Environmental Engineering Reference
In-Depth Information
imperatives of heterotrophic life, and it may be more
rewarding to look at several important links between for-
aging and energy expenditure.
Garland (1983) has reviewed these critical relations—
daily movement distance (DMD), incremental cost of
locomotion (ICL), daily energy expenditure (DEE), and
ecological cost of transport (ECT)—for mammalian spe-
cies. DMD (in km, for animals ranging from 56 g,
Dipodomys, to 6 t, elephant) scaled as 1.038M
0
:
25
(M in
kg), but it varied almost 2 OM for a given body mass
(fig. 4.8). Carnivores move much more than herbivores
(respective M multiples are 3.87 and 0.87) but the expo-
nents are identical (0.22). ICL (in J/km, a constant in-
dependent of speed) scaled as 10,078M
0
:
7
, and DEE
was a relatively uncertain variable that averaged about
800M
0
:
71
. ECT, expressed as a percentage of DEE
(ECT
¼
100 [DM
ICL/DEE]) scaled as 5.17M
0
:
21
for carnivores and as 1.17M
0
:
21
for other mammals.
ECT of small noncarnivorous animals may thus be less
than 1% of their DEE, whereas large carnivores may
spend 10%-15% or even a larger share of their DEE on
locomotion.
living within epiphytic tree ferns has doubled the esti-
mate of invertebrate zoomass in a rain forest canopy
(Ellwood and Foster 2004). Extrapolation and global ag-
gregation of uncertain information carries large errors
that are further increased when these totals are converted
to energy equivalents. Carbon dominates the elemental
composition of heterotrophs, but higher protein content
accounts for much higher shares of N and S than in
plants. Invertebrates are about 80% water, fish average
about 75%, arthropods, birds, and mammals about 70%.
Dry matter of metabolizing tissues is nearly pure protein,
averaging about 22.5 kJ/g.
Differences in the energy density of heterotrophs are
thus a function of mean body ash contents (about 17%
of dry fat-free weight in mammals, 12% in birds) and ac-
cumulation of lipids. Both these shares change with age,
and fat content in adult animals fluctuates frequently
with the season and feeding opportunities or necessities.
For most vertebrate species there is at least a twofold dif-
ference of energy densities depending on their growth
stage; seasonal variations on the order of 20%-30% are
common; and energy contents of different developmental
stages or castes of insects differ by up to 40%. Approxi-
mate energy densities (all in kJ/g) are 18.5 for bacteria,
22 for fungi, 19 for protists, molluscs, and annelids, 23
for arthropods, 21 for fish, and 23 for mammals and
birds. The only certainty is that endothermy limits the
share of heterotrophic biomass; consumers must be
mostly prokaryotes and invertebrates.
Available estimates do not distinguish between auto-
trophic and heterotrophic species, but the bulk of bacte-
rial mass is clearly heterotrophic (decomposers, N fixers).
Given the wide range of bacterial presence in soils (10
1
-
10
3
g/m
2
) (Paul and Clark 1989; Coleman and Crossley
1996), global estimates differ substantially. Whitman,
4.4 Biomasses and Productivities
Global assessments of heterotrophic biomass can aim at
nothing more than the right orders of magnitude. Large
and unknown number of species, their highly variable
densities, and their occupation of many extreme niches
are the main factors militating against any reliable assess-
ments of prokaryotic biomass. Enormous uncertainty
regarding the overall diversity of arthropods, which ac-
count for most of the described heterotrophic species,
and the variety, mobility, and variability of vertebrates
pose additional difficulties for quantification of other het-
erotrophic biomass. For example, collection of biomass
