An ecosystem is a natural unit consisting of all plants, animals and micro-organisms (biotic factors) in an area functioning together with all of the physical (abiotic) factors of the environment. Ecosystems can be permanent or temporary. An ecosystem is a unit of interdependent organisms which share the same habitat. Ecosystems usually form a number of food webs. Ecosystem services are fundamental life-support services upon which human civilization depends and can be direct or indirect. Examples of direct ecosystem services are: pollination, wood and erosion prevention.