Invasive Alien Plants In South Africa Research Articles

Economic evaluation of chemical and biological control of four aquatic weeds in South Africa

ABSTRACT Invasive alien plants (IAPs) pose a threat to biodiversity and the economy of the countries they invade. In South Africa, the Department of Environment, Forestry and Fisheries, Natural Resources Management Programmes, previously The Working for Water Programme (WfW) is tasked with controlling IAPs in a way that protects the environment, as well as producing maximum return to society through poverty alleviation. Biological control is one of the management tools used to control IAPs in South Africa. Four aquatic weeds, Pista stratiotes, Salvinia molesta, Azolla filiculoides and Myriophyllum aquaticum, are under complete biological control in South Africa. However, in the absence of biological agents, the WfW programme would have used herbicides to control these weeds. This paper presents a retrospective analysis of the relative herbicide cost-saving associated with the use of biological control instead of chemical control. The study used cost benefit analysis (CBA) framework with an 8% discount rate. The estimated cost of the biological control on all four aquatic weeds was about R7.8 million, while the estimated cost of chemical control to achieve the same level of control varied between R150 million and R1 billion, depending on the method of application and number of follow up operations. Benefit to cost ratios varied between 90:1 and 631:1, again depending on method of application and number of follow up sprays. The results remained robust under a 5% and 10% sensitivity test and show that biological control is the most cost-effective management option for aquatic weeds in South Africa.

Assessing the status of biological control as a management tool for suppression of invasive alien plants in South Africa

Background: Biological control of invasive alien plants (IAPs) using introduced natural enemies contributes significantly to sustained, cost-effective management of natural resources in South Africa. The status of, and prospects for, biological control is therefore integral to National Status Reports (NSRs) on Biological Invasions, the first of which is due in 2017. Objectives: Our aim was to evaluate the status of, and prospects for, biological control of IAPs in South Africa. We discuss expansion of biological control and suggest indicators to be used in the upcoming NSR to assess sufficient growth. Method: We used published literature, unpublished work and personal communication to assess the status of biological control of IAPs. We propose indicators based on the targets for biological control that were proposed in the 2014 ‘National Strategy for dealing with biological invasions in South Africa’. To prioritise targets for future efforts, we used published lists of damaging IAPs and assessed the prospects for their biological control. Recommendations for using biological control as a management tool were made after discussion among the authors and with colleagues. Results: Significant control of several Cactaceae, Australian Acacia species and floating aquatic plants, and many other IAPs has been achieved in South Africa since 1913. Recently, biological control has benefited from improved international collaboration, a streamlined application process for the release of new biological control agents (resulting in the approval of 19 agents against 13 IAP species since 2013), and increased funding and capacity. There is still a need to improve implementation and to better integrate biological control with other control methods. In order to maximise benefits from biological control, increased investment is required, particularly in implementation and post-release evaluation, and in targeting new IAPs. Proposed targets for growth between 2017 and 2020 include an increase in financial investment in research by 29%, implementation by 28% and mass-rearing by 68%. Research capacity should increase by 29%, implementation capacity by 63% and mass-rearing capacity by 61%. New research projects should be initiated on 12 new IAP targets, while post-release monitoring efforts should be expanded to another 31 IAPs. Conclusion: Biological control of IAPs has contributed substantially to their management in South Africa, and continues to do so. Further investment in targeted aspects of IAP biological control will increase this contribution.

Plant invasions as a biogeographical assay: Vegetation biomes constrain the distribution of invasive alien species assemblages

Plant assemblages define vegetation patterns at different scales, from plant communities at the scale of small plots to broad biomes. Species assemblages are traditionally investigated with a focus on native species, and the spatial patterns and dynamics of alien species assemblages have received much less attention. Here, we explore the biogeography of a subset of invasive alien plants (IAPs) in South Africa and derive several “alien biomes” based on the alien plant assemblages and associated environmental drivers. We propose six hypotheses (the Weed-Shaped Hole; the Biome Decides; Goldilocks; a New World Order; Something In The Way You Move; and Random Tessellation) based on different drivers (disturbance, competition, climate, global change, introduction dynamics, and null respectively) that might explain distribution patterns. In particular, we explore whether invasive plant assemblages are controlled by the same fundamental factors that define native plant assemblages and biomes. A cluster analysis of the spatial distribution of 69 invasive alien plant species revealed five clearly delineated geographic clusters, three of them significantly aligned with the distribution of vegetation biomes (fynbos, grassland and savanna). The major determinants of the distribution of IAP clusters were identified based on a classification tree analysis. We found that broad environmental variables, especially vegetation biomes, explained the distribution of IAP clusters (60% classification accuracy). We could not find a strong relationship with anthropogenic factors, such as land cover or anthromes, even at a finer scale. Our results indicate that vegetation biomes are characterised by hard environmental barriers which also constrain the distribution of IAPs in South Africa. This supports the development of biome-level strategies for the control of alien plant species in South Africa.