Examples of improved products?

What examples exist where human civilizations improved the baseline environmental quality of their territory?

• I am looking for examples where civilizations in some way improved the ecological conditions for all or most organisms that inhabited or interacted with their territory. An indicator might be increase in endemic biomass, more stable population levels across species, healthier individuals across species, etc. I want to stay away from examples where humans merely improved environmental conditions for their benefit irrespective of endemic species populations.

• Answer:

  I'm not sure if improved, but at least made it possible for life, it was the Aztec Empire at their beginning. When they first arrived to the Mexico Valley (nowadays Mexico City) they were sent to the worst part of all the valley, a big swamp known as Texcoco Lake. The pre-aztec civilizations chose this part because they knew that no city could be built there, because of the swampy condition of the lake. The Aztecs developed a way to construct and use the swamp to their convenience creating the "Chinampas" that were artificial "islands" where they were able to grow corn, the essential food of the Aztec empire.  This made the place able for living, and the major part of the city was built over this Chinampas. It's a shame that after 600 years the Texcoco Lake is almost dry consequence of the over population of the zone, Mexico City. But in the beginning the idea of the Aztecs was to live with the lake, not its disappearance.