An ecosystem is balanced when the biotic (living) and abiotic (non-living) parts of the ecosystem are in equilibrium. This means that the nutrients are able to cycle efficiently, and no community of organisms or natural phenomena is interrupting the flow of energy and nutrients to other parts of the ecosystem.
Although energy must be constantly inputted into an ecosystem, the nutrients are recycled through various levels of the ecosystem. This recycling allows the biogeochemical cycle to be a closed system, where the nutrients are neither entering nor leaving the system.
The water cycle is one example of biogeochemical cycling where, in an abbreviated description, water is evaporated from oceans, condenses into clouds, precipitates onto land, and then flows through rivers back to the ocean.
Nutrient cycles exist for carbon, nitrogen, sulfur, oxygen and phosphorous.
Abiotic factors in the ecosystem may be held in one step of the cycle for extended periods of time, resulting in an accumulation of that factor at one stage. The oceans are an example of a reservoir for water.
Part of the balanced ecosystem involves population control of the living organisms. For example, a population increase of one species could adversely affect the population of competitors and prey but benefit predators, which, in turn, could harm or benefit other species.