Megadiverse fynbos shrublands to invasive wattle tree monoculture
The southwestern tip of Africa is home to the Fynbos biome (Cape Floristic Region), which is characterised by highly diverse plant groups, many of which are endemic and occur nowhere else in the world. Numerous species of Australian acacia (wattles) were introduced to South Africa for multiple reasons, including sand/dune stabilisation, ornamental purposes, and forestry. Many species have subsequently naturalised and some are widespread invaders. The Australian wattles have filled an empty niche (trees in a virtually treeless system) causing a regime shift. This shift has induced many negative impacts to the social-ecological system in the area. These include alterations to fire and hydrological systems, changes in soil nutrient cycles, biodiversity loss, and negative impacts on local livelihoods and human well-being through loss of grazing, water supply, ecotourism and increased exposure to natural hazards. Ongoing management interventions include mechanical and chemical control and the use of biological control agents.
Type of regime shift
- Introduction of aline species (Biological invasions)
- Mediterranean shrubs (egFynbos)
- Small-scale subsistence crop cultivation
- Large-scale commercial crop cultivation
- Extensive livestock production (natural rangelands)
- Timber production
Spatial scale of the case study
- Local/landscape (e.g. lake, catchment, community)
Continent or Ocean
- Western Cape, South Africa
- South Africa
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Key direct drivers
- Species introduction or removal
- Small-scale subsistence crop cultivation
- Large-scale commercial crop cultivation
- Extensive livestock production (rangelands)
- Timber production
- Mediterranean shrubs (eg Fynbos)
Key Ecosystem Processes
- Soil formation
- Primary production
- Nutrient cycling
- Water cycling
- Food crops
- Wild animal and plant products
- Fuel and fiber crops
- Climate regulation
- Water purification
- Water regulation
- Regulation of soil erosion
- Pest & Disease regulation
- Natural hazard regulation
- Aesthetic values
- Food and nutrition
- Livelihoods and economic activity
- Security of housing & infrastructure
- Cultural, aesthetic and recreational values
- Cultural identity
- Social conflict
Spatial scale of RS
Time scale of RS
- Contemporary observations
Confidence: Existence of RS
- Well established – Wide agreement in the literature that the RS exists
Confidence: Mechanism underlying RS
- Well established – Wide agreement on the underlying mechanism
Megadiverse shrubland (Fynbos biome)
In its natural state the fynbos biome is dominated by shrublands. These shrublands have extremely high plant diversity and endemism and the region is listed as one of the world’s biodiversity hotspots. Soils are acidic and nutrient poor and intense fires at intervals of 5-15 years are a key driver of ecosystem dynamics (Allsopp, 2014).
Invasive tree (Australian wattle) monoculture
The introduction, naturalization, and invasion of Australian wattle (Acacia) tree species, especially Acacia cyclops, A. longifolia, A. saligna A. mearnsii in the fynbos biome, has resulted in a shift from a megadiverse shrubland vegetation to landscapes dominated by monocultures of wattles and other invasive trees and shrubs, with the absence or significantly reduced abundance of native species. This has resulted in reduced water flow, altered fire regimes (increased frequency and intensity), and changes in soil nutrient cycles (changes in pH, elevated soil nutrient levels, and the addition of new biotic interactions, e.g. mycorrhiza).
Drivers and causes of the regime shift
The main driver of the regime shift is the introduction of non-native trees into a virtually treeless environment. In South Africa, numerous Australian wattle species were introduced for a variety of reasons, inlcuding dune stabilisation, agroforestry, tannin production and as ornamental trees. These trees escaped from their planted areas and have become widespread in the environment (invasive) (van Wilgen et al. 2011). The invasion process can take a long time from years to decades and is often dependent on the reason for introduction (dune stabilisation vs. ornamental purposes) and the propagule pressure (number of seeds introduced) (Blackburn et al. 2011).
Land transformation, degradation and climate change facilitate the establishment and spread of invasive species, and enhance their competitive advantage over some native species. Forestry, as well as urbanisation in the Cape which required dune stabilisation, are important indirect drivers, as they led to the introduction and spread of Australian wattle species.
How the regime shift worked
(1) The megadiverse fynbos shrubland (Mediterranean type vegetation), was maintained by a 5 to 15-year fire cycle, nutrient-poor soils, and biotic interactions.
(2) Wattle species were transported out of Australian and planted all over the Cape for forestry, dune stabilisation and ornamental purposes and many survived in this new region. With time many species became naturalised meaning they established small, localized, self-sustaining populations. In time, these naturalised populations then spread over large areas, became dominant and thus invasive. Fulfilment of an empty niche, release from natural predators and superior competitive abilities enabled these wattles to become invasive. Millions of hectares are now covered by wattle species in the fynbos biome and these invasions have dramatically altered the landscape structure and many ecosystem processes.
Several feedbacks maintain the invaded regime and have long-lasting legacy effects, making it difficult to reverse the shift. Massive production of long-lived seeds that accumulate in the soil allows for the dominance of these wattles over time. Wattles also have a high competitive ability allowing them to outcompete native fynbos species, in that they fill an empty niche (trees introduced into shrublands) and have been released from their natural enemies (enemy relsease hypothesis). Furthermore, wattles alter soil nutrient levels to suit their growth which reinforces their dominance. They also alter fire regimes which help to promote their germination and growth, while having negative effects on the growth and survival of native species. Wattles further alter biotic interactions, such as the presence of soil bacteria, which facilitates their growth and survival.
Australian wattle invasion outcompetes native plants, resulting in losses of biodiversity and associated livelihood practices associated. Altered fire cycles, changes in soil nutrients and altered water flow as a result of the invasion inhibit native species growth and induce long-lasting legacy effects that thwart post-management recovery and reduce options for restoration (Le Maitre et al. 2011 Gaertner et al. 2014).
Impacts on ecosystem services and human well-being
This regime shift has many negative as well as some positive impacts on ecosystem services. Negative effects on provisioning services include loss of water delivery from catchments, reduced agricultural potential, and the loss of various provisioning ecosystem services (non-timber forest products). Wattles grow in very high densities and suck up large amounts of surface and ground water that negatively affect dam levels (Le Maitre et al. 1996). Thick invasive stands of wattles are impenetrable and outcompete palatable grass and shrub species thus reducing grazing capacity. Wattle invasions also displace native plants species that are harvested such as flowers from Protea species and plants (Restio) used for thatching (Tupie et al. 2003).
Key regulating services that are negatively impacted are fire regimes, nutrient cycles, hydrological and biotic interactions. Wattles increase fire intensity and frequency having negative effects on biodiversity, soil and humans though increasing the chances of natural hazards (Gaertner et al. 2014). Wattles change soil nutrients through increased litter loads and novel interactions with soil bacteria (Le Maitre et al. 2011). Wattles also alter micro climates that have negative effects on native species. High densities of wattles in narrow catchments can also increase flood and erosion risks, and reduce water purification services by increasing flow rates. Furthermore, they compete for pollinators with native plant species.
These invasions also have negative implications for cultural services, including recreational, aesthetic and spiritual values. For example, wattles invasion reduces access to sacred pools which are culturally and spiritually important for the Xhosa people (Shackleton et al. 2007). Thy also invade areas important for ecotourism and hiking thus negatively affecting recreation and aesthetic values.
Some benefits from shifts to invasive monocultures are the increased availability of fuelwood (energy), dune stability and aesthetic values. Many invasive wattle species are still planted on properties to provide shade and beautification. Fuelwood is harvested by rural poor communities, and some people even make a business out of selling wattle fuelwood in urban areas. Furthermore, in many areas, sand stabilisation due to wattle invasion has increased the availability of land for building, especially in the flat areas around Cape Town.
The negative impacts on ecosystem services have substantial implications for local livelihoods and human well-being. These include losses of income/economic productivity due to reduced cropping and livestock production, reduced water supply, losses in ecotourism, and loss of non-timber forest products (Shackleton et al. 2007). This in turn has implications for food security. Wattle invasion also increase peoples exposure to natural hazards and disasters (fires, droughts, floods), which can impact housing and infrastructure. Dense stands are used as hideouts by criminals and therefore create safety issues for local communities. Wattle stands also reduce access to areas of cultural importance. On the other hand, some people benefit culturally through aesthetic value from garden trees and directly through the use and sale of fuelwood, and the ability to build houses on stabilised soils. Others make a profit through using wattle species for forestry. The tradeoff between benefits and costs often lead to conflicts of interest between different stakeholders.
Numerous actions can be taken to prevent the shift. This includes preventing both purposeful and accidental introduction of wattle species. This can be done through border checks, risk assessments and ensuring vectors of spread are managed (e.g. imported equipment is washed down or sterilised). Furthermore, there are several Australian wattle species that are currently in the naturalised phase (only present in small self-sustaining populations). They should be targeted for eradication before they become invasive and induce shifts and negative impacts on ecosystem services and human well-being. This is done through monitoring land areas to find naturalised populations, and where present using manual and chemical clearing methods to remove them. Follow up clearing will have to be conducted for a number of years afterwards until the seed bank is depleted.
Numerous management interventions are currently available and being implemented to reverse and reduce the impacts of the shift (van Wilgen et al. 2011). This includes mechanical and chemical control by private land owners and the state run poverty relief Working for Water (WfW) program (van Wilgen et al. 2012). The WfW program is a sate rung initiative that aims to restore ecosystem services and landscapes after invasions and to employ rural communities. Mechanical and chemical control involves cutting down and pulling out wattle trees. Cut stumps need herbicide to prevent coppicing and seedlings can be killed with foliar spray. Biological control is also being utilized and is showing success. Biological control is the introduction of natural enemies (insects, diseases, fungi) of the invasive species from their native range. Many biological control agents for Australian acacia species have been released in South Africa, to target seed production and increase tree mortality. Some have taken well and species like A. cyclops are reducing in extent.
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