Environmental Engineering Reference
In-Depth Information
the aim would be to maintain the vegetation at some optimal sub-climax
composition so as to maximize secondary production, depending on the
management objectives. What is important to note is that carrying capac-
ity in this context is dependent on the management objectives and is not
an inherent feature of the ecosystem. Dif erent carrying capacities will
be desired, for example, for commercial meat production or subsistence
pastoralism.
Equilibrium theory as applied to sub-Saharan Africa has been criticized
for not adequately describing the reality of rangelands and recently this
criticism has led to the development of an alternative paradigm for range
ecology: the non-equilibrium theory (Behnke and Scoones, 1993; Warren,
1995; Brown and MacLeod, 1996; Briske et al., 2003). This paradigm sug-
gests that there is no single successional pathway and that rangeland can
exist in multiple stable states. Rangelands are then seen as mosaics of plant
communities created by a variety of factors rather than evidence of early
succession stages caused by disturbance (Sullivan and Rohde, 2002). What
is important for non-equilibrium theory is that density-dependent inter-
action between plants and animals is weak or non-existent and instead
abiotic factors, and in particular rainfall, are the dominant driving factors
of vegetation production, distribution and composition. This is an impor-
tant distinction from equilibrium theory because it recognizes that the
highly variable climate and rainfall patterns are more important in deter-
mining primary production and composition than grazing by herbivores.
In non-equilibrium theory l uctuations in rainfall create l uctuations in
vegetation productivity, which in turn cause herbivore numbers to l uctu-
ate. Rainfall is such an important driver of the system that vegetation and
herbivores do not reach equilibrium and so it is not possible to calculate a
carrying capacity.
There is still controversy over whether equilibrium or non-equilibrium
paradigms apply to African rangelands. Evidence has been presented that
supports both views (Illius and O'Connor, 1999; Sullivan and Rohde,
2002). Illius and O'Connor (1999) in particular present the view that there
are still density-dependent interactions between herbivores and vegetation
in key resource areas. Therefore carrying capacity is maintained for these
areas even when it does not correspond to the whole range. In contrast
Sullivan and Rohde (2002) argue that even key resource areas are subject to
highly dynamic rainfall patterns so that primary production is more related
to rainfall than to grazing. It has been suggested that rather than focusing
on the dichotomy the theories should be considered as positions along a
continuum where rangeland can exhibit equilibrium or non-equilibrium
dynamics, or a combination of both (Briske et al., 2003). Empirical
studies are therefore necessary to determine where along this continuum
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