Life on Earth
3.
National 5 Biology
Energy in Ecosystems
3.1
Energy Transfer and Loss
3.2
Pyramid of energy
3.3
Pyramid of Numbers and Pyramid Biomass
Learning Intentions
At the end of this section you should be able to:
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
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Explain how energy is lost is a food web/ food chain
Calculate how much energy is lost or passed on between different levels
Describe and draw a pyramid of energy
Describe and draw a pyramid of numbers
Explain why some pyramids of numbers have irregular shapes
Describe and draw a pyramid of biomass
3. Energy in Ecosystems
3.1 Energy Transfer and Loss
Food chains and food webs show the direction that energy flows in an ecosystem.
Only 10% of the energy present at one trophic level in a food chain is passed on to the
next.
At each level in a food chain 90% of energy is lost as heat, movement or indigestible
materials. 10% of the energy at each level is used for growth and this can be passed to
the next level of the food chain.
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Life on Earth
National 5 Biology
How is energy lost?
When an organism eats food, some of it is converted to waste material and therefore
the energy content of the waste material is lost from the food chain.
During
respiration, some of the food is used to provide energy for movement and some is
released as heat. This energy is also lost from the food chain. Only energy used for
growth can be passed on to the next organism in the food chain.
3.2 Pyramid of Energy
The energy passed on from one stage to the next in a food chain can be represented as
a pyramid-shaped bar chart. The chemical energy present in all the organisms at each
stage in a food chain must be measured in kJ/m 2/year to obtain values to draw a
pyramid of energy.
For example, some plants in a garden may be consumed by snails which in turn are eaten
by thrushes.
The total energy content of the plants gives the bottom bar in the
pyramid, then the total energy content of the snails forms the next bar up and the
energy content of the thrushes forms the topmost bar.
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Life on Earth
National 5 Biology
Use graph paper to draw to scale a pyramid of energy from the following data for a
freshwater food chain.
Organisms in food chain
Energy content (kJ/m2/year)
algae
800
water fleas
100
sticklebacks
10
1. Describe what happens to the energy content of the organisms as you go up each
level of a pyramid of energy.
2. Name the process taking place in an organism which results in energy being passed on
to the subsequent members of a food chain.
3. Collect diagram 3.1 Energy Transfer and Loss and paste it into your notes. Name 3
processes which result in energy being lost from a food chain.
4. What percentage of the energy available from the algae is actually passed onto the
sticklebacks?
5. Insert and complete the Energy Loss in Food Chains diagram from your pack into
your notes.
6. Why does a pyramid of energy always take the form of a pyramid?
3.3 Pyramids of Numbers and Pyramids of Biomass
As it is very difficult to measure the energy value at each stage of a food chain
another method to show the loss of energy at each stage is to count the number of
organisms at each stage in the chain. The energy loss usually means that the number of
organisms that can be supported at each stage along the food chain is fewer and fewer.
This can be represented by a diagram called a pyramid of numbers.
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Life on Earth
National 5 Biology
Pyramid of numbers
A typical pyramid of numbers might look like this:
Consider the following food chain:
Copy the food chain and draw a pyramid of numbers for it.
In some cases a pyramid of numbers can become distorted. Take the simple food chain
below.
oak tree  caterpillar  shrews  owl
The pyramid of numbers for this food chain would look like the one below:
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Life on Earth
National 5 Biology
With the people at your table, try to come up with an explanation for the shape of this
pyramid of numbers.
One oak tree may be home to thousands of caterpillars – which provide the food for
dozens of shrews, and in turn are preyed upon by a few owls.
Normally the number of producers would be greatest, but in this case an irregular
pyramid of numbers is formed.
1. Explain the meaning of the term pyramid of numbers.
2. Explain why the numbers decrease as you go up the levels in the pyramid for the
grassland food chain.
4. Explain the unusual shape of the pyramid of numbers for the woodland food chain.
5. Explain why it is possible for just one organism to support the entire food chain in
the woodland example.
Extension
In the last section you learned about parasites. What do you think a parasitic pyramid
of numbers would like? Using the resources available to you to find out about and draw
a parasitic pyramid of numbers.
Pyramid of biomass
A pyramid of biomass is a better way to represent a food chain – it shows the total
mass of the organisms at each step in a food chain, and rarely becomes distorted.
Pyramids of biomass are very similar to pyramids of energy because the biomass
produced at each stage is dependent on the energy available.
In the food chain above the mass of the oak tree (producer) is much more than the
mass of the entire caterpillar population (primary consumer), and so the pyramid of
biomass would look like the one shown below.
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Life on Earth
National 5 Biology
Use graph paper to draw to scale two pyramids of numbers from the following data for
different food chains.
GRASSLAND
WOODLAND
Organisms in food
Number of
Organisms in food
Number of
chain
organisms
chain
organisms
grasses
200
oak tree
1
slugs
25
caterpillars
100
hedgehogs
5
small birds
6
1. Explain the meaning of the term pyramid of biomass.
2. Describe what happens as you move up the pyramid to the:

size of the organisms

mass of the organisms.
3. Explain why the biomass is different at each level in a pyramid.
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Life on Earth
National 5 Biology
4. Explain why a pyramid of biomass for a food chain may not have the same shape as
its pyramid of numbers.
5. Insert and complete the Pyramid of Biomass diagram from your pack into your
notes.
Check your understanding
Below are some typical exam style questions. Check your understanding by
completing these. Compare your answers with the people at your table.
1. The diagram opposite is a pyramid of energy
for a food chain. There is least energy at level X in the
pyramid because:
a. Energy is stored in each level and not passed on
b. Energy is lost at each level
c. The energy is concentrated in fewer organisms
d. Organisms at level X are very small
2. The diagram opposite shows a pyramid of
Biomass. W represents the:
a. Producers
b. Prey
c. Predators
d. Herbivores
3. The pyramid of energy opposite represents
Energy flow in the following food chain:
Assuming 10% of energy is be passed on at each
Level, how many kJ of energy present in the rose is
passed on to the blackbird?
a. 1000
b. 100
c. 10
d. 1
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Life on Earth
National 5 Biology
4. The diagram below shows the pyramid of numbers for the food chain of a
coastal bay.
a. Present this information as a food chain
b. Use the diagram above to explain what is meant by a pyramid of numbers
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Life on Earth
National 5 Biology
Self Evaluation - How well did I do?
How well can you describe the key words/phrases below? Take turns at
explaining what they mean to the person sitting next to you, and then let them
explain them to you.
Producer, consumer, tertiary, biomass, numbers, energy transfer, irregular,
trophic level, consumed, food chain, distorted, growth, indigestible
Use the success criteria sheet to reflect on and evaluate your learning from
this section.
Think carefully and answer the following questions:
1. What skills have you developed over the past few lessons? How do you
know and how could you prove it to someone?
2. What areas have you found difficult? What strategies are you going to
use to overcome these problems?
Discuss this with your teacher and set yourself a learning target – this is what
you will do to improve your knowledge and skills. Update Didbook with a
comment about what you have done well, what you want to improve at doing and
how you will do this.
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