NEWS
Loss of biodiversity
Invasions by alien plants have been
shown to cause a displacement of na-
tive 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 (Par-
thenium hysterophorus). Invasions are
common in KwaZulu-Natal and Mpuma-
langa.
Famine weed is poisonous to animals
and humans and invades crops and pas-
tures. It owes its common name to the
fact that farmers must often abandon
their land when it invades crops.
Figure 1: Wetlands provide water for animals during dry seasons. (Photo: Farai
Tererai)
A secondary effect of famine weed in-
vasion is high farm costs. Loss of biodi-
versity often results in multiple-use eco-
systems 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 tim-
ber, and diverse wetland ecosystems
might be replaced by a stand of river
red gums, as is the case in the Berg Riv-
er area. Some invasive alien plants have
been associated with pollination disrup-
tion 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 eco-
tourism. For example, indigenous bird
species are less diverse in stands of Eu-
calyptus species. Some invasive species
alter the physical and chemical charac-
teristics 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 resto-
ration after removal of the main invader.
Threat to water security
Invasive alien plants are known for ex-
cessive water consumption due to their
high transpiration rates. This poses a
significant threat to water security, par-
ticularly in water-scarce areas. The im-
pacts are devastating during drought,
and this is a major threat to irrigated
agriculture and animal watering.
Wetlands are known to buffer these ef-
fects 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 gener-
ated on farms need to be flushed regu-
larly, but invasive alien plants’ excessive
water consumption also threatens this
23
Figure 2: Single-species stands have replaced indigenous species in this riparian
zone.
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 groundwa-
ter 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 al-
ien plant species generally produce
much more plant material that is highly
flammable, such as the leaf and twig lit-
ter of pines and wattles, which substan-
tially 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 in-
digenous 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,
Grassroots
Vol 20
No 1
March 2020