Environmental Engineering Reference
In-Depth Information
European and North American pastures no more than 25%-50% of their annual
primary productivity is eaten by grazing livestock (Detling 1988; Hodgson and Illius
1996; Lemaire et al. 2000). Assuming that between 30% and 40% of grassland NPP
(1.5-2 t/ha) is consumed by domesticated grazers would yield a total phytomass
removal of at least 5 and as much as nearly 7 Gt. More elaborate alternatives to
estimate the grazing claim are possible, but all of them depend on concatenated
assumptions and may not yield any superior results.
Detailed reconstructions start with average body masses of grazers (they vary
substantially even for the same species, depending on breeds and typical feeding),
their typical metabolic rates, take-off rates (animal shares killed annually), average
ages at slaughter, and typical meat yields per carcass (or typical lactation rates).
Wirsenius (2000) acknowledged these complications but undertook such a detailed
examination on the global and regional levels: he concluded that during the mid-
1990s, domesticated grazers consumed about 5.8 Gt of dry phytomass (23.3 Gt of
fresh weight). Comparisons with other estimates of phytomass “appropriated” by
grazing show the extent of unavoidable uncertainties: the totals were 10.6 Gt in
Vitousek et al. (1986) and 8.89 Gt (assuming twice the total of 4.45 Gt C) in Haberl
et al. (2007), compared to the 5-7 Gt I estimated by doing just two multiplications.
Aquatic Species
Because marine species now provide about 15% of the world's dietary protein and
because much of the world's ocean either has been exploited to capacity or is now
overexploited, it is important to estimate the evolution of global i sh and inverte-
brate harvests. In contrast to the relatively abundant research on terrestrial harvests,
little has been published on marine catch and the ocean's primary production. Most
notably, all recent studies of the HANPP have chosen to ignore the marine share of
zoomass harvests. But Vitousek et al. (1986) estimated the human claim on marine
phytomass by converting 75 Mt of fresh-weight landings of i sh and aquatic inver-
tebrates to 20 Mt of dry weight and then assuming that the average catch takes
place at the second trophic level and that the transfer efi ciencies are 10% in each
case. This would, obviously, result in about 2 Gt/year of oceanic phytomass con-
sumed by the aquatic zoomass harvested by humans, and Vitousek et al. (1986) put
that rate at 2.2% of the total oceanic NPP.
The assumption of an average energy transfer efi ciency of 10% has been com-
monly made when calculating interlevel transfers ever since Lindeman's (1942)
pioneering research on energy transfers in Wisconsin's Lake Mendota. Moreover,
