Ecosystem

BiologyEcosystemFor NEET aspirants

An ecosystem is a functional unit of nature in which living organisms interact among themselves and with their physical environment. This page covers ecosystem structure and function, primary and secondary productivity, the five steps of decomposition, energy flow through food chains and trophic levels, the 10 per cent law, and the pyramids of number, biomass and energy. It follows the Class 12 NCERT chapter Ecosystem. NEET often asks about GPP and NPP, the order of the decomposition steps, PAR values and why the pyramid of energy is never inverted.

On this page1Ecosystem2Structure and function3Productivity4Decomposition5Energy flow6Ecological pyramids7Nutrient cycling8Pairs to match9Quick revision10Solved examples11Practice

Key Points at a Glance

★ Must learn Ecosystem: a functional unit of nature where organisms interact with each other and with the physical environment.
Structure: species composition and stratification. Function: productivity, decomposition, energy flow and nutrient cycling.
★ Must learn $GPP - R = NPP$ ; NPP is the biomass available to herbivores and decomposers.
Annual NPP of the biosphere is about 170 billion tons (dry weight); oceans give only 55 billion tons despite covering about 70% of the surface.
★ Must learn Decomposition: fragmentation, leaching, catabolism, humification, mineralisation.
Lignin and chitin, cold and anaerobiosis slow decomposition; nitrogen, sugars, warmth and moisture speed it up.
★ Must learn PAR is less than 50% of incident solar radiation; plants capture only 2-10% of PAR.
Energy flow is unidirectional: sun → producers → consumers, with heat lost at every step.
★ Must learn 10 per cent law: only 10% of the energy passes from one trophic level to the next.
The grazing food chain dominates in water; in terrestrial ecosystems more energy flows through the detritus food chain.
★ Must learn The pyramid of energy is always upright; the pyramid of biomass in the sea is generally inverted.
Pyramids ignore species at two or more levels, food webs and saprophytes.

1. What is an Ecosystem?

1.1 Definition and size

★ Very important An ecosystem is a functional unit of nature in which living organisms interact among themselves and with their surrounding physical environment.

★ Exam imp Its size varies greatly: 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.
The biosphere is too big and complex to study at one time. So it is divided into two basic categories: terrestrial and aquatic.

1.2 Types of ecosystems

Type	Examples
Terrestrial	Forest, grassland, desert
Aquatic	Pond, lake, wetland, river, estuary
Man-made	Crop fields, aquarium

Memory Trick

Aquatic examples: Please Leave Wet River Edges: pond, lake, wetland, river, estuary.

1.3 How an ecosystem is studied

First, the structure of the ecosystem is examined.
Then its input (productivity), transfer of energy (food chain or web, nutrient cycling) and output (degradation and energy loss).
These energy flows create relationships, namely cycles, chains and webs, which are linked with one another.

Key idea

An ecosystem may be as small as a pond or as large as the biosphere; it is studied through its structure, its input, the transfer of energy and its output.

2. Ecosystem: Structure and Function

2.1 Structure

The environment has abiotic (non-living) and biotic (living) components. Here they are studied together, as one integrated system.
This integrated view shows how energy flows within the components of the ecosystem.
★ Exam imp The interaction of biotic and abiotic components produces a physical structure that is characteristic of each type of ecosystem.
Species composition: the identification and enumeration (naming and counting) of the plant and animal species of an ecosystem.
Stratification: the vertical distribution of different species occupying different levels.

Layer of a forest	Occupied by
Top vertical stratum (layer)	Trees
Second layer	Shrubs
Bottom layers	Herbs and grasses

Species composition and stratification are the two main structural features of an ecosystem.

Memory Trick

Forest layers from the top: Trees Stand Highest: trees, then shrubs, then herbs and grasses.

2.2 Function: four aspects

The components of an ecosystem work as one unit through four aspects:
(i) Productivity; (ii) Decomposition; (iii) Energy flow; (iv) Nutrient cycling.

Memory Trick

Please Don't Eat Nuts: productivity, decomposition, energy flow, nutrient cycling.

2.3 A pond as an example

A pond is a shallow water body. It is a fairly self-sustainable unit.
It is a simple example that still explains the complex interactions of an aquatic ecosystem.
All four functional aspects of an ecosystem are clearly seen in a pond.

Pond component	What it includes
Abiotic component	Water with dissolved inorganic and organic substances, and the rich soil deposit at the bottom
Regulating factors	Solar input, cycle of temperature, day-length and other climatic conditions; they regulate the rate of function of the entire pond
Autotrophs (producers)	Phytoplankton, some algae, and the floating, submerged and marginal plants at the edges
Consumers	Zooplankton, and the free-swimming and bottom-dwelling forms
Decomposers	Fungi, bacteria and flagellates, especially abundant at the bottom of the pond

2.4 What a pond does

A pond performs all the functions of any ecosystem, and of the biosphere as a whole:

Autotrophs convert inorganic material into organic material, using the radiant energy of the sun.
Heterotrophs consume the autotrophs.
Dead matter is decomposed and mineralised, releasing materials for reuse by the autotrophs.
These events are repeated over and over again.

★ Exam imp Energy moves unidirectionally towards higher trophic levels. It is dissipated and lost as heat to the environment.

Key idea

Structure means species composition and stratification; function means productivity, decomposition, energy flow and nutrient cycling, all of which a pond shows clearly.

3. Productivity

3.1 Primary production and productivity

A constant input of solar energy is the basic requirement for any ecosystem to function and sustain itself.

Primary production: the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. Productivity is the rate of biomass production.

Primary production (an amount)

Weight: $g m^{- 2}$

Energy: $kcal m^{- 2}$

Productivity (a rate)

$g m^{- 2} yr^{- 1}$ or $(kcal m^{- 2}) yr^{- 1}$

Used to compare the productivity of different ecosystems

3.2 GPP, NPP and secondary productivity

Primary productivity is divided into gross primary productivity (GPP) and net primary productivity (NPP).
★ Exam imp GPP: the rate of production of organic matter during photosynthesis. Put another way, the rate of capture of solar energy, or the total production of organic matter.
A considerable amount of GPP is used by plants in respiration (R).

★ Very important Gross primary productivity minus respiration losses is the net primary productivity:

GPP - R = NPP

NPP is the biomass available for consumption by heterotrophs: herbivores and decomposers.
NPP is the biomass, or energy, left after the producers have used what they need.
★ Exam imp Secondary productivity: the rate of formation of new organic matter by consumers. It is also described as the rate of assimilation of food energy by consumers.

Term	Meaning	Made by
Primary productivity	Rate of capture of solar energy, or of biomass production	Producers
GPP	Rate of production of organic matter during photosynthesis	Producers
NPP	GPP minus respiration losses; available to herbivores and decomposers	Producers
Secondary productivity	Rate of formation of new organic matter (assimilation of food energy)	Consumers

Tips and Tricks

Gross is the full total; net is what remains after the plant pays for its own respiration. If a statement says NPP is greater than GPP, it is wrong. Secondary productivity always belongs to consumers, never to plants.

3.3 What primary productivity depends on

The plant species inhabiting a particular area.
A variety of environmental factors.
The availability of nutrients.
The photosynthetic capacity of plants.
So primary productivity varies in different types of ecosystems.

3.4 Productivity of the biosphere

★ Exam imp The annual NPP of the whole biosphere is approximately 170 billion tons (dry weight) of organic matter.
Oceans occupy about 70% of the Earth's surface. Yet their productivity is only 55 billion tons.
The rest, about 115 billion tons ( $170 - 55$ ), is produced on land.

Memory Trick

Seventy per cent of the surface, but only about one-third of the NPP: the oceans make 55 of the 170 billion tons.

Extra Depth: Why is ocean productivity low? Sunlight reaches only the upper layers of water, so light limits productivity in aquatic ecosystems. Open-ocean surface water is also poor in nutrients such as nitrogen and phosphorus.

Key idea

Productivity is a rate; NPP equals GPP minus respiration and feeds herbivores and decomposers, while consumers carry out secondary productivity.

4. Decomposition

4.1 What decomposition is

The earthworm is called the farmer's friend. It helps break down complex organic matter and loosens the soil.

Decomposition: the breakdown of complex organic matter by decomposers into inorganic substances such as

C O_{2}

, water and nutrients.

Detritus: dead plant remains (leaves, bark, flowers) and dead remains of animals, including fecal matter.
Detritus is the raw material for decomposition.
★ Exam imp The important steps of decomposition are fragmentation, leaching, catabolism, humification and mineralisation.

Memory Trick

Farmers Like Clean, Healthy Meadows: fragmentation, leaching, catabolism, humification, mineralisation.

4.2 Steps of decomposition

Fragmentation: detritivores (for example, the earthworm) break detritus down 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: leads to the accumulation of humus, a dark-coloured amorphous substance.
Mineralisation: some microbes further degrade humus, and inorganic nutrients are released.

★ Exam imp Fragmentation, leaching and catabolism all operate simultaneously on the detritus.
These three are the core processes of decomposition. Humification and mineralisation occur during decomposition in the soil.

Humus	Property
Appearance	Dark-coloured and amorphous (without a definite form)
Microbial action	Highly resistant; decomposes at an extremely slow rate
Nature	Colloidal
Role	Serves as a reservoir of nutrients

Test yourself: hide labels

Figure 1: Decomposition cycle in a terrestrial ecosystem. Fallen leaves are eaten, broken down by decomposers and leached. Most of this matter ends up as organic rich soil that supports the tree again, while some enters the food web.

4.3 Rate of decomposition

Decomposition is largely an oxygen-requiring process.
Its rate is controlled by the chemical composition of detritus and by climatic factors.

Slower decomposition

Detritus rich in lignin and chitin
Low temperature
Anaerobiosis (lack of oxygen)
Result: organic materials build up

Faster decomposition

Detritus rich in nitrogen
Detritus rich in water-soluble substances such as sugars
Warm and moist environment

★ Exam imp Temperature and soil moisture are the most important climatic factors. They act through their effects on the activities of soil microbes.

Memory Trick

Lignin and chitin linger; nitrogen and sugars are nimble. Cold and airless soil also makes detritus linger.

Tips and Tricks

Detritivore or decomposer? A detritivore such as the earthworm fragments detritus. Decomposers such as bacteria and fungi carry out catabolism with their enzymes. Leaching does not make nutrients available; it precipitates them as unavailable salts. Mineralisation is the step that releases inorganic nutrients.

Key idea

Detritus is fragmented, leached and catabolised at the same time; humus forms and is slowly mineralised, fastest in warm, moist and well-aerated soil.

5. Energy Flow

5.1 The source of energy

★ Exam imp Except for the deep sea hydro-thermal ecosystem, the sun is the only source of energy for all ecosystems on Earth.
Of the incident solar radiation, less than 50% is photosynthetically active radiation (PAR).
Autotrophs (plants and photosynthetic bacteria) fix the sun's radiant energy to make food from simple inorganic materials.
★ Exam imp Plants capture only 2-10% of the PAR. This small amount of energy sustains the entire living world.
All organisms depend on producers for food, directly or indirectly. So energy flows unidirectionally: from the sun to producers, and then to consumers.
Ecosystems are not exempt from the second law of thermodynamics. They need a constant supply of energy to synthesise the molecules they require.
This energy counteracts the universal tendency towards increasing disorderliness.

5.2 Producers and consumers

Producers: the green plants of an ecosystem.

Ecosystem	Major producers
Terrestrial	Herbaceous and woody plants
Aquatic	Phytoplankton, algae and higher plants

Food chains, or rather webs, start from the producers. An animal feeds on a plant or on another animal, and is in turn food for another.
This interdependency forms the chain or web.
★ Exam imp No energy trapped in an organism stays in it forever. It is either passed on to a consumer, or the organism dies.
The death of an organism is the beginning of the detritus food chain/web.
Consumers, also called heterotrophs: all animals, because they depend on plants directly or indirectly for food.

Consumer	Feeds on	Also called
Primary consumer	Producers (plants or their produce)	Herbivore
Secondary consumer	Animals that eat plants (herbivores)	Primary carnivore
Tertiary consumer	Primary carnivores	Secondary carnivore

Common herbivores: insects, birds and mammals in terrestrial ecosystems, and molluscs in aquatic ecosystems.

Memory Trick

Consumer number = carnivore number + 1. A primary carnivore is a secondary consumer, and a secondary carnivore is a tertiary consumer. Count trophic levels from the producer, which is level one.

5.3 Grazing and detritus food chains

A simple grazing food chain (GFC): grass (producer) → goat (primary consumer) → man (secondary consumer).
The detritus food chain (DFC) begins with dead organic matter. It is made up of decomposers: heterotrophic organisms, mainly fungi and bacteria.
Decomposers meet their energy and nutrient needs by degrading dead organic matter (detritus).
★ Exam imp They are also known as saprotrophs (sapro: to decompose).
Decomposers secrete digestive enzymes that break down dead and waste materials into simple inorganic materials. They then absorb these materials.
★ Exam imp In an aquatic ecosystem, the GFC is the major conduit for energy flow. In a terrestrial ecosystem, a much larger fraction of energy flows through the DFC.

Feature	Grazing food chain (GFC)	Detritus food chain (DFC)
Begins with	Living producers (green plants)	Dead organic matter (detritus)
Made up of	Producers, herbivores and carnivores	Decomposers (saprotrophs), mainly fungi and bacteria
Major energy route in	Aquatic ecosystems	Terrestrial ecosystems
Example	Grass → goat → man	Detritus → decomposers (fungi and bacteria)

Memory Trick

Water grazes, land decays: the grazing chain dominates in water; on land more energy flows through the detritus chain.

5.4 Food web

The DFC may be connected with the GFC at some levels. Some organisms of the DFC are prey to GFC animals.
Some animals, such as cockroaches and crows, are omnivores in a natural ecosystem.
This natural interconnection of food chains makes a food web.

Food chain

A single linear sequence of who eats whom.

Example: grass → goat → man.

Food web

A network of interconnected food chains.

Links the GFC and DFC; omnivores feed at several levels.

5.5 Trophic levels

Trophic level: the specific place an organism occupies in a food chain, based on the source of its nutrition or food.

Organisms occupy a place in the community according to their feeding relationship with other organisms.
★ Exam imp Producers belong to the first trophic level, herbivores (primary consumers) to the second, and carnivores (secondary consumers) to the third.

Test yourself: hide labels

Figure 2: Trophic levels in an ecosystem. Each level obtains its food from the level below it, from producers at the base to top carnivores.

★ Exam imp The amount of energy decreases at successive trophic levels.
When any organism dies, it becomes detritus (dead biomass), which is an energy source for decomposers.
Organisms at each trophic level depend on those at the lower trophic level for their energy.

5.6 Standing crop

★ Exam imp Standing crop: the mass of living material at each trophic level at a particular time.
It is measured as the mass of living organisms (biomass) or as their number in a unit area.
The biomass of a species is expressed as fresh weight or dry weight. Dry weight is the more accurate measure.

5.7 The 10 per cent law

★ Very important 10 per cent law: only 10% of the energy is transferred to each trophic level from the lower trophic level.

This law restricts the number of trophic levels in a grazing food chain.
Levels possible in a GFC: producer, herbivore, primary carnivore and secondary carnivore.

Test yourself: hide labels

Figure 3: Energy flow through different trophic levels. Part of the energy is lost as heat at every transfer, including the transfers to detritivores and decomposers, so less energy reaches each higher level.

NEET Focus

Four numbers that statement questions swap:

Less than 50%: the share of incident solar radiation that is PAR.
2-10%: the share of PAR that plants capture.
1%: the share of the sunlight energy available to producers that becomes NPP in the ideal energy pyramid.
10%: the share of energy passed from one trophic level to the next.

1% and 2-10% do not conflict: the first is a share of total sunlight, the second a share of PAR.

Exceptions

The deep sea hydro-thermal ecosystem is the only exception to the sun being the source of energy.
In terrestrial ecosystems the DFC, not the GFC, carries most of the energy.
Omnivores such as cockroaches and crows do not stay in one food chain; they link chains into a web.

Quick Recall: tap to check

Is the one-way flow of energy in keeping with the first law of thermodynamics?

Yes. Energy is neither created nor destroyed. Solar energy becomes the chemical energy of food and is finally lost as heat, so the total is conserved.

How would you classify human beings in a food chain?

As omnivores. They eat both plant and animal food, so they belong to a food web rather than a single chain.

Why is dry weight a more accurate measure of biomass than fresh weight?

Water content varies widely between organisms and over time. Dry weight removes this variation and measures only the organic matter.

Is there a similar limit on the number of levels in a detritus food chain?

Not in the same way. Detritivores and decomposers receive dead matter from every trophic level (Figure 3), not only from the level just below them.

Key idea

Energy enters through producers that capture 2-10% of PAR, flows one way through trophic levels with only 10% passing on each time, and is lost as heat at every step.

6. Ecological Pyramids

6.1 What a pyramid shows

A pyramid has a broad base and narrows towards the apex. Food or energy relationships between organisms at different trophic levels give a similar shape.
The relationship is expressed in terms of number, biomass or energy.
★ Exam imp The base represents the producers (first trophic level). The apex represents the tertiary or top-level consumer.
The three pyramids usually studied: (a) pyramid of number, (b) pyramid of biomass and (c) pyramid of energy.

6.2 Pyramid of numbers

In a grassland: 5,842,000 producers, 708,000 primary consumers, 354,000 secondary consumers and only 3 tertiary consumers.

Test yourself: hide labels

Figure 4(a): Pyramid of numbers in a grassland ecosystem. Nearly 6 million plants support only 3 top carnivores.

Memory Trick

Halve, then crash: 708,000 primary consumers halve to 354,000 secondary consumers, which then support only 3 top carnivores.

6.3 Pyramid of biomass

A pyramid of biomass is usually upright: biomass shows a sharp decrease at higher trophic levels.

Test yourself: hide labels

Figure 4(b): Pyramid of biomass. Biomass falls sharply at higher trophic levels, from 809 to 37, 11 and 1.5

g m^{- 2}

(dry weight).

★ Exam imp The pyramid of biomass in the sea is generally inverted, because the biomass of fishes far exceeds that of phytoplankton.

Test yourself: hide labels

Figure 4(c): Inverted pyramid of biomass. A small standing crop of phytoplankton (4) supports a large standing crop of zooplankton (21).

6.4 Pyramid of energy

★ Very important The pyramid of energy is always upright; it can never be inverted. Some energy is always lost as heat when energy flows from one trophic level to the next.

Each bar of an energy pyramid shows the energy present at a trophic level in a given time, or annually per unit area.

Test yourself: hide labels

Figure 4(d): An ideal pyramid of energy. Producers convert only 1% of the 1,000,000 J of sunlight available to them into NPP (10,000 J), and each higher level receives one-tenth of the level below.

Memory Trick

Drop two zeros, then one zero per level: 1,000,000 J of sunlight gives 10,000 J in producers (1%), then 1000 J, 100 J and 10 J (10% each).

6.5 Counting correctly: trophic level is a function

Any calculation of energy content, biomass or numbers must include all organisms at that trophic level.
No generalisation will hold if only a few individuals at a trophic level are counted.
★ Exam imp A trophic level is a functional level, not a species as such. A given species may occupy more than one trophic level in the same ecosystem at the same time.
★ Exam imp Example: a sparrow is a primary consumer when it eats seeds, fruits and peas. It is a secondary consumer when it eats insects and worms.

6.6 Upright and inverted pyramids

In most ecosystems, the pyramids of number, energy and biomass are all upright.
Producers are more in number and biomass than herbivores. Herbivores are more in number and biomass than carnivores.
Energy at a lower trophic level is always more than at a higher level.

Feature	Upright pyramid	Inverted pyramid
Shape	Broad base, narrow apex	Narrow base, broader level above it
Meaning	Each lower level has more number, biomass or energy than the level above	A higher level has more number or biomass than the level below
Examples	Most ecosystems; grassland pyramid of numbers; every pyramid of energy	Numbers: insects feeding on one big tree. Biomass: the sea

Exceptions

Pyramid of numbers on one big tree: a single tree supports many insects, so the base is narrower than the level above and the pyramid is inverted. The small and large birds above the insects make the levels taper again.
Pyramid of biomass in the sea is generally inverted.
The pyramid of energy has no exception: it is never inverted.

NEET Focus

Only numbers and biomass can be inverted, never energy. A common trap reads "the pyramid of biomass in a grassland is inverted"; this is false. Inverted biomass belongs to the sea, where a small standing crop of phytoplankton supports a larger standing crop of zooplankton and fishes.

6.7 Limitations of ecological pyramids

They do not take into account the same species belonging to two or more trophic levels.
They assume a simple food chain, which almost never exists in nature. They do not accommodate a food web.
Saprophytes are given no place, even though they play a vital role in the ecosystem.

Memory Trick

Three S gaps: same species at two levels, simple chain only, saprophytes left out.

Quick Recall: tap to check

How many trophic levels can human beings function at?

More than one. Eating plant food, humans are primary consumers (second level). Eating a goat, they are secondary consumers (third level), and they also appear as top carnivores (fourth level).

What pyramid of numbers do you get by counting the insects feeding on one big tree?

An inverted pyramid, because one tree (the base) supports a far larger number of insects.

The biomass of fishes in the sea far exceeds that of phytoplankton. Why is this not a paradox?

A pyramid of biomass shows the standing crop at one moment. Phytoplankton multiply quickly and are eaten quickly, so a small standing crop can still support a larger consumer biomass.

In an ecological pyramid, what do the base and the apex represent?

The base represents producers (first trophic level); the apex represents tertiary or top-level consumers.

Key idea

Pyramids of number and biomass are usually upright but can be inverted; the pyramid of energy is always upright because heat is lost at every transfer.

7. Nutrient Cycling and Ecosystem Services

Nutrient cycling: the storage and movement of nutrient elements through the various components of the ecosystem.
Through nutrient cycling, nutrients are used repeatedly.
Nutrient cycles are of two types: gaseous and sedimentary.

Type of cycle	Reservoir	Example
Gaseous	Atmosphere or hydrosphere	Carbon
Sedimentary	Earth's crust	Phosphorus

Ecosystem services: the products of ecosystem processes.
Example: the purification of air and water by forests.

Memory Trick

Carbon floats, phosphorus sinks: carbon is stored in air and water; phosphorus is stored in the Earth's crust.

Key idea

Nutrients cycle through gaseous (carbon) or sedimentary (phosphorus) routes, and forests that purify air and water show what ecosystem services are.

8. Pairs to Match, Numbers and Examples

Pairs to Match

List I	List II
Earthworm	Detritivore; fragmentation; farmer's friend
Bacterial and fungal enzymes	Catabolism of detritus
Leaching	Nutrients precipitated as unavailable salts
Humus	Dark, amorphous, colloidal reservoir of nutrients
Lignin and chitin in detritus	Slow decomposition
Deep sea hydro-thermal ecosystem	Energy source other than the sun
Grass → goat → man	Grazing food chain
Fungi and bacteria (saprotrophs)	Detritus food chain
Cockroaches, crows	Omnivores that link food chains into a web
Sparrow	Primary and secondary consumer at the same time
Grassland	5,842,000 producers support 3 top carnivores
Sea	Inverted pyramid of biomass
Carbon	Gaseous cycle; atmosphere or hydrosphere as reservoir
Phosphorus	Sedimentary cycle; Earth's crust as reservoir
Forests	Purify air and water (ecosystem service)

Numbers to Remember

Annual NPP of the biosphere: about 170 billion tons (dry weight) of organic matter.
Oceans: about 70% of the Earth's surface but only 55 billion tons of NPP; land produces the rest (about 115 billion tons).
PAR: less than 50% of incident solar radiation.
Plants capture 2-10% of PAR.
10 per cent law: 10% of energy passes to the next trophic level.
Ideal energy pyramid: 1,000,000 J of sunlight → 10,000 J (1%, as NPP) → 1000 J → 100 J → 10 J.
Grassland pyramid of numbers: 5,842,000 → 708,000 → 354,000 → 3.
Pyramid of biomass (dry weight, $g m^{- 2}$ ): 809 → 37 → 11 → 1.5.
Inverted pyramid of biomass: phytoplankton 4, zooplankton 21.
Units: production in $g m^{- 2}$ or $kcal m^{- 2}$ ; productivity in $g m^{- 2} yr^{- 1}$ or $(kcal m^{- 2}) yr^{- 1}$ .

Examples to Remember

Group	Examples
Terrestrial ecosystems	Forest, grassland, desert
Aquatic ecosystems	Pond, lake, wetland, river, estuary
Man-made ecosystems	Crop fields, aquarium
Pond producers	Phytoplankton, some algae, floating, submerged and marginal plants
Pond consumers	Zooplankton, free-swimming and bottom-dwelling forms
Pond decomposers	Fungi, bacteria, flagellates
Producers on land / in water	Herbaceous and woody plants / phytoplankton, algae, higher plants
Herbivores	Insects, birds, mammals (land); molluscs (water)
Detritus	Leaves, bark, flowers, dead remains of animals, fecal matter
Detritivore	Earthworm
Decomposers (saprotrophs)	Fungi, bacteria
Omnivores	Cockroaches, crows
First trophic level	Phytoplankton, grass, trees
Second trophic level	Zooplankton, grasshopper, cow
Third trophic level	Birds, fishes, wolf
Fourth trophic level	Man, lion

Quick Recall: name it

Name the trophic level to which zooplankton belong.

Second trophic level (herbivores, primary consumers).

Name the trophic level of the wolf in Figure 2.

Third trophic level (carnivore, secondary consumer).

In Figure 1, which organisms carry out further decomposition of litter?

Earthworms, bacteria, soil mites and fungi.

In Figure 4(d), how much energy reaches the tertiary consumers?

10 J, out of 1,000,000 J of sunlight.

9. Quick Revision

Ecosystem: a structural and functional unit of nature with abiotic and biotic components.
Abiotic components: inorganic materials such as air, water and soil. Biotic components: producers, consumers and decomposers.
Structural features: species composition and stratification (trees, shrubs, herbs and grasses).
Functional aspects: productivity, decomposition, energy flow and nutrient cycling.
$GPP - R = NPP$ ; secondary productivity is carried out by consumers.
Biosphere NPP is about 170 billion tons a year; oceans give only 55 billion tons.
Decomposition: fragmentation, leaching, catabolism, humification and mineralisation.
Humus is dark, amorphous, colloidal and highly resistant to microbes; it is a nutrient reservoir.
Decomposition is largely aerobic (oxygen-requiring); lignin, chitin, cold and anaerobiosis slow it.
PAR is less than 50% of sunlight; plants capture 2-10% of PAR.
Energy flow is unidirectional; only 10% passes to the next trophic level.
GFC dominates in aquatic ecosystems; DFC carries more energy in terrestrial ecosystems.
A trophic level is a functional level; a sparrow can be a primary and a secondary consumer.
Numbers and biomass pyramids may be inverted (tree, sea); the energy pyramid never is.
Pyramids ignore multi-level species, food webs and saprophytes.

10. Solved Examples

Solved Example 1

Match List I with List II.
List I: A. Fragmentation, B. Leaching, C. Catabolism, D. Mineralisation
List II: I. Bacterial and fungal enzymes degrade detritus into simpler inorganic substances, II. Detritivores break detritus into smaller particles, III. Humus is degraded by microbes and inorganic nutrients are released, IV. Water-soluble nutrients enter the soil horizon and precipitate as unavailable salts
Choose the correct answer:
(A) A-II, B-IV, C-I, D-III
(B) A-II, B-I, C-IV, D-III
(C) A-IV, B-II, C-I, D-III
(D) A-II, B-IV, C-III, D-I

Solution:

Answer: (A). Detritivores fragment detritus (II); leaching moves soluble nutrients down and precipitates them (IV); enzymes of bacteria and fungi catabolise detritus (I); mineralisation of humus releases inorganic nutrients (III).

Solved Example 2

Read the statements about productivity.
A. Productivity is the rate of biomass production.
B. NPP is equal to GPP plus respiration losses.
C. NPP is available for consumption by herbivores and decomposers.
D. Secondary productivity is the rate of formation of new organic matter by producers.
E. The annual NPP of the biosphere is about 170 billion tons (dry weight).
Choose the correct answer:
(A) A, B and C only
(B) A, C and E only
(C) C, D and E only
(D) A, D and E only

Solution:

Answer: (B). B is wrong: $NPP = GPP - R$ . D is wrong: secondary productivity belongs to consumers. A, C and E are correct.

Solved Example 3

Arrange the steps of decomposition in the order in which they are usually listed, ending with the release of nutrients.
A. Humification
B. Fragmentation
C. Mineralisation
D. Catabolism
E. Leaching
Choose the correct answer:
(A) B, D, E, C, A
(B) E, B, D, C, A
(C) B, E, D, C, A
(D) B, E, D, A, C

Solution:

Answer: (D). Fragmentation, leaching and catabolism act on the detritus together and are listed in this order. Humification forms humus, and mineralisation of humus comes last. Every other option puts mineralisation before humification, which is impossible because humus must form first.

Solved Example 4

Producers in a grazing food chain fix 20,000 J of energy. Following the 10 per cent law, how much energy reaches the secondary carnivores?
(A) 2000 J
(B) 200 J
(C) 20 J
(D) 2 J

Solution:

Answer: (C). Herbivores get $20, 000 \times 0.1 = 2000$ J, primary carnivores $200$ J and secondary carnivores $20$ J. Secondary carnivores are tertiary consumers, the fourth trophic level.

Solved Example 5

Which one of the following statements about ecological pyramids is NOT correct?
(A) The base represents the producers.
(B) The pyramid of energy is inverted in some aquatic ecosystems.
(C) The pyramid of biomass in the sea is generally inverted.
(D) Saprophytes are given no place in ecological pyramids.

Solution:

Answer: (B). The pyramid of energy is always upright, because some energy is lost as heat at every transfer. The other three statements are correct.

Solved Example 6

Statement I: In a terrestrial ecosystem, a much larger fraction of energy flows through the detritus food chain than through the grazing food chain.
Statement II: The decomposers of the detritus food chain are autotrophic organisms, mainly fungi and bacteria.
(A) Both Statement I and Statement II are correct
(B) Both Statement I and Statement II are incorrect
(C) Statement I is correct but Statement II is incorrect
(D) Statement I is incorrect but Statement II is correct

Solution:

Answer: (C). Statement I is correct. Statement II is wrong: decomposers are heterotrophic (saprotrophs).

11. Practice Questions

Practice Questions

Match List I with List II.
List I: A. Phytoplankton, B. Zooplankton, C. Wolf, D. Lion
List II: I. Fourth trophic level, II. First trophic level, III. Third trophic level, IV. Second trophic level
(A) A-II, B-III, C-IV, D-I
(B) A-IV, B-II, C-III, D-I
(C) A-II, B-IV, C-III, D-I
(D) A-II, B-IV, C-I, D-IIIAnswer: (C). Phytoplankton are producers, zooplankton herbivores, the wolf a carnivore and the lion a top carnivore.
Read the statements.
A. Decomposition is largely an oxygen-requiring process.
B. Detritus rich in lignin and chitin decomposes quickly.
C. Temperature and soil moisture are the most important climatic factors for decomposition.
D. Anaerobiosis favours decomposition.
E. Humus is highly resistant to microbial action.
(A) A, B and C only
(B) A, C and E only
(C) B, D and E only
(D) C, D and E onlyAnswer: (B). Lignin and chitin slow decomposition, and anaerobiosis inhibits it.
Arrange the layers of a forest from the top downwards.
A. Herbs and grasses
B. Trees
C. Shrubs
(A) C, B, A
(B) B, A, C
(C) A, C, B
(D) B, C, AAnswer: (D). Trees occupy the top stratum, shrubs the second, and herbs and grasses the bottom layers.
Which one of the following is NOT a function of a pond ecosystem?
(A) Conversion of inorganic into organic material by autotrophs
(B) Consumption of autotrophs by heterotrophs
(C) Return of energy from consumers back to producers
(D) Mineralisation of dead matter for reuse by autotrophsAnswer: (C). Energy flow is unidirectional; it is lost as heat and never returns to producers.
Statement I: The pyramid of energy can never be inverted.
Statement II: When energy flows from one trophic level to the next, some energy is always lost as heat.
(A) Both Statement I and Statement II are correct
(B) Both Statement I and Statement II are incorrect
(C) Statement I is correct but Statement II is incorrect
(D) Statement I is incorrect but Statement II is correctAnswer: (A). Both are correct, and the heat loss in Statement II is the reason for Statement I.
The GPP of a grassland is $1500 g m^{- 2} yr^{- 1}$ and respiration losses are $600 g m^{- 2} yr^{- 1}$ . What is its NPP?Answer: $NPP = GPP - R = 1500 - 600 = 900 g m^{- 2} yr^{- 1}$ .
Plants capture only 2-10 per cent of the
(A) incident solar radiation
(B) photosynthetically active radiation
(C) net primary productivity
(D) gross primary productivityAnswer: (B). The 2-10% figure is a share of PAR, which is itself less than 50% of incident solar radiation.

Practice Questions: Short and Long Answer

Fill in the blanks.
(a) Plants are called ______ because they fix carbon dioxide.
(b) In an ecosystem dominated by trees, the pyramid (of numbers) is ______ type.
(c) In aquatic ecosystems, the limiting factor for productivity is ______.
(d) Common detritivores in our ecosystem are ______.
(e) The major reservoir of carbon on Earth is ______.Answer: (a) producers (autotrophs); (b) inverted; (c) light; (d) earthworms; (e) the oceans (hydrosphere).
Which one of the following has the largest population in a food chain?
(a) Producers
(b) Primary consumers
(c) Secondary consumers
(d) DecomposersAnswer: (d) Decomposers. Bacteria and fungi are microscopic and far outnumber the organisms at every other level. If decomposers are left out, as in ecological pyramids, producers are the most numerous.
The second trophic level in a lake is
(a) Phytoplankton
(b) Zooplankton
(c) Benthos
(d) FishesAnswer: (b) Zooplankton. They are the herbivores (primary consumers) of the lake.
Secondary producers are
(a) Herbivores
(b) Producers
(c) Carnivores
(d) None of the aboveAnswer: (d) None of the above. Only green plants (autotrophs) are producers. Consumers show secondary productivity, but they are not called producers. Some answer keys accept (a) Herbivores for this reason.
What is the percentage of photosynthetically active radiation (PAR) in the incident solar radiation?
(a) 100%
(b) 50%
(c) 1-5%
(d) 2-10%Answer: (b) 50%; more precisely, PAR is less than 50% of the incident solar radiation.
Distinguish between: (a) grazing food chain and detritus food chain; (b) production and decomposition; (c) upright and inverted pyramid; (d) food chain and food web; (e) litter and detritus; (f) primary and secondary productivity.Answer: (a) A GFC starts with living green plants and passes through herbivores; a DFC starts with dead organic matter and is made of decomposers. (b) Production is the making of organic matter by plants in photosynthesis; decomposition is the breakdown of organic matter into $C O_{2}$ , water and nutrients. (c) In an upright pyramid each lower level exceeds the level above; in an inverted pyramid a higher level exceeds the level below. (d) A food chain is one linear sequence; a food web is a network of interconnected chains. (e) Litter is fallen plant matter lying on the ground; detritus is all dead organic matter, including dead animals and fecal matter. (f) Primary productivity is the rate of biomass production by producers; secondary productivity is the rate of formation of new organic matter by consumers.
Describe the components of an ecosystem.Answer: Abiotic components: inorganic materials (air, water, soil) and climatic factors such as solar input, temperature and day-length. Biotic components: producers, consumers and decomposers. In a pond these are phytoplankton and plants, zooplankton and other animals, and fungi, bacteria and flagellates.
Define ecological pyramids and describe, with examples, pyramids of number and biomass.Answer: An ecological pyramid shows the number, biomass or energy at successive trophic levels, with producers at the base. Numbers: a grassland has 5,842,000 producers but only 3 top carnivores (upright); one tree with many insects gives an inverted pyramid. Biomass: 809, 37, 11 and 1.5 units from producers upwards (upright); in the sea, phytoplankton 4 support zooplankton 21 (inverted).
What is primary productivity? Give a brief description of the factors that affect it.Answer: It is the rate of biomass production by plants during photosynthesis, in units such as $g m^{- 2} yr^{- 1}$ . It depends on the plant species of the area, environmental factors, the availability of nutrients and the photosynthetic capacity of the plants.
Define decomposition and describe the processes and products of decomposition.Answer: Decomposition is the breakdown of complex organic matter into inorganic substances by decomposers. Its steps are fragmentation, leaching, catabolism, humification and mineralisation. The products are $C O_{2}$ , water, inorganic nutrients and humus.
Give an account of energy flow in an ecosystem.Answer: The sun supplies the energy; less than 50% of it is PAR, and plants capture 2-10% of PAR. Energy flows one way from producers to herbivores and carnivores. Only 10% passes to each next level; the rest is used in respiration and lost as heat, or passes to decomposers as dead matter (detritus food chain).

Common Mistakes to Avoid

Watch out

Writing $NPP = GPP + R$ . Correct: $NPP = GPP - R$ , because respiration uses up part of the GPP.
Saying plants capture 2-10% of the incident solar radiation. Correct: 2-10% of the PAR.
Calling a primary carnivore a primary consumer. Correct: a primary carnivore is a secondary consumer.
Believing the grazing food chain carries most energy on land. Correct: in terrestrial ecosystems more energy flows through the detritus food chain.
Thinking leaching makes nutrients available to plants. Correct: leached nutrients are precipitated as unavailable salts.
Describing humus as quickly decomposed. Correct: humus is highly resistant to microbes and decomposes at an extremely slow rate.
Stating that the pyramid of energy can be inverted. Correct: it is always upright.
Placing saprophytes at the base of ecological pyramids. Correct: pyramids give saprophytes no place at all.

Frequently Asked Questions

What is an ecosystem and what are its components?

An ecosystem is a functional unit of nature in which organisms interact with each other and with their physical environment. Its abiotic components are inorganic materials such as air, water and soil. Its biotic components are producers, consumers and decomposers. Species composition and stratification are its two main structural features.

What is the difference between GPP and NPP?

Gross primary productivity (GPP) is the rate of production of organic matter during photosynthesis. Plants use a considerable part of it in respiration. What remains is net primary productivity: NPP equals GPP minus respiration losses. NPP is the biomass available to herbivores and decomposers.

Why is the productivity of oceans low although they cover about 70 per cent of the Earth?

Of the 170 billion tons of annual NPP, oceans produce only about 55 billion tons. Light reaches only the upper layers of water, and open-ocean surface water is poor in nutrients such as nitrogen and phosphorus. Both factors limit the photosynthesis of phytoplankton.

What are the steps of decomposition?

Decomposition has five steps: fragmentation by detritivores such as earthworms, leaching of soluble nutrients as unavailable salts, catabolism by bacterial and fungal enzymes, humification that forms humus, and mineralisation that releases inorganic nutrients from humus. The first three act on detritus at the same time.

What is the 10 per cent law of energy flow?

The 10 per cent law states that only 10 per cent of the energy at one trophic level is transferred to the next higher level. The rest is used or lost as heat. This restricts the number of trophic levels in a grazing food chain, such as producer, herbivore, primary carnivore and secondary carnivore.

Why is the pyramid of energy always upright?

When energy flows from one trophic level to the next, some energy is always lost as heat. So every higher level holds less energy than the level below it, and the pyramid can never be inverted. Pyramids of number and biomass, in contrast, can be inverted.

Why is the pyramid of biomass in the sea inverted?

In the sea, the biomass of consumers such as zooplankton and fishes far exceeds the standing crop of phytoplankton. A pyramid of biomass shows the standing crop at one time. Phytoplankton multiply and are eaten quickly, so a small standing crop supports a larger consumer biomass.

Which ecosystem topics are most important for NEET?

NEET questions stay close to the NCERT text. Learn GPP minus R equals NPP, the five steps of decomposition in their listed order, the factors that slow decomposition, PAR and the 2-10 per cent figure, the 10 per cent law, the examples at each trophic level, and which pyramids can be inverted.

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