When there are fewer lions, the zebra population will increase. Ecosystems The first steps in the
evolution of agriculture were the tending of particular plant species and the taming of useful animal
species.
The next steps were (a) domestication of these species so as to gain control of their
reproduction thereby enabling selective breeding of more productive types and (b) creation of
special environmental conditions which would enable these improved types to realize their higher
production potential.
These environmental modifications involve soil tillage, soil water management,
weeding and pest control. The resulting combination of humans, domesticated plant and animal
species and their modified environments is an agro-ecosystem, in contrast to natural ecosystems in
which humans play no special role. In agro-ecosystems, the farmer is an essential ecological variable,
influencing or determining the composition, functioning and stability of the system.
Agroecosystems may be viewed as food procurement systems in which the natural ecosystem has been
modified to various degrees in order to increase output of food and other useful products of value to
humans.
The dominants in agro-ecosystems are selected plant and animal species which are tended
and harvested by humans for particular purposes. According to the nature of the modifications,
agro ecosystems range from shifting agriculture, nomadic pastoralism, and non-industrial continuous
agriculture to ranching, industrial agriculture and feedlot animal production.
The first three systems
are practiced primarily for subsistence, and may therefore be called subsistence agro-ecosystems,
while the last three are industrial agro- ecosystems which are geared to a market economy.
Agro ecosystems which involved field crop husbandry viz., shifting agriculture, non-industrial
continuous agriculture and industrial agriculture are also referred to as field crop ecosystems.
Intensive and extensive agro ecosystems Agro-ecosystems are classifiable according to whether
theey are extensive or intensive. Extensive.
Extensive systems may be defined as those where the
annual output of consumable nitrogen is less than 20 kg per ha. Outputs of crop or livestock
products per unit area are low, and these outputs are dependent largely on natural soil nutrient
reserves and management which conserves these reserves. Forms of subsistence agriculture such as
nomadic pastoralism and shifting agriculture are widespread examples.
In intensive agroecosystems, very high outputs are maintained by large inputs of nutrients. Both the volume and rate
of nutrient cycling are much higher than inextensive systems, particularly in industrial agriculture.
Since nutrient inputs are almost entirely in the form of inorganic fertilizers, nitrogen fixation and soil
organic matter are both depressed to very low levels. Losses of nutrients from the system through
exports of produce are great, while considerable leaching losses, of both soil nutrient reserves and
nutrient inputs occur particularly in wetter environments when land is bare during part of the
growing season.
Agro-ecosystems which involve a significant livestock sub-system as well as a
cropping sub- system are known as mixed farming systems. They are usually intermediate in
intensity between extensive and intensive agro-ecosystems. Shifting agriculture Shifting agriculture
is a very widespread agro-ecosystem in the tropics.
It includes a wide range of different localized
systems which have developed in response to local environmental and cultural conditions. The
essential features of the agro-ecosystem are that fields are rotated rather than crops, and a fallow
period restores soil fertility. Disturbance to the soil is also negligible since there is no soil tillage.
The system is well suited to nutrient poor soils in areas of low population density. Provided the fallow period is long enough, relative to the cropping period, the agro-ecosystem is sustainable indefinitely.
The cropping phase is just another human induced and managed disturbance in the natural,
continuing pattern of gap creation and secondaryDisturbance to the soil is also negligible since there
is no soil tillage. The system is well suited to nutrient poor soils in areas of low population density.
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Provided the fallow period is long enough, relative to the cropping period, the agro-ecosystem is
sustainable indefinitely. The cropping phase is just another human induced and managed
disturbance in the natural, continuing pattern of gap creation and secondary succession in forest.
The cleared area, during the period of cropping, is often referred to as a swidden. Because
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