Ecosystem services are essential biological functions that are provided
free of charge by living organisms and which benefit humankind.
They include oxygen production by plants, climate control by forests,
nutrient cycling, natural pest control, pollination of crop plants, etc.
(Daily 1999). These services have been valued at $US33 trillion (1012) per
year, almost double the $US18 trillion yearly global national product
(Costanza et al. 1997).