12.1 1. Ecosystem — What It Is, Its Types & Its Structure
An ecosystem is a functional unit of nature where living organisms interact among themselves AND with the surrounding physical environment.
Scale: An ecosystem varies greatly in size — from a small pond to a large forest or a sea. Many ecologists regard the entire biosphere as a global ecosystem, a composite of all local ecosystems on Earth. Because that is too big and complex to study at once, it is divided into two basic categories.
| Category | NCERT examples |
|---|
| Terrestrial | Forest, Grassland, Desert |
| Aquatic | Pond, Lake, Wetland, River, Estuary |
| Man-made | Crop fields, Aquarium |
Structural features — the two S's
The interaction of biotic and abiotic components produces a physical structure that is characteristic for each type of ecosystem. Its two structural features are:
| Feature | Definition | Example |
|---|
| Species composition | Identification and enumeration of the plant and animal species of an ecosystem | Listing every species present in a forest |
| Stratification | VERTICAL distribution of different species occupying different levels | Trees = top strata; Shrubs = second layer; Herbs and grasses = bottom layers |
Components of an ecosystem
| Type | What it includes |
|---|
| ABIOTIC | Inorganic materials — air, water and soil |
| BIOTIC | Producers, Consumers and Decomposers |
The FOUR functional aspects (the whole chapter, in one list)
1. Productivity→
2. Decomposition→
3. Energy Flow→
4. Nutrient Cycling
Frame it as INPUT → TRANSFER → OUTPUT:
Input = productivity | Transfer = food chain/web + nutrient cycling | Output = degradation and energy loss.
What the 2023-24 rationalisation DELETED from this chapter (do not waste time on these unless your coaching insists):
• Ecological Succession (hydrarch, xerarch, pioneer species, climax community)
• The detailed Carbon cycle and Phosphorus cycle diagrams
• Ecosystem Services with Robert Constanza's valuation figures
What SURVIVES only in the Summary + Exercises: the definition of nutrient cycling, gaseous vs sedimentary cycles, and a one-line mention of ecosystem services. Section 8 below covers exactly that much — no more.
12.2 2. The Pond — A Complete Model Ecosystem
NCERT uses a small pond as the model because it is a fairly self-sustainable unit and a rather simple example that still explains even the complex interactions of an aquatic ecosystem. A pond is a shallow water body in which all four basic functional components are well exhibited.
| Component | In the pond |
|---|
| ABIOTIC |
The water with all the dissolved inorganic and organic substances, and the rich soil deposit at the bottom of the pond |
| Regulating factors |
The solar input, the cycle of temperature, day-length and other climatic conditions — these regulate the rate of function of the entire pond |
| AUTOTROPHS |
Phytoplankton, some algae, and the floating, submerged and marginal plants found at the edges |
| CONSUMERS |
Zooplankton, the free-swimming forms and the bottom-dwelling forms |
| DECOMPOSERS |
Fungi, bacteria and flagellates — especially abundant at the bottom of the pond |
The four functions the pond performs (same as the whole biosphere)
Autotrophs convert INORGANIC → ORGANIC using the sun's radiant energy→
Heterotrophs CONSUME the autotrophs→
Dead matter is DECOMPOSED and MINERALISED→
Nutrients RELEASED back for reuse by autotrophs→
Repeat, over and over again
The one-sentence truth of the whole chapter: There is UNIDIRECTIONAL movement of ENERGY towards the higher trophic levels, and its dissipation and loss as HEAT to the environment.
Compare: NUTRIENTS CYCLE (they are reused again and again). ENERGY DOES NOT CYCLE (it flows one way and is lost as heat).
12.3 3. Productivity — GPP, NPP & Secondary Productivity
A constant input of solar energy is the basic requirement for any ecosystem to function and sustain.
Production vs Productivity — the distinction NCERT tests
| Term | Definition | Units |
|---|
| Primary PRODUCTION |
The AMOUNT of biomass or organic matter produced per unit area over a time period by plants during photosynthesis |
Weight: g m−2
Energy: kcal m−2 |
| PRODUCTIVITY |
The RATE of biomass production |
g m−2 yr−1
or kcal m−2 yr−1
(used to COMPARE different ecosystems) |
The three productivities
| Type | Definition |
|---|
| Gross Primary Productivity (GPP) |
The rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration. |
| Net Primary Productivity (NPP) |
GPP minus respiration losses (R). This is the biomass AVAILABLE for consumption by heterotrophs — i.e., herbivores AND decomposers. |
| Secondary Productivity |
The rate of formation of new organic matter by CONSUMERS. |
THE FORMULA: GPP − R = NPP
Think of it like income: GPP = what you EARN. R = what you SPEND on staying alive. NPP = what you actually KEEP (and what everyone else in the ecosystem gets to eat).
What primary productivity depends on
- The plant species inhabiting the particular area
- Environmental factors
- Availability of nutrients
- Photosynthetic capacity of plants
Because of these, productivity varies in different types of ecosystems.
Global numbers (learn these cold)
| Quantity | Value |
|---|
| Annual NPP of the whole biosphere | ~170 billion tons (dry weight) of organic matter |
| Annual NPP of the oceans | only 55 billion tons |
| Annual NPP of land | The rest — ~115 billion tons |
| Fraction of Earth's surface covered by oceans | ~70 per cent |
The paradox NCERT asks you to explain: Oceans occupy ~70% of the surface yet contribute only ~32% of global NPP. Why so low?
Because LIGHT is the limiting factor in aquatic ecosystems. Sunlight penetrates only the thin upper (photic) layer, so photosynthesis is confined to a shallow film of water. Nutrient scarcity in open ocean surface waters compounds it.
(This is also NCERT Exercise 1(c): "In aquatic ecosystems, the limiting factor for the productivity is LIGHT.")
12.4 4. Decomposition — The Five Steps
The earthworm is called the farmer's 'friend' because it helps in the breakdown of complex organic matter and in the loosening of the soil.
Decomposition: the process by which decomposers break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients.
Detritus — the raw material
Detritus = dead plant remains (leaves, bark, flowers) + dead remains of animals, INCLUDING faecal matter. This is the raw material for decomposition.
The five steps (they all operate SIMULTANEOUSLY)
DETRITUS→
1. Fragmentation→
2. Leaching→
3. Catabolism→
4. Humification→
5. Mineralisation→
INORGANIC NUTRIENTS
| Step | What happens | Who does it |
|---|
| 1. Fragmentation |
Detritus is broken down into smaller particles |
Detritivores — e.g., the earthworm |
| 2. Leaching |
Water-soluble inorganic nutrients go DOWN into the soil horizon and get precipitated as UNAVAILABLE salts |
Water (physical process) |
| 3. Catabolism |
Detritus is degraded into simpler inorganic substances |
Bacterial and fungal ENZYMES |
| 4. Humification |
Leads to the accumulation of HUMUS — a dark-coloured amorphous substance |
Occurs in the soil |
| 5. Mineralisation |
Humus is further degraded and inorganic nutrients are RELEASED |
Some microbes; occurs in the soil |
HUMUS — three properties NCERT wants:
1. Dark-coloured and amorphous
2. Highly resistant to microbial action; decomposes at an extremely slow rate
3. Colloidal in nature → serves as a RESERVOIR OF NUTRIENTS
Trap alert: The SUMMARY of the NCERT chapter says decomposition involves THREE processes (fragmentation, leaching, catabolism), while the MAIN TEXT lists FIVE (adding humification and mineralisation). Both appear in exams. The five-step version is the complete and correct one; humification and mineralisation are the two that occur in the soil.
Rate of decomposition — what controls it
Decomposition is largely an OXYGEN-REQUIRING (aerobic) process. Its rate is controlled by (a) the chemical composition of the detritus and (b) climatic factors.
| Factor | SLOWS decomposition | SPEEDS UP decomposition |
|---|
| Chemical composition of detritus |
Rich in LIGNIN and CHITIN |
Rich in NITROGEN and water-soluble substances like SUGARS |
| Climate |
Low temperature and anaerobiosis → inhibit decomposition → BUILD-UP of organic materials |
Warm and moist environment |
The two most important climatic factors regulating decomposition are TEMPERATURE and SOIL MOISTURE — through their effects on the activities of soil microbes.
12.5 5. Energy Flow — Sun → PAR → Producers
Except for the deep-sea hydro-thermal ecosystem, the SUN is the ONLY source of energy for all ecosystems on Earth. (The hydrothermal exception is the single most-tested one-liner in this section.)
The energy cascade — four numbers you MUST know
Incident solar radiation→
< 50% is PAR→
Plants capture only 2–10% of the PAR→
~1% of available sunlight ends up as NPP→
10% transferred per trophic level
| Number | What it refers to |
|---|
| Less than 50% | of the incident solar radiation is Photosynthetically Active Radiation (PAR) |
| 2–10% | of the PAR is captured by plants — and this small amount sustains the entire living world |
| ~1% | In an ideal pyramid of energy, primary producers convert only 1% of the energy in the sunlight available to them into NPP |
| 10% | The 10 per cent law — only 10% of energy is transferred to each trophic level from the lower trophic level |
THE CLASSIC NEET TRAP:
Q: "What is the percentage of PAR in the incident solar radiation?" → 50% (i.e., less than 50%).
Q: "What percentage of PAR do plants capture?" → 2–10%.
Students swap these two constantly. 50 is about PAR; 2–10 is about capture.
Thermodynamics of an ecosystem
| Law | How the ecosystem obeys it |
|---|
| First Law | Energy is neither created nor destroyed — the sun's energy is converted (fixed by autotrophs into food), not made. Flow is unidirectional: sun → producers → consumers. |
| Second Law | Ecosystems are not exempt. They need a constant supply of energy to synthesise the molecules they require, in order to counteract the universal tendency toward increasing disorderliness (entropy). |
Producers
| Ecosystem | Major producers |
|---|
| Terrestrial | Herbaceous and woody plants |
| Aquatic | Phytoplankton, algae and higher plants |
Key line: "No energy that is trapped into an organism remains in it forever." The energy trapped by the producer is either passed on to a consumer, or the organism dies. Death of an organism is the BEGINNING of the detritus food chain/web.
12.6 6. Food Chains, Food Webs & Trophic Levels
Consumers
All animals depend on plants directly or indirectly for their food — they are consumers, also called heterotrophs.
| Level | Name | Also called | Examples |
|---|
| Feed on producers | Primary consumers | Herbivores | Terrestrial: insects, birds, mammals
Aquatic: molluscs |
| Feed on herbivores | Secondary consumers | Primary carnivores | Frog, small fish |
| Feed on primary carnivores | Tertiary consumers | Secondary carnivores | Snake, hawk |
Grazing Food Chain (GFC) — NCERT's exact example
Grass
(Producer)→
Goat
(Primary Consumer)→
Man
(Secondary Consumer)
Detritus Food Chain (DFC)
- Begins with DEAD ORGANIC MATTER.
- Made up of decomposers — heterotrophic organisms, mainly FUNGI and BACTERIA.
- They meet their energy and nutrient requirements by degrading dead organic matter (detritus).
- Also known as SAPROTROPHS (sapro = to decompose).
- They secrete digestive enzymes that break down dead and waste materials into simple inorganic materials, which are subsequently ABSORBED by them.
GFC vs DFC — HIGH-YIELD comparison
| Feature | Grazing Food Chain (GFC) | Detritus Food Chain (DFC) |
|---|
| Starts with | Living producers (plants) | Dead organic matter (detritus) |
| Energy source | Sun (directly) | Detritus |
| Main organisms | Herbivores → carnivores | Decomposers / saprotrophs (fungi, bacteria) |
| Dominant in AQUATIC ecosystems | YES — GFC is the major conduit for energy flow | Minor |
| Dominant in TERRESTRIAL ecosystems | Minor | YES — a much larger fraction of energy flows through the DFC |
| Trophic levels | Restricted (by the 10% law) | Not similarly restricted |
Memory hook: "Water grazes, Land rots." Aquatic → GFC dominates. Terrestrial → DFC dominates.
Food web
The DFC may be connected with the GFC at some levels: some organisms of the DFC are prey to GFC animals, and in a natural ecosystem some animals — like cockroaches and crows — are omnivores. These natural interconnections of food chains make it a FOOD WEB.
| Food Chain | Food Web |
|---|
| A single, linear sequence of who-eats-whom | Many interconnected food chains |
| An organism occupies one position | An organism may occupy several positions |
| Almost never exists in nature in pure form | What actually exists in nature |
Trophic levels
Based on the source of their nutrition or food, organisms occupy a specific place in the food chain known as their trophic level.
| Trophic Level | Occupant |
|---|
| First (T1) | Producers |
| Second (T2) | Herbivores (primary consumers) |
| Third (T3) | Carnivores (secondary consumers) |
| Fourth (T4) | Secondary carnivores (tertiary consumers) |
CRITICAL CONCEPT: A trophic level represents a FUNCTIONAL LEVEL, NOT a species. A given species may occupy more than one trophic level in the SAME ecosystem at the SAME time.
NCERT's example: A sparrow is a primary consumer when it eats seeds, fruits and peas, and a secondary consumer when it eats insects and worms.
Standing crop
Each trophic level has a certain mass of living material at a particular time — its standing crop.
- Measured as the mass of living organisms (biomass) or the number in a unit area.
- Biomass of a species is expressed in fresh or dry weight.
- Measurement in DRY WEIGHT is MORE ACCURATE — because fresh weight varies with water content, which fluctuates with hydration, humidity and species, and water contributes no organic matter or energy.
The 10 per cent law
Only 10 per cent of the energy is transferred to each trophic level from the lower trophic level.
Consequence: the number of trophic levels in the grazing food chain is RESTRICTED — there simply isn't enough energy left to support a fifth or sixth level. The amount of energy DECREASES at successive trophic levels, and organisms at each level depend on those at the lower level for their energy demands.
Worked example: 10,000 J at producers → 1,000 J at herbivores → 100 J at primary carnivores → 10 J at secondary carnivores.
When any organism dies it is converted to detritus or dead biomass, which serves as an energy source for decomposers.
12.7 7. Ecological Pyramids — Number, Biomass & Energy
An ecological pyramid expresses the food or energy relationship between organisms at different trophic levels, in terms of number, biomass or energy.
- The BASE of each pyramid represents the producers (first trophic level).
- The APEX represents the tertiary or top-level consumer.
The three types
| Pyramid | What each bar shows | Can it be inverted? | NCERT's example |
|---|
| Pyramid of NUMBERS |
Number of individuals at each trophic level |
YES — in a TREE ecosystem |
Grassland: nearly 6 million plants support only 3 top carnivores (upright) |
| Pyramid of BIOMASS |
Standing crop (mass of living material) at each level |
YES — in the SEA |
Inverted in the sea: a small standing crop of phytoplankton supports a large standing crop of zooplankton / the biomass of fishes far exceeds that of phytoplankton |
| Pyramid of ENERGY |
Amount of energy present at each trophic level in a given time or annually per unit area |
NEVER. ALWAYS UPRIGHT. |
Ideal energy pyramid: primary producers convert only 1% of the sunlight available to them into NPP |
WHY the pyramid of energy can NEVER be inverted:
Because when energy flows from a particular trophic level to the next, some energy is ALWAYS lost as HEAT at each step. Energy at a lower trophic level is therefore always more than at a higher level. There is no physical way around it.
The sea-biomass PARADOX, resolved: How can a small phytoplankton biomass feed a large zooplankton/fish biomass?
Because biomass is a STANDING CROP — a snapshot at one instant, not a rate. Phytoplankton have an extremely high turnover rate: they reproduce and are eaten within hours to days. Over a year they PRODUCE an enormous amount of biomass, but at any given moment very little of it is standing around. The long-lived fish accumulate their biomass and hold it. Energy still flows correctly downhill — which is exactly why the energy pyramid stays upright even when the biomass pyramid flips.
The exceptions — memorise these three lines
TREE flips NUMBERS|
SEA flips BIOMASS|
NOTHING flips ENERGY
In most ecosystems all three pyramids are upright: producers are more in number and biomass than herbivores, and herbivores more than carnivores; and energy at a lower trophic level is always more than at a higher level.
Rules for constructing pyramids
- Any calculation of energy content, biomass or numbers must include ALL organisms at that trophic level. No generalisation is true if only a few individuals are counted.
- A given organism may occupy more than one trophic level simultaneously — the trophic level is a functional level, not a species.
LIMITATIONS of ecological pyramids (the three S's)
| Limitation | Why it matters |
|---|
| Same species, two levels | It does not take into account the same species belonging to two or more trophic levels (e.g., the sparrow). |
| Simple chain assumed | It assumes a simple food chain — something that almost never exists in nature. It does not accommodate a food web. |
| Saprophytes skipped | Saprophytes are not given any place in ecological pyramids, even though they play a vital role in the ecosystem. |
12.8 8. Nutrient Cycling & Ecosystem Services (Summary-level only)
Read this first: The rationalised 2023-24 chapter deleted the full sections on nutrient cycling (carbon cycle, phosphorus cycle diagrams) and ecosystem services. What is written below is everything that still survives, in the chapter's SUMMARY and EXERCISES. Learn exactly this much — no more.
Nutrient cycling
Nutrient cycling is the storage and movement of nutrient elements through the various components of the ecosystem. Through this process, nutrients are used repeatedly.
| Type of cycle | Reservoir | NCERT's example element |
|---|
| GASEOUS | Atmosphere or Hydrosphere | Carbon |
| SEDIMENTARY | Earth's crust | Phosphorus |
Mnemonic: "Carbon flies, Phosphorus lies." Carbon is in the air (gaseous); phosphorus lies in the rocks (sedimentary).
Exercise 1(e): "The major reservoir of carbon on earth is ______" → OCEANS. (Roughly 71% of Earth's carbon is dissolved in the oceans, which act as a huge sink regulating atmospheric CO2.)
The energy/nutrient contrast — the single most examined idea in this chapter
| Feature | ENERGY | NUTRIENTS |
|---|
| Path | UNIDIRECTIONAL (flows) | CYCLIC (cycles) |
| Fate | Lost as HEAT at every step | Reused repeatedly |
| Source | Must be constantly resupplied by the sun | Drawn from a reservoir (atmosphere/hydrosphere or Earth's crust) |
| Can it be recycled? | NO | YES |
Ecosystem services
Ecosystem services = the products of ecosystem processes.
NCERT's example: the purification of air and water by forests.
12.9 9. Cheat Sheet — Every Number, Formula, Distinction & Exercise Answer
Formula & number bank
| Item | Value / Formula |
|---|
| Net primary productivity | NPP = GPP − R |
| Units of primary PRODUCTION | g m−2 or kcal m−2 |
| Units of PRODUCTIVITY | g m−2 yr−1 or kcal m−2 yr−1 |
| PAR in incident solar radiation | less than 50% |
| PAR captured by plants | 2–10% |
| Sunlight converted to NPP (ideal energy pyramid) | ~1% |
| Energy transfer between trophic levels | 10% law |
| Annual NPP of the biosphere | ~170 billion tons (dry weight) |
| Annual NPP of the oceans | 55 billion tons |
| Ocean share of Earth's surface | ~70% |
| Grassland pyramid of numbers | ~6 million plants support 3 top carnivores |
The six NCERT "Distinguish between" pairs (Exercise 6)
| Pair | The key line |
|---|
| (a) GFC vs DFC | GFC starts with living producers and dominates AQUATIC ecosystems; DFC starts with dead organic matter and dominates TERRESTRIAL ecosystems. |
| (b) Production vs Decomposition | Production BUILDS complex organic matter from inorganic (photosynthesis, needs sunlight); Decomposition BREAKS DOWN complex organic matter into inorganic (needs oxygen, done by decomposers). |
| (c) Upright vs Inverted pyramid | Upright: base (producers) is the largest, narrowing to the apex. Inverted: base is smaller than the levels above (tree → numbers; sea → biomass). |
| (d) Food chain vs Food web | Food chain = a single linear sequence; food web = many interconnected food chains. Only the web actually exists in nature. |
| (e) Litter vs Detritus | Litter = the surface layer of dead PLANT material (fallen leaves, twigs, bark) lying on the soil. Detritus is broader — all dead organic matter: dead plant remains AND dead animal remains AND faecal matter. Litter is one part of detritus. |
| (f) Primary vs Secondary productivity | Primary = rate of biomass production by PRODUCERS (photosynthesis). Secondary = rate of formation of new organic matter by CONSUMERS. |
Exercise answer key (fill-in-the-blanks & MCQs)
| Question | Answer |
|---|
| 1(a) Plants are called ___ because they fix carbon dioxide | Producers / Autotrophs |
| 1(b) In an ecosystem dominated by trees, the pyramid of numbers is ___ type | INVERTED |
| 1(c) In aquatic ecosystems, the limiting factor for productivity is ___ | LIGHT |
| 1(d) Common detritivores in our ecosystem are ___ | Earthworms |
| 1(e) The major reservoir of carbon on earth is ___ | OCEANS |
| 2. Largest population in a food chain | Producers |
| 3. The second trophic level in a lake | Zooplankton (phytoplankton are the FIRST) |
| 4. Secondary producers are | Herbivores — they are the first consumers, and secondary productivity is the rate of new organic matter formed by consumers |
| 5. Percentage of PAR in incident solar radiation | 50% (option (d) "2–10%" is the trap — that's the fraction of PAR CAPTURED) |
The "always/never" list
- Energy flow is ALWAYS unidirectional.
- The pyramid of energy is ALWAYS upright — it can NEVER be inverted.
- Energy at a lower trophic level is ALWAYS more than at a higher level.
- The sun is the ONLY energy source for all ecosystems — except the deep-sea hydrothermal ecosystem.
- All five decomposition steps operate SIMULTANEOUSLY on the detritus.
- Dry weight is ALWAYS more accurate than fresh weight for measuring biomass.