GEOG211 ASSIGNMENT COVER PAGE
STUDENT NUMBER:
32211155
SURNAME AND INITIALS:
NXUMALO PJ
ASSIGNMENT TOPIC:
ROLES AND EFFECTIVENESS OF BIOLOGICAL
CONTROL AGENTS IN REDUCING THE IMPACTS OF
INVASIVE ALIEN PLANT SPECIES ON ECOSYSTEMS
IN SOUTH AFRICA.
DUE DATE:
23 APRIL 2020
I hereby declare that this assignment is my own work. I further declare that:
1. The text and reference list reflect the sources I have consulted;
2. Where I have made reproductions of any literary or graphic work(s) from someone
else, I have obtained the necessary prior written approval of the relevant author(s)/
publisher(s)/creator(s) of such works and/or, where applicable, from the Dramatic,
Artistic and Literary Rights Organisation (DALRO);
3. Sections with no source referrals are my own ideas, arguments and/or conclusions;
4. All sources consulted have been properly acknowledged according to the Harvard
referencing style.
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CONTENTS
INTRODUCTION ................................................................................................................. 3
BIOLOGICAL CONTROL AGENTS .................................................................................... 3
ADVANTAGES AND DISADVANTAGES OF BIOLOGICAL CONTROL ....................... 3
INVASIVE ALIEN SPECIES ................................................................................................ 4
THE IMPACTS OF INVASIVE ALIEN SPECIES ............................................................. 5
IMPACTS OF INVASIVE ALIEN SPECIES ON THE ECONOMY ................................ 5
IMPACTS OF INVASIVE ALIEN PLANT SPECIES ON ECOSYSTEM SERVICES .... 5
IMPACTS OF INVASIVE ALIEN PLANT SPECIES ON THE ENVIRONMENT ........... 5
IMPACTS OF INVASIVE ALIEN PLANT SPECIES ON HUMAN HEALTH ................. 6
HOW BIOLOGICAL CONTROL AGENTS REDUCE IMPACTS OF INVASIVE ALIEN
PLANT SPECIES IN SOUTH AFRICA’S ECOSYSTEM ..................................................... 7
THE EFFECTIVENESS OF USING BIOLOGICAL CONTROL AGENTS IN REDUCING
IMPACTS OF INVASIVE ALIEN SPECIES IN SOUTH AFRICA’S ECOSYSTEM ............. 7
HOW TO SUSTAIN INVASIVE ALIEN PLANT SPECIES IN SOUTH AFRICA .................. 8
CONCLUSION ..................................................................................................................... 9
REFERENCE LIST ............................................................................................................ 10
INTRODUCTION
Biological control is a method or system of controlling pests and plants, mostly invasive
species by introducing a natural organism in the environment. Invasive alien plant species,
on the other hand, are species which spread outside their natural habitat threatening
biological diversity, hence they are considered a major threat in the biodiversity. In this
assignment, I will discuss what biological control agents are and what invasive alien species
are, I will furtherly discuss and explain the role and effectiveness of biological control agents
in reducing the impacts of invasive alien plant species on ecosystems in South Africa and
possible measures to help sustain invasive alien species in South Africa.
BIOLOGICAL CONTROL AGENTS
Biological control agents are used to reducing the spread of pests or outside species, the
biological control agent (also known as biocontrol) is the plants, natural enemy. Biocontrol
agents used frequently to control alien plants are mites, insects and pathogens (fungus is
an example). The biocontrol agents target on significant features on plants, like their leaves,
stem and leaves or reproductive features like flowers or seeds and fruits. Choosing a
biological control agent depends upon the purpose of the project, for example, if the purpose
is to eliminate all invasive species in the plant, then the scientist will create a biocontrol
agent that will only target on destroying the alien species in the plant. This can have its
complications at the end like damaging useful parts of the plant. Five species of seeddestroying Melanterius weevil species have been used in South Africa since mid-1980 as
biocontrol agents to eliminate invasive Australian Acacia tree (Hoffmann, Impson, 2019:1).
ADVANTAGES AND DISADVANTAGES OF BIOLOGICAL CONTROL
Some of the listed advantages of biological control in South Africa are that it is
environmentally friendly because it does not pollute the environment, it only focuses on the
targeted invasive alien plant. Also, biological control is self-sustaining thus making it
permanent, it is also productive with a cost. The last advantage of biological control in South
Africa is that it does not create empty holes where other alien plants could invade. It is a
considerable thing that there are more advantages of biological control than disadvantages,
this makes it to be the best lenient solution to lessen invasive alien species. The listed
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disadvantage of biological control is that it does not completely remove all invading plants,
but it allows native plants to recover gradually.
INVASIVE ALIEN SPECIES
An invasive alien species is a growing thread on global sustainability. Invasive alien species
are one of the major drivers of loss of biodiversity, therefore changing the ecosystem
services and socio-economic conditions. Now how do they journey from one place to
another? A simple answer is by human activity, which mostly happens to be unintentional.
Some examples of invasive alien species found in South Africa are Opuntia stricta (it is
native in USA Mexico and Cuba), Acacia saligna (it is native in Australia), Sesbania punicea,
Hukea sericea (native to several parts of Australia), Opuntia humifusa (O. humifusa) and
more. These alien species affect native plants by using up more Carbon dioxide (CO2) and
taking up a large number of nutrients, this, as a result, reduces native plants, as they will die
out from lack of nutrients and CO2.
Figure 1 below shows a map and a table. The map indicates the growth of O. humifusa on
all provinces of South Africa. An increased number of O. humifusa began in 1980 at the
border between Limpopo and Mpumalanga then spread out to all continents (Rule &
Hoffman, 2018:7). The graph (labelled "B") shows how O. humifusa increased from 1980.
According to Rule & Hoffman, there were still lesser reports in 1980 because it was thought
to be under biological control of Cactoblastic cactorum. In between 1988 and 1996 the
population grown of O. humifusa decreased and increased again in 2012.
Figure 1. A map showing an increase of O. humifusa in different provinces and a graph (B)
showing the rate of occurrence of O. humifusa from 1980 to 2012 (Rule & Hoffman, 2018:8).
THE IMPACTS OF INVASIVE ALIEN SPECIES
Impacts of Invasive alien species are found to be mostly negative towards the ecosystem
services and global climate change, they can also reduce diversity and affect the country's
economy, and however, there are some positive impacts of invasive alien species. Some of
the negative impacts of invasive alien species in an ecosystem are that it reduces the
diversity of native plant and animals, promotes remarkable changes in characteristics of soil
and has an enhancement in ecosystems response towards altered fire regimes (Rai &
Singh, 2020:7). Other negative impacts of invasive alien plant species (IAPS) which will be
briefly explained in details below are economic impacts, ecosystem services impacts,
environmental impacts and impacts on human health.
IMPACTS OF INVASIVE ALIEN SPECIES ON THE ECONOMY
In African countries (not specified) an IAPS (Opunitia stricta) was found to have caused an
economic loss of $500-1000 in each household per year through participatory rural appraisal
technique. Also in other African countries, alien invaders resulted in a loss of $1 billion by
causing damage to crops (Rai & Singh, 2020:8).
IMPACTS OF INVASIVE ALIEN PLANT SPECIES ON ECOSYSTEM SERVICES
Most of the IAPS are known for their influence on ecosystem services like recreational,
cultural and regulatory (Rai & Singh, 2020:8). It is known that the IAPS tend to temper with
the navigation of water navigation, they are also known to impact unfavourably the
recreation and tourism services. Many IAPS are also known to impact the regular ecosystem
impacts such as water treatment, pollination, pest management, climate change, etc. Since
the Opunitia stricta invaded in Africa, it has affected the environment, economy and also
affected the livelihood of local people by reducing livestock health.
IMPACTS OF INVASIVE ALIEN PLANT SPECIES ON THE ENVIRONMENT
As mentioned earlier on that it has been demonstrated that IAPS affect the ecosystem
functioning through (i) reduction in the diversity of native plants and animals, (ii) remarkable
changes in Physico-chemical characteristics of soils and (iii) enhancement in ecosystems
response towards altered fire regime (Rai & Singh, 2020:7). The documented impact of IAPS
is to reduce the biodiversity of native plants, which as a result may lead to implications for
the environment's normal functioning, ecosystem services and climate change.
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IMPACTS OF INVASIVE ALIEN PLANT SPECIES ON HUMAN HEALTH
Biodiversity is indeed linked with human health, both positively and negatively. Humans are
dependant on plants for medical remedies, so if an invasive alien specie takes over, it will
deplete the existence of the native plant and results to poor health for humans because even
some medicines are made from plants. Another way which invasive species affect human
health is by environmental contamination. An example is a Long-leaved wattle (an invasive
specie) increases chances of fire which may result to air pollution and loss of native plants.
The table below (figure 2) shows more examples (briefly explained) of invasive alien species
in South Africa and what impacts they have, including biomes they are found in. The first
invasive alien specie listed is Jointed cactus, it has invaded the thicket biome mostly, the
savannah and grassland, it overgrazes and erodes the areas, it injures animals and
damages skin. The second listed invasive alien specie is Red sesbian, it has invaded the
banks of perennial rivers and streams all around South Africa. The negative impacts of the
Red sesbania are that it increases evapotranspiration thus resulting to loss of water
(however a good advantage to this negative impact is that it results to precipitation), it also
causes erosion, and this impacts the ecosystem negatively.
Table 1. A table showing negative impacts of invasive alien plants in South Africa (Wilgen,
de Wit, Anderson, Le Maitre, Kotze, Brown, Ndala & Rapholo, 2004:116).
HOW BIOLOGICAL CONTROL AGENTS REDUCE IMPACTS OF INVASIVE ALIEN
PLANT SPECIES IN SOUTH AFRICA’S ECOSYSTEM
Over the past few years, biological control agents have been used to reduce the population
of invasive alien species. According to Wilgen et all, a research was done to calculate the
increasing rate of invasive alien species without using biological control agents, with the
results it was, later on, discovered that invasive alien species increased greatly without the
use of biological control agents. This means that without the use of biological control agents
invasive alien species are likely to grow in numbers. By using biological control agents, the
land of the ecosystem is likely to recover from the danger of alien species, also natural
vegetation is likely to grow back.
Biological control agents do not completely remove invasive alien species, but it reduces
their growth and allow native species to grow back. An example is Salvinia molesta (an
aquatic weed), it originated from South America. This aquatic weed was firstly documented
in South Africa in the early 1900s, by 1960 it was considered as one of the worst aquatic
weed in South Africa (Martin, Coetzee, Weyl, Parkinson & Hill, 2018:74). According to Martin
et al, Salvinia molesta infestations increased massively between 1995 and 2006 across
Mpumalanga, Limpopo and Western Cape, this was before a biological control agent was
used. The level of Salvinia molesta started to decrease after scientist introduced the
biological control agent C. salvinae. The current status of Salvinia molesta in South Africa
has reduced from 65% to 12%.
THE EFFECTIVENESS OF USING BIOLOGICAL CONTROL AGENTS IN REDUCING
IMPACTS OF INVASIVE ALIEN SPECIES IN SOUTH AFRICA’S ECOSYSTEM
Biological control agents have been used from the 1900s till current date, I can confidently
say that they have been very well effective. However not all the time the results of using
biological control agents will be effective. As it was mentioned earlier on that invasive alien
species can result in economic loss, putting the health of humans at risk and more, biological
control agents can help in stabilising most. Some of the positive results can be increasing
human health because when invasive alien species reduce, native plants grow and this
secures herbs to help heal humanity, and vegetation will not be at risk. Another positive
impact of using biological control is that it is environmentally friendly and can reduce the
growth of invasive alien species which results in polluting the air such as Long-leaved wattle.
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By so doing human health will be well. On the other hand, biological control is also less
costly but invasive alien species costs the country more because important plants are loss,
some even become extinct thus reducing tourists attraction, hence using biocontrol agents
can help the country by spending less and will be able to recover damages.
The table below shows seven invasive alien species, the spread of invasive alien species,
focus of biological control and the effectiveness of using biological control. The jointed
cactus is spread by animals, however with the use of biological control agents the amount
of the alien specie has reduced and controlled efficiently. The Red sesbania produces
several seeds which are spread by water and soil, biological control focused on terminating
the seed feeders to kill the plant and it was a success because all Red sesbania was
completely terminated. Lastly, the Long-weaved wattle also produces hard-coated seeds
which are spread by water and soil, biological control focused on reducing the seed
production which as a result prevented the spread effectively and the overall control
achieved was substantial.
Table 2. A table showing the modes of dispersal, focus and effectiveness biological control
on seven invasive alien species in South Africa (Wilgen, de Wit, Anderson, Le Maitre, Kotze,
Brown, Ndala & Rapholo, 2004:115).
HOW TO SUSTAIN INVASIVE ALIEN PLANT SPECIES IN SOUTH AFRICA
There are fewer possible ways of sustaining IAPS in South Africa in which some are that
there must be a cultivation of multi-purpose trees and shrubs to boost bioenergy and
industrial sectors. Although multi-purpose trees and vegetation benefit human [n which in
South Africa Prosopts sp. (mesquite) was introduced] can affect ecosystem services.
Another way to help sustain the spread of invasive alien species in South Africa is by
ensuring that those who travel do not return with foreign organisms, otherwise thorough
checking must be done and completed, however, this might cost the economy a little bit
more.
CONCLUSION
In the final analysis, invasive alien species will always have an impact on native plants,
humans including the land. But with the help of scientists, biological control can help to
reduce the population of invasive species. With so many new researches that have failed
and less obliging, more scientific research are to be done and more possible and effective
ways to reduce the spread and population of alien species are yet to be done and tested.
Scientists are still looking for more convenient and cheaper ways to reduce invasive alien
species. The effectiveness of biological control on South Africa’s ecosystem, on the other
hand, is indeed helping to secure the ecosystem and keep it in a good way.
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