Energy Pyramids

Essential Question

Why is less energy available at each higher trophic level?

A field can feed thousands of grasshoppers but only a few hawks. In this lesson, you’ll find out why. You’ll use the 10 percent rule to calculate how much energy reaches each trophic level, read energy budgets and pyramids, and explain why food chains rarely have more than four or five links.

The 10 Percent Rule

• About 10% of energy passes to the next level
• About 90% is not passed on
• Up one level: × 0.10. Down one level: × 10

An energy pyramid shows how much energy is available at each trophic level. On average, only about 10% of the energy in one level ends up stored in the bodies of the next level. If the grass in a field stores 10,000 kcal of energy, the grasshoppers that eat it store only about 1,000 kcal, the mice about 100 kcal, and the snakes about 10 kcal. To move up one level, multiply by 0.10. To move down one level, multiply by 10.

Where Does the Energy Go?

• Some food is not eaten or not digested
• Cellular respiration releases energy as heat
• Only energy stored in growth passes on

Why is so much energy lost? This energy budget shows what happens when a caterpillar eats 200 J of leaf. Half the energy (100 J) passes out in feces, because the caterpillar cannot digest it. About a third (67 J) is used in cellular respiration to power movement, growth, and other life processes, and that energy ends up as heat. Only 33 J, about 17%, is stored in new body tissue that a bird could eat. Over a whole trophic level, including the many plants and animals that are never eaten, the average works out to about 10%.

Pyramids of Numbers and Biomass

• Pyramid of numbers: how many organisms
• Pyramid of biomass: total mass of living things
• A pyramid of numbers can be upside down

Ecologists also make pyramids of numbers and of biomass (the total mass of living things). Biomass usually shrinks at each level, just like energy. A pyramid of numbers can look very different. One oak tree can feed 2,000 caterpillars, so the bottom of the pyramid of numbers is narrower than the level above it. The tree’s huge biomass still makes the base of the biomass pyramid the widest. An energy pyramid, unlike a pyramid of numbers, is widest at the bottom.

Why Food Chains Are Short

• Each level gets about 10% of the level below
• After four or five levels, little energy is left
• Top predators are few and need large areas
• Persistent chemicals, such as DDT, can build up in top predators

Because energy drops by about 90% at each step, the fifth level of a food chain gets only about 0.01% of the energy captured by producers: 10,000 kcal shrinks to just 1 kcal. There is not enough energy left to support another level. This is why top predators, such as hawks, wolves, and sharks, are rare and must hunt over large areas. On this log-scale graph, each bar stands for one tenth the energy of the bar to its left.

Chemicals Build Up in Top Predators

• Some chemicals, such as DDT and mercury, are not easily broken down or removed from the body
• A consumer takes in the chemical from the many organisms it eats
• The concentration usually rises at each higher trophic level

Organism	Trophic level	DDT in body tissue (ppm)
Algae	Producer	0.04
Zooplankton	Primary consumer	0.23
Small fish	Secondary consumer	1.2
Fish-eating bird	Tertiary consumer	24

Energy is not the only thing that moves up a food chain. Some pollutants, such as the pesticide DDT and the metal mercury, are not easily broken down or removed from the body, so they stay in an organism’s tissues. A consumer eats many organisms from the level below, so it collects the chemical from all of them, while most of the energy in that food is lost, as you saw above. As a result, the concentration of the chemical usually rises at each higher trophic level. This is called biomagnification. The table shows sample data for DDT in a lake food chain. The concentration in the fish-eating bird is about 600 times the concentration in the algae, even though the bird holds the least energy. High concentrations can harm top predators. For example, DDT thinned the eggshells of birds such as bald eagles and peregrine falcons, and their populations fell until DDT was banned in the United States in 1972.

Eating Lower on the Food Chain

• Eating plants skips a trophic level
• About 10 times as much food energy reaches people
• More people can be fed from the same land

People can eat at different trophic levels. When people eat corn, they are primary consumers. When cattle eat the corn and people eat the beef, people are secondary consumers, and about 90% of the corn’s energy is lost along the way. So the same field of corn provides roughly 10 times as much food energy when people eat it directly. This is one reason diets with more plant foods can feed more people from the same amount of land.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Energy Up an Ocean Food Chain

Phytoplankton in a coastal area of ocean capture 5,000 kcal/m² of energy per year. Use the 10 percent rule to find the energy available to the zooplankton, the small fish, and the tuna.

Show the solution

1. Zooplankton: 5,000 kcal/m²/yr × 0.10 = 500 kcal/m²/yr
2. Small fish: 500 kcal/m²/yr × 0.10 = 50 kcal/m²/yr
3. Tuna: 50 kcal/m²/yr × 0.10 = 5 kcal/m²/yr
4. Check: 5 ÷ 5,000 = 0.001, so the tuna get only 0.1% of the producers’ energy.
5. Answer: 500, 50, and 5 kcal/m² per year.

Example 2: Feeding a Hawk

A hawk needs about 150 kcal of energy per day from its food. In the food chain grass → mouse → snake → hawk, how much energy must the grass capture each day to support it?

Show the solution

1. Work down the chain: each lower level must have about 10 times as much energy.
2. Snakes: 150 kcal × 10 = 1,500 kcal
3. Mice: 1,500 kcal × 10 = 15,000 kcal
4. Grass: 15,000 kcal × 10 = 150,000 kcal
5. Answer: About 150,000 kcal per day, 1,000 times what the hawk gets.

Example 3: A Deer’s Energy Budget

A deer eats plants containing 10,000 kJ of energy. Of that, 4,000 kJ passes out in feces and 5,800 kJ is used in cellular respiration. How much energy is stored in new growth? What percent is that? Explain why it is less than the caterpillar’s 17%.

Where the energy goes	Energy (kJ)
Eaten	10,000
Lost in feces	4,000
Used in cellular respiration	5,800
Stored in new growth	?

Show the solution

1. Growth: 10,000 kJ − 4,000 kJ − 5,800 kJ = 200 kJ
2. Percent: 200 kJ ÷ 10,000 kJ × 100 = 2%
3. Why lower: a deer is warm-blooded. It burns a lot of energy in cellular respiration just to keep its body warm, while a caterpillar does not.
4. Answer: 200 kJ, which is only 2% of the energy it ate.

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Practice

Complete an energy pyramid, read an energy budget, explain a pyramid of numbers, and use data to explain how a chemical builds up in a top predator. Try it on your own first, then check your work with the answer key.

• Energy Pyramids Practice Worksheet (PDF)
• Energy Pyramids Practice Answer Key (PDF)

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Lesson Slides

• 1.6 Energy Pyramids Slides (PDF)

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Big Ideas

• About 10% of energy passes to the next level
• Energy not passed on is lost as heat or left in wastes
• An energy pyramid is widest at the bottom
• Little energy is left after four or five levels
• Persistent chemicals can build up in top predators

Back to the essential question: at each trophic level, organisms lose a large share of the energy they take in. Some food is never eaten or digested, and much of the rest is used in cellular respiration and ends up as heat. Only about 10% is stored in their bodies for the next level to eat. That is why energy pyramids shrink at each level, food chains are short, and top predators are rare. Some persistent chemicals, such as DDT, are not used up, so they can build up in top predators. This wraps up our unit on matter and energy in ecosystems.

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