Evidence, perceptions, and trade-offs associated with invasive alien plant control in the Table Mountain National Park, South Africa

The Table Mountain National Park is a 265 km2 protected area embedded within a city of 3.5 million people. The park contains an extremely diverse flora with many endemic species, and has been granted World Heritage Site status in recognition of this unique biodiversity. Invasive alien plants are arguably the most significant threat to the conservation of this biodiversity, and the past decade has seen the implementation of aggressive programs aimed at the removal of invasions by these plants. These invasive alien plants include several species of trees, notably pines (Pinus species) and eucalypts (Eucalyptus species), which historically have been grown in plantations, and which are utilized for recreation by the city’s residents. In addition, many citizens regard the trees as attractive and ecologically beneficial, and for these reasons the alien plant control programs have been controversial. I briefly outline the legal obligations to deal with invasive alien plants, the history of control operations and the scientific rationale for their implementation, and the concerns that have been raised about the operations. Evidence in support of control includes the aggressive invasive nature of many species, and the fact that they displace native biodiversity (often irreversibly) and have negative impacts on hydrology, fire intensity, and soil stability. Those against control cite aesthetic concerns, the value of pine plantations for recreation, the (perceived) unattractive nature of the treeless natural vegetation, and the (incorrect) belief that trees bring additional rainfall. The debate has been conducted through the press, and examples of perceptions and official responses are given. Despite opposition, the policy promoting alien plant removal has remained in place, and considerable progress has been made towards clearing pine plantations and invasive populations. This conservation success story owes much to political support, arising largely from job creation, and a strong body of scientific evidence that could be cited in support of the program

Challenges and trade-offs in the management of invasive alien trees

Over 430 alien tree species worldwide are known to be invasive, and the list is growing as more tree species are moved around the world and become established in novel environments. Alien trees can simultaneously bring many benefits and cause substantial environmental harm, very often leading to conflicts over how they should be managed. The impacts grow over time as invasions spread, and societal perceptions of the value of alien trees also change as understanding grows and as values shift. This leads to a dynamic environment in which trade-offs are required to maximise benefits and minimise harm. The management of alien tree populations needs to be strategic and adaptive, combining all possible management interventions to promote the sustainable delivery of optimal outcomes. We use examples, mainly from South Africa (where issues relating to invasive alien trees introduced for forestry have received most attention), to argue for holistic and collaborative approaches to alien tree management. Such approaches need to include bold steps, such as phasing out unsustainable plantation forestry that is based on highly invasive species, and in which the costs are externalised. Furthermore, it would be advisable to impose much stricter controls on the introduction of alien trees to new environments, so that problems that would arise from subsequent invasions can be avoided

The recent fire history of the table Mountain National Park and implications for fire management

This paper provides an assessment of fire regimes in the Table Mountain National Park over the past four decades. We compiled a GIS database of all fires between 1970 and 2007 and analysed the fire regime in terms of the frequency, season and size of fires and the relationship between fire occurrence and fire weather. Most fires (90.5% of area burnt) occurred in summer and autumn, the ecologically acceptable season for fires. However, mean fire return intervals declined by 18.1 years, from 31.6 to 13.5 years, between the first and last decades of the record respectively. The area subjected to short (≤ six years) intervals between fires covered > 16% of the park in the last two decades of the record, compared to ~ 4% in the first two decades. A relatively small number of large fires dominated in terms of area burnt. Of the 373 fires recorded, 40 fires > 300 ha burnt 75% of the area, while 216 fires < 25 ha burnt 3.4% of the area. Fires occurred under a wide range of weather conditions, but large fires were restricted to periods of high fire danger. Prescribed burning was a relatively unimportant cause of fires, and most (> 85%) of the area burnt in wildfires. Areas subjected to short fire return intervals should be considered for management interventions. These could include the re-establishment of extirpated fire-sensitive species, the clearing of invasive alien plants and increased precautions for the prevention or rapid suppression of future accidental fires.Centre of Excellence for Invasion Biolog

The costs and benefits of biological control of invasive alien plants in South Africa

This paper provides a brief review of the environmental and economic costs of invasive alien plants in South Africa as a background to assessments of returns on investment in weed biological control. The understanding of impacts and their economic costs is far from perfect, but estimates indicate that some costs (of lost water, grazing and biodiversity) are currently about R6.5 billion per annum (about 0.3 % of South Africa’s GDP of around R2000 billion: R7=about US$1), and could rise to >5%ofGDPif invasive plants are allowed to reach their full potential. By comparing the costs of biological control research and implementation to the benefits of restored ecosystem services, or avoided costs, and avoided ongoing control costs, biological control has been shown to be extremely beneficial in economic terms: estimated benefit:cost ratios ranged from 8:1 up to 3726:1. Currently, spending on biological control is far lower than on other forms of control (about 5 % and 14 % of that spent on mechanical and chemical control, respectively), despite the significantly better returns on investment from biological control. In aggregate these assessments indicate that higher levels of spending on biological control research would generate extremely attractive returns on investment.Centre of Excellence for Invasion Biolog

Reconstructing the spread of invasive alien plants on privately-owned land in the Cape Floristic Region: Vergelegen Wine Estate as a case study.

Invasive alien plants (IAPs) are a major threat to biodiversity in South Africa’s Cape Floristic Region (CFR). Activities like agricultural development, the establishment of forestry plantations and urbanization change environments and provide opportunities for IAPs to spread. We examine the factors that have contributed to plant invasions on a privately-owned farm in the CFR. The objectives of the study were: (1) to determine what land-use changes took place that could have influenced the spread of IAPs; and (2) to document the patterns of spread of IAPs. Aerial photographs from 1938, 1966, 1977, 1989 and 2004 were used to determine the types and rates of land-use change over time. Key land-use changes involved agriculture, commercial plantations, urban development and the spread of IAPs. The spatial analysis identified the main drivers of change. Agricultural development was the main land-use change driver and affected 40% of the study area by 1977. Areas occupied by IAPs increased with land-use change from 8% cover in 1938 to 40% in 2004, and then declined to 15% in 2013 following the initiation of control operations in 2004. The study illustrated a clear link between human activities, changes in land-use and the spread and proliferation of IAPs

An assessment of the distribution and potential ecological impacts of invasive alien plant species in eastern Africa

With a few exceptions, comprehensive lists of alien plants that invade natural ecosystems are lacking in sub-Saharan Africa. Some available lists are either preliminary or localised, or focus on agricultural weeds. This study set out to compile a list of alien plant species that are invading natural ecosystems and rangelands in five countries in eastern Africa, and to map the distribution of the species that threaten ecosystem integrity and productivity. The location of all alien plant species seen during surveys between 2008 and 2016 was recorded using a hand-held GPS device, as well as their status in terms of either being present and/or naturalised, or invasive and spreading. Individual occurrence records were summarised at the scale of half degree grid cells (∼55 km × 55 km). The survey covered almost half (522) of the 1063 grid cells in Ethiopia, Kenya, Tanzania, Uganda and Rwanda. We recorded 164 invasive alien species in 110 genera and 47 families. We provide further information on the distribution and impacts of 30 species considered to have the greatest impacts in terms of transforming natural ecosystems, as well as on a further 21 species with limited distributions that could potentially become ecosystem transformers. Invasive alien plants are clearly a widespread and growing problem in eastern Africa, and capacity to manage them effectively remains a problem. A great deal of work needs to be done to raise awareness of the problem, and to identify appropriate responses that will be effective in resource-poor countries

Re-establishment of Protea repens after clearing invasive Acacia saligna: Consequences of soil legacy effects and a native nitrophilic weedy species

Invasive Australian acacias can alter soil chemistry and microbial communities in areas they invade. After clearing invasive acacias, these changes can persist, and previously invaded areas can become dominated by nitrophilic weedy species. Restoration of viable native plant communities in cleared sites often fails due to a lack of native species re-establishment. Therefore, to improve restoration outcomes, it is important to understand the effects of soil chemical and biotic legacies, and of nitrophilic weedy species, on native species re-establishment. To investigate the effect of soil chemical legacies, we germinated and grew Protea repens seedlings (a native proteoid shrub) as an indicator species in soil taken from areas cleared of Acacia saligna in lowland fynbos, as well as from non-invaded areas under controlled conditions. To investigate the effect of soil biotic legacies, we sterilized half the soil from each cleared or non-invaded area. We grew Ehrharta calycina (a native nitrophilic weedy grass species) in half of each treatment and measured the effect of treatments on P. repens germination and growth. Germination percentage, root and shoot dry mass of P. repens did not significantly differ between altered and native soil chemistry. The germination percentage of P. repens was significantly greater (93%) in the presence of soil microbial communities than in their absence. The presence of E. calycina significantly increased (29%) the root-to-shoot ratio of P. repens than their absence. Since the legacy of altered soil chemistry did not have a direct negative effect on P. repens germination and growth; we conclude that restoration efforts do not always have to manage altered soil chemistry after clearing invasive A. saligna

Quantifying the social and economic benefits of the biological control of invasive alien plants in natural ecosystems

Invasive alien plants reduce ecosystem service delivery, resulting in environmental, economic and social costs. Here we review the returns on investment from biological control of alien plants that invade natural ecosystems. Quantifying the economic benefits of biological control requires estimates of the reductions in ecosystem goods and services arising from invasion. It also requires post-release monitoring to assess whether biological control can restore them, and conversion of these estimates to monetary values, which has seldom been done. Past studies, mainly from Australia and South Africa, indicate that biological control delivers positive and substantial returns on investment, with benefit:cost ratios ranging from 8:1 to over 3000:1. Recent studies are rare, but they confirm that successful biological control delivers attractive returns on investment, which increase over time as the value of avoided impacts accumulates

A biome-scale assessment of the impact of invasive alien plants on ecosystem services in South Africa

This paper reports an assessment of the current and potential impacts of invasive alien plants on selected ecosystem services in South Africa. We used data on the current and potential future distribution of 56 invasive alien plant species to estimate their impact on four services (surface water runoff, groundwater recharge, livestock production and biodiversity) in five terrestrial biomes. The estimated reductions in surface water runoff as a result of current invasions were >3000 million m3 (about 7% of the national total), most of which is from the fynbos (shrubland) and grassland biomes; the potential reductions would be more than eight times greater if invasive alien plants were to occupy the full extent of their potential range. Impacts on groundwater recharge would be less severe, potentially amounting to approximately 1.5% of the estimated maximum reductions in surface water runoff. Reductions in grazing capacity as a result of current levels of invasion amounted to just over 1% of the potential number of livestock that could be supported. However, future impacts could increase to 71%. A ‘biodiversity intactness index’ (the remaining proportion of premodern populations) ranged from 89% to 71% for the five biomes. With the exception of the fynbos biome, current invasions have almost no impact on biodiversity intactness. Under future levels of invasion, however, these intactness values decrease to around 30% for the savanna, fynbos and grassland biomes, but to even lower values (13% and 4%) for the two karoo biomes. Thus, while the current impacts of invasive alien plants are relatively low (with the exception of those on surface water runoff), the future impacts could be very high. While the errors in these estimates are likely to be substantial, the predicted impacts are sufficiently large to suggest that there is serious cause for concern.Centre of Excellence for Invasion Biolog

An assessment of the effectiveness of a long-term ecosystem restoration project in a fynbos shrubland catchment in South Africa

The long-term effectiveness of ecological restoration projects is seldom reported in the scientific literature. This paper reports on the outcomes of ecosystem restoration following the clearing of alien Pinus plantations and associated alien plant invasions over 13 years from an 8000 ha mountain catchment in the Western Cape Province, South Africa. We examined the goals, methods and costs of management, and the ecological outcomes in terms of reduced alien plant cover and native vegetation recovery. While the goals were not explicitly formulated at the outset, they were implicitly focussed on the conservation of water resources, the restoration of biodiversity, and the provision of employment. Initially, most (>90% of the area) was occupied by Pinus and Acacia invasions, mostly at low densities. The cost of control (initial clearing and up to 16 follow-up visits to remove emergent seedlings) amounted to almost ZAR 50 million (14 ZAR ~ 1US$). Although the cover of alien plants was greatly reduced, over 1000 ha still support dense or medium invasions (>25% cover), and the area occupied by scattered Pinus plants increased by over 3000 ha to >5700 ha. A reliance on passive restoration had not yet resulted in full recovery of the natural vegetation. The mean number of species, and total projected canopy cover on 50 m2 plots was lower in cleared than in comparable reference sites with pristine vegetation (21 vs 32 species/plot, and 94 vs 168% cover respectively). While the project is ongoing, we conclude that the entire area could revert to a more densely-invaded state in the event of a reduction of funding. Several changes to the management approach (including the integrated use of fire, a greater use of power tools, and active re-seeding of cleared areas with indigenous shrubs) would substantially increase the future effectiveness of the project and the sustainability of its outcomes