Net primary production (NPP), the amount of solar energy that is converted to plant organic matter through photosynthesis, represents the primary source of food for all ecosystems on earth, including humans. The more net primary production that is hoarded by humans, the less food there is for the rest of the living planet. Imhoff et al. (2004) calculate the terrestrial NPP and the human appropriated NPP (HANPP), or the amount used by humans, for all regions of the globe. As a percentage of NPP, they find that HANPP ranges from 6% in South America to over 80% in South Central Asia, with 20% being their estimate of the global average. In North America the value is about 24%. These estimates reveal that some areas of the globe will find it hard to maintain natural ecosystems, since humans are consuming such a large fraction of the basic food supply.
Reference
Imhoff, M. L., L. Bounoua, T. Ricketts, C.Loucks, R. Harris, and W. T. Lawrence, 2004: Global patterns in human consumption of net primary production. Nature, 429, 870-873.