Food chains and webs are useful for understanding the feeding relationships in an ecosystem. However, pyramids of numbers provide even more information. They are a type of bar chart that show the population of organisms at each stage in a food chain.
For example, here is a pyramid of numbers diagram for the food chain: Grass → Rabbit → Fox.
In a pyramid of numbers diagram, the bars are drawn to scale and each level is represented as a separate bar. The width of each bar represents the population size of organisms at that stage of the food chain, and the height stays the same. The more organisms there are at that stage, the wider the bar will be.
In this case, the producer, which is the grass, is located at the bottom of the pyramid. As you can see, the width of the grass bar is relatively large, as a lot of grass is needed to support the population of rabbits.
The width of the rabbit bar is larger than the width of the fox bar, as more rabbits are needed to support the fox population. For example, one fox may eat many rabbits, and those rabbits eat many blades of grass.
The example above resembles an energy pyramid, with the number of organisms decreasing as you go up the food chain. This is due to a loss of energy.
A pyramid of numbers diagram can vary in shape depending on the population sizes of organisms at each stage of the food chain. When the producer, at the bottom of the pyramid, has a large population, it may not appear as a typical pyramid shape.
For example, below is a pyramid of numbers diagram for the food chain: Oak tree → Caterpillar → Blue Tit
The producer, which is the oak tree, goes at the bottom. As you can see, the width of the oak tree block is relatively small, despite being the producer. This is because there are many caterpillars eating the oak tree.
Also, the width of the blue tit block is smaller than the width of the caterpillar block. This shows that there is a large number of caterpillars supporting the population of blue-tit birds.