Environmental Engineering Reference
In-Depth Information
Coleman, and Wiebe (1998), assuming an average of
about 450 g/m
2
, ended up with 57 Gt of soil bacteria.
A more conservative assumption of 250 g/m
2
yields
about 33 Gt. But these uncertainties are minor compared
to those we face in estimating the mass of subterra-
nean (and subsea) prokaryotes. Whitman, Coleman, and
Wiebe (1998) offered a range of 48-477 Gt, but even
this span may be much too narrow. Fungal biomass in
soils also ranges widely, from 10
0
g/m
2
to 10
2
g/m
2
(Bowen 1966; Reagan and Waide 1996). A fairly conser-
vative global aggregate would be 7 Gt, but a mass twice
as large (about 100 g/m
2
of ice-free surface) may not be
excessive.
Earthworms are the most conspicuous soil inverte-
brates, but their mass is usually just around 5 g/m
2
, al-
though in cultivated soils it may be well over 10 g/m
2
(Hartenstein 1986). Nematodes average about one-tenth
of annelid biomass, and ants and termites each add typi-
cally no more than 0.1 g/m
2
(Brian 1978). Invertebrates
(dominated by annelids, nematodes, and microarthro-
pods) range between 7 g/m
2
and 10 g/m
2
, and their
global biomass may be close to 1 Gt, roughly 20 EJ.
Reptiles and amphibians often dominate vertebrate
zoomass in tropical forests, and their biomass density
may rival that of invertebrates (Reagan and Waide 1996).
Densities of rain forest mammals are generally low, 0.1-
2 g/m
2
, whereas in the ungulate-rich savannas they
can surpass 3-4 g/m
2
(Plumptree and Harris 1995).
Small mammals, mostly rodents, add generally less than
0.2 g/m
2
, and zoomasses of insectivorous mammals are
mostly below 0.05 g/m
2
(Golley and Medina 1975).
And the total for all carnivores in the Ngorongoro
Crater, one of the world's best places for large predators
to hunt ungulates,
Similarly, total avifaunas usually do not surpass 0.05
g/m
2
(Reagan and Waide 1996). In aggregate, verte-
brate zoomass rarely sums to more than 1 g/m
2
over
large areas, and uncertainties in its quantification thus
hardly matter in the overall biospheric count. Errors
inherent in estimating prokaryotic biomass are easily 1
OM larger. Published estimates of terrestrial invertebrate
and vertebrate biomass (excluding domestic animals)
range between 1 Gt and 2.1 Gt, with wild mammals con-
tributing less than 10 Mt (Bowen 1966; Whittaker and
Likens 1975; Smil 2001). Great depth of the inhabited
medium, and extraordinary patchiness and mobility of
many oceanic heterotrophs, makes the quantification
of marine zoomass exceedingly difficult. Published esti-
mates range from 0.7 Gt to 1.1 Gt, with invertebrates
dominant and mammals contributing less than 5% of the
total. Mann (1984) estimated total fish biomass at 300
Mt of fresh weight. The global grand total of close to 10
Gt, or roughly 200 EJ, of heterotrophic biomass equals
less than 0.2% of all phytomass stores.
Although it is possible to construct similar sets of plan-
etary totals for heterotrophic productivity (production/
year), a high degree of uncertainty involved in these ex-
ercises, particularly because of the rapid turnovers of
decomposers and invertebrates, may result in totals of a
wrong order of magnitude. Sexual reproduction domi-
nates the production in metazoa as genetic recombina-
tion provides a mechanism for purging deleterious
mutations and improving chances of environmental
adaptation. But sexual reproduction is not inherently su-
perior; many successful organisms are asexual (fungi) or,
as some parasitic protozoa, clonal (Ayala 1998). Sexual
heterotrophs have a continuum of reproductive strategies
ranging from a single prodigious reproductive bout
(semelparity) to successive breeding (iteroparity), which
is less than 0.03 g/m
2
(Schaller
1972).
