Flow of energy in an ecosystem is as follows.\xa0\tThe green plants in a terrestrial ecosystem capture about 1% of the energy of sunlight that falls on their leaves and convert it into food energy.\xa0\tWhen green plants are eaten by primary consumers, a great deal of energy is lost as heat to the environment, some amount goes into digestion and in doing work and the rest goes towards growth and reproduction.\xa0\tAn average of 10% of the food eaten is turned into its own body and made available for the next level of consumers.\tTherefore, 10% can be taken as the average value for the amount of organic matter that is present at each step and reaches the next level of consumers.\xa0\tSince so little energy is available for the next level of consumers, food chains generally consist of only three or four steps.\xa0\tThe loss of energy at each step is so great that very little usable energy remains after four trophic levels.\xa0\tThere are generally a greater number of individuals at the lower trophic levels of an ecosystem, the greatest number is of the producers.\tThe length and complexity of food chains vary greatly.\xa0\tEach organism is generally eaten by two or more other kinds of organisms which in turn are eaten by several other organisms.\xa0\tSo instead of a straight line food chain, the relationship can be shown as a series of branching lines called a food web.Flow of energy is unidirectional because the energy that is captured by the autotrophs does not revert back to the solar input and the energy which passes to the herbivores does not come back to autotrophs.\xa0As it moves progressively through the various trophic levels it is no longer available to the previous level.Secondly, the energy available at each trophic level gets diminished progressively due to loss of energy at each level.