An ecosystem may be considered as a unit within which an assemblage of living organisms interact with each other and with the chemical and physical environment. The resulting natural processes establish a series of complex ecological balances. Ecosystems may operate at a wide range of scales, from long-term global systems such as oceans, to very small, localised or ephemeral systems such as freshwater pools that persist for only short periods.
Some of the interactions both between organisms and with their physical habitats (biophysical interactions) result in ecological processes that interact at different scales to deliver ‘ecosystem services’ or ‘natural capital’ that have value to people. The Millennium Ecosystem Assessment grouped ecosystem services into four broad categories:
- Supporting services, such as nutrient cycling, oxygen production and soil formation. These underpin the provision of the other ‘service’ categories.
- Provisioning services, such as food, fibre, fuel and water.
- Regulating services, such as climate regulation, water purification and flood protection.
- Cultural services, such as education, recreation, and aesthetic value.
For example, the structures within woodland habitats can slow the passage of water into water courses, thereby contributing to the ecosystem regulating service of flood protection. The ecological processes that contribute to ecosystem services, in this case slowing the passage of water, are referred to as ecosystem functions. The habitats and organisms that give rise to the ecological processes are usually described as the ecological assets, and these can be protected to ensure ecosystem services are maintained.