Food chains, food webs and trophic levels
A food chain is a series of organisms, each eating the one before it. Each step is a trophic level, and linked food chains form a food web.
Draw it: a food chain in a grassland
Arrows point from the food to the eater, the direction in which food and energy move.
Trophic levels in three food chains
| Trophic level | Who is here | Grassland | Pond | Forest |
|---|---|---|---|---|
| First (T1) | Producers | Grass | Phytoplankton (tiny floating plants) | Plants |
| Second (T2) | Herbivores, or primary consumers | Grasshopper | Zooplankton (tiny floating animals) | Deer |
| Third (T3) | Carnivores, or secondary consumers | Frog | Small fish | Tiger |
| Fourth (T4) | Large carnivores, or tertiary consumers | Snake | Big fish | None: this chain ends at the tiger |
Food webs
In nature, one organism is often eaten by more than one kind of animal, and most animals eat more than one kind of food. A frog may eat grasshoppers and other insects. A snake may eat frogs and rats. So food chains cross and join, forming a food web: a network of interconnected food chains. A food web gives animals other choices, so if one food runs short they can eat another.
How to draw a food chain or a food web
- Write the producer first, on the left or at the bottom.
- Write each eater after it, in order, with an arrow pointing from the food to the eater.
- Under each organism, write its trophic level: T1, T2, T3, T4.
- For a food web, draw several chains that share organisms. For example: grass → rat → snake, and grass → grasshopper → frog → snake, both leading on to the eagle.
- Never draw an arrow backwards: it shows the direction in which food and energy move.
When we eat rice, dal and vegetables we feed at the second trophic level, like herbivores. In paddy fields, owls and snakes eat rats and so protect the grain: a food chain at work.
2 marks: “What are trophic levels? Write a food chain and name the trophic level of each organism in it.” Each step of a food chain. Grass (T1, producer) → grasshopper (T2, herbivore) → frog (T3, carnivore) → snake (T4, large carnivore).
3 marks: “Distinguish between a food chain and a food web. Why do food webs form in nature?” Food chain: one straight series of who eats whom. Food web: many interconnected food chains. Webs form because animals eat, and are eaten by, more than one kind of organism.
1 marks: “Which organisms always occupy the first trophic level?” Producers, such as green plants.
The arrow means 'is eaten by'. Grass, grasshopper, frog, snake, eagle: Green Grasshoppers Feed Snakes Eventually.