Abstract:
QUESTIONS : Vegetation change in arid regions with a coefficient of rainfall
exceeding 33% usually displays non-equilibrium dynamics, where abiotic factors
override internal biotic controls. Irreversible changes have nonetheless also
been described for arid regions. What are the contributions of internal/equilibriumvs
external/non-equilibriumfactors to vegetation dynamics and can degradation
due to overstocking be reversed after removal of livestock?
LOCATION : Goegap Nature Reserve, Namaqualand, South Africa.
METHODS : The descending point method was conducted annually from 1974 at
two transects. Vegetation change was assessed in terms of vegetation cover, species
composition, life-form composition, range condition, species richness and
diversity. Principal coordinates analysis was used to illustrate the trajectories in
floristic data, and the effects of stocking density and rainfall were examined with
redundancy analysis.
RESULTS : Vegetation cover, species richness and Shannon-Wiener index of
diversity showed an increase and range condition improved with time. These
positive changes could be related to the removal of high numbers of livestock
and low wildlife numbers in the first years of survey. A gradual decline in the
rate of increase in some of these parameters could be related to high grazing
pressure during the later monitored years. There was a notable increase in nonsucculent
chamaephytes, but the initial increase in succulent chamaephytes was
not sustained. The directional change evident in perennial species composition,
supports the equilibrium concept, whereby the negative changes induced by
heavy grazing were partially reversed. Within the directional change, four
quasi-stable states could be distinguished, which could be reconciled with the
state-and-transition model. The annual component showed no directional
change, but displayed event-driven, non-equilibrium dynamics by fluctuating
in reaction to the timing and quantity of rainfall.
CONCLUSIONS : The vegetation change displayed elements of both equilibrium
and non-equilibrium dynamics, and demonstrated that the effects of heavy grazing
in the Succulent Karoo were reversible. Overall, the recovery process proceeded
slowly and was primarily detected in the perennial component of the
vegetation. The increase in wildlife numbers in the later studied years and
decline in perennial vegetation cover stress the need for active management of
animal numbers to avoid vegetation degradation.
