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🌿 Chapter 12: Ecosystem
A functional unit of nature where living organisms interact with each other and the physical environment.
By Persue Classes || Teacher: Asfak Hossain, NEET Biology Expert
🔸 12.1 Ecosystem – Structure and Function
🌎 What is an Ecosystem?
Definition
A functional unit of nature where living organisms interact with each other and the physical environment.
Types
  • Terrestrial: Forests, grasslands, deserts
  • Aquatic: Pond, lake, river, estuary
  • Man-made: Crop fields, aquariums
Global View
Biosphere = largest ecosystem (sum of all local ecosystems)
🌱 Structural Components
Biotic (living):
  • Producers (e.g., plants, algae)
  • Consumers (herbivores, carnivores)
  • Decomposers (bacteria, fungi)
Abiotic (non-living):
  • Sunlight
  • Water
  • Soil
  • Temperature
🌿 Stratification
Vertical arrangement of species:
Top
Trees
Middle
Shrubs
Bottom
Herbs/Grasses
🔸 12.2 Productivity
⚡ Key Concepts
Primary Productivity
Rate of biomass generation by producers
  • Gross Primary Productivity (GPP) = total energy captured
  • Net Primary Productivity (NPP) = energy left after respiration
  • NPP = GPP - R
Secondary Productivity
Biomass generation by consumers
🧠 Factors Affecting Productivity
Species diversity
Nutrient availability
Light and temperature
Photosynthetic efficiency
🌍 Global Stats
Biosphere NPP ≈ 170 billion tons/year
Oceans (70% of surface) only ≈ 55 billion tons → Low productivity due to limited nutrient availability
🔸 12.3 Decomposition
🔁 Process Overview
Breakdown of organic matter (detritus) into inorganic nutrients.
🧬 Steps in Decomposition
Fragmentation
Detritivores (earthworms) reduce organic matter size
Leaching
Nutrients dissolve into soil
Catabolism
Enzymes break detritus into simpler substances
Humification
Forms humus (dark, nutrient-rich, stable compound)
Acts as a long-term nutrient reservoir
Mineralisation
Nutrients released back into ecosystem
🌦️ Influencing Conditions
Fast decomposition: nitrogen-rich, warm & moist climates
Slow decomposition: lignin/chitin-rich, cold/anaerobic conditions
🔸 12.4 Energy Flow
🌞 Key Principle
Sunlight is the main energy source
Only 2–10% of PAR (Photosynthetically Active Radiation) is captured by producers
🔄 Flow Pattern
Unidirectional: Sun → Producers → Consumers → Decomposers
🧃 Food Chains
Grazing Food Chain (GFC)
Producer → Herbivore → Carnivore
Detritus Food Chain (DFC)
Dead matter → Decomposers → Detritivores
🌐 Food Webs
Interconnected chains
Omnivores (e.g., humans, crows) function at multiple trophic levels
📊 Trophic Levels
1
Producers
(plants)
2
Primary consumers
(herbivores)
3
Secondary consumers
(carnivores)
4
Tertiary consumers
(top carnivores)
⚖️ Standing Crop
Biomass or population at a trophic level
Measured in dry weight for accuracy
🔟 Ten Percent Law
Only 10% of energy is transferred between trophic levels
🔸 12.5 Ecological Pyramids
🔺 Types
1
Pyramid of Numbers
Example: Grassland – many plants, few top carnivores
2
Pyramid of Biomass
Terrestrial: Upright
Aquatic: Inverted (phytoplankton < fishes)
3
Pyramid of Energy
Always upright due to energy loss as heat
🔍 Limitations
  • Doesn’t show organisms at multiple trophic levels
  • Ignores food webs
  • No role assigned to decomposers/saprophytes
📝 Summary Snapshot (Slide Ready)
Ecosystem
Biotic + Abiotic interactions
Core Functions
  • Productivity: Biomass creation (GPP, NPP)
  • Decomposition: Organic matter breakdown
  • Energy Flow: Solar energy through trophic levels
  • Nutrient Cycling: Gaseous (e.g., carbon) & Sedimentary (e.g., phosphorus)
Energy Flow
Energy flow is unidirectional and follows the 10% rule
Pyramids
Pyramids help visualize biomass, numbers & energy dynamics
Real-world applications
forest purification, soil fertility, food resource mapping
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