Trophic Levels

All living things need energy to stay alive. In an ecosystem this energy starts as sunlight and is passed from organism to organism when one thing eats another. The path the energy takes is called a food chain.

A trophic level is simply a feeding level — a single step in that food chain. We give each step a name depending on what it feeds on:

• the producer makes its own food,
• the primary consumer eats the producer,
• the secondary consumer eats the primary consumer,
• the tertiary consumer eats the secondary consumer.

Reading a food chain is easy once you know the rule about the arrows. Each arrow means "is eaten by" and points in the direction the energy flows — from the food to the feeder:

grass → rabbit → fox

Here the grass is eaten by the rabbit, and the rabbit is eaten by the fox. The arrows are not pointing at what each animal eats — they point the way the energy travels.

Exam tip. If you are asked which way the arrows point, the safe answer is "in the direction of energy flow / from the organism being eaten to the organism that eats it". Drawing the arrows backwards is a very common, easily avoided error.

Every food chain begins with a producer. A producer is an organism that makes its own food by photosynthesis — it uses light energy from the Sun, together with carbon dioxide and water, to build glucose.

Producers are nearly always green plants (such as grass, trees and crops) or algae in water. Because they trap the Sun's energy and bring it into the living world, producers are always the first trophic level.

Feature	Producer
How it gets food	Photosynthesis (makes its own)
Examples	Grass, oak tree, seaweed, algae, phytoplankton
Position in chain	Always first (trophic level 1)
Energy source	Sunlight

Without producers there would be no food chain at all, because no other organism can capture the Sun's energy directly. Every consumer further along the chain ultimately depends on the food the producer made.

Exam tip. When you define a producer, say it "makes its own food by photosynthesis" — naming photosynthesis is the marking point. Writing only "a plant" is not enough.

A consumer is an organism that cannot make its own food, so it must eat other organisms to get energy. Consumers are named according to where they sit in the food chain:

1. A primary consumer eats the producer. Because it eats plants, a primary consumer is a herbivore (e.g. a rabbit eating grass).
2. A secondary consumer eats the primary consumer (e.g. a fox eating a rabbit).
3. A tertiary consumer eats the secondary consumer (e.g. an eagle eating the fox).

Each step is one trophic level higher than the one before it. The same animal can be a different level in different chains, depending on what it is eating — so always work out the level from the chain in front of you, not from memory of the animal.

Here is the labelled four-step chain so you can see how the levels build up:

Exam tip. Name the level from its position in the chain, not from the animal. A fox is a secondary consumer in "grass → rabbit → fox", but it would be a tertiary consumer in a longer chain that has another animal before it.

Not all organisms feed by eating living things. Decomposers are organisms that break down dead material — dead plants, dead animals and waste such as droppings.

The main decomposers are bacteria and fungi. As they feed on dead material they cause it to decay (rot), releasing the nutrients locked inside it back into the soil so that producers can use them again. In this way decomposers are vital for recycling nutrients through the ecosystem.

Trophic level	What it does	Examples
Producer	Photosynthesises to make food	Grass, algae
Primary consumer	Herbivore — eats the producer	Rabbit, caterpillar
Secondary consumer	Eats the primary consumer	Fox, frog
Tertiary consumer	Eats the secondary consumer	Eagle, large fish
Decomposer	Breaks down dead material	Bacteria, fungi

Decomposers do not usually appear in a straight food-chain line, but they act on organisms at every level — whenever any plant or animal dies, decomposers break it down.

Exam tip. Define a decomposer as something that "breaks down dead organisms / dead material" and give an example such as bacteria or fungi. Do not confuse a decomposer with a consumer that simply scavenges live-killed prey.

When you are given a food chain in the exam, label it step by step using two simple rules:

1. The first organism is the producer (it photosynthesises).
2. Each organism after that is a consumer, named by counting along: the first consumer is primary, the next is secondary, the next is tertiary, and so on.

Worked through on the classic example:

grass  →  rabbit  →  fox  →  eagle

• grass — makes its own food by photosynthesis → producer (level 1)
• rabbit — eats the grass (a herbivore) → primary consumer (level 2)
• fox — eats the rabbit → secondary consumer (level 3)
• eagle — eats the fox → tertiary consumer (level 4)

Remember the arrows show energy flowing along the chain, starting with the sunlight captured by the grass. Whenever any of these organisms dies, decomposers (bacteria and fungi) break it down and return the nutrients to the soil.

Quick rule: producer first → then count the consumers along the chain (primary, secondary, tertiary). Decomposers act on the dead remains at every level.

Exam tip. Always check the producer is the organism at the start of the chain and that you have counted the consumers in order. Mislabelling because you started counting from the wrong end is a frequent slip.