GCSE · Topic 7 Ecology

AQA GCSE Biology: Trophic Levels and Biomass

A Triple award GCSE Biology revision guide to trophic levels and biomass: the trophic levels, pyramids of biomass, and why so much biomass is lost between levels.

This whole guide is Triple award only. Every organism occupies a position in a food chain, and biomass passes from one level to the next, but most of it is lost along the way. This guide covers the trophic levels, pyramids of biomass, and why biomass transfer is so inefficient. The key figure to remember is that only about 10% passes to the next level.

Trophic levels

A trophic level is the position a species occupies in a food chain. There are four levels.

Level 1: producers, plants or algae that make their own food.
Level 2: primary consumers, herbivores that eat plants or algae.
Level 3: secondary consumers, carnivores that eat herbivores.
Level 4: tertiary consumers, carnivores that eat other carnivores.

A carnivore with no predators of its own is an apex predator. Decomposers break down dead plant and animal matter by secreting enzymes into the environment, and the small soluble molecules produced then diffuse back into the microorganism.

Pyramids of biomass

A pyramid of biomass shows the relative amount of biomass in each trophic level, with level 1 (the producers) at the bottom. Biomass is the mass of living tissue of the organisms in an area, and the pyramid gets narrower towards the top.

Four horizontal bars stacked into a pyramid, each one narrower than the one below it. The widest bar at the bottom is level one, the producers, with a biomass of 10 000 arbitrary units. Above it level two, the primary consumers, at 1000. Above that level three, the secondary consumers, at 100. The narrowest bar at the top is level four, the tertiary consumers, at 10. Each level holds about a tenth of the biomass of the level below it

The producers are at the bottom and the bars get narrower going up, which is the shape a pyramid of biomass always has. The numbers beside it are there to be used: each level holds about a tenth of the biomass of the one below, so 10 000 becomes 1000, then 100, then 10.

Those are the figures an efficiency calculation uses. From level 1 to level 2 it is 1000 ÷ 10 000 × 100 = 10%, and the same arithmetic works for any pair of levels a question gives you.

One honest word about the drawing: the bars are not to scale. A bar a tenth as wide as the one below it would make the top two invisible at this size, which is itself the reason a food chain rarely has more than four or five levels. There is not enough biomass left to support another one.

The transfer of biomass

Only a small fraction of biomass passes from one level to the next. Producers transfer only about 1% of the light energy that falls on them into biomass, and only about 10% of the biomass at one level is passed to the level above.

Biomass is lost between levels because:

not all ingested material is absorbed, so some is egested as faeces (and not all parts are even eaten, such as bones);
some absorbed material is lost as waste, for example carbon dioxide from respiration and urea in urine;
large amounts of glucose are used in respiration to release energy.

The efficiency of transfer can be calculated as the biomass transferred to the next level ÷ the biomass available at the previous level, × 100. Because transfers are so inefficient, there are usually only a few trophic levels, with fewer organisms at the higher levels.

How this fits together

Understanding how inefficiently biomass transfers explains why it is more efficient to eat plants than animals, which links to feeding a growing population, the subject of the food security and production guide.