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dispersion), and the potential areas that invasive species would
be likely to occupy if no action was taken to control them. It
will also be necessary to develop a better understanding of the
effectiveness of clearing operations. Finally, the role that biologi-
cal control plays in changing the population dynamics of
invasive alien plants, and contributing to the long-term mainte-
nance of cleared areas, needs to be better understood.
The choice of appropriate courses of action regarding the
clearing of invasive alien plant infestations can be assisted by the
development of decision-support models based on the above
understanding. For example, managers need to decide whether
available labour and funds should be directed towards clearing
light or dense infestations where these occur together, as these
choicescouldhavedifferentoutcomesbecauseofthedifferences
in costs and in the rate of spread of different species.10 Such
models have been developed11,12 but have not yet been used in
practice. The development of the project information system
reported on here will provide an opportunity to develop these
models further as well as to apply them for the first time in
practice.
We thank the Working for Water Programme for funding, Kevin Meyer and Aukje
Coleman for data extraction, and Kasey Voges for data on the costs of herbicides.
1. Van Wilgen B.W., Marais C., Magadlela D., Jezele N. and Stevens D. (2002).
Win-Win-Win:South Africa’s Working for Water programme. In Mainstreaming
Biodiversity in Development: Case studies from South Africa, eds S.M. Pierce, R.M.
Cowling, T. Sandwith and K. MacKinnon, pp. 5–20. The World Bank,
Washington, D.C.
2. Anon. (2002). The Working for Water Annual Reports for the years 1996/97 to
2001/02. Department of Water Affairs and Forestry, Cape Town.
3. Versfeld D.B., Le Maitre D.C. and Chapman R.A. (1998). Alien invading plants
and water resources in South Africa: a preliminary assessment. WRC Report
No. TT 99/98. Water Research Commission, Pretoria.
4. Zimmermann H.G., Moran V.C. and Hoffmann J.H.(2004). Biological control in
the management of invasive alien plants in South Africa, and the role of the
Working for Water Programme. S. Afr. J. Sci. 100, 34–40.
5. Nel J.L., Richardson D.M., Rouget M., Mgidi T., Mdzeke N.P., Le Maitre D.C.,
van Wilgen B.W., Schonegevel L., Henderson L. and Neser S. (2004). A
proposed classification of invasive alien plant species in South Africa: towards
prioritising species and areas for management action. S. Afr. J. Sci. 100, 53–64.
6. Harper J.L. (1977). Population Biology of Plants. Academic Press, London.
7. Mack R.N. (1985). Invading plants: their potential contribution to population
biology. In Studies on Plant Demography: A festschrift for John L. Harper, ed. J.
White, pp. 127–142. Academic Press, London.
8. Birks H.J.B. (1989). Holocene isochrone maps and patterns of tree-spreading in
the British Isles. J. Biogeog. 16, 503–540.
9. Holmes P.M. and Richardson D.M. (1999). Protocols for restoration based on
recruitment dynamics, community structure, and ecosystem function:
perspectives from South African fynbos. Restoration Ecol. 7, 215–230.
10. Van Wilgen B.W., Richardson D.M. and Higgins S (2000). Integrated control of
alien plants in terrestrial ecosystems. In Best Management Practices for Preventing
and Controlling Invasive Alien Species, eds G. Preston, G. Brown and E. van Wyk,
pp. 118–128. Working for Water Programme, Cape Town.
11. Higgins S.I., Richardson D.M. and Cowling R.M. (2000). Using a dynamic
landscape model for planning the management of alien plant invasions. Ecol.
Appl. 10, 1833–1848.
12. Higgins S.I., Richardson D.M., Cowling R.M. and Trinder-Smith T.H. (1999).
Predicting the landscape-scale distribution of alien plants and their threat to
plant diversity. Conserv. Biol. 13, 303 – 313.
Working for Water South African Journal of Science 100, January/February 2004 103
Alien plant invasions in South Africa: driving
forces and the human dimension
David C. Le Maitrea, David M. Richardson and R. Arthur Chapman
Introduction
Invasivealienplants,hereaftercalledinvaders,arewidelycon-
sidered as important a threat to biodiversity as direct human
transformation of the natural environments and production of
greenhouse gases.1,2 Invaders also threaten ecosystem services,
including water purification, soil generation, waste decomposi-
tion and nutrient cycling, which are critical to human survival. A
recent overview for seven different countries estimates the
globalcostsofcontrolprogrammesplusthetotalcostsofdamage
caused by invaders to be of the order of US$314 billion per year.3
Invaders cost South Africans tens of billions of rand annually in
lost agricultural productivity and resources spent on weed
control.4An assessment of the economic impact of black wattle
(Acacia mearnsii) gave a net present cost of $1.4 billion (R9.8
billion)5. The costs associated with invasion by black wattles are
at least partly offset by the substantial social and economic
Invasive alien plants pose a substantial threat to the rich bio-
diversity of South Africa, and to the sustained delivery of a wide
range of ecosystem services. Biological invasions are driven by
human activities and mediated by culturally shaped values and
ethics. This paper explores the human dimensions of alien plant
invasions in South Africa. We consider four primary forces, those
which directly influence the likelihood and rate of invasion — arrival
of propagules; changes in disturbance regimes; changes in the
availability of limiting factors; and fragmentation of the land-
scape — and the roles of 22 secondary driving forces in shaping
the outcomes of the four primary driving forces. Human societies
and their dynamics and activities are an integral part of each of the
secondary driving forces. A map of the interactions between and
among the primary and secondary driving forces shows how they
are interlinked and influence each other — either positively or nega-
tively, or switching between the two. There are two key points for
intervention: prevention of the introduction of propagules of poten-
tially invasive species and developing collaborative initiatives with
enterprises that rely largely on alien species (for example, horticul-
ture, agriculture and forestry, including community forestry) to
minimize the introduction and use of potentially invasive species.
An example of the first type of intervention would be to implement
more effective inspection systems at international border and
customs posts. This type of intervention can only be effective if
those who are directly affected — whether businessmen, tourists or
migrants — understand the requirement for these measures, and
collaborate. The need to build public awareness of the critical
importance of the human dimension of invasions emerges as a
key theme from this analysis and is the basis for better-informed
decisions, more effective control programmes and a reduction of
further invasions.
aCSIR Water, Environment and Forestry Technology, P.O. Box 320, Stellenbosch 7599,
South Africa.
bInstitute for Plant Conservation, Botany Department, University of Cape Town, Private
Bag, Rondebosch 7701, South Africa.
*Author for correspondence. E-mail: dlmaitre@csir.co.za
104 South African Journal of Science 100, January/February 2004 Working for Water
benefits derived from the wattle industry. Many other invading
species do not have any commercial value or use to offset their
costs to society and their impacts on the environment. South
Africa is facing critical water shortages, both regionally and
nationally,6,7 and cannot afford the loss of 7% of its renewable
surface water resources8to invading woody plants.
The most obvious and direct solution to a dilemma of this
magnitude is to implement nationwide control operations, like
the Working for Water programme, but these actions address
only one aspect of this multi-faceted and complex problem.
Other actions that are needed include: preventing invaders
entering the country or escaping from cultivation, early detec-
tion before the invader becomes a major problem, flexible
responses to events that trigger invasions, and rehabilitation
of the cleared areas.9Before we can tackle any of these actions,
we need to face up to the simple fact that human actions and
activities play critical roles in facilitating invasions. We will not
find effective solutions until we identify inappropriate actions
and behaviours, and where and how to intervene to change
them. This paper begins by reviewing invasion processes and
describing the primary driving forces of invasion — those pro-
cesses which have a direct influence on the rate and likelihood of
invasion. Then we describe the secondary driving forces for
invasion and how they influence the primary driving forces. We
show that the primary driving forces are closely related to envi-
ronmental conditions and perturbations, whereas the second-
ary driving forces are largely human-mediated vectors and
pathways. We also show that the secondary forces can interact
with the primary forces either to promote or to retard invasion.
This gives us scope to identify and alter aspects of human behav-
iour and activities that facilitate alien plant invasions in South
Africa.
Processes of invasions: understanding the driving forces
Understanding of the biology and ecology of invasive organ-
isms has improved substantially in the half century since
Charles Elton’s12 seminal synthesis launched the field of inva-
sion ecology. The last two decades have been especially produc-
tive in this regard. Most studies have concentrated on under-
standing what attributes distinguish successful from unsuccess-
ful invaders, what features predispose ecosystems to invasions,
how these aspects are linked, and on deriving management
strategies.13–16 There have been important advances in the scien-
tific understanding of the process of invasions, particularly for
plants, and a reasonably robust set of generalizations has
emerged.17–20 In considering aspects of invasions, it is useful to
conceptualize processes that limit or facilitate invasions as an
invader negotiating a series of barriers.17,21 Much work has been
done since the 1980s to develop a better understanding of
the determinants of these barriers and how invaders succeed
in crossing three main categories of barriers: (a) geographic
barriers, which propagules of a species (seeds, spores, cuttings or
other parts able to develop into a new organism) must overcome
to arrive in the new environment; (b) habitat barriers (general
climatic and edaphic conditions and the prevailing disturbance
regime); and (c) biotic barriers (resident biota that occupy space
and compete for resources). Human activities influence the
potency of these barriers in a number of key ways,22 thus mediat-
ing what we have defined as the four primary driving forces
for invasions, namely: arrival of new propagules, distur-
bance regimes, changes in limiting factors, and fragmentation of
the landscape by human activities. Each of these is described
below.
Primary driving forces
Arrival of new propagules
Humans have moved organisms around since prehistoric
times by transporting crops and associated weeds from the
Middle East across Europe and Asia, but relatively few species
were involved.23,24 Large-scale and long-distance movement of
thousands of plant species began with the establishment of
European colonies across the world and has continued ever
since;23,25–28 a process that has been called the ‘MacDonaldization’
of the world.29 A key motivating factor has been the need to
ensure reliable and sustainable supplies of food, fuel, forage and
medicines, but many species have also been moved for reasons
of fashion and novelty.30,31
Disturbance regimes
Very few environments are unaffected by disturbances such
as fires, droughts or floods. Disturbances typically release
resources and reduce the domination of the existing communi-
ties of plants. For example, fires and floods create areas of open
soil for plants to colonize and change nutrient and water
availability.32 Plants in disturbance-prone environments are
adapted to particular combinations of the frequency, seasonality
and intensity of key disturbances.33 Land transformation often
leads, directly or indirectly, to changes in disturbance regimes.
For example, it is more difficult for fires to spread through areas
where the transformed patches are not fire-prone.
Human activities often change disturbance regimes to such an
extent that they expose native species to novel conditions.
Where such modified regimes are beyond the tolerance of some
native species, such species decline or become extinct, and
certain alien species can capitalize on the available resources. For
example, fire suppression in grasslands can favour the incursion
of woody species or herbs, while increased fire frequency and
intensity can rapidly eliminate woody elements.34
Changes in limiting factors
Modern industrial and agricultural practices alter biogeo-
chemical cycles by, for example, increasing atmospheric CO2and
nutrient availability.35 Most of South Africa’s indigenous grasses
have the C4photosynthetic pathways (one that is most efficient
in areas with cool, temperate climates and high altitudes),
whereas most of the introduced grasses have the C4type (more
efficient in warm tropical climates).80 Increases in atmospheric
CO2(amongst other changes) enhance the growth of species
with the C3photosynthetic pathway more than species with
other photosynthetic pathways, particularly the C4grass
species.80 This may favour the invasion of predominantly C4
grasslandsbyC3grassesandwoodyspecies.36 Heavyuseoffertil-
izers and atmospheric pollution leads to increased nutrient
deposition in natural ecosystems.35 Invaders often benefit more
from the increased nutrient levels than native species, particu-
larly in nutrient-poor environments.1,37–40 Some invaders also
alter natural ecosystem processes in ways that facilitate inva-
sions by themselves and/or by other species. For example, the
nitrogen-fixing shrub Myrica faya invades grasslands on Hawaii,
which lack native nitrogen fixers.41 Nitrogen enrichment follow-
ing entry of this species facilitates massive invasion of nitro-
philous grasses.42
Fragmentation of the landscape
Fragmentation arises when patches of land are transformed.
Transformed areas are rarely contiguous, typically resulting in a
multi-scale mosaic of patches of natural ecosystems with high
perimeter-to-area ratios set in a matrix of transformed land.43
The perimeter of the patches forms a transition zone, or ecotone,
between the matrix and the patch and is often highly vulnerable
to invasions.44–46 The disturbance regimes in the patches are typi-
cally different from those in the unfragmented ecosystem,
increasing their vulnerability to invasion32,47, as discussed earlier.
As patches get smaller they become increasingly exposed to
external influences. For example, small patches will receive a
greater load of fertilizers from adjacent farmlands than larger
patches and this can alter limiting factors (see above).
Interactions
The four driving forces are inextricably linked and can operate
individually or together to facilitate or to hinder invasions. Land
transformation can affect a number of the driving forces simulta-
neously or differently over time. For example, there may be a
delay between the establishment of a plantation and invasion of
adjacent natural ecosystems by invaders inadvertently intro-
duced during establishment and management of the plantation.
Or land transformation from a natural to an industrial area can
result in increased atmospheric deposition of a range of pollut-
ants on the adjacent natural areas, potentially altering the limit-
ingfactorsinthenaturalecosystems.Humanactivitiesindirectly
affect all four primary driving forces and thus we call these activ-
itiessecondary driving forces. The following section explores the
secondary driving forces and how they interact with each other
and with the primary driving forces.
Secondary driving forces
We have identified 22 ‘secondary driving forces’, which are
those factors that influence the primary driving forces. We have
grouped them according to whether they operate predomi-
nantly at the international or at the national scale. This distinc-
tion is important because those operating at the national scale
can be dealt with by national bodies, whereas those operating
at the international scale require international alliances and
partnerships, which are inherently more difficult to direct. The
secondary driving forces are explained below.
International secondary driving forces
•Human population growth and migration
Population increases lead to greater demands for food and
materials. These demands are met by increasing productivity
through fertilization, adoption of new species, and by increasing
the area under production.1,35,37,48 Where demands cannot be
met, people tend to migrate, taking propagules of their custom-
ary food and medicinal plant species with them31,49 and inadver-
tently carrying invasive species.
•Expanding network of international trade and travel links
The extent of international links, and therefore the number of
potential invasion pathways, increased dramatically during the
last century. Many of the significant invasions that originated
through trade have been of marine organisms which are trans-
portedin ship ballast water,50but seedsof many plant species can
be moved, for example, in containers, transport vehicles, soil or
contaminated seed lots.26,27 The desire of tourists for souvenirs
hasalso increased the movement of undesirable species, particu-
larly during the second half of the 20th century.27 Since 1994,
South Africa’s importance as a hub for international trade and
commerce with the rest of Africa has increased considerably.51
Growth in inter-African trade and the associated infrastructure
is a key element of the New Partnership for Africa’s Develop-
ment (NEPAD) initiative of the African Union.52 If these develop-
ments come about, they will increase the quantity and variety of
goods being traded and thus the risk of propagule movement.
•Increasing magnitude of international trade
As the volume of trade grows, the number of alien plants or
propagules arriving in a country increases53. The enhanced
propagule pressure increases the probability of the successful
establishment of an alien species in the country to which it has
been introduced47. It also increases the likelihood of sampling a
greater genetic diversity of the new species and thus the poten-
tial fitness of an alien for its new environment.
•Globalization of economies
International trade is expanding and linking economies
across the world ever more directly, a process known as global-
ization. Historically, most international trade was in products or
commodities, but the past few decades have seen significant
growth in non-commodity items and services such as direct
monetary exchanges. An example of this is the financial trading
mechanisms proposed under the Kyoto Protocol. These allow
CO2-generating industries to offset or reduce the proposed
carbon taxes by establishing carbon banking or storage indus-
triesin other countries.54 This could encourage developing coun-
tries to establish plantations of fast-growing tree species,55 many
of which are highly invasive.56,57
•Global economic trends
Recent events have demonstrated the vulnerability of the
South African economy to turmoil in international financial
markets, especially those of other ‘emerging economies’.58 South
Africa has also opened, to some extent, its markets to trade and
monetary flows, which has resulted in major fluctuations in
monetary exchange rates. For example, changes in the rand–
dollar trading rates have greatly increased the amount of
money that South Africa has to spend on repaying international
debts and for its arms procurement programme, reducing
expenditure on other items, including invasive plant control
programmes. The volatile exchange rates are also having a
significant influence on sectors such as mining, agriculture and
forestry. Farmers are being forced to experiment with new crops
or abandon formerly cultivated land.
•Globalization of the forestry/agro-forestry enterprise
Commercial plantation forestry is a significant source of
alien invasion. First, the tree species themselves are often
highly invasive.57 Second, forestry operations have often facili-
tated invasions through poor weed control in the nurseries,59
and by transporting invader seeds on forestry machinery. Third,
the growing demand for forestry products is leading to an
increase in the area under plantations60 and, therefore, in propa-
gule sources. Agricultural and forestry enterprises are finding
and testing new plant species and varieties with commercial
value or potential. The exchange and trade in plant materials
often involves species known to be invasive elsewhere in the
world but this is ignored because of the perceived benefits of the
‘miracle’ plants.27,31,56 Two lists of species recommended by South
Africa’s national forestry authority for fuel wood and agroforest-
ry include taxa known to spread naturally61 or to be invasive.62
Forestry need not facilitate invasions only. Environmental
certification has been implemented by many forestry companies
andrequires them to limit the distribution of invader propagules
and support control programmes for invasive plants.63 The
timber trade in many developed countries, for example the
Working for Water South African Journal of Science 100, January/February 2004 105
United States and countries of the European Union, will not
import products lacking this certification. A certified product
will specify how that item was grown, and whether or not the
product is invasive, where it was produced and what control
measures were adopted.64
•Improved communication methods (Internet, global
databases)
Modern communication methods such as the Internet may
speed the flow of information on the useful characteristics of a
plant, prompting horticulturalists and other users to import
these species without first considering their invasiveness. Many
websites already contain information on plants with agricul-
tural, horticultural and herbal value, some of which are run by
commercial companies.65 There are also several organizations
which specialize in studying and researching the commercial
potential of agricultural crops and tree species — such as the
Central America and Mexico Coniferous Resources Cooperative
(CAMCORE),abodytowhichsomecommercialforestrycompa-
nies in South Africa belong66 — or the Consultative Group on
International Agricultural Research (CGIAR), which includes 16
research centres worldwide, many of which specialize in agro-
forestry systems.67
This trend may be countered, however, by the growing num-
ber of global databases on problem plants and their control.68,69
These databases are also used by organizations responsible for
quarantining and screening imports, for example in Australia
and New Zealand.56,70 Several countries maintain websites
which list their major invaders with information on how to iden-
tify and control them.69,71,72 International e-mail groups (such as
the Aliens-L list server maintained by the Invasive Species
Specialist Group of the IUCN in Switzerland) share information
on invasive plants.
•Growth and maturation of invasion ecology into a robust,
predictive science
Although the SCOPE programme on invasive species led to
substantial and significant advances in invasion ecology at
national and international levels13, it is still a growing science.
The recently launched Global Invasive Species Programme73
was motivated by new information on the enormous economic
costs of alien invasions and their increasing pervasiveness27,74.
Increases in our understanding of the processes of invasions and
their impacts should lead to improved polices and laws, strate-
gies for control, and methods for screening plants for invasive
potential.
•Global climate change
Global climate change is highly likely to lead to more frequent
and intense extreme events, such as droughts and floods, which
will create severe disturbances in the affected areas.75 Climate
change can, therefore, have a direct effect on disturbance regimes
— one of the primary driving forces of invasions. For example,
floods disturb river banks and re-route water courses, providing
recruitment sites for colonizing plants with water-borne propa-
gulessuchasblackwattle(Acacia mearnsii) and other invaders.76–78
The climates of areas may also change in ways that favour
invasions. For example, humid areas tend to be more susceptible
to invasions than drier ones and the same is true of wetter
habitats in dry regions.78,79 An increase in long-term rainfall in a
region therefore is likely to result in an increase in the level of
invasion. Similarly, regional warming may also lead to condi-
tions for invaders becoming more favourable at higher altitudes.
This could be the case for woody invaders on the escarpment
and in the headwater catchments of both sides of the Drakens-
berg.80
Increases in atmospheric CO2are likely to have major impacts
on invasions via several mechanisms. Enhanced CO2will alter
the competitive balance between plants with the C3and C4
photosynthetic pathways. The changed dynamics could favour
invasions by C3grasses of our natural grasslands, which have so
far proved to be remarkably resistant to such invasions.80
Elevated CO2levels could also favour tree invasions in grass-
lands;81 there is evidence that this is already happening with
Prosopis in North America.82 This could lead to bush encroach-
ment by indigenous species and promote invasions by alien
species; both outcomes will reduce the productivity of natural
pastures, alter the water balance by increasing evapotranspira-
tion, and change fire regimes.
•International treaties
Some international treaties and conventions, such as the Con-
vention on Biological Diversity, which South Africa has ratified,
address aspects of the problems caused by invasive alien
species.83 Most governments that have ratified these conven-
tions lack the political will and resources to implement them
effectively. South Africa is also a signatory to the World Trade
Organization trade protocols and other trade agreements which
encourage the free movement of goods, including invasive and
potentially invasive organisms.84,85 In some cases these trade
agreements even prevent countries from controlling trade using
the precautionary principle on issues such as the prevention of
alien invasions.
National secondary driving forces
•Human population dynamics
Urbanization in South Africa increased rapidly after 1986,
when laws controlling the movement of black people were
abolished.86 This is reducing the population in rural areas, partic-
ularly the economically active segment, leaving the children and
elderly behind. The depopulation of these rural areas is decreas-
ing the local demand for wood resources. HIV/AIDS infection
rates, which are reaching 20–30% in some communities, are
further reducing population growth rates.6,86–88 People aged
between 20 and 40 will be worst affected, leading to marked
changes in livelihood strategies.89 Rural depopulation, diversion
of resources to caring for the sick and the loss of adult labour
could all lead to abandonment of land and lower rates of
wood-cutting, facilitating invasions.
•Economic trends
South Africa’s economic performance is controlled primarily
by the performance of the global economy (see Global economic
trends above) and by the national economy’s efficiency and
stability and the level of good governance in the country.90 A
collapse in the gold price, for example, may lead to large-scale
unemployment and therefore to increased disturbance and
fragmentation of the land as people return to the rural areas to
subsist.
A country with a strong economy has the resources to invest in
infrastructure (see Expanding infrastructure below) and orderly
and effective land reform (see Post-apartheid transformation
below). It also can invest in research on methods of managing
invasive species, for example through biological control. A weak
economy restricts options and the ability to implement policies
or enforce regulations. This driving force can, therefore, have
either a positive or negative effect on invasions.
106 South African Journal of Science 100, January/February 2004 Working for Water
•Expanding infrastructure
The development of new infrastructure such as road and rail
networksprovides a vector for the transport of propagules along
new routes.12,91,92 Seeds carried by earth-moving and construc-
tion equipment, and in construction materials, also are a signifi-
cant source of invasions. In addition, road and rail networks
promote the movement of propagules in vehicles operating
along these routes. Inter-basin transfers of water are a mecha-
nism for the transport of invasive aquatic organisms.83,93
•Post-apartheid transformation
One of the stated aims of the post-1994 government is to con-
duct land reforms, in which there is a substantial movement of
dispossessed people back onto the land and an increase in the
number of individual black farmers.94–96 This process could range
from orderly to chaotic, depending on the manner in which the
government conducts its operations and the resources avail-
able97 (see Economic trends above). If the process is orderly, then
there is a stronger likelihood of controlling invaders than if the
process is chaotic.
•Changes in laws, policies and regulations
The government has recently promulgated new regulations
for controlling invading plants98 and there are initiatives to
ensure that the relevant provisions of the Convention on Biolog-
ical Diversity (see International treaties above) are realized.99 A
project to develop a National Biodiversity Strategy was
launched during 2003. This initiative will address invasive
species and feed into the planning for the National Weeds Strat-
egy. Experience in Australia100 shows that such strategies could
lead to more coherence through integrated and effective poli-
cies, laws and regulations for all aspects of alien invaders.
•Changing agricultural practices
Increasing fragmentation of the landscape is likely as state
land is made available to small farmers96 and existing farms are
subdivided to accommodate more people. This will change the
regional mosaic, as there are shifts between subsistence and
commercial agriculture. Subdivision is much more likely where
the land is relatively fertile and there are good water resources.
This will enable those settled on the land to make a living by
farming cash crops without requiring large amounts of capital
investment. Intensive use of the land in this way is likely to lead
to a reduction in aliens.
The development of woodlots will encourage invasions.
Where fertilizer use increases, higher nutrient loads will encour-
age invasion. While more people may be settled on the land,
some traditional practises may not change — the storage of
wealth in livestock may continue, but within a smaller area —
leading to increased soil disturbance and hence scope for inva-
sion.101 Livestock exchange will continue to be a vector of seed
movement. Land care policies and programmes are being devel-
oped and encouraged by the national and provincial depart-
ments of agriculture.94,101 This will have a direct effect on the
status of invasions as people realize the negative consequences
of aliens and control them.
There is now a strong emphasis by biotechnology companies
on the development of genetically modified organisms. One of
the key concerns is the insertion into crops of genes for the pro-
duction of insect and pest toxins and herbicide resistance.102
There are conflicting claims about the biosafety of these organ-
isms, and there are many concerns which have not yet been
resolved.
•Options available for alien invader control
Alien plants can be controlled by mechanical clearing, use of
herbicides and through biological control, or combinations of
these. Biological control is one of the most cost-effective means
of restricting invasive alien plants over 5–10 or more years.103,104
Once the necessary agent has been released onto its target
plants, little further investment is required. However, thorough
research needs to be carried out before the agents can be
released, delaying their deployment for several years.
Screening and quarantine protocols are an effective method
forpreventing invasive organisms from arriving in the country.70
Phytosanitary controls are already in place in South Africa,83 but
local customs authorities lack the resources to search cargo con-
tainers, vessels, vehicles or personal baggage systematically.
Screening is limited to commercial imports. There are signs indi-
cating forbidden goods at customs entry points but there is little
attempt to enforce such warnings. Compared with the compre-
hensive, strictly enforced and well-publicized systems adopted
by Australia and New Zealand, the measures in South Africa are
lax and ineffective. These systems require considerable invest-
ment and it is not clear what the South African government’s
intentions are in this regard.
An obvious way of controlling alien invaders is for landowners
to control invaders on their own land. Innovative approaches
are needed to help landowners realize that the expenditure is in
their own best interest.
•Fire
Changes in land ownership patterns and use (for instance,
tourism or recreation) could lead to an increased frequency and
altered seasonal patterns of fire. However, the projected greater
fragmentation is likely to prevent individual fires from burning
over large areas. Changing climates could also alter fire regimes
by raising temperatures or reducing rainfall, or both, thus
extending the duration and risk of fires as well as their inten-
sity.105 Invasive trees and shrubs increase fuel loads and fire
intensities and frequencies.106,107
•Afforestation (commercial forestry, agroforestry, social
forestry)
The pattern of ownership of plantation forestry is changing.
New plantations are owned and managed by small growers
(family operations), unlike the extensive plantations owned and
managed by multinational corporations.108,109 One of the driving
forces behind these changes is the requirement for more social
investment by the big forestry companies, as well as the way in
which afforestation permits are granted. The result of this may
be less effective management of alien invaders because small
owners may lack the resources to address invaders in a system-
atic manner unless they work together, which requires signifi-
cant organizational effort.
The increase in the number of woodlots will result in more
sources for the spread of propagules, and a bigger perimeter to
area ratio (contact zone) with the natural vegetation. These
characteristics favour the spread of invaders.110 New regions are
being exposed to invasions as the area of new land under planta-
tions increases. Thus plantations are now expanding into the
montane grassland areas of the Eastern Cape province, and may
extend into the Wild Coast area of the Eastern Cape and
KwaZulu-Natal.
More than a century of plantation forestry in South Africa has
resulted in a long residence time for some major forestry species,
allowing for acclimatization and some hybridization.83 This, in
conjunction with large planted areas, increases the potential of
Working for Water South African Journal of Science 100, January/February 2004 107
an invader to capitalize on rare events that
facilitate spread. Plantation forestry (and
agriculture) involves the frequent movement
of heavy machinery. This differs from road
and dam construction, which typically is a
once-off event. Seeds of problem invaders
picked up with plantation soil are often trans-
ported over large distances. Riparian zones
also become conduits for rapid dispersal of
seeds out of forestry zones. On the other
hand, the development of an informal eco-
nomic sector based on alien plant species uti-
lization may limit invasions. The use of
Australian wattles (Acacia cyclops and A.
mearnsii) for firewood111,112 is an example of
this.
•Changing perspectives and paradigms in
nature conservation
The approach to wildlife conservation in
South Africa has changed substantially over
the past few decades. Conservation areas are
no longer seen as inviolate sanctuaries which
must be sustained in isolation from the
surrounding areas and their human commu-
nities.113,114 The new approach includes
limited access to reserves for harvesting and
use of materials like thatching grass, increas-
ing the probability of moving propagules
into conserved areas. Similarly, trans-fron-
tier parks may promote cross-border move-
ments of propagules, although the benefits
from more coordinated cross-border control
programmes may cancel detrimental
effects.115
•Horticulture
Changing fashions continue to drive a
demand for new species of flowers, trees,
bushes and shrubs. The horticultural trade
generally promotes alien species at the
expense of indigenous ones.116,83 Many plant
nurseries are still selling species which have
been declared weeds for several years.117 A
high proportion of the total number of invaders are
ornamentals, barrier plants and groundcovers.118 Smuggling
seeds through phytosanitary controls increases the risk of intro-
ducing invasive species.
•Changing perspectives regarding alien plants
The attitude of the general public towards alien plants is
changing as the impacts become more evident and information
is more widely publicized. Cultures of ‘non-tolerance’, ‘indige-
nous is better’ and more precautionary approaches to introduc-
tions and cultivation practices are developing. Small, but
vocal, groups of opponents of invasive-plant management
programmes need to be engaged with a view to resolving
conflicts of interest.
Interactions between forces
All the driving forces interact directly and indirectly with both
the primary and the secondary driving forces and within and
across the national and international contexts where they oper-
ate (Fig. 1). All the international secondary driving forces act
indirectly via one or more of the national secondary driving
forcestoinfluencetheprimary driving forces for invasions. Only
twoof these international forces, human population growth and
movements and global climate change, are likely also to have a
direct effect on the primary driving forces of propagule arrival
and changes in limiting factors, respectively. The influence of in-
ternational human population dynamics and human activities
as secondary driving forces in the international context is shown
by the number of links which connect these secondary driving
forcestoeachotherandtodrivingforcesinthenationalcontext.
In the national context, the key role of the driving force of
national economic trends is emphasized by the network of
connections linking it directly and indirectly to many other
driving forces, particularly the agricultural, horticultural and
forestry enterprises. There are also important and direct link-
ages with the expansion of infrastructure, human population
growth and post-apartheid transformation. Economic growth,
or the lack of it, will play a key role in determining the strength of
these secondary forces as well as their direction.
Human population growth at the national scale is also impor-
108 South African Journal of Science 100, January/February 2004 Working for Water
Fig. 1. A summary of the primary and secondary driving forces for biological invasions, illustrating their
multi-dimensional nature and the main linkages which characterize their complex interactions.119 The
primary driving forces for invasion are shown in bold near the bottom of the diagram. The secondary driving
forces, which operate internationally, are grouped in the upper half; those operating in the national context
are grouped in the lower half. The more important and better-connected driving forces are shown in italics.
tant and has a significant effect on economic success. Rapid
population growth diverts resources into the provision of infra-
structure, facilities and services as well as increasing the need to
provide more jobs. HIV/AIDS probably will reduce population
growth rates and lessen the pressure but, at the same time, it will
limit the most productive age group in the population and
increase the demand for health and other social support
services. It is difficult to determine the net influence of these
complex and multi-directional linkages on invasions but they
are likely to reduce the availability of resources to combat alien
plant invaders.
Changes in national laws and regulations can play a critical
roleindetermining the impact of agricultural, forestry and horti-
cultural enterprises on invasions. Regulation to control the
import, planting and management of known and potential
invader species could minimize the adverse consequences of
these industries. Poor economic growth could lead to policies
which promote the use of any and all species in search of
short-term benefits and result in significant environmental
degradation and costs to the economy. However, policies, laws
and regulations relating to invading plants will remain ineffec-
tive if the broad public does not understand why they are impor-
tant and put pressure on their political, corporate and civil
leadership to ensure that sufficient resources are allocated to
implementing and maintaining them.
Although global climate is not directly linked to local fire
regimes, it can have a significant impact by affecting plant
growth, and thus fuel accumulation, and by changing the
driving factors such as rainfall and dry periods.41,120 This interac-
tion would alter disturbance regimes, perhaps favouring invad-
ers, as well as exacerbating the influences of changes in limiting
factors.
Insights into invasion ecology have been used to develop
screening systems54,121 and new understanding is being gained
from ongoing research.20 But screening systems still have a high
risk of identifying both false negatives (species that will be
approved but can become invaders) and false positives (species
that are rejected with a potential loss of economic benefits).122
This makes these systems difficult to defend against powerful
commercial interests or other national imperatives. Precaution-
ary approaches are often unacceptable to politicians and
commercial interests, especially in a country like South Africa,
which urgently needs its economy to grow.
Uncertainty
This review has exposed some uncertainties which could play
a decisive role in determining the future of invasions in South
Africa. The root of these uncertainties lies in both the complex
and diverse roles of humans as drivers of invasions and in uncer-
tainties about the ecology of biological invasions. The key
sources of ecological uncertainty include:
•Our poor understanding of the invasive ability and the adapt-
ability of many invaders, and the importance of idiosyncratic
factors — which weaken our ability to make reliable predic-
tions and develop effective screening systems16.
•The significant role that accidental introductions have played
in the introduction of the most important invaders1,27,35,123
means that there will always be unpredictable and unwelcome
surprises. South African examples are the triffid weed
(Chromolaena odorata) and Argentine ant (Linepithema humile)
which apparently were introduced to South Africa acciden-
tally in provisions during the Second World War and the
Anglo-Boer War, respectively.124,125
•The significant role of time lags between introduction and
noticeable invasions for several invaders implies that many
speciesalreadyintroducedandinusemayyetemergeasmajor
invaders, perhaps triggered by unusual events.126,127 A local
exampleisredsesbania(Sesbaniapunicea)whichwasfirstnoted
as a rapidly spreading invader only 50–60 years after it was
introduced.126 The emergence of mesquite (Prosopis species) as
a major invader appears to have been due to some very wet
years and large floods in the Karoo during the 1970s and
1980s.128 Hybridization between Prosopis species may also
have played a role as the most aggressive invader is a hybrid
form.129
Scenario planning offers a means for exploring the impacts of
uncertainties and identifying key interventions that can help to
shape the future of nations, organizations and people.130 This
makes the approach ideally suited to dealing with complex
problems involving uncertainties about both ecological and
socio-economic dimensions.119
The human dimension
Most research on biological invasions has focused on the
biophysical aspects but there is growing realization that issues
relating to invasive organisms cannot be dealt with solely by
concentratingontheseaspects.15,123,131 Humansareboth the cause
of invasions and, potentially, the solution to the problem.132
Choices about whether or not to control invaders are deter-
mined largely by culturally-shaped values and ethics.131–133 The
issues extend to, and almost always conflict with, national and
international trade policies and practices.15,85 The human dimen-
sion adds a new layer of complexity and uncertainty to an
already multifaceted and rapidly escalating problem.
The complexities that can arise due to the way humans behave
both individually and collectively have been touched on above.
Even a cursory examination would highlight the importance of,
for example, acclimatization societies and their desire to intro-
duce species to make human colonists more at home in the new
regions.134 Many of the species introduced in this way were
highly unlikely to succeed unaided in the new environment but
became major invaders with human assistance. Examples
include poplars, pines and eucalypts. Another example is the
fashion-driven introductions of species for horticulture such as
several of the less significant but still important Australian Acacia
species, or the jacaranda and syringa, when South Africa already
has a wealth of attractive ornamental tree species which are
suited to the same environments. A more important problem is
the introduction, promotion and poorly controlled dissemina-
tion and planting of ‘miracle’ species for use in land rehabilita-
tion, community forestry and agroforestry.85 Most of these
species are highly aggressive and successful invaders, often
with cultivars, races or genotypes specifically selected for
traits favouring invasions such as abundant seed production,
rapid growth (even in degraded environments) and vigorous
coppicing.135 There is a need for species suitable for growing
under very adverse conditions, and also for multiple uses, but
often the alien is favoured over the local for no apparent reason.
In addition, little care is taken to ensure that there is any form of
monitoring to detect and deal with unwanted invasions as early
as possible. These examples highlight only some of the more
direct roles; there are many other more indirect ways in which
humans can facilitate or limit invasions.
Conclusions
Human value systems, and the acts and behaviour these give
rise to, clearly have a significant influence on invasions.31,49 The
effects are expressed in many different ways, ranging from the
Working for Water South African Journal of Science 100, January/February 2004 109
110 South African Journal of Science 100, January/February 2004 Working for Water
direct and deliberate introduction and cultivation of invasive
species to the indirect effects of global climate change. Many of
these acts began in prehistoric times, for example, the spreading
of newly domesticated crops from the Middle East, but their
scope and consequences altered dramatically during the colo-
nial periods and have continued unabated despite the increas-
ingly evident adverse effects on the ecosystem services that
sustain our socio-economic systems3.
The outcomes of the four primary driving forces — arrival of
new propagules, changes in disturbance regimes, changes in the
availability of limiting factors and fragmentation of the land-
scape — are influenced by human actions through a network of
22 secondary driving forces. The analysis of the network of links
between these 22 forces, 10 acting primarily internationally and
12 primarily nationally, shows that some are more influential
than others. The most powerful secondary forces are the move-
ments of people and goods both nationally and across the globe,
and human policies and legal systems which influence these
movements.
Agriculture, forestry and horticulture are the most important
activities driving the deliberate introduction of plant species and
promoting their cultivation. This makes these businesses a key
target for initiatives that seek to reduce the risk of invasions by
preventing the introduction of invasive species or ensuring that
these enterprises accept responsibility for managing the un-
wanted consequences of their propagation. Effective control
and management of invasions cannot be divorced from the
human dimension of invasions. Human value systems and
ethics can both promote and control invasions. The outcome
depends on how we, individually and collectively, respond to
the challenges that invading plants pose to our well-being. The
Working for Water programme has shown that the choice need
not be whether to control invaders or create jobs and improve
human well-being; both can be achieved simultaneously.
The most effective intervention prevents propagules arriving
by establishing screening and early-warning systems. These
only will work if people accept the need for the constraints they
will place on the movements of themselves and their goods, and
the associated costs. Australia and New Zealand have shown
that a properly informed public will accept these constraints and
costs. The challenge for South Africans lies in motivating the
adoption of similar practices in a much more diverse society, and
in a developing country which is not an island and, thus, is more
easily invaded by species from adjacent countries.
This paper has shown that the human dimension is complex
but tractable, and that a greater understanding of it is critical if
we are to succeed in gaining the upper hand over the invasive
species which threaten the well-being and livelihoods of all
South Africans.
Wethank the Water Research Commission for funding, and the Workingfor Water
programme for supporting our research on this topic. We also thank our three
anonymous reviewers for their pertinent comments which significantly enhanced
the quality of this paper.
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