Students should be able to describe the differences between the trophic…

The idea

Trophic levels describe an organism's feeding position in a food chain, defined by how many energy-transfer steps separate it from the producer. Level 1 holds producers such as grass, algae and phytoplankton, which make their own food by photosynthesis. Level 2 holds primary consumers, herbivores such as rabbits or zooplankton, which eat producers. Level 3 holds secondary consumers, carnivores such as foxes or small fish, which eat herbivores. Level 4 holds tertiary consumers, predators such as owls or sharks. The key differences are therefore the type of food eaten, the source of energy and the number of transfers from the original producer. Decomposers are not assigned a numbered trophic level because they feed at every level.

What earns the marks

• Level 1 contains producers, which synthesise their own organic nutrients by photosynthesis rather than consuming other organisms.
• Level 2 contains primary consumers, which are herbivores feeding directly on producers.
• Level 3 contains secondary consumers, which are carnivores or omnivores feeding on primary consumers.
• Level 4 contains tertiary consumers, which feed on secondary consumers and sit further along the chain.
• The trophic level number counts the feeding steps from the producer, so each level differs by what the organism eats and how far it is from the original energy source.
• Decomposers break down dead material at all levels and are not given a numbered trophic level in a food chain.

Common mistakes and how to put them right

• Treating trophic level as a measure of body size or physical height in the habitat.

  Put it right by linking the number to feeding position and distance from the producer.

• Calling every consumer a carnivore.

  Put it right by stating that level 2 organisms are herbivores eating producers, while higher consumers eat other animals.

• Giving decomposers a numbered trophic level such as level 5.

  Put it right by explaining that decomposers act at every level and are shown separately from the numbered chain.

Exam tips

• Name the producer and each consumer in a stated food chain, then quote the level number for each organism to show the difference clearly.
• Use the terms producer, primary consumer, secondary consumer and tertiary consumer rather than vague words such as plant-eater or top animal.
• When asked to describe differences, compare pairs of levels directly, for example state what a level 2 organism eats compared with a level 3 organism.