Seals are part of the ecosystem in the Danish Wadden Sea.
Photo: Svend Tougaard
An ecosystem is a functional unit consisting of all the living organisms (plants, animals, and microbes) in a given area, and all the non-living physical and chemical elements in their environment, linked together through nutrient cycling and energy flow. Ecosystems vary in size, from a log, a pond, a field, a forest, at one end of the scale, to the earth's biosphere, at the other. However, they always function as a whole unit.