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Secondary 4 Pure Biology Ecology Quiz
Free Sec 4 Pure Biology Ecology quiz, LongCat Exam version, with questions, answers, and O Level-style practice for Singapore students.
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Secondary 4 Pure Biology Quiz - Ecology
Answer Key
Section A: Multiple Choice Questions
1. B — Dissolved oxygen [1]
Note: Abiotic factors are non-living physical/chemical components. Algae, water fleas, and aquatic plants are biotic.
2. B — population [1]
Note: A community includes all species; an ecosystem includes abiotic factors; a habitat is the physical environment.
3. A — Grass → Grasshopper → Snake → Eagle [1]
Note: Food chains must begin with a producer and flow in the direction of energy transfer.
4. D — omnivores [1]
5. C — Respiration and combustion [1]
Note: Decomposition also releases CO₂, but the best answer from the options is C as it captures two major processes. Photosynthesis removes CO₂.
6. A — the producer is very large, such as a single tree supporting many insects [1]
7. C — Reduced water vapour in the atmosphere [1]
Note: Deforestation reduces transpiration, which decreases water vapour returned to the atmosphere.
8. B — accumulation of non-biodegradable toxins in organisms at higher trophic levels [1]
9. B — A network of interconnected food chains [1]
10. A — atmospheric nitrogen into ammonia [1]
Section B: Structured Response Questions
11. (a) Grass plants [1]
(b) Grass → Grasshopper → Field mouse → Snake → Hawk [2]
Marking: 1 mark for correct order, 1 mark for including all five organisms. Direction arrows must be correct.
(c) Energy is lost at each trophic level (as heat through respiration, uneaten parts, faeces) [1]. Therefore, less energy is available at higher trophic levels, supporting fewer organisms / smaller populations [1].
Common mistake: Students may say "hawks eat fewer things" without explaining energy loss.
12. (a) Photosynthesis [1]
(b) Any two of: Respiration, Combustion (of fossil fuels), Decomposition [2 — 1 mark each]
(c) Burning fossil fuels releases carbon that was stored underground over millions of years [1]. This adds extra CO₂ to the atmosphere faster than natural processes can remove it, increasing the atmospheric CO₂ concentration / enhancing the greenhouse effect / contributing to global warming [1].
13. (a) Species P: Prey [1] — Reason: Its population rises first, followed by the rise in Species Q (the predator). When P decreases, Q subsequently decreases.
Species Q: Predator [1] — Reason: Its population changes lag behind those of Species P; it depends on P as a food source.
(b) Predator–prey relationship [1]
(c) The population of species Q would decrease sharply / become extinct [1]. Species Q depends on species P as a food source; without P, Q would starve and its population would decline [1].
14. An ecological pyramid of energy is a diagram that shows the amount of energy available at each trophic level in an ecosystem [1]. Energy transfer between trophic levels is inefficient because: energy is lost as heat during respiration / metabolic processes [1]; not all parts of the organism are consumed or digested, and energy is lost in faeces and uneaten material [1].
Note: Typically only about 10% of energy is transferred from one trophic level to the next.
15. Way 1: Burning of fossil fuels [1]
Effect: Releases large amounts of CO₂ into the atmosphere, enhancing the greenhouse effect and contributing to global warming / climate change [1].
Way 2: Deforestation / clearing of forests for agriculture or development [1]
Effect: Reduces the number of trees available to absorb CO₂ through photosynthesis; also releases stored carbon when trees are burned or decompose, increasing atmospheric CO₂ [1].
Acceptable alternatives for Way 2: Industrial processes, vehicle emissions, slash-and-burn agriculture. Must link to environmental effect for full marks.
16. (a) Mangrove leaves → Insects → Mudskippers → Herons [1]
(Also acceptable: Algae → Shrimps → Mudskippers → Herons)
(b) Tertiary consumer / Third trophic level as consumer (fourth trophic level overall) [1]
Note: The heron feeds on mudskippers (secondary consumers), making it a tertiary consumer.
(c) The heron would be most affected [1]. Due to biological magnification, the non-biodegradable pesticide accumulates at each trophic level. The heron, being at the highest trophic level, would accumulate the highest concentration of the toxin, which could cause illness, reproductive failure, or death [1].
Section C: Data-Based / Source-Based Question
17. (a) The number of Species A cells increases from 200 at 0 m to a peak of 350 at 2 m [1], then decreases steadily to 5 at 8 m [1].
(b) Species A thrives at moderate light intensities (around 60 arbitrary units at 2 m depth) [1]. At the surface (0 m), very high light intensity may be damaging / cause photoinhibition. At greater depths, light intensity is too low for efficient photosynthesis, so the population declines [1].
18. (a) As light intensity decreases (with increasing depth), the number of Species B cells increases [1], reaching a maximum at 6 m depth (300 cells per cm³ at 10 light units), then decreases slightly at 8 m [1].
(b) Species B is better adapted to low light conditions / has a lower light compensation point / possesses more chlorophyll or accessory pigments to capture light more efficiently at low intensities [1].
19. The two species compete for the same resource — light [1]. Species A dominates at shallower depths (higher light) while Species B dominates at greater depths (lower light) [1]. This is resource partitioning — the two species utilise different parts of the habitat (different depths/light intensities), reducing direct competition and allowing them to coexist in the same lake [1].
20. The population of Species B would increase [1]. With Species A removed, competition for light and other resources (e.g., nutrients) would be reduced, allowing Species B to expand into shallower depths and grow to a larger population size [1].
End of Answer Key