U.S. Fish and Wildlife Service
What is an Ecosystem?
The U.S. Fish and Wildlife Service defines an ecosystem as a geographic area and all its living components (e.g., people, plants, animals, and microorganisms), their physical surroundings (e.g., soil, water, and air), and the natural cycles that sustain them (e.g., precipitation, drought, fire, grazing). The term ecosystem was coined in 1935 by the British ecologist Sir Arthur George Tansley, who described natural systems in "constant interchange" among their living and non-living parts.
Since that time, scientists have devoted much time and resources toward gaining a better understanding of ecosystem structure and function from the global to localized scale. Part of this work has included developing a National Hierarchical Framework for classifying and mapping ecosystems at different geographical scales. Recognition of ecosystem scale and hierarchy provides managers context from which ecosystem assessment, analysis, and management can occur.
Sitting at the top of the ecosystem hierarchy is the planet's entire living environment, known as the Biosphere. Within the biosphere there are several categories of living communities referred to as ecological units (e.g., domains, divisions, provinces, section, sub-section, etc). Ecological units are generally characterized by their dominant vegetation, such as grasslands, forests, or deserts, and by their different biological and physical potentials.
The structure and function of an ecosystem is largely determined by energy, moisture, nutrient, and disturbance regimes, which in turn are influenced by a variety of biological and non-biological factors, including climate, geology, flora, fire, hydrology, and wind. Ecosystem function depends on inputs, outputs, and the cycling of materials and energy.
Ecosystem function is often described biologically in terms of trophic levels. Plant matter in an ecosystem makes up the first trophic level and thus are known as primary producers. A plant's ability to convert energy from the sun into food is a process known as photosynthesis. The second trophic level of an ecosystem, the primary consumers (herbivores), are the animals and insects that obtain their energy solely by eating the primary producers (plants). The third trophic level is composed of secondary consumers, carnivorous animals that feed on herbivores. At the fourth level are the tertiary consumers, carnivores that feed on other carnivores. Finally, the fifth trophic level consists of the decomposers, organisms such as fungi and bacteria that break down dead or dying matter into nutrients that can be used again.
Some or all of these trophic levels combine to form what is known as a food web, the ecosystem's mechanism for circulating and recycling energy and materials. In aquatic ecosystems for instance, microscopic plants (algae) use sunlight to produce energy in the form of carbohydrates. Primary consumers such as zooplankton and small fish feed on the microscopic plant matter, and are in turn eaten by secondary consumers, larger fish such as salmon and trout. Bears play the role of the tertiary consumers in the aquatic food web by catching and eating the salmon and trout. Bacteria and fungi that feed on and decompose the salmon and trout carcasses left behind by the bear enable chemical nutrients tied up in the fish to leach back into the soil and water, where they are absorbed by algae and plants. In this way energy, originally captured by the algae from sunlight, is recycled back into the ecosystem through a complex food web.