Ecological pyramids: pyramids of number and biomass
Ecological pyramids show numbers, biomass or energy at each trophic level. Pyramids of number and biomass are usually upright but can be inverted, as in a tree or the sea.
What is an ecological pyramid?
A pyramid has a broad base and narrows to an apex. The feeding relationships in an ecosystem give a similar shape when we show the number, biomass or energy at each trophic level. The base is the producers (first trophic level), and the apex is the top consumers. Three pyramids are usually studied: the pyramid of number, the pyramid of biomass and the pyramid of energy. Any calculation must include all the organisms at a trophic level; a few individuals give a false picture.
Pyramid of number
| Ecosystem | Shape | Why |
|---|---|---|
| Grassland | Upright | Huge numbers of grass plants support fewer herbivores, still fewer carnivores and very few top carnivores. Your textbook's figure: primary producers 5,842,000; primary consumers 708,000; secondary consumers 354,000; tertiary consumers 3. So nearly 6 million plants supported only 3 top carnivores. |
| One large tree with insects and birds | Inverted at the base | One tree feeds thousands of insects; the insects feed fewer small birds, which feed a few large birds. Counting ignores the huge size of the tree. |
Pyramid of biomass
| Ecosystem | Shape | Why |
|---|---|---|
| Forest or grassland | Upright | Biomass falls sharply at higher trophic levels. Your textbook's figure (dry weight, kg m⁻²): producers 809, primary consumers 37, secondary consumers 11, tertiary consumers 1.5. |
| Sea | Inverted | A small standing crop of phytoplankton supports a larger standing crop of zooplankton and fish (your textbook's figure: phytoplankton 4, zooplankton 21). Phytoplankton are tiny but reproduce very fast, so a small biomass present at any moment can feed a larger biomass of consumers. |
How to draw pyramids of number and biomass
- Draw stacked horizontal bars. The bottom bar is the primary producers (PP); above it are the primary consumers (PC), secondary consumers (SC) and top carnivores (TC).
- Make each bar's length show the number or biomass at that level.
- Upright pyramid of number (grassland): the longest bar at the base (grasses), each bar shorter going up.
- Inverted pyramid of number (tree): a short bar at the base (one tree), a long bar above it (insects), then shorter bars for small birds and large birds.
- Inverted pyramid of biomass (sea): a short base bar (phytoplankton) and a longer bar above it (zooplankton and fish).
- Label each bar with its trophic level, and write a title under each pyramid.
In a mango orchard, one tree can carry thousands of insects that feed many birds, so the pyramid of number is inverted. In the sea, a small weight of fast-growing phytoplankton feeds a larger weight of fish, the food base of India's coastal fisheries.
2 marks: “Why is the pyramid of biomass in the sea generally inverted?” The biomass of fishes and zooplankton far exceeds the small standing crop of phytoplankton at any moment. Phytoplankton reproduce fast, so a small biomass can support a larger biomass of consumers.
3 marks: “Draw pyramids of number for a grassland and for a large tree with insects and birds. Explain why they differ.” Grassland: upright, many grasses to very few top carnivores. Tree: inverted at the base, since one tree supports many insects, which support fewer birds. Numbers do not reflect the size of individuals.
One tree, many bugs: number flips. Small plankton, big fish: biomass flips. Energy never flips.