What Is a Food Chain?
A food chain is a simple diagram that shows how energy and matter move through an ecosystem from one organism to another. Each link in the chain shows a feeding relationship: the organism at each step eats the one before it.
Food chains always begin with a producer (a plant or other photosynthetic organism) and end with a top predator (an animal that is not regularly eaten by any other animal). Arrows in a food chain point in the direction that energy flows — from food to feeder.
Example (grassland): Grass → Grasshopper → Frog → Snake → Hawk
This means: grass is eaten by grasshoppers, grasshoppers are eaten by frogs, frogs are eaten by snakes, and snakes are eaten by hawks.
Trophic Levels
Each position in a food chain is called a trophic level (from the Greek trophe, meaning food):
- Trophic level 1 (producers) — Plants and other photosynthetic organisms that make food energy from sunlight.
- Trophic level 2 (primary consumers / herbivores) — Animals that eat producers. Examples: caterpillars, rabbits, deer, grasshoppers.
- Trophic level 3 (secondary consumers) — Animals that eat primary consumers. Often carnivores or omnivores. Examples: frogs, foxes, small birds.
- Trophic level 4 (tertiary consumers) — Animals that eat secondary consumers. Usually apex predators. Examples: eagles, orca whales, lions.
Most food chains have 3–5 trophic levels. Very few chains have more than five links because so much energy is lost at each step.
Energy Loss Along the Chain
Energy is transferred from one trophic level to the next, but each step is inefficient. On average, only about 10% of the energy at one trophic level is passed on to the next. The rest is:
- Used by the organism in respiration (to fuel movement, maintenance, and heat)
- Lost as heat to the environment
- Not eaten or not digested (remains in faeces and decay)
This is why food chains rarely have more than five links — there simply is not enough energy left to support another level.
The "10% rule" explains why there are always many more plants than herbivores, and many more herbivores than carnivores in any ecosystem. Think of a pyramid of numbers: lots of grass, fewer rabbits, fewer still foxes.
Decomposers: The Final Link
Decomposers — mainly bacteria and fungi — break down dead organisms and waste products from all levels of the food chain. This releases nutrients back into the soil for plants to absorb. Without decomposers, nutrients would be locked up in dead matter and ecosystems would collapse.
Changes to a Food Chain
Removing a species from a food chain can have dramatically damaging effects:
- If the snake population collapses (from disease or hunting), frogs may overpopulate, eating more grasshoppers, leading to grasshopper scarcity and then grass overgrowing.
- These knock-on effects show why every species matters in an ecosystem.
Real world: Garden food chain
A simple garden food chain: grass → caterpillar → robin → sparrowhawk. The grass (producer) is eaten by caterpillars (primary consumers), robins eat caterpillars (secondary consumers), and sparrowhawks hunt robins (tertiary consumers). You can observe the beginning of this chain whenever you see birds on a lawn searching for grubs.
Real world: Farm food chains and pest control
On a wheat farm: wheat → aphid → ladybird → blue tit. Aphids are primary consumers that damage crops. Ladybirds (secondary consumers) eat aphids, naturally controlling pest numbers. Farmers who encourage ladybird populations reduce the need for pesticides — they are working with the food chain rather than against it.
Real world: Ocean food chain and fishing
A simple ocean food chain: phytoplankton → krill → herring → seal → orca. Overfishing herring removes a middle link — seals have less to eat, affecting their population, and orca have fewer seals to hunt. Understanding food chains explains why fisheries management is essential to prevent the collapse of ocean ecosystems.
Where do you see this in real life? Every meal you eat involves food chains. A beef burger represents: grass → cow → human (short chain). A tuna sandwich involves a much longer ocean chain. Eating lower on the food chain (plants rather than meat) is more energy-efficient because less energy is lost at each step.
Practice Problems
In the food chain: Seaweed → Sea urchin → Starfish → Sea otter, identify (a) the producer, (b) the primary consumer, (c) the secondary consumer.
Show answer
If 10,000 kJ of energy is available at the producer level, roughly how much is available to tertiary consumers (three levels up), assuming 10% efficiency at each level?