Environmental Engineering Reference
In-Depth Information
(overgrazing, excessive erosion),” and the authors thought this total “seems a
reasonable statement of human impact on the biosphere” (Vitousek et al. 1986,
368). These additions brought the grand total to 58.1 Gt, an equivalent of 38.8%
of the global NPP estimate of Ajtay et al. (1979). This i nding led to the most quoted
sentence of the entire report: “Thus, humans now appropriate nearly 40% of poten-
tial terrestrial productivity ” (Vitousek et al. 1986, 372), and the authors added that
the humans also affect much of the remaining 60%, “often heavily,” but offered no
quantii cations of that claim.
The second attempt to quantify the human appropriation on the Earth's NPP
was expressed in energy terms. Wright (1990) estimated this impact in order to
i nd the likely effect on the survival of wild species whose existence depends on
metabolizing the available primary production, the supply of which is reduced by
food harvesting, wood gathering, and other human interferences. Wright sought to
compare human appropriations not with the actual NPP but with the amount of
primary production that would be available in natural ecosystems “in the absence
of human impact by expressing them as a percentage of pre-impact global net
primary production” (Wright 1990, 189). The entire exercise was thus predicated
on a fairly accurate knowledge of potential primary production, and Wright based
his estimate on the already cited work of Bazilevich, Rodin, and Rozov (1971) and
Olson, Watts, and Allison (1983), and opted for the total of 160 Gt/year of phyto-
mass, equivalent to about 2,800 EJ.
Wright's appropriation count focused on what he called production effects, that
is, impacts that changed (lowered) the natural potential of the NPP; such impacts
included forage consumption by livestock (64 EJ), habitat destruction (the conver-
sion of natural ecosystems to cropland and urban areas, altogether worth 480 EJ),
and habitat degradation (the conversion of forests to grazing land, desertii cation,
and the production of secondary growth in forest and i eld patches, adding up
to 112 EJ). The grand total of 656 EJ was equal to 23.5% of the Earth's annual
potential production, and by using specii c energy curves for groups of heterotrophs,
Wright estimated that this human diversion of natural energy l ows translated into
endangerment of 2%-7% of terrestrial species and predicted (conservatively, as
he noted) that by the year 2000, 3%-9% of the world's species could be extinct or
endangered.
The third attempt to calculate the human appropriation of primary production
(Rojstaczer, Sterling, and Moore 2001) was essentially an update of the 1986
assessment: the authors followed the original template for the intermediate calcula-
tion, arguing that the low estimate, which dealt only with direct consumption,
