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Flow of Energy in Ecosystems

Energy is transferred between organisms in one direction in a food chain; interconnected food chains make a food web.

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Flow of Energy in Ecosystems

What is the source of energy for almost all ecosystems?

The Sun supports most of Earth's ecosystems. Plants create chemical energy (sugar) from abiotic factors that include solar energy. The food energy created by producers is passed to consumers, scavengers, and decomposers. This is known as a food chain.

Trophic Levels

Energy flows through an ecosystem in only one direction. Energy is passed from organisms at one trophic level or energy level to organisms in the next trophic level. The energy or ecological pyramid is a more complex way to model a food chain.  Which organisms do you think are at the first trophic level (Figure below)?

Producers are always the first trophic level, herbivores the second, the carnivores that eat herbivores the third, and so on.

Most of the energy at a trophic level – about 90% – is used at that trophic level. Organisms need it for movement, maintaining stable internal conditions, and reproduction. So animals at the second trophic level have only about 10% as much energy available to them as do organisms at the first trophic level. Animals at the third level have only 10% as much available to them as those at the second level, or only 1% of the energy the producers started with.

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What are the consequences of this loss of energy at each trophic level? The result is that each trophic level can support fewer and fewer individual organisms.  See the pyramid diagrams.

What does this mean for the range of the osprey (or lion, or other top predator)? A top predator must have a very large range in which to hunt so that it can get enough energy to live.Why do most food chains have only four or five trophic levels? There is not enough energy to support organisms in a sixth trophic level. Food chains of ocean animals are longer than those of land-based animals because ocean conditions are more stable.

Why do organisms at higher trophic levels tend to be larger than those at lower levels? The reason for this is simple: a large fish must be able to eat a small fish, but the small fish does not have to be able to eat the large fish (Figure below).

Food Webs

What is a more accurate way to depict the passage of energy in an ecosystem? A food web (Figure below) recognizes that many organisms eat at multiple trophic levels.

A food web includes the relationships between producers, consumers, and decomposers.

Humans are an important part of both of food webs; we are at the top of a food web, since nothing regularly eats us. That means that we are top predators.


  • food chain: An energy pathway that includes all organisms that are linked as they pass along food energy, beginning with a producer and moving on to consumers.
  • food web: Interwoven food chains that show each organism eating from different trophic levels.
  • trophic level: Energy levels within a food chain or food web.


  • A food chain describes the passage of energy between trophic levels.
  • A food web is a set of interconnected and overlapping food chains.
  • Food webs are interconnected, such as nearby land and a marine food webs.
  • Organisms occupy a trophic level in a food chain, depending on how far removed they are from producers.


Use this resource 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 and ecosystem?


1. What does a food chain depict? Why do scientists usually use a food web instead of a food chain?

2. Start with the Sun and describe what happens to energy through the trophic levels. Why does this not go on forever (with many more trophic levels)?

3. What trophic level do you inhabit? Do all humans inhabit the same trophic level?

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