This MCQ module is based on: Energy Flow Ecological Pyramids
TOPIC 58 OF 64
Energy Flow Ecological Pyramids
🎓 Class 12
Biology
CBSE
Theory
Ch 12 – Ecosystem
⏱ ~14 min
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This assessment will be based on: Energy Flow Ecological Pyramids
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Energy Flow and Ecological Pyramids
Except for the deep-sea hydro-thermal ecosystem, the sun is the only source of energy for all ecosystems on Earth. This part follows that energy from the sunlight falling on a leaf to the last carnivore that can be supported — and explains why the chain has to stop where it does.
How little of the sunlight is used
Of the incident solar radiation, less than 50 per cent is photosynthetically active radiation (PAR). Plants and photosynthetic bacteria — the autotrophs — fix the Sun's radiant energy to make food from simple inorganic materials, but they capture only 2–10 per cent of the PAR.
And this small amount of energy sustains the entire living world.
Do the arithmetic once and remember it. Less than half the arriving sunlight is even usable, and plants capture only a few per cent of that usable fraction. Every animal, fungus and bacterium on Earth, and every human civilisation that has ever existed, has lived on that remainder. It is the most important small number in ecology.
Energy flow and the laws of thermodynamics
All organisms are dependent for their food on producers, either directly or indirectly. So you find a unidirectional flow of energy from the sun to producers and then to consumers.
Is this in keeping with the first law of thermodynamics? Yes — energy is neither created nor destroyed in an ecosystem. What arrives as sunlight is either stored in chemical bonds, used to do work, or dissipated as heat; none of it vanishes and none is made.
Further, ecosystems are not exempt from the Second Law of thermodynamics. They need a constant supply of energy to synthesise the molecules they require, so as to counteract the universal tendency towards increasing disorderliness.
Producers, consumers and trophic levels
The green plants in an ecosystem are called producers. In a terrestrial ecosystem the major producers are herbaceous and woody plants; in an aquatic ecosystem they are species such as phytoplankton, algae and higher plants.
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 — and the death of an organism is the beginning of the detritus food chain or web.
All animals depend on plants, directly or indirectly, for their food needs. They are hence called consumers, and also heterotrophs.
| Trophic level | Occupied by | Also called | Examples |
|---|---|---|---|
| First | Producers | Autotrophs | Herbaceous and woody plants; phytoplankton, algae |
| Second | Primary consumers | Herbivores | Insects, birds and mammals in terrestrial ecosystems; molluscs in aquatic ecosystems |
| Third | Secondary consumers | Primary carnivores | Animals that eat the herbivores |
| Fourth | Tertiary consumers | Secondary carnivores | Animals that depend on the primary carnivores for food |
Organisms occupy a place in the natural surroundings or in a community according to their feeding relationship with other organisms. Based on the source of their nutrition or food, organisms occupy a specific place in the food chain that is known as their trophic level. Producers belong to the first trophic level, herbivores or primary consumers to the second, and carnivores or secondary consumers to the third.
A trophic level is a functional level, not a species. A given species may occupy more than one trophic level in the same ecosystem at the same time. A sparrow is a primary consumer when it eats seeds, fruits and peas, and a secondary consumer when it eats insects and worms. Work out for yourself how many trophic levels human beings function at.
Food chains and food webs
The grazing food chain
Starting from the plants, or producers, food chains — or rather webs — are formed such that an animal feeds on a plant or on another animal, and in turn is food for another. The chain or web is formed because of this interdependency. A simple grazing food chain (GFC):
Grass (producer) → Goat (primary consumer) → Man (secondary consumer)
The detritus food chain
The detritus food chain (DFC) begins with dead organic matter. It is made up of decomposers, which are heterotrophic organisms, mainly fungi and bacteria. They meet their energy and nutrient requirements by degrading dead organic matter or detritus. These are also known as saprotrophs (sapro: to decompose).
Decomposers secrete digestive enzymes that break down dead and waste materials into simple inorganic materials, which are subsequently absorbed by them.
Which chain carries more energy depends on where you are. In an aquatic ecosystem the grazing food chain is the major conduit for energy flow. In a terrestrial ecosystem, a much larger fraction of energy flows through the detritus food chain than through the GFC.
The reason is that most terrestrial plant material — wood, bark, roots, fallen leaves — is never eaten alive; it dies and enters the DFC. In water, the producers are mostly small, soft phytoplankton that are grazed almost immediately.
How the two connect
The detritus food chain may be connected with the grazing food chain at some levels: some of the organisms of the DFC are prey to the GFC animals, and in a natural ecosystem some animals such as cockroaches and crows are omnivores. These natural interconnections of food chains make it a food web.
The 10 per cent law
The important point to note is that the amount of energy decreases at successive trophic levels. When any organism dies it is converted to detritus or dead biomass, which serves as an energy source for decomposers. Organisms at each trophic level depend on those at the lower trophic level for their energy demands.
Each trophic level has a certain mass of living material at a particular time, called the standing crop. It is measured as the mass of living organisms (biomass) or the number in a unit area. The biomass of a species is expressed in terms of fresh or dry weight, and measurement in terms of dry weight is more accurate — because the water content of living tissue varies greatly and contributes no energy.
The number of trophic levels in the grazing food chain is restricted, because the transfer of energy follows the 10 per cent law: only 10 per cent of the energy is transferred to each trophic level from the lower trophic level.
📐 Activity: how long can a food chain be?
Suppose the producers in a grassland fix 10,000 kJ m−2 yr−1 as net primary productivity, and energy transfer follows the 10 per cent law.
Predict: how much energy reaches the fourth trophic level, and how much would reach a sixth? What does this tell you about the length of food chains?
Work it down, one tenth at a time.
First trophic level (producers): 10,000 kJ m−2 yr−1
Second (herbivores): 1,000
Third (primary carnivores): 100
Fourth (secondary carnivores): 10
Fifth: 1
Sixth: 0.1 kJ m−2 yr−1
What this means. At the sixth level, an entire square metre of grassland yields a tenth of a kilojoule a year. To support a single predator of any size, the area required becomes impossibly large. This is why the number of trophic levels in the grazing food chain is restricted, and why in nature it is possible to have levels such as producer, herbivore, primary carnivore and secondary carnivore — and rarely more.
The consequence for large predators. A tiger needs an enormous territory not because it is fussy but because it sits near the top of a chain that has already discarded 99.9 per cent of the energy the plants captured. It also explains why top predators are always rare, and always the first to disappear when an ecosystem is reduced in area.
And a question worth asking. Is there the same limitation in a detritus food chain? Not in the same way — detritus is repeatedly recolonised and partially decomposed material is passed among many organisms, so the DFC is less a ladder with a fixed number of rungs than a web of organisms working over the same material.
Ecological pyramids
The base of a pyramid is broad and it narrows towards the apex. One gets a similar shape whether the food or energy relationship between organisms at different trophic levels is expressed in terms of number, biomass or energy. The base of each pyramid represents the producers, or first trophic level, while the apex represents the tertiary or top-level consumer.
The three types of ecological pyramids usually studied are the pyramid of number, the pyramid of biomass and the pyramid of energy.
The general rule, and its exceptions
In most ecosystems all the pyramids — of number, of energy and of biomass — are upright: producers are more in number and biomass than the herbivores, and herbivores are more in number and biomass than the carnivores. Also, energy at a lower trophic level is always more than at a higher level.
But there are exceptions to this generalisation.
An inverted pyramid of numbers. Count the insects feeding on a single big tree, then the small birds depending on the insects, then the larger birds eating the smaller. One tree supports thousands of insects — so the pyramid is narrow at the base and broad above.
An inverted pyramid of biomass. The pyramid of biomass in the sea is generally inverted, because the biomass of fishes far exceeds that of phytoplankton. Isn't that a paradox? The resolution is turnover: phytoplankton live only a few days, so a small standing crop can supply a very large amount of production over a year, which supports a much greater standing crop of longer-lived consumers.
The pyramid of energy is always upright and 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. Each bar in the energy pyramid indicates the amount of energy present at each trophic level in a given time, or annually per unit area.
Rules for drawing pyramids
Any calculation of energy content, biomass or numbers has to include all organisms at that trophic level. No generalisation we make will be true if we take only a few individuals at any trophic level into account. Also, a given organism may occupy more than one trophic level simultaneously.
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, something that almost never exists in nature; they do not accommodate a food web.
- Saprophytes are given no place in ecological pyramids, even though they play a vital role in the ecosystem.
Nutrient cycling and ecosystem services
The storage and movement of nutrient elements through the various components of the ecosystem is called nutrient cycling; nutrients are repeatedly used through this process. Nutrient cycling is of two types:
| Type | Reservoir | Example element |
|---|---|---|
| Gaseous | Atmosphere or hydrosphere | Carbon — the oceans are the major reservoir of carbon on Earth |
| Sedimentary | Earth's crust | Phosphorus |
The products of ecosystem processes are named ecosystem services — for example the purification of air and water by forests.
🎯 Interactive: Read the Pyramid
Six situations. Choose one to see what shape the pyramid takes and why.
🎯 Competency-Based Questions
Q1. Give an account of energy flow in an ecosystem.
The source. 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 per cent is photosynthetically active radiation, and plants capture only 2 to 10 per cent of that PAR. This small amount of energy sustains the entire living world.
Entry into the living system. Plants and photosynthetic bacteria, the autotrophs, fix the Sun's radiant energy to make food from simple inorganic materials. The green plants are called producers — herbaceous and woody plants in terrestrial ecosystems, phytoplankton, algae and higher plants in aquatic ones.
The direction of flow. All organisms depend for their food on producers, either directly or indirectly, so the flow of energy is unidirectional: from the sun to producers and then to consumers. This accords with the first law of thermodynamics, since no energy is created or destroyed. Ecosystems are also not exempt from the second law: they need a constant supply of energy to synthesise the molecules they require, to counteract the universal tendency towards increasing disorderliness.
Through the trophic levels. Based on the source of their nutrition, organisms occupy a trophic level: producers the first, herbivores or primary consumers the second, carnivores or secondary consumers the third, and so on. A simple grazing food chain runs grass → goat → man. Alongside it runs the detritus food chain, which begins with dead organic matter and is made up of decomposers, mainly fungi and bacteria, also called saprotrophs. In aquatic ecosystems the GFC is the major conduit for energy flow, while in terrestrial ecosystems a much larger fraction flows through the DFC. The interconnection of these chains makes a food web.
The loss at each step. The amount of energy decreases at successive trophic levels, because the transfer of energy follows the 10 per cent law — only 10 per cent is transferred to each level from the one below. Energy not passed on is lost as heat to the environment.
The consequence. The number of trophic levels in the grazing food chain is restricted, and the pyramid of energy is always upright and can never be inverted. No energy trapped in an organism remains there forever: it is either passed to a consumer, or the organism dies and its energy begins the detritus food chain.
Q2. Define ecological pyramids and describe, with examples, the pyramids of number and biomass.
Definition. An ecological pyramid is a representation of the food or energy relationship between organisms at different trophic levels, expressed in terms of number, biomass or energy. The base of each pyramid represents the producers, that is the first trophic level, while the apex represents the tertiary or top-level consumer. Like any pyramid, the base is broad and it narrows towards the apex.
The pyramid of number. Each bar represents the number of individuals at that trophic level.
Upright example. In a grassland ecosystem, only three top carnivores are supported in an ecosystem based on the production of nearly 6 million plants — a very broad base tapering to a narrow apex.
Inverted example. Count the insects feeding on one big tree, then the small birds depending on those insects, then the larger birds eating the smaller birds. A single tree supports thousands of insects, so the base is narrower than the level above it and the pyramid is inverted. This shows the weakness of counting individuals: a count ignores size, and one tree may outweigh every animal in the ecosystem.
The pyramid of biomass. Each bar represents the mass of living material — the standing crop — at that trophic level, measured as fresh or dry weight, dry weight being more accurate.
Upright example. In most terrestrial ecosystems the pyramid of biomass shows a sharp decrease in biomass at higher trophic levels, since producers have far more biomass than herbivores and herbivores more than carnivores.
Inverted example. The pyramid of biomass in the sea is generally inverted, because the biomass of fishes far exceeds that of phytoplankton — a small standing crop of phytoplankton supports a large standing crop of zooplankton and fish. The paradox is resolved by turnover: phytoplankton live only a few days, so a small standing crop produces an enormous amount of biomass over a year.
The contrast with the pyramid of energy. Both number and biomass can invert because both are measured at an instant and ignore the rate of production. A pyramid of energy is always upright and can never be inverted, because energy is always lost as heat at each transfer.
Q3. Distinguish between the grazing food chain and the detritus food chain. Which dominates where, and why?
| Feature | Grazing food chain (GFC) | Detritus food chain (DFC) |
|---|---|---|
| Starts with | Living producers — plants | Dead organic matter, that is detritus |
| Made up of | Herbivores and carnivores | Decomposers — heterotrophic organisms, mainly fungi and bacteria, also called saprotrophs |
| Energy source | Solar energy fixed by producers, taken by grazing | The same energy, taken from dead and waste material |
| Mode of feeding | Ingestion of living tissue | Decomposers secrete digestive enzymes that break dead and waste materials into simple inorganic materials, which they then absorb |
| Example | Grass → goat → man | Leaf litter → fungi and bacteria → inorganic nutrients |
Which dominates where. 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 than through the GFC.
Why. Terrestrial producers are mostly large, long-lived plants built largely of wood, bark and roots — material that is tough, rich in lignin and mostly never eaten alive. It dies where it stands and enters the detritus chain. Aquatic producers, by contrast, are mostly microscopic, soft-bodied phytoplankton with lifespans of days, which are grazed almost as fast as they are produced.
How the two are joined. The DFC may be connected with the GFC at some levels: some organisms of the DFC are prey to GFC animals, and some animals such as cockroaches and crows are omnivores. These natural interconnections of food chains make a food web.
Q4. Why is the pyramid of biomass in the sea generally inverted while the pyramid of energy never is? Resolve the apparent paradox.
The observation. The pyramid of biomass in the sea is generally inverted, because the biomass of fishes far exceeds that of phytoplankton: a small standing crop of phytoplankton supports a large standing crop of zooplankton and fish. Taken at face value this looks impossible — how can more living material be supported by less?
The resolution: standing crop is not production. Standing crop is the mass of living material present at a particular time. Phytoplankton are microscopic and live only a few days, so they are eaten and replaced continuously. Over a year, a standing crop of a few grams per square metre may produce many hundreds of grams of new biomass. Fish, by contrast, live for years and accumulate their biomass, so their standing crop represents the summed production of many phytoplankton generations.
Why energy cannot behave this way. A pyramid of energy measures the amount of energy present at each trophic level in a given time, or annually per unit area — a rate, not a snapshot. And when energy flows from a particular trophic level to the next, some energy is always lost as heat at each step, following the 10 per cent law. A higher trophic level therefore receives strictly less energy per year than the level below, and no amount of rapid turnover can change that. The pyramid of energy is always upright and can never be inverted.
The general lesson. Numbers and biomass are measured at an instant and can mislead; energy is measured as a flow and cannot. This is why the energy pyramid is the reliable representation of trophic structure.
Q5. State the limitations of ecological pyramids, and explain how each limitation arises from something the pyramid model has to assume.
(i) They do not take into account the same species belonging to two or more trophic levels. A pyramid has to place each bar at one level, which forces every organism into a single position. But a trophic level represents a functional level, not a species: a sparrow is a primary consumer when it eats seeds, fruits and peas, and a secondary consumer when it eats insects and worms. A given organism may occupy more than one trophic level simultaneously, and the pyramid has no way to show it.
(ii) They assume a simple food chain, something that almost never exists in nature. The whole diagram is a stack of discrete levels, which presupposes a linear chain. But natural interconnections of food chains make a food web: the detritus food chain is connected with the grazing food chain at several levels, and animals such as cockroaches and crows are omnivores. A pyramid cannot accommodate a food web.
(iii) Saprophytes are given no place in ecological pyramids, even though they play a vital role in the ecosystem. The pyramid is built on the grazing chain, in which energy is passed upwards from level to level. Decomposers do not fit that geometry: they take energy from every level at once, from the dead remains of producers, herbivores and carnivores alike. This is a serious omission, since in a terrestrial ecosystem a much larger fraction of energy flows through the detritus food chain than through the grazing food chain.
A fourth caution, on method rather than model. Any calculation of energy content, biomass or numbers has to include all organisms at that trophic level. No generalisation will be true if only a few individuals at any level are taken into account.
The verdict. Ecological pyramids remain useful because they make the loss of energy at successive levels immediately visible. But they are a simplification of a web, and should be read as a summary of energy flow rather than as a map of who eats whom.
🧠 Assertion–Reason Questions
For each pair, decide whether both statements are true and whether the reason correctly explains the assertion.
Assertion (A): The number of trophic levels in a grazing food chain is restricted.
Reason (R): Only 10 per cent of the energy is transferred to each trophic level from the lower trophic level.
Both A and R are true, and R is the correct explanation of A.
Losing nine tenths of the energy at every step means that after four or five levels there is too little left to support a viable population. This is also why top predators are always rare and need very large territories.
Assertion (A): A pyramid of energy can never be inverted.
Reason (R): When energy flows from one trophic level to the next, some energy is always lost as heat at each step.
Both A and R are true, and R is the correct explanation of A.
Since each level receives strictly less energy per unit time than the level below it, the bars must decrease upwards. Pyramids of number and biomass can invert because they are snapshots that ignore turnover; the energy pyramid measures a flow, and a flow cannot increase as it loses.
Assertion (A): In a terrestrial ecosystem, most energy flows through the grazing food chain.
Reason (R): The detritus food chain begins with dead organic matter and is made up of decomposers.
A is false but R is true.
R correctly describes the detritus food chain, but A reverses the facts. In an aquatic ecosystem the grazing food chain is the major conduit for energy flow; in a terrestrial ecosystem a much larger fraction of energy flows through the detritus food chain than through the GFC — because most terrestrial plant material is wood, bark and litter that dies rather than being eaten alive.
Frequently Asked Questions - Energy Flow and Ecological Pyramids
What percentage of incident solar radiation is photosynthetically active radiation, and how much do plants capture?
Of the incident solar radiation, less than 50 per cent is photosynthetically active radiation. Plants capture only 2 to 10 per cent of that PAR, and this small amount of energy sustains the entire living world.
Why is energy flow in an ecosystem unidirectional?
Because all organisms depend for their food on producers, either directly or indirectly, so energy flows from the sun to producers and then to consumers, and at each step some is dissipated as heat and never returns. This accords with the first law of thermodynamics, and ecosystems are also not exempt from the second law: they need a constant supply of energy to synthesise the molecules they require, to counteract the universal tendency towards increasing disorderliness.
What is a trophic level?
Based on the source of their nutrition or food, organisms occupy a specific place in the food chain that is known as their trophic level. Producers belong to the first trophic level, herbivores or primary consumers to the second, and carnivores or secondary consumers to the third. A trophic level represents a functional level, not a species: a sparrow is a primary consumer when it eats seeds and a secondary consumer when it eats insects.
What is the difference between the grazing food chain and the detritus food chain?
The grazing food chain begins with the producers, as in grass to goat to man. The detritus food chain begins with dead organic matter and is made up of decomposers, heterotrophic organisms mainly fungi and bacteria, also known as saprotrophs, which secrete digestive enzymes that break down dead and waste materials into simple inorganic materials that they then absorb. In an aquatic ecosystem the grazing food chain is the major conduit for energy flow, while in a terrestrial ecosystem a much larger fraction of energy flows through the detritus food chain.
What is a food web?
The detritus food chain may be connected with the grazing food chain at some levels: some organisms of the DFC are prey to GFC animals, and in a natural ecosystem some animals such as cockroaches and crows are omnivores. These natural interconnections of food chains make a food web.
What is standing crop?
Each trophic level has a certain mass of living material at a particular time called the standing crop. It is measured as the mass of living organisms, that is biomass, or the number in a unit area. Biomass is expressed in terms of fresh or dry weight, and measurement in dry weight is more accurate.
What is the 10 per cent law?
The number of trophic levels in the grazing food chain is restricted because the transfer of energy follows the 10 per cent law: only 10 per cent of the energy is transferred to each trophic level from the lower trophic level.
What are ecological pyramids and what are the three types?
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 or first trophic level while the apex represents the tertiary or top level consumer. The three types usually studied are the pyramid of number, the pyramid of biomass and the pyramid of energy.
Why is the pyramid of biomass in the sea inverted, and why can the pyramid of energy never be inverted?
The pyramid of biomass in the sea is generally inverted because the biomass of fishes far exceeds that of phytoplankton: a small standing crop of phytoplankton supports a large standing crop of consumers, since phytoplankton live only a few days and are replaced continuously. The pyramid of energy is always upright and 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.
What are the 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, something that almost never exists in nature, and cannot accommodate a food web. And saprophytes are given no place in ecological pyramids even though they play a vital role in the ecosystem.
What is nutrient cycling and what are its two types?
Nutrient cycling is the storage and movement of nutrient elements through the various components of the ecosystem, by which nutrients are repeatedly used. It is of two types: gaseous, for which the atmosphere or hydrosphere is the reservoir, as with carbon, and sedimentary, for which the Earth's crust is the reservoir, as with phosphorus.
What are ecosystem services?
Ecosystem services are the products of ecosystem processes, for example the purification of air and water by forests.
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