Wetlands provide water for animals during dry seasons. (Photo: Farai Tererai)

Invasive alien plants multiply rapidly once they are established in an area, which increases clearing costs and compounds adverse effects on the environment. These species have established sustained self-propagating populations and spread considerable distances from the site of introduction.

Invasive plants and soil health

Invasive alien plants are introduced either intentionally or unintentionally. Plants that were introduced intentionally were brought into the country for horticulture, aquaculture, agriculture and forestry. Plants that were introduced unintentionally entered the country as commodity contaminants, or as stowaways on trucks, aeroplanes and ships.

How alien plants spread

Once these species arrive, some naturalise to the local conditions, including climate factors. They accomplish this through superior competitive capabilities, which include fast growth, high reproductive output and the ability to adapt to a wide range of physical environments. In many cases these invasive alien plants establish dominance and form monocultures or homogeneous populations.

These species spread rapidly in rivers or streams and their margins, collectively known as riparian zones, as well as wetlands, because these systems are inherently highly dynamic and connected.

Riparian zones and wetlands, which usually cover only a small part of the landscape, experience much disturbance due to the natural processes of flooding, erosion, deposition, river damming, and land use next to rivers. Consequently, they are highly susceptible to invasion.

Rivers serve as conduits for substantial fluxes of materials and energy, thus dispersing alien plants. Wetlands are known for trapping material transported by rivers, which is why rivers and wetlands are the most heavily invaded ecosystems. This phenomenon causes substantial changes to ecosystem structure and function.

Common invaders of wetlands

Approximately 26 alien plant species are listed as invasive in the inland aquatic ecosystems of South Africa, which includes wetlands.

The Southern African Plant Invaders Atlas database indicates the most common invaders of rivers and their fringes. These plants include:

  • Weeping willow (Salix babylonica L).
  • White poplar (Populus alba).
  • Grey poplar (Populus × canescens).
  • Black wattle (Acacia mearnsii), silver wattle (Acacia dealbata), Port Jackson wattle (Acacia saligna).
  • Honey mesquite (Prosopis glandulosa var. torreyana/velutina).
  • Bugweed (Solanum mauritianum).
  • Red sesbania (Sesbania punicea).
  • Common lantana (Lantana camara).
  • Siam weed (Chromolaena odorata).
  • River red gum (Eucalyptus camaldulensis).

Common invasive in-stream species include giant reed (Arundo donax L.), water fern (Azolla filiculoides), and the notorious water hyacinth (Eichhornia crassipes).

The top five wetland invaders are water hyacinth, water fern, spear thistle (Cirsium vulgare), honey mesquite and purpletop vervain (Verbena bonariensis L.).

Table 1: Invasive species known to have adverse effects on South Africa’s inland aquatic biome. (Source: National Biodiversity Assessment, 2018)

Species Regulatory category Extent (Quarter-degree grid cells occupied) Examples of impact
Silver wattle (Acacia dealbata) 2 240 Forms closed-canopy stands, excluding most other species in riparian zones. Uses excessive amounts of water.
Long-leaved wattle (Acacia longifolia) 1b 53 Forms closed-canopy stands, excluding most other species. Uses excessive amounts of water.
Black wattle (Acacia mearnsii) and its hybrids 2 369 Forms closed-canopy stands, excluding most other species in riparian zones. Uses excessive amounts of water.
Port Jackson willow (Acacia saligna) 1b 126 Forms closed-canopy stands, excluding most other species.
Creeping bentgrass (Agrostis stolonifera) Context specific* Offshore islands Forms extensive clonal patches by means of long stolons, which affects indigenous plant species on offshore islands.
Velvet mesquite (Prosopis velutina) Context specific* 5 Many well-documented impacts on biodiversity, groundwater supplies, rangeland productivity and human livelihoods.

* Applies to species that have been placed into various categories depending on their location.

Loss of biodiversity

Invasions by alien plants have been shown to cause a displacement of native indigenous plant species – in some cases replacing them with less usable single-species stands (monocultures), such as river red gum and black wattle.

Highly diverse ecosystems often have high grazing value compared to single-species stands. A typical example of an invasion problem in grazing is that of dense, widespread famine weed (Parthenium hysterophorus). Invasions are common in KwaZulu-Natal and Mpumalanga.

Single-species stands have replaced indigenous species in this riparian zone.

Famine weed is poisonous to animals and humans and invades crops and pastures. It owes its common name to the fact that farmers must often abandon their land when it invades crops.

A secondary effect of famine weed invasion is high farm costs. Loss of biodiversity often results in multiple-use ecosystems and habitats being replaced by those that have a single use. Areas that were used for indigenous medicinal use and grazing may now be used for timber, and diverse wetland ecosystems might be replaced by a stand of river red gums, as is the case in the Berg River area. Some invasive alien plants have been associated with pollination disruption in crops, which reduces crop yield.

The unique and diverse habitats of many flora and fauna are also lost to the detriment of farms that depend on ecotourism. For example, indigenous bird species are less diverse in stands of Eucalyptus species. Some invasive species alter the physical and chemical characteristics of soil and some of them, such as Eucalyptus species, are known for depositing chemicals that are harmful to other species, which not only restricts land use but also limits unassisted restoration after removal of the main invader.

Threat to water security

Invasive alien plants are known for excessive water consumption due to their high transpiration rates. This poses a significant threat to water security, particularly in water-scarce areas. The impacts are devastating during drought, and this is a major threat to irrigated agriculture and animal watering.

These photos show riverbank erosion due to falling invasive trees in the riparian zone (a) and consequent channel blockage by debris (b), which causes further erosion, in the Berg River in the Western Cape.

Wetlands are known to buffer these effects because of their ability to store floodwater and release it slowly after rain has fallen, but invasive plants threaten this regulating service. The nutrient and chemical contaminants that are generated on farms need to be flushed regularly, but invasive alien plants’ excessive water consumption also threatens this function and the availability of clean water on farms for various uses. Honey mesquite, which is known to invade the Karoo and arid savanna, reduces native species’ diversity, depletes groundwater and reduces rangeland quality.

Changes in fire regimes

Fires are a natural phenomenon that farmers use on their land to stimulate grazing, especially in the grassland and savanna biomes. However, invasive alien plant species generally produce much more plant material that is highly flammable, such as the leaf and twig litter of pines and wattles, which substantially increase the frequency, magnitude and intensity of fires. The result of such fires is a significant loss of farm property and life.

The vicious Knysna and George fires in 2017 and 2018 were compounded by the dense pine and hakea invasions in the mountains. Such fires result in indigenous species not coping with the new fire regimes and other species completely burning out with their soil seed stores. New fire regimes favour the propagation of species such as black wattle over that of indigenous species.

Invasions cause wetlands and peatland, a type of wetland with high carbon content, to dry up. When a fire breaks out in peatland, it burns carbon stored in these wetlands, and these fires can last a long time. Many peatlands in South Africa, along with their associated benefits, have disappeared because of peat fires.

The role of landowners

Seeing that invasions in wetlands have potentially significant impacts locally, landowners or users have a role to play in the management of these invasions.

If each landowner manages invasive alien plants on his or her land, the landscape could be free of these species and their harmful effects. Landowners can participate in the management of invasive alien plants by avoiding certain pathways of introduction of these species, clearing and following up, and whistleblowing.

The responsibility to maintain land that is free of invasive alien plants primarily lies with landowners, as stated in the Conservation of Agricultural Resources Act, 1983 (Act 43 of 1983). However, it is acknowledged that the magnitude of invasions may be beyond the capacity of individual landowners and the sources of these invasions may be beyond landowners’ boundaries, which warrants support from the government.

The Department of Environment, Forestry and Fisheries (DEFF) has been clearing invasive alien plants through the Working for Water programme since 1995. The department has cleared vast tracks of land in various land tenure systems, but some of the biggest challenges relate to inadequate follow-up and landowner co-operation. – Farai Tererai, deputy director: Planning, Monitoring and Evaluation, Working for Wetlands

For more information, send an email to the author at ftererai@environment.gov.za.