Karoo Biomes (Walker et al. 2018), on which continuous grazing by small-stock is
practised (Todd and Hoffman 2009). More than 30% of the Grassland Biome has
been transformed, <20% is under communal tenure, and the remainder is used
mainly for commercial production of sheep and cattle (Mucina and Rutherford
2006). The Savanna Biome contains approximately 100,000 km
2 of communal
area, the backbone of state wildlife conservation areas including the Kruger National
(19,000 km
2 ) and Kalahari Gemsbok National (9590 km
2 ) Parks, commercial
ranching based mainly on cattle but including sheep and goats in semi-arid regions,
and private wildlife properties that have emerged as an increasingly important land
use since the 1960s (Carruthers 2008). Albany Thicket was used mainly for goat
farming following European settlement but commercial wildlife enterprises are now
well established. The overall value of rangeland in South Africa is reflected by the
annual value of commercial livestock production estimated to be ZAR 18 billion
(Meissner et al. 2013), that of the commercial wildlife industry ZAR 10 billion
(Department of Environmental Affairs 2018b), and that 19 million people reside in
rural areas.
Here we discuss four components of change within the rangelands described
above—shifts in the composition of the herbivores, increases in the intensity of
grazing, changes to fire regimes, and fencing—and their consequences for invasions
(see also Box 16.1 for the evidence of the impact of grazing and fire).
Box 16.1 Experimental Evidence for the Role of Fire and Grazing
in Promoting or Preventing Invasion
South Africa has a rich legacy of long-term experiments in which regimes of
fire and grazing management have been manipulated in order to improve our
understanding of their ecological effects. In addition, the ecological outcomes
of different long-standing management regimes can commonly be compared
across the fence line separating two properties. While the effects of these
regimes on invasion by alien plants were not included in the formal experimental designs, the experiments or fence-line comparisons themselves offer an
opportunity to examine outcomes in this regard, and to look for generalities.
These experiments and contrasts have been collated in Supplementary Appendix 16.1 to provide a novel data base about the influence of management on the
success of alien plants.
Exclusion of fire and grazing was investigated at eight locations ranging
from semi-arid savanna through to moist grassland; investigation of fire returninterval was similarly represented along a climate gradient. Moist and mesic
areas grow sufficient fuel that can support frequent fires, semi-arid systems do
not. The effects of complete fire and grazing exclusion in semi-arid systems
may therefore be attributed mainly to grazing exclusion; those in mesic or
moist systems are usually attributed mainly to exclusion of fire. The effects of
fire have not been experimentally investigated in the semi-arid Albany
(continued)
16 The Impact of Invasive Alien Plants on Rangelands in South Africa
463
practised (Todd and Hoffman 2009). More than 30% of the Grassland Biome has
been transformed, <20% is under communal tenure, and the remainder is used
mainly for commercial production of sheep and cattle (Mucina and Rutherford
2006). The Savanna Biome contains approximately 100,000 km
2 of communal
area, the backbone of state wildlife conservation areas including the Kruger National
(19,000 km
2 ) and Kalahari Gemsbok National (9590 km
2 ) Parks, commercial
ranching based mainly on cattle but including sheep and goats in semi-arid regions,
and private wildlife properties that have emerged as an increasingly important land
use since the 1960s (Carruthers 2008). Albany Thicket was used mainly for goat
farming following European settlement but commercial wildlife enterprises are now
well established. The overall value of rangeland in South Africa is reflected by the
annual value of commercial livestock production estimated to be ZAR 18 billion
(Meissner et al. 2013), that of the commercial wildlife industry ZAR 10 billion
(Department of Environmental Affairs 2018b), and that 19 million people reside in
rural areas.
Here we discuss four components of change within the rangelands described
above—shifts in the composition of the herbivores, increases in the intensity of
grazing, changes to fire regimes, and fencing—and their consequences for invasions
(see also Box 16.1 for the evidence of the impact of grazing and fire).
Box 16.1 Experimental Evidence for the Role of Fire and Grazing
in Promoting or Preventing Invasion
South Africa has a rich legacy of long-term experiments in which regimes of
fire and grazing management have been manipulated in order to improve our
understanding of their ecological effects. In addition, the ecological outcomes
of different long-standing management regimes can commonly be compared
across the fence line separating two properties. While the effects of these
regimes on invasion by alien plants were not included in the formal experimental designs, the experiments or fence-line comparisons themselves offer an
opportunity to examine outcomes in this regard, and to look for generalities.
These experiments and contrasts have been collated in Supplementary Appendix 16.1 to provide a novel data base about the influence of management on the
success of alien plants.
Exclusion of fire and grazing was investigated at eight locations ranging
from semi-arid savanna through to moist grassland; investigation of fire returninterval was similarly represented along a climate gradient. Moist and mesic
areas grow sufficient fuel that can support frequent fires, semi-arid systems do
not. The effects of complete fire and grazing exclusion in semi-arid systems
may therefore be attributed mainly to grazing exclusion; those in mesic or
moist systems are usually attributed mainly to exclusion of fire. The effects of
fire have not been experimentally investigated in the semi-arid Albany
(continued)
16 The Impact of Invasive Alien Plants on Rangelands in South Africa
463