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Trophic levels and biomass transfer

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Summary

Trophic levels and biomass transfer

In a nutshell

Trophic levels are the different stages of a food chain or food web. Biomass is the dry mass of living material. Biomass can be measured at different trophic levels and this can be represented as a pyramid of biomass. A pyramid structure is formed because not all of the biomass will be transferred from one trophic level to the next. Some biomass is lost in waste and not all biomass can be transferred because not every part of an organism is edible. The efficiency of biomass transfer can be easily calculated.

Trophic levels

Definition

Trophic levels are the different stages of a food chain or food web. They are numbered after their location in the food chain.

Biology; Ecosystems and material cycles; KS4 Year 10; Trophic levels and biomass transfer

Trophic level	Organism type	Description	Example
1.	Producers	Producers make their own food through photosynthesis. To do this, they use energy from the sun.	Plants
2.	Primary consumers	Primary consumers are often herbivores or omnivores that eat the producers.	Grasshopper
3.	Secondary consumers	Secondary consumers eat the primary consumers. These are often omnivores or carnivores that eat meat.	Frog
4.	Tertiary consumers	Tertiary consumers eat the secondary consumers. These are often carnivores. Carnivores that have no predators are the highest trophic level and they are called apex predators.	Snake

Note: There may be more than four trophic levels but lots of energy is lost between each step in the food chain so there is usually only four or five trophic levels.

Decomposers

Decomposers, like certain bacteria and fungi, break down dead plant and animal matter by secreting (releasing) enzymes. The enzymes break the dead matter down into small food molecules. The organism then absorbs the food molecules. This process also releases nutrients into the soil which helps the producers grow.

Pyramids of biomass

Biomass is the dry mass of living material. Biomass can be measured at different trophic levels and this can be represented as a pyramid of biomass. Each bar on the pyramid represents the relative mass of living material.

At each progressive stage of a food chain, there will be less energy and less biomass.

1.	The producer is the dandelion. This will have the largest biomass.
2.	The primary consumer is the rabbit. Not all of the biomass is passed from the producer to the primary consumer.
3.	The secondary consumer is the fox. Not all of the biomass is passed from the primary consumer to the secondary consumer.
4.	The tertiary consumer is the flea. Not all of the biomass is passed from the secondary consumer to the tertiary consumer.

Biomass transfer

Producers, like plants and algae, use energy from the sun to make glucose during photosynthesis. Not all of the light energy the plants receive will be used for photosynthesis.

Some of the glucose that plants produce will be used by the plants themselves to make biological molecules like cellulose and starch. This is what makes up the plant's biomass.

Biomass is then transferred through the trophic levels of the food chain when one organism eats another. However, not all of the biomass from one trophic level will be passed to the next level, only around $10\%$ of it will.

There are various reasons for this loss of biomass.

Reason	Description
Organisms don't eat every part of their food.	Not all materials, like bone, are edible. So, not all of the energy will be transferred to the next trophic level.
Not everything is absorbed in an organism.	Some of the material will be egested (released) through faeces.
Some biomass is converted into other substances and lost through waste.	Glucose is an energy molecule transferred by organisms. The glucose is broken down in respiration to provide energy for processes such as protein synthesis. Respiration produces water and carbon dioxide as waste products so this decreases the biomass available to transfer to the next trophic level. Proteins in biomass can also be broken down to form urea. Urea is excreted in urine.

Calculating efficiency

The efficiency of biomass transfer can be calculated using the formula below.

$Efficiency=\dfrac{biomass\ transferred\ to\ the\ next\ level}{biomass\ available\ at\ the\ previous\ level}\times100$

Example

Using the table below, calculate the efficiency of biomass transfer from the grass to the grasshopper.

Trophic Level	Organism	Biomass
1.	Grass	$30kg$
2.	Grasshopper	$3.5kg$
3.	Frog	$0.3kg$

$\begin{aligned}Efficiency&=\dfrac{biomass\ transferred\ to\ the\ next\ level}{biomass\ available\ at\ the\ previous\ level}\times100\\\\&=\dfrac{3.5}{30}\times100\\\\&=\underline{11.7\%}\end{aligned}$

$\underline{Therefore,\ 11.7\%\ of\ the\ biomass\ was\ transferred\ to\ the\ grasshopper.}$

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