1. Ecosystem: Concept & Definition

• An ecosystem is the basic structural and functional unit of ecology.
• Odum (1963):
  “An ecosystem is a functional unit of nature that includes living organisms interacting with the physical environment, resulting in the flow of energy and cycling of materials.”
• Includes:
  • Biotic components (living)
  • Abiotic components (non-living)
• Interaction between these components creates a characteristic physical structure and species composition.

2. Structure of an Ecosystem

A. Abiotic (Non-living) Components

These regulate the distribution, abundance, and functioning of organisms.

• Climate: Temperature, rainfall, humidity
• Soil: Texture, nutrients, pH, moisture
• Water: Availability and quality
• Light: Controls photosynthesis
• Topography: Altitude, slope, drainage
• Air: Oxygen, CO₂ concentration
• Natural disturbances: Fire, floods, storms

Warm and moist climates → high productivity and faster decomposition

B. Biotic (Living) Components

1. Producers (Autotrophs)

They synthesize organic matter from inorganic substances.

(a) Photoautotrophs

• Use sunlight for photosynthesis
• Contain chlorophyll
• Examples:
  • Green plants
  • Algae
  • Cyanobacteria

(b) Chemoautotrophs

• Use chemical energy (not sunlight)
• Found in extreme environments
• Examples:
  • Nitrifying bacteria
  • Sulphur bacteria
  • Archaea in hydrothermal vents

2. Consumers (Heterotrophs)

• Cannot synthesize food
• Depend on producers or other consumers

Types:

• Primary consumers: Herbivores (deer, cattle)
• Secondary consumers: Carnivores (frog, lizard)
• Tertiary consumers: Top carnivores (tiger, eagle)

Macroconsumers

• Large consumers at higher trophic levels
• Example: Lion, Shark

3. Decomposers / Microconsumers

• Mainly bacteria and fungi
• Feed on dead organic matter (detritus)
• Convert organic matter into inorganic nutrients
• Also called saprotrophs

3. Interaction of Biotic & Abiotic Components

• Leads to:
  • Species composition
  • Vertical stratification

Stratification (Vertical Distribution)

• Example (Forest):
  • Top layer: Trees
  • Middle layer: Shrubs
  • Bottom layer: Herbs & grasses

4. Functional Aspects of Ecosystem

(i) Productivity

Primary Productivity

• Rate of biomass production by plants
• Expressed as weight or energy per unit area per time

Types:

• Gross Primary Productivity (GPP):
  Total photosynthetic production
• Net Primary Productivity (NPP):
  Biomass available to consumers

Formula (Very Important for Prelims):
NPP = GPP − Respiration (R)

• Secondary Productivity:
  Rate of biomass formation by consumers

Highest NPP: Tropical rainforests, estuaries
Lowest NPP: Deserts, tundra

(ii) Decomposition

• Breakdown of dead organic matter into inorganic substances
• Raw material: Detritus (dead leaves, bark, animal remains, faecal matter)

Steps of Decomposition (Mnemonic: FLC-HM)

1. Fragmentation – Detritivores (earthworms) break detritus
2. Leaching – Soluble nutrients seep into the soil
3. Catabolism – Microbial enzymes degrade detritus
4. Humification – Formation of humus (dark, resistant, colloidal)
5. Mineralisation – Release of inorganic nutrients

Faster decomposition:

• Warm, moist, aerobic conditions
• Detritus rich in nitrogen & sugars

Slower decomposition:

• Cold, dry, anaerobic conditions
• Detritus rich in lignin & chitin

(iii) Energy Flow

• Energy flows unidirectionally
• From:
  • Sun → Producers → Consumers → Decomposers
• 10% law: Only ~10% energy is transferred to the next trophic level

Energy is never recycled; nutrients are recycled

(iv) Nutrient Cycling

• Cyclic movement of nutrients like:
  • Carbon
  • Nitrogen
  • Phosphorus
• Maintains ecosystem stability

5. Types of Ecosystems

A. Terrestrial Ecosystems

1. Forest Ecosystem

• High biodiversity
• Types:
  • Tropical rainforests
  • Temperate forests
  • Boreal (Taiga)

2. Grassland Ecosystem

• Moderate rainfall
• Dominated by grasses
• Types:
  • Savannah
  • Prairie
  • Steppe

3. Desert Ecosystem

• Low rainfall
• Sparse vegetation
• Types:
  • Hot desert (Sahara)
  • Cold desert (Gobi)

4. Tundra Ecosystem

• Arctic & alpine regions
• Permafrost
• Mosses, lichens, dwarf shrubs

B. Aquatic Ecosystems

1. Freshwater

• Rivers, lakes, ponds, wetlands

2. Marine

• Oceans, seas, coral reefs
• Highest biodiversity

3. Estuarine

• Freshwater meets seawater
• Highly productive
• Brackish water species

C. Artificial Ecosystems

• Urban ecosystems: Cities, towns
• Agricultural ecosystems: Croplands, livestock farms

D. Specialised Ecosystems

• Wetland ecosystems: Marshes, swamps, bogs
• Mountain ecosystems: Altitudinal zonation, endemic species