Home/ Biology/ Chapter 12 — Ecosystem
Class 12 Biology

Ecosystem

Structure and function, productivity (GPP, NPP), decomposition, energy flow, food chains, the 10 per cent law and ecological pyramids — with SVG diagrams, an energy-flow calculator, MCQs, A-R and PYQs.

5NCERT Sections
12MCQs
6Assertion–Reason
5PYQs
Section 01

Ecosystem — Structure and Function

12.0What is an Ecosystem?

Ecosystem

A functional unit of nature where living organisms interact among themselves and with the surrounding physical environment. It ranges from a small pond to a large forest or sea; the entire biosphere is a global ecosystem.

  • Terrestrial: forest, grassland, desert. Aquatic: pond, lake, wetland, river, estuary. Man-made: crop fields, aquarium.

12.1Structure and the Four Functions

  • Species composition — identification and enumeration of plant and animal species. Stratification — vertical distribution of species at different levels (trees → shrubs → herbs and grasses in a forest).
  • Components: abiotic (air, water, soil — inorganic materials) and biotic (producers, consumers, decomposers).
  • Components function as a unit through: (i) productivity, (ii) decomposition, (iii) energy flow, (iv) nutrient cycling.
V1 · A pond as a self-sustaining ecosystem Solar input Soil deposit at the bottom (abiotic) Abiotic: water + dissolved inorganic and organic substances · climate, day-length regulate function PRODUCERS phytoplankton, algae, floating, submerged & marginal plants CONSUMERS zooplankton, free-swimming and bottom-dwelling forms DECOMPOSERS fungi, bacteria, flagellates (bottom) Heat loss Heat loss minerals reused Unidirectional energy, cycling nutrients
Section 02

Productivity

12.2Primary and Secondary Productivity

  • A constant input of solar energy is required. Primary production = biomass/organic matter produced per unit area over time by plants in photosynthesis (g m⁻² or kcal m⁻²). The rate of biomass production = productivity (g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹).
  • Gross primary productivity (GPP) = rate of production of organic matter during photosynthesis. Part is used in plant respiration (R).
GPP − R = NPP
  • Net primary productivity (NPP) = biomass available to heterotrophs (herbivores and decomposers).
  • Secondary productivity = rate of formation of new organic matter by consumers.
  • Depends on plant species, environmental factors, nutrient availability and photosynthetic capacity — so it varies among ecosystems.
  • Annual NPP of the whole biosphere ≈ 170 billion tons (dry weight). Though oceans cover ~70% of the surface, their productivity is only 55 billion tons (the rest is on land). In aquatic ecosystems light is a major limiting factor for productivity.
V2 · From GPP to NPP and secondary productivity Gross primary productivity (GPP) — total photosynthetic production R (respiration) NPP = GPP − R Food for heterotrophs: herbivores + decomposers Secondary productivity = new organic matter formed by consumers
Section 03

Decomposition

12.3Steps of Decomposition

Earthworms are the farmer's friend — they break down complex organic matter and loosen soil. Decomposers break complex organic matter into inorganic substances (CO₂, water, nutrients). Detritus = dead plant remains (leaves, bark, flowers) and dead animals including faecal matter — the raw material.

Step What happens
Fragmentation Detritivores (e.g. earthworm) break detritus into smaller particles.
Leaching Water-soluble inorganic nutrients percolate into the soil and get precipitated as unavailable salts.
Catabolism Bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
Humification Accumulation of dark amorphous humus — highly resistant to microbial action, decomposes extremely slowly; colloidal, a reservoir of nutrients.
Mineralisation Some microbes degrade humus and release inorganic nutrients.
V3 · Decomposition of detritus Detritus dead remains, faeces Fragmentation detritivores (earthworm) Leaching soluble nutrients wash into soil Catabolism bacterial & fungal enzymes Humification humus formed (slow) Mineralisation inorganic nutrients released Fragmentation, leaching and catabolism operate simultaneously · humification and mineralisation occur in soil
Rate of decomposition is… Because
Slower Detritus rich in lignin and chitin; low temperature; anaerobiosis (leads to build-up of organic material)
Quicker Detritus rich in nitrogen and water-soluble substances (sugars); warm and moist conditions

Decomposition is largely an oxygen-requiring process. Temperature and soil moisture are the most important climatic factors (they affect soil microbes).

Section 04

Energy Flow

12.4Energy Flow, Food Chains and Trophic Levels

  • Except for deep-sea hydrothermal ecosystems, the Sun is the only energy source. Less than 50% of incident solar radiation is photosynthetically active radiation (PAR); plants capture only 2–10% of PAR.
  • Unidirectional flow of energy: sun → producers → consumers. Ecosystems obey the second law of thermodynamics — they need a constant supply of energy to counter increasing disorder; energy is dissipated as heat.
  • Producers — green plants (terrestrial: herbs and woody plants; aquatic: phytoplankton, algae, higher plants). Consumers (heterotrophs): primary consumers = herbivores; secondary consumers = carnivores that eat herbivores; tertiary consumers beyond.
  • Grazing food chain (GFC): Grass → Goat → Man (producer → primary consumer → secondary consumer). Detritus food chain (DFC): starts with dead organic matter; decomposers (fungi and bacteria — saprotrophs) secrete digestive enzymes and absorb the simple products.
  • Aquatic ecosystem: GFC is the major conduit of energy. Terrestrial ecosystem: a much larger fraction flows through the DFC.
  • Chains are interconnected into a food web — some DFC organisms are prey to GFC animals; omnivores (cockroaches, crows) feed at several levels.
V4 · Grazing and detritus food chains (a food web) Grazing food chain (GFC) Sun Grass Producer · 1st trophic level Goat Primary consumer · 2nd Man Secondary consumer · 3rd ↑ heat loss at every level Detritus food chain (DFC) DEAD ORGANIC MATTER (detritus) — death at every trophic level Decomposers (fungi, bacteria) saprotrophs · degrade detritus Detritivore → predator DFC links to GFC

12.4.1Trophic Levels, Standing Crop and the 10% Law

  • Trophic level = a functional place in the food chain by source of nutrition — producers (1st), herbivores (2nd), carnivores (3rd). It is a functional level, not a species: e.g. a sparrow is a primary consumer when it eats seeds and a secondary consumer when it eats insects.
  • Standing crop = mass of living organisms (biomass) or number per unit area at each level; dry weight is a more accurate measure of biomass.
  • 10 per cent law: only 10% of the energy is transferred from one trophic level to the next. Hence the number of trophic levels in a GFC is restricted (generally 4–5).
Try it · energy flow and the 10% law

Enter incident solar energy and the percentage of PAR that plants capture (2–10%).

Section 05

Ecological Pyramids

12.5Pyramids of Number, Biomass and Energy

  • Base = producers (first trophic level); apex = top consumer. Three types: pyramid of number, of biomass and of energy.
  • In most ecosystems all three are upright. Exceptions: the pyramid of numbers for insects on a big tree is inverted/spindle-shaped; the pyramid of biomass in the sea is generally inverted — biomass of fishes far exceeds that of phytoplankton (phytoplankton have a short life and high turnover).
  • Pyramid of energy is always upright and can never be inverted — some energy is always lost as heat at each step; each bar shows the energy at each level per unit area per year.
V5 · Types of ecological pyramids (a) Number — grassland upright (b) Number — large tree insects ≫ trees: inverted (c) Biomass — sea inverted (d) Energy always upright 10000 J 1000 J 100 J 10 J illustrative values (J m⁻² yr⁻¹) Producers Primary consumers Secondary Tertiary / top
Limitations of pyramids: they do not consider the same species at two or more trophic levels; assume a simple food chain (never exists in nature) and do not fit a food web; and saprophytes have no place although they play a vital role.
Summary note

Nutrient Cycling and Ecosystem Services

12.6From the Chapter Summary

  • Nutrient cycling = storage and movement of nutrient elements through the components of the ecosystem; nutrients are used repeatedly.
  • Gaseous cycle: reservoir = atmosphere or hydrosphere (e.g. carbon). Sedimentary cycle: reservoir = Earth's crust (e.g. phosphorus).
  • Products of ecosystem processes = ecosystem services, e.g. purification of air and water by forests.
Practice

📝 Solved Examples with Step Marking

📋 2 Marks

Q1. The GPP of an ecosystem is 1000 kcal m⁻² yr⁻¹ and plants lose 400 kcal m⁻² yr⁻¹ in respiration. Find NPP.

Formula 1 Mark

NPP = GPP − R.

Calculation 1 Mark

NPP = 1000 − 400 = 600 kcal m⁻² yr⁻¹ — available to herbivores and decomposers.

📋 3 Marks

Q2. If producers fix 20,000 J of energy, how much reaches the tertiary consumer in a grazing food chain (10% law)?

Primary consumer 1 Mark

10% of 20,000 = 2000 J.

Secondary consumer 1 Mark

10% of 2000 = 200 J.

Tertiary consumer 1 Mark

10% of 200 = 20 J.

📋 3 Marks

Q3. Distinguish between grazing food chain and detritus food chain.

Starting point 1 Mark

GFC begins with producers (living plants); DFC begins with dead organic matter (detritus).

Organisms 1 Mark

GFC: herbivores, carnivores. DFC: decomposers (fungi, bacteria — saprotrophs) and detritivores.

Importance 1 Mark

GFC is the major energy conduit in aquatic ecosystems; in terrestrial ecosystems most energy flows through DFC.

Test Yourself

✍️ MCQ Practice

Score: 0 / 12
New Pattern

🎯 Assertion–Reason

(A) Both true, R explains A. (B) Both true, R does NOT explain A. (C) A true, R false. (D) A false, R true.

Score: 0 / 6
Exam Focus

📖 Important Board Questions

NCERT

Q. Describe the components of an ecosystem.

Answer 3 Marks

Abiotic: air, water, soil, inorganic materials, solar input. Biotic: producers (autotrophs), consumers (herbivores, carnivores, omnivores) and decomposers (fungi, bacteria). They function through productivity, decomposition, energy flow and nutrient cycling.

NCERT

Q. What is primary productivity? Describe factors affecting it.

Answer 3 Marks

Primary productivity is the rate of biomass production by plants in photosynthesis (GPP; NPP = GPP − R). It depends on plant species, environmental factors (light, temperature, water), availability of nutrients and the photosynthetic capacity of plants.

NCERT

Q. Define decomposition and describe its processes and products.

Answer 5 Marks

Decomposition converts complex organic matter (detritus) into CO₂, water and inorganic nutrients. Processes: fragmentation, leaching, catabolism, humification, mineralisation. Products: humus, CO₂, water and mineral nutrients. Faster in warm, moist, N-rich detritus; slower with lignin/chitin, low temperature or anaerobiosis.

NCERT

Q. Define ecological pyramids and describe pyramids of number and biomass with examples.

Answer 5 Marks

Ecological pyramids show food/energy relationships among trophic levels (base = producers). Number and biomass pyramids are usually upright (grassland); the number pyramid for insects on a big tree is inverted; the biomass pyramid in the sea is inverted (fish biomass exceeds phytoplankton). Energy pyramids are always upright.

NCERT

Q. Give an account of energy flow in an ecosystem.

Answer 5 Marks

Sun is the only source; <50% is PAR; plants capture 2–10% of PAR. Energy flows unidirectionally producers → consumers → decomposers through GFC and DFC (food web). Only 10% passes to the next trophic level (10% law); the rest is lost as heat, so trophic levels are limited and energy pyramids are always upright.