Environmental Engineering Reference
In-Depth Information
and potentially rewarding option to exploit the limited primary productivity of arid
ecosystems (Rodriguez 2008).
But pastoral societies have been in retreat for centuries owing to a combination
of natural and human factors. Unpredictable production in arid climates can be
easily aggravated by overgrazing; once other options become available (such as
moving to nearby farm settlements or migrating to distant cities), prolonged droughts
pushed many pastoralists permanently off their ancestral lands; rangeland is reduced
through the encroachment of neighboring agricultural societies coveting the best
grazing lands; and recurrent attempts by centralized governments either restrict
nomadic roaming or force the populations to resettle.
For these reasons, a gradual, and since the beginning of the modern era a surpris-
ingly rapid, expansion of land devoted to animal grazing is not an indicator of a
relative increase in the world's pastoral populations but a rel ection of two separate
processes. First, the higher population growth that i rst became evident during the
early modern era (average annual growth rates were less than 0.1% before 1700,
and during the eighteenth century they averaged about 0.5%) pushed increasing
numbers of pastoralists into less inviting areas, whose poor grazing translated into
lower productivity. Second, the extension of large-scale commercial cattle production
primarily onto the vast grasslands of North America and Australia during the nine-
teenth century (particularly its latter half) and onto the South American and African
grasslands throughout most the twentieth century added nearly twice as much pas-
tureland in 200 years as the total arable land accumulated after millennia of settled
cultivation—without any expansion of true pastoralist populations.
Approximate estimates of the world's grazing land show a slow growth during
the i rst 1,500 years of the Common Era, from about 100 Mha at its beginning to
about 220 Mha in the early sixteenth century (HYDE 2011). By 1800 that total
had more than doubled, to just over 500 Mha, and then it expanded 2.5 times
during the nineteenth century, mainly as a result of the westward population
advances in the United States, Canada, and Australia (for a gain of some 250 Mha
of pastures) and the large expansion of grazing throughout sub-Saharan Africa (a
gain of some 200 Mha) and Central Asia. And the expansion rate was even slightly
higher during the twentieth century (a 2.6-fold gain), when the greatest gains came
in Latin America (quadrupling the 1900 total to about 560 Mha, mainly thanks to
the conversion of Brazilian
cerrado
), followed by sub-Saharan Africa (more than
doubling, to about 900 Mha) and several regions of Asia.
At the beginning of the twenty-i rst century pastures occupied about 34 million
km
2
, or more than twice the area devoted to the cultivation of annual and perennial
