Test 3 : Ecosystem, Food Chain & Energy Flow
Welcome to TNPSC Biology Environment and Ecology – Test 3, a free online test designed to help TNPSC aspirants strengthen their knowledge of Biology, Environment and Ecology. This test covers important concepts related to the environment, ecosystem and ecological relationships, providing an effective way to assess your understanding and identify areas that need further revision. Regular practice with such free online tests can help you build confidence and perform better in the examination.
Note: Once you have attempted all the questions, read the explanations below to reinforce your learning and prepare better for similar questions in the exam.
TNPSC Biology Environment and Ecology Test 3 – Essential Points for Revision
The Detailed Explanations section provides a clear explanation for every question in this TNPSC Biology Environment and Ecology – Test 3. Each answer is explained with relevant concepts from Biology, Environment, Ecosystem and Food Chain, helping you understand not only the correct answer but also the underlying ecological principles. These explanations are useful for revising important Environment and Ecology concepts, correcting mistakes and strengthening your overall TNPSC Biology preparation. Use this section as a quick revision resource after completing the free online test and make a note of the concepts that require additional practice.
1. Which sequence represents a correct food chain?
Answer: A. Grass → Grasshopper → Frog → Snake
A food chain represents the transfer of food and energy from one organism to another. Grass is a producer because it makes its own food through photosynthesis. The grasshopper feeds on grass and acts as a primary consumer. The frog feeds on the grasshopper and acts as a secondary consumer, while the snake feeds on the frog.
Food chain: Grass → Grasshopper → Frog → Snake
Key Point: Energy generally flows from producers → consumers → higher-level consumers.
2. A food web differs from a food chain because it shows
Answer: B. Multiple interconnected food chains
A food chain represents a single pathway of feeding relationships in an ecosystem. In nature, however, an organism may feed on several different organisms and may also be eaten by different predators. These interconnected food chains form a food web.
A food web therefore provides a more realistic representation of feeding relationships within an ecosystem.
Key Point:
Food chain = single feeding pathway
Food web = multiple interconnected food chains
3. An organism that feeds directly on a herbivore is generally called a
Answer: C. Secondary consumer
Herbivores feed directly on producers such as green plants and are therefore called primary consumers. An organism that feeds directly on a primary consumer is generally known as a secondary consumer. Secondary consumers may be carnivores or omnivores, depending on their feeding habits.
For example, in the food chain Grass → Grasshopper → Frog, grass is the producer, the grasshopper is the primary consumer, and the frog is the secondary consumer. Thus, the frog obtains energy by feeding directly on the herbivore.
Key Point: Producer → Primary consumer → Secondary consumer; the secondary consumer generally occupies the third trophic level.
4. Which animal is an omnivore?
Answer: D. Bear
An omnivore is an animal that obtains its food from both plant and animal sources. Bears have a varied diet that can include fruits, nuts, roots, insects, fish and other animal foods. Therefore, the bear is classified as an omnivore.
In comparison, cows and deer are mainly herbivores, as they primarily feed on plant material, while lions are carnivores, as they mainly depend on other animals for food. These feeding habits determine the position of organisms within food chains and food webs.
Key Point:
Herbivores → mainly plants
Carnivores → mainly animals
Omnivores → both plants and animals
5. If the number of producers in an ecosystem decreases sharply, which group is likely to be affected first?
Answer: A. Primary consumers
Producers, such as green plants and algae, form the base of most food chains because they produce organic food using energy from sunlight. Primary consumers such as herbivores depend directly on producers for their food. Therefore, when the number of producers decreases sharply, primary consumers are likely to face a shortage of food and energy first.
The effect can then extend to higher trophic levels. A decline in herbivores may reduce the food available to secondary and tertiary consumers, causing changes throughout the food chain or food web.
Key Point: Producers → Primary consumers → Higher consumers; a decline at the producer level can affect the entire food chain.
6. What happens to the amount of available energy as we move to higher trophic levels?
Answer: C. It decreases
The amount of available energy decreases as we move from one trophic level to the next higher level. Producers capture solar energy and convert it into chemical energy, but only a small portion of this energy becomes available to the organisms at the next trophic level. Much of the energy is used for respiration, movement, growth and other metabolic activities, and a considerable amount is released as heat.
As a result, higher trophic levels receive progressively less energy. This is also why food chains usually contain only a limited number of trophic levels.
Key Point: Higher trophic level → Less available energy; energy flow in an ecosystem is unidirectional.
7. According to the 10% law, approximately what percentage of energy is transferred to the next trophic level?
Answer: B. 10%
According to the 10% law, proposed by ecologist Raymond Lindeman, only about 10% of the energy available at one trophic level is transferred to the next trophic level. The remaining energy is largely used by organisms for metabolic activities such as respiration, movement, growth and maintenance, and much of it is eventually released as heat.
For example, if producers have 10,000 units of energy, approximately 1,000 units may be transferred to primary consumers. Of these 1,000 units, only about 100 units may be transferred to secondary consumers. This explains why the amount of available energy decreases at higher trophic levels.
Key Point: About 10% of energy → transferred to the next trophic level; most energy → used and lost as heat.
8. Which ecological pyramid is always upright?
Answer: C. Pyramid of energy
The pyramid of energy represents the amount of energy available at different trophic levels. Energy decreases as we move from producers to higher consumers. Therefore, the energy pyramid is always upright.
Unlike pyramids of numbers and biomass, an energy pyramid cannot be inverted because the available energy necessarily decreases at successive trophic levels.
Key Point: Pyramid of energy is always upright.
9. Which ecological pyramid shows the number of organisms present at each trophic level?
Answer: B. Pyramid of numbers
The pyramid of numbers represents the number of individual organisms present at each trophic level of an ecosystem.
For example, a grassland ecosystem may contain many grass plants, fewer herbivores and still fewer carnivores. Depending on the ecosystem, the pyramid of numbers can be upright or inverted.
Key Point:
Pyramid of numbers → number of organisms
Pyramid of biomass → mass of living matter
Pyramid of energy → energy available
10. Which process involves the breaking of dead organic matter into smaller pieces during decomposition?
Answer: A. Fragmentation
Fragmentation is the process in which detritivores break dead organic matter, or detritus, into smaller pieces. Organisms such as earthworms and other soil detritivores help carry out this process. Breaking the detritus into smaller particles increases its surface area, making it easier for decomposers to act on the organic matter.
Fragmentation is one of the important steps in the decomposition process. It helps prepare dead organic material for further breakdown and transformation by microorganisms such as bacteria and fungi.
Key Point: Detritus → Smaller pieces = Fragmentation.
11. The formation of humus during decomposition is known as
Answer: B. Humification
Humification is the process during decomposition in which complex organic matter from dead plants and animals is gradually transformed into a dark, amorphous and relatively stable substance called humus. It is an important part of the decomposition process carried out mainly by decomposers such as bacteria and fungi.
Humus accumulates in the soil and plays an important role in maintaining soil fertility and soil structure. It can also improve the soil’s ability to retain moisture and nutrients, supporting plant growth.
Key Point: Decomposing organic matter → Humus = Humification.
12. Which process returns water vapour to the atmosphere from plant leaves?
Answer: C. Transpiration
Transpiration is the process by which plants lose excess water in the form of water vapour, mainly through tiny openings called stomata present on their leaves. It is an important part of the water cycle because it transfers water from plants back into the atmosphere.
Most transpiration occurs through stomata and is therefore called stomatal transpiration. Transpiration also helps in the upward movement of water and minerals from the roots to other parts of the plant through the process of transpiration pull.
Key Point: Water in plants → Water vapour through leaves → Atmosphere = Transpiration.
13. Which process removes carbon dioxide from the atmosphere?
Answer: C. Photosynthesis
During photosynthesis, green plants, algae and some microorganisms absorb carbon dioxide (CO₂) from the atmosphere and use it along with water to produce glucose in the presence of light energy and chlorophyll. In this process, oxygen is released as a by-product.
Photosynthesis is an important part of the carbon cycle because it removes carbon dioxide from the atmosphere and incorporates carbon into organic compounds. In contrast, processes such as respiration, decomposition and combustion release carbon dioxide back into the atmosphere.
Key Point: Photosynthesis → removes CO₂ | Respiration, decomposition and combustion → release CO₂.
14. Which process converts atmospheric nitrogen into a form that can enter the nitrogen cycle?
Answer: B. Nitrogen fixation
Atmospheric nitrogen exists mainly as nitrogen gas (N₂), which most plants and animals cannot use directly because the nitrogen molecules are very stable. Nitrogen fixation converts atmospheric nitrogen into usable nitrogen compounds, such as ammonia (NH₃), allowing nitrogen to enter biological processes and become available for further transformations in the nitrogen cycle.
Nitrogen fixation is carried out mainly by certain nitrogen-fixing microorganisms, such as Rhizobium bacteria associated with the roots of leguminous plants. It can also occur naturally through lightning and industrially through chemical processes.
Key Point: Atmospheric N₂ → Usable nitrogen compounds = Nitrogen fixation.
15. The conversion of ammonia into nitrites and nitrates is called
Answer: A. Nitrification
Nitrification is a microbial process in the nitrogen cycle in which ammonia or ammonium compounds are oxidized first into nitrites (NO₂⁻) and then into nitrates (NO₃⁻). This process is mainly carried out by specialized nitrifying bacteria and other microorganisms in the soil and aquatic environments.
Nitrates produced through nitrification can be absorbed by plants and used to synthesize important nitrogen-containing compounds such as proteins and nucleic acids. Thus, nitrification helps make nitrogen available in a form that plants can utilize.
Key Point: Ammonia/ammonium → Nitrites → Nitrates = Nitrification.
16. Which process returns nitrogen to the atmosphere as nitrogen gas?
Answer: C. Denitrification
Denitrification is a microbial process in the nitrogen cycle in which nitrate compounds are converted into gaseous forms of nitrogen. The process ultimately releases nitrogen gas (N₂) back into the atmosphere, helping maintain the balance of nitrogen between the soil and atmosphere.
Denitrification is mainly carried out by certain microorganisms under low-oxygen or anaerobic conditions, particularly in waterlogged soils and sediments. It is an important step in completing the nitrogen cycle by returning nitrogen from the soil to the atmosphere.
Key Point: Nitrates → Nitrogen gas (N₂) → Denitrification.
17. Which relationship best illustrates the interdependence of organisms in an ecosystem?
Answer: A. Plants and herbivores
Plants and herbivores demonstrate interdependence because plants serve as producers and provide food and energy to herbivores. Herbivores, in turn, form an important link between producers and higher-level consumers in food chains. This relationship shows how organisms depend on one another for the transfer of energy within an ecosystem.
A change in one population can affect other populations. For example, a significant reduction in plant availability can reduce the food available to herbivores and may eventually affect secondary and tertiary consumers that depend on them.
Key Point: Organisms in an ecosystem are interconnected through feeding relationships and energy flow.
18. Which statement about nutrient cycling is correct?
Answer: B. Nutrients circulate between living organisms and the environment
Unlike energy, which flows through an ecosystem and is eventually dissipated as heat, nutrients are continuously recycled between living organisms and their physical environment. Nutrients move from the environment into organisms and return to the environment through processes such as feeding, excretion and decomposition.
Important elements such as carbon, nitrogen and phosphorus participate in biogeochemical cycles involving organisms, soil, water and the atmosphere. This continuous recycling ensures that essential nutrients remain available within ecosystems.
Key Point: Energy flows; nutrients cycle.
19. Which organisms are commonly the pioneer species in primary succession on bare rocks?
Answer: B. Lichens
Primary succession begins in an area where there is no established soil, such as newly exposed bare rock. Lichens are commonly among the first organisms to colonize such harsh surfaces because they can survive under unfavorable conditions. They also help in the gradual breakdown of rocks and contribute to the development of soil.
As soil formation progresses, other organisms such as mosses, grasses and shrubs can establish themselves. Thus, lichens play an important role in initiating the early stages of ecological succession on bare rocks.
Key Point: Bare rock → Lichens → Soil formation → Later communities
20. The relatively stable community formed at the later stage of ecological succession is called the
Answer: C. Climax community
Ecological succession is the gradual and natural change in the composition and structure of a biological community over time. The organisms that first colonize an area are called pioneer species. They modify the environment and create conditions that allow other organisms to establish. These organisms are gradually replaced by different communities during the successive stages of succession.
The relatively stable community that develops at the later stage of succession is traditionally called the climax community. For example, in primary succession on bare rocks, lichens can act as pioneer organisms and contribute to soil formation, allowing grasses, shrubs and eventually larger plants to establish.
Key Point: Pioneer species → Transitional communities → Climax community
Quick Revision Points
Before moving to the next test, remember these important TNPSC points:
- Food chain → Single feeding pathway
- Food web → Interconnected food chains
- Primary consumer → Feeds on producers
- Secondary consumer → Generally feeds on primary consumers
- 10% law → About 10% energy passes to the next trophic level
- Energy pyramid → Always upright
- Pyramid of numbers → Shows number of organisms
- Fragmentation → Breaking detritus into smaller pieces
- Humification → Formation of humus
- Transpiration → Water vapour released by plants
- Photosynthesis → Removes CO₂ from atmosphere
- Nitrogen fixation → Atmospheric nitrogen enters usable forms
- Nitrification → Ammonia/ammonium → nitrites → nitrates
- Denitrification → Returns nitrogen to atmosphere
- Nutrient cycling → Nutrients circulate through organisms and environment
- Lichens → Common pioneer organisms on bare rocks
- Climax community → Relatively stable later-stage community
Further Reference – Environment and Ecology
To Know More About “Environment and Ecology” Topics For further reading and concept clarity, learners can refer to the NCERT textbooks, which provide useful foundational information on environment, ecology, ecosystems, and related topics. This official NCERT resource “Our Environment” can be used alongside the explanations in this quiz for deeper understanding and revision.
