Environmental Engineering Reference
In-Depth Information
feed per unit of edible mass than their unimproved predecessors. But these improve-
ments could not prevent a massive increase in the total biomass required to feed
the domesticated animals: because of enormous increases in their numbers, better
feeding efi ciencies could only reduce the rate of increase in the overall claim.
Hunting and Domestication
Hunting opportunities are fundamentally determined by the efi ciency of energy
transfer between trophic levels. Shares of the NPP that are actually consumed by
herbivores are only 1%-2% in deciduous temperate forests, may surpass 25% in
temperate meadows and wetlands, and reach maxima of 50%-60% in rich tropical
grasslands; in the ocean they can peak at more than 95% in some patches of phy-
toplankton (Crawley 1983; Valiela 1984; Chapin, Matson, and Mooney 2002).
Terrestrial rates between 5% and 10% are perhaps most common, implying annual
transfers of roughly 3-6 Gt C. But when the calculation is restricted to vertebrates,
the primary targets of hunting, it is mostly just around 1% (around 0.5 Gt C). Only
part of this ingested energy is digested: for herbivores (whose diet contains a high
share of digestion-resistant phytomass), this transfer amounts generally to less than
30%, while for carnivores (eating high-lipid, high-protein tissues) it may surpass
90%. The next step is to see what share of digested energy is neither respired nor
spent on reproduction and becomes available for growth of new tissues.
Ectotherms have an inherent advantage (many nonsocial insects channel more
than 40% of ingested energy into growth), for endotherms have to spend a great
deal of ingested energy on maintaining high core body temperature: only about 3%
of ingested energy ends up in growth in large mammals, and for small mammals
and birds the rate is even lower, 1-2% (Humphreys 1979). Multiplying the three
rates—exploitation, assimilation, and production—yields overall trophic efi ciencies
that are invariably well below 10% for most hunted vertebrates, and mostly from
a fraction of 1% to a few percent for the most commonly targeted herbivores. But
this may be largely irrelevant as far as the human claims on wild zoomass were
concerned. Herbivorous mammals and birds have been always the most important
target of hunting, and Hairston, Smith, and Slobodkin (1960) showed that the
numbers of herbivores are most often limited by predators rather than by the energy
available for transfer and that only the predators are resource-limited. This reality
put natural limits on the avian and mammalian zoomass available for hunting, and
the size of hunted animals was a result of particular environmental and trophic
outcomes.
