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O Level Biology Ecology Quiz
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Questions
O-Level Biology Quiz - Ecology
Name: ________________________
Class: ________________________
Date: ________________________
Score: ______ / 40
Duration: 45 minutes
Total Marks: 40
Instructions:
- Answer ALL questions in the spaces provided.
- Write your answers clearly and legibly.
- Marks for each question are indicated in brackets [ ].
- Where calculations are required, show your working.
- Diagrams may be drawn in pencil.
Section A: Short Answer Questions (10 marks)
Answer all questions in this section.
1. Define the term ecosystem. [1]
2. State the role of decomposers in an ecosystem. [1]
3. Name the process by which carbon is removed from the atmosphere by plants. [1]
4. Explain why energy flow in an ecosystem is described as non-cyclical. [2]
5. Distinguish between a food chain and a food web. [2]
Section B: Structured Questions (10 marks)
Answer all questions in this section.
6. State one reason why a pyramid of biomass is generally a better representation of energy relationships in an ecosystem than a pyramid of numbers. [1]
7. Name the process by which nitrogen gas in the atmosphere is converted into nitrates usable by plants. [1]
8. Define the term biodiversity. [1]
9. The diagram below shows a simplified food web in a freshwater pond ecosystem.
Heron
↑
Pike (fish)
↑
┌────────────┼────────────┐
↑ ↑ ↑
Water beetle Dragonfly Tadpole
↑ nymph ↑ ↑
│ │ │
Water flea Mayfly nymph │
↑ ↑ │
└───────┬───────┘ │
↑ │
Algae ←──────────────┘
(a) Name one producer in this food web. [1]
(b) Construct a food chain from this web that contains exactly four trophic levels. [2]
(c) Explain what would happen to the population of water beetles if the pike were removed from the pond. [3]
(d) The heron is a top consumer. Explain why there are usually fewer organisms at the highest trophic level in a food chain. [3]
Section C: Structured Questions (10 marks)
Answer all questions in this section.
10. The carbon cycle describes how carbon moves between the atmosphere, living organisms, and the Earth.
(a) Name two processes that release carbon dioxide into the atmosphere. [2]
(b) Explain how deforestation contributes to an increase in atmospheric carbon dioxide concentration. [3]
(c) Oceans act as carbon sinks. Describe how carbon dioxide from the atmosphere becomes stored in the ocean. [2]
(d) Suggest one way in which an individual can reduce their carbon footprint. [1]
11. The graph below shows changes in the population sizes of a predator (lynx) and its prey (snowshoe hare) over a period of time.
Population
size
↑
│ /\ /\ /\
│ / \ / \ / \
│ / \ / \ / \
│ / \/ \/ \
│/ \
└──────────────────────────→ Time
─── Lynx
─ ─ Hare
(a) Describe the relationship between the population sizes of the lynx and the hare shown in the graph. [2]
(b) Explain why the peak in the lynx population occurs slightly after the peak in the hare population. [2]
Section D: Data-Based and Extended Response Questions (10 marks)
Answer all questions in this section.
12. A student investigated the effect of sewage pollution on the distribution of organisms in a river. Samples were taken at four sites (A, B, C, and D) along the river. Site A was upstream of a sewage outlet. Site B was immediately downstream of the outlet. Sites C and D were further downstream. The table below shows the results.
| Organism | Site A | Site B | Site C | Site D |
|---|---|---|---|---|
| Stonefly nymph | 15 | 0 | 2 | 8 |
| Mayfly nymph | 20 | 1 | 5 | 12 |
| Water louse | 3 | 18 | 10 | 4 |
| Sludgeworm | 0 | 25 | 12 | 2 |
| Dissolved oxygen (mg/dm³) | 8.5 | 2.0 | 4.5 | 7.0 |
(a) Describe the change in dissolved oxygen concentration from Site A to Site B. Suggest a reason for this change. [2]
(b) Stonefly nymphs are sensitive to pollution, while sludgeworms are tolerant of pollution. Explain how the data supports this statement. [2]
(c) Explain why the dissolved oxygen concentration increases from Site B to Site D. [2]
(d) Suggest how the student could ensure the reliability of the data collected. [1]
13. Coral reefs are among the most biodiverse ecosystems on Earth. However, they are threatened by rising sea temperatures, which cause coral bleaching.
(a) Explain how rising sea temperatures lead to coral bleaching. [2]
(b) Describe the importance of conserving coral reef ecosystems. [3]
14. The nitrogen cycle is essential for maintaining soil fertility.
(a) Name the process by which ammonium compounds are converted into nitrites and then nitrates. [1]
(b) Explain the role of denitrifying bacteria in the nitrogen cycle. [2]
15. Explain how the use of chemical fertilizers on farmland can lead to eutrophication in nearby water bodies. [3]
16. Suggest two reasons why it is important to conserve biodiversity. [2]
17. A farmer notices that a population of aphids on his crops is increasing rapidly. He introduces ladybirds, which are natural predators of aphids.
(a) Name this method of pest control. [1]
(b) State one advantage and one disadvantage of this method compared to using chemical pesticides. [2]
18. Explain how burning fossil fuels contributes to global warming. [3]
19. Describe how deforestation can lead to soil erosion. [2]
20. A student set up a sealed mesocosm containing pond water, aquatic plants, small fish, and decomposers. Explain why the mesocosm must be placed in sunlight to sustain life. [3]
END OF QUIZ
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Answers
O-Level Biology Quiz - Ecology: Answer Key and Marking Scheme
Total Marks: 40
Section A: Short Answer Questions (10 marks)
1. Define the term ecosystem. [1]
Answer: An ecosystem is a community of living organisms (biotic factors) interacting with each other and with their non-living (abiotic) environment.
Marking notes:
- Award [1] for reference to both biotic (living organisms/community) AND abiotic (non-living/physical environment) components and their interaction.
- Accept: "A self-sustaining unit consisting of living organisms and their physical environment interacting together."
2. State the role of decomposers in an ecosystem. [1]
Answer: Decomposers break down dead organic matter and waste materials, releasing nutrients (mineral ions/inorganic nutrients) back into the soil/environment for reuse by producers.
Marking notes:
- Award [1] for reference to breakdown of dead matter/waste AND release/recycling of nutrients.
- Accept: "Decomposers recycle nutrients in an ecosystem."
3. Name the process by which carbon is removed from the atmosphere by plants. [1]
Answer: Photosynthesis.
Marking notes:
- Award [1] for "photosynthesis" only.
- Do not accept "respiration" or "carbon fixation" alone.
4. Explain why energy flow in an ecosystem is described as non-cyclical. [2]
Answer: Energy enters the ecosystem as light energy from the sun and is converted to chemical energy by producers through photosynthesis. As energy passes along food chains, some energy is lost at each trophic level as heat through respiration, and some is lost in undigested matter and waste. This lost energy cannot be reused by the ecosystem, so energy must be continuously supplied from the sun. Energy does not cycle back to the beginning of the food chain.
Marking notes:
- Award [1] for stating that energy enters from the sun/is not recycled.
- Award [1] for explaining that energy is lost as heat/respiration at each trophic level and cannot be reused.
- Accept reference to energy loss through metabolic activities/heat loss.
5. Distinguish between a food chain and a food web. [2]
Answer: A food chain is a linear sequence showing the transfer of energy from one organism to another, with each organism feeding on the one before it. A food web consists of multiple interconnected food chains, showing the complex feeding relationships between organisms in an ecosystem.
Marking notes:
- Award [1] for describing a food chain as a single/linear feeding sequence.
- Award [1] for describing a food web as interconnected/multiple food chains showing complex feeding relationships.
- Accept diagrams with clear labels as part of the explanation.
Section B: Structured Questions (10 marks)
6. State one reason why a pyramid of biomass is generally a better representation of energy relationships in an ecosystem than a pyramid of numbers. [1]
Answer: A pyramid of biomass takes into account the mass/size of organisms, whereas a pyramid of numbers only counts the number of organisms regardless of their size. A pyramid of numbers can be inverted (e.g., one large tree supporting many insects), which does not accurately represent energy flow, while a pyramid of biomass is rarely inverted.
Marking notes:
- Award [1] for any valid reason, such as: accounts for organism size/mass; avoids inverted pyramids; better represents the amount of living material/energy available at each trophic level.
- Accept: "Biomass gives a more accurate measure of the energy content at each level."
7. Name the process by which nitrogen gas in the atmosphere is converted into nitrates usable by plants. [1]
Answer: Nitrogen fixation.
Marking notes:
- Award [1] for "nitrogen fixation."
- Accept "nitrification" only if the context clearly refers to the overall conversion from atmospheric nitrogen to nitrates (though technically nitrification is the conversion of ammonium to nitrates).
- Do not accept "denitrification."
8. Define the term biodiversity. [1]
Answer: Biodiversity refers to the variety of living organisms in an ecosystem, including the variety of species, genetic diversity within species, and the variety of ecosystems.
Marking notes:
- Award [1] for reference to variety/diversity of living organisms/species in an area/ecosystem.
- Accept: "The range/variety of different species in a habitat/ecosystem."
9. Freshwater pond food web.
(a) Name one producer in this food web. [1]
Answer: Algae.
Marking notes:
- Award [1] for "algae" only.
(b) Construct a food chain from this web that contains exactly four trophic levels. [2]
Answer: Algae → Water flea → Dragonfly nymph → Pike
OR Algae → Tadpole → Pike → Heron
OR Algae → Mayfly nymph → Dragonfly nymph → Pike
(Any valid four-level chain from the web)
Marking notes:
- Award [1] for correct sequence starting with a producer (algae).
- Award [1] for exactly four trophic levels with arrows pointing in the correct direction (showing energy flow).
- Deduct [1] if arrows point in the wrong direction.
(c) Explain what would happen to the population of water beetles if the pike were removed from the pond. [3]
Answer: The pike is a predator that feeds on organisms such as dragonfly nymphs and possibly water beetles. If pike are removed, the population of dragonfly nymphs would increase due to reduced predation. Dragonfly nymphs compete with water beetles for food (water fleas and mayfly nymphs). With more dragonfly nymphs, there would be increased competition for food, leading to a decrease in the water beetle population. Alternatively, if pike also prey on water beetles directly, their removal might initially cause an increase in water beetles, but increased competition from other prey species would eventually limit their numbers.
Marking notes:
- Award [1] for identifying that pike is a predator of organisms in the web.
- Award [1] for explaining the effect on intermediate organisms (e.g., dragonfly nymph population increases).
- Award [1] for linking to competition for food resources and the effect on water beetle population.
- Accept reasoned arguments for either increase or decrease, provided the explanation is logical and references the food web.
(d) The heron is a top consumer. Explain why there are usually fewer organisms at the highest trophic level in a food chain. [3]
Answer: Energy is lost at each trophic level as it is transferred along the food chain. Only about 10% of the energy from one trophic level is transferred to the next. Energy is lost through respiration (as heat), in undigested food materials (egested as faeces), and in excretory products (urine). Because less energy is available at higher trophic levels, the ecosystem can support fewer organisms at these levels. The heron, as a top consumer, receives only a small fraction of the original energy captured by the producers.
Marking notes:
- Award [1] for stating that energy is lost at each trophic level.
- Award [1] for explaining how energy is lost (respiration/heat, undigested matter, excretion).
- Award [1] for linking reduced energy availability to fewer organisms at higher trophic levels.
- Accept reference to the 10% energy transfer rule.
Section C: Structured Questions (10 marks)
10. The carbon cycle.
(a) Name two processes that release carbon dioxide into the atmosphere. [2]
Answer: Any two from:
- Respiration
- Combustion (burning of fossil fuels/wood)
- Decomposition (by decomposers)
- Volcanic activity
Marking notes:
- Award [1] for each correct process (maximum 2).
- Do not accept "photosynthesis" or "dissolving in oceans."
(b) Explain how deforestation contributes to an increase in atmospheric carbon dioxide concentration. [3]
Answer: Deforestation involves the removal of trees and forests. Trees act as carbon sinks, removing carbon dioxide from the atmosphere through photosynthesis and storing carbon in their biomass. When trees are cut down and burned or left to decompose, the stored carbon is released back into the atmosphere as carbon dioxide. Additionally, fewer trees mean less carbon dioxide is removed from the atmosphere through photosynthesis, leading to a net increase in atmospheric carbon dioxide concentration.
Marking notes:
- Award [1] for stating that trees remove CO₂ through photosynthesis/act as carbon sinks.
- Award [1] for explaining that burning/decomposition of trees releases stored carbon as CO₂.
- Award [1] for linking reduced photosynthesis to less CO₂ removal.
- Accept reference to the use of machinery/transport in deforestation releasing CO₂.
(c) Oceans act as carbon sinks. Describe how carbon dioxide from the atmosphere becomes stored in the ocean. [2]
Answer: Carbon dioxide from the atmosphere dissolves directly into the surface waters of the ocean. Some of this dissolved carbon dioxide is used by marine organisms like phytoplankton for photosynthesis, incorporating carbon into their bodies. When these organisms die, their remains sink to the ocean floor, where the carbon can be stored in sediments for long periods. Some dissolved carbon dioxide also reacts with seawater to form carbonate compounds, which can be used by organisms to build shells.
Marking notes:
- Award [1] for stating that CO₂ dissolves in ocean water.
- Award [1] for describing storage through photosynthesis by marine organisms/sedimentation/formation of carbonates.
- Accept reference to the biological pump or physical dissolution.
(d) Suggest one way in which an individual can reduce their carbon footprint. [1]
Answer: Any one from:
- Using public transport, cycling, or walking instead of driving a car.
- Reducing energy consumption at home (e.g., switching off lights, using energy-efficient appliances).
- Reducing, reusing, and recycling materials.
- Eating less meat, particularly beef.
- Planting trees.
Marking notes:
- Award [1] for any valid and specific suggestion.
- Do not accept vague answers like "be more environmentally friendly."
11. Predator-prey graph.
(a) Describe the relationship between the population sizes of the lynx and the hare shown in the graph. [2]
Answer: The population sizes of the lynx and the hare show a cyclical pattern. When the hare population increases, the lynx population also increases after a short time lag. When the hare population decreases, the lynx population also decreases after a time lag. The peaks and troughs of the lynx population follow those of the hare population.
Marking notes:
- Award [1] for describing the cyclical/oscillating pattern.
- Award [1] for stating that changes in the lynx population follow changes in the hare population (with a time lag).
- Accept reference to predator-prey relationship.
(b) Explain why the peak in the lynx population occurs slightly after the peak in the hare population. [2]
Answer: The hare is the prey, and the lynx is the predator. When the hare population is high, there is an abundant food supply for the lynx, allowing them to survive and reproduce more successfully, leading to an increase in the lynx population. However, this increase takes time due to the lynx's reproductive rate, so the peak in the lynx population occurs after the peak in the hare population. The time lag represents the time needed for the lynx population to respond to the increased food availability through reproduction.
Marking notes:
- Award [1] for stating that increased hare numbers provide more food for lynx, leading to increased lynx reproduction/survival.
- Award [1] for explaining that the time lag is due to the time needed for the lynx population to grow through reproduction.
- Accept reference to the predator-prey cycle dynamics.
Section D: Data-Based and Extended Response Questions (10 marks)
12. Sewage pollution investigation.
(a) Describe the change in dissolved oxygen concentration from Site A to Site B. Suggest a reason for this change. [2]
Answer: The dissolved oxygen concentration decreases sharply from 8.5 mg/dm³ at Site A to 2.0 mg/dm³ at Site B. This is because sewage contains organic matter, which is decomposed by aerobic bacteria. These bacteria use up oxygen from the water for respiration, leading to a decrease in dissolved oxygen concentration.
Marking notes:
- Award [1] for describing the decrease (using data).
- Award [1] for explaining that aerobic bacteria/decomposers use oxygen to break down organic matter in sewage.
- Accept reference to increased biochemical oxygen demand (BOD).
(b) Stonefly nymphs are sensitive to pollution, while sludgeworms are tolerant of pollution. Explain how the data supports this statement. [2]
Answer: At Site A (unpolluted), there are 15 stonefly nymphs and 0 sludgeworms. At Site B (polluted), the number of stonefly nymphs drops to 0, while the number of sludgeworms increases to 25. This shows that stonefly nymphs cannot survive in polluted conditions (low oxygen), whereas sludgeworms thrive in polluted conditions where there is abundant organic matter and they can tolerate low oxygen levels.
Marking notes:
- Award [1] for comparing the numbers of stonefly nymphs at Site A and Site B (decrease/disappearance).
- Award [1] for comparing the numbers of sludgeworms at Site A and Site B (increase) and linking to pollution tolerance.
- Accept reference to dissolved oxygen levels as an indicator of pollution.
(c) Explain why the dissolved oxygen concentration increases from Site B to Site D. [2]
Answer: As the river flows downstream from the sewage outlet, the organic matter from the sewage is gradually broken down by decomposers. This reduces the amount of organic matter available for decomposition, so less oxygen is used by aerobic bacteria. Additionally, the river water is aerated as it flows, and oxygen from the atmosphere dissolves into the water. Photosynthesis by aquatic plants and algae further downstream also releases oxygen into the water.
Marking notes:
- Award [1] for stating that organic matter is broken down/used up, reducing oxygen demand.
- Award [1] for mentioning aeration/mixing with air or photosynthesis by aquatic plants/algae adding oxygen.
- Accept reference to dilution of pollutants.
(d) Suggest how the student could ensure the reliability of the data collected. [1]
Answer: Any one from:
- Take multiple samples at each site and calculate an average.
- Repeat the investigation on different days/seasons.
- Use the same sampling method/equipment at each site.
- Ensure samples are collected at the same time of day.
Marking notes:
- Award [1] for any valid method to improve reliability (replication, standardisation, repetition).
- Do not accept "be careful" or "use accurate equipment" without further detail.
13. Coral reefs and coral bleaching.
(a) Explain how rising sea temperatures lead to coral bleaching. [2]
Answer: Corals have a mutualistic relationship with zooxanthellae (algae) living in their tissues. The zooxanthellae provide the coral with food through photosynthesis and give corals their colour. When sea temperatures rise, the corals become stressed and expel the zooxanthellae. Without the zooxanthellae, the coral loses its colour and appears white (bleached). If the stress is prolonged, the coral may die.
Marking notes:
- Award [1] for stating that corals expel their symbiotic algae/zooxanthellae under heat stress.
- Award [1] for linking the loss of algae to the loss of colour (bleaching).
- Accept reference to the breakdown of the symbiotic relationship.
(b) Describe the importance of conserving coral reef ecosystems. [3]
Answer: Coral reefs are biodiversity hotspots, providing habitat for a vast number of marine species. They act as nurseries for many fish species, supporting fisheries and food security for millions of people. Coral reefs protect coastlines from erosion and storm damage by absorbing wave energy. They are also a source of potential new medicines and provide economic benefits through tourism and recreation.
Marking notes:
- Award [1] for mentioning biodiversity/habitat provision.
- Award [1] for mentioning coastal protection or fisheries/food security.
- Award [1] for mentioning economic value (tourism, medicines) or any other valid ecological/economic reason.
- Accept any three distinct and valid points.
14. The nitrogen cycle.
(a) Name the process by which ammonium compounds are converted into nitrites and then nitrates. [1]
Answer: Nitrification.
Marking notes:
- Award [1] for "nitrification" only.
- Do not accept "nitrogen fixation" or "denitrification."
(b) Explain the role of denitrifying bacteria in the nitrogen cycle. [2]
Answer: Denitrifying bacteria convert nitrates in the soil back into nitrogen gas, which is released into the atmosphere. This process, called denitrification, reduces the amount of nitrates available to plants and completes the nitrogen cycle by returning nitrogen to the atmosphere.
Marking notes:
- Award [1] for stating that they convert nitrates into nitrogen gas.
- Award [1] for stating that this returns nitrogen to the atmosphere/completes the cycle.
- Accept reference to anaerobic conditions often required for denitrification.
15. Explain how the use of chemical fertilizers on farmland can lead to eutrophication in nearby water bodies. [3]
Answer: Chemical fertilizers contain nitrates and phosphates. When excess fertilizer is applied, it can be washed into nearby rivers and lakes by rain (runoff/leaching). These nutrients cause a rapid increase in the growth of algae and aquatic plants (algal bloom). The algal bloom blocks sunlight from reaching submerged plants, causing them to die. When the algae and plants die, they are decomposed by aerobic bacteria, which use up oxygen from the water. This leads to a decrease in dissolved oxygen, causing the death of fish and other aquatic organisms.
Marking notes:
- Award [1] for stating that fertilizers (nitrates/phosphates) are washed into water bodies.
- Award [1] for describing the algal bloom and its effect (blocking light/plant death).
- Award [1] for explaining that decomposition of dead matter uses oxygen, leading to oxygen depletion and death of aquatic life.
- Accept reference to biochemical oxygen demand (BOD).
16. Suggest two reasons why it is important to conserve biodiversity. [2]
Answer: Any two from:
- Ecological stability: Biodiverse ecosystems are more resilient to changes and disturbances.
- Economic value: Many species provide resources like food, medicine, and raw materials.
- Aesthetic and cultural value: Natural environments provide recreational opportunities and have cultural significance.
- Ethical reasons: Every species has a right to exist, and we have a responsibility to protect them for future generations.
- Ecosystem services: Biodiversity supports essential services like pollination, nutrient cycling, and water purification.
Marking notes:
- Award [1] for each valid and distinct reason (maximum 2).
- Accept any reasonable ecological, economic, aesthetic, or ethical reason.
17. Biological pest control.
(a) Name this method of pest control. [1]
Answer: Biological control (or biocontrol).
Marking notes:
- Award [1] for "biological control" or "biocontrol."
- Do not accept "using predators" alone without the term "biological control."
(b) State one advantage and one disadvantage of this method compared to using chemical pesticides. [2]
Answer:
- Advantage: It is specific to the pest, so it does not harm other beneficial organisms / pests do not develop resistance / it is environmentally friendly and does not leave chemical residues / it provides long-term control once established.
- Disadvantage: It may be slower to act than chemical pesticides / the predator may become a pest itself or affect non-target species / it may not completely eradicate the pest / the predator population may be affected by environmental conditions.
Marking notes:
- Award [1] for a valid advantage.
- Award [1] for a valid disadvantage.
- Accept any reasonable advantage and disadvantage.
18. Explain how burning fossil fuels contributes to global warming. [3]
Answer: Burning fossil fuels (coal, oil, natural gas) releases carbon dioxide and other greenhouse gases into the atmosphere. These greenhouse gases trap heat from the sun in the Earth's atmosphere, a process known as the greenhouse effect. An increase in the concentration of greenhouse gases enhances this effect, trapping more heat and causing the Earth's average temperature to rise, leading to global warming and climate change.
Marking notes:
- Award [1] for stating that burning fossil fuels releases CO₂/greenhouse gases.
- Award [1] for explaining the greenhouse effect (trapping heat).
- Award [1] for linking increased greenhouse gases to enhanced greenhouse effect and global warming.
- Accept reference to other greenhouse gases like methane, but CO₂ is the primary one from fossil fuels.
19. Describe how deforestation can lead to soil erosion. [2]
Answer: Tree roots bind soil particles together, holding the soil in place. When trees are removed through deforestation, there are no roots to anchor the soil. The soil becomes loose and is easily washed away by rain (surface runoff) or blown away by wind. The removal of the tree canopy also means that the soil is directly exposed to the impact of raindrops, which can dislodge soil particles and increase erosion.
Marking notes:
- Award [1] for stating that tree roots bind soil/remove of roots leads to loose soil.
- Award [1] for explaining that rain/wind can then easily remove the soil.
- Accept reference to the protective role of the canopy.
20. Mesocosm experiment.
Answer: Sunlight is essential for the aquatic plants to carry out photosynthesis. During photosynthesis, plants use light energy to convert carbon dioxide and water into glucose and oxygen. The oxygen produced is used by the fish and decomposers for respiration. The plants also provide a food source for the fish. Without sunlight, photosynthesis would stop, oxygen would be depleted, and the organisms would eventually die. Sunlight thus sustains the ecosystem by providing the initial energy input.
Marking notes:
- Award [1] for stating that sunlight is needed for photosynthesis by aquatic plants.
- Award [1] for explaining that photosynthesis produces oxygen needed for respiration by fish/decomposers.
- Award [1] for linking the production of oxygen/food to the survival of the organisms in the mesocosm.
- Accept reference to sunlight as the primary energy source for the ecosystem.
END OF ANSWER KEY