Environmental Engineering Reference
In-Depth Information
Xochimilco. By the time of the
conquista
(1519) there were about 12,000 ha of
chinampas
in the Basin of Mexico able to sustain population densities ranging from
fewer than 3 people/ha to as many as 13-16 people/ha, and the mean for the entire
basin was about 4 people/ha (Sanders, Parsons, and Santley 1979). Similar densities
could be supported by raised-i eld cultivation of potatoes around Lake Titicaca,
the core area of the medieval Inca Empire (Denevan 1982). Indigenous North
American cropping has received less attention, but McLauchlan's (2003) pollen
analysis showed that even in southwestern Ohio the cultivation of starchy or oily
seeds began as early as 4,000-3,500 years ago.
Fairly reliable historical statistics show that the U.S. grain yields remained very
low even during the i rst quarter of the twentieth century: in 1866 (at the end of
the Civil War) the wheat harvests averaged just 750 kg/ha, during the next i ve
decades they rose only to a l uctuating plateau between 800 and 900 kg/ha, and
they permanently surpassed 1 t/ha only during the 1940s (USBC 1975); similarly
low rates had prevailed in Canada. But because of the late nineteenth-century
opening up of the U.S. Great Plains and Canada's prairies, these low yields satisi ed
not only the rising domestic food demand but also produced plenty of high-quality
feed for record numbers of draft animals and a surplus for export: in 1900 the land
planted to supply the latter two demands accounted for, respectively, about 25%
and 10% of all cultivated U.S. cropland (Smil 2008).
Well-documented history of traditional European farming shows that a long
period of stagnating and l uctuating medieval yields ended only during the late
eighteenth and the early nineteenth century. During the i rst two centuries of the
Common Era (the apogee of Roman imperial power), cropping in Italy or Gaul
(with typical wheat yields at just 400 kg/ha) could support no more than 1 person/ha,
a millennium later typical wheat yields in Atlantic Europe were only marginally
higher. Reconstructions of English wheat yields indicate average population densities
of no more than 1.5 person/ha (100/km
2
) at the beginning of the thirteenth century
(box 8.4).
There are two simple explanations why food production in traditional agricul-
tural societies—despite its relatively high need for claiming new arable land—had
a limited impact on natural ecosystems: very low population growth rates and very
slow improvements in prevailing diets. Population growth rates averaged no more
than 0.05% during the antiquity and they reached maxima of just 0.07% in medi-
eval Eurasia—resulting in very slow expansion of premodern societies: it took
Europe nearly 1,500 years to double the population it had when Rome became an
