CBSE Class 12 Biology: Ecosystem — Complete Notes 2026

Ecosystem Structure

• Biotic: producers (autotrophs), consumers (heterotrophs), decomposers (detritivores)
• Abiotic: solar radiation, temperature, water, soil nutrients, wind; determine type of ecosystem
• Physical structure: vertical distribution of organisms (stratification); forest has 5 layers (canopy, sub-canopy, shrub, herb, moss)

Productivity

• Gross Primary Productivity (GPP): total energy fixed by photosynthesis per unit area per unit time
• Net Primary Productivity (NPP) = GPP − Respiration; energy available to herbivores
• Secondary productivity: rate of biomass formation by consumers; depends on NPP and assimilation efficiency

Decomposition

• Decomposers (bacteria, fungi) break down detritus (dead organic matter) into inorganic components
• Steps: fragmentation → leaching → catabolism → humification → mineralisation
• Rate depends on: temperature, moisture, chemical composition of detritus (lignin and chitin slow decomposition)

Energy Flow

• Unidirectional: energy flows from sun → producers → herbivores → carnivores; cannot be recycled
• 10% law: only 10% of energy transferred to next trophic level; rest lost as heat
• Ecological pyramid: pyramid of numbers, biomass, energy; energy pyramid always upright; biomass may be inverted in aquatic

Nutrient Cycling (Biogeochemical Cycles)

• Carbon cycle: CO₂ fixed by photosynthesis; returned by respiration, decomposition, fossil fuel burning
• Nitrogen cycle: N₂ → NH₃ (fixation by Rhizobium) → NO₂⁻ → NO₃⁻ (nitrification) → plant uptake → denitrification back to N₂
• Phosphorus cycle: no gaseous phase; weathering → soluble phosphate → plants → animals → decomposers → soil

Ecosystem Services

• Provisioning: food, water, wood, medicines from natural ecosystems
• Regulating: carbon sequestration, water purification, flood control, disease regulation
• Cultural: recreation, spiritual values, ecotourism; supporting: nutrient cycling, soil formation, primary production

CBSE Board Focus

• Ecosystem: 5–7 marks; food chain energy loss, carbon and nitrogen cycle with diagram
• Define: GPP, NPP, biomass, standing crop; calculate energy at trophic level using 10% law
• Nitrogen cycle: name 4 processes (fixation, nitrification, ammonification, denitrification) with organisms