18.8 Trophic Levels and Biomass

A free GCSE Biology lesson for AQA (8461) from OBA School: producers, consumers, apex predators, decomposers and pyramids of biomass drawn to scale. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe the trophic levels in a food chain, using numbers and names
• Identify producers, primary, secondary and tertiary consumers and apex predators
• Explain how decomposers break down dead plant and animal material
• Construct an accurate pyramid of biomass from data, to scale
• Interpret pyramids of biomass

Key words

• Trophic level: The position of an organism in a food chain, food web or pyramid. Level 1 is the producer.
• Producer: An organism (plant or alga) that makes its own food by photosynthesis. Trophic level 1.
• Primary consumer: An animal that eats producers; a herbivore. Trophic level 2.
• Secondary / tertiary consumer: Carnivores that eat primary consumers (level 3), or that eat other carnivores (level 4).
• Apex predator: A carnivore with no natural predators, at the top of a food chain.
• Biomass: The mass of living material in organisms, often measured as dry mass per square metre (g/m²).
• Decomposer: A microorganism (bacterium or fungus) that breaks down dead plant and animal matter.

Trophic levels

• 1; Name Producers; What they do Plants and algae make their own food by photosynthesis, using light energy.
• 2; Name Primary consumers; What they do Herbivores that eat plants or algae.
• 3; Name Secondary consumers; What they do Carnivores that eat primary consumers.
• 4; Name Tertiary consumers; What they do Carnivores that eat other carnivores.

Apex predators are carnivores with no predators, so they are always at the top of the food chain. Most food chains have only four or five trophic levels.

Key fact: An animal can be at different trophic levels in different food chains. A fox eating a rabbit is a secondary consumer, but eating blackberries it is a primary consumer.

Decomposers

Decomposers are bacteria and fungi that break down dead plant and animal matter. They feed on every trophic level once organisms die, and on faeces.

Exam tip: Use the words secrete enzymes and diffuse in your answer. The decomposer does not “eat” the dead leaf in the way an animal does.

Pyramids of biomass

A pyramid of biomass shows the biomass at each trophic level. Each bar is drawn to scale: its width is proportional to the biomass. Producers go at the bottom.

Worked example: drawing a pyramid to scale

A pond has these dry biomasses (g/m²): algae 800, water fleas 120, small fish 15, pike 2.
Choose a scale that fits the page: 1 mm = 10 g/m².
Bar widths: 800 ÷ 10 = 80 mm; 120 ÷ 10 = 12 mm; 15 ÷ 10 = 1.5 mm; 2 ÷ 10 = 0.2 mm.
Draw each bar the same height, centred above the one below, and label each level.

Answer: bars 80 mm, 12 mm, 1.5 mm and 0.2 mm wide

A pyramid of biomass is almost always a pyramid shape, because biomass is lost at each level (see 18.9). A pyramid of numbers can be a strange shape: one oak tree can feed thousands of caterpillars. But one tree has a huge biomass, so the pyramid of biomass is still a pyramid.

Common mistakes

• Drawing the arrows in a food chain the wrong way. Arrows point from the organism eaten to the one that eats it.
• Calling plants “trophic level 0” or calling herbivores “level 1”. Producers are level 1.
• Drawing pyramid bars by eye. Use a scale and calculate each width, and keep all bar heights the same.
• Saying decomposers “eat” dead material. They secrete enzymes, then the small soluble molecules diffuse into their cells.

Revision questions

Try these, then check your answers below each one.

Question 1. Which trophic level contains the producers? (Choose from: Level 0, Level 2, Level 4, Level 1.)

Answer: Level 1

Working: Producers (plants and algae) are trophic level 1.

Question 2. True or false? Decomposers secrete enzymes onto dead material, and the small soluble molecules produced diffuse into their cells.

Answer: True

Working: True. This is how bacteria and fungi break down dead plants and animals.

Question 3. A pyramid of biomass uses the scale 1 cm = 50 g/m². How wide should the bar be for a biomass of 350 g/m²?

Answer: 7 cm

Working: 350 ÷ 50 = 7 cm.

Related lessons

• Previous lesson: 18.7 Maintaining Biodiversity
• Next lesson: 18.9 Biomass Transfers
• In this chapter: 18.1 The Human Population Explosion
• In this chapter: 18.2 Land and Water Pollution
• In this chapter: 18.3 Air Pollution

Continue in the full course

This lesson is section 18.8 (AGCBH1808) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.