The statement that there are fewer organisms at the top of the food pyramid is evidence that the 10% rule applies to food pyramids.
The 10% rule, or the energy pyramid rule, states that only about 10% of the energy from one trophic level is transferred to the next level. This means that as you move up the food pyramid from producers to primary consumers, then to secondary consumers, and so on, energy decreases significantly, which results in fewer organisms being able to be supported at higher levels.
Therefore, the limited availability of energy at higher trophic levels accounts for the smaller number of organisms found there.