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A Level H1 Biology Ecology Quiz

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A-Level Biology H1 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.
You are advised to spend approximately 2 minutes per mark.
Write in clear, concise scientific English. Use appropriate biological terminology.

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

1. Define the term ecosystem. [2]

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

3. Fig. 1.1 shows a pyramid of biomass for a terrestrial ecosystem.
(a) Explain why the pyramid of biomass for this ecosystem is always upright. [2]

(b) Suggest why a pyramid of numbers for the same ecosystem might be inverted. [1]

4. Energy transfer between trophic levels is inefficient.
(a) State two reasons why energy is lost between trophic levels. [2]

(b) Calculate the efficiency of energy transfer if the net primary production is $20,000 \text{ kJ m}^{-2} \text{ year}^{-1}$ and the net secondary production of primary consumers is $2,000 \text{ kJ m}^{-2} \text{ year}^{-1}$ . Show your working. [2]

5. Describe the role of decomposers in the cycling of nutrients within an ecosystem. [3]

Section B: Nutrient Cycles (Questions 6–10)

6. Nitrogen is essential for the synthesis of biological molecules.
(a) Name two biological molecules found in plants that contain nitrogen. [1]

(b) Explain why most plants cannot use atmospheric nitrogen ( $N_2$ ) directly. [1]

7. Describe the process of nitrification in the nitrogen cycle. Include the names of the bacteria involved and the chemical conversions. [3]

8. Fig. 1.2 illustrates the carbon cycle.
(a) Identify the process by which carbon dioxide is removed from the atmosphere by producers. [1]

(b) Identify the process by which carbon dioxide is returned to the atmosphere by all living organisms. [1]

9. Explain how the combustion of fossil fuels contributes to the enhancement of the greenhouse effect. [2]

10. In the phosphorus cycle, phosphate ions are released from rocks.
(a) Name the process that releases phosphate ions from rocks into the soil/water. [1]

(b) Explain why the phosphorus cycle is described as a "sedimentary cycle" rather than a "gaseous cycle". [1]

Section C: Population Ecology (Questions 11–15)

11. Define the term carrying capacity. [1]

12. Fig. 1.3 shows a sigmoid (S-shaped) population growth curve.
(a) Name the phase labeled X where the population growth rate is accelerating. [1]

(b) Explain why the population growth rate slows down as it approaches the carrying capacity (Phase Y). [2]

13. Distinguish between density-dependent and density-independent factors affecting population size. Give one example of each. [3]

14. A student used the mark-release-recapture method to estimate the population size of woodlice in a garden.

First catch: 40 woodlice marked and released.
Second catch (2 days later): 50 woodlice caught, of which 10 were marked.
(a) Calculate the estimated population size. Show your working. [2]

(b) State two assumptions made when using this method. [2]

15. Explain how competition acts as a biotic factor limiting population growth. [2]

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

16. Define biodiversity. [1]

17. Explain two reasons why the conservation of biodiversity is important for humans. [2]

18. Describe how eutrophication occurs in a freshwater lake following the runoff of fertilizers from agricultural land. [4]

19. Global warming is a consequence of increased greenhouse gas concentrations.
(a) Name one greenhouse gas other than carbon dioxide. [1]

(b) Explain one biological consequence of global warming on species distribution. [2]

20. Sustainable fishing practices are required to maintain fish stocks.
(a) Explain what is meant by Maximum Sustainable Yield (MSY). [2]

(b) Suggest one method, other than quotas, used to conserve fish stocks. [1]

*** End of Quiz ***

Answers

A-Level Biology H1 Quiz - Ecology (Answer Key)

Total Marks: 40

Section A: Ecosystems and Energy Flow

1. Define the term ecosystem. [2]

Answer: A community of living organisms (biotic factors) [1] interacting with each other and their physical environment (abiotic factors) [1].
Note: Must mention both biotic and abiotic components and their interaction.

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

Answer: 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 the complex feeding relationships in an ecosystem [1].

3. Pyramid of Biomass. [3]

(a) Why always upright? [2]
- Answer: Biomass decreases at each successive trophic level because energy is lost (as heat/respiration/waste) between levels [1]. Therefore, there is less energy available to support the biomass of the next level [1].
(b) Why might pyramid of numbers be inverted? [1]
- Answer: One large producer (e.g., a tree) can support many primary consumers (e.g., insects), resulting in fewer individuals at the producer level than the primary consumer level.

4. Energy Transfer. [4]

(a) Two reasons for energy loss. [2]
- Answer: (Any two)
  1. Energy lost as heat during respiration.
  2. Energy lost in excretory products (urine/faeces).
  3. Parts of the organism are not eaten/digested (e.g., bones, roots).
(b) Calculate efficiency. [2]
- Answer:
  $\text{Efficiency} = \left( \frac{\text{Net Secondary Production}}{\text{Net Primary Production}} \right) \times 100$ $Efficiency = (\frac{Net Secondary Production}{Net Primary Production}) \times 100$
  $= \left( \frac{2,000}{20,000} \right) \times 100 = 10\%$ $= (\frac{2 , 000}{20 , 000}) \times 100 = 10%$
  - Working: Correct substitution [1]. Answer: 10% [1].

5. Role of decomposers. [3]

Answer: Decomposers (saprophytic bacteria and fungi) secrete extracellular enzymes onto dead organic matter [1]. They digest the material externally and absorb the soluble nutrients [1]. This releases inorganic ions (e.g., nitrates, phosphates) back into the soil/water for use by producers [1].

Section B: Nutrient Cycles

6. Nitrogen in molecules. [2]

(a) Two molecules. [1]
- Answer: Proteins / Amino acids / DNA / RNA / ATP / Chlorophyll. (Any two correct).
(b) Why not use $N_2$ ? [1]
- Answer: The triple bond in atmospheric nitrogen ( $N \equiv N$ ) is very strong and stable, requiring high energy to break, which plants cannot provide. (Or: Plants lack the enzyme nitrogenase).

7. Nitrification. [3]

Answer:
1. Ammonium ions ( $NH_4^+$ ) are converted to nitrites ( $NO_2^-$ ) by Nitrosomonas bacteria [1].
2. Nitrites ( $NO_2^-$ ) are converted to nitrates ( $NO_3^-$ ) by Nitrobacter bacteria [1].
3. This is an aerobic process (requires oxygen) [1].

8. Carbon Cycle processes. [2]

(a) Removal: Photosynthesis [1].
(b) Return: Respiration [1].

9. Fossil fuels and greenhouse effect. [2]

Answer: Combustion releases carbon dioxide that was previously stored/locked away for millions of years [1]. This increases the atmospheric concentration of $CO_2$ , enhancing the trapping of long-wave infrared radiation (heat) [1].

10. Phosphorus Cycle. [2]

(a) Process: Weathering / Erosion [1].
(b) Sedimentary cycle: Phosphorus does not have a significant gaseous phase in the atmosphere (unlike Carbon or Nitrogen); it cycles mainly through soil, water, and sediments [1].

Section C: Population Ecology

11. Carrying capacity. [1]

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

12. Sigmoid Growth Curve. [3]

(a) Phase X: Exponential / Log phase [1].
(b) Slowing down (Phase Y). [2]
- Answer: As population increases, resources (food, space) become limited [1]. This leads to increased competition and/or accumulation of waste, increasing the death rate and/or decreasing the birth rate [1].

13. Density-dependent vs Independent. [3]

Answer:
- Density-dependent: Factors whose effect intensifies as population density increases (e.g., competition, disease, predation) [1]. Example: Disease spread [1].
- Density-independent: Factors that affect population regardless of density (e.g., natural disasters, temperature, pH) [1]. Example: Flood/Fire [1].
- (Note: 1 mark for distinction, 1 mark for each correct example).

14. Mark-Release-Recapture. [4]

(a) Calculation. [2]
- Formula: $N = \frac{n_1 \times n_2}{m_2}$
- $N = \frac{40 \times 50}{10} = \frac{2000}{10} = 200$
- Answer: 200 woodlice [1]. Working shown [1].
(b) Assumptions. [2]
- Answer: (Any two)
  1. Marked individuals mix randomly with the population.
  2. No births, deaths, immigration, or emigration during the interval.
  3. Marks do not affect survival (e.g., make them more visible to predators).
  4. Marks are not lost.

15. Competition. [2]

Answer: Individuals compete for limited resources (food, water, light, mates) [1]. Those better adapted survive and reproduce, while others die or fail to reproduce, limiting the population size [1].

Section D: Conservation and Human Impact

16. Biodiversity. [1]

Answer: The variety of living organisms (species diversity) in a particular area, including genetic diversity and ecosystem diversity [1]. (Accept: Variety of species).

17. Importance of conservation. [2]

Answer: (Any two)
1. Medical: Source of new drugs/medicines (e.g., from plants).
2. Agricultural: Genetic diversity for crop breeding (resistance to disease/pests).
3. Ecological: Maintains ecosystem stability/services (pollination, nutrient cycling).
4. Economic: Tourism/Ecotourism revenue.

18. Eutrophication. [4]

Answer:
1. Fertilizers (nitrates/phosphates) leach into water bodies [1].
2. This causes rapid growth of algae/algal blooms on the surface [1].
3. Algae block sunlight, causing submerged plants to die and decompose [1].
4. Decomposers (aerobic bacteria) multiply and use up dissolved oxygen during decomposition, leading to hypoxia/anoxia and death of aquatic animals (fish kills) [1].

19. Global Warming. [3]

(a) Other gas. [1]
- Answer: Methane ( $CH_4$ ) / Nitrous oxide ( $N_2O$ ) / Water vapour / CFCs.
(b) Consequence on distribution. [2]
- Answer: Species may migrate towards the poles or to higher altitudes to find suitable temperatures [1]. Species unable to migrate or adapt may face extinction/reduced range [1].

20. Sustainable Fishing. [3]

(a) Maximum Sustainable Yield (MSY). [2]
- Answer: The largest catch that can be taken from a fish stock over an indefinite period without depleting the stock [1]. It usually corresponds to the point where the population growth rate is highest (around half carrying capacity) [1].
(b) Other method. [1]
- Answer: (Any one)
  1. Use of larger mesh nets (to allow juveniles to escape).
  2. Closed seasons (breeding seasons).
  3. Marine protected areas / No-take zones.
  4. Limiting fishing effort (number of boats/days).