Food Chains and Food Web

Changes in the Population of Living Things

This science revision note aims to help your child

– Understand food chains (What does the arrow,—>, mean?)

– Predict what happens to the population of organisms.

– proper scientific keywords to explain the concepts clearly.


But before you read on, you might want to download this entire revision notes in PDF format to print it out for your child, or to read it later.

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The food chain below shows the food relationship between plant P, animal Q and animal R.

A disease outbreak killed many animals Q. The disease does not affect plant P and animal R. The line graph below shows the change in the population of plant P and animal R over time.


Which line (1 or 2) shows the population change of Animal R? Explain your answer.



Identify the relationship between Animals Q and R.

To do this, examine the food chain.

The arrows in the food chain show the direction that which energy is transferred. Since the arrow is pointing from Q to R, energy is transferred from Q to R. This means that Animal R feeds on Animal Q.


Note: These are different ways to describe the food relationship between Animals R and Q.

Animal R feeds on Animal Q.

Animal Q is eaten by Animal R.

Animal Q is the prey of Animal R.

Animal R is the predator of Animal Q.

You can use any one of the above to answer this question.



Explain how Animal R will be affected when Animal Q decreases.

Animal Q decreases —-> Animal R will have less food to feed on —> Population of animal R will decrease.



To do so, examine the line graph and identify what each line shows.

  • Line 1 shows an increase in population
  • Line 2 shows a decrease in population

Hence, Line 2 shows the population change in the population of Animal R.


FINALLY, put your answer together

Question: Which line (1 or 2) shows the population change of Animal R? Explain your answer.

Answer: Line 2. Animal R feeds on Animal Q. Hence, when the population of Animal Q decreases, Animal R have less food to feed on, causing some of them to die of starvation. The population of Animal R decreases.



Common mistakes


Wrong answers: Animal Q feeds on Animal R.

The direction of the arrows in the food chain does not mean that P eats Q and that Q eats R. Instead, the arrows show the direction of the flow of energy. Energy flows from the prey to the predator when the prey is eaten by the predator. Hence, this means that Q eats P and that R eats Q.



Final note 

Wrong answer: Animal Q is the predator of plant P.

The word “predator” means an animal feeds on another animal.

In the statement above, P is a plant. Hence, you cannot write Q is the predator of the plant.


Correct answer: Q feeds on P or P is eaten by Q.

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✍️You can read the revision notes on living together and energy in food topics.


(Understand how examiners combine multiple topics into a single question. This is suitable for P6 students.)

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Andy Ling

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