Food chain is a series of depend organisms , they depend on each other for survival . The basic relationship between the trophic level of a food chain is that all organisms depend on each other , if one will extinct this will affects the whole chain . 

Food chains start with plant because plants are only producers and are consumed by the primary consumers . plants only can make their own food from sunlight so they are in first trophic levels. 

Food chain have a limited number of trophic levels as while the transfer of energy takes place 90 % of energy is lost in environment and only 10% is passed to next level.
e.g. : plants have 1000 j of energy then human would receive only 100 j of energy and rest all energy gets lost in environment.

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