ecological pyramids
Being able to present food chains and food webs as a diagram is a useful way of showing the feeding relationships and how energy is transferred.
However these diagrams do not provide any quantitative information and so this is where ecological pyramids come in:
An ecological pyramid is found in 3 key forms:
Pyramids of number
This is the number of each organism in a community. Usually the number of organisms at lower trophic levels is greater than those at higher trophic levels.
A pyramid of numbers is drawn with bars of lengths proportional to the number of the organism.
If we take our simple example of grass, rabbits and foxes in a food chain we will get this:
However these diagrams do not provide any quantitative information and so this is where ecological pyramids come in:
An ecological pyramid is found in 3 key forms:
- Pyramids of number
- Pyramids of biomass
- Pyramids of energy
Pyramids of number
This is the number of each organism in a community. Usually the number of organisms at lower trophic levels is greater than those at higher trophic levels.
A pyramid of numbers is drawn with bars of lengths proportional to the number of the organism.
If we take our simple example of grass, rabbits and foxes in a food chain we will get this:
There are considerably more grass plants than rabbits and there are more rabbits than foxes.
However there are drawbacks to this form of pyramid:
Pyramids of biomass
Instead of a pyramid of numbers, a better way of showing a food chain quantitatively is a pyramid of biomass.
The biomass of an organism is the total dry mass in a particular place at a given time.
Here is an example below:
However there are drawbacks to this form of pyramid:
- The size of the organism isn't taken into account - so a large tree is treated as equal to a tiny insect (so inverted pyramids are sometimes found).
- The number of individuals of a certain organism can be so great that the bar is impossible to show accurately.
Pyramids of biomass
Instead of a pyramid of numbers, a better way of showing a food chain quantitatively is a pyramid of biomass.
The biomass of an organism is the total dry mass in a particular place at a given time.
- It has to be dry as the varying amounts of water in the organisms makes the 'fresh mass' unreliable.
- A downside being that organisms must be killed.
- Biomass is measured in grams per square metre on land (e.g. grassland) and grams per cubic metre in a volume (e.g. sea).
Here is an example below:
Pyramids of energy
The best and most accurate way of demonstrating the energy flow through a food chain is a pyramid of energy.
However it is often difficult to obtain the data for this type of pyramid:
The best and most accurate way of demonstrating the energy flow through a food chain is a pyramid of energy.
However it is often difficult to obtain the data for this type of pyramid:
- It is usually measured over a given area (e.g. square metres)
- And is measured over a certain time period (e.g. a year)
- Ultimately measured in KJ per metre squared per year.