Biodiversity refers to the variety of life on Earth, including all living organisms from the smallest microbes to the largest mammals, as well as their genes and the ecosystems they form. It is a measure of the variety and variability among living organisms, and is often used as a measure of the health of ecosystems. Biodiversity is important for many reasons, including the provision of ecosystem services such as food, water purification and climate regulation. It also provides a source of inspiration, recreation and cultural identity for humans.