Environmental Engineering Reference
In-Depth Information
season with good pastures camels can go up to 90 days
without water, sheep 30, cows only 3, and calves just 1.
Similarly, camels can cover up to 80 km/day to reach
new grazing ground, cows no more than 20, and calves
10. In East Africa, traditional diet (eating only old or dis-
eased animals) required 7 lactating cows (or 4 camels) for
a family of 6.5 adult equivalents, or 14-15 cows in total.
With milch cows being about half of a typical herd, 30-
40 animals per family, 5-6 heads of cattle, 2.5-3 camels,
or 25-30 goats or sheep were a minimum standard per
person.
This represents 2.5-3 livestock units (equivalent to
one camel, two or three cattle, or ten heads of small
stock). African experience shows that 6 ha of a good
grassland may be a typical average to support one adult
head of cattle, with a common range of 4-8 ha and
extremes of less than 2 ha (where green grass is available
all year) and 16 ha (Allan 1965). Some 36 ha of grazing
land would be needed to support six cows per capita,
prorating to an average population density of 2.8/km
2
.
Actual densities (subject to long-term environmental
fluctuations) have been considerably smaller, in East Af-
rica largely between 0.8-2.2 people/km
2
and 0.03-
0.14 heads/ha (Helland 1980; Evangelou 1984; Cough-
enour et al. 1985). The larger number of cattle among
traditional Maasai (13-16 heads per capita) is explained
by the status-seeking accumulation of animals or (in en-
ergetic terms) by the minimum requirements for the sus-
tainable tapping of blood. Blood (harvested by piercing a
tightened jugular vein with an arrow and letting out 2-4
L at intervals of five to six weeks) was nearly as important
as milk. Two heads of cattle must be bled to feed a family
of five or six people, a herd of 80 is needed during peri-
ods of low (drought-induced) lactation, or 13-16 ani-
mals per capita.
Shifting cultivation alternates variable but always short
cropping periods (one to three years) with no less vari-
able but fairly long periods of fallow (a decade or more).
The practice was once ubiquitous on every continent ex-
cept Australia, and it remained of major importance for
tens of millions of families in Africa, Latin America, and
Southeast Asia even in the second half of the twentieth
century (Allan 1965; Spencer 1966; Watters 1971; Grigg
1974; Okigbo 1984). The cycle starts with clearing of
natural vegetation, most often forest, climax, or second-
ary growth. The most labor-intensive tasks are felling of
large trees, trimming and pollarding of smaller trees, and
slashing of younger growth. After drying, the cut phyto-
mass is burned. Fire clears away the litter, prepares the
surface for planting, and reduces the invasion of weeds
and pests. Most of the nitrogen is lost, but minerals are
recycled.
A variety of edible, fiber, and medicinal species, domi-
nated by grains (rice, corn, millet), roots (sweet potatoes,
cassava, yams), and legumes (beans, peanuts) were grown
in gardenlike arrangements with high degrees of inter-
planting and staggered harvesting. Just two to five staples
provided most of the food energy, but there were rarely
fewer than 12 crops, and often 30 to 50 species crowded
a small area. Gardens were often fenced to keep domestic
or wild animals away, and much time was spent in pre-
venting predation and keeping the herbaceous and ligne-
ous competitors in check by repeated weeding. Besides
annual or semiannual harvests of major grain crops, there
was continual digging of roots and picking of seeds,
leaves, and stems.
The energetics of shifting cultures is not easy to study.
Yields of small, scattered, continually harvested plots
are seasonally variable and differ enormously depend-
ing on soils, fallow periods, and the quality of clearing,
