The Biomass Pyramid
Imagine you could weigh everything that lives in a grassland in Chihuahua: all the grass, all the grasshoppers, all the mice, and all the snakes. Which of those groups would weigh the most? And why is there almost never more weight of predators than of prey? To find out, you are going to build a biomass pyramid.
What Is Biomass?
Biomass is the total mass of living matter of a group of organisms in one place. It is measured in kilograms (kg) or in kilograms per square meter.
Each trophic level has its own biomass:
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Producers (grasses, trees, algae): make their own food using sunlight.
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Primary consumers (herbivores): feed on the producers.
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Secondary and tertiary consumers (carnivores): feed on other animals.
When an animal eats, not everything it takes in becomes part of its body. Some of it is used to move, keep warm, and stay alive (it is released as heat), and another part is expelled as waste. That is why a higher level almost always has less mass than the level that feeds it.
Explain the difference between an energy pyramid and a biomass pyramid. Describe in which case the biomass pyramid can appear inverted (give an example of a marine…
Explain the difference between an energy pyramid and a biomass pyramid. Describe in which case the biomass pyramid can appear inverted (give an example of a marine ecosystem with phytoplankton) and why the energy pyramid is never inverted.
How to Build a Biomass Pyramid
A biomass pyramid is a graph of stacked horizontal bars. Each bar is a trophic level, and its length is proportional to its biomass. The base is always the level of the producers.
In a grassland, the biomass of four groups was measured: grasses, 8,000 kg; grasshoppers, 800 kg; mice, 80 kg; snakes, 8 kg. Order the groups into trophic levels, build…
In a grassland, the biomass of four groups was measured: grasses, 8,000 kg; grasshoppers, 800 kg; mice, 80 kg; snakes, 8 kg. Order the groups into trophic levels, build (describing it in words and numbers) the biomass pyramid, and calculate what percentage of the total biomass corresponds to the producers.
Percentage of Biomass by Level
To find out what part of the total belongs to a level, divide its biomass by the total biomass and multiply by 100.
$$\text{porcentaje} = \frac{\text{biomasa del nivel}}{\text{biomasa total}} \times 100$$Inverted Pyramids: The Case of Phytoplankton
In the sea, the situation can change. Phytoplankton are microscopic algae that float in the water and carry out photosynthesis. They reproduce very quickly: in just a few days an entire population is renewed. Small animals (zooplankton) and fish eat them almost as fast as they grow.
At any given moment, if we weigh everything, there can be more biomass of consumers (zooplankton, fish) than of phytoplankton. The biomass pyramid appears inverted: the base is narrower than the level above it.
Why is there no contradiction? Because biomass is a snapshot of one moment: it tells how much matter there is right now. But phytoplankton is renewed so quickly that it produces a huge amount of matter over time, even though at any moment there is little of it.
Explain the difference between an energy pyramid and a biomass pyramid. Describe in which case the biomass pyramid can appear inverted (give an example of a marine…
Explain the difference between an energy pyramid and a biomass pyramid. Describe in which case the biomass pyramid can appear inverted (give an example of a marine ecosystem with phytoplankton) and why the energy pyramid is never inverted.
Biomass Versus Energy
The energy pyramid (previous section) measures how much energy flows through each level over a period, for example a year. The biomass pyramid measures how much living matter there is at one moment.
| Biomass pyramid | Energy pyramid | |
|---|---|---|
| What it measures | Mass of living matter (kg) | Energy passing through the level (kcal or kJ) in a period of time |
| It is like… | A photograph | A movie |
| Can it be inverted? | Yes, in some marine ecosystems | Never |
The energy pyramid is never inverted because energy is lost at each step, mainly as heat, and cannot be recycled. A level can never receive more energy than the previous level had. That is why, in any ecosystem, the available energy decreases going up; the 10% rule says that only about one tenth passes on.
Go back to the beginning: in a grassland, the producers weigh far more than the predators because matter and energy decrease at each step of the chain; that is why there are more kilos of grass than of snakes.
Summary
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Biomass is the mass of living matter at a trophic level; it is measured in kg.
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The biomass pyramid puts the producers at the base and the consumers above, ordered by who eats whom.
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For the percentage of a level: biomass of the level ÷ total biomass × 100.
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In the sea, phytoplankton renews itself so fast that the biomass pyramid can be inverted.
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The energy pyramid is never inverted, because energy is lost as heat at each level.