Question

why are there fewer top - level predators compared to primary consumers?
top - level predators rarely add to the population.
all top - level predators have specific prey to hunt.
there is less available energy at higher trophic levels.

Explanation:

In an ecosystem, energy transfer between trophic levels is inefficient. Only about 10 - 20% of energy is passed on from one level to the next. Since top - level predators are at the highest trophic level, there is less available energy to support a large population compared to primary consumers which are at a lower trophic level.

Answer:

There is less available energy at higher trophic levels.