When a Food Web Changes

By the end of this lesson, you will be able to predict what happens to energy and matter in a food web when something in an ecosystem changes.

Think about a grassy field in Texas. The Sun shines on the grass, and the grass uses that light to make its own food. Grasshoppers eat the grass. Mice eat the grasshoppers. Hawks eat the mice. When these feeding paths cross and connect, they form a food web, which is a picture of who eats whom in one place. Energy from the Sun moves along those arrows, from the grass to the animals that eat it.

Matter moves too. The tiny bits that make up plants and animals get passed along when one thing eats another. When a plant or animal dies, decomposers, which are living things like fungi and bacteria that break down dead material, return those bits to the soil. New grass uses them to grow. That is why we say matter cycles, or goes around again and again.

Now imagine a long, hot drought. Less rain means less grass. With less grass, fewer grasshoppers have food, so their numbers drop. Fewer grasshoppers means less food for mice, and fewer mice means hawks go hungry too. One change at the bottom can change the whole web. Less matter cycles through the soil, and less energy flows up to the hawks.

Changes can also add something. If people plant new trees or a new kind of animal moves in, some populations may grow while others shrink. To predict what happens, follow the arrows. Ask yourself which living things eat the thing that changed, and then which living things eat those.

Try it: If a wildfire burned most of the grass in that Texas field, what would probably happen to the number of mice and hawks?