Environmental Engineering Reference
In-Depth Information
zoomass, 1 OM less than domesticated animals and also
considerably less than planetary anthropomass.
Biospheric energy flows cannot be realistically under-
stood without the appreciation of numerous limiting
factors. Low conversion efficiencies in photosynthesis
(commonly 1 OM below the genetic potential) are
largely due to nutrient deficiencies and limited water
availability. Hence PAR alone is a very poor predictor of
the spatial distribution of low-productivity ecosystems.
Behavior is frequently a major limiting factor for hetero-
trophs, especially because increasing densities promote
territorial strife and aggression reduces the fecundity of
many fish, bird, and mammalian species. Physical features
of the environment (above all, availability of cover and
nesting or denning sites) also limit the spacing, density,
and dispersion of vertebrate species even in the presence
of abundant food. The diversity of heterotrophs also
demonstrates that there are many ways to ensure evolu-
tionary competitiveness.
Although endotherms have an undisputed adaptive
edge, ectotherms are both very abundant and highly
diversified. And whereas larger bodies entail both ther-
moregulatory and locomotive advantages, energy har-
vested daily per unit area is independent of the unit mass
of feeding heterotrophs, and hence no herbivorous
species can become more successful only because of its
bigger size. Energetic considerations alone are also insuf-
ficient to explain the spatial behavior of heterotrophs,
and the degree of their explanatory power clearly
decreases with the advancing complexity of behavior.
The evolution of heterotrophs provides clear evidence of
deviation-amplifying changes. There was a span of some
2.5 Ga between the emergence of the first prokaryotic
cells and more complex eukaryota, but Metazoa were
present only 300 Ma later. Similarly, in human evolution
there is a huge disparity between the duration of stone-
tool cultures (@2 Ma) and the time between first agricul-
tures and industrial civilization (
<
10 ka).
The rapid ascent of Homo sapiens, one of whose conse-
quences has been the decline of wild vertebrate zoomass,
is an impressive but worrisome testimony to the success
of the most versatile and the most adaptive of all hetero-
trophs (fig. 13.2). Many vertebrate species surpass us
in particular functions, but among terrestrial mammals,
humans have no equal as generalists (only muscle-
powered flight is beyond our ability). Decoupling of the
metabolic cost of running from speed, high metabolic
scopes (up to 25 for exceptional individuals), and high
perspiration capacity mean that humans are unsurpassed
endurance runners. Our unrivaled encephalization en-
abled us to construct and use a myriad of exosomatic
aids that have elevated our existence far above the plane
of mammalian heterotrophy and that have conquered all
of the planet's terrestrial environments. But human ener-
getics is still far from perfectly understood. Even intake
recommendations for some essential nutrients required
for human growth, tissue maintenance, and activity are
uncertain.
Lipids have the highest energy density, 38 kJ/g, and
carbohydrates and proteins provide 17 kJ/g. Food
requirements are a complex function of age, gender,
body size, activity, climate, and individual BMR. Adult
BMRs range from 60 W to 90 W and show considerable
variation among equally massive individuals. Kidneys,
heart, liver, and brain account for almost two-thirds of
BMR, but hard mental work requires a negligible energy
markup. The early peak of specific metabolic rates, reach-
ing 2.7 W/kg during the first six months of life, is fol-
lowed by a steady decline to about 1 W/kg by the age
of 70 years. Minimum survival needs are about 1.25
