Environmental Engineering Reference
In-Depth Information
Box 8.2
An Example of Grazing Impacts: East Africa
In East Africa, where a large-scale expansion of grazing began a millennium ago, the
minimum number of cattle needed to produce enough food has been i ve to six head
per capita; as a result, the most commonly encountered population densities were
between just 1-2 people/km
2
(Helland 1980; Evangelou 1984; Coughenour et al. 1985).
Similar population densities could be supported with three camels and about 30 sheep
per capita. Standard metabolic equations can be used to calculate approximate annual
feed requirements, assuming that the average weight of cattle is 200-250 kg, which is
not much greater than for wildebeests and somewhat less massive than adult zebras
(Voeten and Prins 1999).
The bovine feed requirement would be then about 40-45 MJ/day, and with an
average grass phytomass power density of 11 MJ/kg, this would translate to annual
intakes of up to about 1,500 kg of dry-matter feed per head. But this does not mean
that cattle herds have been reducing the NEP of East African grasslands by 1.5 t per
head every year. With lower-intensity grazing (in the absence of any cattle) the accu-
mulation of high-grass phytomass may lead to larger i res rather than providing more
nutrition to wild herbivores (Holdo et al. 2007). More important, exclosure studies
have shown that in high-rainfall years, chronic grazing by a mixture of cattle and wild
ungulates can actually increase annual NPP on nutrient-rich glades (due to deposition
of nitrogen in feces and urine) while suppressing it on nutrient-poor bushland sites
(Augustine and McNaughton 2006).
they may participate in a three-way partitioning with wildebeests and zebras, and
their presence may overlap with zebras' resource use only the early wet season,
and with wildebeest grazing only in the early dry season (Voeten and Prins 1999).
And while in Kenya's arid regions wild grazers completely avoid areas that are
heavily grazed by livestock, they mix with livestock in semiarid rangelands, and may
actually graze close to settlements for protection from predators (Suttie, Reynolds,
and Batello 2005).
Historical sources make it clear that populations of pure pastoralists were rela-
tively common during antiquity as well as during the Middle Ages. Repeated
encounters of settled societies with roaming pastoralists—be it due to long-distance
invasions, protracted armed conl icts, or, particularly in China's case, attempts to
manage the ever-present nomadic threat with walls, treaties, and trade—were one
of the key unpredictable and disruptive factors of their premodern history (Adas
2001). Modern appraisals see a properly practiced pastoralism not as an archaic
way of food production that is conducive to land degradation and can provides
only negligible economic benei ts but as a rational, environmentally appropriate,
