Environmental Engineering Reference
In-Depth Information
Group hunting improved the chances of success and
hence the rates of energy return. The population den-
sities of foragers ranged over 2 OM, from 1-2/100 km
2
in forests to about 100/km
2
in maritime groups, which
were among the first societies to adopt sedentary living.
The evolution of sedentism, with its food storage,
increase of private property, and more complex social
structures, was a multifocal and protracted process;
agricultures coexisted for millennia with gathering and
hunting. This gradual shift from simple, opportunistic,
omnivorous heterotrophs to increasingly more adept
exploiters of photosynthesis through cropping did not
come because of the energetic advantages of farming but
because intensifying crop cultivation is the only way to
sustain higher population densities. That shift also began
civilization's continuing quest for higher energy inputs.
—have persisted for millennia alongside intensifying
farming, clear testimony to the appeal of minimized en-
ergy inputs. For pastoralists, the production of milk,
blood, and (secondarily) meat required very low labor
inputs, often delegated to children, and supported highly
variable but always low population densities. With stock-
ing rates of 0.05-0.15 animal units/ha, grasslands can
carry 0.8-2.7 people/km
2
. Shifting agriculture follows a
sequence of clearing natural or secondary growth, burn-
ing the undergrowth, planting a variety (12-50) of food,
feed, and medicinal species, weeding (often also protect-
ing) the plots or gardens, and staggered harvesting. It
has typical energy inputs of 1-2 GJ/ha and high net en-
ergy returns of 15-30, and it supports 10-30 people/
km
2
, 1 OM more than pastoralism.
Continuing population increases shortened the fallow
periods of shifting agriculture and eventually led to inten-
sive field cropping. Plowing, requiring animal draft, was
its energetic hallmark. Working animals, from donkeys
to large horses, delivered drafts of 30-80 kg at speeds of
0.6-1 m/s for effective sustained power of 100-800 W.
Horses were the most powerful choice only when prop-
erly harnessed and adequately fed. Only the universal
adoption of the collar harness and feeding of grains
opened the way for heavy horse labor. A good horse
could easily do the work of six to eight men but its feed
of 110-120 MJ/day meant that relatively large shares of
farmland (as much as 25% in the United States) had to
be devoted to pastures and feed crops for the animals.
In agricultures relying on oxen and water buffalo the
shares were considerably lower (5%-7% in India and
China). Improved designs and diversification of animal-
drawn implements came in the eighteenth century. Steel
plows, seeders, reapers, mechanical harvesters, and com-
13.2 Energy and Civilization
With agriculture humans ceased to be simple hetero-
trophs and became increasingly sophisticated manipula-
tors of solar flows and builders of complex societies.
Although energy returns in early farming were typically
no better (and often somewhat lower) than gains in for-
aging, agriculture could support larger populations from
smaller areas of land. Higher population densities pro-
moted further intensification of farming, that is, higher
energy subsidies with attendant technical and organiza-
tional advances. The resulting growth of permanent set-
tlements, diversification of manufactures, accumulation
of knowledge and private possessions, and evolution of
hierarchical structures were key ingredients in the devel-
opment of civilizations.
The most extensive forms of managed food pro-
duction—nomadic pastoralism and shifting agriculture
