Counting organisms and measuring biomass answer different questions. A count tells you how many individuals there are, and biomass tells you how much living material there is at each trophic level. Both appear in ecology and energy flow, and questions often ask you to explain why the two diagrams differ.
How do numbers and biomass differ?
A pyramid of numbers draws a bar for each trophic level whose width shows the number of individuals. It does not care whether those individuals are tiny or huge, so it can look odd. One oak tree can support thousands of caterpillars.
A pyramid of biomass uses the total mass of living material in each level, usually dry mass. Because energy is lost between levels, each level can only support less mass than the one below, so biomass pyramids are usually narrower at each higher level.
Total biomass = number of organisms × mean mass of one organism. Keep the units consistent, and remember 1 kg = 1 000 g.
Worked example
A woodland food chain (invented data for practice): oak tree → caterpillar → insect-eating bird.
| Level | Number of organisms | Mean dry mass of one | Total biomass |
|---|---|---|---|
| Oak tree | 1 | 500 kg | ? |
| Caterpillar | 40 000 | 0.2 g | ? |
| Bird | 30 | 20 g | ? |
Complete the table, then say what the numbers and biomass pyramids look like.
Step 1, oak: 1 × 500 kg = 500 kg.
Step 2, caterpillars: 40 000 × 0.2 g = 8 000 g. Convert: 8 000 ÷ 1 000 = 8 kg.
Step 3, birds: 30 × 20 g = 600 g = 0.6 kg.
Step 4, compare: numbers go 1, 40 000, 30, so the middle bar is the widest and it is not a pyramid. Biomass goes 500 kg, 8 kg, 0.6 kg, which narrows at each step and is a pyramid.
Step 5, explain: the count ignores size. The tree is huge, the caterpillars are small, and mass falls at each level because energy is lost as heat and in waste.
Check step 2 again: 40 000 × 0.2 = 40 000 ÷ 5 = 8 000. Check step 3: 30 × 20 = 600.
The mistake to watch for
Students often treat the number of organisms as the same as the amount of living matter.
Mistaken answer: “There are 40 000 caterpillars and only 1 oak tree, so the caterpillars have more biomass.”
The student compared counts and forgot the mass of each organism.
The correction is to multiply by the mass of one organism before comparing. The oak tree has 500 kg, the caterpillars only 8 kg. If the question says “biomass”, do the multiplication first.
Check yourself
1. A pond sample (invented): 50 000 algal cells of mean dry mass 0.00004 g; 400 water fleas of 0.002 g each; 2 fish of 0.15 g each. Find the total biomass of each level.
Show answer
Algae: 50 000 × 0.00004 = 2 g. Water fleas: 400 × 0.002 = 0.8 g. Fish: 2 × 0.15 = 0.3 g.
2. Using question 1, does the pond have a pyramid of biomass, and does it have a pyramid of numbers?
Show answer
Biomass is 2 g, 0.8 g, 0.3 g, so it is a pyramid (narrowing upwards). Numbers are 50 000, 400, 2, which also narrows, so it is a pyramid of numbers too.
3. Why is dry mass used rather than fresh mass when measuring biomass?
Show answer
Fresh mass includes water, and the water content varies between organisms and with conditions such as recent rain. Dry mass measures the actual living material, so comparisons are fairer.
Where this leads next
This lesson pulls together energy loss from the energy lessons and scaling from sampling estimates. Test the whole module on the mixed practice set.
If you want feedback on how you explain the difference in your own words, online one-to-one Biology tuition is designed for that.