Keywords

1 Introduction

Mountains in South Africa (SA) support critically important ecosystem services (ES) – notably water production – and are exceptionally rich in floral and faunal biodiversity and endemics (Mucina & Rutherford, 2006). For example, almost the entire Cape Fold Mountains falls within the Fynbos biome which is a biodiversity hotspot (Goldblatt & Manning, 2000), while most of the eastern Great Escarpment falls into the endangered Grassland Biome (Mucina & Rutherford, 2006). Yet these montane habitats are often marginalised regions, and are under threat from detrimental land-uses, unsustainable use of natural resources, climate change, poor governance, and invasive alien plants (IAPs) (Clark et al., 2019). Invasive alien plants are direct drivers of change in these ecosystems and threaten their role in the provision of nature’s contributions to people (NCP). Yet the extent to which mountains have been invaded has not been accurately assessed. This lack of knowledge is a reflection of the fact that SA’s mountains have been poorly studied. No mountain range in southern Africa has been comprehensively assessed for the purposes of valuing ecosystem services (ES), assessing threats or making policy recommendations (Clark et al., 2019).

While IAPs are known to have ecological consequences in adventive ranges, they are often deliberately introduced for their material or cultural benefits to communities. This is particularly true for mountain areas whereby given the socio-economic conditions, people are often more reliant on the natural resources around them. In light of the potential conflicts of interest, there is a need to expand research so that management interventions are guided by accurate information from both ecological and social perspectives. This chapter explores available information by (1) first describing the mountainous areas of SA, (2) discussing the most abundant IAPs in these areas, (3) outlining their major impacts on ES, and (4) addressing management challenges.

2 Defining Mountain Areas

South Africa is dominated by an extensive interior plateau >1200 m (3,900 ft) above sea level (asl) that covers c. 40% of its surface area; this interior plateau is bounded to the west, south, and east by the 5000 km-long southern African Great Escarpment (Clark et al., 2011), reaching its highest elevations in the Maloti-Drakensberg (3450 m, Mafadi Peak). In the south-western part of the Cape region, rugged fold mountains dominate the landscape (reaching 2325 m in the Klein Swartberg Peak); these are situated 100–150 km south of the southernmost section of the Great Escarpment. Mountains in SA reach relatively low elevations compared to other regions globally (Clark et al., 2011). There are six major mountain ranges in SA with the Great Escarpment and the Cape Fold Mountains together providing most of the mountainous terrain (some 226,061 km2, or 18.5% of the country), while smaller ranges (e.g. Magaliesberg, Soutpansberg, Waterberg, Witwatersrand; elevation rarely exceeds 1800 m) occur as linear outliers in the north.

For the purpose of this chapter, we delineated these mountain areas using a combination of the Topographical Positional Index (an algorithm used to measure topographic slope positions and to automate landform classifications) and roughness surfaces (Shepard et al., 2001); these were used to produce mountain area layers in ArcMap 10.3 based on altitude (Fig. 14.1).

Fig. 14.1
figure 1

The six major mountain ranges in South Africa. Black shading superimposed over colours indicates areas with the highest elevation of >1600 m. The Western Great Escarpment (i.e. Richtersveld and Namaqualand) are arid, receive winter rainfall, and occur in the Succulent Karoo Biome. The Southern Great Escarpment (Hantam–Roggeveld and Nuweveldberge) is arid to semi-arid winter, receives rain throughout the year and occurs in the Nama Karoo Biome. The Eastern Great Escarpment (Sneeuberg to Wolkberg) receives rainfall mainly in summer, and mesic grasslands and Afro-temperate forest are the typical vegetation. The Cape Fold Mountains (Cederberg to Makhanda), with predominantly winter rainfall, occurs mainly in the Fynbos Biome, but some areas are covered by Afro-temperate forest. The Sub-tropical/tropical Cuestas (Witwatersrand, Magaliesberg, Waterberg, Soutpansberg), receives summer rainfall and occurs in the Savanna Biome. The Central Griqualand Mountains receives sparse rainfall in summer, is arid, and forms a marginal part of the Savanna Biome

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3 Invasive Alien Plants in Montane Areas

There has been a paucity of IAP monitoring targeting mountains specifically in SA. Consequently, current estimates largely rely on national monitoring surveys that often do not fully encompass the more inaccessible mountain areas and thus underestimate their true extent. We collated the IAPs recorded within the mountain areas using ArcMap 10.3. Records were obtained from the two most comprehensive occurrence species datasets available– the Southern African Plant Invaders Atlas (SAPIA) and iNaturalist records – for both high and low elevation mountain areas (Fig. 14.2). The SAPIA and iNaturalist databases vary in how records are obtained, being roadside surveys (Henderson, 2007) and citizen science observations, respectively (Unger et al., 2020). These different approaches are reflected in the variation in IAP records between the databases.

Fig. 14.2
figure 2

The most abundant IAPs in high-elevation (>1600 m) and low-elevation (between 1000–1600 m) mountain areas in South Africa (see Fig. 14.1 for outline of high elevation mountain areas). Records were obtained from two databases: the Southern African Plant Invaders Atlas (SAPIA, downloaded: 2018) and iNaturalist records of naturalised plants (downloaded: 16th February 2021). Records from iNaturalist were filtered to only include Research Grade observations that were indicated to be out of cultivation

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Despite the variation in IAP species and their order of abundances between the databases, they both reflect the commonalities in the types of invasions occurring in mountain areas. Both databases show a higher number of IAPs in the lower-elevation mountain areas with an average of double the records on SAPIA and 65% of the records in iNaturalist. In all montane areas, trees and shrubs make up the majority of IAPs. This woody densification is occurring across most ranges in SA, particularly from fire-driven or fire-tolerant species such as Cluster Pine Pinus pinaster and Black Wattle Acacia mearnsii.

A different suite of IAPs dominate the higher and thus moister and colder montane areas (>1600 m), mostly comprising of the eastern Great Escarpment and higher reaches of the Cape Fold Mountains (Figs. 14.1 and 14.2). High-elevation areas have distinct environmental conditions such as large temperature fluctuations, higher rainfall, and the occurrence of freezing conditions, including on occasion snow (Henderson, 2007; Mucina & Rutherford, 2006). These features are favourable only to certain types of IAPs that can withstand such extremes, thus excluding many common lowland tropical species. For example, a number of invasive Rosaceae species – including Orange Firethorn Pyracantha angustifolia, Nepalese Firethorn P. crenulata, Orange Cotoneaster Cotoneaster franchetii, and Rosehip Rosa rubiginosa – are generally more abundant (or even restricted to) high-elevation areas, where extended minimum winter temperatures are needed to trigger flowering, fruiting, and seed germination.

4 Invasive Alien Plants and Their Impact on Nature’s Contribution to People

The IAPs that are most abundant in SA’s mountain areas are largely trees and shrub, as discussed in the previous section (Fig. 14.2). The establishment of these woody species both threaten and support ES and human well-being for both local communities in montane areas, and those in nearby lowland areas.

The increasing encroachment of woody growth forms is a direct driver of change in these naturally open-habitat ecosystems. In montane grasslands, landscapes are transformed into dense woody thickets; in montane fynbos, woody IAPs outcompete the shorter fynbos shrubland through their sheer adult size and numbers. These IAPs are therefore exploiting a missing ecological niche, which can result in displacing local species and transforming the vegetation structure (O’Connor & Van Wilgen, 2020).

The transformation of montane areas can result in a loss of ES and thus community access to food, natural medicines and fibre, firewood, building materials, and agricultural productivity that supports the livelihoods and economies of commercial, small-scale and communal farmers (Shackleton et al., 2007). The most concerning ES at risk is the impact of woody encroachment on SA’s water security, as these areas are the source regions for the country’s river systems. Woody species, especially those in the genera Acacia, Eucalyptus, Pinus and Populus, can alter the hydrology of the watercourses and reduce streamflow (Le Maitre et al., 2020). It is estimated that IAPs in mountain catchments consume more than 4% of all registered water use; if left uncontrolled this figure might become as high as 16% (Blignaut et al., 2007). In addition, IAPs can undermine water quality, thus increasing water purification costs and directly assaulting human health (Chamier et al., 2012). Water is becoming a limiting factor to development in SA, with an average precipitation of approximately 500 mm/annum, well below the world average of about 860 mm/annum (DWAF, 2013). Furthermore, SA shares its major mountain range, the Maloti-Drakensberg, with the Kingdom of Lesotho and most of the water reserve lies within the latter’s territory (Hoag, 2019). The potential for IAPs to reduce the country’s already limited water resources is therefore a major regional geopolitical issue (see case study of the Manica Highlands, in Clark et al. (2019)).

Invasive alien plants can also hold benefits for local people, particularly in low income, natural resource dependent communities. Most of these species were deliberately introduced for their provisioning services such as for food, aesthetic value, building material, medicine and fuel (Shackleton et al., 2007). For example, several Acacia species are widespread invaders with little economic importance yet are still widely used for firewood and construction material in marginalised rural areas (Kull & Rangan, 2008). Further, mountain harvested plants are an important feature in the muthi trade (traditional medicine) and many alien plants have been culturally integrated into these practices. Communities also benefit from their regulating and cultural services including shade and erosion control. We discuss in the case study an example of invasive Rosaceae that are largely restricted to high elevation areas whereby their invasions outline species that are providing resources but also give rise to disservices to these areas.

4.1 Case Study: Invasive Rosaceae

Several Rosaceae species from the northern temperate regions of the world are becoming increasingly invasive in mountainous regions of SA, particularly the Eastern Great Escarpment. Most were introduced through the horticultural industry, as they were highly regarded for their ability to withstand cold temperatures, and for their aesthetics including displays of red, yellow, and orange berries in autumn and winter months. These berries are now fuelling their invasion; Chari et al. (2020) showed that invasions of Orange Firethorn could be producing up to five million seeds per square meter of invaded land per annum.

South African Afromontane grasslands are typically poor in fleshy-fruited plant species, and the berries of alien Rosaceae species are particularly attractive to frugivorous birds and small mammals which facilitate their spread, not only through dispersal but also through enhanced germination rates. The spread of invasions have been rapid; Orange Firethorn only began invading the Grassland Biome in the early 1980s and is already one of the most widespread and abundant IAPs in the biome (see Fig. 14.3 for images of a typical invasion; Chari et al., 2020).

Fig. 14.3
figure 3

A Rosaceae invasion (Cotoneaster, Pyracantha, and Rosa spp.) in the Clarens Nature Reserve, Free State province (part of the Eastern Great Escarpment mountains) showing (a) urban area at base of mountain with cultivated Rosaceae, (b) plants escaping cultivation and moving up the slopes of the mountain and (c) an established stand forming natural monocultures in the damp ravines. (Photos taken by K. and S. Canavan)

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Unlike the situation with other tree invaders such as wattles (Acacia spp.), for which impacts have been relatively well studied in SA (Le Maitre et al., 2011), the impacts of invasive Rosaceae are still being assessed. Current evidence shows that the impacts in invaded ecosystems are likely to be significant, both environmentally and economically (Martin, 2021a). However, in the high-elevation regions of SA, natural resources are limited and some Rosaceae species contribute provisioning ES for the communities living there. Feral rosehip for example, is harvested by rural communities and sold to private companies for the global food and herbal tea market. The rosehip market is a substantial and important economic component for communities with over 5000 tons of rosehip harvested annually, providing informal income for approximately 30,000 people in SA and Lesotho (Martin, 2021a). Introduced Rubus spp. (brambles and blackberries) also contribute to sustaining livelihoods, including harvesting the wild fruits for commercial uses, supplementing diets, and generating income in rural communities where it is sold for use in small scale secondary industries. The use of these IAPs poses challenging conflicts of interest for landowners and conservationists (Zengeya et al., 2017).

Management is often required for these introduced Rosaceae species, particularly when infestations impede on the provision of other ES such as when limiting access to arable land. However, many invasive populations occur on steep slopes and dangerous terrain where conventional control methods are difficult and expensive to implement. Where access is possible, some of these species form dense natural monocultures that hamper control efforts (Martin, 2021a). Careful evaluations need to be conducted for each species, as some may be important components to communities living in these resource-limited environments (see multi-valued based approaches in discussion).

5 Management Challenges

Invasive alien plants have been described as a wicked problem (Woodford et al., 2016) as they can have both positive and negative impacts on ES and the stakeholders who benefit from them. While it is clear that increasing woody densification threatens montane ecosystems, it is also imperative to explore how these plants have been integrated into these communities. Such stakeholder engagement needs to be considered and unified with any management interventions. Here we discuss management of IAP in mountain areas according to (1) what legislation is currently available, (2) multi-valued based approaches to management, (3) which nature-based solutions could be considered, and (4) current implementation programmes.

5.1 Legislation

The SA government has instituted several initiatives for sustainable development, yet there is almost no mention of mountain areas in these policies. The ‘invisibility’ of mountainous areas in the environmental policy domain is probably because they cover a fairly small part of the total territory, have small human populations, and are thus of limited political importance (Browne et al., 2004). However, with growing recognition of their value, particularly for water security, there has been increased investment into managing these areas. In the 1970s the Mountain Catchment Areas Act (Act 63 of 1970) was published (Van Wilgen et al., 2020). This Act was intended to protect mountain catchments by authorising the destruction of alien vegetation within five kilometres of a boundary of a mountain catchment area (Van Wilgen et al., 2020). Since then, the management of IAPs has also been addressed through the national alien-plant control programme known as Working for Water (WfW) under the Department of Environment, Forestry and Fisheries: Natural Resource Management Programme (DEFF: NRM) (Bennet & Van Sittert, 2019).

5.2 Multi-Valued Based Approaches to Management

The overall aim of any environmental management intervention is to improve the sustainability and resilience of target systems to ensure that they continue to deliver key ES. Mountain areas in SA have distinct socio-economic characteristics, especially with respect to poverty indicators. Although they house a small proportion of the total human population of the country, those communities are generally poorer, geographically isolated from resources and markets than those of most other areas and have lower political influence (Browne et al., 2004). Communities in mountains, unlike those in higher-income areas, do not often have economic diversification and still are highly reliant on provisioning ES. Strategies for controlling IAPs in these regions must recognise that some IAPs provide critical resources and this needs to be accommodated to achieve sustainable solutions (Linders et al., 2021).

The value or worth of an IAP can vary over time and between cultures. Given this interplay of multiple perspectives by different stakeholders, invasion biology has been met with problematic circumstances and in some cases direct public opposition (Zengeya et al., 2017). Such social disagreements can lead to obstruction in control efforts and demonstrate the practical need to ensure public support for successful management projects. Value-based conflicts are generally challenging to resolve (Estévez et al., 2015). Multi-value based approaches should be applied to invasive species management such as the structured decision-making process (Liu et al., 2012) and the ecosystem service multifunctionality approach (Manning et al., 2018). These strategies promote the identification of each stakeholder’s objectives and potential synergies which in turn are related to ecological models and management alternatives. Such approaches share a commonality in the central role of traditional ecological knowledge in the management of ecosystems (Dean et al., 2021) and strive to maximise the multiple benefits of the invaded landscape to help communities to derive requisite benefits. Selection of appropriate control options is carried out to ensure sustainable use of multiple ecosystem services and to provide meaningful outcomes.

5.3 Nature-Based Solutions

Invasive alien plants in high-elevation areas typically occupy steep slopes and inaccessible terrain which often makes conventional control methods dangerous or impossible (e.g. see Van Wilgen & Richardson, 2012 for discussion of problems in this regard for invasive pines in mountains). One area that offers promise and reflects a nature-based solution, is in the adoption of biological control that provides an effective, sustainable and herbicide-free management option. This approach is ideal for mountain systems that are inaccessible to clearing efforts. The Northern Temperate Weeds programme was established in 2017 with the aim of targeting Northern temperate weeds that are common, widespread and problematic primarily in high-elevation mountains of SA for biological control. The programme has also helped establish an IAP working group for southern Africa mountains with the aim of bringing together interested and affected parties to both improve collaboration and coordinate management efforts (Martin, 2021b).

5.4 Implementing Programmes

Invasive alien plant management may benefit from the strong momentum being generated by NGOs around stewardship programmes in SA mountains. Although often appealing to private landowners and traditional authorities as a mechanism to exclude commercial afforestation programmes and mineral exploitation on their land, these stewardship programmes also offer increased incentive for co-ordinated IAP management in these areas. Such IAP programmes increase the ecological value of the land, and make the future deproclamation of such Protected Environments more difficult to motivate for (by e.g. mining interests). Examples of such stewardship programmes in SA mountains are the Ekangala Grasslands Project (Carbutt et al., 2008) and the Upper Umzimvubu Watershed (with an active IAP control programme) (CEPF, 2017).

6 Lessons Learnt

Invasive alien plants will continue to expand into montane areas and become endogenous pressures on these mountain systems that can alter quality of life. For example, Carbutt (2012) performed an early detection study in the Drakensberg Alpine Centre of the eastern Great Escarpment and found 23 emerging IAPs that are likely to become increasingly problematic. Yet many of these IAPs also provide material benefits that contribute to local economies and livelihoods. Due to the multiple complex interlinkages between IAPs as drivers of change, ES, and human well-being in mountains; greater collaboration across humanities, social sciences and natural science is needed (Martín-López et al., 2019).

These invasions are probably being driven by continued habitat degradation and climate change, as well as that they have not saturated their total potential area of invasion. Establishing research priorities for SA mountains in order to untangle the mechanisms driving IAP spread is essential, so that these outcomes can feed into policy, and align with broader habitat protection goals. For example, increasing the rollout of the Mountain Invasion Research Network’s (MIREN) long-term monitoring protocols (Kueffer et al., 2014) in SA mountains would be a valuable start. More emphasis on transboundary collaborations – particularly between SA, and the Kingdoms of Lesotho and Eswatini – will help improve our understanding of the movement of IAPs in southern African mountains (both temporally and spatially), and help inform co-ordinated management strategies alongside the protection of livelihoods.