ECOSYSTEM: Definition, Structure and Functions

Learning Objectives

After studying this topic, you should be able to:

• Define an ecosystem.
• Explain the structure of an ecosystem.
• Identify the biotic and abiotic components.
• Describe the four major ecosystem functions.
• Differentiate between GPP and NPP.
• Understand the importance of decomposition and nutrient cycling.

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Definition of Ecosystem

Ecosystem

An ecosystem is the structural and functional unit of the biosphere in which biotic (living) and abiotic (non-living) components interact with each other.

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Structure of an Ecosystem

The ecosystem consists of two major components:

               Ecosystem
                   │
      ┌────────────┴────────────┐
      │                         │
Abiotic Components        Biotic Components
 (Non-living)               (Living)

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1. Abiotic Components (Non-Living)

Abiotic components are the physical and chemical factors of the environment.

A. Climatic Factors

• Sunlight (Insolation)
• Temperature
• Rainfall
• Wind
• Moisture

B. Edaphic Factors (Soil Factors)

• Soil pH
• Mineral content
• Soil texture
• Topography

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2. Biotic Components (Living)

Biotic components include all living organisms in an ecosystem.

They are classified into:

A. Producers (Autotrophs)

Definition

Organisms that prepare their own food.

Types

i. Phototrophs

• Prepare food using sunlight (photosynthesis).
• Examples:
  • Green plants
  • Phytoplankton

ii. Chemotrophs

• Prepare food using chemical reactions (chemosynthesis).
• Example:
  • Thiobacillus bacteria

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B. Consumers (Heterotrophs)

Definition

Organisms that depend on producers or other organisms for food.

Types

• Primary Consumers → Herbivores
• Secondary Consumers → Carnivores
• Tertiary Consumers
• Quaternary Consumers

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C. Decomposers

Definition

Organisms that break down dead organic matter into simpler inorganic substances.

Characteristics

• Perform external digestion.
• Release enzymes outside the body.
• Recycle nutrients.

Examples

• Bacteria
• Fungi
• Protozoa

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D. Detritivores

Definition

Organisms that feed directly on dead organic matter.

Characteristics

• Perform internal digestion.
• Consume detritus physically.

Examples

• Earthworm
• Millipede
• Woodlouse
• Termite

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Functions of an Ecosystem

An ecosystem performs four major functions:

Productivity
      ↓
Energy Flow
      ↓
Decomposition
      ↓
Nutrient Cycling

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1. Productivity

Definition

The rate at which producers generate organic matter (biomass).

Types

Gross Primary Productivity (GPP)

• Total organic matter produced by producers.
• Includes the energy used for respiration.

Net Primary Productivity (NPP)

• Energy stored as biomass.
• Available for the next trophic level.

Formula

$$\boxed{\textbf{NPP = GPP - Respiration}}$$

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2. Energy Flow

Definition

Transfer of energy from one trophic level to another.

Characteristics

• Unidirectional.
• Follows Lindeman's 10% Law.
• Only 10% of energy is transferred to the next trophic level.
• Remaining 90% is lost mainly as heat during metabolic activities.

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3. Decomposition

Definition

Breakdown of dead plants, animals and organic wastes into simpler inorganic substances.

Importance

• Removes dead organic matter.
• Releases nutrients.
• Maintains ecological balance.

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4. Nutrient Cycling

Definition

Recycling of nutrients between living organisms and the physical environment.

Process

• Nutrients released during decomposition return to soil or water.
• Producers absorb them again.
• Cycle continues repeatedly.

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Flowchart: Functions of an Ecosystem

Producers
(Productivity)
        │
        ▼
Consumers
(Energy Flow)
        │
        ▼
Dead Organic Matter
        │
        ▼
Decomposers
(Decomposition)
        │
        ▼
Inorganic Nutrients
        │
        ▼
Soil / Water
        │
        ▼
Absorbed by Producers
(Nutrient Cycling)

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Comparison Tables

Producers vs Consumers

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Decomposers vs Detritivores

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Gross Primary Productivity (GPP) vs Net Primary Productivity (NPP)

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Important Definitions

Ecosystem

The structural and functional unit of the biosphere.

Biomass

Stored organic matter available as food for the next trophic level.

Trophic Level

The feeding position of an organism in a food chain.

Edaphic Factors

Abiotic factors related to soil.

Chemosynthesis

Food synthesis using chemical energy instead of sunlight.

Saprotrophs

Another name for decomposers.

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Frequently Confused Concepts

Decomposers vs Detritivores

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GPP vs NPP

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Energy Flow vs Nutrient Cycling

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Examination Focus (High Probability MCQs)

Frequently Asked Topics

• Definition of ecosystem.
• Structural and functional unit of the biosphere.
• Components of an ecosystem.
• Climatic and edaphic factors.
• Producers, consumers and decomposers.
• Four functions of an ecosystem.
• NPP = GPP − Respiration
• Lindeman's 10% Law.
• Difference between decomposers and detritivores.
• Difference between GPP and NPP.

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Common Mistakes

❌ Energy flow is cyclic.

✔ Energy flow is unidirectional.

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❌ Nutrients are lost permanently.

✔ Nutrients are recycled continuously.

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❌ Earthworms are decomposers.

✔ Earthworms are detritivores.

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❌ Viruses are decomposers.

✔ Viruses are not decomposers.

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Memory Tricks

Ecosystem Functions

Penguins Enjoy Delicious Noodles

• P → Productivity
• E → Energy Flow
• D → Decomposition
• N → Nutrient Cycling

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Ecosystem Components

A before B

• A → Abiotic (Non-living)
• B → Biotic (Living)

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Quick Revision

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One-Line Revision

• Ecosystem → Structural and functional unit of the biosphere.
• Abiotic Components → Climatic + Edaphic factors.
• Biotic Components → Producers, Consumers, Decomposers, Detritivores.
• Productivity → Biomass production (GPP & NPP).
• Energy Flow → Unidirectional; follows the 10% Law.
• Decomposition → Breakdown of dead organic matter.
• Nutrient Cycling → Continuous recycling of nutrients between organisms and the environment.