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Secondary 3 Biology Ecology Quiz
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Questions
Secondary 3 Biology Quiz - Ecology
Name: ___________________________
Class: ___________________________
Date: ___________________________
Score: ________ / 50
Duration: 60 minutes
Total Marks: 50
Instructions
- Answer all questions in the spaces provided.
- Read each question carefully before writing your answer.
- Marks are indicated in brackets [ ] at the end of each question or part-question.
- The number of marks allocated reflects the depth of response expected.
- Where diagrams or data are provided, use them to support your answers.
- Write your answers clearly and in complete sentences where required.
Section A: Multiple Choice Questions (10 marks)
Questions 1–5: Choose the most appropriate answer and write its letter in the space provided.
1. Which of the following is an abiotic factor in a freshwater pond ecosystem?
A. Water lilies
B. Dissolved oxygen
C. Freshwater snails
D. Algae
Answer: ________ [1]
2. A food chain in a grassland ecosystem is shown below:
grass → grasshopper → frog → snake → hawk
Which organism is a secondary consumer?
A. Grass
B. Grasshopper
C. Frog
D. Hawk
Answer: ________ [1]
3. Which of the following best describes a population?
A. All the living organisms in a forest
B. All the oak trees in a park
C. All the fish, plants, and bacteria in a pond
D. The soil, water, and air in a grassland
Answer: ________ [1]
4. In a food web, energy is lost between trophic levels primarily as:
A. Light energy reflected by producers
B. Heat energy from respiration
C. Chemical energy stored in faeces only
D. Kinetic energy from movement of consumers
Answer: ________ [1]
5. Which of the following correctly shows the direction of energy flow in an ecosystem?
A. Sun → decomposers → producers → consumers
B. Sun → consumers → producers → decomposers
C. Sun → producers → consumers → decomposers
D. Producers → Sun → consumers → decomposers
Answer: ________ [1]
6. The diagram below shows part of a food web in a mangrove ecosystem.
heron
/ \
crab fish
| / \
mangrove algae shrimp
leaves |
plankton
How many food chains are present in this food web?
A. 2
B. 3
C. 4
D. 5
Answer: ________ [1]
7. Which of the following is a role of decomposers in an ecosystem?
A. Convert solar energy into chemical energy
B. Break down dead organic matter and recycle nutrients
C. Produce oxygen through photosynthesis
D. Consume herbivores to regulate population size
Answer: ________ [1]
8. A pyramid of numbers for a grassland ecosystem is shown below (not drawn to scale):
hawk (5)
frog (50)
grasshopper (500)
grass (very large number)
Which statement best explains why the pyramid of numbers has this shape?
A. Hawks have the largest biomass in the ecosystem.
B. Each grasshopper consumes a large number of grass plants.
C. Grass is a producer and supports many primary consumers.
D. Frogs are more abundant than grasshoppers.
Answer: ________ [1]
9. Which of the following processes returns carbon dioxide to the atmosphere?
A. Photosynthesis only
B. Respiration only
C. Respiration and combustion only
D. Respiration, combustion, and decomposition
Answer: ________ [1]
10. The nitrogen cycle involves several processes. Which process converts atmospheric nitrogen (N₂) into a form usable by plants?
A. Denitrification
B. Nitrification
C. Nitrogen fixation
D. Ammonification
Answer: ________ [1]
Section B: Structured Questions (25 marks)
Questions 11–15: Answer the questions in the spaces provided.
11. Fig. 11.1 shows a simplified food web in a tropical rainforest ecosystem.
jaguar
/ \
monkey snake
/ \ \
fruit leaves frog
/ \
insect small fish
| |
leaves algae
(a) Name the producer(s) shown in the food web. [1]
(b) Identify the tertiary consumer(s) shown in the food web. [1]
(c) Write a complete food chain from the food web that contains exactly four organisms. [1]
(d) Explain why the jaguar population is typically much smaller than the insect population in this ecosystem. [2]
12. A student investigated the population of woodlice (also known as pill bugs) in a school garden. The garden has four areas: under a log, in a flower bed, on a paved path, and under a bush. The student used a quadrat sampling method and recorded the number of woodlice in each area over five days.
Table 12.1: Number of woodlice recorded in each area over five days
| Area | Day 1 | Day 2 | Day 3 | Day 4 | Day 5 | Average |
|---|---|---|---|---|---|---|
| Under a log | 18 | 22 | 20 | 19 | 21 | 20 |
| In a flower bed | 8 | 10 | 9 | 11 | 12 | 10 |
| On a paved path | 0 | 1 | 0 | 0 | 1 | 0.4 |
| Under a bush | 14 | 16 | 15 | 13 | 17 | 15 |
(a) Calculate the total average number of woodlice across all four areas. Show your working. [1]
(b) Suggest two abiotic factors that could explain why the highest average number of woodlice was found under the log. [2]
(c) Explain why the student sampled over five days rather than on a single day. [1]
13. Fig. 13.1 shows a simplified carbon cycle.
CO₂ in atmosphere
↑↓
┌────┴────┐
↓ ↑
Producers → Consumers
↓ ↓
└────┬────┘
↓
Decomposers
↓
Fossil fuels
↑
Combustion
(a) Name process X that removes carbon dioxide from the atmosphere. [1]
(b) Name process Y that releases carbon dioxide from decomposers into the atmosphere. [1]
(c) Describe two ways in which human activities have disrupted the carbon cycle. [2]
(d) Explain the consequence of increased atmospheric carbon dioxide on global temperatures. [2]
14. Fig. 14.1 shows the population sizes of a predator (fox) and its prey (rabbit) over a period of 10 years.
Population (thousands)
25 | *--* *--*
| * * * *
20 | * * * *
| * * * *
15 | * * * *
| * ** *
10 |* *
| * * * * * * * * * * (rabbits)
5 |* * * * * * * * * * *
|________________________________________________
0 1 2 3 4 5 6 7 8 9 10
Year
(Solid line = rabbits, Dashed line = foxes)
(a) Describe the relationship between the fox and rabbit populations over the 10-year period. [2]
(b) In which year was the fox population at its lowest? [1]
(c) Explain why the fox population peaks after the rabbit population peaks. [2]
15. A farmer noticed that the yield of his rice crop decreased significantly after clearing a nearby forest. He also observed that the river near his farm became more polluted with soil runoff.
(a) Explain how the removal of the forest could have led to increased soil runoff into the river. [2]
(b) Suggest two ways the farmer could practise sustainable agriculture to reduce the environmental impact on the river. [2]
(c) Explain why the decrease in forest cover could lead to a decrease in rice crop yield. [2]
Section C: Extended Response (15 marks)
Questions 16–20: Answer the questions in the spaces provided. Use complete sentences and provide detailed explanations where required.
16. Fig. 16.1 shows a simplified nitrogen cycle in an agricultural ecosystem.
Atmospheric N₂
↑↓
Nitrogen fixation
(by lightning &
nitrogen-fixing bacteria)
↓
Ammonia (NH₃)
↓
Nitrifying bacteria
↓
Nitrites (NO₂⁻)
↓
Nitrifying bacteria
↓
Nitrates (NO₃⁻)
↑↓ ↓
Denitrifying Absorbed by
bacteria plant roots
↓ ↓
Atmospheric Proteins in
N₂ plants & animals
↓
Decomposers
↓
Ammonia (NH₃)
(a) Name the process by which atmospheric nitrogen is converted into ammonia. [1]
(b) Explain why nitrates are important for plant growth. [2]
(c) Describe the role of decomposers in the nitrogen cycle. [2]
(d) Farmers often add nitrogen-containing fertilisers to their fields. Explain one environmental problem that can result from the excessive use of such fertilisers. [2]
17. Fig. 17.1 shows the energy flow through a marine ecosystem.
Sunlight (100,000 kJ/m²/year)
↓
Phytoplankton (producers)
1% of sunlight captured = 1,000 kJ
↓ (10% transferred)
Zooplankton (primary consumers)
100 kJ
↓ (10% transferred)
Small fish (secondary consumers)
10 kJ
↓ (10% transferred)
Large fish (tertiary consumers)
1 kJ
(a) Calculate the percentage of sunlight energy captured by phytoplankton. Show your working. [1]
(b) State the approximate percentage of energy transferred from one trophic level to the next. [1]
(c) Explain two reasons why energy is lost between trophic levels. [2]
(d) Explain why food chains rarely have more than four or five trophic levels. [2]
18. An oil spill occurred off the coast of a tropical island. The oil spread across the surface of the sea, affecting marine life and coastal ecosystems.
(a) Explain how the oil spill would affect the producers (phytoplankton and seaweed) in the marine ecosystem. [2]
(b) Describe how the oil spill could affect organisms at higher trophic levels in the food web. [2]
(c) Suggest two long-term effects of the oil spill on the biodiversity of the coastal ecosystem. [2]
19. A student set up two sealed terrariums (Terrarium A and Terrarium B) to study ecosystem sustainability. Both terrariums contained soil, water, small plants, and a small snail. Terrarium A was placed in sunlight, while Terrarium B was kept in complete darkness. The student observed both terrariums over four weeks.
(a) Predict what would happen to the snail in Terrarium B after four weeks. Explain your prediction. [2]
(b) Explain why Terrarium A is more likely to remain sustainable over a longer period than Terrarium B. [2]
(c) Name one biotic factor and one abiotic factor that are essential for the sustainability of Terrarium A. [1]
20. Fig. 20.1 shows the population growth of an invasive species (Species X) and a native species (Species Y) in a lake ecosystem over 20 years.
Population (thousands)
40 | *--* Species X (invasive)
| * *
35 | * *
| * *
30 | * *
| * *
25 | *
| *
20 | * *--*--*--*--*--*--*--*--*--*--* Species Y (native)
| * *
15 |* *
| *
10 | *
| *
5 |*
|________________________________________________
0 2 4 6 8 10 12 14 16 18 20
Year
(a) Describe the population trend of Species Y after the introduction of Species X. [2]
(b) Suggest two possible reasons why Species X was able to outcompete Species Y. [2]
(c) Explain one method that could be used to control the population of Species X and protect the native ecosystem. [1]
Answers
Secondary 3 Biology Quiz - Ecology
Answer Key
Section A: Multiple Choice Questions (10 marks)
1. B [1]
Dissolved oxygen is a non-living (abiotic) factor. Water lilies, snails, and algae are living (biotic) components.
Common mistake: Students may select algae, not realising it is a living organism (biotic).
2. C [1]
The frog feeds on the grasshopper (primary consumer), making it a secondary consumer.
Common mistake: Selecting the hawk, which is a tertiary consumer.
3. B [1]
A population refers to all individuals of the same species in a given area. All the oak trees in a park represent one species.
Common mistake: Selecting option A or C, which describe communities (multiple species).
4. B [1]
Energy is lost between trophic levels primarily as heat energy released during respiration.
Common mistake: Selecting option C — while some energy is lost in faeces, the majority is lost as heat from metabolic processes.
5. C [1]
Energy flows from the Sun to producers (via photosynthesis), then to consumers, and finally to decomposers.
Common mistake: Selecting option A — decomposers do not receive energy directly from the Sun.
6. C [1]
The four food chains are:
(i) mangrove leaves → crab → heron
(ii) mangrove leaves → insect → frog → snake
(iii) algae → shrimp → fish → heron
(iv) plankton → shrimp → fish → heron
Common mistake: Counting only the most obvious chains and missing the plankton-based chain.
7. B [1]
Decomposers break down dead organic matter, releasing nutrients back into the soil for reuse by producers.
Common mistake: Selecting option A, which describes the role of producers.
8. C [1]
Producers (grass) are the most numerous because they support all higher trophic levels. The pyramid narrows because energy is lost at each transfer.
Common mistake: Selecting option B — while true, it does not explain the overall pyramid shape.
9. D [1]
Respiration (by all living organisms), combustion (burning of fossil fuels), and decomposition (by decomposers) all release CO₂ into the atmosphere. Photosynthesis removes CO₂.
Common mistake: Selecting option B and overlooking decomposition and combustion.
10. C [1]
Nitrogen fixation converts atmospheric N₂ into ammonia (NH₃) or ammonium compounds that plants can absorb.
Common mistake: Confusing nitrogen fixation with nitrification (which converts ammonia to nitrites/nitrates).
Section B: Structured Questions (25 marks)
11.
(a) Producer(s): Mangrove leaves, algae [1]
(Both must be named for the mark. "Leaves" alone is insufficient — must specify mangrove leaves.)
(b) Tertiary consumer(s): Heron, snake [1]
(Both feed on secondary consumers — heron eats crab/fish; snake eats frog.)
(c) Any one of the following food chains (4 organisms): [1]
- Mangrove leaves → crab → heron
- Mangrove leaves → insect → frog → snake
- Algae → shrimp → fish → heron
- Plankton → shrimp → fish → heron
(Must have exactly 4 organisms in correct order: producer → primary consumer → secondary consumer → tertiary consumer.)
(d) [2]
Energy is lost at each trophic level (as heat from respiration, in faeces, and through movement). [1]
Therefore, only a small amount of energy is available at higher trophic levels, supporting fewer organisms. [1]
(Award 1 mark for mentioning energy loss between trophic levels; 1 mark for linking this to smaller population size at higher levels.)
12.
(a) Total average = 20 + 10 + 0.4 + 15 = 45.4 (or approximately 45) [1]
Working: 20 + 10 + 0.4 + 15 = 45.4
(b) Any two of the following: [2]
- Higher moisture/humidity under the log (woodlice lose water easily and prefer damp conditions)
- Lower light intensity under the log (woodlice are nocturnal/avoid light)
- Cooler temperature under the log (protection from direct sunlight)
- Greater availability of food/decaying organic matter under the log
(Award 1 mark per valid abiotic factor. "Food" alone is biotic; "decaying leaves" as a food source is acceptable if linked to the sheltered environment.)
(c) Sampling over multiple days reduces the effect of random variation/anomalies and gives a more reliable/representative average of the woodlouse population in each area. [1]
(Key idea: increases reliability/accuracy of data.)
13.
(a) Process X: Photosynthesis [1]
(b) Process Y: Respiration [1]
(Decomposers respire, releasing CO₂ back into the atmosphere.)
(c) Any two of the following: [2]
- Deforestation — reduces the number of trees that absorb CO₂ through photosynthesis, increasing atmospheric CO₂
- Burning of fossil fuels — releases large amounts of CO₂ that were stored underground for millions of years
- Industrial processes — manufacturing and cement production release additional CO₂
(Award 1 mark per valid human activity with a brief explanation.)
(d) [2]
Increased CO₂ in the atmosphere enhances the greenhouse effect. [1]
CO₂ traps more heat/infrared radiation in the atmosphere, leading to an increase in global temperatures (global warming). [1]
(Award 1 mark for identifying the greenhouse effect; 1 mark for explaining the consequence — rising temperatures.)
14.
(a) [2]
The fox and rabbit populations show a cyclical/oscillating pattern over the 10-year period. [1]
The fox population peaks after the rabbit population peaks, and the fox population declines when the rabbit population declines, showing a predator-prey relationship. [1]
(Award 1 mark for describing the cyclical pattern; 1 mark for identifying the predator-prey relationship with the time lag.)
(b) Year 1 (or the first year shown) [1]
(The fox population is at its lowest at the beginning of the observation period.)
(c) [2]
When the rabbit population increases, there is more food available for foxes. [1]
This allows foxes to reproduce more successfully, leading to an increase in the fox population after a time lag. [1]
(Award 1 mark for linking rabbit abundance to food availability for foxes; 1 mark for explaining the delayed response/time lag in population increase.)
15.
(a) [2]
Tree roots hold the soil together and reduce soil erosion. [1]
Without tree roots, rain washes away the topsoil more easily, carrying it into the river as runoff. [1]
(Award 1 mark for the role of roots in soil stability; 1 mark for linking removal to increased erosion/runoff.)
(b) Any two of the following: [2]
- Plant cover crops or maintain vegetation buffer strips along the riverbank to reduce soil runoff
- Use terracing on slopes to slow water flow and reduce erosion
- Practise crop rotation to maintain soil fertility and structure
- Reduce the use of chemical fertilisers/pesticides that can pollute the river
- Use organic farming methods to reduce chemical runoff
(Award 1 mark per valid sustainable practice.)
(c) [2]
Forests help maintain the water cycle through transpiration, which contributes to local rainfall. [1]
Without the forest, there may be less rainfall and poorer soil quality (due to erosion), both of which reduce rice crop yield. [1]
(Alternative: Forests provide habitats for pollinators and natural pest controllers; their removal reduces these services, lowering crop yield.)
(Award 1 mark for identifying a valid reason; 1 mark for explaining how it affects crop yield.)
Section C: Extended Response (15 marks)
16.
(a) Nitrogen fixation [1]
(b) [2]
Nitrates contain nitrogen, which is an essential element for the synthesis of amino acids and proteins in plants. [1]
Proteins are needed for growth, enzyme production, and cell division, so nitrates are critical for healthy plant development. [1]
(Award 1 mark for identifying that nitrates provide nitrogen; 1 mark for explaining the role of nitrogen in plant growth/protein synthesis.)
(c) [2]
Decomposers break down dead organisms and waste products (faeces, urine). [1]
They convert the nitrogen-containing compounds (proteins, urea) in these materials back into ammonia (NH₃), which can then be converted into nitrates by nitrifying bacteria, making nitrogen available to plants again. [1]
(Award 1 mark for describing decomposition of organic matter; 1 mark for explaining the return of nitrogen to the cycle as ammonia.)
(d) [2]
Excessive fertilisers can be washed into rivers and lakes by rain (leaching/runoff), causing eutrophication. [1]
This leads to algal blooms that block sunlight and deplete oxygen in the water when the algae die and are decomposed, killing fish and other aquatic organisms. [1]
(Alternative: Excess nitrates in drinking water can be harmful to human health, particularly infants — "blue baby syndrome".)
(Award 1 mark for identifying eutrophication or water pollution; 1 mark for explaining the ecological consequence.)
17.
(a) Percentage = (1,000 ÷ 100,000) × 100 = 1% [1]
Working: (1,000 / 100,000) × 100 = 1%
(b) Approximately 10% [1]
(c) Any two of the following: [2]
- Energy is lost as heat during respiration/metabolism
- Energy is lost in faeces and undigested material (not all food is absorbed)
- Energy is used for movement and other life processes and is lost as kinetic/heat energy
- Some organisms die without being eaten, so their energy is not passed to the next level
(Award 1 mark per valid reason.)
(d) [2]
Only about 10% of energy is transferred from one trophic level to the next. [1]
After four or five trophic levels, there is insufficient energy left to support a viable population at a higher level. [1]
(Award 1 mark for stating the 10% energy transfer rule; 1 mark for explaining that energy becomes too limited to sustain another level.)
18.
(a) [2]
The oil forms a layer on the water surface, blocking sunlight from reaching phytoplankton and seaweed below. [1]
Without sunlight, photosynthesis cannot occur, so producers cannot produce food/energy and will eventually die. [1]
(Award 1 mark for describing the physical barrier; 1 mark for explaining the effect on photosynthesis.)
(b) [2]
Organisms at higher trophic levels (fish, seabirds, marine mammals) are affected because:
- They ingest oil directly or consume contaminated prey, leading to poisoning/internal damage [1]
- Their food source (producers and smaller consumers) declines, leading to starvation [1]
(Award 1 mark for direct toxicity/ingestion; 1 mark for food chain disruption.)
(c) Any two of the following: [2]
- Loss of species — some species may die out locally if they cannot survive the pollution or recover quickly
- Reduced biodiversity — the ecosystem becomes less diverse as sensitive species are eliminated
- Disruption of food webs — the loss of key species affects the entire ecosystem structure
- Habitat destruction — coastal habitats like mangroves and coral reefs may be damaged long-term
(Award 1 mark per valid long-term effect.)
19.
(a) [2]
The snail in Terrarium B would die after four weeks. [1]
Without light, the plants cannot carry out photosynthesis and will die, so there will be no food and no oxygen for the snail to survive. [1]
(Award 1 mark for predicting death; 1 mark for explaining the lack of food/oxygen due to absence of photosynthesis.)
(b) [2]
In Terrarium A, sunlight allows plants to carry out photosynthesis, producing oxygen and glucose/food. [1]
This creates a self-sustaining cycle: plants produce oxygen and food for the snail; the snail produces CO₂ for the plant's photosynthesis; decomposers recycle nutrients from waste. [1]
(Award 1 mark for photosynthesis producing oxygen/food; 1 mark for describing the sustainable cycle.)
(c) [1]
- Biotic factor: Plants (or snail, or decomposers/microorganisms in soil)
- Abiotic factor: Sunlight (or water, oxygen, carbon dioxide, minerals in soil)
(Award 1 mark for one valid biotic AND one valid abiotic factor. Both must be correct.)
20.
(a) [2]
After the introduction of Species X, the population of Species Y decreases significantly/declines rapidly. [1]
Species Y is outcompeted by Species X for resources such as food, space, or light, leading to a sharp drop in its population. [1]
(Award 1 mark for describing the decline; 1 mark for explaining the cause — competition.)
(b) Any two of the following: [2]
- Species X may have no natural predators in the new ecosystem, allowing its population to grow unchecked
- Species X may be a more efficient competitor for food/space/resources than Species Y
- Species X may reproduce faster than Species Y
- Species X may be able to tolerate a wider range of environmental conditions than Species Y
- Species X may prey on Species Y or its food source
(Award 1 mark per valid reason.)
(c) Any one of the following: [1]
- Physical removal — manually or mechanically removing Species X from the lake
- Biological control — introducing a natural predator or disease that targets Species X (with caution)
- Chemical control — using targeted pesticides/herbicides (with environmental safeguards)
- Prevention — stricter regulations on the transport and release of non-native species
(Award 1 mark for a valid control method.)