1. National 5 Biology Section 17 – Energy in Ecosystems
Unit 3 – Life on Earth
Section 17 – Energy in Ecosystems

2. Section 3. 17 – Energy in Ecosystems Pupils should be able to:
By the end of this section you should be able to:
Describe how energy transfers from one level to another in a food chain.
Describe how energy is lost as heat, movement and undigested materials.
State that a small amount is used for growth and is therefore available at the next level in a food chain.
Describe and define a pyramid of energy.
Describe and define a pyramid of numbers.

3. What is a Food Chain? Corn Locust Lizard Snake Primary Consumer
Food chains and food webs show the relationship between the organisms in an ecosystem.
Corn
Locust
Lizard
Snake
Primary
Consumer
Secondary
Consumer
Tertiary
Consumer
Producer

4. How is Energy Transferred in a Food Chain?
Food chains show the passage of energy from a producer through a series of organisms.
Pondweed
Tadpole
Perch
At the start of every food chain there is a green plant. Green plants use sunlight to produce food energy by photosynthesis. Because they produce their own food they are known as producers.
A consumer is an organism which eats another organism.

5. A consumer which eats the producers are known as primary consumers.
A consumer which eats the primary consumer is known as the secondary consumers.
A consumer which eats the secondary consumer is known as the tertiary consumers and so on……..
The arrows in a food chain show the direction of the energy flow. e.g. Plant Rabbit
A food chain only follows just one path as animals find food. A food web shows the many different paths plants and animals are connected.

6. Pike
Perch
Water Beetle
Stickleback
Tadpole
Snail
Caddis Fly Larva
Water Flea
Pond weed
Algae

7. How is Energy Transferred in a Food Chain?
Energy is transferred in the form of chemical energy in food from plants to animals and then to other animals. Every time an organism eats another organism energy is transferred.
At each stage in a food chain only about 10% of the energy is passed on.
Lettuce Rabbit Fox
10% %
Units of energy
present

8. How is Energy Lost in a Food Chain?
Oak leaves
Caterpillar
Shrew
Badger
When the caterpillar eats the leaves it does so to get energy.
It uses this energy for:-
Movement
Heat
Undigested Material - There are parts of the leaves which are indigestible and pass straight through as faeces

9. This energy loss occurs at each step of a food chain
These uses of energy can be divided into 2 groups:-
Used (not in caterpillar)
Unused ( still in caterpillar)
Movement
Heat
Chemical reactions
Faeces
Growth and repair
90%
10%
Therefore when a caterpillar eats a leaf 90% of the energy it gets is used up (lost) and only 10% is available for the shrew.
This energy loss occurs at each step of a food chain
Oak leaves
Caterpillar
Shrew
Badger
100%
10% 1%
0.1%
Therefore to survive the caterpillar has to eat lots of leaves and the shrew has to eat lots of caterpillar and so on along the food chain

10. Energy captured by wheat
sunlight
Energy lost through cellular activities
Energy captured by wheat
Energy built into wheat cells
Energy lost by movement/heat
Energy lost in faeces/ undigested food/ decomposition
Energy captured by mouse when it eats the wheat
Energy built into mouse tissue

11. WHEAT HUMAN
WHEAT CATTLE HUMAN 1. 2.
In which food chain is less energy lost?
Which food chain is most energy efficient?
Why?
FOOD CHAIN 1
FOOD CHAIN 1
Less energy is being lost because there are fewer links in the food chain.

12. What is a Pyramid of Energy?
Food chains and food webs show the paths which _________ flows in an ecosystem. 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/m2/year to obtain values to draw a _______________. Only energy used for growth can be passed on to the next organism in the food chain.
energy
pyramid of energy

13. Pyramids of Energy
A pyramid of energy represents the quantity of energy (kJ/m2/year) at each stage in a food chain.
Secondary consumer e.g. thrushes
Primary consumer e.g. snails
Producer e.g. plants
Length of bar represents the amount of energy present
A pyramid of energy always takes the form of a true pyramid since only a proportion of energy is transferred from one level to the next. The rest is lost to the surrounding environment.

15. What is a Pyramid of Numbers?
A pyramid of number represents the number of organisms present at each level in a food chain.
Consider the following food chain
Lettuce  Snail  Thrush  Sparrowhawk
The producer, lettuce, is most numerous followed by the primary consumer, snail, followed by the secondary consumer etc. this numerical relationship is known as a pyramid of number.
The producer is usually the most numerous
The final consumer is usually least numerous
As we go up the pyramid, energy is lost.

16. Pyramid of Numbers

17. Pyramid of Numbers
A pyramid of numbers can be used to show the number of organisms at each stage of a food chain
Small numbers of large organisms
1 fox
30 Rabbits
Large numbers of small organisms
100,000 blades of grass

18. Pyramid of Numbers
SECONDARY CONSUMER
PRIMARY CONSUMER
PRODUCER
A pyramid of numbers indicates the relative number of organisms at each stage in a food chain.
Producers are found to be the most numerous, followed by primary consumers, with the final consumer being the least numerous.

19. Pyramid of Numbers What happens to
Green Plants
Slug
Frog
Fox
What happens to
The size of the organisms as you go along the food chain?
The no. of organisms as you go along the food chain?
The organisms get bigger and fewer as you go along a food chain

20. What happens to the energy content of the organisms as you go up each level of a pyramid of energy? _________________________________ Name the process taking place in an organism which results in energy being passed on to the subsequent members of a feed chain ________________________________ Name 3 processes which result in energy being lost from a food chain ___________________
It decreases
Growth and repair
Heat loss
Movement
Waste material in faeces

21. Sometimes a pyramid of numbers is not pyramidal in shape. e.g.
Rose Bush
Green fly
Ladybird
BlackBird
BLACKBIRD
LADYBIRD
GREEN FLY
ROSE BUSH
This pyramid is not accurate, “it would fall down” . A more accurate way to explain this food chain would be to weigh all the organisms at each level.

22. oak tree caterpillar shrew owl 1 200 5 1 owl shrew caterpillar

24. Organisms in food chain
Energy content (kJ/m2/year)
algae
water fleas
sticklebacks
800
100 10
Use graph paper to draw to scale a pyramid of energy from the following data for a freshwater food chain.