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 forests. 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 waterbody, or maybe the lake plus its terrestrial watershed. Because all of these units consist of organisms and...