L10. Energy Flows Through Food Chains and Webs

Matter, Energy, and Organisms

R-report

Energy Flows Through Food Chains and Webs

Why can a hawk hunt many mice, but a forest cannot support thousands of hawks? The answer is hidden in how energy moves through a food web.

Sunlight enters through producers

Almost all energy in a food web begins with the Sun. Plants, algae, and some bacteria are producers: they capture sunlight during photosynthesis and store it in the chemical bonds of sugars. When a caterpillar eats a leaf, the caterpillar gains some of that stored energy. The plant is the first level of the food chain, so every animal in the chain depends on producers even if it never eats a plant directly.

Energy moves from food to eater

A food chain shows one path: grass is eaten by a grasshopper, the grasshopper by a frog, and the frog by a snake. Each arrow means energy moves from the organism being eaten to the organism that eats it. In nature, most organisms eat more than one kind of food and are eaten by several others. Those connected chains form a food web, which is a better picture of a real ecosystem.

Less energy is available at each step

An organism uses most of the energy in its food to move, grow, repair cells, and keep its body working. Much of that energy leaves as heat, and some material is not digested. Only a small portion becomes new body tissue that the next eater can use. Because less usable energy remains at every step, food webs usually have many producers, fewer herbivores, and even fewer top predators.

Energy flows in one direction

A food web connects every eater to the energy stored in producers. Sunlight enters the ecosystem, passes through plants and animals, and gradually leaves as heat. The web can reuse matter, but it cannot reuse energy that has already escaped.

This one-way flow explains why a healthy habitat needs many plants to support fewer herbivores and still fewer predators. When one species changes, the effects can travel through all of the connected feeding paths.