CBSE Class 12 Biology Chapter 12 Revision Notes

An ecosystem is a functional unit of nature where living organisms interact among themselves and with their physical environment. It consists of biotic (living) and abiotic (non-living) components that function together through four key processes: productivity, decomposition, energy flow, and nutrient cycling. Energy enters the system from the sun, is captured by producers, and flows unidirectionally through various trophic levels, while nutrients are recycled repeatedly. Understanding these interactions, from simple food chains to complex ecological pyramids, is essential for studying the biosphere's sustainability.

1. Ecosystem Structure

The interaction of biotic and abiotic components creates a physical structure unique to each ecosystem type.

• Stratification: This is the vertical distribution of different species occupying different levels.
• Top Layer: Trees in a forest.
• Middle Layer: Shrubs.
• Bottom Layer: Herbs and grasses.
• Species Composition: The identification and enumeration of plant and animal species within an ecosystem.
• Types of Ecosystems:
• Terrestrial: Forest, grassland, desert.
• Aquatic: Pond, lake, wetland, river, estuary.
• Man-made: Crop fields and aquariums.

2. Productivity

Productivity refers to the rate of biomass production.

Exam Tip: The annual NPP of the whole biosphere is ~170 billion tons; despite covering 70% of Earth, oceans contribute only 55 billion tons.

3. Decomposition

Decomposition is the process where decomposers break down complex organic matter into inorganic substances like CO₂, water, and nutrients.

• Detritus: The raw material for decomposition, consisting of dead plant remains (leaves, bark) and animal fecal matter.
• The Five Steps of Decomposition:
• Fragmentation: Detritivores (like earthworms) break detritus into smaller particles.
• Leaching: Water-soluble inorganic nutrients sink into the soil and precipitate as unavailable salts.
• Catabolism: Bacterial and fungal enzymes degrade detritus into simple inorganic substances.
• Humification: Accumulation of humus, a dark, amorphous substance resistant to microbes that serves as a nutrient reservoir.
• Mineralisation: Further degradation of humus by microbes to release inorganic nutrients.

Factors Affecting Decomposition Rate:

• Quick: Detritus rich in nitrogen and water-soluble substances (sugars); warm and moist environments.
• Slow: Detritus rich in lignin and chitin; low temperature and anaerobic conditions.

4. Energy Flow

Energy flow in an ecosystem is unidirectional (Sun → Producers → Consumers).

• PAR (Photosynthetically Active Radiation): Less than 50% of incident solar radiation is PAR.
• Capture: Plants capture only 2-10% of PAR to sustain the entire living world.
• 10% Law: Only 10% of energy is transferred to each successive trophic level; the rest is lost as heat.

Comparison: Food Chains

Food Web: The natural interconnection of multiple food chains.

5. Ecological Pyramids

Ecological pyramids represent the relationship between organisms at different trophic levels in terms of number, biomass, or energy.

• Trophic Levels:
• First: Producers (Plants/Phytoplankton).
• Second: Herbivores/Primary Consumers (Zooplankton/Insects).
• Third: Carnivores/Secondary Consumers (Birds/Fishes).
• Types of Pyramids:
• Pyramid of Number: Usually upright (e.g., grassland), but can be different in tree-dominated systems.
• Pyramid of Biomass: Usually upright on land; inverted in the sea because the biomass of fish exceeds that of phytoplankton.
• Pyramid of Energy: Always upright and can never be inverted because energy is lost as heat at every step.

Limitations of Ecological Pyramids:

• They do not account for species belonging to two or more trophic levels.
• They assume a simple food chain and do not accommodate food webs.
• Saprophytes (decomposers) are not given a place in the pyramids.

Frequently Asked Questions:

1. What is the difference between Gross Primary Productivity (GPP) and Net Primary Productivity (NPP)?
   
   GPP is the total rate at which plants produce organic matter through photosynthesis, while NPP is what remains after the plants use some of that energy for respiration. NPP represents the actual biomass available for consumption by herbivores and decomposers.
   
   
2. Why is the pyramid of energy always upright?
   
   According to the 10% law, energy is lost as heat at each step as it moves from one trophic level to the next. Because the energy available at a lower level is always greater than the energy at a higher level, the pyramid can never be inverted.
   
   
3. What are the main factors that inhibit the rate of decomposition?
   
   Decomposition is slowed down if the detritus is rich in lignin and chitin. Additionally, physical factors like low temperatures and anaerobic (low oxygen) conditions inhibit microbial activity, leading to a buildup of organic material.