At each trophic level in a food chain, a large portion of the energy is utilized for the maintenance of organisms which occur at that trophic level and lost as heat. As a result of this, organisms in each trophic level pass on less and less energy to the next trophic levels, than they receive.The longer the food chain, the less is the energy available to the final member of food chain. Food chains generally consist of three or four steps (three or four organisms) because after that the energy available for the next organism will be so small that it will be insufficient to sustain the life of that organism.There are, however, some food chains containing five steps (or five organisms) but there are rarely more than five steps (or five organisms) in a food chain. We will now discuss the accumulation of harmful chemicals in food chains.
On an average only 10 percent of food available to a tropic level is transferred to next tropic leve.since amount of available energy becomes as we move to higher tropic levels so very little usable energy remains afyer four tropic levels. that's why food chain has 3 to 4 tropic levels.