Ecosystem — Class 12 Biology NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 12 Biology chapter "Ecosystem" — 8 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 12 Biology chapter "Ecosystem" — 8 important questions with detailed answers for CBSE board exam preparati…
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NCERT Solutions for Class 12 Biology — Ecosystem. Step-by-step answers to all 8 textbook questions from this chapter, written for CBSE board preparation and free to use.
Ecosystem — All 8 Questions Solved
Q1. Define an ecosystem. What are its main components?
An ecosystem is a functional unit of nature comprising all living organisms (biotic components) and their physical environment (abiotic components) interacting together through energy flow and nutrient cycling.
MAIN COMPONENTS:
BIOTIC COMPONENTS:
1. Producers (Autotrophs)
- Green plants, photosynthetic bacteria, algae
- Convert solar energy into chemical energy (food)
- Form base of food chains
2. Consumers (Heterotrophs)
- Primary consumers: Herbivores eating plants
- Secondary consumers: Carnivores eating primary consumers
- Tertiary consumers: Top carnivores
- Omnivores: Eating both plants and animals
3. Decomposers
- Bacteria, fungi, saprophytic organisms
- Break down dead organic matter
- Release nutrients back to soil
- Complete nutrient cycles
ABIOTIC COMPONENTS:
1. Physical factors
- Temperature, light, humidity, atmospheric pressure
- Wind, water current
- Soil pH, salinity
2. Chemical factors
- Oxygen, carbon dioxide, nitrogen
- Minerals: phosphorus, potassium, magnesium
- Water
FUNCTIONAL RELATIONSHIPS:
- Energy flow: Unidirectional, decreases at each trophic level
- Nutrient cycling: Circular movement of nutrients
- Food chains and food webs: Transfer of energy between organisms
Q2. Explain energy flow in ecosystems. What is the 10% law?
Energy flow is the movement of energy through an ecosystem from sun through organisms to environment as heat. It is unidirectional and decreases at each level.
Q3. Describe nutrient cycling. Explain the carbon cycle and nitrogen cycle.
Nutrient cycling (biogeochemical cycles) involves circular movement of nutrients between biotic and abiotic components of ecosystem. Nutrients are recycled indefinitely.
Q4. What are food chains and food webs? Explain the concept of trophic levels.
Food chains and webs show transfer of energy and nutrients through different organisms in ecosystem.
Q5. Define ecological pyramid. Explain pyramids of numbers, biomass, and energy.
Ecological pyramids are graphical representations showing quantitative relationships between organisms at different trophic levels.
Q6. What is ecological efficiency? Calculate energy available at different trophic levels.
Ecological efficiency refers to percentage of energy transferred from one trophic level to next, typically 10%.
Q7. Explain the concept of carrying capacity and how ecosystems maintain stability.
Carrying capacity is maximum population size ecosystem can sustain indefinitely given available resources.
Q8. What are ecosystem services? Give examples and explain their importance.
Ecosystem services are benefits humans derive from ecosystems including provisioning, regulating, supporting, and cultural services essential for human survival and well-being.
Ecosystem — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. Which of the following describes the functional role of a species in an ecosystem?
- A. Biome
- B. Niche
- C. Habitat
- D. Population
Answer: B. Niche — Niche refers to the unique functional position or role of a species within an ecosystem, encompassing its habitat, food source, and interactions with other species.
MCQ 2. Gross Primary Productivity (GPP) is the rate of biomass production during photosynthesis. What does Net Primary Productivity (NPP) represent?
- A. GPP minus energy lost to predators
- B. GPP minus energy lost during respiration by producers
- C. GPP plus energy gained from consumers
- D. Total biomass available to decomposers
Answer: B. GPP minus energy lost during respiration by producers — Net Primary Productivity (NPP) is the amount of biomass remaining after producers have utilized some energy for their own metabolic activities (respiration), calculated as GPP minus respiration (R).
MCQ 3. Which of the following is NOT a step involved in the decomposition process?
- A. Fragmentation
- B. Leaching
- C. Photosynthesis
- D. Mineralization
Answer: C. Photosynthesis — Photosynthesis is the process by which producers convert light energy into chemical energy, not a step in the decomposition of organic matter. Fragmentation, leaching, catabolism, humification, and mineralization are the key steps in decomposition.
MCQ 4. The 10% law of energy transfer in an ecosystem states that:
- A. 10% of the energy is transferred from abiotic to biotic components.
- B. Only 10% of the energy is transferred from one trophic level to the next.
- C. 10% of the energy is lost as heat at each trophic level.
- D. Organisms consume only 10% of their body weight daily.
Answer: B. Only 10% of the energy is transferred from one trophic level to the next. — The 10% law, proposed by Lindeman, states that only about 10% of the energy from one trophic level is transferred and incorporated into the biomass of the next successive trophic level, while the remaining 90% is lost, primarily as heat.
MCQ 5. In an inverted pyramid of biomass, which of the following ecosystems is most likely represented?
- A. Grassland ecosystem
- B. Forest ecosystem
- C. Pond ecosystem
- D. Desert ecosystem
Answer: C. Pond ecosystem — A pond ecosystem often exhibits an inverted pyramid of biomass, where the biomass of producers (phytoplankton) at the base is smaller than the biomass of primary consumers (zooplankton) they support for a given time.
MCQ 6. What is the primary factor limiting productivity in deep oceans?
- A. Temperature
- B. Oxygen concentration
- C. Light availability
- D. Salinity
Answer: C. Light availability — Light is the primary limiting factor for productivity in deep oceans because photosynthetic organisms cannot survive without sufficient sunlight, which penetrates only to a certain depth.
MCQ 7. Which of the following is a characteristic of a pioneer species in ecological succession?
- A. Slow growth rate
- B. Long lifespan
- C. Tolerance to harsh conditions
- D. Requires fertile soil
Answer: C. Tolerance to harsh conditions — Pioneer species are the first organisms to colonize a barren area. They are typically hardy, fast-growing, and tolerant to extreme environmental conditions like bare rock, low nutrient availability, and strong sunlight.
MCQ 8. The major reservoir for the carbon cycle is:
- A. Atmosphere
- B. Oceans
- C. Fossil fuels
- D. Living organisms
Answer: B. Oceans — Oceans are the largest reservoir for carbon, holding approximately 50 times more carbon than the atmosphere, primarily in dissolved forms like bicarbonate and carbonate ions.
MCQ 9. Which type of ecological pyramid is always upright and can never be inverted?
- A. Pyramid of biomass
- B. Pyramid of number
- C. Pyramid of energy
- D. All of the above
Answer: C. Pyramid of energy — The pyramid of energy is always upright because energy flows unidirectionally and diminishes at each successive trophic level, meaning there is always less energy available at higher trophic levels.
MCQ 10. Detritus food chain begins with:
- A. Producers
- B. Herbivores
- C. Carnivores
- D. Dead organic matter
Answer: D. Dead organic matter — The detritus food chain starts with dead organic matter (detritus), which is consumed by decomposers like fungi and bacteria, and detritivores, unlike grazing food chains that begin with producers.
MCQ 11. What is the role of chemosynthetic bacteria in an ecosystem?
- A. Primary consumers
- B. Secondary consumers
- C. Producers in aphotic zones
- D. Decomposers
Answer: C. Producers in aphotic zones — Chemosynthetic bacteria act as producers in environments lacking sunlight (aphotic zones), synthesizing organic matter using chemical energy from oxidation of inorganic substances, forming the base of certain food webs.
MCQ 12. The rate of formation of new organic matter by consumers is called:
- A. Gross Primary Productivity
- B. Net Primary Productivity
- C. Secondary Productivity
- D. Standing Crop
Answer: C. Secondary Productivity — Secondary productivity refers to the rate of assimilation of energy and formation of new organic matter by consumers, as they convert the energy from their food into their own biomass.
MCQ 13. Which of the following biogeochemical cycles does not have a gaseous atmospheric phase?
- A. Carbon cycle
- B. Nitrogen cycle
- C. Phosphorus cycle
- D. Water cycle
Answer: C. Phosphorus cycle — The phosphorus cycle is a sedimentary cycle, meaning its major reservoir is in the Earth's crust (rocks) and it does not involve a significant gaseous phase in the atmosphere, unlike carbon, nitrogen, and water cycles.
MCQ 14. What is humification during decomposition?
- A. Breaking down of detritus into smaller particles
- B. Release of inorganic nutrients from organic matter
- C. Accumulation of dark, amorphous colloidal substance called humus
- D. Leaching of water-soluble inorganic nutrients into the soil
Answer: C. Accumulation of dark, amorphous colloidal substance called humus — Humification is the process where dead organic matter is converted into a dark-colored, amorphous, and highly resistant colloidal substance called humus, which is crucial for soil fertility.
MCQ 15. A stable, mature, and self-sustaining community that develops at the end of ecological succession is called a:
- A. Pioneer community
- B. Seral community
- C. Climax community
- D. Biotic community
Answer: C. Climax community — A climax community is the final, stable, and self-perpetuating community in ecological succession that is in equilibrium with the prevailing environmental conditions and undergoes little further change.
MCQ 16. Which of the following statements about ecological succession is incorrect?
- A. Primary succession occurs on newly exposed land.
- B. Secondary succession is faster than primary succession.
- C. Lichen is typically a pioneer species in hydrosere.
- D. The species diversity increases during succession.
Answer: C. Lichen is typically a pioneer species in hydrosere. — Lichen is typically a pioneer species in xerosere (succession on bare rock), not hydrosere (succession in water). In hydrosere, pioneer species are usually phytoplankton and submerged plants.
MCQ 17. If 10,000 J of energy is available at the producer level, how much energy will be available to the tertiary consumers?
- A. 1000 J
- B. 100 J
- C. 10 J
- D. 1 J
Answer: C. 10 J — According to the 10% law, energy transfer: Producers (10,000 J) → Primary Consumers (1,000 J) → Secondary Consumers (100 J) → Tertiary Consumers (10 J).
MCQ 18. Ecosystem services are the benefits that humans receive from ecosystems. Which of the following is NOT an ecosystem service?
- A. Pollination of crops
- B. Climate regulation
- C. Production of synthetic fertilizers
- D. Nutrient cycling
Answer: C. Production of synthetic fertilizers — Production of synthetic fertilizers is an anthropogenic (human-made) process, not a natural ecosystem service. Ecosystem services are the natural life-supporting functions provided by ecosystems.
MCQ 19. What is 'standing crop' in an ecosystem?
- A. The total amount of energy flowing through the ecosystem.
- B. The total biomass of living organisms in an ecosystem at a particular time.
- C. The rate of production of biomass over a period.
- D. The number of species found in an ecosystem.
Answer: B. The total biomass of living organisms in an ecosystem at a particular time. — Standing crop refers to the total mass (biomass) or number of living organisms of a population or ecosystem present at a given time, typically measured in units like kg/m² or number/m².
MCQ 20. Which one of the following is an example of an artificial ecosystem?
- A. Forest
- B. Grassland
- C. Crop field
- D. Pond
Answer: C. Crop field — A crop field is an artificial or man-made ecosystem, as it is created and maintained by humans for agricultural purposes, unlike natural ecosystems like forests, grasslands, and ponds.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.