Ecosystem Class 12 Notes पारितंत्र — कक्षा 12 जीव विज्ञान अध्याय 12

Revise these first (prerequisites)

• Basic equation of photosynthesis (Class 10–11)
• Food chain / food web (Class 10 'Our Environment')
• Meaning of producer, consumer, decomposer
• Chapter 11: population interactions

1. Ecosystem = organisms + environment

   When all the organisms of a place (plants, animals, microbes) and their non-living environment (water, soil, light, air) work together as one unit, it is called an ecosystem (पारितंत्र). A small pond is an ecosystem, and so is an entire forest.

2. Pond — a small model

   A pond has water and soil (abiotic), phytoplankton and algae (producers), zooplankton and fish (consumers), and fungi and bacteria (decomposers). Each does its own 'job' — like in a household, there is someone who cooks, someone who eats and someone who cleans.

3. Productivity = rate of food production

   Plants make organic matter by photosynthesis using sunlight. The amount of organic matter produced in an area over a period of time is primary production. Plants use some of it in their own respiration; what remains is available to other organisms.

4. Decomposition = nature's recycling

   Decomposers break down fallen leaves, dead animals and excreta (detritus) into small fragments and simple inorganic substances. This returns nutrients to the soil. Without decomposers, the Earth would be filled with waste!

5. The journey of energy — one way only

   Energy flows from the sun to plants, from plants to herbivores, and from them to carnivores — but it never flows back (unidirectional). At every step most of the energy is lost as heat, which is why food chains are not long.

6. Pyramid — an ecosystem in one picture

   If each trophic level is drawn as a block, with producers at the bottom and consumers above, a pyramid is formed. It can be a pyramid of number, biomass or energy. The pyramid of energy is always upright.

Quick check ✔

Basics clear? Now move on to Learn in Detail 👆

As per NCERT 2026-27 — every point you need for the board exam.

1. Ecosystem

   Interaction of biotic + abiotic components. Terrestrial (forest, grassland, desert) and aquatic (pond, lake, wetland, river, estuary). Man-made: crop fields, aquarium.

2. Structure

   Stratification — vertical layers (in a forest, trees at the top, then shrubs, then herbs/grasses). Components: productivity, decomposition, energy flow, nutrient cycling.

3. Pond ecosystem

   Abiotic: water, dissolved substances, soil. Producers: phytoplankton, algae, floating/submerged plants. Consumers: zooplankton, free-swimming/bottom dwellers. Decomposers: fungi, bacteria, flagellates.

4. Productivity

   Primary production: biomass/organic matter produced per unit area over a time period (g⁻² yr⁻¹ or kcal m⁻² yr⁻¹). GPP = total rate of photosynthesis. NPP = GPP − R (respiration loss). Secondary productivity = new organic matter formed by consumers. Annual NPP of the biosphere is ~170 billion tons (dry weight); the oceans contribute only ~55 billion tons.

5. Decomposition

   Detritus (dead leaves, bark, flowers, animal remains). Steps: fragmentation (detritivores, earthworm) → leaching (dissolving in water) → catabolism (enzymes of bacteria/fungi) → humification (humus — dark, amorphous, colloidal, decomposes slowly) → mineralisation (inorganic nutrients).

6. Decomposition factors

   Chemical composition (more lignin, chitin → slower; nitrogen, sugars → faster), climate: warm and moist → faster; cold, anaerobic → slower.

7. Energy flow

   Sun = the only source. Plants capture only 2–10% of PAR (photosynthetically active radiation). Energy flow is unidirectional (sun → producers → consumers). Follows the laws of thermodynamics.

8. 10 percent law

   At each trophic level, only about 10% of the energy of the previous level is passed on. That is why a food chain has a limited number of trophic levels (3–4).

9. Food chains

   Grazing food chain (GFC): grass → goat → man. Detritus food chain (DFC): begins with decomposers (saprotrophs). In aquatic ecosystems GFC carries more energy; in terrestrial ecosystems more energy flows through DFC. Food web = interconnected food chains.

10. Trophic levels

    Producers (1st), primary consumers / herbivores (2nd), secondary consumers (3rd), tertiary consumers (4th). Standing crop = the living material (biomass) present at a trophic level at a given time.

11. Ecological pyramids

    Pyramids of number, biomass and energy. The energy pyramid is always upright. The pyramid of biomass in the sea is inverted (phytoplankton have less biomass, fish have more). The pyramid of number for insects on a single tree is inverted.

12. Limitations of ecological pyramids

    It does not account for the same species occurring at two levels; it assumes a simple food chain (not a food web); saprophytes are given no place.

The concept in depth — the why and the how. Everything stays within the NCERT syllabus.

1. GPP, NPP and 'available' food

   GPP = total rate of production of organic matter during photosynthesis. Plants use a part of it themselves in respiration (R): NPP = GPP − R. NPP is the biomass available to heterotrophs (herbivores, decomposers). The rate of formation of new organic matter by consumers is secondary productivity. The annual NPP of the whole biosphere is ~170 billion tons (dry weight), of which oceans contribute only ~55 billion tons — despite covering 70% of the surface, because nutrients and light are limited there.

2. Steps and rate of decomposition

   Steps: fragmentation (by detritivores such as earthworms), leaching (water-soluble nutrients go down into the soil and become unavailable salts), catabolism (bacterial and fungal enzymes), humification and mineralisation. Detritus rich in lignin and chitin decomposes slowly; detritus rich in nitrogen and water-soluble substances (such as sugars) decomposes quickly. Warm and moist weather increases decomposition; low temperature and anaerobiosis inhibit it — this is why organic matter keeps accumulating in cold/marshy areas.

3. Energy flow numbers

   Less than 50% of the incoming solar radiation is PAR (photosynthetically active radiation), and plants capture only 2–10% of PAR — the entire living world runs on this small fraction. According to the 10 percent law, only ~10% of the energy of the previous level passes on at each trophic level. So if producers have 10,000 J, only ~10 J reaches the tertiary consumer.

4. GFC vs DFC — where is more energy?

   The grazing food chain (GFC) starts with producers; the detritus food chain (DFC) starts with dead organic matter, with decomposers (saprotrophs). In aquatic ecosystems the GFC is the main pathway of energy flow, whereas in terrestrial ecosystems a much larger amount of energy flows through the DFC. The two chains are interconnected at many points to form a food web.

5. Inverted pyramids and limitations

   Many insects on one large tree — the pyramid of number is inverted. In the sea, the biomass of fish is greater than that of phytoplankton — the pyramid of biomass is inverted. The energy pyramid is never inverted. Limitations: it does not show one species occupying more than one trophic level, it assumes a simple food chain (not a food web), and it gives no place to saprophytes.

Common confusions — cleared up

1. Standing crop vs Productivity

   Standing crop = the living material (biomass) present in a trophic level at a given time. Productivity = the rate of formation of organic matter (per unit area per unit time).

2. Detritivore vs Decomposer

   Detritivores (such as earthworms) break detritus into smaller pieces (fragmentation). Decomposers (bacteria, fungi) break it down chemically using enzymes.

3. Humification vs Mineralisation

   Humification forms dark, amorphous, highly resistant humus, which decomposes very slowly. In mineralisation, inorganic nutrients are released from humus and organic matter.

4. Trophic level vs Species

   A trophic level is a functional position (producer, primary consumer…); the same species can occupy different trophic levels in different food chains.

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