigation
- Criterion-linked questions
- Challenge questions
- Topic check
- Review your mistakes
- Your progress
Learning objectives
By the end of this topic you should be able to:
- identify producers, consumers, predators, prey and decomposers
- construct and read food chains and food webs
- describe how energy flows through a food chain
- predict the effects of changes in a food web
- analyse predator–prey data and plan fair tests
- discuss pesticides and natural pest control
Before you start
You will use these skills. If any feel shaky, review them first.
- characteristics and classification of living things (see Characteristics and classification)
- plants need light to make food
- reading tables and patterns
Key vocabulary
- Ecosystem
- All the living things in an area and their surroundings.
- Producer
- An organism that makes its own food, usually a plant.
- Consumer
- An organism that eats other organisms.
- Predator / prey
- An animal that hunts / an animal that is hunted.
- Food web
- A diagram of many linked food chains.
- Decomposer
- An organism that breaks down dead material and waste.
Understanding the ideas
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What is it?
Every ecosystem has producers (plants), which capture energy from sunlight, and consumers, which get energy by eating. Herbivores eat plants, carnivores eat animals and omnivores eat both. Decomposers recycle nutrients from dead material.
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Why does it happen?
A food chain shows how energy passes from one organism to the next; arrows point from the food to the eater. Most animals eat more than one thing, so food chains link together into food webs. Energy is lost at every step, so food chains are short and top predators are few.
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How do we know?
Ecologists study food webs by watching animals, counting populations over many years, and even examining owl pellets to see what owls have eaten.
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Why does it matter?
Changes such as pesticides, hunting or disease can affect many species at once. Using natural predators, such as ladybirds and owls, can control pests without poisons.
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What does it connect to?
Food webs link to classification, nutrient cycles, photosynthesis and human impacts on ecosystems later in the course.
Reading a food web
- Arrows point from the food to the eater (the direction energy flows).
- Producer → primary consumer (herbivore) → secondary consumer → tertiary consumer.
- If a prey species decreases, its predators may decrease too.
- If a predator is removed, its prey may increase, which can then affect their food.
- Animals with several food sources are less affected by the loss of one.
Food webs in the real world
Farmers in many countries put up barn owl boxes to control rats without poison. Losing vultures in South Asia, after they were poisoned by a medicine given to cattle, left carcasses uneaten and caused disease to spread. Ladybirds are sold to gardeners to eat aphids.
Worked examples
Example 2: predicting a change
In Figure 1, what would happen to grasshoppers if frogs disappeared?
- Frogs eat grasshoppers.
- With fewer predators, grasshoppers would probably increase.
- More grasshoppers might then eat more grass.
Assessment tips
Food web questions often ask you to read diagrams and predict changes. Expect to:
- Identify producers, consumers, predators and prey.
- Write food chains from webs, with arrows the right way round.
- Predict and explain the effects of changes.
- Analyse population data and plan investigations.
Common mistakes: drawing arrows from the eater to the food; forgetting that animals may have several food sources; saying energy is recycled (nutrients are recycled, energy is not); and ignoring knock-on effects.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Investigation: what did the owl eat?
Partially guided investigation · about 45 minutes · pairs
- Research question
- What does a barn owl eat, based on the bones found in its pellets?
- Scientific background
- Owls swallow small animals whole and cough up pellets of fur and bones they cannot digest. The bones show which animals the owl ate.
- Hypothesis
- Write your own prediction, with a scientific justification.
- Variables
- Identify your independent, dependent and control variables, and explain how you will control them.
- Apparatus
- Sterilised owl pellets (bought from a supplier), tweezers, cocktail sticks, paper plates, hand lens, bone identification chart, gloves.
- Method
- Put on gloves and gently pull the pellet apart on a paper plate.
- Separate the bones from the fur.
- Use the chart to identify skulls and jaws (mouse, shrew, bird, frog).
- Count how many of each prey animal the pellet contains and combine class results.
Safety. Use only sterilised pellets, wear gloves, keep hands away from your face, and wash hands thoroughly afterwards.
Then evaluate: how could you use class results to draw part of the owl's food web, and how representative is one pellet?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
| Skill | Correct | Status |
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