Every step or level of the food chain forms a trophic level. Autotrophs are considered to be at the first trophic level of all food chains because they are the green plants, which are the only organisms which can fix up the solar energy and make it available for heterotrophs or the consumers. The main reason for the limited number of trophic levels in nature is because a very little amount of energy is available to the last trophic level as energy is lost at every level to the environment.