Final answer:
The length of a food chain, usually consisting of 4-5 links, is referred to as trophic levels. These levels indicate the position of organisms within the food chain, which is limited by the inefficiency of energy transfer between levels.
Step-by-step explanation:
Charles Elton first pointed out that the length of a food chain is usually 4-5 links, which he called trophic levels. The concept of trophic levels is a way to describe the different feeding positions within a food chain or food web. Energy transfer between these levels is typically inefficient, with a great deal of energy lost as heat due to the second law of thermodynamics. Therefore, after four to six energy transfers, there is usually not enough energy left to support an additional trophic level. This inefficiency is why most food chains in ecosystems tend to have a limited number of levels.