Olibits / Science / Ecosystem Balance
Primary 6 Science: Ecosystem Balance Practice
Real Primary 6 Science questions on Ecosystem Balance, pulled directly from the Olibits question bank — the same questions a child would actually see, not separately written for this page.
Curriculum alignment
Science Teaching & Learning Syllabus, Primary Three to Six (MOE, 2023 syllabus)
Science is not a taught subject at P1/P2 under MOE's own syllabus — our P1/P2 Science content is offered as enrichment beyond the official syllabus, not as a claim of syllabus coverage at those grades.
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Real Ecosystem Balance practice questions
In a rocky shore food chain, algae are eaten by limpets, and limpets are eaten by starfish. A disease wipes out the starfish population. With no starfish to prey on them, what is the most likely change in the limpet population, and its effect on the algae?
Limpet population falls, and algae becomes abundant
Both limpets and algae disappear completely
Algae population rises because it needs starfish to reproduce
Limpet population rises and grazes down more algae, so algae becomes scarcer ✓
Without starfish as a predator, limpet numbers are no longer kept in check, so their population grows and grazes more heavily on algae, reducing how much algae is left.
In a woodland food web, mice are eaten by both owls and foxes, and owls and foxes do not eat each other. One year, a disease wipes out most of the fox population. Because mice are part of a food web with more than one predator, what is the most likely effect on the mice population?
The mice population crashes completely, since foxes are gone
The mice population is likely to rise, since owls alone may not eat enough mice to keep numbers as low as before ✓
The mice population is unaffected, because food webs never change
The owls will also disappear once the foxes are gone
In a food web, mice have more than one predator; losing foxes removes only part of the predation pressure, so with fewer predators overall the mice population is likely to increase.
Sea otters prey on sea urchins, and sea urchins graze on kelp (a large seaweed) in a coastal ecosystem. When sea otters were hunted until very few remained, the sea urchin population grew rapidly. What effect did this most likely have on the kelp?
The kelp forest grew larger because urchins protected it from other grazers
The kelp was unaffected, since kelp is a producer and cannot be eaten by urchins
The kelp population increased because otters normally eat kelp directly
The kelp forest shrank sharply as the growing urchin population overgrazed it ✓
With fewer otters to control urchin numbers, the urchin population grew and grazed heavily on the kelp, causing the kelp forest to decline - a knock-on effect called a trophic cascade.
In a grassland, ecologists tracked rabbit and fox populations over 4 years. Rabbit numbers rose steadily from 150 to 450, providing more food for foxes, and fox numbers also rose from 15 to 40 over the same period. Which statement best explains why the fox population increased?
Foxes reproduce faster whenever the weather turns warmer, regardless of food supply
Rabbits provide shelter that allows more foxes to survive, even without extra food
With more rabbits (their main food source) available, more foxes could find enough food to survive and raise young ✓
The fox population increased because grass became more plentiful
Foxes are predators of rabbits, so when rabbit numbers rise there is more food available, allowing more foxes to survive and reproduce successfully.