NEWS
Invasive alien plants and
their impact on wetlands
Current Address: Deputy Director: Planning, Monitoring and Evaluation, Working for Wetlands
E-mail Address: [email protected]
Reprinted From: http://bit.ly/3204wnR
Farai Tererai
I
nvasive alien plants multiply rapidly
once they are established in an area,
which increases clearing costs and com-
pounds adverse effects on the environ-
ment. These species have established
sustained self-propagating populations
and spread considerable distances from
the site of introduction.
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 unintention-
ally entered the country as commodity
contaminants, or as stowaways on trucks,
aeroplanes and ships.
How alien plants spread
Once these species arrive, some natu-
ralise to the local conditions, including
climate factors. They accomplish this
through superior competitive capabili-
ties, which include fast growth, high re-
productive output and the ability to adapt
to a wide range of physical environments.
In many cases these invasive alien plants
establish dominance and form mono-
cultures or homogeneous populations.
These species spread rapidly in rivers or
streams and their margins, collectively
known as riparian zones, as well as wet-
lands, because these systems are inher-
ently highly dynamic and connected.
Riparian zones and wetlands, which usu-
ally cover only a small part of the land-
scape, experience much disturbance due
to the natural processes of flooding, ero-
sion, 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 dis-
persing alien plants. Wetlands are known
for trapping material transported by riv-
ers, 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 in-
cludes wetlands. The Southern African
Plant Invaders Atlas database indicates
the most common invaders of rivers and
their fringes. These plants include:
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Weeping willow (Salix babylonica L).
White poplar (Populus alba).
Grey poplar (Populus × canescens).
Black wattle (Acacia mearnsii), silver
wattle (Acacia dealbata), Port Jack-
son wattle (Acacia saligna).
Honey mesquite (Prosopis glandu-
losa var. torreyana/velutina).
Bugweed (Solanum mauritianum).
Red sesbania (Sesbania punicea).
Common lantana (Lantana camara).
Siam weed (Chromolaena odorata).
River red gum (Eucalyptus camaldu-
lensis).
Common invasive in-stream species in-
clude giant reed (Arundo donax L.), water
fern (Azolla filiculoides), and the notori-
ous 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 Biodiver-
sity Assessment, 2018)
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 spe-
cies. 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 spe-
cies.
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, groundwa-
ter supplies, rangeland productivity and human livelihoods.
Species
* Applies to species that have been placed into various categories depending on their location.
Grassroots
Vol 20
No 1
March 2020
22