livestock species than wildlife species and, consequently, less of a range in body size
and feeding ecology. However, the foraging ecology of individual livestock species
is similar to that of individual wildlife species. In the Karoo biomes, the feeding
ecology and impact of caprines is comparable with that of the springbuck they have
replaced. Within the Savanna Biome, cattle are comparable with buffalo (Syncerus
caffer) in terms of body size and social aggregation, but there is no natural counterpart for cattle in the Grassland Biome. Moreover, mesic montane grasslands are
stocked at 35-fold higher biomass by commercial producers than that occurring
within natural wildlife ecosystems (Rowe-Rowe and Scotcher 1986).
16.2.3 Increases in the Intensity of Grazing
Grazing systems involving paddocks are the norm for commercial livestock production (O’Reagain and Turner 1992), with rotation among paddocks meaning that a
paddock is subjected to heavy grazing then rested. Communal areas are more
commonly grazed continuously but at up to three times higher stocking rates than
corresponding commercial ranches (Tapson 1993; Rutherford et al. 2012b).
O’Reagain and Turner (1992) concluded that stocking rate rather than grazing
system was the most important management variable affecting rangeland vegetation;
chronic (consistent and heavy) grazing over time has been responsible for depressed
herbaceous biomass that may offer establishment opportunities for alien plants.
Livestock may also create piospheres, a gradient of grazing impact diminishing
with distance from water points (Thrash 1998), and nutrient-enriched patches where
they repeatedly rest.
16.2.4 The Role of Fire
Fire is integral to the management of grassland and savanna, and is employed in
grassland mainly for removing moribund material and in savanna for controlling
bush encroachment (Tainton 1999). Fire-return interval across savanna and grassland is inversely related to mean annual precipitation (Archibald et al. 2009), which
varies from 200 mm to 1200 mm; fire-return interval correspondingly varies from
>10 years to 1 year (van Wilgen and Scholes 1997). Woody plants establish in moist
grasslands within 6 years if fire is excluded (Titshall et al. 2000). Accordingly, a
biennial fire regime is commonly imposed for their control. Savanna trends toward
dry thicket if fire is excluded (O’Connor et al. 2014). In semi-arid savanna or
grassland, fire may occur as infrequently as once a decade, or be completely
excluded if herbaceous fuel production is lessened by chronic grazing (Archibald
et al. 2009). Alien plants invading grassland or savanna would therefore be expected
to be fire-adapted, or to exploit fire refugia, localised areas of low fire intensity, or
areas in which fire has been excluded for long enough to allow for the development
16 The Impact of Invasive Alien Plants on Rangelands in South Africa
465
and feeding ecology. However, the foraging ecology of individual livestock species
is similar to that of individual wildlife species. In the Karoo biomes, the feeding
ecology and impact of caprines is comparable with that of the springbuck they have
replaced. Within the Savanna Biome, cattle are comparable with buffalo (Syncerus
caffer) in terms of body size and social aggregation, but there is no natural counterpart for cattle in the Grassland Biome. Moreover, mesic montane grasslands are
stocked at 35-fold higher biomass by commercial producers than that occurring
within natural wildlife ecosystems (Rowe-Rowe and Scotcher 1986).
16.2.3 Increases in the Intensity of Grazing
Grazing systems involving paddocks are the norm for commercial livestock production (O’Reagain and Turner 1992), with rotation among paddocks meaning that a
paddock is subjected to heavy grazing then rested. Communal areas are more
commonly grazed continuously but at up to three times higher stocking rates than
corresponding commercial ranches (Tapson 1993; Rutherford et al. 2012b).
O’Reagain and Turner (1992) concluded that stocking rate rather than grazing
system was the most important management variable affecting rangeland vegetation;
chronic (consistent and heavy) grazing over time has been responsible for depressed
herbaceous biomass that may offer establishment opportunities for alien plants.
Livestock may also create piospheres, a gradient of grazing impact diminishing
with distance from water points (Thrash 1998), and nutrient-enriched patches where
they repeatedly rest.
16.2.4 The Role of Fire
Fire is integral to the management of grassland and savanna, and is employed in
grassland mainly for removing moribund material and in savanna for controlling
bush encroachment (Tainton 1999). Fire-return interval across savanna and grassland is inversely related to mean annual precipitation (Archibald et al. 2009), which
varies from 200 mm to 1200 mm; fire-return interval correspondingly varies from
>10 years to 1 year (van Wilgen and Scholes 1997). Woody plants establish in moist
grasslands within 6 years if fire is excluded (Titshall et al. 2000). Accordingly, a
biennial fire regime is commonly imposed for their control. Savanna trends toward
dry thicket if fire is excluded (O’Connor et al. 2014). In semi-arid savanna or
grassland, fire may occur as infrequently as once a decade, or be completely
excluded if herbaceous fuel production is lessened by chronic grazing (Archibald
et al. 2009). Alien plants invading grassland or savanna would therefore be expected
to be fire-adapted, or to exploit fire refugia, localised areas of low fire intensity, or
areas in which fire has been excluded for long enough to allow for the development
16 The Impact of Invasive Alien Plants on Rangelands in South Africa
465