5. Energy Pyramids

Introduction

Although food chains show who eats whom, they do not show how much energy is available at each stage. Scientists use energy pyramids to illustrate how energy decreases as it moves through an ecosystem. These diagrams help explain why ecosystems contain many plants, fewer herbivores, and even fewer top predators.

Energy pyramids are one of the most important tools in ecology because they show that the amount of energy available becomes smaller at each trophic level. This decreasing energy supply limits the number of organisms that an ecosystem can support and explains why food chains are usually only four or five trophic levels long.

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What Is an Energy Pyramid?

An energy pyramid is a diagram that shows the amount of energy available at each trophic level in an ecosystem.

The pyramid is widest at the bottom because producers contain the greatest amount of energy.

Each higher level becomes narrower because less energy is available.

A typical energy pyramid looks like this:

• Top: Tertiary consumers
• Secondary consumers
• Primary consumers
• Bottom: Producers

The width of each level represents the amount of energy available.

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Figure 1. An energy pyramid shows that the amount of available energy decreases at each trophic level.

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Why Is the Pyramid Wide at the Bottom?

Producers capture energy directly from the Sun through photosynthesis.

Because they receive energy first, producers contain the largest amount of available energy.

Examples of producers include:

• Grass
• Trees
• Algae
• Phytoplankton

All other organisms depend directly or indirectly on the energy stored by producers.

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Why Does Energy Decrease?

When organisms consume food, they do not pass all of the stored energy to the next trophic level.

Energy is used for:

• Movement
• Growth
• Reproduction
• Respiration
• Repair of body tissues
• Maintaining body temperature (in mammals and birds)

Much of this energy is eventually released into the environment as heat.

Some energy also remains in:

• Bones
• Hair or fur
• Feathers
• Waste products
• Uneaten body parts

Only a small fraction becomes available to the next trophic level.

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Figure 2. Most energy is used by organisms or lost as heat, leaving only a small amount to pass to the next trophic level.

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The 10% Rule

Ecologists often use the 10% Rule as a simple model of energy transfer.

It states that, on average:

Only about 10% of the energy at one trophic level is transferred to the next trophic level.

Approximately:

• 90% is used or lost.
• 10% becomes available to the next consumer.

Example

This dramatic decrease explains why ecosystems cannot support large numbers of top predators.

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Figure 3. The 10% Rule illustrates the rapid decrease in available energy between trophic levels.

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Interpreting Energy Pyramids

When reading an energy pyramid, remember:

• The bottom contains the most energy.
• The top contains the least energy.
• Each level represents a trophic level.
• The pyramid narrows because available energy decreases.

For example:

If producers contain 50,000 kJ, the approximate energy available to:

• Primary consumers = 5,000 kJ
• Secondary consumers = 500 kJ
• Tertiary consumers = 50 kJ

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Energy Availability and Population Size

The amount of available energy influences how many organisms can survive at each trophic level.

Generally:

• Many producers
• Fewer herbivores
• Even fewer carnivores
• Very few apex predators

For example:

A grassland may contain:

• Millions of blades of grass.
• Thousands of grasshoppers.
• Hundreds of frogs.
• Dozens of snakes.
• Only a few hawks.

As available energy decreases, fewer organisms can be supported.

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Figure 4. As energy decreases, ecosystems can support progressively fewer organisms at higher trophic levels.

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Why Food Chains Are Short

Most food chains contain only four or five trophic levels.

If food chains became much longer:

• Very little energy would remain.
• Top consumers would not obtain enough food to survive.

This is why:

• Large predators require extensive hunting territories.
• Apex predators are usually rare.
• Food chains are naturally limited in length.

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Energy Pyramid vs Food Chain

Both diagrams are useful and complement one another.

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Worked Example

Question

An energy pyramid contains:

• Producers = 40,000 kJ

Using the 10% Rule, estimate the amount of energy available at each trophic level.

Solution

Only about 10% of the energy is transferred from one level to the next.

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Real-World Connection

Energy pyramids help farmers, fisheries managers, and conservationists make informed decisions about sustainable resource use. For example, raising herbivorous fish generally requires less energy than raising large predatory fish because fewer trophic levels are involved. Similarly, protecting producers such as forests, grasslands, and phytoplankton is essential because they provide the energy that supports entire ecosystems.

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Did You Know?

Although phytoplankton are microscopic, they capture enormous amounts of solar energy every day. They form the foundation of nearly all marine food chains and support some of the largest animals on Earth, including whales. Without these tiny producers, most ocean ecosystems would collapse.

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Key Terms

Apex predator – A predator at the highest trophic level with few or no natural predators.

Energy pyramid – A diagram showing the amount of energy available at each trophic level.

Energy transfer – The movement of energy from one trophic level to another.

Primary consumer – A herbivore that feeds directly on producers.

Producer – An organism that captures energy from sunlight or chemicals to make its own food.

Secondary consumer – A consumer that feeds on primary consumers.

Tertiary consumer – A consumer that feeds on secondary consumers.

Trophic level – The feeding position an organism occupies in a food chain or food web.

10% Rule – The guideline that, on average, only about 10% of the energy at one trophic level is transferred to the next.

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Key Takeaways

• An energy pyramid shows the amount of energy available at each trophic level.
• Producers form the broad base of the pyramid because they contain the greatest amount of energy.
• Only about 10% of energy is transferred from one trophic level to the next, while the rest is used by organisms or lost as heat.
• The decreasing availability of energy means that higher trophic levels can support fewer organisms.
• Energy loss limits the length of food chains, so most ecosystems have only four or five trophic levels.