Integrated management of invasive alien plants in South Africa: A case for biological control

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This brochure is published by the Working for Water Programme, a multi-department programme led by the Department of Water Affairs and Forestry.

Contact details: The Depurty Director: Strategic Services, Working for Water Programme, Private Bag X4390, Cape Town, 8000

ISBN: 978-0-7988-5566-2

Many statements in this document are numbered, and explanatory notes and references are provided at the end of the document.

We acknowledge with gratitude the many photographic illustrations that were provided by the Agricultural Research Council (Plant Protection Research Institute).

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B.W. van Wilgen* and V.C. Moran+ July 2007*Centre for Invasion Biology, CSIR Natural Resources and the Environment, Stellenbosch + Department of Zoology, University of Cape Town

Integrated management of invasive alien plants in South

Africa: A case for biological control

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1. The demands of our growing population are dramatically altering the land cover of the Earth.

2. Biological control, using host-specific insects, is an indispensable part of managing invasive alien plants such as chromolaena.

3. Chemical control involves the application of registered herbicides to invasive alien plants such as jointed cactus

4. Mechanical control can be labour-intensive and is often only a temporaty solution.

5. The research capacity needs to be transformed to meet the demands in the 21st century.

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Executive summary• Global change in the 21st century is bringing important challenges as humanity strives to reduce poverty and

achieve sustainable development. The phenomenon of invasive alien plants is one element of this change.

• This report aims to inform key decision-makers about: (i) the role that integrated control of invasive alien plants will play in reducing poverty in Southern Africa; (ii) the pivotal importance of well-directed research, particularly in biological control, to optimise the integrated management of invasive alien plants; and (iii) the steps needed to build sustainable capacity in this field.

• Invasive alien plants inflict a significant cost on the South African economy, and they exacerbate poverty. Potentially, the cost could be in excess of tens of billions of rands. These costs will grow as existing infestations spread, and as new invasive species establish. It is imperative that this problem be effectively addressed.

• South Africa boasts a history of relatively effective management of the problem, built on decades of scientific research. This scientifially-based management has already conferred large benefits.

• The effective management of invasive alien plants requires the integration of a range of strategies, including prevention, mechanical, chemical and biological control, and ecosystem restoration. Indications are that the current emphasis on mechanical and chemical control will not overcome the problem, which is large and growing.

• This report, therefore, emphasises the importance of research aimed at biological control as an essential element of effective integrated management. Biological control offers significant advantages, including the fact that it is relatively cheap, that it carries very low risks, and that it is sustainable over the long term. Properly implemented, biological control significantly augments conventional controls using chemical and mechanical means.

• Effective control also requires research to improve our understanding of the ecology of invasive species and the ecosystems they invade. Past experience suggests that failure to recognise this has led to much wasted effort.

• The existing capacity to conduct the essential research into biological control in South Africa is not adequate to meet projected needs. Existing skills need to be harnessed to rebuild and transform the research capacity to meet the demands in the 21st century. Time is of the essence – if the existing capacity is lost, it will be difficult if not impossible to rebuild.

• Research funding should be in line with the size of the problem. Most fields of endeavour would invest 1- 5% of their value into research. Given that the potential for impacts in South Africa is in the order of tens of billions of rands, the research investment should be substantial. A proportion of 2% of R50 billion (a plausible net present value for impacts), would suggest an appropriate research investment of R1 billion. At current interest rates, this would generate an annual research budget of R70 million – at least 3.5 times the current level of funding.

• In the expectation that sufficient funding can be secured to optimise the use of biological controls in integrated management, we recommend that the Working for Water programme, in line with its responsibility as the country’s leading alien plant control agency, and as part of its existing and continuing efforts in this respect, takes the responsibility for drafting a business plan for the required research and implementation of a programme of biological control:

• recognizing the national imperative to gain control of the invasive alien plant problem, and the vital role that biological control must play;

• defining an achievable goal for the capacity needed to establish, maintain and implement a programme of biological control appropriate to the size of the problem; and

• detailing the steps and funds needed to achieve this goal within an appropriate time frame.

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Only detailed studies and research will allow us to generate the knowledge necessary to find solutions to the challenge of invasive alien plants.

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IntroductionGlobal change and povertyWe are entering a period of unprecedented change in the world. Growth of the human population and its demands over the past two centuries, the increased technological capacity to alter Earth processes, and the expansion of human activities into all parts of the world have dramatically altered the land cover of the Earth, its atmospheric composition, the flow and chemistry of its rivers, the biology of its oceans and the climate of the planet. There has been greater change to the capacity of the Earth to provide the services on which we depend over the past fifty years than at any time in human history 1.

Together, these changes in the biosphere constitute a syndrome called ‘Global Change’. They are unlikely to threaten the persistence of life on Earth, but they are sufficient in combination to threaten the continuance of life-as-we-know-it. The Earth cannot sustain even the current level of consumption by 6 billion people: the anticipated peak human population of around 9 billion in mid-century, of whom the vast majority will live in cities, will require profound ecological, technological and economic adaptation.

The root cause of all of these changes is increases in the human consumption of natural resources, and the concomitant production of wastes. This is partly caused by the growth in the human population. Mostly it is due to increases in consumption per capita, induced by rising wealth and urbanisation. Currently the human population and its domestic animals appropriate over half of the net primary production of the world for their exclusive consumption. As resources become scarcer, poverty increases, especially in rural areas in developing countries. We urgently need to find ways to effectively address this situation.

What can South Africans do about global change?Many of the drivers of global change are just that – global. The burning of vast amounts of fossil fuels, and the enormous consumption of resources in developed countries, cannot be addressed effectively by developing countries with relatively small economies. At best, the developing world can participate in global agreements, but their actions will do little to reverse the broader trends.

There are, however, many things that developing countries like South Africa can do that would effectively address their own needs. Broad examples include the protection and wise use of our own resources (such as water, arable land, and biodiversity), while at the same time growing our own economy in a responsible and sustainable manner.

The invasion of ecosystems by alien species is an aspect of global change that has long been recognised as a threat to rural livelihoods and as a major threat to conservation of biodiversity and the maintenance of a healthy environment – elements which, in many profound ways, make life worth living. Invasions erode and eliminate valuable natural resources and contribute significantly to the exacerbation of poverty. Our ability to effectively use the land and to gain from the benefits of a healthy environment, are grossly compromised by these invasions. These are problems that we can, and do, address. However, there is an urgent need for improvement and a greater impetus. In this report, we highlight the risks that will accompany inefficient or inadequate management of the problem. If we are able to address the problem more effectively, we can make a significant contribution to the reduction of poverty – and if we do not, the levels of poverty will increase as the resource base is eroded.

The purpose and focus of this reportThis report focuses on the management of invasive alien plants in South Africa, in terrestrial and fresh-water environments, with an emphasis on the need for increased activity in and capacity for research into biological controls. (We deal perfunctorily with the problems of management of weeds in an agricultural context, and we do not include the huge problems of invasive alien organisms in the marine environment: these are inordinately complex and important topics but beyond our present focus.)

This report emphasizes the need for detailed studies and research that will allow us to generate the knowledge and solutions necessary to design and execute effective control programmes against invasive alien plants. South Africa is fortunate in having a history of relatively effective management of invasive alien plants, built in turn on decades of scientific research. However, the capacity to do this research is eroding rapidly. If we are to address the needs of the future, existing skills need to be harnessed to rebuild and transform the research capacity to meet the demands of South Africa in the 21st century.

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How serious is the problem of invasive alien plants?The invasion of ecosystems by alien species is an

important and growing aspect of global change. Up until

the late 19th century, mountains, large rivers, deserts and

oceans provided formidable barriers to the movement

and migration of species. As a result, ecosystems

evolved in relative isolation. Early human migration saw

the first intentional introductions of alien species as

our ancestors attempted to satisfy their physical and

social needs by domesticating wild animals and planting

crops.However, the magnitude and frequency of these

introductions were minor compared to those associated

with today’s global agriculture and vast volumes of trade

and passenger movements. The ongoing and increasing

human redistribution of species to support agriculture,

forestry, horticulture and recreation supplies a continuous

pool of species from which invasive aliens are recruited.

Invasive alien species are also a by-product of accidental

introductions, and include disease organisms, agricultural

weeds and insect pests.

The problem is growing in severity and geographic extent

as global trade and travel accelerate, and as human-

mediated disturbance, changes in the world’s climate

and biogeochemical cycling make ecosystems more

susceptible to invasion by alien species. As a result, all

human communities and natural ecosystems are under

siege from a growing number of destructive invasive alien

species that erode natural capital, compromise ecosystem

stability and threaten economic productivity.

Invasive alien plants cause damage estimated at billions of rands worldwide (Box 1). The situation is of major concern in South Africa, where invasive alien

Box 1. The global cost of invasive alien plantsEstimating the global costs of invasive alien plants is an elusive challenge. In many of the world’s countries, there is simply no information on which to base such estimates. However, in those countries that have attempted to estimate the costs, the indications are that they are considerable, and that they run into many billions of rands annually. Some examples include:

• In Australia, costs to crop systems, pasture land and the horticultural industries alone amount to R12 billion annually.

• In the United Kingdom, damage and control costs associated with invasive alien plants in crop systems amounts to about R2.5 billion annually.

• In New Zealand, damage and control costs associated with invasive alien plants in crop systems amounts to about R600 million annually.

• In the United States, most invasive alien plant species impact on natural ecosystems in the south and west, where combined damage and control costs amount to over R200 billion annually.

• Costs to human health, although difficult to quantify in monetary terms, can also be significant. Parthenium weed (invasive in India and Australia, and now also found in South Africa) poses a threat to agriculture, biodiversity and human health. Regular contact with the plant over a prolonged period produces an acute form of dermatitis and asthma in humans. Sensitised persons develop itching eruptions on the face, neck, hands and back of the knees. In fact, sensitised farmers in India have been known to commit suicide. Meat and milk from animals that have eaten the plant are unsuitable (distasteful) for human consumption.

Source – Pimentel (2002); Mc Neely (2001).

Water hyacinth, originally imported as an ornamental plant, now creates serious problems in many of our waterways.

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plants affect almost 10 million hectares (over 8%) of the country, and are spreading rapidly2. These plants have a range of negative impacts on ecosystem services and agricultural practices. Perhaps best known in South Africa are the impacts that invading tree species have on water resources. However, there are many others. They include the exacerbation of problems with wildfires, the reduction of the grazing potential of the land by replacing palatable plants with unpalatable or poisonous plants, the reduction of biodiversity and the potential extinction of many unique indigenous species, and the degradation of water bodies by invading aquatic weeds. Arriving at a comprehensive figure for the total costs of invasive plants is difficult because relatively few studies have attempted to quantify the costs. However, the indications are that the total costs could become substantial, and a number of examples can be given to support this contention (Box 2).

The problem with attempting to place a value on the cost of invasions is exacerbated by the fact that economic studies have varied in their scale and scope, as well as in the ‘currency’ of evaluation (such as financial or economic). While some costs may have been over-estimated, the high discount rates used by some economists tend to under-estimate future benefits3. It is clear, though, that the actual costs of invasions in South Africa probably amount to billions of rands, and potential costs (if larger areas were to become invaded over time) would reach tens of billions of rands.

Indications are that many of the problems caused by invasive organisms will continue to worsen. New invasive species continue to arrive, and many potential invasive species are probably already here – but not yet invading. Many serious invasions have exhibited a ‘lag period’

Box 2. Examples of the cost of invasions in South AfricaThe cost of invasive alien plant infestations is difficult to estimate at a national scale, given the lack of studies in this

field. However, several more focussed studies, some listed here, indicate that the phenomenon comes at considerable

cost.

• The value of a hypothetical 4 km2 (4000 ha) mountain fynbos ecosystem was estimated to be between R18 million

(with no management of alien plants)4, and R300 million (with effective management of alien plants) . Given that

there are over 1 million ha of protected fynbos areas in South Africa, the potential reduction in value due to invasion

could amount to over R70 billion.

• The value of water lost due to invasions was estimated to be R1 141/ha on the Agulhas Plain area of South Africa5,

thus, if 20 000 ha of this area became invaded (20 000 ha is the target area to be incorporated in the proposed

Agulhas National Park), the total cost could be over R20 billion.

• A study on black wattle (Acacia mearnsii) invasions revealed a ‘net present cost‘’ of R50 billion attributed to black

wattle invasions (this study considered only black wattles, and not the many other invasive trees in the country)6.

• The discounted cost to clear the alien plant invasions in South Africa was initially estimated to be around R5.4

billion7, or roughly R600 million per year for the estimated 20 years that it would take to deal with the problem.

However, this was almost certainly an underestimate.20

• Jointed cactus was estimated to cause economic losses of R310/ha per year in areas where it became established8.

When jointed cactus was at its peak levels of infestation, it occupied almost 1 million ha, implying that it would have

caused damages valued at over R300 million each year.

in which the introduced species may occur at very low population levels for several decades before becoming invasive, sometimes suddenly. This could be the result of exponential population growth, a period of selection of genotypes suited to the newly invaded environment, or the occurrence of a changes in environmental conditions or climate that encourage or exacerbate the invasions. With the rapid growth in the rate of introduction of new species, most introductions of alien species have occurred recently. It is therefore likely that a large number of new potential invasions are currently in a lag phase, and the rate of new invasive species problems will increase dramatically in future.

Invasion by the South American silverleaf nightshade has ruined numerous previously wealthy crop farmers because it defies all attempts to control it.

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The imperative for actionThe above commentary has highlighted two important points. First, alien plants already inflict a significant cost on the South African economy. Secondly, if the problem is not addressed effectively, then economic impacts will persist and grow, as shown in a number of studies10. For example, one study estimated that, of 71 important invasive alien plant species in southern Africa, most were currently confined to 10% or less of the region, but could potentially invade up to 40% – four times the current extent of invasion. Another study estimated that four important invasive species could potentially cover areas of between 2.5 and 15 times greater than the current levels of infestation. In addition, many more invasive species will almost certainly be added to the list over time, and these could invade additional areas not suitable for the current selection of invaders. The potential is therefore clearly there for the negative impacts of alien plant invasions, in South Africa, to increase significantly.

The costs of addressing the problem are large. For example, the Working for Water programme (the country’s largest funder of control efforts) has invested about R2.4 billion in control operations over the past 10 years11. More will certainly need to be invested, or at least the available funds will have to be better utilized, as current levels of intervention are not going to contain the problem (see below). However, given that the current costs associated with invasive alien plants are already significant, and given that they are almost certainly set to grow, the costs

Slash pine (Pinus elliottii) currently occurs in the areas covered by squares, but could potentially cover all of the shaded areas if it is not controlled. This is just one of many such cases. Data from Rouget et al. (2004).

of not addressing the problem will be far greater (to the economy, the environment, and human-wellbeing) than the current costs of control and management operations. We cannot afford to neglect this burgeoning and debilitating problem.

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Invasive alien plant management in South Africa

Box 3. Effective control of alien plant invasions: Lessons from the Cape of Good Hope.

documented, the existing evidence shows that poor understanding of the ecology of invasive species, as well as a lack of follow-through when clearing was done,

led to much wasted effort and money (Box 3). The early,

erratic control efforts were replaced later by co-ordinated

control programmes in the 1970s and 1980s (largely in

the southern and south-western parts of the (then) Cape

Province). At the same time, considerable efforts were

put into research, in order to develop sound, scientifically-

based control options12.

Increasing demands on government spending in a

beleaguered South Africa under sanctions, led to funding

cutbacks and a loss of momentum, in the late 1980s, with

deleterious consequences for the skilled capacity and

expertise that had been built up locally to deal with these

issues. Research programmes also suffered cutbacks, and

Approaches in the pastThe realisation that invasive alien plants pose serious threats to the ecology of South Africa is not new. Early botanists, including Peter MacOwan (in 1888), and Rudolf Marloth (in 1908) raised concerns about the potential for introduced plants to replace natural vegetation, with detrimental consequences. Despite these warnings, the problem only became more widely regarded as serious during the latter part of the 20th century. It is important to note that when control efforts started in the second half of the 20th century, they were done mostly for reasons of conserving natural vegetation, and not for any direct agricultural benefits that might have accrued.

The initial attempts at the control of invasive plants were at best uncoordinated and erratic, and did little to stem their spread. Although few campaigns were adequately

The Cape of Good Hope Nature Reserve, at the tip of the Cape Peninsula, is an important component of the Table Mountain National Park. This park, a significant component of the nation’s protected area network, is an important tourist attraction as well as a reserve of incredible natural beauty and high levels of biological diversity and endemism. During the 20th Century, this area became badly invaded by alien plants, and despite decades of control operations, the menace persisted and grew. Virtually all of the considerable attempts at control in the early years were wasted. The problem was brought under control only when systematic programme of control operations, with adequate funding and scheduled follow-up operations, was introduced in 1974. The important lessons that arose from this exercise included:

• The necessity for understanding the ecology of the target species;

• The need for a systematic, long-term approach to control operations;

• The recognition of the importance of the problem by managers, and its adoption as a management priority; and

• The value of appointing trained ecologists to the management staff.

Reference – Macdonald, Clark & Taylor (1989).

The Cape of Good Hope section of the Table Mountain National Park is now comparatively free of invading alien plants.

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the net result was that the substantial public investment

in the control of alien plants was at least partly nullified,

and alien plants again began to re-invade cleared areas.

Following the election of the country’s first democratic

government, in 1994, ecologists used the opportunity to

revitalize the programme by demonstrating the water and job-creation advantages of alien plant control (Box 4). Well-funded and revitalised clearing programmes were resumed under the banner of the ‘Working for Water‘’ programme13 .

Are current control efforts sufficient to deal with the problem?Whether or not current control attempts are having real impacts on the overall status of invasive alien plant

Box 4. The demonstration of water benefitsThe fact that alien plants use large amounts of water constituted a powerful argument for the revitalization of invasive alien plant control programmes in South Africa in the 1990s. Researchers used models based on plantation forestry to demonstrate that alien plant invasions can have significant effects on water resources, as the invasive trees use more water than the grasslands or fynbos that they replace. Estimates showed that this could reduce the amount of water available to Cape Town, for example, by as much as 33%. A team of ecologists, water resource engineers, and economists used these estimates to demonstrate that controlling invasive alien plant invasions in catchment areas could be as effective (and in some cases more effective) than building new dams. It was also shown that conducting clearing operations, sooner rather than later, was the best approach to follow, both to reduce costs and increase benefits9. These arguments resulted in the establishment of South Africa’s ‘Working for Water‘’ programme, and would not have come about had the country not invested in research in the past.

A plantation of pines in the Jonkershoek valley, Western Cape. Hydrological studies conducted here for 50 years (since 1936) were used to demonstrate that invasions by pines and other alien trees could reduce surface water runoff and have serious impacts on water resources.

infestations in South Africa is an important question. For

example, the Working for Water programme was initially

put forward as a 20-year activity, but it appears that

achieving the goal of clearing major infestations within

that timeframe will not be possible. Data from the Working

for Water programme suggest that, at current rates of

clearing, infestations of several important species would

only be cleared within 30 – 85 years14. These estimates are

extremely conservative15, and more realistic indications

are that, at current rates of management, the problem

will not be contained; at best, only some species will be

controlled, and some areas will be kept clear of invasive

species. This is a sobering prognosis, and it highlights the

need to find sustainable solutions if significant impacts are

to be avoided, and management efforts optimised.

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Optimal approaches to invasive alien plant controlThe concept of integrated controlEffective management of invading alien plants in natural

and semi-natural ecosystems is an imperative if we are

to prevent enormous negative impacts. The effective

management of plant invasions will involve a suite of

approaches. These approaches are usually combined,

tailored and locally-focussed to suit the problem at

hand, and a combination of approaches is referred to

as integrated control. Integrated control is essential, as

no single approach can be successful in stemming the

invasion of ecosystems by alien plants. The design of

effective integrated control approaches requires a detailed

understanding of the ecology of both the ecosystem

being invaded and the invasive weed species. Elements

of integrated control include prevention; early detection

and eradication; mechanical control; chemical control;

biological control; and rehabilitation (Box 5).

Box 5. The concept of integrated control

with burning. It can be labour-intensive, and requires several follow-up treatments to ensure that all individuals have been found and removed.

• Chemical control involves the application of herbicides, either directly to infestations of invasive plants, or in conjunction with mechanical clearing.

• Biological control involves the use of species-specific insects, mites and diseases from the alien plant’s country or region of origin to either kill the target alien plant species, or lessen its ‘weediness’ by reducing its vigour or lessening the number of seeds it produces (see Box 6).

• Ecosystem rehabilitation is necessary to restore the stability of ecosystems once invasive alien plant

infestations have been removed.

Medium - term

Prevention

Early detection and eradication

Mechanical & Chemical Control

Follow-up & rehabilitation

Biological Control

Short - term Long - term

Integrated control refers to a combination of approaches

that are aimed, collectively, at the sustainable and effective

control of invasive alien plant infestations. The combination

of elements in the design of an appropriate integrated

control programme for an area or species depends on

the characteristics and dynamics of the ecosystem being

invaded, the life history attributes of the weed, the time

since introduction, and the size of invasive population.

Elements of integrated control of invasive alien plants,

showing their relative effectiveness in the short, medium

and long terms.

• Prevention involves the development and enforcing

of policies about the importation of new species

into the country, minimising the risks of accidental

introductions, effective policing at border posts, and

so on.

• Early detection and eradication requires widespread

vigilance on the part of knowledgeable people,

to identify problem plants as early as possible.

Eradication is also only possible when populations

of invasive species are very small, and confined to a

limited area. • Mechanical control involves the clearing, felling, or

removal of invading alien plants, often in conjunction

These sap-sucking lace bugs are examples of host-specific biological control agents on South American bugweed, where they cause extensive leaf loss and die-back.

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significant proportion going to chemical control (Table 1)17. The allocation to biological control is small. The relative amount spent on ecosystem rehabilitation is also not known, although it also appears to be a relatively minor component. In summary, it can probably be said that some progress has been made in integrating mechanical and chemical control approaches to address short to medium-term control objectives. Long-term objectives need greater attention by apportioning more of the available funds and research to biological control and ecosystem rehabilitation.

The importance of biological controlBiological control (Box 6) is an essential element of integrated control, offering significant advantages. These include the fact that it is relatively cheap, that it carries very low risks, and that it is sustainable over the long term. Biological control already plays an important and highly

How well is integrated control practiced in South Africa?The degree to which each of the elements of integrated control is addressed in South Africa, currently, is difficult to ascertain. The responsibility for reducing the risk of new invasive species entering the country is placed on the National Department of Agriculture; however, their priority is on preventing diseases and insect pests from entering the country (prevention), and until recently, very little attention has been paid to the invasive potential of the alien plants themselves. Although the new Biodiversity Act caters for the assessment of the risk of invasion, no regulations have yet been promulgated in this regard. Early detection is at best opportunistic. Data from the Working for Water programme (the largest single operation addressing integrated control in the country) currently spends over half of its funds on mechanical control, with a

Box 6. Biological control – a vital element of integrated control of invasive alien plants.

Aphanasium australe, a new ‘agent introduced to combat Australian hakeas

Galls on an Australian acacia, caused by an introduced wasp. The galling largely stops formation of seeds, and reduces the invasiveness of the acacias significantly.

For nearly 150 years, in many countries around the world, biological control has contributed to the successful management of alien, invasive problem plants. Biological control of weeds is a practice in which introduced agents from the country of origin (plant-feeding insects and mites, and plant pathogens) are rigorously tested under quarantine conditions, usually for several years, to ensure that they do not feed or develop on any agricultural crop or indigenous plant species. They are only released into the field once it has been deemed safe to do so where they then variously damage or kill the target weed.

The biological control ‘agents’ usually reduce the fecundity, ‘fitness’ and ‘invasiveness’ of the target weed, often resulting in declining populations and reducing the rate of spread of the problem plants. These circumstances can provide complete control of the problem plant, or else augment and reduce the costs of integrated and conventional control methods such as herbicidal controls and mechanical clearing.

As a management tool, biological control of weeds, using plant-feeding insects and mites, and pathogens, is an attractive option because: (i) it is relatively cheap and very safe compared with the costs and risks associated with herbicide development and deployment; (ii) biological control can be successfully integrated with other management practices; and (iii), most compelling of all, biological control is self-sustaining. Without biological control, weed management would be beyond the capabilities and resources of most countries.

Biological control of weeds has an excellent safety record because: (i) the agents are very carefully chosen and meticulously screened (on average, in South Africa, each is subjected to nearly 4 years of scientific testing) before

release; (ii) various regulatory procedures provide critical scrutiny of these tests; (iii) approval is required at a high administrative level before the agents are released; and (iv) the biology of the agents themselves ensures a high degree of safety. Biological control agents are almost always chosen because they are specific to the target weed, i.e. they cannot and do not feed on any other plants. Even when the host plant is killed the agents do not switch hosts. The safe practice of biological control of weeds relies on good scientific research, good judgement and a careful assessment of the possible risks and benefits.

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Table 1. An approximation of the funds currently spent annually on elements of integrated control of invasive alien plants in South Africa.

Element of integrated control Annual budget currently spent on element (R million)

Prevention 5 (?)

Early detection and eradication

Very little

Mechanical control 303.8

Chemical control 216.2

Biological control 10

Ecosystem rehabilitation Unknown – included in mechanical control.

efficient role in the control of invasive species in South Africa, despite the relatively small proportion of funds expended on it. In South Africa, 63 species of biological control agents have become successfully established on 44 invasive alien plant species since 1913. Eleven (25%) of the 44 target weeds have been completely controlled biologically – that is, no other control methods are needed to reduce the weed populations to acceptable levels. Sixteen species (36%) have been substantially controlled, in that expenditure on other forms of management has been greatly reduced. Thus, for at least 27 invasive alien plant species, the need for expensive mechanical and chemical control has largely been eliminated18. The introduction of biological control agents has been shown in a number of cases (see Box 7) to be extremely beneficial in South Africa19.

At current rates of clearing, indications are that combined mechanical and chemical approaches will not contain the problem, even with more generous and increased funding20. This highlights the imperative to find additional solutions to the problem, and biological control is an

obvious candidate for consideration in this regard. In many

cases, it can offer the best, most sustainable, effective

and inexpensive solution to the most intractable of the

invasive alien plant problems. Mechanical (and chemical)

clearing is necessary to deal with existing infestations,

and it brings additional benefits in the form of labour-

intensive employment, and by-products such as firewood.

However, in the long term, it will not be sustainable over

large areas, it will be extremely costly, and it will take

many decades more than originally envisaged, if it can be

achieved at all20. While biological control will never be able

to solve or lessen all of the problems, it should be used

far more extensively than it currently is – the principle

should be to find biological control solutions to as many

of the invasive alien plant species as possible, and to find

them earlier rather than later21. Biological control should

be seen, in general terms, as a supplement to, and not as

a replacement for more conventional methods of control

(i.e. mechanical clearing and herbicide use). Biological

control is a cheap and effective expedient to ‘soften up’

problematic plants, making them less ‘fit’, less competitive

and less invasive, thus making them easier and less

expensive to control through conventional means. This in

essence is the rationale behind the integration of biological

control with other methods of management. We must use

all the weapons in our armoury if we are to have any hope

at all of succeeding in the long term.

One of the most spectacular biological control successes in South Africa has been achieved by a minute weevil released against the red water fern. The two photos show the same river near Cradock, Eastern Cape, before and after the release of the weevil.

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Box 7. Biological control can generate large savingsproduction, wildlife farming and tourism. While some of the remaining prickly pear stands are still problematic, many people rely on the prickly pear fruits as a source of income, or depend on the spineless varieties as fodder during droughts. In aggregate an impressive win-win situation for the country as a whole and for biological control.

• Hakea infestations in the mountains of the Cape have been reduced by almost 350 000 ha through integrated control programmes22, including biological control, with significant benefits for fynbos biodiversity and the protection of important water catchment areas.

• Red water fern, that was a major problem on still-water bodies throughout the country, has been completely controlled biologically, resulting in significant savings to farmers. For the year 2000, the benefits accrued in relationship to expenditure on biological control research and implementation was a ratio of 2.5: 1, which increased to a ratio of 13:1 in 2005 and will be about 15: 1 by 2010.

• Invasive Australian wattles (acacias) are major problems in many parts of the country, in native vegetation, conservation areas and riparian zones. A range of seed-feeding weevil species, and also insects and pathogens that induce the plants to produce ‘galls’ (‘cancer-like’ growths on the plants which sap the nutrients of the plants and thus, among other debilitating effects, diminish seeding) have been introduced. These biological control agents reduce the ‘invasiveness’ of the wattle species and make them much more amenable to integrated control operations involving chemical and mechanical clearing.

The introduction of biological control agents has been shown in many cases to have highly beneficial economic impacts. A weevil introduced to control red water fern (a floating fern native to South America) reduced damages by R3500 per hectare per year, and the returns on research investment were calculated to be 15:1. Another study showed that biological control reduced impacts by between R300 and R3600 per ha per year for various species. By taking future savings into account, it was estimated that benefits gained in relation to the cost of biological control research and implementation ranged from 34:1 to 4333:1 for the same species. This means that, for every rand invested in research, between 34 and 4333 rands of benefits were generated.

The graphs below depict how, in the case of four important invasive species in South Africa, biological control has significantly reduced the area invaded, and thus the economic impacts of invasive alien plants. The solid curve represents the actual area under infestation, while the dashed line represents the most likely increases in infestation that would have occurred had biological control agents not been introduced (dates of introduction of biocontrol agents are shown as arrows). The shaded area represents the area ‘saved‘’ from infestation as a result of biological control.

• Prickly pear infestations, mostly in the Eastern Cape and Karoo regions, which were the subjects of parliamentary debates at the turn of the last century because of their major socio-economic consequences for the country have been effectively reduced to about 10% of their former area of infestation. Over a million ha of karoo veld has been saved for livestock

These graphs show how the introduction of biological control agents has made a difference to the area invaded by four serious invasive plant species over time.

11

The role of research in support-ing integrated managementThe value of researchResearch generates the knowledge necessary to underpin

effective control programmes (Box 8). Attempting control

operations in the absence of a good understanding of

the biology and ecology of weed species is a recipe for

failure – this is the striking lesson from past experience.

Many control attempts initially fail, or waste large sums of

money, until an adequate understanding of the problem

provides appropriate solutions. While there are many

examples of this, a few cases will suffice to illustrate

the point. The first is the case of prickly pear, which in

the early 1930s threatened the very existence of rural

communities over hundreds of thousands of hectares

of the karoo and Eastern Cape (Box 9). This problem

was almost completely solved by the establishment of

populations of two insect species, which have effectively

kept this otherwise dominant weed species under control.

The second is the case of water hyacinth, an aquatic weed

species from South America, which dominates many

water bodies throughout Africa (Box 10). This species threatened the livelihoods of many African communities dependent on wetland ecosystems, and proved almost impossible to control using conventional approaches such as herbicide application or mechanical harvesting. Biological controls provided sustainable and effective, long-term solution to these problems. Another example is provided by Chromolaena odorata (also known as triffid weed), arguably one of the world’s worst weeds (Box 11). This species is a rapidly growing problem in Southern Africa, but researchers have strived to find solutions to reducing the impact of this species from an early stage; indications are that this research may be paying off, as a promising biological control agent has recently been established in KwaZulu/Natal.

How much is currently invested in research, and is this enough?It is generally recognized that research is an indispensable component of any enterprise (Box 8). Research on invasive

Box 8. What is research?Research is an activity that generates new knowledge. It plays a vital role in almost all human endeavours. In the context of alien plant invasions, research seeks to understand:

• Why some plants become invasive;• How they reproduce and spread;• What impacts they have;• What approaches could be used to control them;• What the consequences of invasions are; and• What combination of methods would most effectively

bring the problems under control.

Research involves the conceptualization and testing of ideas regarding a particular problem. It requires the dedicated and careful collection of evidence to test ideas and to build an understanding. It leads to the rejection of ideas that are not supported by hard evidence, and the acceptance of those that are. It is the domain of specialists who train for many years to master the required techniques. These specialists publish their findings in the open literature, where they are subjected to rigorous peer-review to ensure their quality. Ultimately, it is this body of knowledge that forms an indispensable component in the formulation of effective and efficient control options, including biological control.

Research involves, among other things, the collection of a large and reliable body of data on which to base an understanding of the ecology of invasive species, and this ultimately forms the basis for the development of sound and effective approaches to their control. Here a researcher is collecting seeds of an invasive Australian acacia species, to determine the levels of seed damage achieved by biological control agents. Typically, these estimates have to be made at regular intervals over several years before a sound understanding can be developed.

1212

Organization Funding source Amount (Millions of rands per year)

Notes

The Working for Water programme

Department of Water Affairs and Forestry

10 Bulk (80%) goes to biological control research. Research carried out by consultant researchers.

The Plant Protection Research Institute

Agricultural Research Council Parliamentary Grant

3 Majority of funds go to biological control research

The DST/NRF Centre for Invasion Biology

National Research Foundation

5 Bulk goes to student bursaries – no biological control research

CSIR Natural Resources and the Environment

CSIR Parliamentary Grant

1 Focus is on plant population dynamics, and economic impacts.

Various Universities Salaries of academic and support staff

2? Estimated time spent on research and student supervision.

Table 2. Current funding available for research into the ecology and control of invasive alien plants.

species ecology and their control currently amounts to

about R21 million per year (Table 2). More than half of this

funding comes from the Working for Water programme.

The bulk of this funding, together with funding from the

Agricultural Research Council – Plant Protection Research

Institute, goes into elements of biological control research.

The National Research Foundation supports a centre of

research in invasion biology with funds supplied by the

Department of Science and Technology. The question is

whether or not the present expenditure on these activities

is adequate.

Many developed countries spend significant amounts on

research. For example, Singapore spends 2.1% of its GDP

on research, Japan 3.1% and Israel 5%. South Africa, by

comparison, spends 0.81% of GDP (or roughly R10.1

billion) on research, and employs about 2.2 researchers

per 1000 employees (compared to 10.6 in Sweden, and

7.4 in Russia, for example). The South African Government

has recognized that research spending needs to be

increased, and the immediate goal is to dedicate 1% of

GDP to research23. In South Africa, different sectors of the

Table 3. Research investment for different sectors of the economy in South Africa in relation to the value of the sector.

Sector of the economy Total annual value generated

Estimated amount spent on research, annually

Percentage of ‘value‘’ spent on research

Fishing R2.5 billion R141 million 5.6%

Forestry R19 billion R380 million 2%

Citrus R2 billion R19 million 1%

economy invest between 1 and 5% of turnover in research (Table 3).

In the case of alien plant invasions, it is not possible to determine the value of the ‘sector’, but an appropriate investment into research could be determined by the cost of negative impacts that are avoided by having conducted the appropriate research. Given that the potential for impacts from invasive alien plants in South Africa is in the order of tens of billions of rands, the research investment should be substantial. If one assumes a proportion of 2% of, for argument’s sake, R50 billion (a plausible net present value for impacts), then a research investment of at least R1 billion would be appropriate. At current interest rates, this would generate an annual research budget of about R70 million. This approach provides a ‘top-down’ estimate of the magnitude of funds that should be spent on research – about 3.5 times the current amount. A ‘bottom-up’ approach (in which the funds required to develop an appropriate suite of research and implementation projects over time are added up) would provide a more accurate figure of what is required. This would require a detailed analysis of the needs (see recommendations).

13

Box 9. Biological control of prickly pear

Box 10. Biological control as a means of dealing with water hyacinth infestations.

The host-specific, sap-sucking cochineal insects were extremely

effective in controlling prickly pear in South Africa.

Larvae of the cactus moth have also made an important contribution to the control of prickly pear.

Fonteinplaas near Graaf Reinet, Eastern Cape, invaded by prickly pear during the 1930s (above), and the same area during the 1970s, following the introduction of biological control.

Water hyacinth, a floating aquatic weed from South America, has become a huge problem in many areas in Africa – notably so in some of the central African lakes. Many proposed control approaches, involving expensive mechanical harvesters, as well as herbicides were proposed by aid agencies – they proved largely ineffective due to high costs, constant recovery of the weeds, and a lack of technical expertise. It was only when biological control researchers, based in South Africa, developed simple methods to raise effective numbers of biological control agents (using low-tech rearing facilities – plastic Chemical spraying of water hyacinth

The case of prickly pear (Opuntia ficus-indica) provides a striking example of what can be achieved through biological control research. The prickly pear had densely infested almost 1 million ha of the karoo and Eastern Cape at the beginning of the 20th century. This species severely impacted on the livestock industry, and caused significant declines in land prices. The social problems associated with this invasion were also significant – people were displaced from the land, and the infestations themselves provided cover for criminals. It was only with the introduction of biological control agents in the 1930s that this significant problem was solved, rapidly and at a very small cost relative to the benefits generated.

An interesting aspect of this intervention is that most people will not remember how big the problem was, now that it is solved. It is an important lesson – we need to constantly remind ourselves that were it not for this intervention, the karoo today would be a very different place – and we need continue to address new problems to ensure similar sustainable benefits in future.

1414

Triffid weed (Chromolaena odorata), an invasive plant introduced from Central America, is arguably one of the world’s worst weeds. It has been growing in importance and impact in South Africa over the past decade or two, to the extent where it is now recognized as one of the biggest threats we face in terms of alien plant invasions. For example, the extent to which it had invaded the Hluhluwe-Imfolozi Park in KwaZulu/Natal threatened to completely over-run the habitat for black rhinos, and to potentially bring down the lucrative tourism industry in the area. This prompted the KwaZulu/Natal provincial government to initiate an extensive (and extremely expensive) clearing programme. The prospects for long-term control of this weed, which is spreading rapidly in many areas in southeastern Africa, were bleak, as no effective biological control agents had yet been found. However, scientists

Box 11. Chromolaena – a potential success story?

The larva of the recently-established

released biocontrol agent

– Paracheutes insulata.

In the foreground, dead Chromolaena (triffid weed) plants in KwaZulu/Natal following their attack by the biological control agent. Prior to the release, this infestation of triffid weed below the trees was 3 m tall, and impenetrable. Large areas of triffid weed on the adjoining hillside show the impressive effects of Paracheutes attack.

swimming pools fertilized with cow dung) that effective, community-driven control was achieved.

Prior to the introduction of biological control agents (left), Lake Victoria in East Africa was heavily infested with water hyacinth, which disrupted shipping, fishing, and many other activities, impacting on people who rely on the lake for a livelihood. Following the introduction of biological control agents (right), the problem was effectively reduced to manageable levels. The expensive mechanical harvesters in the picture were prevented from functioning by the

A mechanical harvester intended to clear infestations of water hyacinth

The same harvester following the introduction of biological control agents

sheer pressure of the weeds that confined the harvesters to the lakeside.

The application of expensive herbicides to infestations of water hyacinth is widely practiced in South Africa, but needs to be repeated annually at significant cost (about R10 million per year) to keep the weeds under control. The use of biological control to deal with these infestations has been extensively but certainly not exhaustively investigated, and this presents a significant opportunity to save costs and achieve sustainable control.

recognized the threats presented by this weed and began research to find biological control agents. Recently, after enormous efforts and after several years of research in testing one of the agents (a moth species, Pareuchaetes insulata), in rearing the larvae under quarantine, and releasing nearly a million of the adult moths in the field, the agent has become established in South Africa. At some of the release sites, on the KwaZulu/Natal south coast, the triffid weed has been heavily damaged or killed by the leaf-feeding moth larvae. Although it is much too early to predict widespread success, the impacts of these biocontrol agents are most impressive, and if they reach their full potential will save the country hundreds of millions of rands. Again, this demonstrates the benefits of careful and persistent research by specialist organisations and their expert scientists and support staff.

15

Scenarios for future research supportFinding sustainable solutions to the problem of invasive alien plants will require appropriate levels of research funding. While it would be very difficult to predict what these levels will be in future, it is possible to examine, in broad terms, the likely consequences of different levels of funding. For this purpose, we can consider three plausible scenarios with regard to potential funding for research, listed below:

A. Research funding is reduced or curtailed. Under this scenario, funding for research would be reduced to levels well below those that currently prevail. This scenario is quite plausible, given the tenuous nature of much of the funding for research in invasive alien plants currently.

B. Maintain the status quo. Under this scenario, funding will continue at current levels in real terms, and increases will only reflect adjustments to accommodate inflation.

C. Increase research funding. Under this scenario, levels of funding will be substantially increased, commensurate with the size of the problem and the predicted returns on investment.

Consequences of these respective scenarios

It can confidently be expected that: (i) new alien plant species will continue to enter the country, both through legal, illegal, and accidental introductions; and (ii) these new species, combined with the species already here but not yet showing signs of invading, will constitute a pool from which invasive alien species will be continually recruited; these will be added to those already established, and together they will cause economic losses and other environmental problems (Figure 1).

Scenario A. (Funding for research either is stopped or reduced to levels where the research capacity erodes to unsustainable levels). Under this scenario, very little or no new understanding would be generated, and there would be very little or no capacity to deal with manager’s requests for advice or new approaches. No new biological control agents would be found or released. Invasions of alien plants would grow in extent and impact as new species become invasive. Research capacity would ultimately be lost completely, and it would be extremely difficult, if not impossible, to rebuild. Under this scenario, natural capital will erode.

Scenario B. (Research funding and capacity are both maintained at current levels, which are sub-optimal). Under

Figure 1. A schematic representation of the number of alien plant

species introduced into South Africa over time, and the proportion

that have become invasive. Introductions began prior to European

colonization in 1650, and by the year 2000 about 8000 alien plant

species had been introduced. At least 250 of these species have

displayed invasive tendencies, and many had become serious

problem. It can be expected that these numbers will increase24.

1650 2000 2050N

umbe

r of

al

ien

plan

t spe

cies

All Species

InvasiveSpecies

this scenario, understanding through research would still improve, but not at a rate sufficient to keep pace with new problems as more and more species are added to the list of invasives. Some new biological control agents would be found, tested and released, but the rate at which new invasive species emerge and become problematic would in all likelihood be greater than the establishment of new biocontrol agents. Research capacity could be maintained at current levels at best, or it may decline, given that the levels of funding are below the optimum. Under this scenario, natural capital will erode, although not at the same rates as those experienced under scenario A.

Scenario C. (Research funding is increased, with corresponding increases in research capacity). Under this scenario, the development of new understanding could potentially keep pace with new problems as they emerge. Biological control agents would be found and released, and would form an integral part of optimal integrated management, making a real contribution to the sustainable, long-term control of invasives. Invasions of alien plants would shrink in extent and impact as new species are brought under control. Research capacity would grow, and South African researchers could then also play a role in providing capacity in this country and beyond its borders in other African states. Under this scenario, natural capital could well be regained as solutions to the problem of

1616

1980 2050

Are

a in

vade

dan

d ec

onom

ic c

ost

A

B

C

invasions are developed, and many ecosystems could be returned to productive use.

Each of these scenarios, if realised, would directly affect the area of the country invaded by alien plants, and ultimately also on the economic potential of the land, and either improve or exacerbate the plight of the poor (Figure 2). The extent to which any of these scenarios plays out will be affected by the choices we make regarding the funding of research to support invasive alien plant control.

Figure 2. A schematic representation of the area invaded by, and the

economic costs associated with infestations of invasive alien plants

in South Africa, over time, under the three scenarios of research

capacity. The scenarios are: (A) reduced funding; (B) funding

maintained at 2006 levels; and (C) funding increased to appropriate

levels. The area invaded in 2000 was about 10 million ha, and the

economic costs approached approximately R10 billion per annum.

17

RecommendationsIn the expectation that increased funding will be secured for research into the optimal integration of biological control with conventional methods of invasive plant management, we recommend that the Working for Water programme, in line with its responsibility as the country’s leading alien plant control agency, takes the responsibility for the following:• Drafting a business plan for the required research and

implementation of a programme of biological control;• That the business plan should recognize the national

imperative to gain control of the invasive alien plant problem, and the vital role that biological control must play;

• That the process should define an achievable goal for the capacity needed to establish, maintain and implement a programme of biological control appropriate to the size of the problem; and

• That the business plan should provide details of the steps and funds needed to achieve this goal within an appropriate time frame.

1818

Notes 1 Several recent syntheses provide a great deal

of information on the topic of global change and ecosystem health. The Millennium Assessment was a significant global attempt to ascertain the state of health of the Earth’s ecosystems (see W.V. Reid et al. (2005). Ecosystems and human well-being: Synthesis. Island Press, Washington DC.). The International Geosphere-Biosphere Programme recently produced a detailed synthesis of global change (see W. Steffen et al. (2004) Global change and the earth system: A planet under pressure. Springer Verlag, Berlin).

2 A national estimate of the extent of invasions is given by Versfeld et al. (1998). Although this is a preliminary study, it is the only estimate available.

3 See Turpie (2004) for a review of economic studies. The use of discount rates has the effect of reducing the estimated cost of future impacts in current monetary terms, and effectively any impacts felt after 20 – 30 years become trivial in such estimates. However, this is akin to supporting the view that future generations should carry the costs of the current generation’ neglect in not dealing with the problem. Zero discount rates emphasise the full, ongoing future benefits derived from pristine natural ecosystems that have not been adulterated by the invasions of alien plants.

4 Higgins et al. (1997) based their estimates of economic value on six components with economic value in fynbos vegetation: water production, wildflower harvest, hiker visitation, eco-tourist visitation, endemic species and genetic storage. There are 4.3 million ha of fynbos vegetation remaining in South Africa. Data from Versfeld et al. (1998) (Table 3.6) indicate that 33% of the three major fynbos catchments in the Western Cape are invaded to some degree. Thus over 1 million ha are probably invaded to some degree. Currently, these invasions occur at different densities, and the full economic impact will only occur if and when they become fully invaded.

5 For full details, see Turpie and Heydenrych (2000).

6 De Wit, Crookes and van Wilgen (2001) calculated the economic value of stream-flow lost to invasions of black wattle in South Africa, using the opportunity-cost approach. First, the value added by water over the different demand sectors (irrigation, domestic and urban use, mining and industry, the environment and afforestation) was calculated. Secondly, the value added by additional water where black wattles were removed was estimated. These estimates were

adjusted for to allow for evaporation and spillage of

flood water (33% of additional water was assumed to be unusable), changes in the numbers of downstream water users over the next 20 years, and the degree to which water would contribute to the economic value added in each sector. The ‘net present cost‘’ was the calculated as the positive benefits minus the negative impacts.

7 The costs of control were estimated by Versfeld et al. (1998), based on a 20-year clearing programme, an annual budget of R600 million, a rate of expansion of the total invaded area of 5% per year, and were discounted at 8% per year.

8 This work has been documented in detail in the scientific literature, and formed the basis for convincing arguments that led to the initiation of the Working for Water programme. See Le Maitre et al. (1996; 2002); van Wilgen et al. (1996;1997) for details.

9 Estimates of the economic impacts of jointed cactus were made by van Wilgen et al. (2004). Jointed cactus is now mostly under effective biological control, so these impacts have been avoided.

10 Studies quoted here used different approaches. Rouget et al. (2004) used climatic envelope models to derive climatic suitability surfaces for each species; van Wilgen et al. (2004) mapped potential distributions from expert opinion of the extent to which suitable vegetation types would become invaded in the absence of control attempts.

11 The Working for Water programme’s annual report for 2003/04 (the latest available) shows that a total of R2.4 billion was obtained between 1995 and 2004. Annual expenditure rose from R27million in 1995/96 to R415 million in 2003/04.

12 An overview of this work can be found in the published report of the Scientific Committee on Problems of the Environment (SCOPE), which had a focus on invasions in the 1980s. See Macdonald et al. (1986).

13 A detailed history of the establishment of the Working for Water programme, and its achievements, are given in van Wilgen et al. (2002).

14 Marais et al. (2004) used data from the Working for Water programme’s newly-established database to estimate the impact of clearing on Australian wattles, gums, pines, bugweed, mesquite, poplars, hakeas, syringas, lantana and brambles. Their conclusions are preliminary, but they remain the best available estimate of whether or not mechanical and chemical control will control alien plant infestations in the long term.

19

15 Marias et al.’s (2004) approximations made a number of assumptions, each of which reduced the estimate of time that would be needed to clear existing infestations; the estimates are therefore probably serious underestimates. The assumptions included: (1) that infestations are static, and will not spread further while clearing operations are under way; (2) that clearing a site will eradicate the invasive alien species; (3) that areas require only one follow-up treatment; (4) that funding levels will remain at the levels sustained over the past few years; and (5) that we know how big the problem is. They also do not address the emergence of new weed species.

16 Several lists of invasive plant species in Southern Africa have been compiled – see Henderson (1999) and Nel et al. (2004) for examples. However, there is no comprehensive account of the total number of alien species that have been introduced to the region, and estimates range from 8 000 to 15 000.

17 The annual budget for the Working for Water programme is R430 million (Annual report 2003/04). The allocation of this to different elements of integrated control is not reported. The annual allocation to biological control research is about R10 million, approximately 2.3% of the annual budget. The remainder covers implementation and overheads. Data from Marais, Stevens and van Wilgen (2004) suggest that the annual cost of herbicides to Working for Water is R51.6 million, leaving R368 million for labour and overheads. If one assumes that at least one third of the labour costs go into the application of chemicals, then chemical control accounts for R51.6 million plus R122.6 million, or 40.5% of expenditure. This leaves 57.2% for mechanical control. The KwaZulu/Natal provincial government also spends R100 million annually on

Invasive species Cost of clearing (R/ha)+ Total area infested#

(x1000 ha)

Total cost to clear (R billion)*Initial Follow-up Herbicides

Australian wattles 1277 1866 285 720 3.8

Lantana 1658 962 572 26 0.1

Pines 2163 2166 0 77 0.5

Hakeas 1231 1868 0 64 0.3

Mesquite 926 793 804 173 0.6

Brambles 1717 2756 280 26 0.2

Eucalypts 2217 1858 992 63 0.4

Triffid weed 1611 1281 282 43 0.2

Syringas 2937 1856 351 72 0.5

invasive alien plant control. It was assumed that this was spent in the same proportions as is done in the Working for Water programme.

18 In the case of a further five invasive alien plant species (11%), the outcome of biological control is still uncertain. See Moran et al. (2005) for details.

19 The main benefits considered were the prevention of water losses due to excessive transpiration by invasive plants; reductions in the values of land that became invaded; and reductions in value added by biodiversity to ecosystem services. See McChonnachie et al. (2003), and van Wilgen et al. (2004).

20 The initial estimate for clearing infestations in South Africa, given by Versfeld et al. (1996) was R5.4 billion over 20 years. However, recent experience suggests that this was almost certainly an underestimate. Data from Marais et al. (2004) suggest: (1) that the job cannot be completed in 20 years; and (2) that costs will be higher. Simple extrapolation of Marais’ data (table below) for just nine groups of species suggest costs exceeding R6.7 billion, without taking further spread or other species into account.

21 Biological control against incipient or ‘emerging’ weeds (plants in an early stage of invasion) has not been widely practiced in many countries, largely because limited funds tend to be directed at invasions that have already reached detrimental levels. The Working for Water programme has now specifically allocated funds towards the initiation of biological control programmes against a number of ‘emerging weed’ species. This commitment has, for the first time, given formal recognition to the rationale of targeting incipient weeds and bodes well for the future of biological weed control in South Africa. See Olckers (2004) for details.

+ Costs are for dense infestations # Expressed in terms of dense infestations * Assuming two follow-up treatments

2020

22 A careful comparison of the extent and density of infestations of hakeas in 1979 and in 2001 in the Western Cape revealed that control programmes had reduced the overall invaded area by 340 135 ha, and also reduced the density of infestations. Some of this may have been due to mechanical clearing in the early 1980s, but much of it is probably due to the effects of seed-feeding biological control agents. See te Roller 2004 for details.

23 Budget speech, Minister of Science and Technology (Mosibudi Mangena), 26 May 2006.

24 Several lists of invasive plant species in Southern Africa have been compiled – see Henderson (1999) and Nel et al. (2004) for examples. However, there is no comprehensive account of the total number of alien species that have been introduced to the region, and estimates range from 8 000 to 15 000.

21

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