Matter Moves Through a Food Web

Students build and interpret a food web model to explain how matter moves among plants, animals, decomposers, and the environment.

Food Chains and Food Webs

A food chain shows one path that matter can follow when organisms eat one another. A food web combines many food chains because most organisms have more than one food source or predator. In a meadow, a grasshopper eats grass, a frog eats the grasshopper, and a hawk may eat the frog. However, a rabbit also eats grass, and the hawk may eat the rabbit. These connected paths form a food web. In a model, each arrow should point from the organism being eaten to the organism that receives its matter. For example, an arrow from grass to rabbit means that matter in the grass moves into the rabbit when it eats. Food webs help scientists represent and study many ecosystem relationships at once.

Producers, Consumers, and Decomposers

Organisms have different roles in moving matter through an ecosystem. Producers, such as grasses and trees, make sugars using sunlight, carbon dioxide from the air, and water. The carbon dioxide and water supply matter that becomes part of the plant. Consumers obtain matter by eating plants or other animals. A deer is a plant-eating consumer, while a wolf is a consumer that eats animals. Decomposers, including many fungi and bacteria, break down dead organisms and wastes. For example, fungi growing on a fallen log use some of the log's matter. Decomposition also returns matter to the soil and air. Plant roots can take in water and minerals from the soil, beginning new pathways. Together, producers, consumers, and decomposers keep matter moving within the ecosystem.

Tracing Matter Through an Ecosystem

Matter does not disappear when one organism eats another. Its atoms become part of the eater, leave as waste, or return to the environment after the organism dies. Consider a carbon atom in carbon dioxide in the air. A clover plant uses the carbon dioxide to make sugar. A rabbit eats the clover, so the carbon atom may become part of the rabbit's body. Later, waste or the rabbit's remains may be broken down by decomposers. Some carbon returns to the air as carbon dioxide, and other matter enters the soil. Plants can then use matter from the environment again. Sunlight provides energy for this process, but sunlight is not matter. Tracing one atom or group of atoms helps explain the repeated movement of matter through living and nonliving parts of an ecosystem.

Building a Food Web Model

To build a food web model, first choose an ecosystem and identify its organisms and environmental features. A pond map might show shallow, sunny water with algae, deeper water with fish, and muddy edges with insects and frogs. Place each organism in a sensible location. Then draw arrows from each food source to the organism that eats it. For example, draw arrows from algae to snails and tadpoles, from insects to frogs, and from frogs to herons. Add bacteria and fungi, with arrows from dead plants, animals, and waste to these decomposers. A mathematical model can count connections. If the frog has three incoming food arrows, it has three modeled food sources. Use photographs, maps, or reliable texts to check that the organisms and relationships fit the pond's environmental conditions.

Explaining Ecosystem Connections

A food web model can be used to explain how a change in one part of an ecosystem may affect other parts. Suppose drought reduces the grass in a prairie. Grass-eating mice and grasshoppers would have less plant matter available. Hawks might then have fewer mice to eat, while frogs might have fewer grasshoppers to eat. Decomposers would still process wastes and dead organisms, returning matter to the soil and air. To explain these connections, make a claim and support it with specific evidence from the model or a scientific text. For example: “Fewer grasses may lead to fewer mice because the arrow from grass to mouse shows that mice receive matter by eating grass.” Remember that a model shows likely relationships, not an exact prediction. Weather, migration, and other food sources can also influence what happens.