An ecosystem is a community of living organisms in conjunction with the nonliving components of their environment, interacting as a system.[2] These biotic and abiotic components are linked together through nutrient cycles and energy flows.[3] Energy enters the system through photosynthesis and is incorporated into plant tissue. By feeding on plants and on one-another, animals play an important role in the movement of matter and energy through the system. They also influence the quantity of plant and microbial biomass present. By breaking down dead organic matter, decomposers release carbon back to the atmosphere and facilitate nutrient cycling by converting nutrients stored in dead biomass back to a form that can be readily used by plants and other microbes.[4]
Ecosystems are controlled by external and internal factors. External factors such as climate, the parent material which forms the soil and topography, control the overall structure an ecosystem, but are not themselves influenced by the ecosystem.[5]
Ecosystems are dynamic entities—they are subject to periodic disturbances and are in the process of recovering from some past disturbance.[6] Ecosystems in similar environments that are located in different parts of the world can end up doing things very differently simply because they have different pools of species present.[5] Internal factors not only control ecosystem processes but are also controlled by them and are often subject to feedback loops.[5]
Resource inputs are generally controlled by external processes like climate and parent material. Resource availability within the ecosystem is controlled by internal factors like decomposition, root competition or shading.[5] Although humans operate within ecosystems, their cumulative effects are large enough to influence external factors like climate.[5]
Biodiversity affects ecosystem functioning, as do the processes of disturbance and succession. Ecosystems provide a variety of goods and services upon which people depend
Definition
There is no single definition of what constitutes an ecosystem.[7] German ecologist Ernst-Detlef Schulze and coauthors defined an ecosystem as an area which is "uniform regarding the biological turnover, and contains all the fluxes above and below the ground area under consideration." They explicitly reject Gene Likens' use of entire river catchments as "too wide a demarcation" to be a single ecosystem, given the level of heterogeneity within such an area.[8] Other authors have suggested that an ecosystem can encompass a much larger area, even the whole planet.[9] Schulze and coauthors also rejected the idea that a single rotting log could be studied as an ecosystem because the size of the flows between the log and its surroundings are too large, relative to the proportion cycles within the log.[8] Philosopher of science Mark Sagoff considers the failure to define "the kind of object it studies" to be an obstacle to the development of theory in ecosystem ecology.[7]
Ecosystems can be studied in a variety of ways. Those include theoretical studies or more practical studies that monitor specific ecosystems over long periods of time or look at differences between ecosystems to better understand how they work. Some studies involve experimenting with direct manipulation of the ecosystem.[10] Studies can be carried out at a variety of scales, ranging from whole-ecosystem studies to studying microcosms or mesocosms (simplified representations of ecosystems).[11] American ecologist Stephen R. Carpenter has argued that microcosm experiments can be "irrelevant and diversionary" if they are not carried out in conjunction with field studies done at the ecosystem scale. Microcosm experiments often fail to accurately predict ecosystem-level dynamics.[12]
The Hubbard Brook Ecosystem Study started in 1963 to study the White Mountains in New Hampshire. It was the first successful attempt to study an entire watershed as an ecosystem. The study used stream chemistry as a means of monitoring ecosystem properties, and developed a detailed biogeochemical model of the ecosystem.[13] Long-term research at the site led to the discovery of acid rain in North America in 1972. Researchers documented the depletion of soil cations (especially calcium) over the next several decades