Ecosystem

NCERT Line-by-Line Breakdown for NEET 2026

Unit 5: Ecology and Environment

An Ecosystem is a functional unit of nature where living organisms interact among themselves and with the surrounding physical environment. It can be small (a drop of rain) or large (an ocean). A.G. Tansley coined the term.

1. Ecosystem Structure and Function

A. Structure

Interaction of biotic and abiotic components results in a physical structure characteristic of each ecosystem.

Species Composition: Identification and enumeration of plant and animal species.
Stratification: Vertical distribution of different species occupying different levels.
Example in Forest: Trees (Top), Shrubs (Second), Herbs/Grasses (Bottom).

B. Four Key Functions

1. Productivity

2. Decomposition

3. Energy Flow

4. Nutrient Cycling

2. Productivity

Rate of biomass production is called productivity (g/m²/yr).

Type	Definition	Formula
Primary Productivity	Biomass produced by plants (Producers) during photosynthesis.	–
GPP (Gross)	Total rate of photosynthesis (Total capture).	–
NPP (Net)	Biomass available for consumption by herbivores.	NPP = GPP – R (Respiration loss)
Secondary Productivity	Rate of formation of new organic matter by consumers.	–

Annual NPP of whole biosphere is ~170 billion tons (dry weight). Oceans contribute only 55 billion tons despite occupying 70% surface.

3. Decomposition

Breakdown of complex organic matter into inorganic substances (CO₂, water, nutrients). Raw material is Detritus (dead leaves, fecal matter).

Steps of Decomposition

Fragmentation: Detritivores (Earthworms) break detritus into smaller particles.
Leaching: Water-soluble inorganic nutrients go down into soil horizon and precipitate as unavailable salts.
Catabolism: Bacterial/Fungal enzymes degrade detritus into simpler inorganic substances.
Humification: Formation of Humus (dark, amorphous, resistant to microbial action, reservoir of nutrients).
Mineralization: Release of inorganic nutrients from humus.

📉 Factors Inhibiting Decomposition:
Low temperature, Anaerobiosis, Detritus rich in Lignin and Chitin.
📈 Factors Promoting Decomposition:
Warm & moist environment, Detritus rich in Nitrogen and sugars.

4. Energy Flow

Sun is the only source of energy (except deep sea hydrothermal vents). Flow is Unidirectional.

PAR: Photosynthetically Active Radiation (400-700 nm). Plants capture only 2-10% of PAR.
10% Law (Lindeman): Only 10% of energy is transferred from one trophic level to the next. 90% is lost as heat.

Food Chains

Grazing Food Chain (GFC): Producer → Herbivore → Carnivore. (Major conduit in Aquatic ecosystem).
Detritus Food Chain (DFC): Dead organic matter → Decomposers (Saprotrophs). (Major conduit in Terrestrial ecosystem).

5. Ecological Pyramids

Graphical representation of trophic structure.

Pyramid	Shape	Example/Exception
Number	Mostly Upright.	Inverted in Tree ecosystem (1 Tree → Many Birds → Many Parasites).
Biomass	Mostly Upright.	Inverted in Sea (Phytoplankton standing crop is low, supported by high Zooplankton biomass).
Energy	Always Upright.	Energy is always lost as heat at each step. Never inverted.

Limitation: Does not account for the same species belonging to two or more trophic levels (e.g., Sparrow eating seeds and insects). Decomposers are not given any place.

6. Ecological Succession

Gradual and predictable change in species composition of a given area. Final stable community is Climax Community.

Primary Succession: On bare rocks/new lava (No soil initially). Very slow.
Pioneer: Lichens (secrete acids to dissolve rock).
Secondary Succession: On abandoned lands/burnt forests (Soil present). Faster.

Hydrarch (Water) & Xerarch (Dry) Succession: Both proceed towards Mesic (Medium water) conditions.

7. Nutrient Cycling (Biogeochemical Cycles)

A. Carbon Cycle (Gaseous)

Reservoir: Atmosphere & Ocean (71% dissolved carbon).
4 x 10¹³ kg carbon fixed annually via photosynthesis.

B. Phosphorus Cycle (Sedimentary)

Reservoir: Rocks (Phosphates). No respiratory release of phosphorus into atmosphere.

[Image of phosphorus cycle diagram]

💰 Ecosystem Services: Robert Costanza put a price tag on nature’s services. Average: US $33 trillion/year. (Soil formation accounts for 50%).

📝 Rapid Fire MCQs

Q1. Which of the following ecological pyramids is always upright?

A) Pyramid of number
B) Pyramid of biomass
C) Pyramid of energy
D) None of these

Click to check Answer

Answer: C) Pyramid of energy (10% law).

Q2. Secondary productivity is rate of formation of new organic matter by:

A) Consumers
B) Decomposers
C) Producers
D) Parasites