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O Level Biology Ecology Quiz
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Questions
O-Level Biology Quiz - Ecology
Name: _________________________ Class: _________________________ Date: _________________________ Score: ______ / 50
Duration: 45 minutes Total Marks: 50
Instructions:
- This quiz contains 20 questions on the topic of Ecology (Organisms and their Environment).
- Answer ALL questions in the spaces provided.
- Marks for each question are indicated in brackets.
- Read each question carefully before answering.
Section A: Short Answer (10 marks)
Answer all questions in this section.
1. Define the term ecosystem. [1]
2. State the original source of energy for most ecosystems on Earth. [1]
3. Name the process by which green plants convert light energy into chemical energy. [1]
4. What term describes an organism that feeds on both plants and animals? [1]
5. State one reason why energy is lost between trophic levels in a food chain. [1]
6. Define the term population in ecology. [1]
7. Name the type of organism that breaks down dead organic matter and returns nutrients to the soil. [1]
8. What is meant by the term habitat? [1]
9. State the role of producers in an ecosystem. [1]
10. Name the process by which carbon is removed from the atmosphere by plants. [1]
Section B: Structured Questions (24 marks)
Answer all questions in this section.
11. The diagram below shows a simple food chain in a grassland ecosystem.
Grass → Grasshopper → Frog → Snake → Hawk
(a) Identify the producer in this food chain. [1]
(b) Name the secondary consumer. [1]
(c) Explain why the number of organisms generally decreases at each successive trophic level. [3]
(d) A disease kills most of the frogs in this ecosystem. Predict and explain the likely effect on the population of grasshoppers. [3]
12. The carbon cycle describes how carbon moves between the atmosphere, organisms, and the Earth.
(a) Name two processes that release carbon dioxide into the atmosphere. [2]
(b) Explain how deforestation can lead to an increase in atmospheric carbon dioxide concentration. [3]
(c) Describe the role of decomposers in the carbon cycle. [2]
13. A student investigated the distribution of plants in a field. The table below shows the results.
| Quadrat number | Number of dandelion plants |
|---|---|
| 1 | 4 |
| 2 | 7 |
| 3 | 5 |
| 4 | 8 |
| 5 | 6 |
(a) Calculate the mean number of dandelion plants per quadrat. Show your working. [2]
(b) Each quadrat has an area of 0.25 m². The total area of the field is 500 m². Estimate the total population of dandelion plants in the field. Show your working. [3]
(c) Suggest one way the student could improve the reliability of this investigation. [1]
14. The graph below shows changes in a population of rabbits and foxes in a woodland ecosystem over time.
[Imagine a graph with two lines: rabbit population (solid line) and fox population (dashed line). The rabbit population rises and falls in a cycle, and the fox population follows a similar pattern with a slight delay.]
(a) Describe the relationship between the rabbit and fox populations shown in the graph. [2]
(b) Explain why the fox population peaks shortly after the rabbit population peaks. [2]
Section C: Data-Based and Extended Response (16 marks)
Answer all questions in this section.
15. Read the following passage about eutrophication and answer the questions that follow.
Eutrophication occurs when excess nutrients, particularly nitrates and phosphates from agricultural fertilisers, enter water bodies such as lakes and rivers. These nutrients stimulate rapid growth of algae, forming an algal bloom on the water surface. The dense algal layer blocks sunlight from reaching submerged aquatic plants, causing them to die. As the algae and plants die, decomposer bacteria break down the dead organic matter. These bacteria use up dissolved oxygen in the water during aerobic respiration. The depletion of oxygen leads to the death of fish and other aquatic organisms.
(a) State the source of excess nutrients that causes eutrophication. [1]
(b) Explain why submerged aquatic plants die during an algal bloom. [2]
(c) Explain how the activity of decomposer bacteria leads to the death of fish. [3]
(d) Suggest one method farmers could use to reduce the risk of eutrophication in nearby water bodies. [1]
16. Discuss the importance of conserving biodiversity. In your answer, you should include ecological, economic, and ethical reasons. [6]
17. The diagram below shows a pyramid of biomass for a marine ecosystem.
[Imagine a pyramid with four levels, from bottom to top: Phytoplankton (largest), Zooplankton, Small fish, Large fish (smallest).]
(a) Explain why the pyramid of biomass has this shape. [2]
(b) Explain why a pyramid of numbers for this ecosystem might not have the same shape as the pyramid of biomass. [2]
18. Explain how the burning of fossil fuels contributes to global warming and describe two possible effects of global warming on ecosystems. [4]
19. A farmer sprays insecticide on crops to kill insect pests. Explain how this can lead to biomagnification of the insecticide in birds of prey that feed on small birds that eat the insects. [3]
20. Describe how energy flows through an ecosystem, from the Sun to decomposers. Explain why this flow of energy is described as non-cyclical. [4]
END OF QUIZ
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Answers
O-Level Biology Quiz - Ecology: Answer Key and Marking Scheme
Total Marks: 50
Section A: Short Answer (10 marks)
1. Define the term ecosystem.
- Answer: An ecosystem is a community of living organisms (biotic factors) interacting with each other and with their non-living (abiotic) environment. [1]
- Marking note: Accept "a community of organisms and their physical environment interacting as a system." Must include both biotic and abiotic components.
2. State the original source of energy for most ecosystems on Earth.
- Answer: The Sun / sunlight / solar energy. [1]
3. Name the process by which green plants convert light energy into chemical energy.
- Answer: Photosynthesis. [1]
4. What term describes an organism that feeds on both plants and animals?
- Answer: Omnivore. [1]
5. State one reason why energy is lost between trophic levels in a food chain.
- Answer: Any one of: [1]
- Energy is lost as heat during respiration.
- Energy is lost in undigested food / faeces / egested material.
- Energy is lost in excretory products (e.g., urea).
- Not all of the organism is eaten (e.g., bones, roots).
- Energy is used for movement / growth / other life processes.
6. Define the term population in ecology.
- Answer: A population is a group of organisms of the same species living in the same area/habitat at the same time. [1]
- Marking note: Must include "same species" and "same area."
7. Name the type of organism that breaks down dead organic matter and returns nutrients to the soil.
- Answer: Decomposers / decomposer bacteria / fungi / saprophytes. [1]
8. What is meant by the term habitat?
- Answer: A habitat is the natural environment / place where an organism lives. [1]
9. State the role of producers in an ecosystem.
- Answer: Producers convert light energy into chemical energy through photosynthesis / produce food (organic compounds) for themselves and other organisms in the ecosystem. [1]
10. Name the process by which carbon is removed from the atmosphere by plants.
- Answer: Photosynthesis. [1]
Section B: Structured Questions (24 marks)
11. Food chain: Grass → Grasshopper → Frog → Snake → Hawk
(a) Identify the producer in this food chain.
- Answer: Grass. [1]
(b) Name the secondary consumer.
- Answer: Frog. [1]
- Marking note: Secondary consumer is the organism that eats the primary consumer (grasshopper). Snake is tertiary consumer.
(c) Explain why the number of organisms generally decreases at each successive trophic level. [3]
- Answer: Energy is lost at each trophic level, mainly as heat from respiration, and in undigested food and excretory products. [1] Only about 10% of the energy is transferred from one trophic level to the next. [1] With less energy available at higher trophic levels, fewer organisms can be supported. [1]
- Marking note: Must mention energy loss and link to fewer organisms supported.
(d) A disease kills most of the frogs in this ecosystem. Predict and explain the likely effect on the population of grasshoppers. [3]
- Answer: The grasshopper population would increase. [1] With fewer frogs (predators), fewer grasshoppers are eaten. [1] Reduced predation pressure allows the grasshopper population to grow. [1]
- Marking note: Accept logical reasoning. Could also mention potential subsequent decrease due to overgrazing/food shortage.
12. Carbon cycle.
(a) Name two processes that release carbon dioxide into the atmosphere. [2]
- Answer: Any two of: [1 each]
- Respiration (by living organisms)
- Combustion / burning of fossil fuels
- Decomposition / decay by decomposers
- Volcanic activity (accept)
(b) Explain how deforestation can lead to an increase in atmospheric carbon dioxide concentration. [3]
- Answer: Trees/forests act as carbon sinks, absorbing carbon dioxide from the atmosphere through photosynthesis. [1] When trees are cut down, less carbon dioxide is removed from the atmosphere. [1] If the trees are burned, the stored carbon is released as carbon dioxide through combustion. Decomposition of remaining plant matter also releases carbon dioxide. [1]
- Marking note: Must mention reduced photosynthesis and release of stored carbon.
(c) Describe the role of decomposers in the carbon cycle. [2]
- Answer: Decomposers (bacteria and fungi) break down dead organic matter (dead plants and animals) and waste products. [1] During decomposition, they respire and release carbon dioxide back into the atmosphere, recycling carbon. [1]
13. Dandelion investigation.
(a) Calculate the mean number of dandelion plants per quadrat. [2]
- Answer: Mean = (4 + 7 + 5 + 8 + 6) ÷ 5 = 30 ÷ 5 = 6 dandelion plants per quadrat. [2]
- Marking note: Award [1] for correct sum, [1] for correct division and answer. Accept 6.0.
(b) Estimate the total population of dandelion plants in the field. [3]
- Answer: Number of quadrats that fit in the field = 500 ÷ 0.25 = 2000 quadrats. [1] Estimated total population = mean per quadrat × number of quadrats = 6 × 2000 = 12,000 dandelion plants. [2]
- Marking note: Award [1] for calculating number of quadrats, [1] for multiplication, [1] for correct answer with units or clear statement.
(c) Suggest one way the student could improve the reliability of this investigation. [1]
- Answer: Any one of: [1]
- Use more quadrats / take more samples.
- Place quadrats randomly (using random number generator/coordinates).
- Repeat the investigation at different times of year.
14. Rabbit and fox population graph.
(a) Describe the relationship between the rabbit and fox populations shown in the graph. [2]
- Answer: The rabbit and fox populations show a predator-prey relationship / cyclical pattern. [1] When the rabbit population increases, the fox population also increases (with a slight delay), and when the rabbit population decreases, the fox population also decreases. [1]
(b) Explain why the fox population peaks shortly after the rabbit population peaks. [2]
- Answer: When the rabbit population is high, there is an abundant food supply for the foxes. [1] This allows more foxes to survive and reproduce, so the fox population increases. The peak occurs after the rabbit peak because it takes time for the foxes to reproduce in response to the increased food availability. [1]
Section C: Data-Based and Extended Response (16 marks)
15. Eutrophication passage.
(a) State the source of excess nutrients that causes eutrophication.
- Answer: Agricultural fertilisers (containing nitrates and phosphates) / runoff from farmland. [1]
(b) Explain why submerged aquatic plants die during an algal bloom. [2]
- Answer: The dense algal bloom on the water surface blocks sunlight from reaching the submerged plants. [1] Without sufficient light, the submerged plants cannot carry out photosynthesis, so they cannot produce food and eventually die. [1]
(c) Explain how the activity of decomposer bacteria leads to the death of fish. [3]
- Answer: When algae and aquatic plants die, decomposer bacteria break down the dead organic matter. [1] The bacteria respire aerobically, using up dissolved oxygen in the water. [1] The depletion of dissolved oxygen means fish and other aquatic organisms cannot obtain enough oxygen for respiration, leading to their death (suffocation). [1]
(d) Suggest one method farmers could use to reduce the risk of eutrophication in nearby water bodies. [1]
- Answer: Any one of: [1]
- Use fertilisers more sparingly / only apply the required amount.
- Use organic/natural fertilisers instead of inorganic fertilisers.
- Create a buffer zone / strip of vegetation between farmland and water bodies to absorb excess nutrients.
- Avoid applying fertiliser before heavy rain.
16. Discuss the importance of conserving biodiversity. Include ecological, economic, and ethical reasons. [6]
- Answer: Award marks for a well-structured discussion covering the three areas:
Ecological reasons (up to 2 marks):
- Biodiversity maintains stable ecosystems; each species plays a role in nutrient cycling, pollination, seed dispersal, etc. [1]
- Loss of one species can disrupt food chains/webs, affecting other species and ecosystem balance. [1]
- Diverse ecosystems are more resilient to environmental changes (e.g., climate change, disease). [1]
Economic reasons (up to 2 marks):
- Many species provide direct economic benefits: food sources (fish, crops), medicines (plant-derived drugs), raw materials (timber, fibres). [1]
- Biodiversity supports ecotourism and recreation, generating income. [1]
- Genetic diversity in wild species can be used to improve crops and livestock. [1]
Ethical reasons (up to 2 marks):
-
Every species has a right to exist; humans have a moral responsibility to protect other species. [1]
-
Future generations have the right to inherit a planet with rich biodiversity. [1]
-
Many cultures and traditions value nature and wildlife intrinsically. [1]
-
Marking note: Award up to [2] for each category. Must cover all three categories for full marks. Accept other valid points. Quality of written communication and logical structure should be considered.
17. Pyramid of biomass.
(a) Explain why the pyramid of biomass has this shape. [2]
- Answer: Biomass decreases at each successive trophic level. [1] This is because energy is lost at each trophic level (as heat from respiration, in undigested food, and in excretory products), so less energy is available to be converted into biomass at the next level. [1]
(b) Explain why a pyramid of numbers for this ecosystem might not have the same shape as the pyramid of biomass. [2]
- Answer: A pyramid of numbers shows the number of individual organisms at each trophic level, not their mass. [1] In this ecosystem, a single large tree (producer) could support many insects (primary consumers), resulting in an inverted or irregular pyramid of numbers, even though the biomass of the tree is much greater. The phytoplankton are very small but numerous, so the pyramid of numbers could appear different from the pyramid of biomass. [1]
- Marking note: Must explain the difference between numbers and biomass.
18. Explain how the burning of fossil fuels contributes to global warming and describe two possible effects of global warming on ecosystems. [4]
- Answer: Contribution to global warming (up to 2 marks):
- Burning fossil fuels (coal, oil, natural gas) releases carbon dioxide (CO₂) into the atmosphere. [1]
- Carbon dioxide is a greenhouse gas that traps heat/infrared radiation in the Earth's atmosphere (the enhanced greenhouse effect), causing the Earth's average temperature to rise (global warming). [1]
Effects on ecosystems (up to 2 marks, any two):
-
Rising sea levels due to melting polar ice caps and thermal expansion of oceans, leading to flooding of coastal habitats (e.g., mangroves, coral reefs). [1]
-
Changes in temperature and rainfall patterns can alter habitats, causing species to migrate or become extinct if they cannot adapt. [1]
-
Coral bleaching occurs when ocean temperatures rise, damaging coral reef ecosystems. [1]
-
Disruption of life cycles and food webs, e.g., mismatched timing of flowering and pollinator emergence. [1]
-
Marking note: Award [2] for explanation of global warming mechanism, [2] for two valid effects on ecosystems.
19. Explain how spraying insecticide on crops can lead to biomagnification of the insecticide in birds of prey. [3]
- Answer: Insecticide sprayed on crops is absorbed/ingested by insect pests. The insecticide is not easily broken down or excreted, so it accumulates in the insects' bodies. [1] Small birds eat many insects, ingesting the insecticide from each insect. The insecticide accumulates to a higher concentration in the small birds' bodies. [1] Birds of prey eat many small birds, so the insecticide becomes even more concentrated in their bodies. This increasing concentration of a toxic substance at successive trophic levels is called biomagnification. The high concentration can harm or kill the birds of prey. [1]
- Marking note: Must describe the stepwise increase in concentration through the food chain.
20. Describe how energy flows through an ecosystem, from the Sun to decomposers. Explain why this flow of energy is described as non-cyclical. [4]
- Answer: Energy flow (up to 3 marks):
- Light energy from the Sun is captured by producers (green plants) during photosynthesis and converted into chemical energy stored in organic compounds (biomass). [1]
- Primary consumers (herbivores) obtain energy by feeding on producers. Energy is transferred from producers to primary consumers. [1]
- Secondary and tertiary consumers obtain energy by feeding on other consumers. Energy is transferred along the food chain. [1]
- Decomposers obtain energy by breaking down dead organic matter and waste products from all trophic levels. [1]
Non-cyclical nature (1 mark):
-
Energy flow is non-cyclical because energy cannot be recycled. It enters the ecosystem as light energy from the Sun and is eventually lost as heat from respiration at each trophic level. Unlike nutrients (e.g., carbon, nitrogen), energy does not return to the Sun or to producers. [1]
-
Marking note: Award up to [3] for describing energy flow through trophic levels, [1] for explaining why it is non-cyclical (energy is lost as heat, cannot be reused).
END OF ANSWER KEY