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A Level H2 Biology Ecology Quiz

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A-Level Biology H2 Quiz - Ecology

Name: __________________________
Class: __________________________
Date: __________________________
Score: ______ / 40

Duration: 45 minutes
Total Marks: 40
Instructions:

Answer all questions.
Write your answers in the spaces provided.
The number of marks is given in brackets [ ] at the end of each question or part question.
Use clear scientific terminology and refer to specific biological processes where appropriate.

Section A: Ecosystem Dynamics and Energy Flow (Questions 1–5)

1. Define the term ecosystem. [2]

2. Distinguish between a food chain and a food web. [2]

3. Explain why energy transfer between trophic levels is inefficient, citing two specific reasons for energy loss. [2]

4. A student calculated the net primary productivity (NPP) of a grassland ecosystem. The gross primary productivity (GPP) was found to be 20,000 kJ m⁻² yr⁻¹, and the respiratory loss (R) by producers was 15,000 kJ m⁻² yr⁻¹.
Calculate the NPP. Show your working. [2]

5. With reference to the Second Law of Thermodynamics, explain why food chains rarely exceed four or five trophic levels. [3]

Section B: Nutrient Cycling (Questions 6–10)

6. State the role of nitrogen-fixing bacteria in the nitrogen cycle. [2]

7. Explain how nitrifying bacteria contribute to soil fertility. [2]

8. Describe the process of denitrification and state the environmental condition required for it to occur. [2]

9. Fig. 1 shows a simplified carbon cycle.
(Note: Imagine a diagram showing Atmosphere CO₂, Plants, Animals, Decomposers, and Fossil Fuels)
Identify the process labelled X where carbon moves from the atmosphere to plants, and process Y where carbon moves from dead organic matter to decomposers. [2]

10. Explain how the combustion of fossil fuels disrupts the natural carbon cycle and contributes to global warming. [3]

Section C: Populations and Conservation (Questions 11–15)

11. Define carrying capacity in the context of population ecology. [2]

12. Distinguish between density-dependent and density-independent factors, giving one example of each. [2]

13. Explain the concept of competitive exclusion principle. [2]

14. A population of rabbits was introduced to an island with no predators. Describe the expected shape of the population growth curve over time and explain the phases. [3]

15. Suggest two reasons why small, isolated populations are at a higher risk of extinction. [2]

Section D: Human Impact and Environmental Issues (Questions 16–20)

16. Explain the process of eutrophication in a freshwater lake, starting from the addition of nitrate fertilizers. [4]

17. Describe how biomagnification of pesticides (e.g., DDT) affects top predators in a food web. [3]

18. Explain why biodiversity is important for ecosystem stability. [2]

19. Discuss the impact of deforestation on the global carbon cycle. [3]

20. Evaluate the effectiveness of captive breeding programs as a conservation strategy for endangered species. Mention one advantage and one limitation. [3]

Answers

A-Level Biology H2 Quiz - Ecology (Answer Key)

Total Marks: 40

Section A: Ecosystem Dynamics and Energy Flow

1. Define the term ecosystem. [2]

A community of living organisms (biotic factors) [1]
Interacting with their physical environment (abiotic factors) as a system. [1]

2. Distinguish between a food chain and a food web. [2]

A food chain shows a single, linear pathway of energy flow from producer to consumer. [1]
A food web shows multiple interconnected food chains, representing complex feeding relationships in an ecosystem. [1]

3. Explain why energy transfer between trophic levels is inefficient, citing two specific reasons for energy loss. [2]

Energy is lost as heat during respiration/metabolic processes. [1]
Energy is lost in undigested material (faeces) or excretory products (urea). [1]
(Accept: Not all parts of organism eaten)

4. Calculate the NPP. [2]

Formula: NPP = GPP - R [1]
Calculation: 20,000 - 15,000 = 5,000 kJ m⁻² yr⁻¹ [1]

5. With reference to the Second Law of Thermodynamics, explain why food chains rarely exceed four or five trophic levels. [3]

The Second Law states that energy transformations are inefficient and entropy increases (energy is lost as heat). [1]
Only about 10% of energy is transferred to the next trophic level. [1]
After 4-5 levels, insufficient energy remains to support a viable population of higher-order consumers. [1]

Section B: Nutrient Cycling

6. State the role of nitrogen-fixing bacteria in the nitrogen cycle. [2]

They convert atmospheric nitrogen (N₂) into ammonia (NH₃) / ammonium ions (NH₄⁺). [1]
This makes nitrogen available to plants for synthesis of amino acids/proteins. [1]

7. Explain how nitrifying bacteria contribute to soil fertility. [2]

They oxidize ammonium ions (NH₄⁺) into nitrites (NO₂⁻) and then nitrates (NO₃⁻). [1]
Nitrates are the primary form of nitrogen absorbed by plant roots for growth. [1]

8. Describe the process of denitrification and state the environmental condition required for it to occur. [2]

Denitrifying bacteria convert nitrates (NO₃⁻) back into atmospheric nitrogen (N₂). [1]
This occurs in anaerobic conditions (waterlogged soils lacking oxygen). [1]

9. Identify processes X and Y. [2]

X: Photosynthesis [1]
Y: Decomposition / Saprotrophic nutrition [1]

10. Explain how the combustion of fossil fuels disrupts the natural carbon cycle and contributes to global warming. [3]

Combustion releases carbon dioxide (CO₂) that was stored underground for millions of years. [1]
This increases the concentration of CO₂ in the atmosphere faster than natural sinks (oceans/plants) can absorb it. [1]
CO₂ is a greenhouse gas that traps infrared radiation, leading to an enhanced greenhouse effect/global warming. [1]

Section C: Populations and Conservation

11. Define carrying capacity. [2]

The maximum population size of a species [1]
That an environment can sustain indefinitely given the available resources. [1]

12. Distinguish between density-dependent and density-independent factors. [2]

Density-dependent factors (e.g., competition, disease) have a greater effect as population density increases. [1]
Density-independent factors (e.g., natural disasters, temperature) affect population regardless of its density. [1]

13. Explain the concept of competitive exclusion principle. [2]

Two species competing for the exact same limited resources cannot coexist at constant population values. [1]
One species will outcompete the other, leading to the local extinction or displacement of the weaker competitor. [1]

14. Describe the expected shape of the population growth curve and explain the phases. [3]

Sigmoid (S-shaped) curve. [1]
Initial exponential (log) phase due to abundant resources and low competition. [1]
Stationary phase as resources become limited and carrying capacity is reached. [1]

15. Suggest two reasons why small, isolated populations are at a higher risk of extinction. [2]

Reduced genetic diversity leads to inbreeding depression and reduced ability to adapt to environmental changes. [1]
Vulnerability to stochastic events (demographic or environmental fluctuations) which can wipe out the small population. [1]

Section D: Human Impact and Environmental Issues

16. Explain the process of eutrophication. [4]

Leaching of nitrates/phosphates from fertilizers into water bodies. [1]
Causes rapid growth of algae (algal bloom) on the surface. [1]
Algae block sunlight, causing submerged plants to die and decompose. [1]
Decomposers (bacteria) use up dissolved oxygen during decomposition, leading to hypoxia/anoxia and death of aquatic animals. [1]

17. Describe how biomagnification affects top predators. [3]

Pesticides (e.g., DDT) are non-biodegradable and fat-soluble, so they accumulate in organism tissues. [1]
Concentration of the toxin increases at each successive trophic level as consumers eat many prey items. [1]
Top predators accumulate toxic levels, leading to reproductive failure (e.g., thin eggshells) or death. [1]

18. Explain why biodiversity is important for ecosystem stability. [2]

High biodiversity ensures functional redundancy; if one species declines, others can fulfill its ecological niche. [1]
Increases resilience to environmental changes and disturbances. [1]

19. Discuss the impact of deforestation on the global carbon cycle. [3]

Trees store large amounts of carbon in their biomass; cutting them releases this carbon (if burned or decomposed). [1]
Removal of trees reduces the rate of photosynthesis, meaning less CO₂ is removed from the atmosphere. [1]
Soil disturbance releases stored soil carbon. [1]

20. Evaluate captive breeding programs. [3]

Advantage: Increases population numbers and genetic diversity through managed breeding; protects from poaching/habitat loss. [1]
Limitation: Animals may lose natural behaviors required for survival in the wild (e.g., hunting, avoiding predators). [1]
Limitation: Expensive and does not address the root cause of extinction (habitat loss). [1]
(Award marks for clear evaluation of one advantage and one limitation)