Living Together and Energy in Food Topics
Testing two topics in one question
This science revision note aims to help your child
– understand how floating plants affect the survival of other organisms. What concepts must you use?
– proper scientific keywords to explain the concepts clearly.
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The diagram below shows a pond and the organisms living in it.
Animal X feeds on plant B. If more of Plant A is added to the pond, how will the population of Animal X be affected?
STEP 1
Consider what physical factors Animal X needs to survive.
Based on the clues found in the question, animal X feeding on plant B.
STEP 2
Does an increase in Plant A directly affect the survival of Animal X?
To answer this, consider whether an increase in Plant A affects the physical factors stated in Step 1:
With more Plant A, the amount of oxygen produced by Plant A through photosynthesis will increase. However, most of this oxygen is released into the air and not into the water. Hence the amount of dissolved oxygen is not directly affected by an increase in Plant A.
Thus, an increase in Plant A does not directly affect the survival of Animal X.
Since an increase in Plant A does not directly affect the survival of Animal X, now consider whether it indirectly affects the survival of Animal X (i.e. Plant A affects the survival of Plant B, and the survival of Plant B affects the survival of Animal X).
STEP 3
Does a decrease in Plant B directly affect the survival of Animal X?
Yes! When the population size of plant B decreases, there is less food for X and the population size of X will decrease.
What then causes a decrease in the population size of plant B?
By analysing the diagram, you will notice floating plants A prevents sunlight from reaching the submerged plants B.
As the population size of plant A increases, less sunlight is absorbed by the submerged plant B. Plants B photosynthesise slower and cannot make enough food. The population size of plant B decreases.
There is not enough plant B for animal X to feed on. The population of animal X decreases as some will die of starvation.
Note: We need to apply the photosynthesis concept to answer this question.
FINALLY, put your answer together
Question: If more of Plant A is added into the pond, how is the population of Animal X affected?
Answer: Population of Animal X decreases.
As the population size of plant A increases, less sunlight is absorbed by the submerged plant B. Plants B photosynthesise slower and cannot make enough food. The population size of plant B decreases.
There is not enough plant B for animal X to feed on. The population of animal X decreases as some will die of starvation.
Common mistakes
Wrong answer: Population of Animal X increases. An increase in Plant A increases amount of oxygen produced through photosynthesis, increasing the amount of oxygen for Animal X.
Animal X is an aquatic animal and requires dissolved oxygen. Plant A is a floating plant with its leaves above the water. Oxygen produced during photosynthesis is released through the leaves, hence oxygen is released into the atmosphere and not into the water. Hence, although it is true that there will be an increase in oxygen produced by Plant A, this oxygen cannot be used by Animal X, hence would not affect the population of Animal X.
Wrong answer: The population of Animal X decreases because an increase in Plant A decreases the amount of sunlight that reaches Animal X.
Animal X is an animal and does not require sunlight to survive. Students need to explain how sunlight affects the population of plant B which then affects animal X.
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Andy Ling
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