Secondary 4 Pure Biology Ecology Quiz

Secondary 4 Pure Biology Quiz - Ecology — Answer Key

Total Marks: 40

Section A: Multiple Choice Questions (5 marks)

1. B — Decomposers break down dead organic matter and return nutrients to the soil.
[1 mark]

2. B — The remaining 90% of energy is lost as heat through respiration and used for metabolic activities.
[1 mark]

3. B — Burning fossil fuels releases large amounts of CO₂, a greenhouse gas, directly contributing to the enhanced greenhouse effect.
[1 mark]

4. B — Energy is lost at each trophic level (approximately 90%), so fewer organisms can be supported at higher trophic levels, resulting in a pyramid shape.
[1 mark]

5. B — Biomagnification is the increase in concentration of toxic substances (e.g., insecticides, heavy metals) in organisms at successively higher trophic levels.
[1 mark]

Section B: Short Answer Questions (15 marks)

6. An ecosystem is a community of living organisms (biotic factors) interacting with each other and with their non-living (abiotic) environment, such as water, soil, and climate, in a defined area.
[2 marks — 1 mark for biotic interaction, 1 mark for abiotic interaction]

7. A food chain is a single linear sequence showing the transfer of energy from one organism to another (e.g., grass → rabbit → fox). A food web consists of multiple interconnected food chains, showing the complex feeding relationships within an ecosystem.
[2 marks — 1 mark for food chain definition, 1 mark for food web definition]

8. Energy flow is non-cyclical because energy enters the ecosystem as sunlight, is converted to chemical energy by producers, and is progressively lost as heat at each trophic level through respiration; it cannot be recycled. Nutrient flow is cyclical because nutrients (e.g., carbon, nitrogen) are taken up by organisms, released through decomposition and waste, and reused by producers, forming a continuous cycle.
[3 marks — 1 mark for energy entering as sunlight, 1 mark for energy lost as heat/non-recyclable, 1 mark for nutrients being recycled via decomposition]

9. The insecticide sprayed on crops is absorbed by pests and may also remain on plant surfaces. Small mammals (e.g., mice) eat the contaminated plants or insects, accumulating the insecticide in their bodies. Since the insecticide is non-biodegradable, it is not broken down and becomes more concentrated in the mammals' tissues. When birds of prey eat many small mammals, they accumulate even higher concentrations of the insecticide through biomagnification, which can cause poisoning, reproductive failure, or death.
[3 marks — 1 mark for insecticide entering food chain, 1 mark for accumulation in small mammals, 1 mark for biomagnification harming birds of prey]

10. Two ways deforestation increases atmospheric CO₂:

1. Fewer trees remain to carry out photosynthesis, so less CO₂ is removed from the atmosphere.
2. Burning or decomposition of felled trees releases stored carbon back into the atmosphere as CO₂.
   [2 marks — 1 mark for each valid point]

Section C: Structured Questions (10 marks)

11. At each trophic level, approximately 90% of energy is lost through respiration, heat, movement, and undigested material. Only about 10% of energy is transferred to the next trophic level. By the fourth trophic level, very little energy remains from the original amount captured by producers. This limited energy can support only a small number of organisms, so there are usually fewer organisms at higher trophic levels.
[3 marks — 1 mark for energy loss at each level, 1 mark for 10% transfer, 1 mark for linking limited energy to fewer organisms]

12.
(a) Algae / Water plants (any one).
[1 mark]

(b) Frog / Kingfisher / Small fish (any one).
[1 mark]

(c) Any valid four-level chain, e.g.: Algae → Mayfly → Dragonfly → Frog
OR: Water plants → Water flea → Small fish → Kingfisher
[2 marks — 1 mark for correct sequence, 1 mark for exactly four trophic levels]

(d) In the short term, the dragonfly population would likely increase. This is because frogs are predators of dragonflies. With fewer frogs, predation pressure on dragonflies is reduced, allowing more dragonflies to survive and reproduce.
[2 marks — 1 mark for population increase, 1 mark for reduced predation]

(e) The heron and kingfisher are in competition because they both feed on small fish (and possibly other shared prey) from the same pond ecosystem. They occupy similar ecological niches and compete for the same limited food resource.
[1 mark — for shared food source and competition for limited resource]

Section D: Data-Based and Extended Response Questions (10 marks)

13.
(a) The population of mayfly nymphs decreased from 45 organisms per m² at Site A to 0 organisms per m² at Site B.
[1 mark — must include data or clear description of decrease]

(b) Tubifex worms are pollution-tolerant organisms that thrive in water with low oxygen concentration. Sewage pollution at Site B introduces large amounts of organic matter. Decomposers break down this organic matter using up dissolved oxygen, creating conditions that favour Tubifex worms while eliminating competitors and predators.
[2 marks — 1 mark for identifying Tubifex as pollution-tolerant, 1 mark for linking to low oxygen from sewage decomposition]

(c) Stickleback fish require high levels of dissolved oxygen to survive. At Site B, sewage pollution leads to a decrease in dissolved oxygen due to decomposition by bacteria. The low oxygen levels cannot support the fish, causing their population to decline to zero.
[2 marks — 1 mark for fish needing high oxygen, 1 mark for oxygen depletion at Site B]

(d) The conclusion is partially supported. The mayfly nymph population increased from 0 at Site B to 10 at Site C, and stickleback fish increased from 0 to 2, indicating some recovery of oxygen-sensitive species. However, the populations are still much lower than at Site A (45 and 8 respectively), showing that full recovery has not occurred. The Tubifex worm population decreased from 120 to 30, further suggesting improving water quality, but still higher than Site A (5).
[3 marks — 1 mark for noting recovery of sensitive species, 1 mark for noting populations still below Site A levels, 1 mark for using specific data to support]

14.
(a) Photosynthesis.
[1 mark]

(b) Any two from: Respiration, Combustion (burning of fossil fuels), Decomposition (by decomposers), Volcanic activity.
[2 marks — 1 mark for each correct process]

(c) Fossil fuels (coal, oil, natural gas) contain carbon that was stored underground for millions of years. Burning them releases this stored carbon into the atmosphere as CO₂ at a rate much faster than natural processes can remove it. This increases atmospheric CO₂ concentration, enhancing the greenhouse effect and disrupting the natural balance of the carbon cycle.
[2 marks — 1 mark for releasing stored carbon, 1 mark for faster release than removal]

(d) Any one valid suggestion with explanation, e.g.:

• Using public transport or cycling instead of driving a car — this reduces the burning of fossil fuels, thereby reducing CO₂ emissions.
• Reducing meat consumption — livestock farming produces significant methane (a greenhouse gas) and requires deforestation for grazing land, so eating less meat lowers an individual's carbon footprint.
• Reducing energy consumption at home (e.g., switching off lights, using energy-efficient appliances) — this reduces demand for electricity generated from fossil fuels.
  [2 marks — 1 mark for valid suggestion, 1 mark for explanation linking to carbon emissions]

15. Conserving tropical rainforests is important for biodiversity because they are home to over half of the world's plant and animal species, many of which are not found anywhere else. Deforestation leads to habitat loss and species extinction, reducing global biodiversity. Rainforests also regulate the Earth's climate by absorbing large amounts of atmospheric CO₂ through photosynthesis, acting as a carbon sink. They influence rainfall patterns and help maintain global temperature stability. Destroying rainforests releases stored carbon, exacerbating climate change.
[4 marks — 1 mark for high biodiversity, 1 mark for habitat loss/extinction, 1 mark for carbon sink/CO₂ absorption, 1 mark for climate regulation]

16. A trophic level is the position an organism occupies in a food chain, defined by its feeding relationship and energy source (e.g., producer, primary consumer, secondary consumer).
[1 mark]

17. An invasive species may have no natural predators in the new ecosystem, allowing its population to grow rapidly. It can outcompete native species for resources (food, space), prey on native species, or introduce diseases, leading to a decline or extinction of native populations and disrupting the existing food web balance.
[2 marks — 1 mark for rapid population growth/lack of predators, 1 mark for outcompeting/preying on native species]

18. Advantage: Biological control is specific to the pest and does not leave harmful chemical residues in the environment.
Disadvantage: The control organism may become invasive itself or take time to reduce the pest population effectively.
[2 marks — 1 mark for valid advantage, 1 mark for valid disadvantage]

19. A pyramid of biomass represents the total dry mass of organisms at each trophic level, which more accurately reflects the amount of energy available. A pyramid of numbers can be misleading, for example, one large tree (producer) can support many insects, creating an inverted pyramid of numbers, whereas the pyramid of biomass would still show a greater mass at the producer level.
[2 marks — 1 mark for biomass reflecting energy/mass, 1 mark for explaining limitation of pyramid of numbers with example]

20. High biodiversity increases ecosystem stability because a greater variety of species provides functional redundancy. If one species is affected by disease or environmental change, other species with similar roles can compensate, maintaining ecosystem processes like nutrient cycling and energy flow.
[2 marks — 1 mark for functional redundancy, 1 mark for maintaining ecosystem processes]

END OF ANSWER KEY