Primary Science


Interactions Within the Environment (Let’s Attempt 2023 WA2 from MGS Q2, Q12)

In the realm of Primary and PSLE Science, grasping the concept of Interactions Within the Environment is crucial. These interactions form the foundation of ecological understanding and are important to help your child excel in the Science exams.

In this article, we’ll be going through essential questions: 2023 WA2 Methodist Girl’s School Weighted Assessment Paper, Question 2 & 12.

But before that, let’s get into something fun, shall we? Let’s see how many Blublu you can spot in this picture (Answer is provided at the end of this article)!


Read more: Must-Know Definitions of a Habitat: A Comprehensive Guide for PSLE Science

Topic: “Interactions Within the Environment”

2003 Methodist Girls’ School WA2 (Question 2)

Q2. Kamal studied the food relationships among four organisms, P, Q, R and S, living in the same habitat. He wrote some notes in his Science journal as shown below.

  • When the population size of Organism S decrease, population of Organism Q will increase.
  • When Organism P move out of the habitat, population size of Organism S will increase.
  • When a disease wipes out Organism R, the population size of the other three organisms will eventually decrease


Based on Kamal’s notes, which of the following food chains is correct?

1) P -> S -> Q -> R

2) P -> Q -> S -> R

3) Q -> S -> R -> P

4) R -> Q -> S -> P

Let’s reveal the answer to this “Interactions within the Environment” question!

To tackle food chain and food web questions, we must be clear on the relationships between the organisms. We need to identify which organism is the:

1. Prey (animal eaten by another animal)

2. Predator (animal which eats other animals)

3. Producer (plant)

Then, we need to draw it out. Annotation is key!

Based on the first statement “when the population size of animal S decreases, population of organism Q will increase”- this tells us that animal S is the predator of organism Q.

So Q is eaten by S.

Q → S

Remember that arrow → represents “is eaten by. Start from the end of the arrow. Draw a pacman if you forget!


Statement 2 says “when organism P moves out of the habitat, population size of organism S will increase”.

This means that when there are less P, there will be more S.

Same thing, P is the predator of S.

So S is eaten by P.

S → P


When you have determined which is the prey, predator and producer, let’s combine the first and second statements.

It will look like:

Q → S → P

P is the apex predator in this food chain.

Statement 3 states that “when a disease wipes out organism R, the population size of the other three organisms will eventually decrease”.

This means that R is a producer. When a producer decreases in population size, all the other organisms will be directly and indirectly affected! Producers are always at the start of the food chain.

Just like how when grass decreases in population size, the cows that eat grass will decrease, then the wolves who eat cows will decrease, followed by the bears who eat the wolves will decrease in population size as well. Everybody is affected 🙁

Combining all three statements will look like:

R → Q → S → P

So the answer is option 4!

We assume that Q is a herbivore, as there is no information on Q eating another animal.

Food chain and food webs are very difficult to “imagine” in your mind’s eye. So annotating out your thoughts are key!

Your paper should look like this:


2003 Methodist Girls’ School WA2 (Question 12)

Now let’s look at Q12 from the same MGS paper:

12. Jane visited an aquarium and saw a sign showing a food web of organisms in a tank as shown below.


A guide told her that a recent disease outbreak caused the population of organism R to decrease significantly.

(a) Identify the two organisms that would be directly affected when organism R decreased significantly. Explain your answer. (2 marks)

Same thing, we must annotate out the relationships between the organisms.

Let’s use smiley faces, sad faces, and increasing/ decreasing arrows to illustrate the changes.


Answer: Organisms Q and U.

When organism R decreases, U will decrease as there is less R to eat.

Organism Q will increase as there are less R to feed on Q.

Jane recorded the different observations about organisms A, B, C, D and E in her home aquarium.

  • B and D are plants
  • C feeds on D
  • E feeds on B and C
  • C and E are eaten by A

(b) Based on her observations, complete the food web below by filling in the boxes with organisms A, B, C, D and E. (2 marks)


Start with the producers- the plants are B and D. They are the only organisms that are eaten by animals and does not eat other animals, so the arrows will be coming out from the boxes only, representing “is eaten by.”


  • B and D are plants ☑️ – tick every given information once done


  • C feeds on D ☑️– tick, D is eaten by C.


  • E feeds on B and C☑️– tick, C and B are eaten by E.


  • C and E are eaten by A ☑️– tick, C and E are eaten by A.

So the key takeaway is to understand what the arrows mean (start from the end of the arrow- “is eaten by” – the direction matters). Remember the pacman!


Read the question carefully and write out the relationships.

For food webs, tick the given information once done so you don’t miss out anything.

P.s.: this is annotating your thoughts on paper!

Hope this helps 😀


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Post: Interactions Within the Environment (Let’s Attempt 2023 WA2 from MGS Q2, Q12)