How Is Biomass Transferred Between Trophic Levels?

Biomass is transferred between trophic levels when one organism eats another. Only part of the biomass eaten becomes new body tissue, so only about $10\%$ is typically transferred to the next level.

How does the transfer work?

Biomass is the mass of living material in an organism. A trophic level is an organism’s position in a food chain. Plants and algae are producers: they make molecules by photosynthesis. A food chain begins with producers at trophic level one. Herbivores that eat them are primary consumers at level two; animals that eat the herbivores are secondary consumers at level three.

When a herbivore eats a plant, it takes in some of the plant’s biomass. If a carnivore eats that herbivore, some biomass passes to the next trophic level. But eating biomass is not the same as adding all of it to your body. Only material that is absorbed and used to build new tissue contributes to biomass available to the next consumer. Material that leaves the body cannot be passed on as body tissue when the organism is eaten.

Why is efficiency low?

Some material eaten is not absorbed and leaves the body as faeces. Some absorbed material is lost as waste, including water and urea in urine. Organisms also use large amounts of glucose in respiration, producing carbon dioxide and water rather than adding that glucose to new tissue. As a result, much less biomass is available to the organism at the next trophic level.

For example, if herbivores eat plants containing $1000\mathrm{g}$ of biomass and gain $100\mathrm{g}$ of new biomass, the transfer efficiency is

$$Efficiency=\frac{100}{1000}\times 100\%=10\%.$$

The $10\%$ figure is approximate, not a fixed rule for every food chain. In an exam, a common mistake is to say that biomass is lost as heat. Respiration transfers energy to the surroundings, but heat is not biomass. Explain the loss of material through faeces, waste products and respiration.