According to strict theory, balance in an ecosystem would be reached when the various populations in the system are neither increasing nor decreasing but holding steady because the various parts of the ecosystem keep each other in perfect balance.
In a coral reef ecosystem, for example, this might look as follows: (1) Coral reef absorbs nutrients directly from the water and from plankton swimming past the reef, (2) small parrotfish feed on the coral reef, and (3) larger fish, such as snappers and barracuda, feed on the smaller fish like the parrotfish. Here is where the theory breaks down, however, because if the coral reef is not growing and expanding its territory, it is considered unhealthy or dying. Similarly, as the reef itself expands, it is able to support more and more animals in categories (2) and (3) respectivelymeaning that none of the populations is actually stable, but rather growing. In reality, then, balance in an ecosystem is a dynamic, changing thing which generally implies a gradual growth of the populations within the ecosystem.
But when we talk about balance in ecosystems, it not only refers to balance within the ecosystem, but also to balance between ecosystems. To wit: no ecosystem exists in a vacuum. Each ecosystem has other systems butting up against it, intersecting it, and often intermeshing with it. To continue the example from above, a coral reef is located in water, putting it in close proximity to a marine ecosystem (or more than one), where there may be even larger fish, sharks, and other animals. In this situation, balance between ecosystems means that the sharks don’t eat all the parrotfish, nor do the fish living on the coral reef suddenly migrate out to the marine ecosystem.
On the other hand, Andrewatha and Birch proposed the idea in their 1954 “The Distribution and bundance of animals” that balance in ecosystems is actually impossible. Because they observed that territorial behavior, rather than a check in food supply, limited population numbers, they viewed the flux in population size as part of normal behavior patterns rather than part of an ecosystem response. Although this view is probably extreme, it illustrates the truth that a static population is not possible in the real world.
Seen from a global perspective, balance in the world ecosystem depends upon balance between ecosystems in the world. Such is the subject of many current headlines about climate change, overfishing, and the like. Yet it is important to keep perspective that neither has the world’s ecosystem remained stagnant over time, populations have increased, decreased, and even gone extinct, entirely independent of human involvement. The question becomes, then: To what extent human involvement has changed the balance in ecosystems beyond what it would have been without any human interference? The answer, right now, is anybody’s guess.