# Ecosystem (NEET Biology Class 12): Productivity, Decomposition, Energy Flow & Ecological Pyramids

Source: https://tracknation.in/story/ecosystem-neet-biology-class-12-productivity-decomposition-energy-flow-ecological-pyramids (TrackNation — free UPSC notes with PYQs)

> NEET Class 12 Ecosystem, mined from NCERT: ecosystem structure (species composition, stratification, the pond as a worked model), a productivity table with GPP − R = NPP and the biosphere/ocean figures, all five decomposition steps tabulated with what speeds or slows the process, energy flow and PAR percentages, a trophic level table plus a grazing-vs-detritus food chain comparison, standing crop and the 10 per cent law, and an ecological pyramids table showing exactly which pyramids can be inve

## Ecosystem Structure, Productivity & Decomposition

📌 **NEET priority: Important.** *2–4 questions a year. **GPP − R = NPP**, the **10 per cent law**, and — above all — **which ecological pyramid can be inverted and which never can** are the three questions this chapter reliably produces.*

### Structure of an ecosystem

- **Species composition** = identifying and enumerating the plant and animal
species of an ecosystem. **Stratification** = the vertical distribution of species at
different levels — **trees occupy the top strata, shrubs the second, and herbs and grasses the
bottom** of a forest.

- The ecosystem functions as a unit through **four aspects**:
**productivity, decomposition, energy flow and nutrient cycling**.

- **The pond as a model ecosystem:** **abiotic** = water with
dissolved inorganic and organic substances + rich bottom soil; **autotrophs** =
phytoplankton, some algae, floating/submerged/marginal plants; **consumers** =
zooplankton, free-swimming and bottom-dwelling forms; **decomposers** = fungi, bacteria
and flagellates, especially abundant at the bottom.

- **Energy movement is UNIDIRECTIONAL** towards higher trophic levels, with
dissipation and **loss as heat**.

### Productivity — the equation NEET asks

| **Term**

 | **Definition**

 | **Units**

 |
| Primary production

 | Amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis

 | Weight (g m⁻²) or energy (kcal m⁻²)

 |
| Productivity

 | The RATE of biomass production

 | g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹

 |
| Gross Primary Productivity (GPP)

 | Rate of production of organic matter during photosynthesis

 | —

 |
| Net Primary Productivity (NPP)

 | GPP MINUS respiration losses. This is the biomass AVAILABLE to heterotrophs (herbivores and decomposers)

 | GPP − R = NPP

 |
| Secondary productivity

 | Rate of formation of new organic matter by CONSUMERS

 | —

 |

- The **annual net primary productivity of the whole biosphere is approximately
170 billion tons** (dry weight) of organic matter. Of this, the **oceans produce only
55 billion tons** — despite occupying about **70 per cent of the earth's
surface**.

### Decomposition — the five steps

- **Detritus** — dead plant remains (leaves, bark, flowers) and dead animal
remains including faecal matter — is the **raw material** for decomposition.

| **Step**

 | **What happens**

 |
| Fragmentation

 | DETRITIVORES (e.g. EARTHWORM) break detritus into smaller particles

 |
| Leaching

 | Water-soluble inorganic nutrients go DOWN into the soil horizon and get precipitated as UNAVAILABLE salts

 |
| Catabolism

 | Bacterial and fungal ENZYMES degrade detritus into simpler inorganic substances

 |
| Humification

 | Accumulation of a dark coloured amorphous substance called HUMUS — highly resistant to microbial action, decomposes extremely slowly, and being COLLOIDAL serves as a nutrient RESERVOIR

 |
| Mineralisation

 | Humus is further degraded by microbes, RELEASING inorganic nutrients

 |

**All these steps operate SIMULTANEOUSLY on the detritus** — they are not
sequential. Decomposition is **largely an OXYGEN-REQUIRING process**. It is
**SLOWER if detritus is rich in LIGNIN and CHITIN**, and **QUICKER if rich in
NITROGEN and water-soluble sugars**. **Warm and moist** conditions favour it;
**low temperature and anaerobiosis inhibit** it, causing organic matter to build up.

## Energy Flow, Food Chains & Trophic Levels

### Energy flow

- The **sun is the only energy source** for all ecosystems —
**except the deep sea hydro-thermal ecosystem**.

- **Less than 50 per cent** of incident solar radiation is
**photosynthetically active radiation (PAR)**, and plants capture only
**2–10 per cent of the PAR**. That small amount sustains the entire living
world.

- Energy flow is **UNIDIRECTIONAL** — sun → producers → consumers.
Ecosystems obey the **Second Law of thermodynamics**, needing a constant energy supply
to counteract increasing disorder.

### Food chains & trophic levels

| **Trophic level**

 | **Who occupies it**

 | **Also called**

 |
| First

 | Producers — herbaceous and woody plants on land; phytoplankton, algae and higher plants in water

 | Autotrophs

 |
| Second

 | Herbivores — insects, birds and mammals on land; MOLLUSCS in aquatic ecosystems

 | Primary consumers

 |
| Third

 | Carnivores feeding on herbivores

 | Secondary consumers / PRIMARY carnivores

 |
| Fourth

 | Animals depending on primary carnivores

 | Tertiary consumers / SECONDARY carnivores

 |

| **Food chain**

 | **Starts with**

 | **Key facts**

 |
| Grazing Food Chain (GFC)

 | Living producers — e.g. Grass → Goat → Man

 | The MAJOR conduit for energy flow in an AQUATIC ecosystem

 |
| Detritus Food Chain (DFC)

 | DEAD organic matter

 | Made of decomposers (mainly fungi and bacteria), also called SAPROTROPHS. They secrete digestive enzymes that break down dead material into simple inorganic materials, then absorb them. In a TERRESTRIAL ecosystem a MUCH LARGER fraction of energy flows through the DFC than through the GFC

 |

The two chains interconnect — some DFC organisms are prey to GFC animals, and omnivores like
**cockroaches and crows** link them — making a **FOOD WEB**.

- **Standing crop** = the mass of living material at each trophic level at a
particular time, measured as **biomass or number per unit area**. Biomass is expressed
as fresh or dry weight, but **DRY weight measurement is more accurate**.

- **The 10 per cent law:** only **10 per cent of energy is transferred
to each trophic level from the lower one**. This is precisely **why the number of
trophic levels in a grazing food chain is RESTRICTED**.

- **A trophic level is a FUNCTIONAL level, not a species.** One species can
occupy several simultaneously — a **sparrow is a primary consumer when eating seeds and a
secondary consumer when eating insects**.

## Ecological Pyramids & Why This Matters for NEET

### Ecological pyramids — which can be inverted

- The **base** of each pyramid represents **producers (first trophic
level)**; the **apex** represents the **top-level consumer**. Three
types are studied: pyramid of **number**, **biomass** and
**energy**.

| **Pyramid type**

 | **Usually**

 | **Exception — the case NEET asks**

 |
| Pyramid of NUMBER

 | Upright

 | INVERTED for a single big TREE: one tree supports many insects, which support fewer small birds, then fewer large birds — the producer level is narrowest

 |
| Pyramid of BIOMASS

 | Upright

 | INVERTED IN THE SEA — the biomass of FISHES far exceeds that of PHYTOPLANKTON at any one moment

 |
| Pyramid of ENERGY

 | ALWAYS upright

 | NEVER inverted — because energy is always LOST AS HEAT at each transfer step. Each bar shows energy at that trophic level per unit area, in a given time or annually

 |

### Limitations of ecological pyramids

- They do **not account for a species belonging to two or more trophic
levels**; they assume a **simple food chain, which almost never exists in
nature**, and cannot accommodate a **food web**; and
**saprophytes are given no place** despite their vital ecosystem role.

### Why this matters for NEET

- High-value one-liners: **GPP − R = NPP**; biosphere NPP =
**170 billion tons**, oceans only **55 billion tons** despite
**70%** of the surface; **PAR < 50%** of incident radiation, plants
capture **2–10%** of PAR; **10 per cent law**; humus is
**colloidal** and a nutrient reservoir; **DFC dominates on land, GFC in
water**.

- **Trap:** the **pyramid of ENERGY can NEVER be inverted**,
but the **pyramid of biomass CAN be** (in the sea) and the **pyramid of number
CAN be** (a single tree). This "which one is never inverted" question appears repeatedly —
the answer is always **energy**, because of heat loss at every step.

- **Second trap:** the five decomposition steps operate
**SIMULTANEOUSLY, not in sequence**. Also, decomposition is **slower**
with lignin/chitin-rich detritus and **faster** with nitrogen-rich detritus — students
often assume tougher material simply takes proportionally longer rather than being chemically
resistant. And **NPP, not GPP, is what heterotrophs actually get**.

## Test Yourself: MCQs, PYQs & Active Recall

Answer these, then close the article and do an Active Recall. Reveal each answer only after you commit to one.

## Practice Questions

**Q1.** The vertical distribution of different species occupying different levels is called:

- (a) Standing crop

- (b) Productivity

- (c) Stratification

- (d) Species composition

**Answer: (c)** — Stratification is the vertical layering — trees at the top, shrubs second, herbs and grasses at the bottom.

**Q2.** Net Primary Productivity is calculated as:

- (a) GPP + R

- (b) R − GPP

- (c) GPP − R

- (d) GPP × R

**Answer: (c)** — NPP = GPP minus respiration losses, and it represents the biomass available to heterotrophs.

**Q3.** Secondary productivity is defined as:

- (a) Respiration loss

- (b) Biomass of decomposers

- (c) The rate of formation of new organic matter by consumers

- (d) The rate of photosynthesis

**Answer: (c)** — Secondary productivity is the rate of formation of new organic matter by consumers.

**Q4.** The annual net primary productivity of the whole biosphere is approximately:

- (a) 170 billion tons

- (b) 17 billion tons

- (c) 500 billion tons

- (d) 55 billion tons

**Answer: (a)** — The biosphere's annual NPP is about 170 billion tons dry weight, of which the oceans contribute only 55 billion tons.

**Q5.** Despite covering about 70 per cent of the earth's surface, the oceans produce only:

- (a) 170 billion tons

- (b) 115 billion tons

- (c) 10 billion tons

- (d) 55 billion tons

**Answer: (d)** — Ocean productivity is only 55 billion tons of the biosphere's 170 billion tons total.

**Q6.** The raw material for decomposition is called:

- (a) Standing crop

- (b) Detritus

- (c) Humus

- (d) Biomass

**Answer: (b)** — Detritus — dead plant and animal remains including faecal matter — is the raw material for decomposition.

**Q7.** Breaking detritus into smaller particles by detritivores such as earthworms is called:

- (a) Humification

- (b) Leaching

- (c) Catabolism

- (d) Fragmentation

**Answer: (d)** — Fragmentation is the physical breakdown of detritus into smaller particles by detritivores.

**Q8.** Humus is best described as:

- (a) An inorganic salt

- (b) A dark amorphous substance highly resistant to microbial action, colloidal and a nutrient reservoir

- (c) A type of detritivore

- (d) Easily decomposed organic matter

**Answer: (b)** — Humus is dark, amorphous, highly resistant to microbial action, decomposes extremely slowly, and being colloidal serves as a nutrient reservoir.

**Q9.** Decomposition is SLOWER if the detritus is rich in:

- (a) Nitrogen

- (b) Water-soluble sugars

- (c) Water

- (d) Lignin and chitin

**Answer: (d)** — Lignin- and chitin-rich detritus decomposes slowly; nitrogen- and sugar-rich detritus decomposes quickly.

**Q10.** The only ecosystem NOT dependent on the sun for energy is:

- (a) The pond

- (b) The deep sea hydro-thermal ecosystem

- (c) The desert

- (d) The tundra

**Answer: (b)** — The sun is the only energy source for all ecosystems except the deep sea hydro-thermal ecosystem.

**Q11.** Photosynthetically active radiation (PAR) constitutes what fraction of incident solar radiation?

- (a) 100 per cent

- (b) About 80 per cent

- (c) Less than 50 per cent

- (d) About 2 per cent

**Answer: (c)** — Less than 50 per cent of incident solar radiation is PAR, and plants capture only 2-10 per cent of that.

**Q12.** Plants capture approximately what percentage of PAR?

- (a) 20-30 per cent

- (b) 50 per cent

- (c) 2-10 per cent

- (d) 90 per cent

**Answer: (c)** — Plants capture only 2 to 10 per cent of the photosynthetically active radiation.

**Q13.** In a terrestrial ecosystem, a larger fraction of energy flows through the:

- (a) Detritus food chain

- (b) Grazing food chain

- (c) Neither

- (d) Both equally

**Answer: (a)** — In terrestrial ecosystems a much larger fraction of energy flows through the DFC; in aquatic ecosystems the GFC is the major conduit.

**Q14.** Decomposers that break down dead organic matter are also known as:

- (a) Detritivores

- (b) Producers

- (c) Saprotrophs

- (d) Autotrophs

**Answer: (c)** — Decomposers are heterotrophs, mainly fungi and bacteria, also known as saprotrophs.

**Q15.** The 10 per cent law explains why:

- (a) Biomass pyramids are inverted

- (b) The number of trophic levels in a grazing food chain is restricted

- (c) Oceans are less productive

- (d) Decomposition is slow

**Answer: (b)** — Since only 10 per cent of energy transfers to each higher trophic level, the number of trophic levels is restricted.

**Q16.** Standing crop is most accurately measured as:

- (a) Volume

- (b) Fresh weight

- (c) Number of individuals only

- (d) Dry weight

**Answer: (d)** — Standing crop is the biomass or number per unit area; measurement of biomass in terms of dry weight is more accurate.

**Q17.** The pyramid of biomass in the sea is generally:

- (a) Absent

- (b) Inverted, because fish biomass exceeds phytoplankton biomass

- (c) Rectangular

- (d) Upright

**Answer: (b)** — The marine pyramid of biomass is inverted because the biomass of fishes far exceeds that of phytoplankton at any moment.

**Q18.** Which ecological pyramid can NEVER be inverted?

- (a) All can be inverted

- (b) Pyramid of energy

- (c) Pyramid of number

- (d) Pyramid of biomass

**Answer: (b)** — The pyramid of energy is always upright because some energy is always lost as heat at each transfer step.

**Q19.** A pyramid of number would be inverted in the case of:

- (a) A pond

- (b) A single big tree supporting many insects

- (c) A grassland

- (d) An ocean

**Answer: (b)** — One tree supports many insects, fewer small birds and even fewer large birds — so the producer level is narrowest.

**Q20.** Which of the following is a limitation of ecological pyramids?

- (a) They only apply to aquatic systems

- (b) They cannot show producers

- (c) They are too complex

- (d) They assume a simple food chain and cannot accommodate a food web; saprophytes get no place

**Answer: (d)** — Pyramids ignore species at multiple trophic levels, assume a simple food chain rather than a web, and give saprophytes no place.

**Q21.** The inverted pyramid of biomass is shown in

- (a) (i)

- (b) (ii)

- (c) None of these

- (d) (iii)

**Answer: (b)** — In an open sea the producers (phytoplankton) have a small standing crop but very high turnover, so the biomass of consumers can exceed that of producers: the pyramid of biomass is inverted. (i) is the usual upright pyramid and (iii) is a spindle-shaped pyramid of numbers.

**Q22.** The organisms marked P in pyramid (ii) are

- (a) zooplankton

- (b) phytoplankton

- (c) small fish

- (d) large fish

**Answer: (b)** — P is the producer level — phytoplankton — which has a small biomass at any moment because it is eaten almost as fast as it multiplies.

**Q23.** Pyramid (iii) is spindle-shaped because, in a single large tree ecosystem,

- (a) energy increases at higher trophic levels

- (b) decomposers are absent

- (c) producers outnumber all consumers

- (d) one tree supports a large number of insects, which are fed on by fewer birds

**Answer: (d)** — A single large tree is one producer individual supporting many herbivorous insects; the numbers then fall again at higher levels, so the pyramid of numbers is spindle-shaped. A pyramid of energy, in contrast, is always upright.

### NEET Previous Year Questions (PYQs)

*Real NEET previous-year questions on this chapter, with explanations in our own words.*

**Q24.** Gross Primary Productivity minus respiration losses gives: (NEET PYQ)

- (a) Net Primary Productivity

- (b) Biomass

- (c) Standing crop

- (d) Secondary productivity

**Answer: (a)** — GPP − R = NPP, which is the biomass available for consumption by heterotrophs.

**Q25.** In the decomposition process, the release of inorganic nutrients from humus is called: (NEET PYQ)

- (a) Leaching

- (b) Catabolism

- (c) Fragmentation

- (d) Mineralisation

**Answer: (d)** — Mineralisation is the microbial degradation of humus releasing inorganic nutrients.

**Q26.** Which process causes water-soluble nutrients to go down into the soil horizon and precipitate as unavailable salts? (NEET PYQ)

- (a) Fragmentation

- (b) Leaching

- (c) Humification

- (d) Mineralisation

**Answer: (b)** — Leaching moves water-soluble inorganic nutrients down the soil horizon where they precipitate as unavailable salts.

**Q27.** Which factor INHIBITS decomposition? (NEET PYQ)

- (a) Warm and moist conditions

- (b) Nitrogen-rich detritus

- (c) Low temperature and anaerobiosis

- (d) Abundant earthworms

**Answer: (c)** — Low temperature and anaerobiosis inhibit decomposition, resulting in build-up of organic materials.

**Q28.** The grazing food chain is the major conduit for energy flow in: (NEET PYQ)

- (a) Forest ecosystems

- (b) Aquatic ecosystems

- (c) Desert ecosystems

- (d) Terrestrial ecosystems

**Answer: (b)** — In an aquatic ecosystem the GFC is the major conduit; in terrestrial ecosystems the DFC carries more energy.

**Q29.** A sparrow eating seeds and then insects occupies: (NEET PYQ)

- (a) More than one trophic level simultaneously

- (b) Only the third trophic level

- (c) No trophic level

- (d) Only the second trophic level

**Answer: (a)** — A trophic level is a functional level, not a species — a sparrow is a primary consumer eating seeds and a secondary consumer eating insects.

**Q30.** Detritivores such as earthworms perform which step of decomposition? (NEET PYQ)

- (a) Fragmentation

- (b) Humification

- (c) Mineralisation

- (d) Catabolism

**Answer: (a)** — Detritivores break down detritus into smaller particles — the process called fragmentation.

**Q31.** The steps of decomposition operate: (NEET PYQ)

- (a) Only underwater

- (b) Simultaneously on the detritus

- (c) Strictly in sequence

- (d) Only in warm climates

**Answer: (b)** — All the steps in decomposition operate simultaneously on the detritus, not one after another.

**Q32.** In an aquatic ecosystem, molluscs are typically: (NEET PYQ)

- (a) Primary consumers (herbivores)

- (b) Producers

- (c) Secondary carnivores

- (d) Decomposers

**Answer: (a)** — Common herbivores are insects, birds and mammals on land, and molluscs in aquatic ecosystems.

**Q33.** Each bar in a pyramid of energy indicates: (NEET PYQ)

- (a) The decomposition rate

- (b) The number of individuals

- (c) The amount of energy present at each trophic level per unit area in a given time

- (d) The dry biomass only

**Answer: (c)** — Each bar in the energy pyramid indicates the amount of energy at each trophic level in a given time or annually per unit area.

### Active Recall Prompt

Write everything you can recall about the **Ecosystem** chapter, naming each part first: **structure** (species composition, stratification, the four functional aspects, the pond as a model with each component); **productivity** (primary production, GPP, NPP with the equation, secondary productivity, and the biosphere/ocean figures); **decomposition** (detritus, all five steps, what humus is, and what speeds up or slows down the process); **energy flow** (PAR percentages, why it is unidirectional); **food chains and trophic levels** (GFC vs DFC, which dominates where, standing crop, the 10 per cent law); and **ecological pyramids** (all three types, exactly which can be inverted and why energy never can, plus the limitations). Begin each fact with its topic and end it with a full stop.