A pyramid of numbers shows the number of individual organisms at each trophic level in a food chain, in a given area at one time. The width of each bar is proportional to the number of organisms.
1. Sample the habitat, for example with quadrats, nets and traps, and count the individuals at each trophic level.
2. Scale the counts up to the whole area if needed.
3. Draw a bar for each level, with producers at the bottom and widths to scale.Worked example: a survey of a meadow finds 10 000 grass plants, 800 grasshoppers, 50 frogs and 2 herons. The number of grasshoppers supporting each frog is:800 ÷ 50 = 16 grasshoppers per frog In most food chains the numbers fall at each trophic level, giving a typical pyramid shape. There are two reasons:
- Energy is lost at each transfer, mainly as heat from respiration, so less energy is available to support organisms at higher levels.
- Predators are usually larger than their prey, so each needs to eat many prey organisms, and fewer predators can be supported.Advantages of pyramids of numbers:
- They are quick and simple to produce, because individuals only need to be counted.
- Organisms do not need to be killed, unlike when measuring biomass or energy.However, every organism counts as one regardless of its size, so a pyramid of numbers is not always pyramid-shaped. Inverted pyramids of numbers and the other limitations are covered in a later lesson.
1. Sample the habitat, for example with quadrats, nets and traps, and count the individuals at each trophic level.
2. Scale the counts up to the whole area if needed.
3. Draw a bar for each level, with producers at the bottom and widths to scale.Worked example: a survey of a meadow finds 10 000 grass plants, 800 grasshoppers, 50 frogs and 2 herons. The number of grasshoppers supporting each frog is:
- Energy is lost at each transfer, mainly as heat from respiration, so less energy is available to support organisms at higher levels.
- Predators are usually larger than their prey, so each needs to eat many prey organisms, and fewer predators can be supported.Advantages of pyramids of numbers:
- They are quick and simple to produce, because individuals only need to be counted.
- Organisms do not need to be killed, unlike when measuring biomass or energy.However, every organism counts as one regardless of its size, so a pyramid of numbers is not always pyramid-shaped. Inverted pyramids of numbers and the other limitations are covered in a later lesson.