Ecological pyramids: number, biomass and energy ·… | StemPy Lab
Course 30.3·Lesson 2 of 3
Ecological pyramids: number, biomass and energy
A pyramid is a picture of the trophic levels with the producers at the base. This lesson draws the pyramids of number, biomass and energy. It shows where they are upright and where they are inverted. It explains why the pyramid of energy is never inverted. Then it lists their limits.
about 30 min1 simulation1 practice question
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Drawing the trophic levels as a pyramid
Take the levels of an ecosystem and draw each as a bar. Put the producers at the bottom, the herbivores above them and so on up to the top consumer. The picture is an ecological pyramid. The base is the first trophic level, the producers. The apex is the top consumer.
You can measure each level in three ways, so there are three pyramids.
The pyramid of number: how many organisms there are.
The pyramid of biomass: the dry mass of the organisms.
The pyramid of energy: the energy that flows through each level in a unit time.
Upright pyramids
In most ecosystems, the pyramids of number and biomass are upright. The base is the widest, and the bars get shorter towards the top. A grassland is the usual example. A huge number of grass plants support a smaller number of herbivores, and they support fewer carnivores.
Three upright pyramids side by side. On the left, a pyramid of number for a grassland, with the number of organisms in 1000 square metres, from the base up: 1200000 grasses, 150000 herbivores, 9000 small carnivores and 40 top carnivores. In the middle, a pyramid of biomass in grams of dry matter for each square metre: 1200, 80, 12 and 1.5. On the right, a pyramid of energy in kJ for each square metre in a year: 10000, 1000, 100 and 10. Every bar is shorter than the one below it. The numbers are model data.
When the pyramid of number is inverted
Count the organisms of a single large tree. There is one tree. It can feed many thousands of insects. A few hundred birds eat the insects, and a few hawks eat the birds. The base, with one tree, is the narrowest bar. The pyramid of number is then inverted at the base, and it is narrow again at the top.
When the pyramid of biomass is inverted
Now think of the sea. The phytoplankton are tiny. At any one time, their biomass is small. But they grow and divide very fast, and the zooplankton eat them very fast. So the biomass of the zooplankton and the fishes can be more than the biomass of the phytoplankton at the same moment. NCERT says that the pyramid of biomass in the sea is generally inverted, because the biomass of the fishes far exceeds that of the phytoplankton.
The pyramid of energy is always upright
The pyramid of energy is drawn for a unit area and for a unit time, such as a year. It is never inverted. At each step, a large part of the energy is lost as heat, so the energy of a level is always less than the energy of the level below it.
How can the sea have a larger biomass above a smaller one and yet an upright pyramid of energy? The answer is the turnover. The phytoplankton make themselves again many times in a year. A small stock can make a large amount in a year.
Three pyramids that are not upright. On the left, a pyramid of number for one large tree: 1 tree at the base, 25000 insects in the second row, 400 insect-eating birds in the third row and 8 hawks at the top, so it is narrow at the base, widest in the second row and narrow at the top. In the middle, a pyramid of biomass in a sea: 5 grams for each square metre of phytoplankton at the base, 18 grams of zooplankton above it and 30 grams of fish at the top, so the bars grow wider upward. On the right, the pyramid of energy of the same sea: 500, 90 and 12 grams of new matter for each square metre in a year, which is upright.
You should find that a low phytoplankton biomass with a high turnover gives an inverted biomass pyramid and an upright pyramid of production.
Limits of the pyramids
NCERT lists what a pyramid does not do.
It does not take into account that the same species may belong to more than one trophic level.
It assumes a simple food chain, which is never found in nature.
It does not accommodate a food web.
Saprophytes (the decomposers) are not given a place in it, although they play a vital role.
A bar chart for a model sea with three groups of bars for phytoplankton, zooplankton and fish. In each group a blue bar shows the biomass in grams for each square metre and an orange bar shows the production in grams for each square metre in a year. Phytoplankton: biomass 5 and production 500. Zooplankton: biomass 18 and production 90. Fish: biomass 30 and production 12. The biomass bars rise from left to right while the production bars fall.
Try it in Python
The program works out the yearly production of a model sea from the biomass and the turnover. Then it tests which pyramid is inverted.
Practise with new numbers
Common mistakes
Saying that all pyramids are upright. The pyramid of number of a tree and the pyramid of biomass of the sea can be inverted.
Saying that the pyramid of energy can be inverted. It cannot. Energy is lost at every step.
Drawing the pyramid of energy without a time. It is a rate, for a unit of area and a unit of time.
Forgetting the turnover. A small standing crop can make a large amount in a year.
Remember these
Base: producers. Apex: top consumer.
Pyramid of number: upright in a grassland, inverted at the base for a large tree.
Pyramid of biomass: upright on land, generally inverted in the sea.
Pyramid of energy: always upright, because energy is lost at each step.
Limits: the same species at more than one level, a simple chain, no web, no saprophytes.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
What does the base of an ecological pyramid stand for? What stands at the top?
Name the three ecological pyramids.
Why is the pyramid of biomass in the sea generally inverted?
Why can the pyramid of energy never be inverted?
Give two limitations of the ecological pyramids.
References
NCERT (2023). Biology, Textbook for Class XII, chapter 12, Ecosystem, section on ecological pyramids.
OpenStax Biology 2e (2018), chapter 46, Ecosystems, section on energy flow through ecosystems.