NCERT Solutions for Class 12 Biology Chapter 12 Ecosystem
NCERT Solutions for Class 12 Biology Chapter 12 Ecosystem covers Ecosystem Structure & Function, Productivity (Primary and Secondary), Decomposition Steps, Energy Flow, Trophic Levels, Ecological Pyramids, and Ecological Succession.
Overview of Topics & Subtopics (Click to Jump)
1. Ecosystem Structure, Function & Productivity
Short Note:
• Ecosystem Components: Biotic (Producers, Consumers, Decomposers) and Abiotic (Light, Temp, Water, Soil).
• Primary Productivity: Rate of biomass/organic matter production per unit area by producers during photosynthesis.
- Gross Primary Productivity (GPP): Total rate of production of organic matter.
- Net Primary Productivity (NPP): GPP minus respiration losses (R). Formula: NPP = GPP - R.
• Secondary Productivity: Rate of formation of new organic matter by consumers.
2. Decomposition & Energy Flow
Short Note:
• Steps in Decomposition:
1. Fragmentation: Breakdown of detritus into smaller particles by detritivores (e.g., Earthworm).
2. Leaching: Water-soluble inorganic nutrients seep down into soil horizon and precipitate as unavailable salts.
3. Catabolism: Enzymatic degradation of detritus by bacterial and fungal enzymes.
4. Humification: Formation of dark-colored, amorphous substance called humus (highly resistant to microbial action).
5. Mineralisation: Humus is further degraded by microbes releasing inorganic nutrients.
• Energy Flow (10% Law): Only 10% of energy is transferred from one trophic level to the next higher level (Lindeman's Law).
3. Ecological Pyramids & Ecological Succession
Short Note:
• Ecological Pyramids:
- Pyramid of Number: Mostly upright, but inverted in a single tree ecosystem (tree → insects → birds).
- Pyramid of Biomass: Mostly upright, but inverted in sea/aquatic ecosystem (small phytoplankton supporting large fish biomass).
- Pyramid of Energy: ALWAYS upright; energy decreases at each progressive trophic level.
• Ecological Succession: Sequential, predictable change in species composition in a given area over time.
- Primary Succession: Begins in biologically bare areas (bare rock, newly cooled lava).
- Secondary Succession: Begins in areas where natural biotic community was destroyed (burnt forest, flooded lands).
Intext & Conceptual Questions
| Feature | Gross Primary Productivity (GPP) | Net Primary Productivity (NPP) |
|---|---|---|
| Definition | Total amount of organic matter synthesized by green plants per unit area per unit time during photosynthesis. | Amount of organic matter remaining stored in producers after accounting for respiratory losses. |
| Respiration Loss | Includes the energy utilized by plants for cellular respiration. | Excludes respiratory losses (GPP - Respiratory Loss). |
| Availability | Not entirely available to herbivores/consumers. | Directly available as biomass for consumption by heterotrophs/herbivores. |
| Equation | GPP = NPP + R |
NPP = GPP - R |
1. According to the 10% Law of Energy Transfer (Lindeman's Law), when energy flows from one trophic level to the next higher level, only about 10% of the energy is stored as organic biomass.
2. The remaining 90% of energy is lost as heat during respiration, metabolic activities, and waste excretion at each transfer level.
3. Consequently, the energy available at successive trophic levels progressively decreases from producers to top carnivores, making the pyramid of energy always upright without exception.
Textbook Chapter End Exercise Questions
Decomposition: Breakdown of complex organic matter into inorganic substances like CO₂, water, and nutrients by decomposers.
Steps in Decomposition:
1. Fragmentation: Detritivores (e.g., earthworms) break down dead plant and animal remains (detritus) into smaller particles.
2. Leaching: Water-soluble inorganic nutrients percolate deep into soil layers and precipitate as unavailable salts.
3. Catabolism: Bacterial and fungal enzymes degrade complex organic matter into simpler inorganic compounds.
4. Humification: Leads to the accumulation of dark, amorphous, nutrient-rich substance called humus, which decomposes at an extremely slow rate.
5. Mineralisation: Humus is further degraded by specific microbes, releasing inorganic minerals into the soil.
Ecological Succession: The gradual, predictable, and directional change in species composition of an ecological community in a given area over time.
| Feature | Primary Succession | Secondary Succession |
|---|---|---|
| Starting Area | Occurs in non-vegetated, biologically barren areas with no pre-existing soil or organic matter (e.g., bare rock, cooled lava). | Occurs in areas where an existing community was destroyed by natural or human disturbance (e.g., burned forest, flooded land). |
| Soil Presence | Soil is completely absent initially; needs time to build up. | Soil and organic matter are already present. |
| Time Taken | Extremely slow process (takes hundreds to thousands of years). | Relatively fast process (takes decades to a few hundred years). |
Inverted Biomass Pyramid in Aquatic Ecosystem:
In aquatic environments (such as oceans or lakes), the pyramid of biomass is typically inverted.
Reason: The biomass of primary producers (phytoplankton) present at any given moment (standing crop) is very small due to their microscopic size and rapid turnover rate (short lifespan). In contrast, the biomass of consumers (fishes, zooplankton) that feed on them is significantly higher, resulting in a inverted pyramid structure where a small producer biomass supports a much larger consumer biomass.
Read More & Chapter Resources
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