deep green, definition of

An ecosystem is a natural unit consisting of all plants, animals and micro-organisms in an area functioning together with all the non-living physical factors of the environment.

Wikipedia Entry

Note: Just be clear, an ecosystem by definition does include 'us' and what we do, we all inhabit the same set of systems. Therefore be careful of ecosystem analysis's done without consideration of what impact (both in the good and bad sense) humans have made. There are very few ecosystems that we do not engage with over the longer term.