Food chains, trophic levels and biomass transfer – GCSE Combined Science Biology

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Every living thing needs energy. Plants get it from sunlight. Animals get it by eating plants or other animals. A food chain shows how that energy and food pass from one living thing to the next.

This matters because it explains why there are many more plants than lions. It also helps farmers to produce more food from the same land.

By the end you will be able to read a food chain and name each trophic level. You will explain a pyramid of biomass. You will calculate how efficiently biomass is passed on.

Food chains

A food chain shows who eats what. It always starts with a producer. A producer is an organism that makes its own food, usually a green plant or an alga, by photosynthesis.

Animals that eat other living things are consumers. A primary consumer eats the producer. A secondary consumer eats the primary consumer. A tertiary consumer eats the secondary consumer.

The arrows show where energy and biomass go next. Biomass is the mass of living material. Think of passing a football. The arrow shows the direction of the pass.

Trophic levels and food webs

Each stage of a food chain is called a trophic level. Producers are level 1. Primary consumers are level 2. Secondary consumers are level 3.

A herbivore eats only plants. A carnivore eats only animals. An omnivore eats both. A predator hunts other animals, and prey is the animal that is hunted. An apex predator has no natural predators.

Think of floors in a building. Producers are on the ground floor, level 1. In a food web many chains are linked, so a change in one species can affect others.

Pyramids of biomass

A pyramid of biomass shows the dry mass of the organisms at each trophic level. Producers are at the bottom. Each bar is drawn in proportion to the biomass.

The shape is usually a pyramid. This is because biomass falls at each level going up the chain. Not all of it is passed on, so fewer organisms can be supported.

Dry mass is used because it is more reliable. Fresh mass includes water, and the amount of water changes.

Why biomass is lost

Only about 10% of the biomass is passed on from one level to the next. The rest is lost in several ways.

Some parts are not eaten, such as bones and roots. Some food is not digested and leaves as faeces. A lot is lost in respiration, when organisms release energy to move and keep warm. Urine also carries some biomass away.

Think of passing a leaky bucket of water along a line. Some spills at every hand-over. So food chains are short, with at most four or five trophic levels.

Calculating transfer efficiency

Efficiency tells you how much biomass is passed on, as a percentage. Use this equation. Efficiency = biomass transferred to the next level ÷ biomass available at the previous level × 100.

Try producers with 2000 kg and primary consumers with 200 kg. The calculation is 200 ÷ 2000 × 100 = 10%.

The answer is a percentage, so it has no unit apart from %. Check that the number on top is the higher trophic level.

Farming and efficiency

Farmers can make food production more efficient by reducing the biomass lost. Keep animals indoors, with limited movement and a warm temperature. Then less energy is lost in respiration for movement and heat.

More of the food goes into growth, so the animals gain biomass faster. Think of how you use energy to keep warm on a cold day.

However, there are ethical concerns about the welfare of the animals. Eating plants instead of animals is also more efficient, because fewer trophic levels are involved.

Recap

A food chain starts with a producer. Arrows show the flow of energy and biomass.

Each stage is a trophic level. A pyramid of biomass shows dry mass at each level.

About 10% is passed on. The rest is lost by respiration, waste and uneaten parts.

Efficiency = transferred ÷ available × 100.