Every time you eat, you’re participating in a food chain. The sandwich you had for lunch started as wheat (a producer), possibly included animal protein (a primary or secondary consumer), and the energy in all of it originated from the sun. Food chains and food webs describe how energy and matter move through ecosystems — and why the number of lions in Africa is so much smaller than the number of zebras.
Key Terms & Definitions
Food chain: A linear sequence showing the transfer of energy from one organism to another through feeding. Each arrow represents “is eaten by” or “energy flows to.”
Food web: An interconnected network of multiple food chains in an ecosystem. More realistic than a food chain — most organisms eat and are eaten by multiple species.
Trophic level: Position in a food chain based on feeding relationships. Producers = first trophic level; primary consumers = second; secondary consumers = third; and so on.
Producer (autotroph): Organism that makes its own food using sunlight (photosynthesis) or chemical energy (chemosynthesis). Examples: plants, algae, cyanobacteria. They form the base of all food chains.
Consumer (heterotroph): Organism that obtains energy by consuming other organisms.
- Primary consumer: Eats producers. Examples: grasshopper, deer, rabbit, cow.
- Secondary consumer: Eats primary consumers. Examples: frog, small birds, fox.
- Tertiary consumer: Eats secondary consumers. Examples: hawk, large snake, large carnivores.
Decomposer (detritivore): Organisms (bacteria, fungi) that break down dead organic matter (detritus) into simple inorganic molecules, releasing nutrients back to the soil. Without decomposers, nutrients would be locked in dead bodies permanently.
Detritus food chain: Starts from dead organic matter (detritus) → decomposers → smaller organisms. Different from the grazing food chain (starts from green plants).
Biomass: Total mass of living organisms at each trophic level. Usually measured as dry mass per unit area.
Productivity: Rate of biomass production. Primary productivity = rate of energy fixation by producers. Net primary productivity (NPP) = gross primary productivity − respiration by producers.
10% Law (Lindeman’s Law, 1942): Only ~10% of the energy at one trophic level is transferred to the next. The rest is lost as heat (respiration) or remains as non-edible parts.
The Food Chain
Grazing Food Chain (GFC)
The most familiar type — starts with living green plants:
Each arrow means “is eaten by” and represents energy transfer. The chain has 5 trophic levels.
A simple aquatic food chain:
Detritus Food Chain (DFC)
Starts with dead organic matter:
In many ecosystems — especially forests and wetlands — the detritus food chain processes more energy than the grazing food chain. Fallen leaves and dead wood contain enormous amounts of energy that decomposers unlock.
The 10% Law and Why it Matters
Lindeman (1942) studied Cedar Bog Lake in Minnesota and found that approximately 10% of the energy at each trophic level is transferred to the next. The other ~90% is lost:
- ~60% as heat through cellular respiration
- ~20% in feces/undigested material
- ~10% in dead organic matter (enters the detritus chain)
- Only ~10% converted to new biomass available to the next level
Why this makes food chains short: Most food chains have only 4–5 trophic levels. After 5 levels, the energy remaining is too small to support a viable population of organisms. Let’s trace what happens to 1000 kJ of energy fixed by producers:
| Trophic Level | Energy Available (kJ) |
|---|---|
| Producers | 1000 |
| Primary consumers | 100 |
| Secondary consumers | 10 |
| Tertiary consumers | 1 |
| Quaternary consumers | 0.1 |
At the 5th level, only 0.1 kJ remains — not enough to sustain a population.
This is why top predators are rare and why herbivores are abundant. An area that can support a thousand kilograms of grass can only support ~100 kg of herbivores, ~10 kg of small carnivores, and ~1 kg of large carnivores. The 10% law explains why there are thousands of zebras for every lion, and why whales must eat millions of krill per day.
Food Webs — Ecological Interconnections
Real ecosystems are far more complex than simple chains. Most animals eat multiple types of food and are eaten by multiple predators. A food web captures this complexity.
Why Food Webs Are More Stable Than Food Chains
In a simple food chain: Grass → Rabbit → Fox If rabbits disappear (disease, hunting), foxes starve immediately, and grass might overgrow.
In a food web, the fox also eats mice, birds, and insects. If rabbits disappear, foxes shift to other prey — the ecosystem is buffered against the loss of any one species.
Ecological resilience: The more connections in a food web, the more robust the ecosystem. This is why biodiversity is important — species-rich ecosystems have more redundancy in their food webs and are more resistant to disturbances.
Keystone Species
Some species have a disproportionate effect on the food web relative to their abundance. Removing a keystone species can cause cascading changes — called a trophic cascade.
Classic example — Sea otters in Pacific kelp forests: Sea otters eat sea urchins. Sea urchins eat kelp. When sea otters were hunted to near-extinction (for fur), sea urchin populations exploded → kelp forests were devastated → entire coastal ecosystems collapsed (hundreds of species of fish and invertebrates lost habitat).
When sea otters were reintroduced, sea urchin populations were controlled → kelp forests recovered → fish returned. The sea otter is a keystone species.
Indian example — Vultures and ecosystem services: In the 1990s–2000s, vulture populations in India collapsed (due to diclofenac, a veterinary anti-inflammatory drug, which is lethal to vultures). Carcasses that vultures normally consumed quickly began decomposing slowly → bacterial contamination of water → increased disease spread. Feral dog populations (which took over as scavengers) increased → more rabies cases. A single species’ removal from the food web had cascading health and ecological consequences.
Ecological Pyramids Revisited
Food chains and food webs are the basis for ecological pyramids (numbers, biomass, energy):
Pyramid of energy: Always upright (10% law operates — always less energy at higher levels). Most informative.
Pyramid of biomass: Usually upright in terrestrial ecosystems. Inverted in aquatic ecosystems because phytoplankton (producers) have small standing biomass but high turnover rate, while zooplankton (primary consumers) build up larger biomass.
Pyramid of numbers: Usually upright, but can be inverted (a single tree supports many herbivorous insects).
Biogeochemical Cycles — Matter Recycling
Unlike energy (which flows one-way through ecosystems and is lost as heat), matter is recycled. Carbon, nitrogen, phosphorus, and water cycle between biotic (living) and abiotic (non-living) components of ecosystems.
Carbon Cycle
Producers fix CO₂ via photosynthesis → incorporated into organic molecules → consumers eat producers, incorporating carbon → decomposers break down dead organisms, releasing CO₂ → CO₂ back to atmosphere.
Combustion of fossil fuels adds CO₂ that was stored for millions of years — disrupting the natural balance.
Nutrient Cycling
Decomposers are essential for nutrient cycling. They break down dead organic matter and release inorganic nutrients (nitrates, phosphates) back to the soil → taken up by producers again. Without decomposers, all nutrients would be locked in dead biomass and producers couldn’t grow.
NEET regularly asks: (1) energy available at each trophic level using 10% law; (2) why food chains are limited to 4–5 links; (3) which ecological pyramid is always upright — pyramid of energy; (4) which pyramid can be inverted in aquatic ecosystems — pyramid of biomass (but NOT energy); (5) definition of keystone species. These five concepts cover most food chain MCQs.
Solved Examples
Example 1 — CBSE Level
Q: In a food chain, grass fixes 500 J of energy. How much energy is available to a secondary consumer?
A: Using 10% law at each trophic level:
- Grass (producers): 500 J
- Primary consumers: 500 × 10% = 50 J
- Secondary consumers: 50 × 10% = 5 J
Example 2 — NEET Level
Q: Which of the following statements about food chains is correct? (a) A food chain always starts with a herbivore (b) The number of trophic levels is unlimited (c) Energy flow in food chains follows the 10% law (d) Food chains are always linear
A: (c). Food chains start with producers (not herbivores) — (a) is wrong. Energy loss at each trophic level limits chains to 4–5 levels — (b) is wrong. Real ecosystems are food webs (not just linear chains) — (d) is wrong. The 10% law correctly describes energy transfer efficiency.
Example 3 — Challenging
Q: Why are pesticides like DDT found in the highest concentration in top predators, even though they are applied to the environment at very low concentrations? Explain the concept used.
A: This is biomagnification (biological magnification). DDT is lipid-soluble — it is stored in fat tissue and is not broken down quickly. At each trophic level, organisms eat many organisms from the level below. Each consumed organism’s accumulated DDT is added to the consumer’s body.
Example: If plankton have 0.01 ppm DDT, small fish eat thousands of plankton and accumulate ~1 ppm. Large fish eat hundreds of small fish and accumulate ~100 ppm. Birds like ospreys or eagles eat large fish and accumulate ~1000 ppm. At the top predator level, DDT concentrations can be millions of times higher than in the environment.
This is why DDT caused reproductive failure in eagles (thin eggshells) and near-extinction of several raptor species in North America in the 1960s. It’s also why PCBs, mercury (as methylmercury), and other persistent organic pollutants are dangerous in seafood — they biomagnify.
Common Mistakes to Avoid
Mistake 1 — Food chain direction: In a food chain, arrows point in the direction of energy flow — from eaten to eater. “Grass → Grasshopper → Frog” means energy flows from grass to grasshopper to frog. Never reverse the arrow direction.
Mistake 2 — 10% law is exact: The 10% figure is an approximation. Real ecosystems show 5–20% efficiency. But for calculation purposes in CBSE/NEET, use 10% unless a different value is specified.
Mistake 3 — Decomposers are in the food chain: Decomposers work on dead matter and are technically separate from the grazing food chain. They form a parallel detritus food chain. In ecological pyramids, they are often not represented — this is a limitation of pyramid diagrams.
Mistake 4 — More trophic levels = better ecosystem: More trophic levels actually mean more energy is lost (10% at each step). An ecosystem can support a larger biomass of primary consumers than of top predators. “More levels” doesn’t mean more stability — it means more energy loss.
Practice Questions
Q1: Why are food chains limited to 4–5 trophic levels?
Due to the 10% law, only ~10% of energy from one trophic level is available to the next. After 4–5 steps, the remaining energy is so small that it cannot support a viable population of organisms. Starting from 1000 kJ at the producer level: primary consumers get 100 kJ, secondary get 10 kJ, tertiary get 1 kJ, quaternary get 0.1 kJ. This fraction is too small to sustain a population, naturally limiting the chain length.
Q2: What is biomagnification? Name one pollutant that shows biomagnification.
Biomagnification is the progressive increase in the concentration of a persistent pollutant at each successive trophic level of a food chain. It occurs because the pollutant is stored in body tissues (especially fat) rather than being broken down, and each organism accumulates the pollutant from all the organisms it eats throughout its lifetime.
Example: DDT (dichlorodiphenyltrichloroethane) — a pesticide that biomagnifies from less than 0.01 ppm in water to over 1000 ppm in top predators like eagles and osprey. Other examples: methylmercury (fish → humans), PCBs (polychlorinated biphenyls).
Q3: Distinguish between grazing and detritus food chains.
The grazing food chain (GFC) starts with living green plants (producers) and passes through herbivores to carnivores: Grass → Rabbit → Fox.
The detritus food chain (DFC) starts with dead organic matter (detritus — dead leaves, fallen trees, dead animals) and passes through decomposers and detritivores: Dead leaves → Earthworms → Birds.
In many ecosystems (especially forests), the DFC processes more energy than the GFC. Both chains are connected — organisms from the GFC die and enter the DFC as detritus.
FAQs
Q: Can an organism belong to multiple trophic levels? Yes — omnivores like bears, raccoons, or humans eat both plants (first trophic level producers → making them primary consumers) and animals (making them secondary or tertiary consumers). In ecological models, omnivores are sometimes assigned to an “average” trophic level between 2 and 3. This is one reason why food webs (which show multiple connections) are more accurate than simple food chains.
Q: What is the difference between food web and food chain stability? A food chain has a single pathway — remove any link and the chain breaks. A food web has multiple pathways between trophic levels — if one species is lost, energy can flow through alternative pathways. This redundancy is why food webs are more stable than food chains, and why biodiversity loss can destabilise ecosystems: fewer species means fewer alternative pathways.
Q: Does the 10% law mean exactly 10% of energy is always transferred? No — the 10% figure is an average/approximation. Actual ecological efficiency ranges from 5–20% depending on organism type, ecosystem, and conditions. Cold-blooded animals (poikilotherms) like insects are more efficient (closer to 20%) because they don’t use energy to regulate body temperature. Warm-blooded animals (homeotherms) like mammals and birds are less efficient (closer to 5–10%) because maintaining body temperature is energetically expensive.
Q: Why is eating lower on the food chain considered more environmentally sustainable? Because of the 10% law, land and resources needed to produce 1 kg of beef are roughly 10× more than to produce 1 kg of grain. The grain must first be grown, then fed to cattle, with ~90% of the energy lost at each step. Eating plants directly instead of feeding them to animals that humans then eat eliminates one or more steps of energy loss — meaning the same land area can feed far more people.