Ecosystem: Definition and Patterns

An ecosystem is the basic functional unit of ecology -- a defined space in which a community of living organisms interacts continuously, both among themselves and with the non-living physical and chemical surroundings they occupy, so that energy and matter flow through the whole system in a broadly self-regulating way. The term was coined by the British ecologist A.G. Tansley in 1935 to capture exactly this idea: that a habitat and the organisms living in it cannot really be studied apart from one another, because each shapes the other. A patch of forest is not just a list of the tree, bird and insect species present -- it is those species together with the soil they root in, the rainfall that waters them, the sunlight that drives their growth and the temperature range they can tolerate, all interacting as one working system.

Ecosystems occur at every scale imaginable. A temporary rain-filled puddle, a garden aquarium, a single rotting log crawling with fungi and invertebrates, and an entire ocean basin are all, in the strict ecological sense, ecosystems -- what changes between them is only their size, complexity and the number of species involved, not the basic definition. Because of this enormous range of scale, ecologists usually sort ecosystems into two broad patterns based on where they occur.

The first pattern divides ecosystems by habitat into terrestrial and aquatic types. Terrestrial ecosystems are land-based -- forests, grasslands, deserts and agricultural land are the commonest examples, each shaped strongly by the local climate, especially rainfall and temperature, and by the type of soil present. Aquatic ecosystems, by contrast, are water-based, and are further split into freshwater ecosystems (ponds, lakes, rivers and streams) and marine ecosystems (estuaries, coastal waters and the open ocean), each with its own characteristic physical conditions such as water depth, salinity, current and light penetration, which in turn govern which organisms can live there.

The second pattern divides ecosystems by origin into natural and artificial (or man-made) types. A natural ecosystem, such as a forest, a natural lake or a coral reef, has developed and continues to function without ongoing human management -- it is self-sustaining, drawing only on sunlight, rainfall and its own internal nutrient cycling to keep going indefinitely. An artificial ecosystem, such as a cultivated crop field, a managed fish pond or a garden, has been deliberately created or substantially modified by human activity to serve a specific purpose, and typically needs continuous human inputs -- irrigation, fertiliser, weeding, feed, or periodic harvesting and re-sowing -- to keep functioning in the form intended; if that human management stops, an artificial ecosystem generally does not persist unchanged but instead begins to shift, through the process of succession, toward a more self-sustaining natural state.

Recognising these two patterns -- terrestrial versus aquatic, and natural versus artificial -- gives a simple but useful first classification for any ecosystem a student encounters, before going on to examine, as the rest of this chapter does, exactly what components make an ecosystem up and how energy and matter move through it once it is running.