Green plants use sunlight and carbon dioxide from the atmosphere to produce carbohydrates by the process of photosynthesis. Since plants produce food for themselves, they are known as producers. So, they are the first link in any food chain. There is only 10% flow of energy from one trophic level to the next higher level. The loss of energy at each step is so great those very little usable remains after four or five trophic levels. Hence only 4 to 5 trophic levels are present in each food chain.