Well, a balanced ecosystem usually has a "measured" amount of herbivores, omnivores, and carnivores. There should always be enough predators to control populations, such as when wolves control a deer population. The deer population then controls the plant population, and this prevents some plants dominating an area and killing other plants for competition for resources. In theory, there should be a balanced food chain. There must almost be optical weather conditions, if there isn't enough rain or sun certain plants will not grow and will effect the "balance" of an ecosystem, depending on the ecosystem of course. A desert would be more equipped for a drought than a forest would, for example.
