Environmental Engineering Reference
In-Depth Information
arid African environments, similar to the efi ciency of food obtained from hunting
large native ungulates (Coughenour et al. 1985).
Milk and blood rather than meat were the primary food products (only old and
ill animals were slaughtered). The variety of environmental settings (from poor
semideserts to tall tropical grasses) and of domesticated herbivores precludes any
easy generalizations about typical population densities of pastoral societies, whose
actual practices have always ranged from the easily sustainable grazing of highly
productive grasslands to the obvious overgrazing of only seasonally green semidesert
grasses of low productivity. Moreover, in many cases the overall rate of phytomass
consumption may not have changed all that much, as domesticated animals simply
displaced wild grazers.
In most arid environments i ve to six head of cattle, or the equivalent zoomass
in other species, are needed to support an average per capita food demand. The
equivalence is commonly expressed in livestock units, whose size varies with breeds
and feed supply: in East Africa, the region of some of the largest remaining pastoral
populations, it is equal to 1.4 head of cattle, one camel, or 10 sheep or goats. When
accurate head counts of grazers are known it is possible to quantify fairly accurately
the effects on the stores and productivity of phytomass for a specii c location or for
a small and relatively homogeneous region, but such an account will miss some
important qualitative differences, above all the impacts of different modes of grazing
by cattle, camels, sheep, and goats. An East African example illustrates the challenge
(box 8.2).
Because of signii cant differences in body weights and pasture qualities, any
global account of grazing impacts can aim only at the right order of magnitude—
providing that good approximations of total numbers of all major grazers are avail-
able. The earliest available global estimate is for 1890, when the totals were just
over 400 million head of cattle, nearly 100 million horses, and about 750 million
goats and sheep (HYDE 2011). The zoomass (live weight) of these domesticated
grazers was about 150 Mt, ten times greater than a likely mass of some i ve million
African elephants that were living in the wild by the end of the nineteenth century.
There is also no simple generalized formula to predict a degree of competition
between domestic and wild herbivores: the outcome depends on the species involved
and their densities, on seasonal stresses, and on other forms of interference (fences,
setting i res to prevent bush and tree growth). Wild grazers on savannas coexist
because of resource partitioning, that is, choosing different kinds, sward heights,
and ages of available forage, and the introduction of cattle may hardly change that.
Although cattle are usually considered to be nonselective roughage grazers, in Africa
