Final answer:
The primary factor that limits the length of food chains in ecosystems is energy availability, as energy is lost as heat between trophic levels due to the second law of thermodynamics, leading to limited trophic efficiency.
Step-by-step explanation:
The primary factor that limits the length of food chains in ecosystems is energy availability. Energy is lost as heat between each trophic level due to the second law of thermodynamics. This loss of energy is so significant that after a few trophic energy transfers, there may not be enough energy left to support another trophic level. For example, after four to six transfers, which is common by the time you reach the apex predators, the remaining energy is insufficient to sustain more levels. This concept is shown in ecosystems like Lake Ontario, where typically only three energy transfers occur from the primary producer to the apex consumer.
This energy loss results in trophic efficiency, which is the percentage of energy transferred from one trophic level to the next. Typically, only about 10% of the energy from one trophic level is transferred to the next, leading to a limit in how long a food chain can be.