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Feeding relationships

Food chains, pyramids of numbers and pyramids of biomass may all be used to describe the feeding relationships existing in ecosystems, such as hedgerows.

The concepts of food chains and pyramids of numbers are usually grasped quickly by pupils, but pyramids of biomass may be more challenging.

Pupils are expected to have encountered pyramids of number and biomass before and to be able to draw pyramids.

Curriculum links

England and Wales - Life Processes and Living Things - 5c (Double) - how food chains may be described quantitatively using pyramids of biomass.
Northern Ireland - Living Organisms and Life Processes - Environment (b) (understand the components of food chains and food webs.

Resources

Student Sheet 1 (click here), graph paper, pens, pencils, ruler.

Age range

14 - 16

Differentiation

Lower ability pupils may wish to start with Activity 5, Ages 11 - 14. This acts as an introduction to pyramids of number and biomass. 

The activity

In this exercise, students construct their own pyramids of numbers and biomass for different hedgerow communities using the information given. They are encouraged to consider the similarities and differences between the two pyramids in relation to the different communities. Which type of pyramid do they think produces more accurate representation, and so gives a fairer comparison between ecosystems?

Extension activities

This exercise could lead into a discussion of the disadvantages as well as advantages of using pyramids of biomass. Disadvantages could include that collecting and weighing the biomass is very destructive to the environment. How practical are pyramids of biomass? Isn't it much easier merely to count the organisms concerned? The advantages of pyramids of biomass are that mass, rather than number is used, and where it is difficult to count numbers, eg numbers of grass plants, they can be easier to use. In addition, weight may sometimes be more critical than number, eg in a marine environment, where there are very heavy organisms, such as whales, eating a high number of very light organisms, such as plankton, and where pyrmids of number would give a distorted impression of the ecosystem. A display or poster illustrating the advantages and disadvantages of using pyramids of numbers and pyramids of biomass could be made.

Teachers' answers

(Answers to Student Sheet 1)

As you can see, pyramids of numbers are not always 'pyramid shaped'. If they are not, they are called 'inverted pyramids'. When do you get inverted pyramids of number?
Where there is a single producer (eg oak tree) or there are parasites in the food chain (e.g. sheep ticks).

From what you have drawn above, are pyramids of biomass usually 'pyramid shaped'?
Yes. Inverted pyramids of biomass may occur where biological material is being imported or exported from the ecosystem in question (eg coastal ecosystems where material is carried in and away with the tides).

Which type of pyramid do you think represents the 'quantity' or 'amount' of organisms within the food chain most accurately?
Pyramids of biomass may be considered as representing the productivity of each organism in the food chain most accurately.