The Impact of Invasive Alien Plants on Rangelands in South Africa

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 their detrimental impact far outweighs advantages as their abundance increases, resulting in a net negative value. An escalating threat of alien plants to rangelands demands innovative responses in addition to biological control and clearing programmes.