CBSE Class 12 Biology (2026–27)
Chapter 12: Ecosystem
20 Important Questions and Answers
1. What is an ecosystem? Explain its main components.
Answer:
An ecosystem is a functional unit of nature where living organisms interact with one another and with their physical environment. It consists of two major components: biotic and abiotic. Biotic components include producers (green plants), consumers (herbivores, carnivores, omnivores), and decomposers (bacteria and fungi). Abiotic components include sunlight, air, water, soil, minerals, temperature, and humidity. Both components interact continuously through energy flow and nutrient cycling. Examples of ecosystems include ponds, forests, grasslands, deserts, and oceans. Ecosystems maintain ecological balance by regulating the transfer of energy and recycling of nutrients among living and non-living components.
2. Differentiate between Gross Primary Productivity (GPP) and Net Primary Productivity (NPP).
Answer:
Gross Primary Productivity (GPP) is the total amount of organic matter produced by green plants through photosynthesis per unit area per unit time. However, plants use some of this energy for respiration. The remaining biomass available for growth and consumption by herbivores is called Net Primary Productivity (NPP). Thus, NPP represents the actual rate of biomass accumulation in an ecosystem. The relationship between them is:
NPP = GPP – Respiration Losses
NPP is an important indicator of ecosystem productivity because it determines the energy available to higher trophic levels. Forests generally have higher NPP than deserts due to favorable environmental conditions.
3. What is decomposition? Mention its major steps.
Answer:
Decomposition is the process by which dead organic matter is broken down into simpler inorganic substances by decomposers such as bacteria and fungi. It plays a vital role in nutrient recycling within ecosystems. The process occurs through five main steps:
- Fragmentation – breakdown of detritus into smaller particles.
- Leaching – dissolution of soluble nutrients into soil.
- Catabolism – microbial enzymes degrade complex compounds.
- Humification – formation of dark-colored humus.
- Mineralization – release of inorganic nutrients into the environment.
Decomposition is faster in warm, moist conditions and slower in cold or dry environments. This process ensures the continuous availability of nutrients for producers.
4. Define detritus and decomposers. Why are decomposers important?
Answer:
Detritus refers to dead remains of plants, animals, and their waste products. Decomposers are microorganisms, mainly bacteria and fungi, that feed on detritus and convert complex organic matter into simple inorganic substances. They are essential components of ecosystems because they recycle nutrients such as nitrogen, phosphorus, and carbon back into the environment. Without decomposers, dead organic matter would accumulate continuously, and nutrients would become unavailable to producers. Thus, decomposers maintain nutrient cycling, soil fertility, and ecosystem stability. Their activities ensure that matter is reused repeatedly within ecosystems, supporting the survival of all living organisms.
5. What are trophic levels? Give an example.
Answer:
Trophic levels are the different feeding levels in a food chain through which energy flows in an ecosystem. Each trophic level represents a particular step in the transfer of food and energy. Producers occupy the first trophic level because they synthesize food through photosynthesis. Herbivores form the second trophic level, primary carnivores the third, and top carnivores the fourth or higher levels.
Example:
Grass → Grasshopper → Frog → Snake
Here, grass is the producer, grasshopper is the primary consumer, frog is the secondary consumer, and snake is the tertiary consumer. Trophic levels help in understanding energy transfer and ecological relationships.
6. Explain the 10% Law of energy transfer.
Answer:
The 10% Law was proposed by Lindeman. It states that only about 10% of the energy available at one trophic level is transferred to the next higher trophic level, while the remaining 90% is lost as heat during metabolic activities. For example, if producers capture 10,000 units of energy, only 1,000 units reach herbivores and about 100 units reach secondary consumers. This law explains why food chains are usually short and why the number of trophic levels is limited. The progressive loss of energy also results in ecological pyramids of energy always being upright.
7. Differentiate between grazing food chain and detritus food chain.
Answer:
A grazing food chain starts with living green plants that capture solar energy through photosynthesis. Herbivores consume these plants, followed by carnivores. Example: Grass → Grasshopper → Frog → Snake.
A detritus food chain begins with dead organic matter called detritus. Decomposers and detritivores break down this material and transfer energy to higher consumers. Example: Dead leaves → Earthworm → Bird.
The grazing food chain directly depends on producers, whereas the detritus food chain depends on dead organic matter. In many terrestrial ecosystems, the detritus food chain contributes significantly to energy flow and nutrient cycling.
8. What is a food web? State its significance.
Answer:
A food web is a network of interconnected food chains present in an ecosystem. Most organisms have multiple food sources and may be consumed by different predators. Therefore, food chains are not isolated but linked together to form a food web.
Food webs increase ecosystem stability because the removal of one organism does not completely disrupt energy flow. Alternative feeding pathways allow organisms to survive despite environmental changes. Food webs also regulate population sizes and maintain ecological balance. They represent natural feeding relationships more accurately than simple food chains and help in understanding ecosystem dynamics and biodiversity conservation.
9. What are ecological pyramids?
Answer:
Ecological pyramids are graphical representations of trophic levels in an ecosystem. They illustrate the number of organisms, biomass, or energy present at successive trophic levels. Three main types are:
- Pyramid of Numbers
- Pyramid of Biomass
- Pyramid of Energy
These pyramids help ecologists understand energy transfer and ecosystem structure. The pyramid of energy is always upright because energy decreases at each trophic level. Ecological pyramids provide valuable information about productivity, food relationships, and ecosystem functioning, making them important tools in ecological studies.
10. Why is the pyramid of energy always upright?
Answer:
The pyramid of energy is always upright because energy decreases at each successive trophic level. During energy transfer, a large portion is lost as heat through respiration, movement, and metabolic processes. According to Lindeman’s 10% Law, only about 10% of the energy is passed to the next trophic level. Since energy cannot be recycled and continuously diminishes, the amount of energy available becomes progressively lower at higher trophic levels. Therefore, the energy pyramid always shows a broad base of producers and a narrow apex of top consumers, making inversion impossible.
11. Why is the pyramid of biomass inverted in aquatic ecosystems?
Answer:
In aquatic ecosystems, producers are mainly phytoplankton. These microscopic organisms have a small standing biomass because they reproduce rapidly and are consumed quickly by zooplankton. Although their biomass at any given moment is low, their productivity is very high. Consequently, the biomass of primary consumers may exceed that of producers. This results in an inverted pyramid of biomass. In contrast, terrestrial ecosystems usually have upright biomass pyramids because large plants accumulate substantial biomass over time. Thus, the unique characteristics of phytoplankton lead to inversion of biomass pyramids in aquatic environments.
12. What factors affect decomposition?
Answer:
The rate of decomposition depends mainly on climatic conditions and the chemical composition of detritus. Warm temperatures and adequate moisture accelerate microbial activity, leading to rapid decomposition. Conversely, low temperatures and dry conditions slow down the process. Detritus rich in nitrogen and soluble sugars decomposes quickly, while materials containing lignin and chitin decompose slowly. Oxygen availability also influences decomposition, with aerobic conditions promoting faster breakdown. These factors determine nutrient release and soil fertility. Hence, decomposition rates vary significantly among ecosystems depending on environmental conditions and the nature of organic matter.
13. What is standing crop and standing state?
Answer:
Standing crop refers to the quantity of living biomass present in an ecosystem at a particular time. It may be measured in terms of number, biomass, or energy. It indicates the productivity and abundance of living organisms.
Standing state refers to the amount of nutrients such as nitrogen, phosphorus, and calcium present in the soil or ecosystem at a given time. Unlike standing crop, it does not involve living organisms.
Both concepts help ecologists assess ecosystem productivity and nutrient availability. Together, they provide information about ecosystem health, nutrient cycling, and biological activity.
14. Explain the role of producers in an ecosystem.
Answer:
Producers are autotrophic organisms, mainly green plants and algae, that synthesize food through photosynthesis using sunlight, carbon dioxide, and water. They form the first trophic level and serve as the primary source of energy for all other organisms. Producers convert solar energy into chemical energy stored in organic compounds. Herbivores feed on producers, while carnivores indirectly depend on them. Producers also release oxygen and help regulate atmospheric carbon dioxide levels. Without producers, energy flow in ecosystems would cease, making life impossible. Thus, producers are the foundation of all food chains and ecological processes.
15. What is secondary productivity?
Answer:
Secondary productivity refers to the rate at which consumers synthesize new biomass from the food they consume. Unlike primary productivity, which is carried out by producers through photosynthesis, secondary productivity depends on the availability of organic matter produced by plants. Herbivores convert plant biomass into animal biomass, and carnivores further transfer this energy. Secondary productivity reflects the efficiency of energy transfer between trophic levels. It varies among ecosystems depending on food availability, environmental conditions, and metabolic rates of organisms. Higher secondary productivity generally indicates efficient utilization of primary production within an ecosystem.
16. Why are food chains usually short?
Answer:
Food chains are generally short because of significant energy loss at each trophic level. According to the 10% Law, only about 10% of the energy is transferred to the next trophic level, while most is lost as heat. As a result, the energy available decreases rapidly with each transfer. After a few trophic levels, insufficient energy remains to support additional consumers. This limitation prevents the formation of very long food chains. Typically, ecosystems contain three to five trophic levels. Short food chains enhance energy efficiency and maintain ecological stability.
17. What are consumers? Mention their types.
Answer:
Consumers are heterotrophic organisms that cannot produce their own food and depend on other organisms for nutrition. They occupy higher trophic levels in food chains. Consumers are classified into:
- Primary consumers – Herbivores that feed on plants (e.g., deer, rabbit).
- Secondary consumers – Carnivores that feed on herbivores (e.g., frog, lizard).
- Tertiary consumers – Carnivores that feed on other carnivores (e.g., snake).
- Top consumers – Apex predators at the highest trophic level (e.g., tiger, eagle).
Consumers play a crucial role in energy transfer and population regulation within ecosystems.
18. What is nutrient cycling? Why is it important?
Answer:
Nutrient cycling is the continuous movement of essential elements such as carbon, nitrogen, phosphorus, and water between biotic and abiotic components of ecosystems. Nutrients are absorbed by producers, transferred through food chains, and eventually returned to the environment through decomposition. This process ensures the repeated use of limited resources. Nutrient cycling maintains soil fertility, supports plant growth, and sustains ecosystem productivity. Without nutrient recycling, essential elements would become depleted, disrupting ecosystem functioning. Thus, nutrient cycling is fundamental for maintaining ecological balance and long-term sustainability of life on Earth.
19. Explain the significance of ecological pyramids.
Answer:
Ecological pyramids provide a visual representation of trophic relationships within ecosystems. They help scientists understand the distribution of organisms, biomass, and energy across trophic levels. These pyramids reveal patterns of energy transfer and productivity and indicate ecosystem efficiency. They also help compare terrestrial and aquatic ecosystems and identify ecological imbalances. Ecological pyramids are useful tools for studying food chains, trophic interactions, and nutrient dynamics. Their analysis contributes to biodiversity conservation and ecosystem management by highlighting the dependence of higher trophic levels on producers.
20. Describe a pond ecosystem as a self-sustaining system.
Answer:
A pond ecosystem is a self-sustaining freshwater ecosystem containing both biotic and abiotic components. Abiotic factors include water, sunlight, dissolved gases, and minerals. Producers such as phytoplankton and aquatic plants synthesize food through photosynthesis. Primary consumers include zooplankton and insects, while fish and amphibians act as higher consumers. Decomposers such as bacteria and fungi break down dead organic matter and recycle nutrients. Energy flows through food chains, while nutrients circulate continuously within the system. Due to these interactions, a pond maintains ecological balance and functions independently without external support, making it a classic example of an ecosystem.