Secondary 3 Biology Ecology Quiz

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.)