12.19: Energy Flow

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How much energy could be gained from the warthog?

If the cheetah is successful in capturing the warthog, it would gain some energy by eating it. But would the cheetah gain as much energy as the warthog has ever consumed? No, the warthog has used up some of the energy it has consumed for its own needs. The cheetah will only gain a fraction of the energy that the warthog has consumed throughout its lifetime.

Energy Pyramids

When an herbivore eats a plant, the energy in the plant tissues is used by the herbivore. But how much of that energy is transferred to the herbivore? Remember that plants are producers, bringing the energy into the ecosystem by converting sunlight into glucose. Does the plant use some of the energy for its own needs? Recall the energy is the ability to do work, and the plant has plenty of "work" to do. So of course it needs and uses energy. It converts the glucose it makes into ATP through cellular respiration just like other organisms. After the plant uses the energy from glucose for its own needs, the excess energy is available to the organism that eats the plant.

The herbivore uses the energy from the plant to power its own life processes and to build more body tissues. However, only about 10% of the total energy from the plant gets stored in the herbivore’s body as extra body tissue. The rest of the energy is used by the herbivore and released as heat. The next consumer on the food chain that eats the herbivore will only store about 10% of the total energy from the herbivore in its own body. This means the carnivore will store only about 1% of the total energy that was originally in the plant. In other words, only about 10% of energy of one step in a food chain is stored in the next step in the food chain. The majority of the energy is used by the organism or released to the environment.

Every time energy is transferred from one organism to another, there is a loss of energy. This loss of energy can be shown in an energy pyramid. An example of an energy pyramid is pictured below (Figure below). Since there is energy loss at each step in a food chain, it takes many producers to support just a few carnivores in a community.

Each step of the food chain in the energy pyramid is called a trophic level. Plants or other photosynthetic organisms (autotrophs) are found on the first trophic level, at the bottom of the pyramid. The next level will be the herbivores, and then the carnivores that eat the herbivores. The energy pyramid (Figure below) shows four levels of a food chain, from producers to carnivores. Because of the high rate of energy loss in food chains, there are usually only 4 or 5 trophic levels in the food chain or energy pyramid. There just is not enough energy to support any additional trophic levels. Heterotrophs are found in all levels of an energy pyramid other than the first level.

Summary

As energy is transferred along a food chain, energy is lost as heat.
Only about 10% of energy of one step in a food chain is stored in the next step in the food chain.

Explore More

Use the resource below to answer the questions that follow.

Ecological Pyramids at http://www.youtube.com/watch?v=NJplkrliUEg (4:03)

What are three types of ecological pyramids? How do their shapes compare?
Do you think it would be possible to construct a pyramid where the number of carnivores was more than the number of herbivores? Why or why not?
Do you think it would be possible to construct a pyramid where the biomass of carnivores was more than the biomass of herbivores? How does this compare to a numbers pyramid.
What consumes energy at each trophic level? How does this contribute to energy loss between trophic levels?

Review

When an herbivore eats a plant, what happens to 90% of the energy obtained from that plant?
What is a trophic level?
Why are the number of trophic levels limited?
In a forest community, caterpillars eat leaves, and birds eat caterpillars. Draw an energy pyramid using this information.