Box 16.1 (continued)
Thicket, Nama-Karoo or Succulent Karoo Biomes. Long-term exclusion of
fire has promoted the success of woody alien species throughout the Grassland
Biome including the Drakensberg, Athole in Mpumulanga, nThabamhlope
and Ukulinga in KwaZulu-Natal, and Döhne in the Eastern Cape. All these
grasslands are high rainfall or at least mesic in character. By contrast for the
Savanna Biome, woody alien species were successful only in the mesic
savannas of the Kruger National Park and Towoomba; woody alien species
did not establish in semi-arid savannas of Kruger National Park or of Mopani,
Limpopo Province. A long fire-return interval (relative to the management
norm of the system) also allowed woody alien species to increase in high
rainfall Drakensberg grassland and in mesic savanna in Kruger National Park.
Woody alien species which gained greatest prominence when fire was
excluded were Acacia mearnsii (Black Wattle) and Rubus cuneifolius (American Bramble) across the Grassland Biome, and Lantana camara (Lantana) in
the Savanna Biome. Exclusion of fire also appeared to promote herbaceous
alien species but this was not consistently evident.
Differences in grazing management had far less of an effect on the success
of alien plant species than differences in fire management. Neither woody nor
herbaceous aliens showed a response to grazing differences in mesic grassland
at Athole, Kokstad, or Ermelo, in semi-arid savanna of the southern Kalahari,
at the Sundays River Valley in the Albany Thicket Biome, in the Nama-Karoo
Biome at Grootfontein, Carnarvon or Sundays River Valley, or in the Succulent Karoo Biome at Tierberg or Hoekdoorn. An increase in alien plants in
response to heavy grazing pressure was recorded only for the Ikwezi district in
the Albany Thicket Biome. The success of the shrub Atriplex lindleyi
(Australian Saltbush) observed at Tierberg in the Succulent Karoo in response
to grazing treatment was the result of invasion onto nutrient-enriched mounds
(“heuweltjies”, see Moore and Picker 1991) (Milton and Dean 2010), a
response also observed following nitrogen addition. Similarly, alien herbaceous plant species are also evident in the grazing experiments in the NamaKaroo Biome at Carnarvon and Grootfontein on accumulated dung piles at
livestock resting spots (<1% of paddock area) that were absent from the
rangeland matrix.
Rangelands that are managed by prescribed grazing and fire within acceptable limits therefore appear to be largely resistant to invasive alien plants.
16.2.2 Shifts in the Composition of Herbivores
Over large parts of South Africa, free-ranging herds of grazing and browsing wild
herbivores have been replaced by domestic livestock. Domestic animals do not have
the same effect as native herbivores on vegetation because there are far fewer
464
T. G. O’Connor and B. W. van Wilgen
Thicket, Nama-Karoo or Succulent Karoo Biomes. Long-term exclusion of
fire has promoted the success of woody alien species throughout the Grassland
Biome including the Drakensberg, Athole in Mpumulanga, nThabamhlope
and Ukulinga in KwaZulu-Natal, and Döhne in the Eastern Cape. All these
grasslands are high rainfall or at least mesic in character. By contrast for the
Savanna Biome, woody alien species were successful only in the mesic
savannas of the Kruger National Park and Towoomba; woody alien species
did not establish in semi-arid savannas of Kruger National Park or of Mopani,
Limpopo Province. A long fire-return interval (relative to the management
norm of the system) also allowed woody alien species to increase in high
rainfall Drakensberg grassland and in mesic savanna in Kruger National Park.
Woody alien species which gained greatest prominence when fire was
excluded were Acacia mearnsii (Black Wattle) and Rubus cuneifolius (American Bramble) across the Grassland Biome, and Lantana camara (Lantana) in
the Savanna Biome. Exclusion of fire also appeared to promote herbaceous
alien species but this was not consistently evident.
Differences in grazing management had far less of an effect on the success
of alien plant species than differences in fire management. Neither woody nor
herbaceous aliens showed a response to grazing differences in mesic grassland
at Athole, Kokstad, or Ermelo, in semi-arid savanna of the southern Kalahari,
at the Sundays River Valley in the Albany Thicket Biome, in the Nama-Karoo
Biome at Grootfontein, Carnarvon or Sundays River Valley, or in the Succulent Karoo Biome at Tierberg or Hoekdoorn. An increase in alien plants in
response to heavy grazing pressure was recorded only for the Ikwezi district in
the Albany Thicket Biome. The success of the shrub Atriplex lindleyi
(Australian Saltbush) observed at Tierberg in the Succulent Karoo in response
to grazing treatment was the result of invasion onto nutrient-enriched mounds
(“heuweltjies”, see Moore and Picker 1991) (Milton and Dean 2010), a
response also observed following nitrogen addition. Similarly, alien herbaceous plant species are also evident in the grazing experiments in the NamaKaroo Biome at Carnarvon and Grootfontein on accumulated dung piles at
livestock resting spots (<1% of paddock area) that were absent from the
rangeland matrix.
Rangelands that are managed by prescribed grazing and fire within acceptable limits therefore appear to be largely resistant to invasive alien plants.
16.2.2 Shifts in the Composition of Herbivores
Over large parts of South Africa, free-ranging herds of grazing and browsing wild
herbivores have been replaced by domestic livestock. Domestic animals do not have
the same effect as native herbivores on vegetation because there are far fewer
464
T. G. O’Connor and B. W. van Wilgen