Energy, water, nitrogen and soil minerals are essential abiotic components of an ecosystem. The energy used by ecosystems comes primarily from the sun, via photosynthesis. Photosynthesis uses energy from the sun and also captures carbon dioxide from the atmosphere. Animals also play an important role in the movement of matter and energy through ecosystems. They influence the amount of plant and microbial biomass that lives in the system. As organic matter dies, carbon is released back into the atmosphere. This process also facilitates nutrient cycling by converting nutrients stored in dead biomass back to a form that can be used again by plants and other microbes.
Ecosystems are controlled by both external and internal factors. External factors such as climate, the parent material that forms the soil, topography and time each affect ecosystems. However, these external factors are not themselves influenced by the ecosystem. Ecosystems are dynamic: they are subject to periodic disturbances and are often in the process of recovering from past disturbances and seeking balance. Internal factors are different: They not only control ecosystem processes but are also controlled by them. Another way of saying this is that internal factors are subject to feedback loops.
Humans operate within ecosystems and can influence both internal and external factors. Global warming is an example of a cumulative effect of human activities. Ecosystems provide benefits, called "ecosystem services", which people depend on for their livelihood. Ecosystem management is more efficient than trying to manage individual species.
There is no single definition of what constitutes an ecosystem. 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. Other authors have suggested that an ecosystem can encompass a much larger area, even the whole planet. 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. 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.
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. Studies can be carried out at a variety of scales, ranging from whole-ecosystem studies to to studying microcosms or mesocosms (simplified representations of ecosystems). 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.
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. 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.