The ecosystem is the major ecological unit, containing both biotic and abiotic components through which nutrients are cycled and energy flows. The function of the ecosystem is related to the flow of energy and the cycling of materials through the structured components of the ecosystem. The total amount of energy that flows through a system depends upon the amount fixed by plants or producers. As energy is transferred from one feeding level to the next, a considerable portion is lost from further transfer. This limits the mass and number of organisms that can be maintained at each feeding level.
Ecosystems tend toward maturity.
In doing so, they pass from a less complex to a more complex state in a process called succession. Early stages are characterized by relatively high energy flow per unit of biomass and high net productivity. Later stages are characterized by low energy flow of biomass and low net production, with most of the energy going into system maintenance. When an ecosystem is exploited and that exploitation is maintained, the maturity of the ecosystem declines. The major functional unit of the ecosystem is the population.