Start with producers
In most ecosystems, producers such as plants and algae capture sunlight and use it to build energy-rich food molecules. Consumers obtain energy by eating producers or other consumers. Decomposers such as many bacteria and fungi use dead material and return nutrients to the environment; energy is not recycled and eventually disperses as heat.
A food chain traces one feeding path. In a chain such as grass → grasshopper → frog, each arrow points from the food to the organism receiving energy from it.
Use a web for the fuller picture
Most organisms have more than one food source and more than one predator, so connected food chains form a food web. A web can reveal both direct effects and changes that spread through several connected populations.
Organisms use much of the energy they obtain for life processes. Only part remains stored in material that can be eaten by the next trophic level, or feeding level, so less usable energy is generally available higher in a food web.
Remember these ideas
Key ideas
- Producers form the energy base of most food webs.
- Consumers obtain energy by eating other organisms.
- Decomposers use dead material and return nutrients—not energy—to the environment.
- Food-web arrows point from the food or prey toward the organism receiving energy.
- Less usable energy is available after each trophic transfer.
Make the reasoning visible
Trace a coastal food chain
- 1
Begin with phytoplankton, producers that capture light energy.
- 2
Draw an arrow from phytoplankton to zooplankton because zooplankton eat phytoplankton and receive energy from them.
- 3
Continue with zooplankton → small fish → tuna to show one possible path.
- 4
If seabirds also eat the small fish, add another arrow from the fish to the seabirds. The branching paths turn the chain into part of a food web.
Try the next step
Read a meadow food web
A diagram contains grass → grasshopper → frog → snake. Identify the producer and explain what the arrow from the frog to the snake means.
Why would adding every food source and predator make this diagram a web rather than a single chain?