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SSC CGL - Detailed Guide 2025

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Ecosystem

Reference: NCERT Class 9-12, Lucent GK

1. Structure of Ecosystem

An ecosystem is a functional unit where living organisms interact with each other and with the non-living environment.

Definition: An ecosystem is a self-sustaining unit comprising biotic (living) and abiotic (non-living) components that interact with each other through energy flow and nutrient cycling.

Component Description Examples
Producers Autotrophs that synthesize food using sunlight or chemicals Green plants, phytoplankton
Consumers Heterotrophs that feed on other organisms Herbivores (deer), Carnivores (lion), Omnivores (humans)
Decomposers Break down dead organic matter and recycle nutrients back to the environment Bacteria, Fungi, Earthworms

Note: Decomposers play a vital role in nutrient recycling.

2. Functions of Ecosystem

Ecosystems perform two key functions: Energy Flow and Nutrient Cycling.

a) Food Chain & Food Web

  • Food Chain: Linear flow of energy from producers to top consumers.
    → Example: Grass → Grasshopper → Frog → Snake → Eagle
  • Food Web: Interconnected food chains showing complex feeding relationships in an ecosystem.
    → Ensures ecological balance and stability.
Illustration of ecosystem by flowchart
Trophic Levels Energy Pyramid

b) Trophic Levels

Trophic levels represent positions in the food chain:

  1. 1st Trophic Level -- Producers (plants)
  2. 2nd -- Primary Consumers (herbivores)
  3. 3rd -- Secondary Consumers (carnivores)
  4. 4th -- Tertiary Consumers (top predators)
  5. Decomposers act at all levels

Note: Energy transfer between trophic levels is only ~10% efficient (10% Law by Lindeman).

c) Energy Flow

  • Unidirectional: Flows from sun → producers → consumers → decomposers.
  • Governed by the 10% Law — most energy is lost as heat during transfer.
  • Energy flow is non-cyclic and non-reversible.
Energy Flow in Ecosystem Diagram
Types of Ecological Pyramids

d) Ecological Pyramids

Graphical representations of trophic structures:

Type Description
Pyramid of Number Number of organisms at each trophic level
Pyramid of Biomass Total mass of organisms at each trophic level
Pyramid of Energy Energy flow at each level (always upright)

Note: Energy pyramid is always upright; others may be inverted (e.g., pyramid of number in a tree ecosystem).

3. Types of Ecosystems

Ecosystems can be classified as:

a) Terrestrial Ecosystems

Type Characteristics
Forest High biodiversity; dense tree cover; heavy rainfall
Grassland Dominated by grasses; moderate rainfall; supports herbivores
Desert Dry, arid; extreme temperatures; xerophytic vegetation

b) Aquatic Ecosystems

Type Characteristics
Freshwater Low salinity; includes rivers, lakes, ponds, streams
Marine High salinity; oceans and seas; stable biodiversity; supports plankton
Estuarine Transition between river and sea; highly productive and nutrient-rich zone

Note: Estuaries act as nurseries for many marine species due to nutrient availability and calm waters.

Quick Exam Highlights

  • • Grasslands in India: Tropical (Savanna type), e.g., Banni Grasslands (Gujarat)
  • • Coral reefs are marine ecosystems rich in biodiversity
  • • Mangroves are coastal wetland ecosystems — found in Sundarbans, Mahanadi Delta, etc.
  • • Pond ecosystem = most studied natural ecosystem in ecology
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