Assertion : Most food chains or webs have only four or five trophic levels
Reason : Trophic efficiencies are generally only about 10 % in different ecosystem .

Assertion and Reason both are true and reason is the correct explanation of the assertion .

Assertion: Most food chains or webs have only four or five trophic levels.

This assertion is generally true. In many ecosystems, food chains or food webs tend to have around four or five trophic levels. A trophic level represents the position of an organism in a food chain, indicating its position as a producer, primary consumer, secondary consumer, etc. As you move up the trophic levels, energy transfer becomes less efficient, and this contributes to the limitation of the number of trophic levels in a typical food chain or web.

Reason: Trophic efficiencies are generally only about 10 % in different ecosystem.

Trophic efficiency is the percentage of energy from one trophic level that is transferred to the next trophic level. As mentioned above, this efficiency is typically only about 10%. This means that for every 100 units of energy that are available at the first trophic level, only about 10 units of energy will be available at the second trophic level.

The low trophic efficiency is due to a number of factors, including the loss of energy as heat, the use of energy for metabolic processes, and the inefficiency of the digestive system. As a result, most food chains or webs have only four or five trophic levels.