The Energy Pyramid
At a street market in Xochimilco they sell lettuce, but nobody sells "jaguars" by the bunch. In the wild, something similar happens: there are tons of plants, quite a few rabbits, and very few pumas. Why are there so many organisms at the bottom and so few at the top?
Energy enters through producers
Every living thing needs energy to move, grow, and reproduce. Almost all the energy in an ecosystem comes from the Sun. Producers (plants, algae, and some bacteria) capture it through photosynthesis and store it in their tissues.
When an herbivore eats the plant, it receives part of that energy. When a carnivore eats the herbivore, it receives part of what the herbivore got. This is how energy passes from one trophic level to another:
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Producers (nopal cacti, grasses, phytoplankton).
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Primary consumers (herbivores: rabbits, deer).
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Secondary consumers (carnivores that eat herbivores: snakes, coyotes).
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Tertiary consumers (carnivores that eat carnivores: eagles, jaguars).
Look at the energy pyramid in the diagram. Identify which trophic level occupies each floor and calculate the missing value, assuming that in every transfer only 10% of…
Look at the energy pyramid in the diagram. Identify which trophic level occupies each floor and calculate the missing value, assuming that in every transfer only 10% of the energy passes to the next level.
Only about 10% passes on
When a rabbit eats a nopal, it doesn't use all of its energy. Part of it isn't digested and leaves the body as feces. Another part the rabbit uses just to live: running, keeping its body temperature up, breathing. That energy is lost as heat and doesn't pass to the next level. Only a small part stays stored in the rabbit's body (its growth) and can reach whoever eats it.
On average, only about 10% of the energy at one level passes to the next. The other 90% is used up or lost as heat. This is the 10% rule.
It's an approximation: in real life the percentage changes depending on the ecosystem, but 10% is a good model for understanding and calculating.
Look at the energy pyramid in the diagram. Identify which trophic level occupies each floor and calculate the missing value, assuming that in every transfer only 10% of…
Look at the energy pyramid in the diagram. Identify which trophic level occupies each floor and calculate the missing value, assuming that in every transfer only 10% of the energy passes to the next level.
The energy pyramid
If we draw each trophic level as a floor, and make the floor with the most energy the widest, we get an energy pyramid. Producers form the base, and each floor above is narrower because it receives only 10% of the one below.
Notice that:
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The base is always the largest: that's where the greatest amount of energy is.
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Each floor has 10 times less energy than the one below it.
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The energy pyramid is never inverted: there can't be more energy at the top than at the bottom, because energy is lost at every step.
Look at the energy pyramid in the diagram. Identify which trophic level occupies each floor and calculate the missing value, assuming that in every transfer only 10% of…
Look at the energy pyramid in the diagram. Identify which trophic level occupies each floor and calculate the missing value, assuming that in every transfer only 10% of the energy passes to the next level.
How much energy do the lower levels need?
The most interesting calculations go the other way: we know how much energy the upper level needs, and we want to know how much the base had to capture.
With every level you go up, the energy the base needs is multiplied by 10. That's why a large carnivore needs a lot of vegetation.
In an ecosystem, the carnivores receive 500 kJ of energy per year. If only 10% of the energy passes to the next level in each transfer, calculate how much energy the…
In an ecosystem, the carnivores receive 500 kJ of energy per year. If only 10% of the energy passes to the next level in each transfer, calculate how much energy the producers had to capture to support those carnivores (producers → herbivores → carnivores). Also explain why food chains rarely have more than four or five levels.
Why are food chains short?
If we follow the calculation, starting with 100,000 kJ in the producers:
| Level | Energy (kJ) |
|---|---|
| Producers | 100,000 |
| 1st consumers | 10,000 |
| 2nd consumers | 1,000 |
| 3rd consumers | 100 |
| 4th consumers | 10 |
At the fourth level of consumers, so little energy is left that it isn't enough to sustain a population of animals. That's why food chains rarely have more than four or five levels.
This also explains why there are few large predators (jaguars, eagles, sharks) and why they are vulnerable: they depend on a lot of energy accumulated below. If the ecosystem is damaged, the animals at the top are the first to suffer.
In an ecosystem, the carnivores receive 500 kJ of energy per year. If only 10% of the energy passes to the next level in each transfer, calculate how much energy the…
In an ecosystem, the carnivores receive 500 kJ of energy per year. If only 10% of the energy passes to the next level in each transfer, calculate how much energy the producers had to capture to support those carnivores (producers → herbivores → carnivores). Also explain why food chains rarely have more than four or five levels.
Back to the beginning: there are lots of plants, quite a few rabbits, and few pumas because at each level about 90% of the energy is lost, and at the top there's only enough for a few individuals.