Assertion [A]: The number of trophic levels in the grazing food chain is restricted.
Reason [R]: Energy decreases from lower trophic level to the higher trophic level
Both statements 1 and 2 are correct and statement 2 is the correct explanation of statement 1.
In any kind of food chain, detritus or grazing, the number of trophic levels is limited to 4-5. In a food chain, energy is passed from one link to another. When a herbivore eats, only a fraction of the energy (that it gets from the plants) becomes new body mass; the rest of the energy is lost as waste or used up by the herbivore to carry out its life processes (e.g. movement, digestion, reproduction). Therefore, when the herbivore is eaten by a carnivore, it passes only a small amount of total received energy to the carnivore. Of the energy transferred from the herbivore to the carnivore, some energy will be “wasted” or “used up” by the carnivore. The carnivore then has to eat many herbivores to get enough energy to grow. Because of a large amount of energy that is lost at each link, the amount of energy that is transferred decreases each time. The further along the food chain you go, the less food and energy remains available. There cannot be too many links in a single food chain because the animals at the end of the chain would not get enough food and energy to stay alive.