- A pyramid of numbers shows how many organisms we are talking about at each level of a food chain.
- The width of the box indicates the number of organisms at that trophic level
- For example, consider the following food chain:
- Ask yourself the following questions:
- Is it likely that there would be more voles in an area than grass plants?
- How many voles might one barn owl need to eat per day? If it’s more than one, is it likely that there are more barn owls in an area than voles?
- So, a pyramid of numbers for this food chain would look like this:
- Despite the name (and the example above), a pyramid of numbers doesn’t always have to be pyramid-shaped, for example:
- This is because the size of the organism is also important – one large organism, like the oak tree in the pyramid above, contains enough energy to support many smaller organisms (the insects)
Rules to remember when drawing a pyramid of numbers:
- You cannot change the trophic level of the organisms – they must stay in the same order as in the food chain with producers on the bottom, followed by primary consumers, then secondary consumers, then tertiary consumers
- Generally, the larger an individual organism is, the less of them there are
- A pyramid of biomass shows how much mass the creatures at each level would have without including all the water that is in the organisms (their ‘dry mass’)
- Pyramids of biomass are ALWAYS pyramid-shaped, regardless of what the pyramid of numbers for that food chain looks like
- This is because the mass of organisms has to decrease as you go up a food chain – if we take our first food chain as an example, it’s impossible to have 10kg of grass feeding 50kg of voles feeding 100kg of barn owls
- Pyramids of biomass provide a much better idea of the quantity of the plant or animal material at each level of a food chain and therefore are a better way of representing interdependence within the food chain
Remember that pyramids of biomass are ALWAYS pyramid-shaped, so they are simple to draw, but pyramids of number can be any shape – so make sure you learn the rules for drawing a pyramid of numbers.