Environmental Engineering Reference
In-Depth Information
as principal staple crops in all extratropical climates, the rotation of cereals and
legumes, cattle used predominantly as draft animals rather than as sources of meat.
But there were also many unique cropping sequences, some involving rotations of
more than a dozen crops; differences in the domestication of animals (dairying
cultures vs. societies abstaining from milk consumption); and distinct diets. Even
the least intensive forms of cropping were able to support more than 100 people/
km
2
, and the best Asian and European practices could feed more than 500 people/
km
2
, that is, more than 5 people/ha (Smil 2008; table 8.1).
But even the best traditional productivities did not sufi ce to meet the post-1850
food demand driven by expanding populations, rapid rates of industrialization and
urbanization, and higher incomes. Modern high-yield cropping has been a recent
creation: its beginnings go back only to the last decades of the nineteenth century
and its greatest returns have come only since the 1950s. Superior new cultivars can
take advantage of new agronomic practices that combine high rates of fertilization,
applications of herbicides and pesticides, supplementary irrigation, and the near
total mechanization of i eld and processing tasks and that can provide excellent
diets by supporting more than 10 people/ha. Moreover, these practices take advan-
tage of comparative environmental advantages and increase the shares of national
diets coming from imports (it is not uncommon to import more than 20% of all
food energy). They also made it possible to consume unprecedented amounts of
animal foods.
Shifting Cultivation and Pastoralism
Dissimilar as they appear to be, these two most extensive ways of food production
are both suited only for low population densities because they both share an inter-
mittent use of land. Shifting cultivation (swidden or slash-and-burn farming) and
pastoralism could be also seen as intermediate evolutionary steps between foraging
and permanent sedentary farming: they share extensive land use with the former
but orderly practices (regular planting of crops, seasonal use of best pastures) with
the latter. Of course, their environments differ greatly: shifting agriculture is best
suited for forests and woodlands, while animal grazing converts grassy phytomass
(indigestible by humans) to high-quality animal foods (milk, blood, meat) and hence
enables people to survive in regions too arid for regular cropping.
As a result, the population densities of some nomadic pastoralists were as low
as those of gatherers and hunters, while shifting agriculture could support popula-
tion densities an order of magnitude higher than even the relatively best-off (seden-
