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18.3: Flow of Energy in Ecosystems

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    What is the source of energy for almost all ecosystems?

    The Sun supports most of Earth's ecosystems. Plants create chemical energy from abiotic factors that include solar energy. The food energy created by producers is passed through the food chain.

    How Energy Flows Through Ecosystems

    All living things need energy. They need it to power the processes of life. For example, it takes energy to grow. It also takes energy to produce offspring. In fact, it takes energy just to stay alive. Remember that energy can’t be created or destroyed. It can only change form. Energy changes form as it moves through ecosystems.

    The Flow of Energy

    Most ecosystems get their energy from the Sun. Only producers can use sunlight to make usable energy. Producers convert the sunlight into chemical energy or food. Consumers get some of that energy when they eat producers. They also pass some of the energy on to other consumers when they are eaten. In this way, energy flows from one living thing to another.

    Food Chains

    A food chain is a simple diagram that shows one way energy flows through an ecosystem. Pictured below is an example of a food chain (Figure below). Producers form the base of all food chains. The consumers that eat producers are called primary consumers. The consumers that eat primary consumers are secondary consumers. This chain can continue to multiple levels.

    Example of a food chain

    What do the arrows stand for in a food chain?

    At each level of a food chain, a lot of energy is lost. Only about ten percent of the energy passes to the next level. Where does that energy go? Some energy is given off as heat. Some energy goes into animal wastes. Energy also goes into growing things that another consumer can't eat, like fur. It's because so much energy is lost that most food chains have just a few levels. There’s not enough energy left for higher levels.

    Food Webs

    Food chains are too simple to represent the real world. They don’t show all the ways that energy flows through an ecosystem. A more complex diagram is called a food web (Figure below). A food web consists of many overlapping food chains. Can you identify the food chains in the figure? How many food chains include the mouse?

    Example of a food web

    The owl in this food web consumes at two different levels. What are they?

    Food webs also overlap. For example, an eagle is part of a land food web. But it might go to the sea to grab a fish. That fish is part of a marine food web.


    • A food chain describes the passage of energy.
    • A food web is a set of interconnected and overlapping food chains.
    • Food webs are interconnected.


    1. What does a food chain depict?
    2. Why do scientists usually use a food web instead of a food chain?
    3. Start with the Sun. Describe what happens to energy as it moves through ecosystems.

    Explore More

    Use the resource below to answer the questions that follow.

    1. What do all organisms require?
    2. What provides the energy required by the ecosystem?
    3. How is energy transferred from one organism to another?
    4. How is some of the energy lost?
    5. How do nutrients move through an ecosystem?
    6. How does the movement of energy and nutrients differ in ecosystems?

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