Environmental Engineering Reference
In-Depth Information
estimate the global total ended up with about 8.7 million for all eukaryotes and 7.8
million for all animals (Mora et al. 2011).
The biosphere's vertebrate zoomass is now dominated by domesticated animals,
in particular by seven genera of mammals—cattle (
Bos
), horse and donkey
(
Equus
), water buffalo (
Bubalus
), pig (
Sus
), sheep (
Ovis
), goat (
Capra
), and camel
(
Camelus
)—and four genera of birds—chicken (
Gallus
), goose (
Anse
r), duck (
Anas
),
and turkey (
Meleagris
). In the year 2000 their live weight added up to about 600
Mt, with cattle and water buffaloes accounting for nearly two-thirds of the total
and pigs for more than 10%. With the water content of empty bodies averaging
55% for bovines and just over 60% for pigs (Garrett and Hinman 1969; Mitchell,
Scholz, and Conway 1998), this translates to about 280 Mt of dry weight and
125 Mt C.
The great volume of the inhabited medium, the high mobility of many marine
organisms, and the extraordinary patchiness of seal oor heterotrophs make the
quantii cation of oceanic zoomass quite difi cult. The greatest challenge is to quan-
tify the smallest and hence the most abundant organisms. To begin with, we do not
even know the number of their species to the nearest two or three orders of mag-
nitude. The Census of Marine Microbes began in 2003 with 6,000 kinds of identii ed
species and with expectations of as many as 600,000, but subsequent sampling has
brought hundreds of thousands of new microbial forms, and expectations are now
for at least 20 million, possibly even billions (Qiu 2010)—and we do not know how
many of these organisms are autotrophs and how many are heterotrophic protists.
There are also considerable uncertainties about the deep-sea zoomass: it may be “an
empire lacking food” (McClain 2010), but sampling has come up with biomass
densities 100 times greater than previously reported for depths below 500 m (De
Leo et al. 2010).
Moreover, all known zooplankton taxa are short-lived, with typical longevities
of up to 10-12 weeks in cold waters but only half as long in the warmest seas
(Allan 1976; Laybourn-Parry 1992). This rapid turnover means that at any given
time, the standing biomass of marine zooplankton will be only a small fraction of
the total mass that is available for capture by species at the next trophic level. For
the i sh zoomass there are two independent estimates prepared by Wilson et al.
(2009). The i rst one uses a model that integrates i sh biomass over 36
36 km
cells and is based on photosynthetic production, trophic transfer efi ciency, and
predator body mass ratios: it resulted in 899 Mt (fresh weight) of i sh zoomass.
The second one, a stratii ed ecosystem model, yielded a total more than twice as
high (2.05 Gt). A highly uneven distribution of this zoomass is best illustrated by
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