Quick answer: A food chain is a single feeding sequence (grass → grasshopper → frog → snake → eagle). A food web is a network of many interlinked food chains. Food webs make an ecosystem more stable, because animals can switch to other foods if one species declines. Energy flows one way, and only about 10% passes from one trophic level to the next, so food chains usually have only 3–4 trophic levels.
Food chain and food web
Food chain: a series of organisms in which each one feeds on the one before it. Example:
Grass → Grasshopper → Frog → Snake → Eagle
Food web: in nature, most animals eat more than one kind of food, and are eaten by more than one kind of animal. So food chains are interconnected, forming a food web.
| Organism | Eats | Is eaten by |
|---|---|---|
| Grass | — (producer) | Grasshopper, rabbit, mouse |
| Grasshopper | Grass | Frog, birds |
| Rabbit | Grass | Snake, eagle |
| Mouse | Grass, seeds | Snake, eagle |
| Frog | Grasshopper | Snake |
| Snake | Frog, mouse, rabbit | Eagle |
| Eagle | Snake, rabbit, mouse | — (top consumer) |
Why food webs make ecosystems more stable
- Alternative food: In a single chain, if one species disappears, the animal that eats it has nothing else to eat. In a food web, it can switch to other food.
- Population control: Each herbivore is eaten by several predators, so herbivore numbers are less likely to rise out of control and destroy the plants.
- Resilience: An ecosystem with many species and many links can usually cope better with disturbances such as disease or drought.
Example: If frogs are killed by pesticides in a simple chain, snakes would starve and grasshoppers might increase greatly. In a food web, snakes can still eat mice and rabbits.
Flow of energy: the 10% law
- Green plants capture only about 1% of the energy of sunlight that falls on their leaves.
- At each step up the chain, only about 10% of the energy is passed on and stored in the next trophic level.
- The remaining energy is used for life processes (such as respiration) and is lost mostly as heat, or remains in undigested and dead matter.
| Level | Energy (example) |
|---|---|
| Sunlight falling on leaves | 1,000,000 J |
| Producers (about 1%) | 10,000 J |
| Herbivores (10%) | 1,000 J |
| Small carnivores (10%) | 100 J |
| Large carnivores (10%) | 10 J |
The 10% figure is an average; it was first described in the 1942 energy-flow work of ecologist Raymond Lindeman.
Why are food chains short?
Because so little energy reaches each higher level, after 3 or 4 steps there is too little energy to support another level of consumers. So food chains generally have only three or four trophic levels. This is also why there are fewer animals at the top of a food chain.
Energy flow is one-way
- Energy captured by plants passes to herbivores and then to carnivores; it does not come back to the earlier level.
- Energy is gradually lost as heat, which living things cannot reuse.
- Nutrients such as carbon and nitrogen, on the other hand, are recycled by decomposers.
Summary table
| Feature | Food chain | Food web |
|---|---|---|
| Structure | Single straight sequence | Network of linked chains |
| Feeding options | One food per level | Several alternatives |
| Stability | Easily disturbed | More stable |
| Energy flow | One-way, about 10% per step | One-way, about 10% per step |
Exam tips
- Draw a food chain with arrows pointing from the food to the eater (grass → grasshopper).
- For energy questions, divide by 10 at each step and show the working.
- Remember both numbers: 1% of sunlight captured by plants, 10% passed between levels.
- Give the reason for short food chains in one line: "energy decreases at each trophic level, so after 3–4 levels too little is left."
Common mistakes
- Drawing arrows from the eater to the food.
- Confusing the 1% (sunlight to plants) with the 10% (between trophic levels).
- Saying energy is recycled. Nutrients are recycled; energy flows one way.
- Thinking 10% is exact. It is an approximate average.
Frequently Asked Questions
What is the difference between a food chain and a food web?
A food chain is a single sequence of organisms in which each feeds on the one before it. A food web is a network of many interconnected food chains in an ecosystem.
Why is a food web more stable than a food chain?
In a food web, animals have alternative foods. If one species declines, its predators can eat something else, so the ecosystem is less easily disturbed.
What is the 10% law?
Only about 10% of the energy at one trophic level is passed on to the next level. The rest is mostly used in life processes and lost as heat.
Why do food chains usually have only three or four trophic levels?
So much energy is lost at each step that after three or four levels there is too little energy left to support another level of consumers.
Why is the flow of energy in an ecosystem unidirectional?
Energy passes from producers to herbivores to carnivores and is lost as heat at each step. It cannot flow back to the earlier levels.