Energy goes from one plant or animal to another. Each time an animal is eaten, its energy is passed on to the animal that eat it.
The energy in most ecosystems flows through the food chain. In terrestrial ecosystems and most marine ecosystems, the energy flow starts in green plants that harvest some of the sun’s energy. The plants are then consumed by animals, bacteria and other creatures, which are themselves eaten by other creatures. At each step in the food chain, energy is lost as heat.
Scientists refer to green plants as producers because they effectively produce the food for the entire food chain. Green plants use the process of photosynthesis to capture light energy, combine it with carbon dioxide and water, and then form sugars to store the energy chemically. The organisms that eat these plants are called consumers. Sometimes a distinction is drawn between primary consumers, which eat green plants, and secondary consumers, which eat other consumers.
The amount of energy present at each step in the process is much less than was in the preceding step. This means that eventually, the energy is gone, and no further energy is left. At the bottom of the food chain, organisms called scavengers feed off the last remaining bits of energy in an organism. These scavengers then release the remaining minerals and nutrients from their food into the environment where they are recycled.