Trophic levels in an ecosystem

Biology · Ecology. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A trophic level is a feeding position in a food chain. Level 1 is the producers. Level 2 is the herbivores that eat them (primary consumers). Level 3 is the carnivores that eat herbivores (secondary consumers), and level 4 is the carnivores that eat other carnivores. A carnivore with no predators is an apex predator.

Biomass is the mass of living material. A pyramid of biomass shows the biomass at each level as a bar, with the producers at the bottom. Each bar is smaller than the one below, because only about 10% of the biomass is passed on at each step.

Decomposers break down dead matter by releasing enzymes onto it.

Rules to remember

Efficiency of transfer = (biomass passed to the higher level ÷ biomass at the lower level) × 100.
Producers transfer about 1% of the incident light energy into biomass.
About 10% of the biomass at one level is transferred to the level above.
Biomass is lost because: not all the food is eaten or absorbed (some is egested as faeces); much glucose is used in respiration, giving waste carbon dioxide and water; water and urea are lost in urine.
Decomposers (bacteria and fungi) secrete enzymes; the small soluble food molecules then diffuse into the microorganism.
In a pyramid of biomass, trophic level 1 is always the bottom bar and bar widths are drawn to scale.

Common mistake

In efficiency calculations students divide the wrong way round. The smaller biomass (the higher level) always goes on top, so the answer is less than 100%.

Worked example

The producers in a lake have a biomass of 6000 kg. 10% is transferred to the primary consumers, and 15% of the biomass of the primary consumers is transferred to the secondary consumers. What is the biomass of the secondary consumers?

Primary consumers = 10% of 6000 = 0.10 × 6000 = 600 kg.
Secondary consumers = 15% of 600 = 0.15 × 600 = 90 kg.

Answer: 90 kg.

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