South Africa has a long history of managing biological invasions. The rapid increase in the scale and complexity of problems associated with invasions calls for new, more strategic management approaches. This paper explores strategic management approaches for cactus invasions in South Africa. Cacti (Cactaceae) have had a long history of socio-economic benefits, considerable negative environmental and socio-economic impacts, and a wide range of management interventions in South Africa.
To guide the future management of cactus invasions, a national strategic framework was developed by the South African Cactus Working Group. The overarching aim of this framework is to reduce the negative impacts of cacti to a point where their benefits significantly outweigh the losses.
Four strategic objectives were proposed: (1) all invasive and potentially invasive cactus species should be prevented from entering the country, (2) new incursions of cactus species must be rapidly detected and eradicated, (3) the impacts of invasive cacti must be reduced and contained and (4) socio-economically useful cacti (both invasive and non-invasive species) must be utilised sustainably to minimise the risk of further negative impacts.
There are currently 35 listed invasive cactus species in the country; 10 species are targeted for eradication and 12 are under partial or complete biological control. We discuss approaches for the management of cactus species, their introduction and spread pathways and spatial prioritisation of control efforts.
A thorough understanding of context-specific invasion processes and stakeholder support is needed when implementing strategies for a group of invasive species.
Biological invasions need to be appropriately managed to prevent and reduce negative environmental and socio-economic impacts (Simberloff et al.
A useful approach to strategic planning for biological invasions is to jointly consider groups of species with similar management requirements (van Wilgen et al.
This article explores strategic management planning using the family Cactaceae in South Africa as a case study. Cacti form a distinct taxonomic group that, with a few exceptions, share similar physiological traits, habitat preferences, spread pathways and negative impacts (Novoa et al.
Cacti are among the most widespread and dominant groups of invasive plants in South Africa (Nel et al.
There is also a long history of cactus management in South Africa (Zimmermann, Moran & Hoffmann
Management decisions need to be based on a clear understanding of the underlying invasion processes involved. Here we discuss the distribution and abundance, the benefits and impacts, and the pathways of cacti in South Africa to provide suitable context for management planning.
An estimated 400 cacti taxa have been introduced to South Africa (Walters et al.
The current extent of cactus invasions in South Africa, showing (a) the distribution and species richness of listed invasive cacti per quarter degree grid cell (QDGC) and (b) range sizes of invasive cacti in South Africa.
Cacti have many important socio-economic benefits in South Africa. Around 300 species of cacti are imported to South Africa annually for ornamental horticultural purposes (Novoa et al.
Cacti also have substantial negative environmental and socio-economic impacts (Barbera, Inglese & Pimienta
There are several main pathways along which cacti are introduced and spread (intentionally or unintentionally) around South Africa (
Invasion pathways for cacti at different scales and recommended management interventions to limit introduction and spread of invasive species. Pathways are listed in descending order of ease of management.
Pathway | International scale | Regional scale | Local scale | Management interventions |
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Horticulture (legal) | Permitted import of seeds by wholesalers | Distribution of plants to towns through nursery industry | Informal trade at garden clubs, markets | Correct listing of prohibited species |
Engagement with nursery stakeholders | ||||
Enforcement of regulations | ||||
Food production | Import of spineless |
Distribution of |
Escape from cultivation and reversion to spiny forms | Biological and chemical control of escaped spiny plants |
Livestock farming (fodder) | Not applicable | Farmers share plant material for fodder production | Movement of plant material by animals. | Increased public awareness |
Enforcement of regulations | ||||
Livestock exclusion in invaded areas | ||||
Horticulture (illegal) | Import of seeds and live plants through online trade without permit | Illegal sale of plants in the nursery trade | Informal trade at garden clubs, markets | At-border screening of imports |
Exchange of plants among gardeners and succulent enthusiasts | Post-border inspections of nurseries and enforcement of regulations | |||
Improved public awareness of illegal cacti | ||||
Animal dispersal | Not applicable | Long-distance dispersal of seeds by birds | Movement of plant material and seeds by animals, for example, elephants and baboons | Contain spread by detecting and removing outlier populations |
Abiotic dispersal | Not applicable | Not applicable | Dispersal of seeds by water and wind | Contain spread by detecting and removing outlier infestations |
Almost all contemporary introductions of cacti are via the horticultural trade to meet a growing demand for ornamental cacti. The horticulture pathway has contributed the most invasive cacti to South Africa (Walters et al.
Illegal horticultural introductions of invasive species are more challenging to manage. Most cacti are imported as seed, which makes detection of illegal imports difficult. Screening seed imports by seed size has recently been proposed as an accurate method of discriminating invasive and potentially invasive from non-invasive cacti: larger seed size is correlated with invasiveness (Novoa et al.
A source of local scale spread of invasive cacti is the utilisation of certain opuntioid cacti for food production and livestock fodder. In the majority of cases, spineless cultivars are used although there is a risk of reversion to spiny populations (Flepu et al. n.d.), which increases the likelihood of escape and invasion. Escape from cultivation is facilitated primarily by animal dispersal of seeds and plant material through fruit consumption and vegetative propagation (Dean & Milton
In response to the need for strategic cactus management, a national working group (the South African Cactus Working Group; SACWG – see
Although strategic decision-making is overseen by the SACWG, which represents organisations that are mandated to regulate and control cacti, the outcomes will clearly affect multiple external stakeholders (
Stakeholder groups and their representative organisations involved in strategic planning and management of cactus invasions in South Africa.
To achieve such engagement in South Africa, all the stakeholders were invited to participate in a workshop with the aim of increasing awareness of different viewpoints and values associated with cactus impacts and benefits (Novoa et al.
A national strategic framework (
National strategic framework and key requirements for the management of cactus invasions in South Africa.
The next step was to determine what management action should be taken for each species. To do this, we developed a protocol with five endpoints (
Decision framework for setting species-based management goals for invasive cactus species.
Implementation of species-based management of cacti in South Africa.
Species-based goal/endpoint | Implementation | Actions to date in South Africa | Reference |
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Do nothing | Species with low risk of invasiveness are not regulated | Risk assessment of Cactaceae | Novoa et al. |
Prevention | Species with high risk of invasiveness or invasive elsewhere are prohibited | Risk assessment of Cactaceae | Novoa et al. |
Four invasive genera prohibited under NEM:BA regulations | |||
Eradication | New incursions of cacti and naturalised species with limited distributions are eradicated from the country | New incursions detected and recorded in SAPIA | Henderson |
Ten cactus species currently being assessed for eradication feasibility | Wilson et al. |
||
Containment | The spread of species with high risk of range expansion is stopped or slowed | No formal assessment of containment feasibility for cacti to date | |
Use of biological control to prevent seed set in some species | Paterson et al. |
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Impact reduction | Populations of widespread invasive species are reduced to tolerable levels | Biological control of 16 cactus species | Paterson et al. |
Strategies for integrated management of cacti in protected areas developed | Lotter and Hoffmann |
NEM:BA,
Unlike within other taxonomic groups (e.g. Australian acacias, van Wilgen et al.
Preventing introductions of high risk species is an important and often highly cost-effective step in reducing the potential impacts of invasions. Risk assessment is needed to distinguish species that pose significant invasive threats from those species that are safe to utilise. All invasive or potentially harmful species should be prohibited. A global assessment of the Cactaceae by Novoa and colleagues (
Given the potentially large invasion debt of cacti in South Africa, new instances of naturalisation are likely to occur. New incursions of cacti should be eradicated where feasible. Feasibility of eradication is assessed on an individual species basis and broadly depends on reproductive and dispersal characteristics and the eradication effort required relative to available resources (Panetta
Species for which eradication is not feasible should be controlled by containment of spread and reduction of negative impacts. Feasibility of containment is also assessed on a species-by-species basis. Containment is considered only for those species that do not occupy their full potential invasive range in South Africa (based on bioclimatic models) and which have the ability to spread. Preventing further range expansion of cacti or slowing the spread would involve stopping seed production through biological control, and setting up barrier zones around existing infestations and regular monitoring of these zones to detect and remove extra-limital incursions (Sharov & Liebhold
Effectiveness of control of listed invasive cacti in South Africa.
Variables | NEM:BA category |
Total | ||
---|---|---|---|---|
1a | 1b | 2 | ||
Taxa with registered herbicides | 0 | 8 | 0 | 8 |
Taxa under complete biological control | 0 | 7 | 0 | 7 |
Taxa under partial biological control | 3 | 8 | 0 | 11 |
Taxa where the only current management option is physical removal | 7 | 2 | 1 | 10 |
10 | 25 | 1 | 36 |
NEM:BA,
Impact reduction is an appropriate goal for widespread, dominant cactus species. Thresholds for impact tolerance may vary by land use type because of relative susceptibility to impacts. For example, protected areas and rangelands will likely have a much lower tolerance to impacts than transformed or urban areas. Consequently, populations will require different management interventions to reduce densities to the required maintenance levels, although further research is needed to determine optimal maintenance levels under different land use scenarios.
A highly effective management tool for reducing the impacts of cacti is biological control (McFadyen
Introduction and spread pathways must be identified and prioritised to prevent and contain the impacts of cacti (
Human-mediated pathways (including both intentional and unintentional introductions) can be managed more strategically. The two broad human-mediated pathways of intentional cactus introductions and dissemination in South Africa are horticulture and agriculture (including legal and illegal trade). To ensure that no prohibited cacti are introduced intentionally by growers through the legal cactus trade, the existing permit-regulation process needs to be well managed. However, intentional illegal trade of cacti is facilitated mainly by international online trading (such as Ebay.com) where suppliers are not necessarily held to the same import regulations as ‘legal’ importers (Humair et al.
We believe that legal horticultural trade of cacti would be relatively easy to regulate through increased awareness of prohibited and regulated cacti among importers and their international suppliers (Novoa et al.
Because of lack of funding and capacity for managing invasions, often species cannot be managed across their entire invasive range, particularly those species that are very widespread. An alternative is to spatially prioritise management efforts to areas where the majority of impacts or potential impacts are likely to occur (Downey et al.
Priority areas for cactus management in South Africa based on assets most vulnerable to the negative impacts of cactus invasions.
For simplicity of management, prioritisation should occur at the scale of land ownership, for example, individual farms or nature reserves. Further research on fine-scale impacts and local invasion hotspots would assist in refining priority areas to enable better allocation of resources. If available resources are insufficient, further prioritisation and trade-offs will need to be made to ensure that strategic objectives are being met at a national scale (van Wilgen et al.
Strategic management needs to incorporate a means of monitoring and assessing the efficacy of strategies towards achieving the desired outcomes. An effective monitoring programme should assess the accuracy of the problem definition, audit the achievement of goals and provide feedback to evaluate policy (Rogers & Biggs
Indicators of progress towards achieving the national strategic objectives for cactus management in South Africa.
Management objective | Indicator | Action |
---|---|---|
Prevention of invasive cactus species | Number of new imports of potentially invasive species | Refine risk assessments based on current data |
Sustainable utilisation of cacti | Number of instances where nursery industry does not comply with invasive species policy | Audit nursery compliance with NEM:BA Alien and Invasive Species regulations |
Eradication of new incursions | New incursions of cacti detected before naturalisation or spread occurs | Active surveillance of cactus invasion hotspots |
Successful eradication of cactus species | Monitor progress of eradication programmes | |
Reduction of impacts of widespread invasive cacti | Stakeholder support for control of invasive cacti | Engage stakeholders in the management of invasive cacti |
Effective control methods are available for invasive species | Develop and test biological and chemical control for all listed invasive species | |
Stable and decreasing population densities | Monitor changes in population densities over time at fixed points |
NEM:BA,
For management to be adaptive, strategies must be based on clear evidence. To this end, the SACWG must ensure that management operations that are encompassed by this framework are well documented and aligned with current knowledge of cacti and best practice. This requires effective cooperation and collaboration among the partner organisations within the SACWG. Likewise, collaboration with relevant international experts is essential. Accordingly, several members of the SACWG are represented on an international cactus working group (
Importantly, this strategy requires ownership in order to ensure implementation and continuity over time. Ownership of the strategy should lie with the South African National Department of Environmental Affairs that represents NEM:BA and the Alien and Invasive Species Regulations.
Strategic management of invasions requires integration of pathway-, area- and species-based interventions. To explicitly incorporate these approaches into strategic planning and management objectives, a good understanding of invasion processes is necessary. We demonstrated this for cacti which have benefited from both a well documented history of management and a large body of research in South Africa. Unfortunately, this is not the case for many other groups of invasive species requiring management. In instances where data and knowledge are insufficient, the formation of taxon-specific working groups, such as the SACWG, is recommended to bring together stakeholders to build the expertise and knowledge necessary for strategic planning at a national level. Coordination and buy-in from stakeholders is essential for successful management of invasive species, especially at a national scale. We believe that future management of cacti in South Africa will be greatly enhanced through the adoption of this proposed strategic framework and with continual coordination and engagement between SACWG stakeholders. More work is needed to improve the framing of issues and problems relating to invasive cacti with ongoing consultation with all stakeholders to identify innovative solutions (Zengeya et al.
We thank members of the South African Cactus Working Group for their inputs into the development of the national strategic framework and for their useful insights and discussion. We also thank E. van Wyk, I. Nänni, T. Xivuri and J. Renteria for their useful comments on the manuscript.
Funding for this work was provided by the Department of Environmental Affairs, through the South African National Biodiversity Institute’s Invasive Species Programme. H.K. and L.H. thank the Agricultural Research Council for facilitating their involvement. D.M.R. acknowledges funding from the DST-NRF Centre of Excellence for Invasion Biology and the National Research Foundation (grant no. 85417).
The authors declare that they have no financial or personal relationship(s) that may have inappropriately influenced them in writing this article.
H.Kaplan led the development of the strategic framework with A.N. and J.R.U.W. H.Kaplan and A.N. wrote the article. All other authors contributed to the strategic framework and the article during workshops held by the SACWG. H.Klein H.G.Z. and P.M. made inputs regarding biological, chemical and integrated control of cacti. P.I. and D.M.R. provided guidance on conceptualisation of the strategic framework. L.H. provided distribution and abundance data for cacti.
The
Towards this end, SANBI initiated a South African National Cactus Working Group (SACWG) to strategically monitor and coordinate management of cactus species in South Africa. The role of the SACWG is to:
Develop a national cactus management strategy;
Co-ordinate nationally work done on cactus;
Assess the risks and management feasibility of cacti;
Ensure best practice control methods are used against target cactus species;
Improve co-ordination and communication among research institutes, invasive species managers and relevant government departments; and
Engage with external stakeholders.
Representatives from all relevant organisations involved in the management or research of cactus and invasive species policy-makers are included in the working group (
The SACWG convened in June 2012 to constitute itself. They will continue to meet biannually with SANBI serving as secretariat.
Member organisations of the South African Cactus Working Group.
Organisation | Relevant expertise | Web link |
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Department of Environmental Affairs – Environmental Programmes | Design and implementation of policies on alien and invasive species in terms of the |
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Department of Agriculture, Forestry and Fisheries – Directorate of Land Use and Soil Management | Design and implementation of policies on alien and invasive species in terms of the |
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South African National Biodiversity Institute: Invasive Species Programme | Detection and assessment of invasive species for eradication | |
DST-NRF Centre for Invasion Biology | Conduct research and development and training in biodiversity science especially as it applies to understanding the impacts of, and managing and preventing biological invasions | |
Agricultural Research Council – Plant Protection Research Institute | Research on the ecology and control of invasive alien plants in South Africa with emphasis on non-native problem plants in conservation and pasture situations. | |
South African National Parks | Management of invasive species in protected areas in South Africa |
Attendees of the South African Cactus Working Group Meeting of 25 April 2013.