Secondary 4 Pure Biology Ecology Quiz

Secondary 4 Pure Biology Quiz - Ecology

Answer Key

Section A: Multiple Choice

1. B
Marking: 2 marks for B only.
Explanation: A population is defined as all the organisms of the same species in a particular habitat at a given time. Option A describes a community, C describes a community/ecosystem, and D describes an ecosystem.
Common mistake: Students often confuse "population" with "community" or "ecosystem."

2. C
Marking: 2 marks for C only.
Explanation: In the food chain, grass is the producer, grasshopper is the primary consumer (feeds on producer), frog is the secondary consumer (feeds on primary consumer), snake is the tertiary consumer, and eagle is the quaternary consumer.
Common mistake: Students may select B (grasshopper) if they confuse primary and secondary consumers.

3. C
Marking: 2 marks for C only.
Explanation: Photosynthesis removes CO₂ from the atmosphere by converting it into glucose. Respiration, combustion, and decomposition all release CO₂ into the atmosphere.
Common mistake: Students may select A (respiration) if they confuse the direction of gas exchange.

4. C
Marking: 2 marks for C only.
Explanation: In a typical pyramid of numbers, the number of organisms decreases at each successive trophic level due to energy loss between levels. Option A is incorrect because biomass of grass is typically greater. Option B is incorrect because energy transfer is never 100% efficient. Option D is incorrect because the eagle has the least biomass.
Common mistake: Students may confuse pyramid of numbers with pyramid of biomass.

5. C
Marking: 2 marks for C only.
Explanation: Biotic factors are living factors. Predatory birds are living organisms that affect fish population through predation. Temperature, dissolved oxygen, and pH are abiotic (non-living) factors.
Common mistake: Students may select B (dissolved oxygen) if they do not distinguish between biotic and abiotic factors.

Section B: Short-Answer and Structured Questions

6. [2 marks]
Answer: An ecosystem is a community of living organisms (biotic factors) interacting with each other and with their non-living (abiotic) environment, functioning together as a unit.
Marking:

• 1 mark for mentioning both living organisms and non-living environment.
• 1 mark for stating that they interact/function together.
  Common mistake: Students may only describe a community (living organisms only) without mentioning abiotic factors.

7. [2 marks]
Answer:
(i) Energy is lost as heat during respiration/metabolism at each trophic level.
(ii) Not all parts of the organism are consumed / not all ingested material is digested and absorbed (some lost as faeces).
Marking: 1 mark each for any two valid reasons. Other acceptable answers: energy used for movement/life processes; some organisms die without being eaten.
Common mistake: Students may state "energy is lost" without specifying how (e.g., as heat through respiration).

8. [3 marks]
(a) [1 mark] Answer: Respiration (or cellular respiration).
Marking: 1 mark for "respiration." "Cellular respiration" also accepted. "Breathing" alone is not accepted at this level.

(b) [1 mark] Answer: Photosynthesis.
Marking: 1 mark for "photosynthesis."

(c) [1 mark] Answer: Any one of: burning fossil fuels / deforestation / combustion of wood / industrial emissions / vehicle exhaust.
Marking: 1 mark for any valid human activity. "Pollution" alone is too vague; a specific activity must be named.

9. [3 marks]
Answer:
Mutualism: A relationship between two species in which both organisms benefit.
Example: Rhizobium bacteria and leguminous plants / cleaner fish and larger fish / bees and flowering plants.

Parasitism: A relationship in which one organism (the parasite) benefits at the expense of the other organism (the host), which is harmed.
Example: Tapeworm in human intestine / fleas on a dog / Plasmodium (malaria parasite) and humans.

Marking:

• 1 mark for correct definition of mutualism.
• 1 mark for correct definition of parasitism.
• 1 mark for two correct named examples (½ mark each if only one example is correct).
  Common mistake: Students may confuse parasitism with commensalism (where one benefits and the other is unaffected).

10. [4 marks]
(a) [2 marks]
Working: Mean = (4 + 7 + 3 + 5 + 6 + 2 + 8 + 4 + 5 + 6) ÷ 10 = 50 ÷ 10 = 5 dandelions per quadrat
Marking:

• 1 mark for correct sum (50) or correct method shown.
• 1 mark for correct final answer (5).
  Common mistake: Students may divide by the wrong number or make addition errors.

(b) [2 marks]
Working: Estimated total population = mean per quadrat × number of quadrats in field
Number of 1 m² quadrats in field = 200 ÷ 1 = 200 quadrats
Estimated total = 5 × 200 = 1000 dandelions
Marking:

• 1 mark for correct method (multiplying mean by number of quadrats or equivalent).
• 1 mark for correct final answer (1000).
  Common mistake: Students may forget to scale up from the sample to the total field area.

11. [2 marks]
Answer: A food web contains multiple interconnected food chains. If one species declines or is removed, consumers have alternative food sources, so the ecosystem is less disrupted. In a single food chain, the loss of one species breaks the entire chain, causing collapse.
Marking:

• 1 mark for stating that food webs have multiple/alternative food chains.
• 1 mark for explaining that this provides stability / allows organisms to switch food sources if one species declines.
  Common mistake: Students may state that food webs are "more complex" without explaining why this increases stability.

12. [2 marks]
Answer: Decomposers (bacteria and fungi) break down dead organic matter and animal waste, releasing ammonium compounds (ammonia) into the soil through the process of ammonification. This returns nitrogen to the soil in a form that can be reused by plants.
Marking:

• 1 mark for stating that decomposers break down dead organisms / organic matter.
• 1 mark for stating that this releases ammonium compounds / ammonia into the soil (ammonification).
  Common mistake: Students may confuse the role of decomposers with nitrifying bacteria.

13. [3 marks]
(a) [1 mark] Answer: The curve is J-shaped / shows exponential growth.
Marking: 1 mark for "J-shaped" or "exponential." "Rapid increase" is acceptable but less precise.

(b) [1 mark] Answer: There are abundant nutrients / no limiting factors initially / no competition for resources / ideal growth conditions.
Marking: 1 mark for any valid reason.

(c) [1 mark] Answer: Depletion of nutrients / accumulation of toxic waste products / competition for space or resources / decrease in pH.
Marking: 1 mark for any valid limiting factor.

14. [3 marks]
Answer:

• Deforestation removes trees, which reduces the rate of photosynthesis, so less CO₂ is removed from the atmosphere. [1]
• When felled trees are burned or decay, the carbon stored in their biomass is released as CO₂ into the atmosphere. [1]
• This increases the atmospheric CO₂ concentration, contributing to the enhanced greenhouse effect and global warming. [1]
  Marking: 1 mark for each valid point, up to 3 marks.
  Common mistake: Students may only mention burning without also addressing the loss of photosynthetic CO₂ uptake.

15. [4 marks]
(a) [1 mark] Answer: Rose bushes → Aphids → Ladybirds (or Rose bushes → Aphids → Ladybirds → Hawks)
Marking: 1 mark for a correct food chain with producer first and correct order. Arrows must point in the correct direction (from food to feeder).

(b) [3 marks]
Answer:

• Rose bushes: Producer
• Aphids: Primary consumer
• Ladybirds: Secondary consumer
• Earthworms: Decomposer
• Hawks: Secondary consumer (or tertiary if placed after ladybirds)
• Fungi: Decomposer

Marking: ½ mark for each correct classification, up to 3 marks (6 items × ½ = 3 marks).
Common mistake: Students may classify earthworms as primary consumers rather than decomposers.

Section C: Data-Based and Extended Response

16. [4 marks]
(a) [2 marks] Answer: The pesticide is non-biodegradable, so it cannot be broken down in the organisms' bodies. It accumulates in fatty tissues and is passed along the food chain when one organism eats another. At each trophic level, organisms consume many organisms from the level below, so the pesticide concentration builds up (bioaccumulation/biomagnification).
Marking:

• 1 mark for stating the pesticide is non-biodegradable / accumulates in tissues.
• 1 mark for explaining biomagnification / concentration increases up the food chain.

(b) [1 mark] Answer: The osprey may suffer from poisoning / reduced reproductive success / weakened immune system / population decline / eggshell thinning.
Marking: 1 mark for any valid consequence.

(c) [1 mark] Answer: The pesticide is non-biodegradable / chemically stable, so it is not broken down by enzymes or metabolic processes in organisms.
Marking: 1 mark for stating that the pesticide cannot be broken down / is persistent in the environment.

17. [4 marks]
(a) [2 marks] Answer: The fox and rabbit populations show cyclical fluctuations. When the rabbit population increases, the fox population also increases shortly after. When the rabbit population decreases, the fox population also decreases shortly after. The two populations are interdependent.
Marking:

• 1 mark for describing the cyclical/fluctuating pattern.
• 1 mark for stating that fox population changes follow rabbit population changes.

(b) [2 marks] Answer: When the rabbit population is high, foxes have more food available, so fox reproduction increases and the fox population grows. However, this response is delayed because it takes time for foxes to reproduce and for offspring to mature. Similarly, when rabbit numbers fall, foxes face food shortage, leading to starvation and reduced reproduction, but this effect is also delayed.
Marking:

• 1 mark for linking abundant prey to increased predator reproduction/survival.
• 1 mark for explaining the time lag / delay in the predator's response.
  Common mistake: Students may describe the relationship without explaining the time lag.

18. [4 marks]
(a) [1 mark] Answer: As light intensity decreases (with increasing distance from the pond edge), the number of plants decreases.
Marking: 1 mark for correctly describing the inverse relationship.

(b) [2 marks] Answer: Light is required for photosynthesis. At greater distances from the pond edge, light intensity is lower, so the rate of photosynthesis is reduced. This means less glucose is produced for growth and reproduction, so fewer plants can survive. Below a certain light intensity, the rate of photosynthesis may fall below the rate of respiration, and the plant cannot sustain itself.
Marking:

• 1 mark for linking light intensity to the rate of photosynthesis.
• 1 mark for explaining that reduced photosynthesis leads to less energy for growth / fewer plants surviving.

(c) [1 mark] Answer: Any one of: temperature / pH of water / concentration of dissolved oxygen / mineral ion concentration / water current / turbidity.
Marking: 1 mark for any valid abiotic factor.

19. [3 marks]
Answer:
Nitrifying bacteria convert ammonium compounds (NH₄⁺) in the soil into nitrites (NO₂⁻) and then into nitrates (NO₃⁻), which plants can absorb. This process is called nitrification. [1½ marks]

Denitrifying bacteria convert nitrates (NO₃⁻) in the soil back into nitrogen gas (N₂), which is released into the atmosphere. This reduces the amount of usable nitrogen in the soil. [1½ marks]

Marking:

• 1½ marks for each bacterium type: 1 mark for the conversion described, ½ mark for naming the process or correct chemical forms.
  Common mistake: Students may confuse nitrifying and denitrifying bacteria, or state that denitrifying bacteria "add" nitrogen to the soil.

20. [4 marks]
Answer (model response):

Human activity 1: Deforestation / clearing of forests for agriculture or development.
Negative impact: Destroys habitats, leading to loss of biodiversity / extinction of species. Disrupts food webs and reduces the ecosystem's ability to regulate the carbon cycle.
Conservation strategy: Establishing nature reserves / protected areas / reforestation programmes / enforcing laws against illegal logging.

Human activity 2: Pollution of water bodies (e.g., discharge of industrial waste, sewage, or agricultural runoff into rivers and seas).
Negative impact: Causes eutrophication / algal blooms that deplete dissolved oxygen, killing aquatic organisms. Toxic substances accumulate in food chains, harming top predators.
Conservation strategy: Implementing stricter regulations on waste discharge / treating industrial effluent before release / promoting organic farming to reduce pesticide runoff.

Marking:

• 1 mark for each human activity correctly identified (2 marks).
• 1 mark for each valid negative impact linked to the activity (2 marks).
• Conservation strategies are embedded within the response and are credited as part of the impact marks.
• Maximum 4 marks. Award marks for any valid combination of activities, impacts, and strategies.
  Common mistake: Students may describe impacts without linking them to specific human activities, or suggest vague conservation strategies (e.g., "stop polluting") without specific measures.

End of Answer Key