Class 12 — Ecosystem

Chapter Overview & Weightage

Ecosystem is part of the Ecology unit in CBSE Class 12 Biology. It typically carries 5-7 marks in board exams and contributes 3-4 questions in NEET. The chapter is content-heavy but formula-light — pure memory work with some diagram understanding.

Key Concepts You Must Know

Prioritized by exam frequency:

Productivity — gross primary productivity (GPP), net primary productivity (NPP), secondary productivity. NPP = GPP - respiration.
Decomposition — fragmentation, leaching, catabolism, humification, mineralisation. The 5-step process is a recurring 5-mark question.
Energy flow — 10% law (only 10% of energy transfers to next trophic level).
Ecological pyramids — pyramid of number, biomass, energy. Energy pyramid is always upright.
Food chains and food webs — grazing food chain (GFC) starts with green plants; detritus food chain (DFC) starts with dead organic matter.
Nutrient cycles — carbon cycle and phosphorus cycle (only two in NCERT).
Ecosystem services — pollination, climate regulation, water cycling, etc. Robert Constanza’s $33 trillion estimate.

Important Numbers and Concepts

NPP = GPP - R (where R = plant respiration)

Available to consumers = NPP

Annual NPP (whole biosphere) = 170 billion tonnes (dry weight) of organic matter

Only 10% of the energy at one trophic level transfers to the next. The other 90% is lost as heat (respiration), undigested matter, and movement.

Solved Previous Year Questions

PYQ 1 (CBSE Class 12, 2023)

Define decomposition. List the steps of decomposition in correct sequence.

Solution: Decomposition is the breakdown of complex organic matter (detritus) into inorganic substances like CO₂, water, and nutrients. Steps in order:

Fragmentation — detritivores break detritus into smaller pieces.
Leaching — water-soluble nutrients are washed into soil.
Catabolism — bacteria and fungi enzymatically degrade detritus into simpler compounds.
Humification — accumulation of dark, amorphous humus.
Mineralisation — humus is further degraded, releasing inorganic nutrients.

PYQ 2 (CBSE Class 12, 2022)

Why is the pyramid of energy always upright? Explain with example.

Solution: At each successive trophic level, only 10% of the energy is passed on (Lindemann’s rule). So energy strictly decreases from producers to top consumers. Example: in a grassland ecosystem, grass (1000 kJ) → grasshoppers (100 kJ) → frogs (10 kJ) → snakes (1 kJ). Pyramid points upward, never inverted.

PYQ 3 (CBSE Class 12, 2024)

What is GPP? How does it differ from NPP?

Solution: GPP (Gross Primary Productivity) is the rate at which organic matter is produced by photosynthesis per unit area per unit time. NPP (Net Primary Productivity) is GPP minus the respiration losses by plants. NPP represents the biomass available for consumption by herbivores.

Difficulty Distribution

Easy (1-2 marks): Definitions, productivity values, decomposition steps — ~40%
Medium (3 marks): Comparisons (GPP vs NPP, pyramids), nutrient cycles — ~40%
Hard (5 marks): Diagrammatic representation of cycles, energy flow with calculations — ~20%

Expert Strategy

Strategy 1: Memorise NCERT numbers exactly. “Plants capture only 2-10% of PAR (photosynthetically active radiation).” “10% energy transfer.” “Annual NPP = 170 billion tonnes.” These specific numbers earn full marks.

Strategy 2: Practice the carbon and phosphorus cycle diagrams. CBSE may ask for either as a 5-mark question. Draw with arrows and labels.

Strategy 3: Inverted pyramid examples. Pyramid of biomass for ocean ecosystem is INVERTED (small phytoplankton biomass supports larger zooplankton biomass — fast turnover). Energy pyramid never inverts.

NEET asks 2-3 ecosystem questions every year. Many are about decomposition steps, productivity numbers, or pyramid types. NEET 2023 included a tricky inverted-biomass-pyramid question — ocean is the standard example.

Common Traps

Trap 1: Confusing GPP and NPP. GPP is total photosynthesis; NPP is what is left after plants use some for respiration. NPP is what feeds herbivores.

Trap 2: Saying decomposition is fast in deserts. Wrong — it’s actually slow due to low moisture. Tropical forests have rapid decomposition because of warmth and humidity.

Trap 3: Inverted energy pyramid. Energy pyramid is ALWAYS upright due to the 10% law. Only number and biomass pyramids can invert.

Trap 4: Forgetting the order of decomposition steps. Mnemonic: “Fragments Leak, Catabolism Humifies, Minerals Released” → Fragmentation, Leaching, Catabolism, Humification, Mineralisation.

This chapter is among the easiest for NEET Biology if NCERT is read carefully. Memorise the exact numbers and step orders for full marks.