Man-Environment Relationship
I. Man-Environment Relationship:
A. Modification of Landforms:
Man's relation with his natural environment is a complex one. While he is
subject to certain controls and events, he also acts as the dominant force in many of
the Earth's physical and biological systems.
Mining and quarrying, deforestation, the introduction of exotic plants and
animals, the use of agricultural machinery, the building and use of tracks and roads,
and the overgrazing of pastures, have all, singly and in combination, profoundly
altered landforms and caused accelerated erosion and deposition to occur.
Man has a direct effect on the shape of landforms by excavating and piling up
earth, reclaiming land from the sea and causing subsidence through mining. These
activities have greatly increased since the Industrial Revolution with the development
of enormous machine power and explosives for moving.
By far the most important of all man's effects on landforms are those
connected with his interference with the natural vegetation, in particular with the
clearing of forest for agricultural purposes. There is a close relationship between the
amount of vegetation cover and erosion rates on hillslopes, and hence with the amount
of sediment in streams.
A stable vegetation cover acts as an effective regulator of natural erosion,
protecting the ground from direct raindrop impact, absorbing some of the runoff, and
making the slope more cohesive. With the removal of the vegetation, the surface loses
its plant litter, causing a loss of soil structure, cohesion and porosity.
Land use type
Open Land:
Cultivated
Pasture
Average annual
Rainfall (cm)
Forest Land:
Abandoned fields
Depleted hardwoods
Pine plantations
Man-Environment Relationship
Average annual
run-off (cm)
Average annual
sediment yield
(tonnes per hectare)
132
129
40
38
50
36
129
129
137
18
13
2.5
0.29
0.22
0.045
Page 1
Runoff and sediment yield from various types of surface (Northern
Mississippi)
Multiple shoe-string rills and gullies on hillsides are often a typical
manifestation of man's indirect effect on slopes. They are presently found in many
parts of the world, notably in semi-arid regions susceptible to tropical downpours.
The alteration of infiltration and run off on slopes by modifying the vegetation
inevitably has a profound effect on adjacent rivers in at least two respects: by
increasing both the discharge and also the sediment supply. These seems little doubt
that many of the folds in mid-latitude rivers would not occur if the vegetation in the
drainage basin were in its natural state.
Another way in which discharge levels may be affected in similar fashion is
through urbanization; the ground surface is rendered impervious by buildings, paths
and roads, and precipitation is channeled directly to rivers through drains and sewers.
The following figure shows the effect of urbanization on flood peaks, with attendant
damage to river banks, properties and farmlands.
Before Urbanization
After Urbanization
B. Modification of the Atmosphere:
Atmospheric circulation systems operate on such a large scale that one is
perhaps inclined to doubt that man's activities would have any appreciable effect on
them. However, it is known that the global heat balance has changed at the last few
decades, and we might ask ourselves how much of this is a result of man polluting the
atmosphere. It is certainly evident that pollution has marked local effects on the
atmosphere.
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To city-dwellers the most obvious way in which man has affected the
atmosphere is through pollution. Pollutants include particulate matter, both solid and
liquid particles, and gaseous substances such as sulphur dioxide, oxides of nitrogen,
carbon monoxide and hydro-carbon compounds. But not all man-made pollution
comes from cities. Isolated industrial activities frequently created a footprint of
atmospheric pollution in areas of countryside downwind from the industrial site.
Mining and quarrying activities also send large amounts of mineral dust into the air.
Even man-induced forest and grass fires as well as bonfires, can greatly add to
particulate pollution at certain times of year.
The harmful effects of atmospheric pollution on plant and animal life are
manifold. For humans, many pollutants are irritant to the eyes and dangerous to the
respiratory system. Ozone in urban smog has a severe effect on plant tissues;
atmospheric sulphuric acid has wiped out lichen growth in many urban areas. Lead
and other toxic metal particles are a particular cause of concern for human health. In
addition, pollution also causes many millions of pounds worth of damage to materials.
In the atmosphere, carbon dioxide and oxygen are the most critical from an
environmental viewpoint.
Gases
N2
O2
Ar
CO2
Ne
He
Kr
O3
H2
Xe
Percentage %
78.084
20.947
0.934
0.030
1.82 x 10-3
5.24 x 10-4
1.14 x 10-4
6.00 x 10-5
5.00 x 10-5
8.70 x 10-6
Before the Industrial Revolution, carbon dioxide levels appear to have been
about 290 parts per million (ppm) in the atmosphere. But in the last hundred years of
so, this amount has increased by about ten per cent, largely because of man's use of
fossil fuels. It has been suggested that an increased level in carbon dioxide content
will increase the temperature of the atmosphere, since the gas is an absorber of
long-wave radiation and enhance the greenhouse effect.
It has been pointed out also that man's large-scale combustion of hydrocarbon
fuels requires a large quantity of oxygen to be withdrawn from the atmosphere and
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Page 3
converted into carbon dioxide and water vapour. There is therefore the possibility of a
lowering of the oxygen content of the atmosphere to levels which might have a
harmful effect on animal life.
Changes in water vapour levels brought about by man through combustion and
alterations to the vegetation cover could in theory markedly affect global radiation
and heat balances in the same manner as changes in carbon dioxide levels.
Meteorological processes close to the ground are extremely sensitive to the
character of the Earth's surface, and man's alteration of this through deforestation,
agricultural practice and urbanization has had several important effects. One result of
these activities is to alter the rate of evapo-transpiration. The complete removal of a
forest cover will sharply reduce transpiration and thus the amount of water returning
to the atmosphere in vapour form. The draining of a swamp will have a similar effect.
Another important consequence of surface change is to alter the temperature
characteristics of the atmosphere nearest the ground. Closely built urban areas
develop their own heat island on calm nights in summer.
A third climatic element that may be modified when man alters the ground
surface is the wind. Trees and hedges effectively brake the wind, causing a
simultaneous diminution in evaporation and in the carbon dioxide exchange close to
the ground.
C. Modification of Ecosystem:
With the beginnings of agriculture, far-reaching effects were introduced into
ecosystems. Man gradually became more sophisticated in knowing just how such to
modify an ecosystem in order to harvest the crop he wanted. In achieving this end, he
has inevitably simplified ecosystems, disrupted nutrient cycling, introduced alien
species and eliminated others, and caused pollution.
The most general effect of man on ecosystems is that he tends to simplify
them. This comes about because man's prime concern is to direct energy and material
cycling in the system towards him so that he can easily crop them. Species other than
the ones he wants to crop are regarded as weeds or pests, and he attempts to eliminate
them. Food webs are then made much simpler in this process.
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Ecosystem simplification often results in disastrous side effects. The potential
for survival in ecosystems is much enhanced in multi-species populations: the greater
the species diversity in any assemblage, the better the chance will be of a balanced
inter-relationship between organisms. Man-created monoculture is thus ecologically
unstable and can only be sustained at the price of high inputs of energy (eg.
machinery, weeding) or matter in the form of chemical fertilizers.
A major consequence of man's simplification of ecosystems is that he
inevitably destroys major nutrient reservoirs, notably the natural vegetation and the
soil system. To maintain yields he attempts to replace the loss by injecting fertilizers
into the system.
When chemical fertilizers are applied to the land, many of the elements
contained in them are retained by the soil. However, certain ions are not retained, and
among them is nitrate, an important constituent of most fertilizers. Nitrate is being
added to the soil from fertilizers and nitrogen-fixing plants at a much faster rate than
it can be broken down by denitrifying agents in the soil. Being soluble, it is rapidly
leached out into rivers and lakes. Here, the increased nitrogen input permits the
accelerated growth of plants, algae and other phytoplankton: this chemical enrichment
resulting in increased productivity is called eutrophication. Unfortunately, in extreme
form the outcome is ultimately harmful, since the plants and organisms die and
decompose at such a rapid rate that oxygen levels fall until aquatic life becomes
impossible.
The extinction or reduction in numbers of plant and animal populations is a
well-known consequence of man's impact on the environment. Often the species
become endangered not so much by hunting or conscious elimination, but by the
disruption and fragmentation of habitats. Some species, particularly large predators,
require an extensive area of specialized habitat in which to breed and hunt, and
fragmentation of this by man's interference has frequently had disastrous effects.
Under natural conditions, ecosystems have been in a state of ecological
equilibrium. With the increasing impact of man, their essential characteristics are
altered, so that now signs of severe imbalance or a declining efficiency are beginning
to be observed in many of them. This is shown, for example, by the progressive
devastation of formerly good fertile agricultural or grazing land through
over-intensive use; in the reduction of species when secondary forest replaces primary
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forest; in a general loss of biological productivity; and in an increasing amount of
pollution.
II. Tropical Rain Forest Landscape:
A. Where is Shifting Agriculture Practiced?
Shifting cultivation is found in a highly scattered distributional pattern
throughout the intertropical world. The actual area under cultivation at any one time is
very small. Cropland is confined to tiny clearings surrounded by vast expanses of
virgin forest or secondary growth, and seldom exceeds 5% of the total area.
The largest region in which shifting cultivation is the prevailing form of land
use is Equatorial Africa, encompassing the rain forests of the Congo Basin and the
West African coast, as well as parts of the savanna woodlands encircling them. In
addition, it includes areas of the east coast and the island of Madagascar.
The second major region is to be found in Central America and throughout the
vast Amazon Basin, where many primitive forest tribes are still to be found.
A third concentration occurs in South-East Asia, where the more remote
islands of Indonesia and the Philippines have this as their dominant form of
cultivation. On the nearby mainland of Asia, shifting cultivation tends to be confined
to the hill and mountain regions of Burma, Laos, Thailand, Cambodia, and Vietnam.
Over much of Papua New Guinea and on some Pacific islands, this form of
cultivation is still practiced by the more primitive tribes.
B. The Characteristics of Shifting Cultivation:
Shifting cultivation is an elementary form of agriculture by which primitive
peoples grow basic food crops in small forest clearings. The clearings are abandoned
after one or two harvests, and the forest allowed regenerating while the cultivators
move on to establish a new site.
In general, shifting cultivation is characterized by:
1. A variety of subsistence crops, but some emphasis upon tuberous plants.
2. "Slash and Burn" methods of clearing the forest before cropping.
3. Low man/ land ratios with population typically in small tribal groups.
4. Primitive methods of cultivation using simple hand tools.
5. Periodic migration leaving the clearing to recover by "bush fallowing".
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6. Relatively low yields from the cropped area necessitating supplementary
gathering from the forest.
7. No. distinctive farm structures apart from simple village huts.
8. The relatively unimportant role of animals in the farm economy.
C. The Relationship between the Practice of Shifting Agriculture and the Natural
Environment:
1. Opportunities and constraints posed by the natural environment:
a. The Climate:
The most distinctive features of the tropical climates are high temperature and
high rainfall which impose no limitations on vegetation growth. Broadly, the
regions of tropical climates are covered in tropical rain forests, characterized
by luxuriant broadleaf trees arranged in tiers and a profusion of lianas and
other exotic plants.
Such climatic conditions also make for a higher level of plant nutrition, rapid
aging of the soil system and rapid leaching rate which is expressed in the way
by the swift decline in the nutrient supplying power of the soil cultivation.
Shifting cultivation is seen by Wright and Twyford as 'one of the methods of
the indigenous farmers to adapt to these conditions'.
b. The Soil and the Nutrient Cycle:
Soils of the tropics are generally infertile. High temperatures and heavy
rainfall mean that chemical processes and bacterial activity are very
pronounced in the soils underlying tropical forests. Thus, the plentiful supplies
of organic matter provided by the luxuriant forests are quickly decomposed.
However, humus accumulation in these tropical soils is quite small. It has been
estimated at Yangambi in the Congo Basin that the forest there provides
between 50 and 63 tonnes of leaves, twigs, lianas, and branches per hectare
per annum. However, the rapid decay of this material and its removal by
leaching results in the soil containing a maximum of only 2% humus,
compared with more than 10% in the fertile soils of the temperate regions.
In tropical soils the mechanisms for maintaining soil fertility depend entirely
on the maintenance of an efficient organic cycle, for most nutrients are locked
up within the plant tissue. There would be little soil depletion as long as the
rain forest covers remains. Tress is able to tap the deeper subsurface supplies
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Page 7
of minerals, and then replenish surface supplies through their own leaf fall
processes. In a sense, therefore, the tropical forest 'lives off itself'.
When the shifting cultivator uses the tropical soils, a dynamic process of
environmental change results. On clearing the forest he disrupts an ecosystem
in which climate, soil, vegetation and fauna are components in an extremely
stable equilibrium. Cultivation in effect takes the form of 'catch crop' that take
advantage of the transient availability of nitrogen and carbon, the main
nutrients of the organic matter lying on the forest floor.
However, once this nutrient cycle is destroyed by clearing, the depleting
effects upon the soil of the tropical climate are accelerated. Without leaf fall,
humus all but disappears and the soil rapidly becomes almost completely
inorganic. Under the prevailing high temperatures, silica in the mineral
fraction of the soil becomes highly mobile, and so is readily leached. As a
result, concentrations of relatively insoluble iron and aluminum are left behind
in the upper horizons, and these may harden into layers known as laterite.
Lateritic crusts are very difficult to cultivate, and their presence leads to
increased runoff and soil erosion because they inhibit the absorption of water
to a marked degree.
However, exposure to the sun's rays leads to increased evaporation, and with
the greater impact of heavy downpours, leads to changes in soil structure. The
steady loss of humus also affects the water holding capacity of the soil as well
as structure. As a result of all these changes in composition and structure, the
soil is likely to produce only one good crop.
2. The farm cycle - a response to the opportunities and constraints:
Because of these opportunities and constraints posed by the climate and soils of
the tropical rain forest ecosystem, a distinctive farming cycle has been developed
by the shifting cultivators. The farming cycle may be divided into six main
phases:
a) selecting the site, b) clearing the forest, c) burning off the dead material, d)
planting the crops, e) weeding and harvesting the crops, f) abandoning the garden.
a) Selecting the Site:
The important considerations in the selection of a site are fertility of the soil
and ease of clearing so it is usual for the forest cultivator to prefer primary
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Page 8
forest for a site, because the dense canopy reduces the amount of bushy
undergrowth that has be cleared. Also, in those places where primeval forest
has remained undisturbed, the humus content of the soil is likely to be at its
maximum. Some tribes, however, do prefer secondary forest that has grown on
former sites. Experience has shown these people that, when felled, secondary
forest dries out more quickly than primary growth, and this reduces the time
lag between clearing and burning.
Other factors like distance from the village and religion are also important
considerations in the selection of a site for cultivation.
b) Clearing the Forest:
Clearing is necessary because of the luxuriant growth of vegetation. Towards
the end of the rainy season, or at the beginning of the dry, the shifting
cultivators clear the selected site of vegetation.
Lianas, undergrowth and saplings are hacked down and large trees are cut
above the buttress roots where the bole is comparatively small. The biggest
trees may be spared because they help to guard against soil erosion and are
very difficult to fell. Once cut, the vegetation is piled into heaps, often around
the stumps of felled trees, and allowed to dry out for one or two months.
c) Burning off the Dead Vegetation:
Late in the dry season the mass of cut vegetation is set on fire. Destruction by
fire is the easiest method of complete clearing available to the shifting
cultivators. Most forest cultivators appear to have recognized the value to be
derived from burning and from the ashes of the burnt vegetation.
Burning leads to an accumulation of potash and valuable phosphates,
immediately prior to planting the crops that will need them. Burning produces
a marked decrease in potential acidity which is especially important in the
more senile lateritic soils.
In addition to providing a primitive fertilizer, burning helps break up the hard
lateritic surface which frequently occur in the wet tropics. This obviates some
of the preparatory working of the soils. Which, to the people who use only
simple digging sticks and hand hoes, is an important advantage. Burning has a
baking effect upon the heavy clay soil, causing it to dry out and develop large
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Page 9
cracks into which the ashes accumulate. This drier soil is much easier to
cultivate than the wet, sticky lateritic crust.
After the burning off is completed, trunks, stumps, and other unconsumed
debris are left to be destroyed by termites and other insects, as well as a whole
range of highly active micro-organisms. In the meantime, the debris helps to
reduce runoff and so minimize erosion. Those trees which survive the fire and
provide shade for the young plants.
In view of these facts, it can be seen the burning is not only part of the shift
cultivator's technology - a device for clearing away vegetation - but also leads
to an improvement in certain properties of the soil which in some areas makes
cultivation possible and generally leads to increased yields during the period
cultivation.
d) Planting the Crops:
Typically, little preparatory cultivation of the soil takes place, and planting is
usually timed to take full advantage of the rainy season. One most universal
feature of shifting cultivation is that of mixed cropping. The basic reason for
this practice is that it enables the maximum return to the obtained for the
minimum effort. The more complete cover provided by a mixture of crops
shades the ground and so inhibits the growth of weeds, thereby reducing the
labour required for weeding.
Moreover, by planting a mixture of several crops that have different growth
habits, different root systems, and make different demand on the soil, the
shifting cultivators are making the best use of the soil, sunshine and rainfall.
Again, mixed cropping provides some insurance against the failure of any one
crop due to abnormal climatic conditions, plant disease, or insect attack.
Multiple-storeyed agriculture is also common. This practice reduces the
amount of space to be cleared and maintained, and also extends the season of
production. It closely approximates the storeyed structure of the virgin
rainforest, suggesting that these primitive tribes have adapted it from nature.
Where shifting cultivators occupy a clearing for more than one season, they
may practice a form of crop rotation designed to prevent soil exhaustion as
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much as possible. It is clear that although implements may be primitive suited
to give a maximum yield for a minimum effort in a difficult environment.
e) Weeding and Harvesting the Crops:
Cultivation and weeding of the ground around the growing crop is minimal,
and some plots receive no attention at all between planting and harvest.
Excessive working of the ground is generally not required as burning usually
increases the friability of the soil. In addition, too much cultivation increases
the hazard of soil erosion.
In some areas, weeding around the crops is carried out, but where there is a
marked dry season, weeds are often left as a mulch to conserve moisture and
protect the ground. During the growing period particular attention is given to
protecting the crops from various pests. Birds are kept away by guards and
sound-producing of thorn bush and sharpened spikes. In general, farmlands
receive little care and attention during the growing period.
f) Abandoning the Garden and Migration:
After one to three years in most places, yields begin to decline. During this
period humus from the former forest and ash fertilizer are thoroughly used and
not replaced by the garden crops, and other nutrients are leached from the
exposed soil by the heavy rains. Also, by this time, weeds and undergrowth
have begun to encroach seriously on the farmland. Together, these two
occurrences become the signal to the shifting cultivator to abandon the site and
seek a fresh site.
Many tribes abandon the site completely, but many of the people of the Pacific
islands plant the old clearing with tree crops such as bananas and coconuts
before moving on. Some tribes even plant young forest saplings to help the
forest regenerate more rapidly. Dyak tribes in Borneo, for example, practice of
allowing the forest to reclaim a clearing and rebuild soil fertility by natural
processes is termed bush fallowing and the replanting techniques mentioned
above are the most advanced forms of it.
As the distance from the main village to the new clearings increases, it often
becomes necessary to move the entire village to a completely new area and
start over again. More culturally advanced groups usually have a permanent
village and, because of greater investment in homes, prefer to rotate the
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clearings around the village in a planned sequence. As the productivity of the
land in the immediate vicinity of the settlement declines, the distance from the
dwelling to the cultivated area becomes quite considerable. Thus temporary
huts are frequently build on the 'farm' and occupied for periods of days or
weeks at a time during the growing season.
In contrast, the more primitive peoples generally build only temporary villages
and practice shifting cultivation in the immediate vicinity of their dwellings,
and then shift completely to a new area.
Where this of complete migration occurs, land is usually only reused after a
long period of bush fallow has restored soil fertility. Clearing, frequent
migration is not a wasteful technique, but rather it is a conservational measure
that helps to restore fertility to depleted soils.
3. Conclusion:
By analyzing the farming cycle, we can see that shifting cultivation is a
response to the opportunities and constraints imposed by the tropical rain forest
environment and it is an effective means of exploiting the rain forest environment
which tend to minimize the problems posed by soil fertility deterioration, soil
erosion and vegetation degradation.
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In ecological terms, the most distinctive positive characteristic of shifting
cultivation is that it is integrated into and when genuinely adoptive, maintains the
general structure of the pre-existing natural ecosystem into which it is projected.
The tropical rain forest and the swidden plots have certain common
characteristics. Both are closed cover system, in part because in swidden some
trees are left standing, in part because some tree crops, such as bananas and
papaya, are planted, but also because food plants are not planted in an open field,
crop-row manner, but helter-skelter in a tightly woven, dense botanical fabrics. It
is a miniaturized tropical forest.
Secondly, shifting cultivation normally involves a wide range of crops,
thereby having a high diversity index like the rain forest itself. And thirdly, both
swidden plots and the rain forest have high quantities of nutrients locked up in the
biotic community compared to that in the soil. The primary concern of slash and
burn activities is not mere clearing of the land but rather the transfer of the rich
store of nutrients locked up in the prolific vegetation of the rain forest to a
botanical complex whose yield to man is a great deal larger.
If the period of cultivation is not long and the period of fallow is long
enough, an equilibrated, no-deteriorating and reasonably productive farming
regime can be sustained in spite of the rather impoverished soil base upon which it
rests.
Just as what Gourou stressed '....... that shifting cultivation is well adapted
to natural conditions.’
Nevertheless, while environment is undoubtedly highly significant in
determining land use in the tropics, it must be considered against the background
of human civilization. Primitive technology resulting from isolation, and sparse
population distribution, are two of the cultural influences that have had a
significant effect upon the nature of shifting cultivation.
Thus the shifting cultivator appears as 'a man struggling in a hard
environment', far from omnipotent over nature nor yet impotent in the face of its
influences. Through the medium of his technology and with the aid in particular of
fire, he is indeed the ecological dominant. But in a stable, integral form of
cultivation, man was still part of the total ecosystem, in harmony with
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Page 13
environmental conditions. Thus it may be said that the shifting cultivator was
aware of the design in nature and he strove always to adapt himself and his culture
to it.
With some qualifications, Sauer finds shifting cultivation, 'in its basic
procedure and crop assemblages' to be '.....most conservative of fertility at high
levels of yield; that being protective and intensive. We might consider it as being
fully suited to the physical and cultural conditions of the areas where it exists'.
C. Shifting Cultivation is Ecologically Destructive:
Shifting cultivation is commonly denounced by foresters and administrators in
S.E. Asian countries, as a disastrously exploitative use of tropical forests and, more
dramatically, as 'the most menacing land use problem of the tropical world.’
When carried out by sparse populations for purely subsistence purposes,
traditional shifting cultivation did little or no permanent damage to the land in the
forest regions. Because the fires did not spread beyond the limits of the small
clearings, and because the fallow periods were long enough to allow the fertility of
the soil to be restored to its original level, traditional methods of shifting cultivation
appear to have been a highly satisfactory adaptation to a difficult environment. One
eminent authority on tropical geography, Pierre Gourou, has said 'We might consider
it as being fully suited to the physical and cultural conditions of the area where it
exists.'
The greatest weakness of the shifting cultivation system is its inability to
support a large or rapidly increasing population without damaging the soil. Since the
turn of the century, population densities throughout the tropics have been on the
increase, and at an accelerating rate in most areas. Thus, in the twenty-five years since
1947, for example, the population of the Australian portion of the island of New
Guinea has risen from 1.3 million to 2.6 million, an increase of 100%. In Zambia over
the same period the number of people has increased from 2.2 million to over 4.5
million. These rapid growth rates have been brought about largely by the removal of
such 'natural checks' as intertribal warfare, famine, disease, and slavery, by the
various governments and administrations. In Zambia, for instance, the death rate has
been reduced from 32 per 1000 in 1950 to 19 per 1000 in 1963. However, the
agricultural effect of these humanitarian advances has been to increase the pressure
upon the food-producing land and reduce the period of essential bush fallow.
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Various estimates have been made of the amount of land required and the
population densities that can be supported by a shifting cultivation system of
agriculture. In the outer islands of Indonesia, for example, it has been estimated that
shifting cultivator and dependents normally require about one hectare of land a year.
Allowing ten years of fallow for every year under crops, this means that an
average-size family of five people would need to have some 10 hectares of suitable
land at its disposal. This would permit a maximum population density of 50 persons
per km2, if all land was suitable for cultivation. However, if it is assumed that half of
the land is physically unsuited to cultivation for various reasons, then this density
figure would also be halved. In other regions, the soil requires a much longer period
of bush fallow to recover, and so the density is much lower again in them.
Where population densities begin to exceed these critical values, shifting
agriculture begins to have destructive effects. The length of the fallow is necessarily
reduced and old clearing are recropped before the forest has sufficiently regenerated.
A second set of problems confronting shifting cultivation as an agricultural
system are the result of outside interference, particularly by Europeans. The chief
impact of the white races throughout the intertropical world has been the introduction
of cash cropping. Many of the traditional crops of the shifting cultivator attracted
commercial interest, and this encouraged the natives to concentrate on their
production at the expense of the land. The commercial production of groundnuts by
natives in West Africa is one example where this has led to widespread soil
exhaustion. Other crops such as cocoa, coconuts, rubber, coffee, tobacco and cotton
are all being increasingly produced by native smallholders throughout the tropics as a
direct result of the cash economy introduced to these regions from Europe.
This partial commercialization initiated by Europeans has also brought about
changes in traditional systems of land tenure. The cash cropping of cocoa by native
West Africans is a good example. Being a perennial crop, cocoa ties up the land for a
longer period than do the typical subsistence crops of the framer. Hence, farmers
acquire a long term interest in a particular piece of land where their cocoa bushes are
planted, and become reluctant to move on and continue their traditional migratory
way of life, especially if their cash income brings them new 'luxuries'.
The introduction of European agricultural techniques has also caused severe
soil erosion in some areas. Geometrically arranged fields of European maize in parts
of Africa have reduced the protective ground coverage provided by the native crops
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and planting methods, and so have accelerated soil depletion through leaching.
European encouragement of the removal of unsightly tree stumps, and the
introduction of deep ploughing techniques with steel ploughs, have also increased
erosion. In the former colony of Southern Rhodesia, the number of steel ploughs in
operation increased from 3400 in 1902 to some 94000 just prior World War II, by
which time 600000 hectares of land had become seriously eroded and an estimated
16% of the territory had been made agriculturally useless.
Finally, European requirements of native labour for portage, road and railway
building, plantations, and mines has seriously disrupted traditional shifting cultivation
many areas. Where men are recruited in large numbers for these tasks, agricultural
work frequently is left to the women. The effect is to shorten the period of bush
fallow since the women are physically incapable of maintaining the same rotation of
clearings. Particularly heavy damage was done in this way in Rhodesia, where the
copper mines drew their heavy labour requirements from native tribes.
Disturbance has occurred in the tropical rain forest in general with the impact
of the West. Rising population and the introduction of a new set of values that came
from the temperate West have set in train a process that has led to rapid changes in the
environment, and destroyed the old balance between man and nature.
D. Impact of Shifting Cultivation
1. The atmosphere
a. Global Climate:
Tropical rain forest is important as a natural filter for the atmosphere so that
the unwanted CO2 is constantly taken away with a constant supply of O2. The
annual O2 supply from the tropical rain forest as a whole is 2.8 tons/ha and
totally the tropical rain forests withhold 40% of the world CO2 Thus clearance
of the forest may lead to local and global climatic change.
It is often said that an increase in CO2 content of the atmosphere will result in
an increase in temperature. CO2 has a similar effect on temperature to that of a
green house. It will allow the short wave length energy through from the sun
but will not allow the longer wave length energy back from the earth. If it is
true the earth will gradually get hotter and hotter. It can, however, be shown
that an increase in temperature on the earth's surface would also result in
greater evaporation of water from the oceans. An increased cloud cover would
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have the opposite effect on earth temperature as clouds reflect the sun's energy
back to space.
Clearly, the earth and its atmosphere are a complex system. Within this
complex system there are many interdependent variables such as temperature,
cloud cover, rainfall and CO2 content but as yet we do not have sufficient
knowledge to say whether there have been permanent changes in our weather.
Moreover, there is little reliable evidence to suggest that regional rainfall is
either significantly increased by afforestation or decreased by deforestation.
Although forest may not necessarily have a proven effect on regional or
continental rainfall levels, they are far more effective than other vegetation
types at tapping other types of precipitation, especially cloud, fog and mist.
Hence deforestation or afforestation can affect water budgets through their
effects on the degree to which non-rainfall precipitation is intercepted.
b. Micro-Climate:
Micro-climate of the tropical rain forest surely has been changed where
shifting cultivation is being practiced. Under forest cover, the forest floor is
moist and little sun's rays reach it. The earth temperature is low, around 32oC
and wind cannot penetrate the trees and humidity is constantly high.
Clearance of forest allows much sunshine to reach the forest floor. This not
only increases the rate of evaporation and lowers the humidity but also
increases the daily range of temperature and the soil temperature to about
65oC. Moreover, clearance of forest means removal of wind breaks hence
wind velocity increases. However, the extent of such changes depends very
much on the size of the clearing.
2. The hydrosphere
The removal of a tree cover not only may affect river flow, stream water
temperature, stream water chemistry but also the shape and size of river channels.
A mature forest probably has a high rainfall interception rate and hence
reduces rates of overland flow. Under natural forest, the floor is covered by thick
layers of organic debris or litter which is highly absorbent. The top soil layer with
a high humus content promotes maximum infiltration. Under such conditions, the
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amount of surface runoff will be low and a large volume of water is entering the
sub-surface system.
When the forest is cleared and burnt, rainfall interception is very much
reduced and as there is no further supply of litter, humus content of the top soil
will progressively reduce and hence the soil structure becomes less absorbent. As
rain drops directly onto the ground and the amount of water entering the soil
decreases, the rate and volume of surface runoff increases.
For example, in Johore (1970-71), a measurement of runoff was made of
two small adjacent catchments of 1) under forest cover and 2) under plantation
cover for a period of 13 months. It was found that the peak storm runoff per unit
area of the catchment under plantation crops was twice that of the forested
catchment and the base flow was roughly halved.
As the clearance of the forest for cultivation usually takes place along the
watershed, when heavy rain comes, the rain water could not seep underground
through infiltration but flows on the surface as surface runoff, increasing the silt
load and the sudden increase of runoff may lead to destructive floods in the
lowland areas. Moreover, as sediments deposit at the lower course of rivers, the
rise in water level also results in flooding in the lower cultivated plains, eg. in the
coastal lowland of Peninsula Malaya.
In many forest large quantities of nutrients are cycled through the
vegetation and in some cases the trees are a great store of the nutrients. If they are
destroyed, the cycle of nutrients is broken and a major store is disrupted. The
nutrients thus released may become available for crops and shifting cultivators, for
example, utilize this nutrient after burning and falling of forest. Some of the
nutrients may however be leached out of the soils and thereby appear as dissolved
load in streams. large increase in dissolved load may have undesirable effects. As
soil nutrients enter streams, they promote algae growth and cause less oxygen left
for fish and marine animals. This will disturb the ecological balance in the river.
Land use changes do not affect only the speed and volume of stream flow.
Deforestation and subsequent cultivation of an area also seem to result in greatly
increased sediment yields in slopes. The sediment being produced on the hill
slopes reaches the river channel. Under natural conditions, the hill slopes and
channel are in dynamic equilibrium with one another. The channel therefore
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retains a fairly stable shape and size. Changes in sediment supply from the slope
will, however, lead to channel changes. Increased supply tends to lead channel
aggradation. While decreased supply seems to result in stream power being
transfer to the erosion of the channel.
3. The soil:
a) Laterization:
In some parts of the tropics there are extensive sheets of a material called
laterite, an iron and / or aluminum rich duricrust. These iron-rich sheets result
naturally, either because of a preferential removal of silica during the course
of extensive weathering (desilication) or because of an absolute accumulation
of these compounds.
One of the properties of laterite is that they harden on exposure to air and
through desiccation. Once hardened they are not favorable to plant growth.
One particular way in which exposure may take place is by accelerated erosion,
while forest removal may so cause a change in micro-climate that desiccation
of the laterite surface can take place. Forest checks the formation of the laterite
in various ways. The trees supply plenty of organic matter and maintain a
good proportion of humus in the soil. The action of capillary attraction is
checked by the loosening of the soil and the bases are retained through the
absorbent capacity of humus. The forest slows down evaporation from the soil
and it reduces percolation and consequently leaching. Removal of vegetation
cover destroy such natural checks. Indeed, one of the main consequences of
the removal of the tropical rain forest is that laterization may occur. this tends
to limit the extent of successful soil utilization and greatly retards the
re-establishment of forest.
b) Breaking down the nutrient cycle:
Nutrient cycling in a mature tropical rain forest is a closed and rapid one. Most
nutrients are stored in or locked up in the living biomass, leaving little
nutrients in the soil, and such nutrients are quickly cycled by the living plants.
Shifting cultivation is a means to tap the accumulated nutrients by releasing
the locked up nutrients into available form. When the shifting cultivators clear
and burn the forest for cultivation, they interrupt the nutrient cycle.
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Fire rapidly alters the amount, form and distribution of plant nutrient in
ecosystems, and compared to normal biological decay of plant remains,
burning rapidly releases some nutrients into a plant available form. Indeed, the
amount of P, Mg, K and Ca released by burning forest and scrub vegetation
are high in relation to both the total and available quantities of these elements
in soils. Burning also leads to some direct nutrient loss by volatilization and
convective transfer of ash or by loss of ash to water erosion and wind deflation.
It has been estimated that firing destroys between 650 and 1150 tons per ha of
organic matter and that from 700 to 1000 kg per ha of nitrogen also goes up in
smoke.
Clearance of forest exposes the latosol to the direct radiation of the sun, the
increase in soil temperature leads to rapid release of nutrients through rapid
decomposition. If soil temperature rises to the point where the destruction of
organic matter by bacteria outstrips the rate of formation, no humus can be
accumulated. But such released nutrients are easily leached away by the heavy
rainfall. Moreover, unshielded ultra-violet rays produce chemical changes in
the soil, resulting in the conversion of nitrogen and carbon dioxide into gas
which escapes into the air. Soil surface deprived of vegetation cover is prone
to erosion. Runoff washes away the available nutrients. Thus an open nutrient
cycle with leakage resulted.
The loss of the plant roots and humus break the cation exchange of the mineral
compounds between the roots and the soil, so that the soil will soon lose the
mineral supply resulting in soil exhaustion as the soil has lost its ability of
nutrient renewal. Further soil exhaustion will lead to soil erosion.
c) Soil Erosion:
Soil erosion is a natural geomorphic process that takes place under all types of
land use. But the rate of soil erosion may be enormously speeded up through
man's activities to result in a state of accelerated erosion, removing the soil
much faster than it can be formed.
Destruction of vegetation by clearing of land for cultivation, or by fire directly
causes great changes in the relative proportion of infiltration to runoff.
Interception of rain by foliage is ended; protection afforded by a ground cover
of fallen leaves and stems is removed. Consequently, the raindrops fall
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directly upon the mineral soil and such direct impact of raindrops destroys the
surface soil structure.
Direct force of falling drops causes a geyser like splashing in which soil
particulate are lifted and then dropped into new positions, a process termed
splash erosion. It is estimated that a violent rainstorm has the ability to disturb
as much as 100 tons of soil per acre. On a sloping ground surface, splash
erosion tends to shift the soil slowly downhill.
A more important effect is to cause the soil surface to become less able to
infiltrate water because the natural soil opening becomes sealed by particles
shifted by raindrops splash. Reduced infiltration permits a much greater
proportion of surface flow to occur from rain of given intensity and duration.
The depth and velocity of overland flow then increases greatly, intensifying
the rate of soil removal and it may lead to sheet erosion and formation of
gullies.
Erosion will lead to sedimentation of streams and this will affect domestic
water supply and aquatic life and recreation. This may also obstruct navigation
and causes higher tendency to flooding. Sedimentation may affect irrigation
work, eg. reservoirs, canals in W. Malaysia, along the Klang River and Kinta
River large scale flood retention works has been carried out. The Klang was
used for navigation by large crafts, but excessive silting has caused the river to
deteriorate so badly that even a small speedboat cannot be operated. Nearly all
rivers have raised their banks affected by flooding. For example, the Kelantan
River in its lower course has its banks being raised as high as 6 to 7 metres.
The Perak River has levees of 3 to 5 metres higher than the surrounding
country.
4. Vegetation and animals:
a) Secondary Rain Forest:
When an area of rain forest that has been cleared is abandoned by man, the
forest begins to regenerate, but for an extended period of years the type of
forest that occurs is secondary forest of which the floristic and structure may
be very different from the virgin forest that it replaces. How different depends
on the availability of seeds of the primary forest trees and the length of
cultivation before abandonment. The longer the time of cultivation, the fewer
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of plant nutrients are available. This in turn will affect which new species can
be grown on that abandoned area.
Secondary forest differs from primary forest in many ways. Firstly, secondary
forest is lower and consists of trees of smaller average dimensions than those
of primary forest, but since it is comparatively rare that an area of primary
forest is clear-felled or completely destroyed by fire, occasional trees much
larger than the average are usually found scattered through secondary forest.
Secondary, very young secondary forest is often remarkably regular and
uniform in structure, though the abundance of small climbers and young
saplings give it a dense and tangled appearance which is unlike that of primary
forest and makes it laborious to penetrate.
Thirdly, secondary forest tends to be much poorer in species than primary
forest, and is sometimes, though by no means always, dominated by a single
species, or a small number of species.
Fourthly, the dominant trees of secondary forest are light demanding and
intolerant of shade, most of the trees possess efficient dispersal mechanisms,
and most of them can grow very quickly. Some species are known to grow at
rates of up to 12m in 3 years, but they tend to be short-lived and to mature and
reproduce early. One consequence of their rapid growth is that their wood
often has a soft texture and low density.
b) Disturbing the Ecological System:
With regard to tropical rain forest, it has been indicated that forest clearance
achieved by fire or by cutting has been going on. The reported regression rates
for tropical rain forest in 13 selected countries representing about 18% of the
world total area for these forests countries are greater than 2 million ha or
1.2% of their tropical rain forest area. Already West Africa has lost 72% of its
rain forest and Southern Asia 63.5%.
This is potentially serious because the tropical rain forest is the centre of the
plant evolution of the world. In fact, much of the flora of the temperate regions
is derived from the tropics. Destruction of the tropical rain forest would
fundamentally change the future course of plant evolution. Moreover, tropical
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rain forest is very important as nearly all our knowledge of plant physiology
and ecology has been gained from studies in tropics.
The transition from forest to agriculture disturbs the ecological system. The
tropical rain forest is the most complex and speciesric ecosystem in which a
large number of animals and insects are living in an ecological balance. They
form a complex food web. Conversion of forest to agricultural land use would
disturb this ecological system because it would encourage certain animals and
insects species to multiply.
Shifting agriculture in the tropical rain forest has a very long history. it is in
equilibrium with its environment as long as the population density is low and
the fallow period is long enough for the soil to recover its fertility. Before the
recent rapid increase in population and the introduction of western farming
practices and technologies, shifting agriculture was not ecologically
destructive, at least not as serious as its present form.
Population pressure has led to an accelerated rate of forest clearance and a
drastic shortening of the bush-fallow period in the swidden cycle. These
changes have triggered off a chain of adverse effects on the tropical rain forest
ecosystem - including the climatic system, landform system and biotic system.
In conclusion, rising population and the introduction of a new set of values
that come form the temperate West have set in train a process that has led to
rapid changes in the environment, and destroyed the old balance between man
and nature. It would not be possible - nor indeed desirable - to try to recreate
the old relationship, the task of this generation is to establish a new
relationship that does not violate the 'design of nature' and yet is consistent
with the needs of a 20th century world.
III. Desert Landscape:
A. Opportunities and Constraints: Nomadic Pastoralism
1. Introduction:
Desert environment is harsh; it makes life difficult and constraints the
development of man's activities. However, it is mined with opportunities. For
centuries, man has utilized the resources of deserts in order to live on these harsh
lands. Traditionally, desert-dwellers engage themselves in activities such as
hunting and gathering, nomadic herding, and oasis cultivation while mining and
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oil industry, tourism and the use of deserts as testing grounds for missiles, bombs
and explosives are man's recent development in the desert.
In the past, human activities in the desert environment were in equilibrium.
Production and
consumption of
primary residents in
the desert ever than
before. Greater supply
of primary products is
required to fulfill
everyone's need.
Population pressure
has increased the
intensity of land use in
the desert. As a result, man has actually destroyed the land he depends upon.
Pastoralism refers to the practice of keeping animals which convert
vegetation inputs into animal products such as meat and milk, fibre and hides.
Nomadic pastoralism - a traditional form of subsistence farming practiced by
tribal people wandering in the semi-arid land in search for grazing land, is the
predominant form of land use in arid areas. This form of land use can be
considered as a refinement of food gathering. Since man is unable to obtain
sufficient food from plants himself, he employs animals which can utilize plant
species which he himself is unable to digest.
In the semi-arid areas rainfall is insufficient, erratic and unpredictable,
arable farming is almost impossible and it is also very difficult to support
domestic animals in one place without irrigated fodder production because
vegetation is sparse and very localized. Moreover, few areas have suitable water
supplies together with the facilities and organization required to make this
extensive cattle farming by grazing their animals over a large area or resources
base. In order to find water source and natural pasture which are scarce and
widely dispersed, the activity of nomads’ shifts from one section of the land to
another.
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The majority of nomads follow relatively fixed patterns of seasonal
movement, determined by rainfall patterns. In other words, the nomads follow the
rain to adjust to the unpredictable rainfall regime in the desert.
In the Sahel zone, for example, the herds are at their furthest south during
the dry season. At the start of the rainy season, they follow the onset of the rains
northwards, grazing the flush of new grasses. During the wet season they graze on
the pasture lands in the northern areas, returning southwards at the end of the rains
and grazing the vegetation which has grown during that season, as well as stubble
and residues from arable cropping. It is because some nomads plant crops in wadis
or other areas where moisture supplies are adequate, and cereals such as barley
and millet may be grown.
The grazing cycle of nomadic pastoralism is similar to that of shifting
cultivation. There is a rotation of grazing land. Such rotations allow the natural
vegetation to regenerate before another cycle of grazing begins. By doing this, soil
structure, soil fertility and hence the carrying capacity of the desert environment
can be conserved.
2. Nomadic pastoralism in Xinjiang and Sahara:
It is in Sahara, the deserts of the Middle East and in Central Asia that true
pastoral nomadism is practiced. This system developed in marginal desert areas
where the soil became so impoverished that cultivation was no longer profitable.
First cattle and sheep were raised but as condition declined only camels and goats
could survive. These days the nomadic system, in which stock owners wander
freely in search of good pastures, is usually found in the most desert areas.
Transhumance, wherein a permanent base is recognized but for part of each year
grazing is obtained elsewhere, may occur where environmental conditions are
somewhat more favourable.
a. In Xinjiang:
Owing to the aridity and consequent paucity of grass, the Tarim Pendi (Basin)
has very little nomadic pastoralism. Although sheltered on the west by 2000 m
high mountains, the Junggar Pendi is exposed through several passes to air
masses from the northwest. This gives the region more rain than the Tarim.
The annual precipitation ranging from 150 - 300 mm. There are more
grassland than in the Tarim Pendi and the traditional economy of nomadic
herding the persists. There are wide grass-covered valleys bordering the desert,
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but the low rainfall makes it necessary for the pastoralists to be always moving
in search of new grass. Herds of sheep, cattle, horses and camels are grazed on
the natural pasture.
There are two types of nomadic grazing. One is an aimless roaming, the routes
varying from year to year. This type of nomadism, though still practiced by
some Kirghiz, is losing in importance on account of the shrinkage of natural
pasture and the commune policy of the government. The other type is
transhumance which has definite encampments for summer and winter
pastures. The mountains are too cold for winter occupancy, but during the
summer nomads take their flocks to the upper meadows.
b. In the Sahara:
True Nomadic Pastoralism in the Sahara:
Tribes have to be continuously on the move since grazing land is too poor and
scattered for flocks to stay because the thin vegetation cover can be eaten
easily. Scouts look for fresh pasture and animals are driven there immediately.
Usually they move once a month. They move in constant routes, depending on
water points, the presence of suitable pastures and where the tribes possess
their customary rights. And for easy movement, they are usually in small
family groups, with minimum baggages. There are two kinds of movements of
these true nomadic pastoralists.
1. Movement within the Sahara:
Some may move westward since Western Sahara has better grazing land
due to the presence of mist and dew near to the west coast.
The Beja, who live in the mountains overlooking the Red Sea carry out
some sorts of transhumance. During the rainy season, they live on the
Western Pediment. During the dry season, they move to the more humid
highlands.
2. Movement out of the desert during the driest season:
Nomads of northern Sahara spend their summer (starting from April) in
the mountains and steppes in the north, and back to the Sahara Desert in
winter.
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Nomads of the southern fringe of the Sahara drive their flocks into the
wetter Sudanese Sahel during the drier period (winter), eg. the Kabadish.
Semi-nomads:
Semi-nomads are found almost entirely among the southern hills of the Atlas
Mountains in the north and the coastal fringe of Libya, as well as the southern
perimeter of the desert in the south. These nomads keep animals like sheep, goats
and cattle and they drive their flocks up to the high plateau according to seasons
and rain. They also carry out subsistence farming and grow barley, wheat and
fruits. The number of these semi-nomads has decreased since grazing land may be
restricted or unavailable because of other development.
3. Conclusion:
Primary production in deserts is very low, unpredictable and except during
brief rainy spells, of poor quality. In addition to these, rainfall events are
frequently very localized.
Nomadic Pastoralism characterized by its frequent movement in response
to the needs of grazing land and water is a flexible and efficient form of livelihood
because water is the most limiting factor for plant growth in deserts. It is well
adapted to the harsh desert environment with a low carrying capacity. As long as
the number of cattle is kept below the carrying capacity of the land, equilibrium
can be maintained by rotating the grazing land by which natural vegetation can
have time to regenerate and recover before another cycle of grazing.
For thousands of years, these nomads make little perceptible impact on the
environment as they are perfectly well integrated ecologically. In fact where the
human and animal population are appropriate to the carrying capacity of the land,
the nomadic system is claimed to be the one of the best adaptation to harsh arid
conditions.
As Wicken and Write (1979) mentioned that 'the nomadic way of life
probably represents the only way in which a maximum amount of food may be
obtained from desert regions with a minimum of damage.’
B. The Problem of Over-grazing:
In the drier areas transhumance and nomadic grazing were practiced in
response to the natural climatic cycles. This mobility, combined with shorter
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Page 27
human life expectancies and lower human and animal populations, meant that man
was more or less in balance with his environment. However, this grazing system
could only support a small population and that over-exploitation could easily
occur.
A relative stability, during which only a very gradual degradation of the
environment occurred. Most of the degradation to more desertic conditions or
even to wasteland has occurred in the last several hundred years in many parts of
the world. For example, the present shrubby Chihuahuan desert of southwestern
United States supported grassland until it was seriously over-grazing in the 19th
century. In South Africa the semi-arid Karoo has increased by 50% during the last
century. In the Negev most destruction has occurred during the 20th century.
Among the many factors, over-grazing is one of the most important causes
of environment degradation and desertification. Why do the nomads overgraze
their environment?
The size of the indigenous population was regulated by a death rate, due to
diseases, starvation or tribal warfare, which balanced the birth rate, keeping the
population size stable. The rapid expansion of improved medical services has
decreased deaths from diseases; international aid programmes have reduced the
level of starvation artificially by supplying food from outside, thus overriding the
laws of supply and demand; and improved law enforcement and tribal settling
programmes have reduced the numbers of deaths caused by warfare. These three
factors combined have produced a population explosion.
In North Africa and in the Near and Middle East, for example, population
has multiplied six-fold since the beginning of this century. The present growth
rate is between 3 and 3.5% per year in most of these countries, a doubling period
of 20 to 23 years. South of the Sahara, the growth is somewhat slower, 2.5 to 3.0
per year, a doubling period of 23 to 28 years. Such rapid population increase
exerts much pressure on food production and demand.
Population increase without alternative means of earning a living forces
the pastoralists to expand their stock. For example, the nomads of the Sahel
increased their herd size substantially during climatically favourable years in
response to increased population size. And they are unwilling to reduce their stock
despite the increasing dryness and restock too quickly when the rain begins to
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come back. With increased herd size, nomads are forced to go back to the fallow
graze land more frequently, meaning that the fallow period in which vegetation
recovers becomes shorter. The never-ending demand for food also forces nomads
to expand further and further into those marginal areas where man only managed
to survive by keeping in precarious balance with nature. These responses of the
nomads lead to overgrazing of the desert environment.
In pre-independence political situations, African countries encourage
nomadism, and there was no restriction on the movement of people (Grove, 1967).
With independence and the establishment of political boundaries, the unrestricted
movement of people and livestock came to an end, mainly for population control,
revenue collection and national boundary adjustment for exploitation of the land
and its resources. The range of migration of nomads is thus restricted and hence
the nomads cannot follow the rain pattern freely as to adjust their activity to the
unpredictable rainfall regime in the desert. Moreover, nomadism is frequently
looked on with disfavour by modern governments and tribesmen are often
encouraged to settle and cultivate. Under such political pressure, more cattle are
being kept on a smaller area, meaning that there is a greater concentration of
grazing pressure on the poor vegetation and a greater danger of over-trampling of
the soil by animals, in another words overgrazing the desert environment which is
of a very low carrying capacity.
Commercial grazing is the grazing of animals for sale. It is quite a recent
development in some desert areas. Probability the first desert area to be used
commercially was that of the 'wild west' in the U.S.A. and Mexico. If water could
be obtained, the desert vegetation could be used to produce meat, and to a lesser
extent wool. The success of commercial grazing in North America and Australia.
Here water is quite often obtained from artificial water holes such as artesian
bores of pipelines.
Today huge areas of land are leased as cattle stations in the Alice Spring
area of the Australian desert, and sheep grazing has been pushed up the west coast
of western Australia into the hot dry Pilbara region. Cattle and sheep grazing have
spread southward from the fertile Pampas of Argentina into the Patagonia desert
of southern South America has settled most of its nomadic herders into
commercial communities based on newly developed water supplies.
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In all these areas, grazing properties have to be very large as the carrying
capacity of the desert land is low, often only one beast per square km. Droughts,
which affect the natural pasture and water supplies, remain the greatest obstacle to
the continued success of this industry. Serious and permanent damage can be done
if the land is overgrazed, particularly in drought periods.
These demographic, political and economical changes lead to the problem
of overgrazing. The tradition of equilibrium with the environment is upset and a
chain of disastrous degradation processes has been triggered.
C. Impacts of Over-grazing:
1. Destruction of vegetation:
Although livestock raising does not involve deliberate eradication of the
vegetation mantle, its effects are mainly negative. Over-stocking - beyond the
carrying capacity of pasture at its seasonal minimum productivity is bad with all
species. Sheep have the additional trait of grazing down to root level, often
destroying beneficial grasses and permanently opening up the ground mat, with
only partial recolonization by shrubby vegetation. Goats are notorious for
indiscriminate grazing, resulting in destruction of trees and their seedlings. Large
aggregation of livestock also serves to destroy the plant cover, not the least
through trampling.
Removal of grassy vegetation eliminate raindrop interception and permit splash
erosion as well as accelerated soil creep and rill erosion. Severe overgrazing
destroys the litter or fermentation horizons that constitute much of the organic mat.
This further reduces infiltration capacity, increasing the volume and velocity of
surface runoff, and exposes bare soil to alternating rain and drought.
The general results of overgrazing are destruction of vegetation cover,
replacement of palatable species by non-palatable grasses or a loss of regeneration
capacity, desiccation of the land, destruction of soil structure and soil erosion. The
situation is particularly serious with the occurrence of droughts.
2. Destruction of soil:
When the drought years came the ecosystem could no longer support the animal
population. The death of large number of their livestock put the human population
at risk. This prompted the drilling of new wells to provide vitally needed water for
the herds. The result however was the concentration of the livestock around these
fixed points. Within a very short time the vegetation in the surrounding areas, up
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to distances of hundreds of km was totally denuded. Severe trampling up of the
soil also results in destruction of soil structure.
On the other hand, droughts force the herds to seek further field for fodder. The
palatable plant species are consumed and eventually eliminated, leaving only the
extremely unpalatable species, which then multiply rapidly in the absence of
competition. If the drought spell lasts long, the influence of over-grazing on the
total plant cover is intensified, when many less palatable species avoided as a rule
in the normal years are grazed.
With the removal of vegetation cover, soil becomes directly exposed to the
desiccation effect of the sun and wind. The surface soil disintegrates into particles
of dust and the pounding of the livestock' hooves also creates a fine surface dust
too. These are picked up by the wind and blow away, and the larger fragments left
behind are not a fertile medium for plant colonization, so the land is left barren,
Subsequent rain, wind and running water further denude the land.
Once soil erosion started, it is usually irreversible and it may eliminate the natural
vegetation of an area forever. Overgrazing can cause the expansion of deserts into
regions that are not naturally deserts, and subsequently can often force a migration
of people into less arid areas, and so the cycle of environmental degradation is
repeated continuously resulting in continuous processes of desertification.
3. Desertification:
a. What is Desertification?
- Warren and Maizels: 'A simple and graphic meaning of the word
"desertification" is the development of desert-like landscapes in areas which
were once green. Its practical meaning ......is a sustained decline in the yield of
useful crops form a dry area accompanying certain kinds of environmental
change, both natural and induced.'
- Kates, Johnson, and Haring: 'It involves destructive processes in which
productive base deteriorates and the social system is imperiled. Unlike drought,
which is usually a short-term diminution of available moisture, the physical
processes involved in desertification are long-term, chronic and pervasive.'
- Anaya Garduno: 'Desertification is the impoverishment of arid, semi-arid and
some subhumid ecosystems by the impact of man's activities. It is the process
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of change in these ecosystems that leads to reduced productivity of desirable
plants, alterations in the biomass and diversity of life forms, accelerated soil
degradation and increased hazards for human occupancy'
- Desertification is the diminution or destruction of the biological potential of
the land, and can lead ultimately to desert-like conditions. It is an aspect of the
widespread deterioration of ecosystems, and has diminished or destroyed the
biological potential, i.e. plant and animal production, for multiple use purposes
at a time when increased productivity is needed to support growing
populations in quest of development.
b. Desertification in the Area of Gourma in Mali:
- Desertification processes are reactions in the terrestrial ecosystem of semi-arid
and arid regions to the impact of man. In the Sahelian regions at the southern
edge of the Sahara, ecosystem have been severely deteriorated. Excessive land
abuse, such as over-grazing, extending cultivation and increased wood-cutting
in combustion with droughts or periods of deficient rainfall, has diminished or
destroyed the biological potential of vast areas.
- One of the effects of this impact on dry land ecosystem is accelerated soil
erosion. The area of Gourma in Mail with three humid months and about
400mm precipitation per year, was originally high-potential grazing land. On
vegetation covered sand sheets and fossil linear dune systems ruddily red
sandy soil have been developed. Relatively nutrient-rich portions of this soil
complex and properties favourable for infiltration and water storage have been
responsible for a relatively high productivity of biomass in the past.
-These areas are exceptionally attractive to the nomads; their livestock have
over-grazed and trampled them to a degree more than this ecosystem can
tolerate, and as a result there is an especially large amount of degradation. The
removal of the original vegetation cover, together with trampling by animals,
initiated interactive water and wind erosion, which destroyed, in a
self-accelerating process, most of the natural potential of this typical Sahelian
ecosystem. In wide areas of the Gourma with a fossil sand cover this land
degradation has reached a very severe stage, where regeneration or restoration
is not feasible, in particular when underlying rocks or lateritic crust formations
have been exposed by erosion. However, there are some areas remaining
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which display an as yet undisturbed equilibrium. Immediate action is
necessary to prevent the complete destruction of this favourable ecosystem.
4. Impact on desert climate:
Man's over-exploitation of his environment has not only directly led to erosion
and hence desertification, but also indirectly affected the desert climate which in
turn reinforces soil erosion and desertification.
The climate of the earth has changed many times in the past, and is probably
changing even now. In the past, these climatic changes were mainly induced by a
combination of natural influences. However, an increasingly important factor has
now been introduced into the complex balance that governs the climate: human
activities. One can no longer neglect the often very substantial and growing
influence that humans exert by polluting the atmosphere regional and globally and
by altering the character of the earth's surface, in some cases very extensively.
Deserts are situated where the large-scale global circulation patterns either deny
moisture to a region or where subsiding air associated with those patterns puts a
lid on rain-producing convection. In addition, there are many localized processes
at work as well, such as the warming - or lack of warming - of the air in contact
with the ground, and such as the degree of the availability of freezing nuclei. The
question now is whether human activities can influence these desert-related
processes. The answer is a qualified 'yes'.
Human activities have increased the surface reflectivity (albedo) of the ground.
Ground covered by plants has an albedo in the range of 10-25%, whereas ground
deprived of a vegetation cover as a result of deforestation and over-grazing (as in
parts of the Sahel) has a very much higher one. This would affect temperature
levels. Otterman (1974) has suggested that this landuse-changed albedo has
produced temperature changes of the order 5oC.
However, the consequences may go beyond changing temperature. Charney and
others (1975) have argued that the increase in surface albedo resulting from a
decrease in plant cover, would lead to a decrease in the incoming radiation, and an
increases in the radioactive cooling of the air. As a consequence, they maintain,
the air would sink to maintain thermal equilibrium by adiabatic compression, and
cumulus convection and its associated rainfall would be suppressed. The lower
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rainfall would in turn have an adverse effect on plants and lead to further decrease
in plant cover.
The effect of trampling the soil and of over-grazing the vegetation cover can also
affect the production of freezing nuclei, according the Schnell (1976) and to Vali
et al (1975). They found that decayed debris may contain a kind of bacteria that
serves as a remarkably efficient freezing nuclei. Removal of vegetation, therefore,
removes a source of efficient freezing nuclei, and theoretically could reduce the
probability of convective rainfall.
Over-grazing not only directly results in destruction of vegetation cover, soil
erosion and hence desertification but also affects the desert climate - by increasing
the surface albedo and reducing freezing nuclei - which in turn intensifies the
process of desert encroachment or desertification.
D. Remedies to Desertification:
Various solutions to the problems have been offered, both by individual North
African countries, and by the region acting together:
 Increase the
production of
meat by keeping
camels instead
of sheep, goats
and cattle
because camels
can graze further
from their
sources of water.
 Plant prickly
pear as a source
of animal fed
and to reduce
runoff from
rainstorms, thereby preventing soil erosion.
 Transfer people to other areas or activities, forcing emigration and providing
education, as well as gaining some control over the use of land and water.
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 Provide a green belt across North Africa to consist of farms, woodlands,
shelter belts etc, designed to offer stable soil and dune conditions, moisture
conservation and afforestation.
To solve the problems created by irrigation, the build up of salt can be reduced by
soaking the ground through tubes, while keeping the soil covered under a plastic
sheet. This method, however, requires a greater volume of water.
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