A Level H2 Biology Ecology Quiz

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

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 gross primary productivity (GPP) and net primary productivity (NPP). [2]

• GPP is the total chemical energy stored in plant biomass/organic molecules via photosynthesis. [1]
• NPP is GPP minus the energy lost/respired by the plants (R). ($NPP = GPP - R$). [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 excretory materials (urine/faeces) or uneaten parts (bones/roots). [1]
  (Accept: Energy used for movement/active transport)

4. A farmer observes that a field of crops has a higher biomass than the herbivores feeding on it. Explain this observation using the concept of the pyramid of biomass. [2]

• Energy transfer is inefficient (only ~10% passed on). [1]
• Therefore, there is insufficient energy to support a biomass of herbivores equal to or greater than the producers. [1]

5. In a freshwater ecosystem...
(a) Identify the trophic level of the small fish. [1]

• Secondary consumer (or Tertiary trophic level). [1]
  (b) Suggest why the population of small fish is usually smaller than the population of zooplankton. [1]
• Less energy is available at the higher trophic level to support individual organisms. [1]

6. Describe the role of saprobionts in the carbon cycle. [2]

• They secrete extracellular enzymes to digest dead organic matter externally. [1]
• They absorb the nutrients and release carbon dioxide back into the atmosphere via respiration. [1]

7. Explain how nitrogen-fixing bacteria in the root nodules of leguminous plants benefit the plant. [2]

• They convert atmospheric nitrogen ($N_2$) into ammonia/ammonium ions. [1]
• The plant uses these ions to synthesize amino acids/proteins/DNA. [1]

8. State the process by which ammonium ions ($NH_4^+$) are converted into nitrites ($NO_2^-$) and then nitrates ($NO_3^-$). Name the type of bacteria involved. [2]

• Process: Nitrification. [1]
• Bacteria: Nitrifying bacteria (e.g., Nitrosomonas and Nitrobacter). [1]

9. Explain why waterlogging of soil can lead to a decrease in soil fertility due to the action of denitrifying bacteria. [2]

• Waterlogging creates anaerobic conditions. [1]
• Denitrifying bacteria convert nitrates back into atmospheric nitrogen gas, removing usable nitrogen from the soil. [1]

10. Describe how the combustion of fossil fuels affects the carbon cycle and contributes to global warming. [2]

• Combustion releases carbon that was stored underground for millions of years as $CO_2$ into the atmosphere. [1]
• Increased atmospheric $CO_2$ enhances the greenhouse effect, trapping heat and raising global temperatures. [1]

11. Define carrying capacity in the context of population growth. [1]

• The maximum population size that an environment can sustain indefinitely. [1]

12. Sketch the general shape of a sigmoid (S-shaped) population growth curve. [2]

• Correct S-shape drawn. [1]
• Labels: Lag phase (initial slow growth), Exponential/Log phase (rapid growth), Stationary phase (plateau at carrying capacity). [1]

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

• Density-dependent: Effect intensifies as population density increases (e.g., competition, disease, predation). [1]
• Density-independent: Effect is not related to population density (e.g., natural disasters, temperature, floods). [1]

14. Explain how interspecific competition differs from intraspecific competition. [2]

• Interspecific: Competition between individuals of different species. [1]
• Intraspecific: Competition between individuals of the same species. [1]

15. A population of deer increases... Explain this stabilization in terms of limiting factors. [2]

• As population increases, resources (food/space) become limited. [1]
• This increases competition and mortality/reduces birth rate until the population stabilizes at carrying capacity. [1]

16. Define biodiversity at the genetic level. [1]

• The variety of different alleles/genes within a species or population. [1]

17. Explain how eutrophication leads to the death of fish in a lake. [3]

• Algal bloom blocks sunlight, causing underwater plants to die. [1]
• Saprobionts/aerobic bacteria decompose the dead matter, consuming large amounts of dissolved oxygen. [1]
• Fish die due to lack of oxygen (hypoxia/anoxia). [1]

18. Describe one method used to conserve biodiversity... [2]

• Example: Seed Banks. Seeds are collected, dried, and stored at low temperatures. [1]
• This preserves genetic diversity and allows for future restoration of plant species. [1]
  (Alternative: Protected Areas – restricts human activity/hunting, allowing habitats to recover.)

19. Explain why indicator species are useful... [2]

• Their presence, absence, or abundance reflects specific environmental conditions/pollution levels. [1]
• Example: Mayfly larvae indicate clean, oxygenated water; their absence indicates pollution. [1]

20. Discuss the ethical and ecological arguments for preserving biodiversity. [2]

• Ethical: Species have a right to exist; humans have a moral stewardship responsibility. [1]
• Ecological: High biodiversity ensures ecosystem stability/resilience and provides ecosystem services (e.g., pollination, nutrient cycling). [1]