Scientists use the term consumer to refer to organisms that eat other organisms. Consumers differ from producers, as they do not produce their own food. Most producers are green plants, but many organisms are consumers.
The distinction between consumers and producers arises as one studies the flow of energy through the Earth's various ecosystems. The vast majority of the energy flowing into the planet each day comes from the sun. Green plants are one of the only organisms that can harness this energy and turn it into a useful form. Green plants accomplish this task through a process called photosynthesis. Most other organisms are unable to create their own food, and so they must subsist on other organisms. Many consumers eat green plants, and scientists call these organisms primary consumers. Other organisms hunt and eat these primary consumers. Scientists often call such animals secondary consumers, but the term predator also applies as well. Secondary consumers have predators of their own, and in practice, all organisms eventually become food for other organisms. Some creatures subsist entirely on dead plants and animals. In the process, these creatures release and recycle the nutrients contained in their food back into the ecosystem. These animals bear the name decomposers, and they help to provide the producers with the raw materials necessary for food production.