of a stand self-protected against fire. The Karoo Biomes and Albany Thicket do not
normally experience fire although a few fires have recently occurred within the
transition between the Nama-Karoo and Grassland Biomes following an increase in
grass biomass resulting from years of above-average rainfall (du Toit et al. 2015).
Fire also interacts strongly with grazing pressure to influence vegetation responses
(Box 16.1).
16.2.5 The Influence of Fencing
Fencing facilitates invasion of alien species through offering a perch site for birds to
disperse the seeds of alien plants, as crows appear to do for Opuntia species (Dean
and Milton 2000), and because fire breaks along fences are commonly prepared by
breaking soil which contributes to establishment success of ruderal including alien
plants (Roux 1969).
16.3 Invasive Plants in Rangelands
South African rangelands harbour hundreds of alien plant species, ranging from an
estimated 129 species in the Nama Karoo Biome to 404 species in the Savanna
Biome (van Wilgen and Wilson 2018), but <10% of these have become seriously
invasive to date (Fig. 16.1). Many alien species remain restricted to land transformed
by crop agriculture, timber plantations, and urban settlements. The wetter biomes
(Indian Ocean Coastal Belt, Savanna, Grassland) and riparian rather than dryland
habitats within each biome have been impacted most by invasive alien plants. Wellmanaged dryland rangeland has proven to be largely resistant to invasion although a
few alien species in riparian habitats have also invaded adjacent dryland; a set of
species have invaded only dryland rangeland. Several invasive alien species have
transformed vegetation structure owing to a larger growth form than those of native
plants, especially evident for invasion by woody plants into grassland or karroid
shrubland. Patches of soil disturbance or nutrient enrichment, often created by
livestock, offer additional pathways of ingress into dryland rangelands. Episodic
proliferation of annual grasses that may fuel fires would threaten transformation of
affected areas of the Succulent Karoo Biome, whose vegetation is not adapted to fire
(Rahlao et al. 2009). Many alien species establish and proliferate in response to
disturbances (Hobbs and Huenneke 1992), of which grazing and fire are the two
most important types in South African rangelands.
To assess biome-level patterns, we focused on species of concern identified by
van Wilgen et al. (2008). We defined these as species which occupy >10% of
quarter degree grid cells of at least one biome (QDGC; 15
0 latitude by 15
0 longitude),
species with limited distributions but severe local impacts (Rouget et al. 2004), and
species which are rapidly becoming major threats to rangelands based on the extent
466
T. G. O’Connor and B. W. van Wilgen
normally experience fire although a few fires have recently occurred within the
transition between the Nama-Karoo and Grassland Biomes following an increase in
grass biomass resulting from years of above-average rainfall (du Toit et al. 2015).
Fire also interacts strongly with grazing pressure to influence vegetation responses
(Box 16.1).
16.2.5 The Influence of Fencing
Fencing facilitates invasion of alien species through offering a perch site for birds to
disperse the seeds of alien plants, as crows appear to do for Opuntia species (Dean
and Milton 2000), and because fire breaks along fences are commonly prepared by
breaking soil which contributes to establishment success of ruderal including alien
plants (Roux 1969).
16.3 Invasive Plants in Rangelands
South African rangelands harbour hundreds of alien plant species, ranging from an
estimated 129 species in the Nama Karoo Biome to 404 species in the Savanna
Biome (van Wilgen and Wilson 2018), but <10% of these have become seriously
invasive to date (Fig. 16.1). Many alien species remain restricted to land transformed
by crop agriculture, timber plantations, and urban settlements. The wetter biomes
(Indian Ocean Coastal Belt, Savanna, Grassland) and riparian rather than dryland
habitats within each biome have been impacted most by invasive alien plants. Wellmanaged dryland rangeland has proven to be largely resistant to invasion although a
few alien species in riparian habitats have also invaded adjacent dryland; a set of
species have invaded only dryland rangeland. Several invasive alien species have
transformed vegetation structure owing to a larger growth form than those of native
plants, especially evident for invasion by woody plants into grassland or karroid
shrubland. Patches of soil disturbance or nutrient enrichment, often created by
livestock, offer additional pathways of ingress into dryland rangelands. Episodic
proliferation of annual grasses that may fuel fires would threaten transformation of
affected areas of the Succulent Karoo Biome, whose vegetation is not adapted to fire
(Rahlao et al. 2009). Many alien species establish and proliferate in response to
disturbances (Hobbs and Huenneke 1992), of which grazing and fire are the two
most important types in South African rangelands.
To assess biome-level patterns, we focused on species of concern identified by
van Wilgen et al. (2008). We defined these as species which occupy >10% of
quarter degree grid cells of at least one biome (QDGC; 15
0 latitude by 15
0 longitude),
species with limited distributions but severe local impacts (Rouget et al. 2004), and
species which are rapidly becoming major threats to rangelands based on the extent
466
T. G. O’Connor and B. W. van Wilgen