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Secondary 3 Biology Ecology Quiz

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Questions

Secondary 3 Biology Quiz - Ecology

Name: ________________________
Class: ________________________
Date: ________________________
Score: _____ / 40

Duration: 45 minutes
Total Marks: 40

Instructions:

Answer all questions in the spaces provided.
The number of marks is given in brackets [ ] at the end of each question or part question.
Write your answers clearly and legibly.
For multiple-choice questions, circle the correct letter.
Diagrams are not drawn to scale unless stated.

Section A: Multiple Choice Questions (10 marks)

Questions 1–10 carry 1 mark each. Circle the correct answer.

Which of the following best defines an ecosystem?
A. All the living organisms in a given area
B. All the populations of different species living and interacting in an area
C. A community of organisms and their physical environment interacting as a system
D. The role and position a species has in its environment
In a food chain, energy is transferred from one trophic level to the next. Approximately what percentage of energy is typically transferred between trophic levels?
A. 1%
B. 10%
C. 50%
D. 90%
The diagram below shows a pyramid of numbers for a grassland ecosystem.

Image pending generation: diagram for Q3.

Which statement correctly explains the shape of this pyramid?
A. Energy is lost at each trophic level, so fewer organisms can be supported at higher levels.
B. Predators are larger than prey, so fewer predators can exist.
C. Producers have a shorter life cycle than consumers.
D. Decomposers recycle nutrients back to the producers.

Which process is carried out by nitrifying bacteria in the nitrogen cycle?
A. Converting atmospheric nitrogen into ammonia
B. Converting ammonia into nitrites and then nitrates
C. Converting nitrates into atmospheric nitrogen
D. Converting dead organic matter into ammonia
A population of deer in a forest reserve was monitored over 10 years. The graph below shows the population size over time.

Image pending generation: graph for Q5.

What is the most likely reason for the sharp decline after the population reached carrying capacity?
A. Increased birth rate
B. Disease outbreak due to high population density
C. Immigration of new individuals
D. Increased food availability

Which of the following is an example of a density-independent factor limiting population growth?
A. Competition for food
B. Predation
C. Forest fire
D. Disease
In the carbon cycle, which process removes carbon dioxide from the atmosphere?
A. Respiration
B. Combustion
C. Photosynthesis
D. Decomposition
The diagram shows a food web in a freshwater pond ecosystem.

Image pending generation: diagram for Q8.

If the population of zooplankton decreases significantly, what is the most immediate effect on the food web?
A. Phytoplankton population decreases
B. Small fish population decreases
C. Large fish population increases
D. Heron population increases

Which interaction describes a relationship where both species benefit?
A. Parasitism
B. Commensalism
C. Mutualism
D. Predation
A student sets up an experiment to investigate the effect of light intensity on the rate of photosynthesis in an aquatic plant. Which of the following is the dependent variable?
A. Light intensity
B. Temperature of water
C. Number of bubbles produced per minute
D. Distance of lamp from plant

Section B: Structured Questions (18 marks)

Answer all questions in the spaces provided.

The diagram below shows part of the carbon cycle.

Image pending generation: diagram for Q11.

(a) Name the process labelled **X** that returns carbon dioxide to the atmosphere from living organisms. [1]  
________________________________________________________________________________

(b) Name the process labelled **Y** that transfers carbon from the atmosphere to plants. [1]  
________________________________________________________________________________

(c) Explain how deforestation and the burning of fossil fuels contribute to increased levels of atmospheric carbon dioxide. [3]  
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________

12. A farmer notices that the yield of his crop decreases when planted in the same field year after year. He decides to rotate the crop with legumes.

(a) Explain why planting legumes improves soil fertility for the next crop. [2]  
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________

(b) Name the type of bacteria found in the root nodules of legumes and state their role in the nitrogen cycle. [2]  
________________________________________________________________________________
________________________________________________________________________________

13. The graph below shows the changes in population size of a predator (lynx) and its prey (hare) over a period of 20 years.

Image pending generation: graph for Q13.

(a) Describe the relationship between the hare and lynx populations shown in the graph. [2]  
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________

(b) Explain why the lynx population peaks after the hare population peaks. [2]  
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________

(c) Suggest one density-dependent factor and one density-independent factor that could affect the hare population. [2]  
________________________________________________________________________________
________________________________________________________________________________

14. The diagram shows a pyramid of biomass for a marine ecosystem.

Image pending generation: diagram for Q14.

(a) Explain why this pyramid of biomass is inverted (biomass increases at higher trophic levels). [2]  
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________

(b) Would a pyramid of energy for this same ecosystem be inverted? Explain your answer. [2]  
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________

15. A student investigated the distribution of daisies in a school field using quadrats. The field has a shaded area under trees and an open sunny area. The student placed 10 quadrats (1 m × 1 m) randomly in each area and counted the number of daisies in each quadrat.

The results are shown in the table below.

| Quadrat Number | Number of Daisies (Shaded Area) | Number of Daisies (Sunny Area) |
|----------------|----------------------------------|--------------------------------|
| 1              | 2                                | 15                             |
| 2              | 0                                | 18                             |
| 3              | 1                                | 12                             |
| 4              | 3                                | 20                             |
| 5              | 0                                | 16                             |
| 6              | 2                                | 14                             |
| 7              | 1                                | 19                             |
| 8              | 0                                | 17                             |
| 9              | 2                                | 13                             |
| 10             | 1                                | 15                             |

(a) Calculate the mean number of daisies per quadrat in the sunny area. [1]  
______________________

(b) Other than light intensity, state two abiotic factors that could explain the difference in daisy distribution between the two areas. [2]  
________________________________________________________________________________
________________________________________________________________________________

(c) Explain why random sampling is important in this investigation. [2]  
________________________________________________________________________________
________________________________________________________________________________

16. The nitrogen cycle involves several groups of bacteria.

(a) Name one genus of free-living nitrogen-fixing bacteria and one genus of symbiotic nitrogen-fixing bacteria. [2]  
________________________________________________________________________________
________________________________________________________________________________

(b) Name the genus of bacteria responsible for each step of nitrification:  
(i) Ammonia → Nitrite [1]  
________________________________________________________________________________
(ii) Nitrite → Nitrate [1]  
________________________________________________________________________________

(c) Describe two negative environmental consequences of excessive nitrate leaching from agricultural fields into waterways. [3]  
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________

17. The diagram below shows the flow of energy through a simple food chain.

Image pending generation: diagram for Q17.

(a) Calculate the percentage of energy transferred from the producer to the primary consumer. [1]  
________________________________________________________________________________

(b) State two ways in which energy is lost between trophic levels. [2]  
________________________________________________________________________________
________________________________________________________________________________

(c) Explain why food chains rarely have more than four or five trophic levels. [2]  
________________________________________________________________________________
________________________________________________________________________________

18. A scientist studied the effect of temperature on the rate of decomposition of leaf litter in a forest ecosystem. The results are shown in the graph below.

Image pending generation: graph for Q18.

(a) Describe the effect of temperature on the rate of decomposition between 0°C and 35°C. [1]  
________________________________________________________________________________

(b) Explain why the rate of decomposition decreases above 35°C. [2]  
________________________________________________________________________________
________________________________________________________________________________

(c) Other than temperature, state one factor that affects the rate of decomposition. [1]  
________________________________________________________________________________

19. The diagram shows a simplified phosphorus cycle.

Image pending generation: diagram for Q19.

(a) Name the process that releases phosphate from rocks into the soil. [1]  
________________________________________________________________________________

(b) Explain why the phosphorus cycle is described as a "sedimentary cycle" rather than a "gaseous cycle" like the carbon or nitrogen cycles. [2]  
________________________________________________________________________________
________________________________________________________________________________

(c) Describe how human use of phosphate fertilisers can disrupt the natural phosphorus cycle. [2]  
________________________________________________________________________________
________________________________________________________________________________

20. An ecologist studied two species of barnacles, Chthamalus and Balanus, on a rocky shore. Balanus grows faster and outcompetes Chthamalus in the lower intertidal zone, but Chthamalus survives in the upper intertidal zone where Balanus cannot tolerate desiccation.

(a) Name the type of interaction between *Chthamalus* and *Balanus* in the lower intertidal zone. [1]  
________________________________________________________________________________

(b) Explain how this interaction leads to the observed distribution pattern of the two species on the rocky shore. [3]  
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________

(c) State the ecological principle illustrated by this example. [1]  
________________________________________________________________________________

End of Quiz

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Answers

Secondary 3 Biology Quiz - Ecology (Answer Key)

Total Marks: 40

Section A: Multiple Choice Questions (10 marks)

C — An ecosystem includes both the community of living organisms (biotic factors) and their physical environment (abiotic factors) interacting as a system. Option A describes a community, B describes populations, D describes a niche. [1]
B — The 10% rule (Lindeman's trophic efficiency rule) states that only about 10% of energy is transferred from one trophic level to the next; the rest is lost as heat, waste, and uneaten parts. [1]
A — Pyramids of numbers typically show fewer organisms at higher trophic levels because energy is lost at each transfer (respiration, heat, waste), so less energy is available to support biomass at higher levels. [1]
B — Nitrifying bacteria (e.g., Nitrosomonas and Nitrobacter) carry out nitrification: ammonia → nitrites → nitrates. Option A is nitrogen fixation, C is denitrification, D is ammonification. [1]
B — At carrying capacity, high population density increases competition, stress, and disease transmission. A disease outbreak is a classic density-dependent factor causing population crashes. [1]
C — Density-independent factors (e.g., forest fires, floods, droughts, temperature extremes) affect populations regardless of their density. Options A, B, D are density-dependent. [1]
C — Photosynthesis by plants, algae, and cyanobacteria removes CO₂ from the atmosphere and fixes it into organic compounds. Respiration, combustion, and decomposition release CO₂. [1]
B — Zooplankton are the direct food source for small fish. A decrease in zooplankton means less food for small fish, so their population would decrease most immediately. Phytoplankton might increase (less grazing), but the question asks for the most immediate effect. [1]
C — Mutualism: both species benefit (e.g., pollination, nitrogen-fixing bacteria in legume roots). Parasitism: one benefits, one harmed. Commensalism: one benefits, one unaffected. Predation: predator benefits, prey harmed. [1]
C — The dependent variable is what is measured (number of bubbles = rate of photosynthesis). Light intensity (A) and distance of lamp (D) are independent variables. Temperature (B) is a controlled variable. [1]

Section B: Structured Questions (18 marks)

(a) Respiration (by plants, animals, or decomposers) — all living organisms respire, releasing CO₂. [1]

(b) Photosynthesis — plants absorb CO₂ from the atmosphere to produce glucose. [1]

(c) Deforestation: Fewer trees mean less photosynthesis, so less CO₂ is removed from the atmosphere. Burning felled trees (combustion) releases stored carbon as CO₂.
Burning fossil fuels: Releases carbon that was locked underground for millions of years as CO₂, adding new carbon to the active carbon cycle.
Marking points: 1 mark for deforestation reducing photosynthesis, 1 mark for combustion from deforestation, 1 mark for fossil fuel combustion releasing ancient carbon. [3]
(a) Legumes have a mutualistic relationship with nitrogen-fixing bacteria (e.g., Rhizobium) in their root nodules. These bacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃), which the plant uses to make proteins. When legumes decompose, nitrogen-rich organic matter enriches the soil for the next crop. [2]
Marking points: 1 mark for nitrogen fixation by bacteria in nodules, 1 mark for nitrogen enrichment of soil upon decomposition.

(b) Bacteria: Rhizobium (or Bradyrhizobium).
Role: Convert atmospheric nitrogen (N₂) into ammonia (NH₃) / ammonium (NH₄⁺) — nitrogen fixation. [2]
Marking points: 1 mark for correct genus, 1 mark for role (nitrogen fixation).
(a) The populations show cyclical fluctuations (predator-prey cycles). The hare population peaks first, followed by the lynx population after a time lag. Both populations rise and fall in repeated cycles (~8–10 years). [2]
Marking points: 1 mark for cyclical pattern, 1 mark for prey peak preceding predator peak.

(b) When hare numbers are high, lynx have abundant food, so lynx survival and reproduction increase. This causes the lynx population to grow, but with a time lag because reproduction and growth take time. By the time lynx peak, they have over-consumed hares, causing the hare population to crash. [2]
Marking points: 1 mark for increased food → increased predator reproduction, 1 mark for time lag explanation.

(c) Density-dependent: Disease, competition for food, predation (by lynx), territoriality.
Density-independent: Extreme weather (harsh winter, drought), fire, flood, human habitat destruction. [2]
Marking points: 1 mark for each correct factor with correct classification.
(a) Phytoplankton (producers) have a very high turnover rate — they reproduce and are consumed rapidly, so their standing biomass at any moment is low despite high productivity. Zooplankton and fish have longer lifespans and accumulate more biomass. [2]
Marking points: 1 mark for high turnover/rapid reproduction of phytoplankton, 1 mark for low standing biomass despite high production.

(b) No, a pyramid of energy would not be inverted. Energy pyramids are always upright because energy is lost (as heat, waste, uneaten parts) at each trophic level (≈90% loss). Only about 10% of energy is transferred upward, so each higher level has less energy flow. [2]
Marking points: 1 mark for "no/not inverted", 1 mark for energy loss at each trophic level (10% rule).
(a) Mean = (15 + 18 + 12 + 20 + 16 + 14 + 19 + 17 + 13 + 15) / 10 = 159 / 10 = 15.9 daisies per quadrat [1]
Accept 15.9 or 16 (rounded).

(b) Any two of: soil moisture, soil pH, temperature, nutrient availability (nitrogen, phosphorus, potassium), soil compaction, wind exposure, water drainage. [2]
Marking points: 1 mark each for any two valid abiotic factors.

(c) Random sampling avoids bias — selecting quadrats where daisies are visible would overestimate the population (biased towards high-density patches). Random sampling gives every part of the field an equal chance of being sampled, providing a representative estimate of the true distribution and allowing valid statistical comparison between areas. [2]
Marking points: 1 mark for avoiding bias/overestimation, 1 mark for representative/unbiased estimate.
(a) Free-living: Azotobacter (or Clostridium, Azotobacter, Cyanobacteria like Nostoc).
Symbiotic: Rhizobium (or Bradyrhizobium, Frankia for non-legumes). [2]
Marking points: 1 mark for each correct genus in correct category.

(b) Step 1 (ammonia → nitrite): Nitrosomonas (or Nitrosococcus).
Step 2 (nitrite → nitrate): Nitrobacter (or Nitrospira). [2]
Marking points: 1 mark for each correct genus matching the correct step.

(c) Consequence 1: Eutrophication — Nitrates leach into water bodies, causing excessive algal growth (algal blooms). When algae die, decomposers respire aerobically, depleting dissolved oxygen, leading to death of aquatic animals.
Consequence 2: Groundwater contamination — High nitrate levels in drinking water can cause methaemoglobinaemia (blue baby syndrome) in infants, where nitrate reduces oxygen-carrying capacity of blood.
Alternative: Soil acidification, loss of biodiversity, N₂O emissions (greenhouse gas). [3]
Marking points: 1 mark for eutrophication with mechanism, 1 mark for human health/groundwater effect, 1 mark for any other valid consequence.
(a) Percentage = (10,000 / 100,000) × 100% = 10% [1]

(b) Any two of:
- Respiration (released as heat)
- Waste products (faeces, urine, excretory products)
- Uneaten parts (bones, fur, roots, stems not consumed)
- Movement and metabolic heat loss [2]
  Marking points: 1 mark each for any two valid energy loss pathways.
(c) Energy is lost at each trophic level (≈90% loss, only ~10% transferred). After 4–5 transfers, insufficient energy remains to support a viable population at the next trophic level. The biomass/energy at higher levels becomes too small to sustain another consumer level. [2]
Marking points: 1 mark for energy loss/10% rule, 1 mark for insufficient energy to support further levels.
(a) As temperature increases from 0°C to 35°C, the rate of decomposition increases (positive correlation). [1]

(b) Decomposition is carried out by enzymes from decomposers (bacteria, fungi). Above the optimum temperature (~35°C), enzymes denature — their active sites change shape, so they can no longer bind substrates effectively, causing the reaction rate to fall sharply. [2]
Marking points: 1 mark for enzyme involvement, 1 mark for denaturation at high temperature.

(c) Any one of: moisture/water availability, oxygen availability (aerobic vs anaerobic), pH, nutrient availability (C:N ratio), particle size/surface area of litter, presence of inhibitors/toxins. [1]
(a) Weathering (physical, chemical, or biological weathering of rocks). [1]

(b) The phosphorus cycle is a sedimentary cycle because phosphorus does not have a significant gaseous phase and does not cycle through the atmosphere. Its main reservoir is rocks and minerals, released by weathering, and its long-term sink is sedimentation forming new rocks. In contrast, carbon and nitrogen cycles have major atmospheric reservoirs (CO₂, N₂) and gaseous phases. [2]
Marking points: 1 mark for no atmospheric/gaseous phase, 1 mark for rock/sediment reservoir distinction.

(c) Mining phosphate rock for fertilisers accelerates the natural weathering rate massively. Runoff from fields carries excess phosphate into waterways, causing eutrophication (algal blooms, oxygen depletion). This short-circuits the slow geological cycle, moving phosphorus rapidly from land to aquatic sediments where it is locked up for long periods, depleting soil phosphorus over time. [2]
Marking points: 1 mark for accelerated mining/runoff, 1 mark for eutrophication or disruption of natural slow cycle.
(a) Interspecific competition (or competitive exclusion). [1]

(b) In the lower intertidal zone, conditions are favourable (submerged longer, less desiccation). Balanus grows faster and outcompetes Chthamalus for space, excluding it (competitive exclusion). In the upper intertidal zone, Balanus cannot survive desiccation stress (abiotic factor), so Chthamalus persists there — its realised niche is restricted to the upper zone by competition in the lower zone, while its fundamental niche includes both zones. This is competitive exclusion mediated by an environmental gradient. [3]
Marking points: 1 mark for competitive exclusion in lower zone, 1 mark for abiotic tolerance difference (desiccation) defining upper zone refuge, 1 mark for realised vs fundamental niche concept.

(c) Competitive Exclusion Principle (or Gause's Law: two species competing for the same limiting resource cannot coexist at constant population values; one will outcompete the other). [1]

End of Answer Key