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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.
Write your answers in the spaces provided.
The number of marks for each question or part question is given in brackets [ ].

Section A: Multiple Choice Questions (Questions 1–5)

Choose the correct answer and write the letter in the box provided.

1. Which of the following statements correctly describes the flow of energy in an ecosystem?
A. Energy flows in a cycle from decomposers to producers.
B. Energy flows in one direction from producers to consumers.
C. Energy is recycled between trophic levels.
D. Energy increases as it moves up the food chain.

Answer: [ ] [1]

2. In a food web, which organism is always a primary consumer?
A. A herbivore
B. A carnivore
C. A decomposer
D. A producer

Answer: [ ] [1]

3. Why is the pyramid of biomass for a terrestrial ecosystem usually upright (broad base, narrow top)?
A. Producers are always larger than consumers.
B. Energy is lost at each trophic level, supporting less biomass at higher levels.
C. Consumers reproduce faster than producers.
D. Decomposers have the highest biomass.

Answer: [ ] [1]

4. Which human activity contributes most directly to the enhancement of the greenhouse effect?
A. Planting more trees in urban areas
B. Burning fossil fuels for electricity generation
C. Using organic fertilisers in agriculture
D. Recycling plastic waste

Answer: [ ] [1]

5. In the nitrogen cycle, which process converts atmospheric nitrogen into nitrates in the soil?
A. Denitrification
B. Nitrification
C. Nitrogen fixation
D. Decomposition

Answer: [ ] [1]

Section B: Structured Questions (Questions 6–10)

6. The diagram below represents a simple food chain in a grassland ecosystem.

Image pending generation for this question.

Grass → Grasshopper → Frog → Snake

(a) Identify the producer in this food chain.
_________________________________________________________________________ [1]

(b) Explain why the snake is considered a tertiary consumer.

_________________________________________________________________________ [2]

7. Energy transfer between trophic levels is inefficient.

(a) State the approximate percentage of energy transferred from one trophic level to the next.
_________________________________________________________________________ [1]

(b) Give two reasons why energy is lost between trophic levels.

_______________________________________________________________________ [2]

8. The carbon cycle involves several key processes.

(a) Name the process by which green plants remove carbon dioxide from the atmosphere.
_________________________________________________________________________ [1]

(b) Name the process by which all living organisms release carbon dioxide into the atmosphere.
_________________________________________________________________________ [1]

_________________________________________________________________________ [2]

9. Deforestation has significant impacts on the environment.

(a) Explain how deforestation contributes to global warming.

_________________________________________________________________________ [3]

(b) Suggest one method to conserve forests while still allowing for economic development.
_________________________________________________________________________ [1]

10. A farmer uses an insecticide to control a pest population in his crop field. The insecticide is not biodegradable.

(a) Define the term biomagnification.

_________________________________________________________________________ [2]

(b) Explain why birds of prey at the top of the food chain are more affected by the insecticide than the insects.

_________________________________________________________________________ [3]

Section C: Structured Questions (Questions 11–15)

11. The graph below shows the population growth of a species of bacteria in a closed culture over time.

Image pending generation for this question.

(Imagine a sigmoid growth curve with phases: Lag, Log, Stationary, Death)

(a) Name the phase where the population growth rate is highest.
_________________________________________________________________________ [1]

(b) Explain why the population enters the stationary phase.

_________________________________________________________________________ [3]

12. Coral reefs are diverse ecosystems but are threatened by climate change.

(a) Explain how rising sea temperatures can lead to coral bleaching.

_________________________________________________________________________ [3]

(b) State one importance of coral reefs to humans.
_________________________________________________________________________ [1]

13. In an ecosystem, decomposers play a vital role.

(a) Name two groups of organisms that act as decomposers.

_______________________________________________________________________ [2]

(b) Explain the role of decomposers in the recycling of nutrients.

_________________________________________________________________________ [3]

14. Eutrophication is a process that can occur in lakes and rivers.

(a) Describe how excessive use of fertilisers on nearby farmland can lead to eutrophication.

_________________________________________________________________________ [3]

(b) State the effect of eutrophication on the fish population in the water body.
_________________________________________________________________________ [1]

15. Sustainable development aims to meet present needs without compromising future generations.

(a) Define sustainable development.

_________________________________________________________________________ [2]

(b) Give one example of sustainable fishing practice.
_________________________________________________________________________ [1]

Section D: Free Response Questions (Questions 16–20)

16. Mangroves are coastal forests found in tropical regions. They provide a habitat for many species.
Construct a food web involving the following organisms found in the mangrove:
Mangrove leaves, Detritus (decaying matter), Crab, Heron, Bacteria.
(Draw the food web in the space below, using arrows to show energy flow.)

Answer space

[4]

17. Explain how the structure of mangrove roots helps the tree survive in waterlogged soil.

_________________________________________________________________________ [4]

18. Mangroves are often cleared for urban development. Discuss the environmental consequences of clearing mangroves.

_________________________________________________________________________ [6]

19. Mangroves are efficient carbon sinks. Explain what this means and why it is important in the context of global warming.

_________________________________________________________________________ [4]

20. Explain why maintaining biodiversity in the mangrove ecosystem is important for its stability.

_________________________________________________________________________ [4]

[End of Quiz]

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Answers

O-Level Biology Quiz - Ecology (Answer Key)

Total Marks: 40

Section A: Multiple Choice Questions

1. B
Reasoning: Energy flows unidirectionally from the sun to producers, then to consumers. It is not recycled.

2. A
Reasoning: Primary consumers feed directly on producers (plants), making them herbivores.

3. B
Reasoning: Energy is lost as heat, in waste, and through respiration at each level, so less energy is available to support biomass at higher trophic levels.

4. B
Reasoning: Burning fossil fuels releases large amounts of stored carbon dioxide into the atmosphere.

5. C
Reasoning: Nitrogen fixation is the conversion of atmospheric nitrogen ( $N_2$ ) into nitrogenous compounds (like nitrates) by bacteria or lightning.

Section B: Structured Questions

6.
(a) Grass [1]
(b) The snake feeds on the frog [1], which is a secondary consumer (feeds on the grasshopper, which feeds on the grass) [1].

7.
(a) 10% [1]
(b) Any two of the following:

Energy is lost as heat during respiration [1].
Energy is lost in excretory products (urine/faeces) or uneaten parts [1].
(Note: "Movement" is also acceptable if linked to energy use).

8.
(a) Photosynthesis [1]
(b) Respiration [1]
(c) Combustion releases carbon dioxide that was stored in fossil fuels [1] into the atmosphere, increasing the concentration of $CO_2$ [1].

9.
(a) Trees absorb carbon dioxide during photosynthesis [1]. Deforestation reduces the number of trees, so less $CO_2$ is removed from the atmosphere [1]. Additionally, burning or decaying trees releases stored carbon as $CO_2$ [1].
(b) Any one valid method:

Replanting trees (afforestation/reforestation) [1].
Establishing protected reserves/national parks [1].
Sustainable logging (selective logging) [1].

10.
(a) Biomagnification is the increase in concentration of a non-biodegradable substance [1] in the tissues of organisms at higher trophic levels of a food chain [1].
(b) The insecticide is not broken down or excreted [1]. As insects are eaten by small birds, and small birds by birds of prey, the insecticide accumulates [1]. The bird of prey consumes many organisms, accumulating the highest concentration [1].

Section C: Structured Questions

11.
(a) Log phase (or Exponential phase) [1]
(b) Nutrients become limited/depleted [1]. Waste products accumulate and become toxic [1]. Space becomes limited, leading to competition [1].

12.
(a) High temperatures cause stress to the coral [1]. The coral expels the symbiotic algae (zooxanthellae) living in its tissues [1]. Without the algae, the coral loses its colour (bleaches) and its main source of food [1].
(b) Any one:

Protection of coastlines from erosion [1].
Source of food/fisheries [1].
Tourism revenue [1].
Biodiversity hotspot [1].

13.
(a) Bacteria and Fungi [2] (1 mark each).
(b) Decomposers secrete enzymes to break down dead organic matter/waste [1]. This releases simple inorganic nutrients/minerals (like nitrates) into the soil [1]. These nutrients are then absorbed by plants for growth [1].

14.
(a) Fertilisers contain nitrates/phosphates [1]. Rain washes these into water bodies (leaching/runoff) [1]. This causes rapid growth of algae (algal bloom) [1].
(Note: Full explanation of eutrophication usually requires mentioning the subsequent death of algae and decomposition, but the question asks how fertilisers lead to it. 3 marks for the causal chain).
(b) Fish die due to lack of oxygen [1]. (Caused by decomposers using up oxygen).

15.
(a) Development that meets the needs of the present generation [1] without compromising the ability of future generations to meet their own needs [1].
(b) Any one:

Using nets with large mesh sizes to allow young fish to escape [1].
Setting fishing quotas/limits [1].
Establishing no-fishing zones [1].

Section D: Free Response Questions

16. Food Web [4]
Marking Scheme:

Correct arrows showing energy flow (from eaten to eater) [1].
Mangrove leaves → Detritus (or directly to Crab if implied detritivore, but Detritus is a pool) [1].
Detritus → Crab [1].
Crab → Heron [1].
Bacteria should be shown decomposing dead matter from all levels or specifically Detritus.
Acceptable Web:
Mangrove Leaves → Detritus → Crab → Heron
(All dead matter) → Bacteria
(Note: Arrows must point correctly. 1 mark for each correct link up to 4).

17. Adaptations of Mangrove Roots [4]

Mangrove soil is waterlogged and lacks oxygen (anaerobic) [1].
Mangroves have pneumatophores (breathing roots) that grow upwards out of the mud [1].
These roots have pores (lenticels) that allow oxygen to enter [1].
This allows the roots to carry out aerobic respiration to generate energy for active transport of minerals [1].

18. Environmental Consequences of Clearing Mangroves [6]
Level 3 (5-6 marks): Detailed explanation of multiple consequences.

Loss of habitat/biodiversity: Many species lose their breeding/nursery grounds, leading to population decline or extinction [1].
Coastal Erosion: Mangrove roots hold soil together. Without them, coastlines are eroded by waves and tides [1].
Increased Flooding: Mangroves act as buffers against storm surges and tsunamis. Removal increases flood risk to inland areas [1].
Carbon Release: Mangroves store large amounts of carbon. Clearing them releases this carbon as $CO_2$ , contributing to global warming [1].
Disruption of Food Webs: Removal of producers/detritus sources affects the entire food chain, including fisheries [1].
Water Quality: Mangroves filter pollutants. Removal leads to poorer water quality in coastal areas [1].
(Award 1 mark per valid point, up to 6. Must be explained, not just listed).

19. Carbon Sink Importance [4]

A carbon sink is a reservoir that absorbs more carbon from the atmosphere than it releases [1].
Mangroves store carbon in their biomass and especially in the soil/sediment [1].
This reduces the amount of $CO_2$ in the atmosphere [1].
Lower atmospheric $CO_2$ reduces the greenhouse effect and mitigates global warming [1].

20. Biodiversity and Stability [4]

High biodiversity means there are many different species and complex food webs [1].
If one species declines or disappears, there are alternative food sources for predators [1].
This makes the ecosystem more resilient to changes or disturbances (e.g., disease, climate change) [1].
Low biodiversity ecosystems are fragile; the loss of one key species can cause the collapse of the whole system [1].

[End of Answer Key]