Ecosystem — Previous Year Questions (Class 12 Biology)
An ecosystem is a self-sustaining functional unit comprising living organisms and their physical environment. This chapter explores ecosystem structure, en
TL;DR: An ecosystem is a self-sustaining functional unit comprising living organisms and their physical environment. This chapter explores ecosystem structur…
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An ecosystem is a self-sustaining functional unit comprising living organisms and their physical environment. This chapter explores ecosystem structure, en
Ecosystem — Previous Year Questions with Solutions
Q (2023, 4 marks): Define ecosystem. Explain the biotic and abiotic components of an ecosystem.
Answer: Ecosystem:
Definition - Community of living organisms interacting with their physical environment, forming a self-sustaining functional unit.
Biotic Components:
1. Producers:
- Green plants (autotrophs)
- Photosynthetic organisms
- Convert solar energy to chemical energy
- Examples: Grass, trees, algae
2. Consumers:
- Primary consumers: Herbivores feeding on plants
Example: Deer, rabbit
- Secondary consumers: Carnivores feeding on herbivores
Example: Lion, hawk
- Tertiary consumers: Carnivores feeding on carnivores
Example: Eagle
3. Decomposers:
- Break down dead organic matter
- Recycle nutrients
- Examples: Bacteria, fungi
Abiotic Components:
1. Climate Factors:
- Temperature: Affects organism metabolism
- Light: Energy source for photosynthesis
- Precipitation: Water availability
- Humidity: Affects water balance
2. Soil Factors:
- Soil composition
- pH level
- Mineral content
3. Water Factors:
- Salinity
- Dissolved oxygen
- pH
4. Atmospheric Factors:
- Oxygen and CO2 concentration
- Wind patterns
5. Topography:
- Altitude and slope
- Geographic location
Q (2022, 5 marks): Explain the flow of energy in an ecosystem with a diagram description.
Answer: Energy Flow in Ecosystem:
Definition - Unidirectional movement of energy from sun through ecosystem components.
Process:
1. Energy Capture:
- Sun provides solar energy
- Producers (plants) capture through photosynthesis
- Store as chemical energy in organic compounds
2. Energy Transfer:
- Primary consumers eat producers
- Consume 10% of energy, rest lost as heat/respiration
- Secondary consumers eat primary consumers
- Lose another 90%
3. Food Chain Example:
Grass (Producer) -> Rabbit (Primary Consumer) -> Fox (Secondary Consumer)
100% -> 10% -> 1%
4. Multiple Food Chains Form Food Web
5. Energy Loss at Each Level:
- Respiration: Used for metabolism
- Heat: Released to environment
- Excretion: Through waste
- Not eaten: Prevents 100% transfer
Trophic Levels:
1st Trophic Level: Producers (100% energy available)
2nd Trophic Level: Primary Consumers (10% of producer energy)
3rd Trophic Level: Secondary Consumers (1% of producer energy)
4th Trophic Level: Tertiary Consumers (0.1% of producer energy)
Pyramid of Energy:
- Shows energy content at each trophic level
- Always pyramid-shaped
- Decreases upward
- Explains why carnivores are rarer than herbivores
Q (2021, 5 marks): Explain the carbon cycle and the role of different organisms in it.
Answer: Carbon Cycle:
Definition - Movement of carbon through atmosphere, biosphere, hydrosphere, and geosphere.
Phases:
1. Atmospheric Phase:
- CO2 in atmosphere
- Comprises 0.04% of air
- Increased by human activities
2. Biotic Phase - Photosynthesis:
- Plants absorb CO2 from atmosphere
- Convert to glucose and organic compounds
- CO2 + H2O -> Glucose + O2
3. Biotic Phase - Respiration:
- Plants release CO2 during respiration
- All organisms respire
- Return CO2 to atmosphere
4. Decomposition:
- Dead organisms broken down
- Decomposers release CO2
5. Fossil Fuel Combustion:
- Coal, petroleum, natural gas burned
- Release stored carbon as CO2
- Increases atmospheric CO2
Role of Organisms:
1. Producers (Plants):
- Absorb CO2 through photosynthesis
- Store carbon in organic compounds
- Release through respiration
2. Consumers (Animals):
- Consume carbon in food
- Release through respiration and excretion
3. Decomposers (Bacteria, Fungi):
- Break down dead matter
- Release CO2 to atmosphere
4. Humans:
- Burn fossil fuels (increase CO2)
- Deforestation (reduce CO2 absorption)
- Industrial processes
Impact:
- Rising atmospheric CO2
- Global warming and climate change
- Ocean acidification
Q (2023, 4 marks): Describe the nitrogen cycle and explain the importance of nitrogen-fixing bacteria.
Answer: Nitrogen Cycle:
Definition - Movement of nitrogen through atmosphere, soil, and living organisms.
Stages:
1. Nitrogen Fixation:
- Atmospheric N2 converted to ammonia (NH3)
- Done by nitrogen-fixing bacteria
- Occurs in soil and root nodules of legumes
- Lightning also fixes small amounts
2. Nitrification:
- Ammonia oxidized to nitrite (NO2-)
- Further oxidized to nitrate (NO3-)
- Done by nitrifying bacteria
- Produces plant-available nitrogen
3. Assimilation:
- Plants absorb nitrates from soil
- Incorporate into amino acids and proteins
- Animals consume plants, obtaining nitrogen
4. Ammonification:
- Dead organisms and waste decomposed
- Nitrogen released as ammonia
- Done by decomposing bacteria
5. Denitrification:
- Nitrates reduced to nitrogen gas (N2)
- Released to atmosphere
- Done by denitrifying bacteria
- Completes cycle
Role of Nitrogen-Fixing Bacteria:
1. Symbiotic Relationship:
- Live in root nodules of legumes
- Legumes: Peas, beans, clover
- Provide carbohydrates to bacteria
- Bacteria provide fixed nitrogen to plant
2. Free-living Bacteria:
- Azotobacter in soil
- Cyanobacteria in water
- Fix nitrogen without host
3. Importance:
- Make atmospheric N2 usable
- Only organisms that fix atmospheric N2
- Essential for nitrogen availability
- Support plant growth and productivity
Q (2022, 4 marks): What is ecological succession? Explain primary and secondary succession.
Answer: Ecological Succession:
Definition - Gradual change in species composition of community over time.
Characteristics:
1. Directional change
2. Predictable sequence
3. Leads to climax community
4. Driven by biotic and abiotic factors
Primary Succession:
Occurs on barren land with no soil:
1. Pioneer Species:
- First organisms to colonize
- Lichens and mosses
- Break rock to form soil
- Tolerate harsh conditions
2. Early Colonizers:
- Grasses and herbaceous plants
- Improve soil further
- Attract insects and animals
3. Intermediate Species:
- Shrubs and small trees
- Increase biodiversity
- Modify environment
4. Climax Community:
- Stable, mature forest
- Various species in equilibrium
- Self-sustaining
- Takes 100+ years
Secondary Succession:
Occurs when existing community is disturbed:
1. Faster than primary succession
2. Soil already present
3. Pioneer species different
- Annual weeds and grasses
- Small shrubs
4. Similar to primary succession:
- Intermediate species
- Eventually reaches climax community
5. Takes 25-50 years usually
Examples:
- Forest regeneration after logging
- Field abandonment recovery
- Recovery after fire
Q (2021, 5 marks): Explain the concept of ecological pyramids. Describe pyramid of numbers, biomass, and energy.
Answer: Ecological Pyramids:
Definition - Graphical representation showing relative amounts of energy, biomass, or organisms at each trophic level.
Characteristics:
1. Producers at base
2. Consumers at upper levels
3. Generally pyramid-shaped
4. Show energy flow and transfer efficiency
Pyramid of Numbers:
1. Shows number of organisms at each trophic level
2. Example: Grass (1000) -> Grasshopper (500) -> Frog (50) -> Snake (10)
3. Decreases upward usually
4. Exception: Inverted in parasitic food chains
- One tree (host) -> Many parasites
5. Limitation: Doesn't show biomass (tiny organisms appear equal to large)
Pyramid of Biomass:
1. Shows total dry mass of organisms
2. Better represents energy than numbers
3. Generally pyramid-shaped
4. Exception: Inverted in aquatic ecosystems
- Phytoplankton (small, high turnover)
- Zooplankton (larger biomass)
5. Unit: grams/m2 or kg/hectare
Pyramid of Energy:
1. Shows energy content at each level
2. Always pyramid-shaped (never inverted)
3. Most accurate representation
4. First trophic level: 100% energy
5. Each subsequent level: 10% of previous
6. Energy lost as heat, respiration, excretion
7. Explains limited carnivore populations
Importance:
- Understand energy transfer
- Predict ecosystem carrying capacity
- Understand food web limitations
Frequently Asked Questions
What is the difference between a food chain and a food web?
A food chain is a linear sequence of organisms showing energy flow (grass -> rabbit -> fox), while a food web is a network of interconnected food chains showing the complex feeding relationships in an ecosystem.
Why are decomposers essential for ecosystem functioning?
Decomposers break down dead organic matter, releasing nutrients back into the soil for plant absorption, recycling nutrients, preventing accumulation of dead matter, and completing nutrient cycles.
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