Which of the following limits the number of trophic levels in a food chain?
(a) Decrease in energy at higher trophic levels
(b) Deficient food supply
(c) Polluted air
(d) Water
(a) When we move from one trophic level to another, energy decreases. This is so because energy is lost as metabolic heat when the organisms from one trophic level are consumed by organisms of the next trophic level.
In this way, the organism at a higher level gets less and less energy at progressive levels. The number of trophic levels is restricted to 3-4 because, from that point forward, the energy accessible for the following level will be too little, i.e., it will be too small to sustain the life of the organisms. Hence, the decrease in energy at higher trophic levels limits the number of trophic levels in a food chain.
Therefore option (A) is correct