Chapter 16
The Impact of Invasive Alien Plants
on Rangelands in South Africa
Timothy G. O’Connor and Brian W. van Wilgen
Abstract Rangeland covers >70% of the land surface of South Africa, and
includes grassland, savanna, thicket, and karroid shrubland vegetation. These
rangelands support domestic livestock and wildlife whose economic value is around
ZAR 30 billion annually. They are invaded by hundreds of alien plant species, of
which 71 have been identified as being of special importance in South African
rangelands. These species are able to proliferate in response to disturbances, of
which grazing and fire are the two most important for South African rangelands;
changes to fire and grazing regimes can therefore promote invasion, especially by
alien trees. These trees replace palatable grasses and are generally unpalatable
themselves. At a national scale, invasive alien plants are estimated to reduce the
value of livestock production by ZAR 340 million annually, but this is expected to
increase dramatically as plant invasions spread, and as additional alien species
become invasive. Invasive species that have increased their range dramatically by
up to 671% between 2006 and 2016 include Campuloclinium macrocephalum
(Pompom Weed), Opuntia engelmannii (Small Round-leaved Prickly Pear),
Opuntia humifusa (Large-Flowered Prickly Pear), Parthenium hysterophorus
(Parthenium Weed), Trichocereus spachianus (Torch Cactus), and Verbena
bonariensis (Wild Verbena). Studies that document the impacts of individual
species on rangeland composition and structure cover only a few species, including
Prosopis species (Mesquite), Acacia mearnsii (Black Wattle) and Parthenium
hysterophorus, all of which can dramatically reduce grass cover and the capacity
to support livestock, especially at high densities. Invasive plants in rangelands
can also be beneficial as sources of firewood, fodder, shade and medicinal
products. Some species may offer considerable value when at low abundance but
T. G. O’Connor (*)
South African Environmental Observation Network, Pietermaritzburg, South Africa
e-mail: tim@saeon.ac.za
B. W. van Wilgen
Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University,
Stellenbosch, South Africa
© The Author(s) 2020
B. W. van Wilgen et al. (eds.), Biological Invasions in South Africa, Invading
Nature - Springer Series in Invasion Ecology 14,
https://doi.org/10.1007/978-3-030-32394-3_16
459
The Impact of Invasive Alien Plants
on Rangelands in South Africa
Timothy G. O’Connor and Brian W. van Wilgen
Abstract Rangeland covers >70% of the land surface of South Africa, and
includes grassland, savanna, thicket, and karroid shrubland vegetation. These
rangelands support domestic livestock and wildlife whose economic value is around
ZAR 30 billion annually. They are invaded by hundreds of alien plant species, of
which 71 have been identified as being of special importance in South African
rangelands. These species are able to proliferate in response to disturbances, of
which grazing and fire are the two most important for South African rangelands;
changes to fire and grazing regimes can therefore promote invasion, especially by
alien trees. These trees replace palatable grasses and are generally unpalatable
themselves. At a national scale, invasive alien plants are estimated to reduce the
value of livestock production by ZAR 340 million annually, but this is expected to
increase dramatically as plant invasions spread, and as additional alien species
become invasive. Invasive species that have increased their range dramatically by
up to 671% between 2006 and 2016 include Campuloclinium macrocephalum
(Pompom Weed), Opuntia engelmannii (Small Round-leaved Prickly Pear),
Opuntia humifusa (Large-Flowered Prickly Pear), Parthenium hysterophorus
(Parthenium Weed), Trichocereus spachianus (Torch Cactus), and Verbena
bonariensis (Wild Verbena). Studies that document the impacts of individual
species on rangeland composition and structure cover only a few species, including
Prosopis species (Mesquite), Acacia mearnsii (Black Wattle) and Parthenium
hysterophorus, all of which can dramatically reduce grass cover and the capacity
to support livestock, especially at high densities. Invasive plants in rangelands
can also be beneficial as sources of firewood, fodder, shade and medicinal
products. Some species may offer considerable value when at low abundance but
T. G. O’Connor (*)
South African Environmental Observation Network, Pietermaritzburg, South Africa
e-mail: tim@saeon.ac.za
B. W. van Wilgen
Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University,
Stellenbosch, South Africa
© The Author(s) 2020
B. W. van Wilgen et al. (eds.), Biological Invasions in South Africa, Invading
Nature - Springer Series in Invasion Ecology 14,
https://doi.org/10.1007/978-3-030-32394-3_16
459