The notion of ecosystem (or ecological system) refers to indeterminate ecological assemblages, consisting of communities of organisms and their environment. Ecosystems can vary greatly in size. Small ecosystems can be considered to occur in tidal pools, in a back yard, or in the rumen of an individual cow. Larger ecosystems might encompass lakes or stands of forest. Landscape-scale ecosystems comprise larger regions, and may include diverse terrestrial and aquatic communities. Ultimately, all of Earth's life and its physical environment could be considered to represent an entire ecosystem, known as the biosphere.
Often, ecologists develop functional boundaries for ecosystems, depending on the particular needs of their work. Depending on the specific interests of an ecologist, an ecosystem might be delineated as the shoreline vegetation around a lake, or perhaps the entire water-body, or maybe the lake plus its terrestrial watershed. Because all of these units consist of organisms and their environment, they can be considered ecosystems.
Through biological productivity and related processes, ecosystems take sources of diffuse energy and simple inorganic materials, and create relatively focused combinations of these, occurring as the biomass of plants, animals, and microorganisms. Solar electromagnetic energy, captured by the chlorophyll of green plants, is the source of diffuse energy most commonly fixed in ecosystems. The most important of the simple inorganic materials are carbon dioxide, water, and ions or small molecules containing nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, and some other nutrients.
Because diffuse energy and simple materials are being ordered into much more highly structured forms such as biochemicals and biomass, ecosystems (and life more generally) represent rare islands in which negative entropy is accumulating within the universe. One of the fundamental characteristics of ecosystems is that they must have access to an external source of energy to drive the biological and ecological processes that produce these localized accumulations of negative entropy. This is in accordance with the second law of thermodynamics, which states that spontaneous transformations of energy can only occur if there is an increase in entropy of the universe; consequently, energy must be put into a system to create negative entropy. Virtually all ecosystems (and life itself) rely on inputs of solar energy to drive the physiological processes by which biomass is synthesized from simple molecules.
To carry out their various functions, ecosystems also need access to materials—the nutrients referred to above. Unlike energy, which can only flow through an ecosystem, nutrients can be utilized repeatedly. Through biogeochemical cycles, nutrients are recycled from dead biomass, through inorganic forms, back into living organisms, and so on.
One of the greatest challenges facing humans and their civilization is understanding the fundamentals of ecosystem organization—how they function and how they are structured. This knowledge is absolutely necessary if humans are to design systems that allow a sustainable utilization of the products and services of ecosystems. Humans are sustained by ecosystems, and there is no tangible alternative to this relationship.