Pyramids of number
Ecological pyramids are made by representing the number of organisms, the biomass or the energy at each trophic level of an ecosystem as a graph-like diagram. The base of the pyramid shows the producers, and the tiers above show the consumers of each level.
There are three main kinds: pyramids of number, pyramids of biomass and pyramids of energy.
Pyramid of number
A graph showing the number of organisms at each trophic level is a pyramid of number. It gives the number of organisms living in a square metre (1 m2).
The number at one trophic level can be smaller or larger than the number at the level above it, so there are upright pyramids of number and inverted ones.
- Tertiary consumers (hawk): 3
- Secondary consumers (robin): 30
- Primary consumers (grasshopper): 200
- Producers (plants): 1000
Figure 15.10 — an upright pyramid of number.
- Secondary consumers (insect parasites): 23
- Primary consumers (coconut beetle): 8
- Producers (coconut palm): 1
Figure 15.11 — an inverted pyramid of number.
The bars are drawn on a log scale, so every level stays visible; the numbers beside them are the counts.
Why a pyramid can turn upside down
A pyramid of number counts organisms without asking how big they are. One coconut palm is a single organism, but it is huge and feeds many beetles, and each beetle carries many tiny parasites, so the counts rise going up and the pyramid inverts. In a grassland, one grass plant is small, so it takes many to feed a grasshopper, and the pyramid is upright. That weakness of counting is why the next page weighs the organisms instead.
Counting the organisms
A pyramid of number shows how many organisms live at each trophic level per m2; it can be upright or inverted.