Environmental Engineering Reference
In-Depth Information
Human Appropriation of the Net Primary Production
The i rst assessment of human claims on the NPP dei ned “appropriation” by resort-
ing to three levels of estimates (Vitousek et al. 1986). The low estimate included
only the share of the NPP that people use directly as food, fuel, i ber, or timber.
Food consumption was calculated for the population of i ve billion people by using
the FAO's average per capita food supply of 2,500 kcal/day. That rate rel ects
average per capita food availability according to the food balance sheets prepared
by the FAO for all of the world's nations, but it is not, as the authors incorrectly
assumed, “average caloric intake”: the actual food intake is signii cantly lower (in
some afl uent countries by as much as 40%) than the mean per capita supply (Smil
2000, 2008).
This low calculation assumed that during the late 1970s people consumed annu-
ally 910 Mt of biomass, including 760 Mt of phytomass and 150 Mt of zoomass,
and that it took about 2.9 Gt of phytomass to produce all animal foodstuffs.
Vitousek et al. (2006) derived the total of 2 Gt of marine phytomass needed to
produce the global i sh harvest of 20 Mt of dry mass by assuming that the average
i sh caught was feeding at the second trophic level, and ended up with 2.2 Gt of
woody phytomass (for timber, pulp, and fuel). This added up to about 7.2 Gt of
phytomass, or roughly 3% of the annual NPP as estimated by Ajtay et al. (1979).
The intermediate calculation added all NPP that was “co-opted” by humans,
that is, “material that human beings use directly or that is used in human-dominated
ecosystems by communities of organisms different from those in corresponding
natural ecosystems,” a taxing dei nition that also includes biomass destroyed
during land clearing or conversion. The count began with the NPP of all croplands
(15 Gt/year) and all pastures that had been converted from other ecosystems
(9.8 Gt/year); to this was added the phytomass of natural grasslands that was
either consumed by grazing livestock (800 Mt) or destroyed in anthropogenic
i res (1 Gt). The forest part of the total account included all phytomass cut and
destroyed during wood harvesting and shifting cultivation and in the course of
establishingf plantations (total of 13.6 Gt). The grand total of 40.6 Gt of “co-opted”
terrestrial phytomass amounted to 30.7% of the global NPP estimated by Ajtay
et al. (1979).
Finally, the high estimate includes all of the phytomass from the intermediate
calculations, as well as all “productive capacity lost as a result of converting
open land to cities and forests to pasture or because of desertii cation or overuse
