The fungi decompose the others and return the nutrients to the Earth for the cycle to begin again. The food web is self regulating. It is a system of negative feedback . More plants mean more primary consumers. More primary consumers mean less plants.
More primary consumers mean more secondary consumers, which means less primary consumers.
Get rid of one, and the others don t work. For example, if you get rid of Wolves, mountain lions, etc. Too many deer and they eat all the plants. Eat all the plants, and they starve. You gotta have balance.