Hint: Until equilibrium is reached, a population will go through the phases of ecological succession. Ecological succession is the concept that describes how a community evolves in terms of the structure and dominant organisms present in that community over time.
Complete answer: Ecological equilibrium refers to a condition that exists in such a manner that a minor interruption or transition is counterbalanced with another change in order to return the environment to its original state.
Thus, it is not in equilibrium when a group goes through several shifts through each stage of succession. New organisms join and take the place of those in the ecosystem and the community does not revert to the previous way it was. The population structure is constant as a community finally approaches the last stage of succession, what is usually referred to as a climax community. Thus, it would counterbalance any minor disruptions. The machine is in relative harmony.
It was conceived of as a predictable phase by the early ecologists who first studied succession, in which a group often went through the same sequence of phases. They also assumed that a permanent, unchanging final condition called a climax culture, primarily decided by an area's climate, was the end product of succession. In the example above, for example, the climax population will be the mature oak and hickory forest.
Note: Climax community is a natural community in which plant or animal species remain intact and live in harmony with each other and their climate. The final stage of succession is a climax culture, remaining largely intact until destroyed by an incident like fire or human intervention.