Three pyramids, three different things being measured, and three different answers to the question “can it be upside down?”. Get those straight and this becomes one of the most reliable sources of marks in the whole topic.
The width of each bar shows how many organisms are at that trophic level. For a chain like grass → vole → owl the shape is a normal pyramid: lots of grass plants, some voles, one owl. Numbers usually fall as you go up, because there is less energy available at each level.
But the name is misleading. Consider oak tree → insects → woodpecker. A single oak supports huge numbers of insects, so the bottom bar is narrow and the pyramid comes out inverted.
| Limitations of pyramids of numbers |
|---|
| They ignore the size of individuals — one oak tree and one grass plant count the same |
| Parasites distort them badly, since one host can support thousands of parasites |
| They say nothing about the energy or biomass at each level |
These show the mass of organisms at each level, measured as dry mass so that water content does not distort the comparison. They are usually pyramid-shaped regardless of what the numbers pyramid looked like, because biomass generally decreases up the chain.
They can still be inverted, though, and the reason is worth understanding properly. In some marine ecosystems the standing crop of phytoplankton is smaller than the mass of the zooplankton feeding on them. That sounds impossible until you remember what a pyramid of biomass actually is: a snapshot in time. Phytoplankton reproduce extremely quickly and are eaten almost as fast as they grow, so at any moment there is not much of them — but their productivity is very high.
| Limitations of pyramids of biomass |
|---|
| They are a snapshot (standing crop) and do not show the rate of production or turnover |
| They can appear inverted in aquatic systems where producers have low standing biomass but high productivity |
| Measuring dry biomass requires organisms to be killed, raising practical and ethical concerns |
| They do not directly show energy flow |
Also called pyramids of productivity. These show the flow of energy through each trophic level — the rate at which energy is being generated, per unit area per unit time. Units are typically kJ m−2 yr−1.
They are always widest at the base and always narrow going up. This is not a coincidence or a convention — it follows directly from the second law of thermodynamics. Energy is lost at every transfer through incomplete consumption, incomplete digestion, heat from respiration and excretion of metabolic waste, so the level above can never receive more than the level below produced.
| Limitations of pyramids of energy |
|---|
| The most accurate representation, but the hardest and most time-consuming to construct |
| They need detailed data on intake, assimilation and loss at every trophic level |
| Data collection usually takes a full year to cover seasonal variation |
A food chain has these biomass values: clover 80 kg, snail 30 kg, thrush 10 kg, sparrowhawk 2 kg. Describe the pyramid you would draw and state the percentage transferred from clover to snail.
Explain why a pyramid of biomass for a marine ecosystem can be inverted, but a pyramid of energy for the same ecosystem cannot.
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