The size of a population in an ecosystem depends on the food sources available, as well as the food chain process. It often happens though, especially nowadays because of the human impact, that a population might become too large or too small for the ecosystem. Usually the populations of the herbivores become too large at certain places, and that is because of the lack of predators. When that happens, they will overgraze and destroy most of the vegetation. That will lead to starvation, eventual death, and not just for that species, but for other species as well, thus resulting in a collapse o the ecosystem. The opposite situation, when there's a too small of a population of a species, usually occurs with the predators. When that happens, they are not able to control the population of the herbivores, thus resulting in the example with the too large population, thus a collapse of the ecosystem.
