Secondary 3 Biology Ecology Quiz

Secondary 3 Biology Quiz - Ecology: Answer Key and Marking Scheme

Total Marks: 40

Section A: Multiple Choice (5 marks)

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

• Explanation: Decomposers (bacteria, fungi) break down dead organisms and waste, releasing nutrients back into the soil for producers to use. Option A describes producers, C describes consumers, D describes plants.

2. B) 10% [1 mark]

• Explanation: Approximately 10% of energy is transferred from one trophic level to the next. The remaining 90% is lost through respiration, movement, heat, and undigested material.

3. B) A tropical rainforest [1 mark]

• Explanation: Carbon sinks absorb more carbon dioxide from the atmosphere than they release. Forests and oceans are major carbon sinks. Volcanic eruptions release carbon, cattle produce methane, and decomposing logs release carbon.

4. B) Energy enters as light and is eventually lost as heat. [1 mark]

• Explanation: Energy flow is non-cyclical because energy enters ecosystems as sunlight, is converted by producers, passed through food chains, and ultimately lost as heat through respiration. It cannot be recycled.

5. B) The accumulation of non-biodegradable substances at higher trophic levels. [1 mark]

• Explanation: Biomagnification is the increasing concentration of toxins (e.g., pesticides, heavy metals) in organisms at successively higher trophic levels. These substances are not broken down and accumulate in fatty tissues.

Section B: Short Answer (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 air, water, and soil. [1 mark]

• Award 1 mark for a definition that includes both biotic and abiotic components and their interaction.

7. A food chain is a single linear sequence showing the transfer of energy from one organism to another (who eats whom). A food web is a network of interconnected food chains showing the multiple feeding relationships in an ecosystem. [2 marks]

• Award 1 mark for defining food chain correctly.
• Award 1 mark for defining food web and highlighting the interconnected/multiple pathway nature.

8. There are fewer organisms at higher trophic levels because only about 10% of energy is transferred from one trophic level to the next. Most energy is lost through respiration, movement, heat, and undigested material. With less energy available at each successive level, the ecosystem can support fewer organisms. [2 marks]

• Award 1 mark for mentioning energy loss between trophic levels (10% transfer).
• Award 1 mark for explaining that less energy available means fewer organisms can be supported.

9. Any two of the following (1 mark each, max 2 marks):

• Energy is lost as heat through respiration.
• Energy is used for movement and other life processes.
• Not all parts of the organism are eaten (e.g., bones, roots).
• Some food is not digested and is lost as faeces.
• Energy is lost through excretion (e.g., urine).

10. (a) Percentage transferred = (100 ÷ 1000) × 100 = 10% [1 mark]

• Award 1 mark for correct calculation and answer.

(b) Any one of the following [1 mark]:

• Energy is lost between trophic levels (only ~10% transferred).
• Frogs use energy for respiration, movement, and other life processes.
• Not all grasshoppers are eaten by frogs.
• Some parts of grasshoppers are indigestible.

11. Forests absorb carbon dioxide from the atmosphere through photosynthesis and store carbon in their biomass (wood, leaves, roots). Oceans absorb carbon dioxide from the atmosphere; some dissolves directly, and some is used by marine phytoplankton for photosynthesis. Both act as carbon sinks because they remove more carbon dioxide than they release. [2 marks]

• Award 1 mark for explaining the role of forests (photosynthesis, carbon storage in biomass).
• Award 1 mark for explaining the role of oceans (dissolution, phytoplankton photosynthesis).

12. Burning fossil fuels (coal, oil, natural gas) releases carbon dioxide that was stored underground for millions of years into the atmosphere. This increases the concentration of greenhouse gases. Carbon dioxide traps heat in the Earth's atmosphere (enhanced greenhouse effect), causing global temperatures to rise (global warming). [2 marks]

• Award 1 mark for stating that burning fossil fuels releases stored carbon as CO₂.
• Award 1 mark for explaining the greenhouse effect link (CO₂ traps heat, causing warming).

13. Any two of the following (1 mark each, max 2 marks):

• Rising sea levels due to melting ice caps and thermal expansion, leading to coastal flooding and habitat loss.
• Changes in weather patterns (more extreme weather events, droughts, floods).
• Disruption of ecosystems and loss of biodiversity as species cannot adapt quickly enough.
• Coral bleaching due to increased ocean temperatures.
• Shifts in species distribution as organisms move to cooler areas.

Section C: Structured and Data-Based Questions (20 marks)

14. (a) Algae [1 mark]

• Accept: Any organism that produces its own food through photosynthesis.

(b) Water beetle OR Small fish [1 mark]

• Explanation: Secondary consumers eat primary consumers. Water beetles eat water fleas (primary consumers); small fish eat water beetles/tadpoles.

(c) Algae → Water flea → Water beetle → Small fish OR Algae → Tadpole → Small fish → Kingfisher [1 mark]

• Award 1 mark for any correct four-level food chain from the web.

(d) If all small fish were removed, the population of water fleas would increase. This is because small fish prey on water beetles, which prey on water fleas. With small fish removed, the water beetle population would increase, leading to more predation on water fleas. However, if water beetles also eat water fleas directly, the prediction may vary. A more direct answer: Water fleas are eaten by water beetles and small fish. If small fish are removed, water beetle populations may increase, potentially decreasing water flea populations. [2 marks]

• Award 1 mark for a clear prediction.
• Award 1 mark for a logical explanation based on feeding relationships in the food web.
• Note: Accept any well-reasoned prediction that demonstrates understanding of food web dynamics.

15. (a) Dissolved oxygen increases as distance from the sewage outlet increases. [1 mark]

• Award 1 mark for correctly describing the trend.

(b) At the sewage outlet, there is a high concentration of organic matter from the sewage. Microorganisms (decomposers) break down this organic matter through aerobic respiration, using up large amounts of dissolved oxygen. This results in a low dissolved oxygen level at the outlet. [2 marks]

• Award 1 mark for mentioning decomposition of organic matter by microorganisms.
• Award 1 mark for linking decomposition to oxygen consumption (aerobic respiration).

(c) Mayfly nymphs are only found in higher numbers further downstream where dissolved oxygen is high and pollution is low, indicating clean water. Sludge worms are found in high numbers near the sewage outlet where dissolved oxygen is low and pollution is high, indicating polluted water. The presence or absence of these indicator species can be used to assess water quality. [2 marks]

• Award 1 mark for linking mayfly distribution to clean water.
• Award 1 mark for linking sludge worm distribution to polluted water and explaining the concept of indicator species.

(d) Any one of the following (award 1 mark for type, 1 mark for effect, max 2 marks):

• Plastic pollution: Plastic waste can be ingested by aquatic animals, causing injury or death. Microplastics can enter food chains and undergo biomagnification.
• Chemical/industrial waste: Toxic chemicals can poison aquatic life directly or accumulate in food chains.
• Fertiliser runoff (eutrophication): Excess nutrients cause algal blooms, which deplete oxygen when they decompose, killing fish and other aquatic life.
• Oil spills: Oil coats the surface, reducing oxygen exchange, and coats animals, affecting their insulation and buoyancy.

16. Conservation is important to maintain biodiversity, preserve ecosystems for future generations, and ensure the sustainable use of natural resources. Two sustainable practices include:

1. Sustainable forestry: Replanting trees after logging to ensure forests are not depleted and continue to act as carbon sinks and habitats.
2. Sustainable fishing: Setting catch limits and using fishing methods that avoid bycatch to prevent overfishing and maintain fish populations. [3 marks]

• Award 1 mark for explaining the importance of conservation.
• Award 1 mark for each well-described sustainable practice (max 2 marks).

17. Insecticides sprayed on crops may be non-biodegradable. These chemicals are absorbed by plants (producers) and accumulate in their tissues. When primary consumers (e.g., insects) eat the plants, they ingest the insecticide. The insecticide is not broken down and accumulates in their bodies. At each successive trophic level, the concentration of insecticide increases (biomagnification). Top predators consume many contaminated prey items, resulting in very high concentrations of insecticide in their bodies, which can cause illness, reproductive failure, or death. [2 marks]

• Award 1 mark for explaining the transfer through the food chain.
• Award 1 mark for explaining biomagnification (increasing concentration at higher trophic levels).

18. Any two of the following (1 mark each, max 2 marks):

• Deforestation: Cutting down forests reduces the number of trees available to absorb CO₂ through photosynthesis. Burning forests also directly releases stored carbon as CO₂.
• Agriculture (livestock farming): Cattle and other ruminants produce methane (a greenhouse gas) through digestion. Rice paddies also release methane.
• Industrial processes (cement production): The production of cement releases CO₂ as a by-product.
• Land use change: Converting natural ecosystems to agricultural or urban land reduces carbon storage capacity.

19. (a) The pyramid of numbers is not pyramid-shaped because one large producer (oak tree) supports many small primary consumers (caterpillars). The base of the pyramid (one tree) is smaller than the level above it (thousands of caterpillars). [1 mark]

• Award 1 mark for explaining that the producer is a single large organism supporting many smaller consumers.

(b) A pyramid of biomass would be a better representation because it shows the total mass of living material at each trophic level rather than just the number of organisms. The biomass of one oak tree is much greater than the biomass of thousands of caterpillars, so the pyramid would have the typical pyramid shape with a broad base. [1 mark]

• Award 1 mark for explaining that biomass accounts for the size/mass of organisms, giving a more accurate representation of energy available.

20. Energy flow is non-cyclical because energy enters the ecosystem as light energy from the sun, is converted to chemical energy by producers through photosynthesis, and is passed along food chains. At each trophic level, energy is lost as heat through respiration and cannot be reused by the ecosystem. Energy must be continuously supplied by the sun.

Nutrient flow (e.g., carbon) is cyclical because nutrients are recycled within the ecosystem. In the carbon cycle, carbon dioxide is absorbed by plants during photosynthesis and converted to organic compounds. When organisms respire, carbon is returned to the atmosphere as CO₂. When organisms die, decomposers break down their remains, releasing carbon back into the soil and atmosphere. Fossil fuels store carbon for millions of years, but burning them releases this carbon back into the cycle. Thus, carbon atoms are reused continuously, unlike energy which flows in one direction. [3 marks]

• Award 1 mark for explaining that energy enters as light and is lost as heat (non-cyclical).
• Award 1 mark for explaining that carbon is recycled through photosynthesis, respiration, and decomposition.
• Award 1 mark for clearly contrasting the two processes with reference to the carbon cycle.

END OF ANSWER KEY