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

Feasible or foolish: Attempting restoration of a Parthenium hysterophorus invaded savanna using perennial grass seed

“…The suppression of P. hysterophorus growth, namely plant height and leaf area, occurred only in the plots which were sown or cleared and sown. This growth suppression is likely to have resulted from the lowered resources available to P. hysterophorus given increases in competition imposed by surrounding vegetation, particularly grasses (Strathie and McConnachie, 2019;O'Connor and van Wilgen, 2020). Under low to moderate grazing pressure, such as this, perennial grasses remain highly competitive, growing rapidly and establishing large tufts which acquire various above-and below-ground resources (Van Der Laan et al, 2008;Khan et al, 2013Khan et al, , 2019.…”

“…As a result of this intensive and often prolonged overuse, many of these savannas systems have become degraded, suffering invasion by unwanted non-native plants termed 'weeds' (DiTomaso et al, 2010;Roodt, 2011). Numerous factors facilitate the invasion of savannas and their rangelands by weeds, however, the most severe invasions occur predominantly in systems that are poorly managed, particularly when overgrazed (DiTomaso et al, 2010;O'Connor and van Wilgen, 2020). Overgrazing and weed invasions frequently co-occur given that intensive grazing, without respite, increases disturbance, lowers vegetation cover and limits the growth and recovery of pastures or rangelands, creating ideal conditions for the establishment and spread of invasive plants (Shabbir et al, 2013;Davies and Johnson, 2011).…”

Host suitability of three Opuntia taxa for the Dactylopius opuntiae (Hemiptera: Dactylopiidae) ‘stricta’ lineage

“…A fourth distinct lineage of O. engelmannii is found in Kenya (Githae, 2019). Dense infestations of O. engelmannii lineages and O. humifusa (Raf) Raf (creeping prickly pear) decrease the grazing potential of land and reduce access to livestock (O'Connor & van Wilgen, 2020). The long spines on the cladodes can injure livestock and the tiny glochids irritate their mouths in such a way that they are eventually unable to feed (O'Connor & van Wilgen, 2020).…”

“…Dense infestations of O. engelmannii lineages and O. humifusa (Raf) Raf (creeping prickly pear) decrease the grazing potential of land and reduce access to livestock (O'Connor & van Wilgen, 2020). The long spines on the cladodes can injure livestock and the tiny glochids irritate their mouths in such a way that they are eventually unable to feed (O'Connor & van Wilgen, 2020). Dense infestations of Opuntia species displace native flora which results in serious ecological impacts (Novoa et al, 2016) and socioeconomic impacts (Reynolds et al, 2020;Shackleton et al, 2017).…”

Functional Traits Plasticity of the Invasive Herb Argemone ochroleuca Sweet in Different Arid Habitats

“…Invasive alien plants have become a serious threat to native plant diversity and ecosystem function in many regions of the world [ 1 ]. A major challenge for plant ecologists is to highlight how invasive species respond to the variation in environmental conditions [ 2 , 3 ].…”