Environmental Engineering Reference
In-Depth Information
versus environment' discourse where local people are seen as damaging
to the environment. This discourse implies a need to manage people and
the environment separately. The other is the 'people and environment'
discourse that seeks to encourage integrated management of people and
resources by recognizing the importance of the reciprocal relationship
between people and their environment. These policy discourses are under-
pinned by two dif erent ecological and social paradigms upon which is
dei ned the conceptualization of management problems.
Ecological paradigms
Until relatively recently the predominant paradigm for understanding
the ecology of semi-arid regions, and rangelands in particular, has been
equilibrium theory. This paradigm developed from a European and
US perspective of rangelands in temperate climates (Clements, 1916).
Equilibrium theory is based on the assumption that vegetation composi-
tion will progress towards a climax community, depending on climate and
soil conditions, through the processes of succession (Begon et al., 1990;
Behnke and Scoones, 1993). The climax community is therefore consid-
ered to be the most stable plant community that could occur. Disturbance,
either natural or anthropogenic, will result in the vegetation being 'pushed
back' to an earlier successional stage and once the disturbance ceases or
is removed then natural processes will lead back to the climax condition
(Briske et al., 2003). The most commonly used example of succession is
the clearing of forest for agriculture in temperate regions. Once the land
is abandoned then it is assumed that a climax forest community will
eventually return (Behnke and Scoones, 1993). These ideas were applied
to grazed rangelands in sub-Saharan Africa, as well as elsewhere, where
dif erent grass communities were considered to represent dif erent suc-
cessional stages with certain ecological communities representing climax
(Brown and MacLeod, 1996).
An important element of equilibrium theory as applied to grazed range-
lands is the concept of carrying capacity. If, as suggested by equilibrium
theory, secondary production is linearly related to the successional status
of vegetation then it is possible to calculate carrying capacity in terms of
wild herbivores or livestock that can be supported on the range. In equi-
librium theory the interaction between plants and animals is an important
negative feedback mechanism that regulates the ecosystem (Briske et al.,
2003). This density dependence relationship means that at ecological
carrying capacity herbivore numbers are controlled by primary produc-
tion so that the number of births equals the numbers of deaths and the
vegetation is held back from reaching the climax composition that would
occur without such disturbance from grazing. For managers of rangelands
