Ecosystem Class 12 NCERT: Concepts, Solved Examples, and Board Practice

Structure and Stratification of an Ecosystem

To understand any ecosystem, we must first study its structural components. Broadly, these are divided into Biotic (living) and Abiotic (non-living) factors. When these components interact, they form a distinct physical structure. One crucial structural feature is Stratification, which is defined as the vertical distribution of different species occupying different levels in a community. For instance, in a dense forest ecosystem, trees occupy the top vertical strata or layer, shrubs occupy the middle layer, and herbs and grasses occupy the bottom-most ground layer.

Every functional ecosystem requires four basic processes to work in harmony:

1. Productivity: The rate of biomass production by plants.
2. Decomposition: The breakdown of complex organic matter into simple inorganic raw materials.
3. Energy Flow: The unidirectional transfer of energy from producers to consumers.
4. Nutrient Cycling: The movement and storage of nutrient elements through the ecosystem.

The Step-by-Step Mechanism of Decomposition

1. Fragmentation — Detritivores (such as earthworms) feed on dead organic matter (detritus) and break it down into much smaller, manageable particles.
2. Leaching — Water-soluble inorganic nutrients percolate deep down into the soil horizon and precipitate as unavailable salts.
3. Catabolism — Bacterial and fungal enzymes break down complex organic compounds from detritus into simple inorganic compounds.
4. Humification — An amorphous, dark-colored substance called humus accumulates. Humus is highly resistant to microbial action and decomposes at an extremely slow rate.
5. Mineralisation — The humus is further degraded by microbes to release inorganic nutrients (such as carbon dioxide, water, and ions) back into the soil.

Ecological Pyramids: Board Exam Traps & Rules

When drawing and solving questions about ecological pyramids, CBSE board examiners love to test you on exceptions. Here are two critical rules you must memorize:

• The Pyramid of Biomass in Sea is Inverted: This is a classic exception! The biomass of trophic level 1 (phytoplankton) is surprisingly smaller than that of trophic level 2 (zooplankton) because phytoplanktons have a high turnover rate. They multiply and are consumed rapidly. Do not draw this as an upright pyramid.
• The Pyramid of Energy is ALWAYS Upright: Energy transfer is strictly unidirectional and obeys thermodynamics. Energy is lost as heat at every trophic level (Lindeman's 10% law). Therefore, the pyramid of energy can never be inverted under any circumstances.

Practice Questions with Solutions

• Q: Distinguish between Gross Primary Productivity (GPP) and Net Primary Productivity (NPP). Write the equation that relates them. A: Step 1: Define Gross Primary Productivity (GPP). GPP is the total rate at which solar energy is captured and converted into organic matter (biomass) by producers during photosynthesis in a given area over a specific time period. Step 2: Define Net Primary Productivity (NPP). NPP is the actual organic matter stored by producers after accounting for respiratory losses (R). This is the biomass available for consumption by herbivores and decomposers. Step 3: Write the relating equation. The mathematical relationship is: NPP = GPP - R where 'R' represents respiratory loss. Final answer: GPP is the total organic matter produced, while NPP is the remaining organic matter after plants use energy for respiration. The relationship is NPP = GPP - R.
• Q: Calculate the amount of energy available to the tiger (tertiary consumer) if 10,000 Joules of energy is present at the level of grass (producer). Show all steps using Lindeman's 10% law. A: Step 1: Identify the trophic levels in the food chain. The food chain is: Grass (Producer, T1) -> Deer (Primary Consumer, T2) -> Jackal (Secondary Consumer, T3) -> Tiger (Tertiary Consumer, T4). Step 2: Calculate energy at the Primary Consumer level (Deer). According to Lindeman's 10% law, only 10% of the energy is transferred to the next level. Energy at T2 (Deer) = 10% of 10,000 J = 0.10 10,000 = 1,000 J. Step 3: Calculate energy at the Secondary Consumer level (Jackal). Energy at T3 (Jackal) = 10% of 1,000 J = 0.10 1,000 = 100 J. Step 4: Calculate energy at the Tertiary Consumer level (Tiger). Energy at T4 (Tiger) = 10% of 100 J = 0.10 * 100 = 10 J. Final answer: The energy available to the tiger at the tertiary consumer level is 10 Joules.
• Q: Explain why decomposition is slower in anaerobic conditions and colder temperatures. A: Step 1: Understand the nature of decomposition. Decomposition is primarily an oxygen-requiring process carried out by aerobic microbes (fungi and bacteria). Step 2: Analyze the effect of oxygen availability. Under anaerobic (low-oxygen) conditions, the metabolic activities of aerobic decomposers are severely restricted, slowing down the fragmentation and catabolism steps. Step 3: Analyze the effect of temperature. Decomposition is enzymatically driven. Colder temperatures inhibit the activity of microbial enzymes, reducing the rate of chemical breakdown. Final answer: Low temperatures and anaerobiosis inhibit microbial and enzymatic activity, which severely delays the rate of decomposition.
• Q: What is primary ecological succession? How does it differ from secondary ecological succession? A: Step 1: Define primary succession. Primary succession is the process of biotic colonization occurring in a newly formed, completely barren area where no life or soil previously existed (e.g., bare rock, cooled lava, or a newly formed pond). Step 2: Define secondary succession. Secondary succession occurs in areas where a natural community was destroyed or disturbed (e.g., burned forest, abandoned farm) but where some soil and organic matter are already present. Step 3: Compare their rates of progress. Since soil formation is a slow process, primary succession is extremely slow and can take thousands of years, whereas secondary succession is much faster as the soil is already present. Final answer: Primary succession begins on bare, soil-less surfaces and is very slow. Secondary succession starts in disturbed areas with pre-existing soil and proceeds much faster.

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