Pyramid of numbers
Like food chains and food webs, 'pyramids of numbers' are used to represent the feeding relationships between plants and animals in ecosystems.
They are, in effect, another way of describing the connections within ecosystems, which is a challenging task: they can be complex.
England and Wales - Life Processes and Living Things - 5e (how food chains can be quantified using pyramids of numbers)
Scotland - Understanding Living Things and the Processes of Life - Interactions of living things with their environment (food webs and food pyramids)
Northern Ireland - Living Organisms and Life Processes - Environment (e) (understand the components of food chains and food webs)
Student sheet 1, graph paper, pens, pencils, rulers
11 - 14.
More able groups could be given this sheet as a homework activity, having covered the basics of pyramids of number in class. Less able groups could use it as a whole class exercise.
In this exercise, students use the information given to construct their own pyramids of numbers for different hedgerow communities. The idea that pyramids of numbers are not always pyramid-shaped is also introduced. Students are encouraged to think about whether pyramids of numbers are accurate ways of representing and comparing ecosystems. Perhaps there are better ways: you could introduce pyramids of biomass at this point.
This activity could lead onto a discussion of where you might find an inverted pyramid of numbers. You could also discuss the advantages and disadvantages of using pyramids of biomass rather than pyramids of numbers. The disadvantage of the pyramid of biomass is that collecting and weighing can be difficult, and may be destructive to the environment. The advantage of a biomass pyramid is that it can be used where it is difficult to count numbers of things, for example, large numbers of grass plants.
Answers to Student Sheet 1
Grass plants = 100 plants
Aphids = 50
Ladybirds = 5
Are pyramids of numbers for all ecosystems pyramid-shaped? No. When they are not this shape, they are called 'inverted pyramids'. Inverted pyramids may occur where there is a single producer (eg a tree or bush), or one of the animals in the chain is a parasite (eg grass plants -> sheep -> sheep ticks).
How else could you measure the 'amount' of animals or plants at each trophic level? What could you do instead of counting numbers? You could measure the mass of organisms at each level and construct 'pyramids of biomass'.