All food chains must begin with a producer, an autotroph which can convert raw materials into organic compounds that can be used by the other organisms in its ecosystem. In most cases the autotroph is a plant using photosynthesis to capture cunlight in order to convert carbon dioxide into glucose and oxygen. Some bacteria, e.g. deep-sea bacteria found near volcanic vents, can use chemosynthesis to obtain their energy and these bacteria begin the food chain in their ecosystem.
If you drew all the food chains that exist in a natural ecosystem, you would form a food web. Food webs show all the interacting and interconnecting food relationships. Each step along a food chain is a trophic level. Producers are the first trophic level, first order consumers occupy the second trophic level, second order consumers occupy the third trophic level etc. In a food web, an organism may occupy several trophic levels, depending upon which food source you are investigating, e.g. a bear is a first order consumer when it eats berries but a second order consumer when it eats a fish that ate algae and a third order consumer when it eats a fish that are a fish that ate algae.
Unlike materials, which flow through natural cycles, energy passes in a one-way flow. This flow of energy can be shown as a food chain, beginning with the Sun being the original source of energy for nearly every food chain.
The flow of energy begins when radiant energy is captured by autotrophs in photosynthesis and the...