(1978) noted that “Prickly pears . . . are problem plants from two points of view:
firstly, they form impenetrable thickets over wide areas, overwhelming the native
vegetation and completely preventing normal agricultural activities: and secondly,
livestock eat the fruit and cladodes, the spines and glochids of which may lead to
severe or fatal gastro-intestinal disorders”. Van Sittert (2002) notes that in the
1890s, farmers “urged the colonial state take immediate action against opuntia to
save the pastoral economy from the threatened ruination of its stock holdings,
labour and pastures”, and that “opuntia cut deep into the material base of the settler
economy and rendering it unfit for pastoral farming.” Now that the species is under
effective biological control, these impacts have largely been forgotten (Hill et al.
2020, Chap. 19).
16.6 Management of Rangelands
Rangelands are managed using manipulations of herbivory and fire that may influence the success of alien plants. Sources of information for considering these factors
were studies which have contrasted the influence on botanical composition of
commercial ranching, communal tenure, or conservation estate, and the results of
field trials or fence-line contrasts investigating fire and grazing management.
Communal rangeland tends to support a greater diversity and abundance of alien
plants than commercial rangeland in moist but not in semi-arid or arid environments.
Across KwaZulu-Natal [670–1120 mm mean annual precipitation (MAP)], three
alien Paspalum grass species dominated wetter communal sites but not wetter
commercial or semi-arid communal sites (O’Connor et al. 2003). In the southern
Drakensberg region of KwaZulu-Natal, the alien dicotyledon Richardia brasiliensis
(Tropical Richardia) was a sub-dominant species on dryland communal rangeland
(13% aerial cover) but this and other alien species were almost absent from commercial or conservation grassland (0.03% cover of R. brasiliensis) despite the
proximity of transformed land (cultivated grass pastures, maize fields, plantations)
that were dominated by a richness (50) of alien herbaceous species (O’Connor
2005). Scott-Shaw and Morris (2015) describe grazing gradients in the KwaZuluNatal midlands along which alien forb species replace native forb species lost to
chronic grazing. This evidence suggests that well-managed rangeland is resistant to
invasion. Wetlands were more severely invaded than drylands in this region; their
total of 47 alien species included 6 of the 30 most abundant species, with wetlands in
communal areas more densely invaded than those in commercial or protected areas
(Walters et al. 2006). By contrast, alien species were absent or very scarce on both
sides of the fence in semi-arid areas. Such fence-line contrasts included chronicallygrazed communal rangeland versus moderately-stocked commercial rangeland or
nature reserves in the Succulent Karoo (Todd and Hoffman 2009), the transition
from the Nama-Karoo to the Grassland Biome (Rutherford et al. 2012a), Thornveld
Savanna (Rutherford and Powrie 2011), and Mopane Savanna (Rutherford et al.
2012b). Within Albany Thicket, chronic continuous grazing by domestic livestock,
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T. G. O’Connor and B. W. van Wilgen
firstly, they form impenetrable thickets over wide areas, overwhelming the native
vegetation and completely preventing normal agricultural activities: and secondly,
livestock eat the fruit and cladodes, the spines and glochids of which may lead to
severe or fatal gastro-intestinal disorders”. Van Sittert (2002) notes that in the
1890s, farmers “urged the colonial state take immediate action against opuntia to
save the pastoral economy from the threatened ruination of its stock holdings,
labour and pastures”, and that “opuntia cut deep into the material base of the settler
economy and rendering it unfit for pastoral farming.” Now that the species is under
effective biological control, these impacts have largely been forgotten (Hill et al.
2020, Chap. 19).
16.6 Management of Rangelands
Rangelands are managed using manipulations of herbivory and fire that may influence the success of alien plants. Sources of information for considering these factors
were studies which have contrasted the influence on botanical composition of
commercial ranching, communal tenure, or conservation estate, and the results of
field trials or fence-line contrasts investigating fire and grazing management.
Communal rangeland tends to support a greater diversity and abundance of alien
plants than commercial rangeland in moist but not in semi-arid or arid environments.
Across KwaZulu-Natal [670–1120 mm mean annual precipitation (MAP)], three
alien Paspalum grass species dominated wetter communal sites but not wetter
commercial or semi-arid communal sites (O’Connor et al. 2003). In the southern
Drakensberg region of KwaZulu-Natal, the alien dicotyledon Richardia brasiliensis
(Tropical Richardia) was a sub-dominant species on dryland communal rangeland
(13% aerial cover) but this and other alien species were almost absent from commercial or conservation grassland (0.03% cover of R. brasiliensis) despite the
proximity of transformed land (cultivated grass pastures, maize fields, plantations)
that were dominated by a richness (50) of alien herbaceous species (O’Connor
2005). Scott-Shaw and Morris (2015) describe grazing gradients in the KwaZuluNatal midlands along which alien forb species replace native forb species lost to
chronic grazing. This evidence suggests that well-managed rangeland is resistant to
invasion. Wetlands were more severely invaded than drylands in this region; their
total of 47 alien species included 6 of the 30 most abundant species, with wetlands in
communal areas more densely invaded than those in commercial or protected areas
(Walters et al. 2006). By contrast, alien species were absent or very scarce on both
sides of the fence in semi-arid areas. Such fence-line contrasts included chronicallygrazed communal rangeland versus moderately-stocked commercial rangeland or
nature reserves in the Succulent Karoo (Todd and Hoffman 2009), the transition
from the Nama-Karoo to the Grassland Biome (Rutherford et al. 2012a), Thornveld
Savanna (Rutherford and Powrie 2011), and Mopane Savanna (Rutherford et al.
2012b). Within Albany Thicket, chronic continuous grazing by domestic livestock,
480
T. G. O’Connor and B. W. van Wilgen