1 Introduction

This book on biological invasions in South Africa has focussed on the current state of invasions in South Africa and the processes that have led us to this point. It has highlighted the fascinating interplay between socio-economic factors and biological processes that have determined which alien species have been introduced, where they have spread to, what impacts have occurred, and how South African society has responded. This is largely because the book set out to be encyclopaedic (van Wilgen et al. 2020a, Chap. 1). However, biological invasions are fundamentally dynamic and are an important component of global change (van Wilgen et al. 2020b, Chap. 29). It would therefore be remiss not to conclude with an evaluation of what the future might bring. This chapter examines possible scenarios for biological invasions globally and in South Africa, and aims to show how different events and decisions could set us on radically different trajectories.

There has been an increasing wave of interest in conducting horizon-scanning, both for conservation generally and for invasion science specifically, with the aim of anticipating and preparing for problems (Ricciardi et al. 2017; Sutherland and Woodroof 2009). However, such exercises typically only consider what could happen over the next few planning cycles. This chapter takes a different approach. Although we rely heavily on existing projections, we focus first on the long-term (i.e. on what the “end-points” might be), and work back through time. Our approach was inspired by a recent exercise that considered potential futures for human civilisation and identified four basic trajectories: civilisation could conquer space; technological transformations could be such that what we now recognise as ‘human’ would no longer be relevant; civilisation could continue to develop, but with no transformative changes (status quo); or there could be a catastrophic end (Baum et al. 2019). These trajectories form the basis for evaluating the consequences of actions taken now. Thinking in this way balances short-termism that permeates most planning and political cycles and pitches thinking back into ‘Long Now’ time scales consistent with the functioning of ecosystems (Brand 2008). On this basis, and noting that the focus is on biological invasions rather than other global change drivers , we consider invasions over three time-periods:

  • The long-term: the transfer of species across biogeographical barriers by humans in South Africa started slowly probably around 2000 years before present and has accelerated particularly over the last 200 years (Deacon 1986; Faulkner et al. 2020, Chap. 12). What will the situation look like in South Africa 200–2000 years from now? We assume that there will have been no significant shifts in tectonic plates (although there might be significant tectonic activity, important shifts in ocean currents, and sea-level changes), and assume that substantially new and diverse phylogenetic lineages will not yet have evolved.

  • 2070: the need for inter-generational thinking is a principle embedded within conservation science; the choice of a 50-year time horizon is meant to reflect this. Specifically, what will South Africa look like when children born today become the decision makers? (although the age profile of decision-makers might and maybe should shift).

  • 2025: current decisions are, of course, still made in the context of policy and management planning horizons, usually covering no more than the next 5 years (e.g. elections or government funding cycles).

For the 2070 and 2025 time periods we consider how different events and drivers are likely to put us on a trajectory to one of the long-term scenarios.

2 The Long-Term: What Will Invasions Look Like 200–2000 Years from Now?

Below we sketch out four long-term scenarios (Table 31.1). The inspiration for these came largely from the scenarios of Baum et al. (2019) and post-lunch discussions at the wine farm Lovane (close to Stellenbosch). However, they undoubtedly also arose from nascent ideas buried deep in our memories of concepts more eloquently expressed by other authors.

Table 31.1 The possible causes, outcomes, and consequences for South Africa of four long-term scenarios for biological invasions (i.e. 200–2000 years from now)

First, we consider “Collapse of civilisation, but no return to Eden”. In this scenario, a disastrous event (or series of events) leads to the extinction of Homo sapiens, or, of more specific relevance to biological invasions, leads to a situation where there is no longer an advanced civilisation that is capable of the inter-continental dispersal of species in a manner akin to either mass dispersal or cultivation (Wilson et al. 2009). The consequences of existing biological invasions would play out in full (Rouget et al. 2016), but there would be no new human-mediated introductions (or the few that occur would be akin to natural dispersal).

Second, global trade and transport continue to accelerate, and the rate of introduction of species and the subsequent invasions are not (possibly cannot) be controlled. Biogeographical barriers become fully eroded such that there is essentially global dispersal—the Earth could then be considered as a single continent from a biogeographic perspective. The concept of a “New Pangea” has a long pedigree, with some of the potential consequences codified by Rosenzweig (2001). In this scenario, local variation disappears and biotic homogenisation associated with globalisation becomes complete. This scenario has been termed a World of Weeds (Quammen 1998), although it is important to note that the New Pangea would also consist of globalised crops, livestock, and pets (McKinney 2005). Indeed, the beginnings of this can be seen with the globalisation of agriculture. For example, a McDonald’s hamburger with a coffee contains at least 19 plant species from all of the eight global centres of cultivated plant diversity identified by Vavilov (1926). All of these species are cultivated all around the world (Procheş et al. 2008b). However, the lack of effort to retain or protect non-utilitarian species and natural biogeographic distributions would lead to steep declines in biodiversity at a global scale.

Third, we consider a scenario that is somewhat similar to the Earth we know today—“Preserve or Use”. The Earth is divided into broad use types: areas that are transformed; areas used for sustainable agriculture, forestry production, or harvesting (e.g. fishing); and natural areas that are protected. Current levels of protection vary—around 14.7% of land area and 4.1% of the oceans are formally protected (UNEP-WCMC and IUCN 2016). This does not mean that such areas are devoid of alien species (Foxcroft et al. 2013) or that the eradication of alien species from such areas is possible or in some cases desirable—a third of all formally protected land is still subjected to intense human pressures (Jones et al. 2018). There are also significant moves to ensure that biodiversity is appreciated and considered everywhere. For example, in urban ecosystems the native/alien dichotomy is but one of many factors considered when formulating management strategies for “the whole landscape” (Hobbs et al. 2014; Potgieter et al. 2020, Chap. 11). Nonetheless, the distinction between alien and native is important and should be made explicit if biodiversity is to be conserved (Pauchard et al. 2018). This scenario requires a societal consensus that persists over time (e.g. in a “Preserve or Use” Earth a sense of enormous well-being is gained both by conserving native wildlife and by feeding the pigeons and sparrows too). The overall area that should be set aside is the subject of on-going debate, with recent proposals suggesting it should be as high as 50% (Buscher et al. 2017; Noss et al. 2012; Wilson 2016).

Although Earth is currently in a “Preserve and Use” state, we do not consider this scenario to be the status quo as we do not believe the current situation is sustainable. While some progress has been made controlling biological invasions, especially in protected areas (Foxcroft et al. 2013) and on islands (Greve et al. 2020, Chap. 8; Jones et al. 2016), problems with invasions are worsening in most cases (Millenium Ecosystem Assessment 2005), and globally the number of alien species that naturalise and become invasive in new areas keeps climbing, with no indication that it will plateau soon for most taxonomic groups (Seebens et al. 2017). Based on current drivers, we believe we are drifting towards the New Pangea.

Finally, we propose a “Conservation Earth” scenario in which the whole planet is conserved as a ‘cradle of life’. Human-mediated dispersal of organisms stops; invasions are eradicated; other human-mediated drivers of global change are reversed; and the Earth is actively restored to how it was before widespread human influence (including before biological invasions). For this scenario to be realised, humans would need to have developed radically advanced technologies in ecological restoration; there would need to be profound modifications to current biodiversity (from genes to ecosystems) and physico-chemical processes (e.g. the creation of soils); and the impact of humans on Earth (i.e. their footprint) would have to decline to negligible levels. However, once “Conservation Earth” was achieved, further human interventions could cease. This is perhaps the most sci-fi of our four long-term scenarios, but it is compatible with, and perhaps a likely outcome of, two of Baum et al. (2019)’s trajectories for human civilisation—the technological transformation trajectories, and the astronomical trajectories.

There is somewhat of a continuum between “New Pangea” and “Conservation Earth”, with “Preserve or Use” as an intermediate and possibly unstable state. There are, however, some qualitative differences. “Preserve or Use” differs from “New Pangea” in the retention of significant historical biogeographical patterns (e.g. Australia has a unique recognisable fauna, and the fishes of the Amazon are distinct from those of the Mekong or the Nile). “Preserve or Use” differs from “Conservation Earth” in the constant need for human intervention to ensure sustainability while maintaining biosecurity. Notably, if civilisation were to collapse, we suspect that it might already have moved significantly towards a “New Pangea” scenario. Therefore, while under both “Collapse of civilisation, but no return to Eden” and “Conservation Earth” there might be few if any humans left on Earth, these scenarios would look very different in terms of biogeography.

These scenarios are also not exhaustive, and we acknowledge that they deal with the interaction of global change drivers rather crudely. For example, climate change alone might lead to a complete reorganisation of the world’s biomes. These novel biomes might be distinct and separated by biogeographical barriers maintained by future civilisations, and so might be valued both for their intrinsic uniqueness and their utilitarian value. We feel, however, that the four potential futures we outline are useful as they provide a small set of different trajectories against which current events and decisions in biological invasions can be assessed.

3 The Year 2070: What Will Biological Invasions Look Like in South Africa When Children Born Today Are the Decision Makers?

In considering what South Africa might look like 50 years from now we considered five main themes that have emerged from recent horizon-scanning exercises in invasion science (Caffrey et al. 2014; Dehnen-Schmutz et al. 2018; Ricciardi et al. 2017): technological advances; the political socio-economic milieu; trade; the link to global change drivers ; and potential evolutionary and ecological responses. We tried to envisage potential changes, and how these might influence biological invasions consistent with one of the long-term scenarios [excluding the collapse of civilisation scenario where the influence of catastrophic events on biological invasions would be irrelevant compared to the catastrophe itself] (Table 31.2). These projections are our own, but were inspired by horizon scanning; studies of the current and future trends in the Anthropocene ; and deliberations during a 1-day workshop entitled “Where to with invasion science in South Africa?” organised by the Centre for Invasion Biology (CñIñB) in November 2018.

Table 31.2 How changes over the next 50 years in technological advances; the political socio-economic milieu; trade; global change drivers; and evolutionary and ecological responses might place biological invasions in South Africa along a trajectory to one of the scenarios outlined in Table 31.1

4 The Year 2025: What Will Biological Invasions Looks Like After the Next Funding/Political Cycle?

The choice of specific events over the next 5 years that are likely to happen or that are already happening (e.g. challenges to the current regulations) are our own, but as before were inspired by: the recent report on the national status of biological invasions in South Africa (van Wilgen and Wilson 2018); South Africa’s draft National Strategy on Biological Invasions (van Wilgen et al. 2014); the C∙I∙B’s strategic plan for 2025; and insights from the 2018 C∙I∙B workshop on “Where to with invasion science in South Africa?”. We found it difficult, however, to link these events to the long-term trajectories. Therefore we categorised events in terms of whether they are likely to cause biological invasions to worsen (consistent with “Pangea Earth”); keep invasions roughly static (“Preserve or Use”); or reduce the impacts seen (consistent with either “Preserve or Use” or “Conservation Earth”) (Table 31.3). We also selected and discuss events under themes that we feel operate over this time-scale—planning, regulation, funding, public support, and research.

Table 31.3 How changes over the next 5 years in planning, regulation, funding, public support, and research might place biological invasions in South Africa along a trajectory to one of the scenarios outlined in Table 31.1

5 Possible Ways Forward: Examples from South Africa

God, grant me the serenity to accept the things I cannot change,

courage to change the things I can,

and wisdom to know the difference.

The Serenity Prayer (Reinhold Niebuhr)

In this chapter, we have outlined four long-term scenarios, and have described how events over the next 5–50 years will place us on a trajectory to one of these. Which end point is desirable is a choice for society, and some of the issues are highly contentious and incompatible [e.g. the right of your neighbour to keep a pet cat in their garden affects your right to enjoy a diversity of birds in your garden (Potgieter et al. 2020, Chap. 11)]. Such issues can, of course, also vary over space and time. Introduced species might increase local diversity over the short-term but reduce global diversity and even local diversity over longer time frames, due to the interplay between invasion and extinction debts (Rouget et al. 2016; Tilman et al. 1994). We illustrate these scenarios not to proselytise, but to highlight how the choices we make now could influence the future state of the Earth and what options (if any) are available to future generations.

Importantly, business-as-usual will ensure that current trends continue and that biological invasions will worsen due to an increasing number of alien species, growth in the extent of invasions, increasing impacts, and the continuing problems around conflict-generating species , ineffective management, and insufficient management capacity (van Wilgen and Wilson 2018). There are few studies on the impacts of alien species but available studies show that the reductions in the value of ecosystem services, productivity of rangelands, and in biodiversity intactness caused by alien species are low at present, but expected to grow rapidly (van Wilgen et al. 2008; Zengeya et al. 2020, Chap. 17). The challenge in South Africa will be to combine the current funding model (where most government funding to manage biological invasions is primarily for job creation), with one that also focuses on improving the efficiency of management and the outcomes in terms of reduced impacts and threats from invasions (van Wilgen and Wannenburgh 2016). Shifting our focus from control to prevention would also improve returns on investment, but siphoning funds from current problems might exacerbate them. Practicing conservation triage, with a focus on priority areas, could lead to patchy successes, but is likely to meet stiff resistance as people are reluctant to admit that some areas have to be abandoned to save others. Similarly, the distribution of funding has been based on political and social concerns (e.g. the desire to spread funding across the country). Shifting this to a funding system based on ecological and environmental needs would be unpopular and might see a decline in political support and ultimately funding. Increasing investment in biological control would also increase returns on investment (Hill et al. 2020, Chap. 19), but is less politically attractive as it is not labour-intensive. Important questions remain unanswered. What will be required to turn this around, and will it be politically possible? What is the future of South Africa’s legislative framework in the face of legal challenges (cf. Lukey and Hall 2020, Chap. 18)? There are, however, plenty of examples where change is possible. Continuing investment in the management of biological invasions can be both vital for sustainable and equitable development and cost-effective, especially if economic incentives for invasive species management and overall restoration are implemented (Milton et al. 2003). Regardless of the trajectory and how we deal with the issue, we expect that in 50 years’ time the most widespread invaders that cause the most impacts will be similar to those that occur now [for comparison, invasions in the Fynbos Biome are largely, though not entirely, the same as those 70 years ago with acacias , hakeas and pines dominating (van Wilgen et al. 2016)]. However, there will inevitably be some big surprises (e.g., the discovery of the Polyphagous Shot-Hole Borer, Paap et al. 2018; Box 11.3, Potgieter et al. 2020, Chap. 11).

South Africa as a society will need to make decisions as to what and how to prioritise for management. In the rest of this chapter we outline selected case-studies from this book to illustrate how decisions made over the next 5–50 years will determine the trajectory of biological invasions in the future.

5.1 Coastal vs. Off-Shore Ecosystems

Most of South Africa’s rocky seashore has been transformed by the introduction of alien mussel species. This was not a deliberate choice and no technologies currently exist to alter this situation (Robinson et al. 2020, Chap. 7). The novel ecosystems created by these invasions have some benefits, and interesting impacts on biodiversity (Griffiths et al. 1992; Robinson et al. 2020, Chap. 7). Despite the current regulations, it will be difficult, but not impossible, to prevent new invasions of coastal species. There are also moves to protect large areas from habitat transformation. All this suggests that, for coastal systems, we are in a “Preserve or Use” state that is much closer to “New Pangea” than “Conservation Earth”. In sharp contrast, very few off-shore marine invasions have been recorded, and there are no examples of invasive marine fish in South Africa. It might be possible to preserve this situation, and stay on a trajectory closer to “Conservation Earth”, though this depends on the degree to which a sustainable blue economy can be achieved without leading to more species introductions and more impacts.

5.2 The Management of Invasions in Arid Rangelands: Prosopis Species

A large proportion of the land surface of South Africa is taken up by arid rangelands (Table 16.1, O’Connor and van Wilgen 2020, Chap. 16). These rangelands are being threatened by rapidly-expanding invasions of Mesquite (Prosopis) trees that reduce groundwater resources on which many towns and communities in the region are dependent (Le Maitre et al. 2020, Chap. 15), and reduce the capacity of rangelands to produce livestock. If Prosopis invasions continue to increase, there could be total economic collapse in these regions, similar to that experienced in the Karoo in the 1920s as a result of invasion by Opuntia ficus-indica (Mission Prickly Pear) (O’Connor and van Wilgen 2020, Chap. 16). There is a need to diversify land-use activities to increase income in these areas, for example by combining livestock farming with game viewing, hunting and tourism (Milton et al. 2003). If successful, some of the income could be channelled back into Prosopis control. There are also initiatives that will explore the possibility of triple bottom-line accounting, and using this to underpin a system of tax incentives to allow landowners to recoup the costs of alien plant control. This, combined with more effective biological control, could reverse the negative trend in Prosopis invasions. Currently, however, we are in a “Preserve or Use” state that is shifting rapidly towards “New Pangea”, and if the similar on-going Prosopis invasions in Kenya and Ethiopia continue, many of these landscapes will become physically and functionally identical (and provide few ecosystem services).

5.3 The Need for Taxonomic Services and Well-Curated Comprehensive Lists of Alien Species

The status of knowledge of alien species varies markedly—high for mammals (Measey et al. 2020, Chap. 5), lower for plants (Richardson et al. 2020b, Chap. 3), lower still in marine systems (Robinson et al. 2016, 2020, Chap. 9), and almost non-existent for many soil and microbial groups (Janion-Scheepers et al. 2016; Wood 2017). But even for well-studied groups, there are errors and omissions in the lists of invasive species (Magona et al. 2018). South Africa lacks a comprehensive consolidated list of alien taxa (cf. van Wilgen and Wilson 2018). This is a problem as many alien species that are known invaders elsewhere in the world are present in South Africa but not yet incorporated into long-term planning and strategies. Continuing investment in taxonomy would increase our ability to identify and respond to incursions before they become widespread, and understanding the target species can be essential for management (Jacobs et al. 2017; Pyšek et al. 2013). By contrast, a dramatic reduction in research funding would see lists quickly become out of date which would undermine both risk analysis efforts (Kumschick et al. 2020, Chap. 20) and public support. Taxonomic services and alien species lists provide the foundational biodiversity information necessary for us to be able to choose between a “New Pangea” or “Conservation Earth” trajectory.

5.4 Regulatory Directions

South Africa is one of the few countries that has comprehensive regulations in place to manage biological invasions, and many parts of the regulations are innovative (van Wilgen and Wilson 2018). While this is certainly commendable, there are many challenges to the effective implementation of these regulations, not least of which is a lack of capacity to monitor and, if necessary, enforce them. Section 18.8.2 of Lukey and Hall (2020, Chap. 18) highlights that compliance with the regulations by 90% of society will only be achieved if the 10% that do not comply are brought to book, which is not the case at present. Compliance will also only be achieved if the regulations are broadly regarded as just and equitable; this may not be the case with the current approach of “faultless liability” whereby landowners are responsible for the control of species they did not introduce. Currently, the regulations are either ignored, or people are unaware of them (van Wilgen and Wilson 2018), and some people have mounted legal challenges to them (Lukey and Hall 2020, Chap. 18). If the regulations remain ineffective, or are removed as a result of legal challenges, we may be heading towards “New Pangea”. A change in approach might be required to move in other directions, including subsidies and tax breaks, but a major step would be the development of a national policy on biological invasions to provide the basis for strategic and regulatory developments.

5.5 A New Green Deal and Landscape Stewards

The idea of linking environmental management to employment creation (i.e. labour-intensive alien plant clearing programmes) was an innovative solution to the need to raise funds for invasive plant control in South Africa in the post-apartheid consensus (when funds were also desperately needed for education, health, infrastructure development, security, and welfare). However, the management of invasions is still tied primarily to welfare and job creation, and while the allocation of funds has grown, managers are still assessed on input indicators (e.g. numbers of jobs created, and money spent) rather than output or outcome indicators (e.g. reductions in the area invaded and the impacts caused) (Wilson et al. 2018). Moreover, the approach limits the implementation of more effective high-tech solutions in some cases (van Wilgen and Wilson 2018). This “green deal” has thus failed to stem the spread of invasive species at a national scale, and business-as-usual would set us on a trajectory towards “New Pangea”. A combination of a new green deal and a ‘landscape steward’ approach could reverse these trends. More effective, goal-directed planning and implementation supported by a greater focus on training on project management and monitoring control effectiveness, and judicious use of new technologies (e.g. drones, precision control, DNA barcoding , remote sensing and monitoring, and improved taxonomic capacity), could improve management effectiveness and returns on investment. Continuous monitoring and maintenance of project outcomes as well as the development of nuanced interventions that are appropriate for the specific context would be more realistic if a more permanent connection is made between managers and the land they are managing, e.g. through a landscape steward type approach.

This would require a societal consensus around the need to avoid the longer-term impacts associated with invasions (i.e. beyond current political and funding cycles); the need to balance all the benefits of invasions (timber, fuel, fodder, carbon, food and recreation) against their negative impacts (on water, rangeland productivity, biodiversity, fire hazard and human health); an appreciation of the threat invasions pose to economic and social prospects and ultimately sustainable development; and an increased focus on supporting bottom-up community driven connections to the land that is being managed. But the idea of linking environmental sustainability and job creation is as valid now as it was 25 years ago. A new green deal based on explicit and commonly shared goals of environmental and social sustainability would set South Africa on the path to “Preserve and Use”, and ultimately to ensure that South Africa retains its unique character.

6 Conclusions

It’s hard to make predictions, especially about the future.

Provenance uncertain, probably Danish (made famous by Niels Bohr)

While efforts to predict invasions are becoming more sophisticated (e.g., Essl et al. 2019; Gallien et al. 2019) and metrics exist for projecting how current indicators might change over time (Rouget et al. 2016; Wilson et al. 2018), scenarios for biological invasions will remain uncertain, particularly over longer time horizons. Most projections are implicitly or explicitly based on experience with invasions in the recent past. Conditions, including many drivers of invasions, are changing rapidly. Uncertainties are implicit in invasion science and will be best dealt with by clearly circumscribing invasion phenomena, measuring and providing clear evidence for such phenomena, and understanding their drivers and the mechanisms that generate consequences (Latombe et al. 2019). In the last section, we highlighted a few of the things that, for future generations to continue to have the choice of which scenario they want, will likely be needed: different priorities for different ecosystems (e.g. coastal vs. off-shore); the development and implementation of strategies for particular invasions (e.g. for Prosopis invasions); improvements in our foundational knowledge (e.g. through well-curated and comprehensive lists of alien species); a wide range of regulatory and other policy approaches; and novel ways to facilitate land management. However, it seems likely to us that in the next 200–2000 years we will reach a point when either the concept of biological invasions is irrelevant; invasions continue to be managed in the context of complex competing needs and interests; we have advanced to a stage where we can turn Earth as a whole into a biodiversity reserve; or civilisation collapses. We believe that the policy and management decisions we make over the coming years and decades will set us on one of these trajectories (Figs. 31.1 and 31.2). If we can develop a shared vision of how we want South Africa to look (e.g. a national policy on biological invasions), then this will provide us with a focal point for our efforts.

Fig. 31.1
figure 1

A schematic showing the themes discussed in this chapter, i.e. how events and drivers over the next 5–50 years might place us on a trajectory towards “New Pangea”, “Preserve or Use”, or “Conservation Earth”. We have not included events that might lead to “Collapse of civilisation but no return to Eden” as these are often typified by unpredictable events not directly related to invasions; suffice to say that if civilisation were to collapse the impacts of invasions on biodiversity and biogeographic processes would not cease

Fig. 31.2
figure 2

Photographs illustrating potential futures of biological invasions in South Africa taken from the Western Cape in areas close to Stellenbosch. In the panels next to each photograph, the outlines of different species or vegetation types are numbered, and coloured according to whether they represent native, alien, or cultivated. (a) South Africa is transformed to a novel ecosystem composed largely of alien species (“Collapse but no return to a Garden of Eden”). (1) Acacia saligna, (2) Pinus sp., (3) Leptospermum laevigatum, and (4) a Restionaceae. The photo was taken near Betty’s Bay by Brian van Wilgen. (b) The landscape is composed of a matrix of utilised areas and invaded areas that have little if any conservation value in terms of native species (“New Pangea”). (1) Acacia saligna; (2) Carpobrotus sp.; (3) a wheat field. The photo was taken on the Agulhas Plain by John Measey. (c) The landscape is composed of a matrix of utilised areas and areas that have significant conservation value in terms of native species (“Preserve or Use”). (1) A mix of vegetation types including Kouebokkeveld Shale Fynbos, North Hex Sandstone Fynbos, and Altimontane Sandstone Fynbos; (2) Eucalyptus sp. planted close to a homestead; (3) agricultural land; (4) Ceres Shale Renosterveld. The photograph was taken close to Ceres by John Wilson. (d) The unique vegetation of South Africa is conserved for future generations and natural per-human ecosystem processes are allowed to continue (“Conservation Earth”). (1) Agulhas limestone fynbos; (2) Protea compacta in flower; (3) Leucadendron sp.; (4) a Restionaceae. The photograph was taken near Betty’s Bay by Brian van Wilgen