How does energy move through a food web?

A hawk can obtain energy that first entered grass as sunlight. Grass uses light in photosynthesis to make energy-rich organic material from carbon dioxide and water. Eating transfers some of that material and its chemical energy to another organism.

In the small hypothetical web, a rabbit and a grasshopper eat grass. A hawk eats these consumers. Each arrow points from food toward eater: it follows the direction of energy-rich material being taken in. Two routes therefore connect grass to the hawk, one through each intermediate consumer.

These connections do not mean equal amounts of energy travel along every arrow. An animal uses food energy for movement, growth and maintaining its body. Much is transferred to the surroundings as heat. Some food is not eaten, and some eaten material leaves as waste. Only a portion becomes body material available to another eater.

Dead organisms and waste provide food for decomposers and other organisms in pathways beyond this small drawing. Their activity helps return matter to forms that organisms can use again. Matter can circulate through many such routes. Usable energy continually spreads into less useful forms, so the ecosystem needs continuing energy input.

A food web is consequently both a map and a selective view. It reveals possible feeding connections, not every diet, population count or amount of energy. The hawk’s two routes illustrate how organisms are linked, while the heat arrows explain why adding more steps does not multiply the energy available.