Energy and Matter in a Food Web
Students analyze a food web to explain how energy and matter move among producers, consumers, decomposers, and the environment.
Illustrations are auto-generated and may be placeholders. They can be refreshed to match the narration.
Meet the Ecosystem Roles
Every organism in an ecosystem has one or more roles. Producers, such as grasses and oak trees, use sunlight, carbon dioxide, and water to make sugars. Consumers get energy and matter by eating plants or other animals. A grasshopper that eats grass is a consumer, and a frog that eats the grasshopper is also a consumer. Decomposers, including many fungi and bacteria, break down wastes and dead organisms. This returns matter to the soil, water, and air. The environment includes nonliving parts such as sunlight, air, water, rocks, and soil. In a meadow, grass may feed rabbits and insects, while hawks eat rabbits and frogs eat insects. Because organisms can have several food connections, these relationships form a food web rather than one simple chain.
Trace Energy from the Sun
Most energy in a food web begins with the Sun. A plant captures sunlight and stores some of that energy in sugars. When a caterpillar eats a leaf, some stored energy moves to the caterpillar. If a robin eats the caterpillar, energy moves to the robin. Food-web arrows point from the organism being eaten toward the eater, showing the direction of energy transfer. Less usable energy is available at each step because organisms use energy to move, grow, stay warm, and carry out life processes. Much of that energy eventually leaves the ecosystem as heat. For example, a model might show 10,000 energy units captured by plants, 1,000 available to plant eaters, and 100 available to their predators. These numbers are simplified estimates, but they show why food webs usually have many producers and fewer top predators.
Follow Matter Through a Food Web
Matter includes the atoms that make up air, water, soil, and living things. Unlike energy, matter can move through an ecosystem again and again. A plant takes in carbon dioxide from the air, water through its roots, and mineral nutrients from the soil. It uses these materials to build leaves, stems, and sugars. When a deer eats the plant, plant matter becomes part of the deer or leaves its body as waste. A wolf that eats the deer receives some of the same matter. After organisms produce waste or die, decomposers break their remains into simpler substances. Carbon dioxide returns to the air, while water and nutrients return to the environment. On a forest food-web map, arrows from shrubs to deer and from deer to wolves show where food matter moves between organisms.
The Work of Decomposers
Decomposers are essential recyclers in a food web. Many bacteria and fungi obtain energy by breaking down dead organisms and wastes. Detritivores, such as earthworms and millipedes, first shred or eat pieces of dead material, making it easier for decomposers to act. Imagine a fallen leaf on a forest floor. An earthworm tears the leaf into smaller pieces, and fungi and bacteria continue breaking down its matter. Nutrients from the leaf enter the soil, where tree roots can absorb them. Some carbon returns to the air as carbon dioxide during the decomposers’ life processes. Decomposers do not return energy to the Sun or create new energy. They use chemical energy in dead material, and much of it eventually leaves as heat. Without decomposition, dead matter would pile up and fewer nutrients would be available to producers.
Build and Explain a Food Web Model
To build a food web model, first choose a real location and identify its environmental characteristics. A pond map might show open water, a muddy shore, sunlight, and nearby plants. List producers, consumers, and decomposers that can live there. Then draw arrows from each food source to the organism that eats it. For example, draw arrows from algae to snails and tadpoles, from tadpoles to bass, and from bass to a heron. Add arrows from dead organisms and waste to bacteria and fungi. Use numbers to model a change: if algae decrease from 500 units to 250 units, less food matter and energy may be available to algae eaters. Explain the relationships using evidence from the model, such as, “The tadpole population may decrease because its producer food source was cut in half.” Also describe how the pond’s water and shoreline support particular organisms.
