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PLANT INVASIONS IN ARID AREAS OF SOUTH AFRICA — SPECIAL PROBLEMS AND SOLUTIONS

The face of arid areas across the globe is continuously shaped by the spread of invasive alien plant species. Plant invasions create a number of problems that have negative consequences for the natural and agricultural ecosystems of these areas and the services they provide. Among the problems that invasive alien plants cause are the reduction of available water resources, invasion of areas critical for livestock grazing and, more importantly, and negative impacts on biodiversity. When it comes to plant invasions in arid areas, South Africa is not any different. It is well documented that invasive alien plants in arid ecosystems of South Africa include species that are globally known for their negative impacts on these ecosystems. Finding solutions and management practices to prevent the introduction and spread of invasive plant species in arid areas is therefore critical to ensure the sustainability of these services.

A recent review paper by former C·I·B core team member, Sue Milton and co-author Richard Dean in the journal Biological Invasions, shed light on the special problems associated with the management of invasive alien plants in arid areas of South Africa and the challenges involved in finding solutions to these problems.

One of the foremost reasons for plant invasions in arid areas of South Africa is that they are often overlooked until they are so widespread that eradication is no longer possible. This is mainly because research facilities, farmers and agricultural extension officers are few and far between in these areas. Early detection is further complicated by the close resemblance between some invaders and their native equivalents. One example is the Australian saltbush Atriplex nummularia that looks very similar to the African A. vestita. The lack of early detection may also cause poorly adapted invasive species to improve through selective breeding and hybridisation. For example, Prosopis glandulosa, P. juliflora and P. velutina were introduced to South Africa during the late 1800s but large scale invasions began only 60 years later. Most of the invasive stands have been identified as hybrids of P. glandulosa and the other introduced Prosopis species. Invasive alien plant species were originally imported and planted for the many goods and services they provide. These include shade in treeless areas, firewood, drought fodder, building material, and soil stabilisation. Negative ecological impacts associated with invasive alien plants include changes in nutrient cycling in the soil, out-competing of native species, changed vegetation flammability, reduced soil stability and hybridization between invasive and closely related indigenous species.

The paper also suggests possible solutions and opportunities where managers and farmers can get involved to prevent further introductions of invaders. Management interventions can include the prioritisation of areas for clearing based on the impact of the invasion on key resources such as groundwater sources, wetlands and riverbeds. For example, when funds are limited, phreatophytes including Arundo donax and Tamarix ramosissima should be targeted. Other management strategies should include the allocation of funding to follow-up control and the prevention of activities that favour invasive species. For example, the national road agency has for many years applied roadside management practices that encouraged the spread of invasive alien species, including removal of indigenous vegetation and planting of invasive alien trees. The South American Schinus molle is a familiar sight at many road side picnic areas. Unfortunately, the seeds from these trees are easily spread by animals, cars, and storm-water culverts into neighbouring farmlands where they quickly find a new “home” [read more about this species]. One practical solution that can be implemented by inhabitants of arid areas is the use of indigenous plants to provide goods and services that are currently provided by invasive alien plants. In South Africa little research has been conducted on the commercial development of indigenous plants for these goods and services.

To access the paper, click HERE.

Arid zone rivers unnaturally stabilised by invasive alien vegetation: (A) Pennisetum clandestinum near Pearston, (B) Arundo donax near Calitzdorp (photos: S.J. Milton)

(A1) Prosopis planted at roadside picnic sites are spread into rangeland (A2) by domestic livestock that eat pods that fall over the fence; (B1) Clearing of indigenous vegetation from roadside make opportunities (B2) for colonisation of the bare ground by invasive alien plants such as Salsola kali and Bromus spp. (photos: S.J. Milton).