When one prey population falls, check whether its predator has another food source in the food web. A second prey may help the predator survive, so the web alone usually cannot tell you exactly how much the predator population will change.
The MOE Primary Science syllabus covers feeding relationships and energy pathways. This example shows how to make a prediction that uses all the relevant arrows.
- Read each arrow from food to eater.
- List every prey shown for the predator.
- Explain what changes when one prey declines.
- Use the available evidence to qualify the prediction.
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Read one arrow at a time
Imagine a simple food web where grass is eaten by grasshoppers and caterpillars. Both insects are eaten by the same kind of bird. Draw an arrow from grass to each insect, then from each insect to the bird. Each arrow shows the direction in which energy passes.
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Worked example | Fewer grasshoppers
If the number of grasshoppers falls, the birds have fewer grasshoppers available as food. The birds can still eat caterpillars. Their numbers may fall if the remaining food is insufficient. The web does not give the number of caterpillars available or how much food the birds need.
For a short written answer, name both feeding links. Explain that the birds lose some of one food source and can still feed on the other. Avoid predicting that all birds will disappear unless the question supplies evidence for that conclusion.
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Add evidence and refine the prediction
Suppose the question also says caterpillars are scarce. The prediction that the birds may decline is now stronger because both prey sources are limited. If the question instead says caterpillars are plentiful and the birds feed on them readily, the birds may be less affected.
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Frequently asked questions
Does an arrow from caterpillar to bird mean the caterpillar eats the bird?
The arrow points towards the eater. The bird eats the caterpillar.
What can a parent ask during revision?
Ask the child to point to all arrows ending at the bird. Then remove one arrow on a sketch and ask what food remains.