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 to consumers, scavengers, and decomposers through the food web.
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.
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. 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 locomotion, heating themselves, 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.
A food chain is a simple diagram that shows one way energy flows through an ecosystem. You can see an example of a food chain in 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.
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 10 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.
In the simple depiction of a food chain all organisms eat at only one trophic level (Figure below).
A simple food chain in a lake. The producers, algae, are not shown. For the predatory bird at the top, how much of the original energy is left?
What are the consequences of the loss of energy at each trophic level? Each trophic level can support fewer organisms.
How many osprey are there relative to the number of shrimp?
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).
In this image the predators (wolves) are smaller than the prey (bison), which goes against the rule placed above. How does this relationship work? Many wolves are acting together to take down the bison.
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. You can see an example in 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?
The owl in this food web consumes at two different levels. What are they?
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.
Even food webs are interconnected. 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. All organisms depend on two global food webs. The base of one is phytoplankton and the other is land plants. How are these two webs interconnected? Birds or bears that live on land may eat fish, which connects the two food webs.
Humans are an important part of both of these food webs; we are at the top of a food web, since nothing eats us. That means that we are top predators.
- 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.
Food Web Slide Show and Vocabulary
Slide show with vocabulary words include: ecosystem, food chain, food web, producer, autotroph, consumer, heterotroph, herbivore, primary consumer, carnivore, secondary consumer, omnivore, decomposer, scavenger, detrivore, predator, and trophic level.
Open the resource in a new window.
- food chain: An energy pathway that from a producer and moving on to consumers.
- food web: Interwoven food chains.
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?
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?