First off, we need to establish what secondary consumers are. Primary consumers are all herbivores; they eat the producers. The secondary consumers are on the next trophic level up; they eat the primary consumers. They thus help to control the level/number of primary consumers; otherwise, the number of primary consumers would grow too high, and the producers would all be WIPED OUT.
Now, you may be wondering what controls the amount of secondary consumers? The answer is tertiary consumers. What controls the tertiary consumers? At this point, energy does the job of limiting the population. As you move up trophic levels in an ecosystem, the amount of energy in each trophic level goes down by 90%. So like if you started w/ 1000000 joules of energy in the producer level, that would be 100000 in the primary, 10000 in the secondary, and only 1000 in the tertiary.
