7.16 Trophic levels and pyramids of biomass

Trophic levels

Trophic levels are numbered according to the position of the organism in a food chain:

• Level 1: producers, such as plants and algae, which make their own food by photosynthesis.
• Level 2: primary consumers, herbivores that eat plants.
• Level 3: secondary consumers, carnivores that eat herbivores.
• Level 4: tertiary consumers, carnivores that eat other carnivores. Apex predators are carnivores with no predators.

Decomposers (bacteria and fungi) break down dead plant and animal matter by secreting enzymes into the environment. Small soluble molecules then diffuse into the microorganisms.

Pyramids of biomass

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

• The producers are always at the bottom.
• The biomass at each level is less than the level below, so the diagram is a pyramid shape.
• A pyramid of numbers can be an odd shape (one oak tree feeds thousands of caterpillars), but a pyramid of biomass is almost always a true pyramid.

Drawing one

1. Use a scale, such as 1 cm = 10 kg.
2. Draw the producers as a bar at the bottom, then each level above, centred.
3. Label each bar with the trophic level or organism.