NCERT grounding
NCERT Class 12 Biology, Chapter 12 Ecosystem, section 12.4 Energy Flow, anchors this subtopic. The textbook states plainly that “except for the deep sea hydro-thermal ecosystem, 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 (PAR), and plants capture only 2–10 per cent of that PAR. The chapter establishes that energy flow is unidirectional, that organisms occupy trophic levels, and that transfer between levels obeys the 10 per cent law.
“You find unidirectional flow of energy from the sun to producers and then to consumers.”
NCERT Class 12 Biology · Chapter 12, Energy Flow
The same idea appears in the NIOS Senior Secondary biology course (Chapter 25, Principles of Ecology), which describes the conversion of solar radiation into plant biomass, the passage of this chemical energy through trophic levels, and the 10 per cent rule of thumb. Both sources agree that ecosystems are not exempt from the laws of thermodynamics — they need a constant input of energy to counteract the universal tendency toward disorder.
How energy flows through an ecosystem
A constant input of solar energy is the basic requirement for any ecosystem to function and sustain itself. Energy enters the living system at one door only — photosynthesis by green plants and photosynthetic bacteria, the autotrophs. These producers fix the sun’s radiant energy and use simple inorganic raw materials to manufacture organic food. The chemical energy now locked inside plant biomass is the currency that every other organism in the ecosystem must spend. All organisms depend on producers for food, either directly or indirectly, so the route of energy is fixed: sun → producers → consumers.
No energy trapped by an organism stays in it forever. The energy a producer captures is either passed on to a consumer that eats it, or, when the organism dies, it becomes detritus that fuels the detritus food chain. At each transfer, two losses occur. A part of the ingested energy is simply not utilised — it passes out undigested or is never consumed at all. A second, larger part is used up by the organism in its own metabolism and is released as heat through respiration. Neither fraction can be recovered by the ecosystem.
This is why the flow is described as unidirectional. Energy moves from the sun to producers and then to consumers, and it never moves in the reverse direction. A herbivore cannot return energy to the grass; the grass cannot return energy to the sun. Coupled with the steady loss as heat at every step, this one-way movement means the energy available at successive trophic levels keeps decreasing.
Thermodynamics in ecosystems
Energy flow obeys the first law — energy is converted from light to chemical to heat, never created or destroyed. Ecosystems are not exempt from the second law: they need a constant supply of energy to counteract the universal tendency toward increasing disorderliness.
Producers are the green plants of the ecosystem. In a terrestrial ecosystem the major producers are herbaceous and woody plants; in an aquatic ecosystem the producers are phytoplankton, algae and higher plants. The animals that depend on plants — directly or indirectly — are the consumers, also called heterotrophs. Consumers that feed on producers are primary consumers; animals that eat other animals are secondary or tertiary consumers. This feeding sequence, where an organism eats and is in turn eaten, builds the food chain along which energy travels.
Figure 1. Energy enters as sunlight, is fixed by producers, and decreases at each successive trophic level. Respiratory heat is lost at every step and cannot return — the flow is strictly unidirectional.
PAR and the 2–10% capture
Not all of the sunlight striking the Earth can be used to make food. Of the total incident solar radiation, less than 50 per cent is photosynthetically active radiation — the PAR. PAR is the band of wavelengths (broadly the visible region) that chlorophyll and accessory pigments can absorb to drive photosynthesis. The remaining radiation lies outside this usable band or is reflected, and plays no part in fixing energy.
Even within PAR, plants are remarkably inefficient capturers. Producers fix only 2–10 per cent of the PAR that reaches them. It is a small fraction — yet this tiny slice of the sun’s output is the entire energy budget that sustains every herbivore, carnivore and decomposer on the planet. Understanding how that small captured amount flows onward is the central question of this subtopic.
PAR captured by producers
Less than 50% of incident solar radiation is PAR; of that PAR, plants fix only 2 to 10 per cent. This small captured amount of energy sustains the entire living world.
A common exam point: PAR is a fraction of the incident solar radiation, while the 2–10 per cent figure is the fraction of PAR (not of total radiation) that plants actually fix. Keeping these two percentages in the correct order — radiation, then PAR, then captured energy — prevents the most frequent mistake in statement-based questions.
Trophic levels in a food chain
Organisms occupy a place in a community according to their feeding relationship with other organisms. Based on the source of its nutrition, each organism sits at a specific trophic level in the food chain. The trophic level represents a functional level, not a species as such — it describes what an organism eats, not what it is.
Four trophic levels of the grazing food chain. Producers form the base; energy and biomass decrease as you move up.
First level
Producers — green plants and photosynthetic autotrophs. They fix solar energy and enter it into the living system.
Second level
Herbivores (primary consumers) — insects, birds and mammals on land, molluscs in water. They eat producers.
Third level
Primary carnivores (secondary consumers) — animals that feed on herbivores.
Fourth level
Top carnivores (tertiary consumers, or secondary carnivores) — animals that feed on primary carnivores.
A textbook grazing food chain — Grass → Goat → Man — runs across three trophic levels: grass is the producer, the goat is the primary consumer (herbivore), and man is the secondary consumer here. The amount of energy decreases at every successive trophic level, because organisms at each level depend on the lower level for energy and lose much of what they receive as heat. When an organism at any level dies, it converts to detritus or dead biomass and becomes an energy source for decomposers.
Grazing vs detritus food chains
Energy does not flow along a single kind of chain. NCERT recognises two distinct food chains, defined by where they begin. The grazing food chain (GFC) starts with living producers — green plants — and passes to herbivores and then to carnivores. The detritus food chain (DFC) starts not with living plants but with dead organic matter — detritus. The DFC is made up of decomposers, mainly fungi and bacteria, that are heterotrophic. They meet their energy and nutrient needs by degrading detritus, which is why they are also called saprotrophs. Decomposers secrete digestive enzymes that break down dead and waste materials into simple inorganic substances, which are then absorbed.
Grazing food chain (GFC)
Producers
Begins with living green plants
- Producer → herbivore → carnivore sequence
- Powered directly by captured solar energy
- Major conduit of energy in aquatic ecosystems
- Example: Grass → Goat → Man
Detritus food chain (DFC)
Detritus
Begins with dead organic matter
- Detritus → decomposers / detritivores
- Powered by energy in dead biomass
- Carries a much larger fraction of energy on land
- Decomposers (fungi, bacteria) are saprotrophs
The balance between these two chains depends on the ecosystem. In an aquatic ecosystem, the GFC is the major conduit for energy flow — most energy passes through grazing on phytoplankton and algae. In a terrestrial ecosystem, the situation reverses: a much larger fraction of energy flows through the detritus food chain than through the GFC, because so much plant material (fallen leaves, bark, wood) is never grazed and instead enters the soil as detritus.
Food webs and standing crop
The grazing and detritus food chains are not isolated sequences. They are interconnected at several levels. Some organisms of the DFC are prey to GFC animals, and in a natural ecosystem many animals — cockroaches, crows and humans among them — are omnivores that feed at more than one level. These natural interconnections of food chains make a food web. A food web is a more realistic picture of nature than a single straight chain, because in it one trophic level can be linked to several others.
Figure 2. The grazing food chain (top) and detritus food chain (bottom) are linked by dashed connections: dead organisms at every level feed detritus, and DFC organisms can be prey to GFC animals. Together they form a food web.
Each trophic level carries a certain mass of living material at a particular time, called the standing crop. The standing crop is measured as the mass of living organisms — the biomass — or as the number of organisms in a unit area. Biomass is expressed in fresh or dry weight, and measurement in terms of dry weight is more accurate, because the water content of fresh material varies and would distort comparisons. Standing crop should not be confused with standing state, which refers to the amount of inorganic nutrients held in the soil at a given time.
The 10 per cent law
The single most examined number in this subtopic is the transfer efficiency between trophic levels. The 10 per cent law, proposed by Lindeman, states that only about 10 per cent of the energy at one trophic level is transferred to the next higher trophic level. The other roughly 90 per cent is lost — largely as heat in respiration, with a further part remaining unutilised. This is the quantitative expression of the energy decrease seen at successive trophic levels.
The 10% law in numbers
-
Level 1
Producers
1000 kcal of energy fixed in plant biomass.
100% -
Level 2
Herbivores
Only 100 kcal becomes herbivore tissue.
10% -
Level 3
Primary carnivores
Only 10 kcal passes to the first carnivore.
1% -
Level 4
Top carnivores
Only 1 kcal reaches the top carnivore.
0.1%
The consequence of this steep loss is profound: it sets a hard limit on the length of food chains. Because energy shrinks roughly tenfold at every step, after four or five levels there is too little energy left to support a viable higher trophic level. This is why, in nature, the number of trophic levels in a grazing food chain is restricted, usually to about four or five. NCERT lists the typical levels as producer, herbivore, primary carnivore and secondary carnivore in the grazing food chain.
The 10 per cent law also explains why a pyramid of energy is always upright and can never be inverted — energy is always lost as heat moving up, so a higher level can never hold more energy than the one below it. The law applies to the energy passed up the chain; it does not mean every level loses exactly 90 per cent, but it is the accepted rule of thumb for NEET problem-solving.
Worked examples
In a grassland ecosystem the producers fix 20,000 kcal m −2 yr−1. Applying the 10 per cent law, how much energy is available to the primary carnivores (third trophic level)?
The third trophic level is two steps above the producers. At each step only 10 per cent transfers. Producers = 20,000 kcal → herbivores = 10% of 20,000 = 2,000 kcal → primary carnivores = 10% of 2,000 = 200 kcal m−2 yr−1. Each trophic level holds one-tenth of the level below it.
A statement reads: “In an ecosystem there is unidirectional flow of energy from the sun to producers and then to consumers.” Is this correct, and why?
It is correct. The sun is the only source of energy for almost all ecosystems. Energy enters through producers and passes to consumers, and it cannot move in the reverse direction because a large part is lost as heat at every transfer and cannot be recaptured. This one-way movement is exactly what unidirectional flow means.
Match the trophic levels of a grassland to organisms: grass, rabbit, crow, vulture. Which level does each occupy?
Grass is the producer — first trophic level. The rabbit eats grass, so it is a herbivore — second trophic level. The crow feeds on the rabbit (a primary carnivore) — third trophic level. The vulture, a top carnivore, occupies the fourth trophic level.
Common confusion & NEET traps
Energy flow questions are rarely about a single fact — they pair a true statement with a false one and ask which holds. The reliable defence is to keep the percentages, the direction of flow and the food-chain types cleanly separated.