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A proposed classification of invasive alien plant species in South Africa: Towards prioritizing species and areas for management action
Abstract
Many invasive alien plant species in South Africa are already well-established and cause substantial damage, while scores of others are at the early stages of invasion (only recently introduced and/or entering a phase of rapid population growth). Management programmes must target well-established invaders, but must also give appropriate attention to emerging problems. Protocols for objectively prioritizing species in the two groups for management action are lacking. To this end, the authors describe the objective derivation of two lists of invasive alien plants in South Africa, using available quantitative data and expert knowledge on current patterns of distribution and abundance, life-history traits, and (for emerging invaders) estimates of potential habitat. 'Major invaders' are those invasive alien species that are well-established, and which already have a substantial impact on natural and semi-natural ecosystems. 'Emerging invaders' currently have less influence, but have attributes and potentially suitable habitat that could result in increased range and consequences in the next few decades. We describe the derivation of lists that contain 117 major invaders (categorized into groups based on geographical range and abundance) and 84 emerging invaders (categorized into groups based on current propagule-pool size and potentially invasible habitat). The main lists, and groupings within them, provide a useful means for prioritizing species for a range of management interventions at national, regional and local scales.
... Prosopis species have been shown differences in their leaves and fruits due to exposure to variable stresses (Villagra et al., 2010) [32] . The introduced and naturalized or invasive species devise different strategies for environmental management (Nel et al., 2004) [22] . The recorded taxonomic identity of introduced and invasive Prosopis taxa remains questionable (Zimmermann, 1991) [33] . ...
... Prosopis species have been shown differences in their leaves and fruits due to exposure to variable stresses (Villagra et al., 2010) [32] . The introduced and naturalized or invasive species devise different strategies for environmental management (Nel et al., 2004) [22] . The recorded taxonomic identity of introduced and invasive Prosopis taxa remains questionable (Zimmermann, 1991) [33] . ...
... Nel et al. (2004) 2011 Earliest record of Phytophthora cinnamomi on Q. cerris in South Africa. Somerset West, Western Cape Oh et al. (2011) 2012 First record of Polyphagous Shot Hole Borer in South Africa KwaZulu-Natal Van Rooyen et al. (2021) 2015 Earliest recorded truffle harvest in South Africa Near Dullstroom, Mpumalanga Hall et al. ...
This study reviews various aspects of the genus Quercus in South Africa. We determine the number of oak species present, assess their introduction status, map their distribution, highlight areas of putative naturalisation, elucidate environmental factors that can mediate invasibility in a riparian zone, and evaluate the usefulness of the community science platform iNaturalist for assessing these factors.
A literature search was conducted to review events associated with the introduction, cultivation and naturalisation of oaks and associated pests and diseases in South Africa. Records of Quercus species in South Africa were collated from herbaria, arboreta, the Southern African Plant Invader Atlas, the Plants of Southern Africa database and iNaturalist. A field study was conducted along the Eerste River in Stellenbosch to elucidate the role of environmental factors in mediating naturalisation.
We found records of 47 Quercus taxa in South Africa which were refined to a list of 22 to 34 species likely present. The earliest record was for Quercus robur in 1656 but first records for most species date from between the late 1800s to the early 1900s. More than 99% of presence and naturalisation records were of the ten commonly cultivated species. Quercus palustris, Q. robur, and Q. suber were categorised as invasive, and naturalisation was documented for Q. acutissima, Q. canariensis, Q. cerris, Q. ilex, and Q. nigra. Most naturalisation occurred at the urban-wildland interface and in riparian vegetation in Cape Town and Stellenbosch. Steepness of the riverbank was found to affect seedling recruitment in riparian areas. iNaturalist records shed new light on the presence, distribution, and introduction status of Quercus species in South Africa. However, taxonomic complexity and data quality complicated attempts to develop a robust inventory of Quercus in South Africa. Molecular studies are needed to achieve better resolution. Nonetheless, this study has greatly improved our understanding of the distribution and status of oaks in South Africa, with implications for their management.
... Of these, at least A. falciformis and A. viscidula already qualify for categorization as invasive. Several mapping exercises have documented the massive scale or wattle invasions nationally (e.g.Rouget et al., 2004) or regionally (e.g.Richardson et al., 1996: Rouget et al., 2003. Le Maitre et al.(2000) estimated that wattle invasions covered 643,000 ba in 2000. ...
This chapter reviews the history of introductions, plantings, usage, naturalization and invasion of Australian Acacia species (‘wattles’) for African countries and several islands that are considered part of Africa in biogeographical or geological contexts. Although many more species have been introduced, our review focussed on 48 species that feature explicitly in discussions about wattles in 45 countries and islands/archipelagos. The history of introductions, plantings, naturalization and invasion is well documented for South Africa, where 40 Acacia species are known to be present, three species are commercially important, several species are important to rural and urban populations for fuelwood and other uses, and 16 species are currently regulated as invasive alien species in national legislation. Less detailed information is available for the rest of the continent. Introductions to South Africa were made mainly for dune stabilization, to create a forestry industry, and for ornamental horticulture, and were largely government-orchestrated. In most other parts of Africa, wattle introductions were mostly for other purposes - to address desertification and soil conservation, for reforestation of degraded ecosystems, for use in diverse forms of agroforestry, to supply fuelwood, for tannin production, and for human food. The interplay of drivers of wattle introductions has changed radically, and sometimes very quickly, over time across the continent. Whereas the implications of wattle introductions for native biodiversity and ecosystems received no attention a century ago, these concerns are now much more important in most areas. Trajectories for wattles in Africa will be affected by changing markets, global change, invasion debt, the effectiveness of management of invasive wattles, the emergence of problems with insect pests and diseases, and by changing requirements for timber, charcoal, fuelwood and food products.
... Most are quite toxic to livestock, native mammals and humans (de Rojas and D'Arcy, 1998;Makokha, 2018). For these reasons, they are labelled as noxious weeds with moderate to high invasive potential (Nel et al., 2004;Henderson, 2007). In South Africa and Australia, extensive programs have been undertaken to clear areas of Cestrum species (Macdonald and Jarman, 1985;Stockard, 1996;Marais and Wannenburgh, 2008). ...
In the montane forest-grassland mosaics of the Western Ghats, land cover conversion to silviculture and agriculture over the last five decades has resulted in both loss of natural habitats and widespread invasion of remnant habitat patches. While invasion of the grassland habitats of the mosaic has been relatively well studied, there have been few attempts to understand the extent to which forest habitats (locally known as sholas) have been affected by the spread of exotic species. Here we examine the patterns and impacts of invasion of shola forest understoreys by Cestrum aurantiacum Lindl., an exotic shrub species. At the landscape scale, we demonstrate that the presence and abundance of this invasive in shola understories is negatively related to distance from tea plantations. Further, the intensity of invasion is higher in areas with greater seasonality of temperature and lower mean annual precipitation. At the patch scale, invasion is greatest at shola edges and away from stream courses. We find that C. aurantiacum abundance has negatively affected the regeneration of native shola tree species as well as the abundance of native shola understorey shrubs. Fifty three percent of invaded plots had no native shrubs present. In plots where both C. aurantiacum and native shrubs were present in large enough numbers, we found evidence of negative spatial dependence between stem locations of C. aurantiacum and native shrubs. Our findings have important implications for the management and conservation of these mosaics.
... This evergreen shrub possesses lanceolate and aromatic leaves and is widespread in Australia, although a couple of them have been introduced to other countries and continents like the USA and Africa. The plant is known as a bottle brush due to its traditional brush that looks like a conventional bottle brush [9,10]. Naturally, Callistemon is commonly found either along a watercourse or at the edge of a swamp. ...
This study carried out a bibliometric analysis centered on Callistemon research, spanning five decades, disclosing their trend and limitations. Published works related to Callistemon between 1966 and 2020 were retrieved from Web of Science (WoS) and SCOPUS databases using the search term Callistemon. A total of 239 documents comprising 237 research and two review articles were obtained from the hybrid databases. The 239 articles were authored by 680 researchers across the globe with an article/author and author/article ratio of 0.351 and 2.85, respectively. An average citation number of 9.657 was recorded per document; the most recurrent keywords obtained in this analysis were Myrtaceae, Callistemon citrinus, Callistemon lanceolatus, 1, 8-cineole, essential oil, essential oil composition, antibacterial, and antioxidants. This analysis offered a comprehensive and general overview of the Callistemon genus from 1966 to 2020 with an insight into the frequency of citations for most cited publications in addition to the trend of articles, journals, countries, top researchers, and the quality of works steering from research on Callistemon genus. This bibliometric study will help researchers in this field of interest understand Callistemon's growth trajectory, identify the scientific areas of evidence, and find exceptional analysis centers for scientific interchange and collaboration.
... The objective of this work is to evaluate the mechanisms controlling the establishment and survival of the invasive tree Gleditsia triacanthos L. in an intensive rotational grazing system in the Pampas. Gleditsia triacanthos has invaded the Pampean grasslands, the Uruguayan Campos, and grasslands in Australia, South Africa, Spain, and countries of Central and Eastern Europe (Csurhes and Kriticos 1994;Nel et al. 2004;Chaneton et al. 2012;Pyšek et al. 2012;Romero et al. 2021;Dana et al. 2022), making this species an appropriate study model for rangeland ecosystems. With two manipulative field experiments in different years, we studied the effect of plant size, as related to resistance and tolerance mechanisms at different development stages (Exp. ...
Grazing is one of the principal drivers triggering woody invasion in grasslands and savannas worldwide, but evidence is opposing. Cattle can promote or limit woody establishment by direct and indirect mechanisms, like consumption, trampling or relaxing grass competition. Despite its increasing relevance, it is unknown if intensively managed cattle grazing can curb woody establishment. We evaluated the effect of cattle grazing with short cycles of high stocking rate and resting periods on Gleditsia triacanthos establishment, an invasive tree in grasslands. By two field experiments in pastures, we manipulated cattle grazing (with or without) and: Exp. (1) initial age-related size of G. triacanthos (small plants: 3 mo.; or large plants: 9 mo.), and Exp. (2) neighbouring vegetation (clipped or intact). Overall, grazing had a negative effect due to tree consumption and a positive effect by relaxing grass competition on tree establishment, but the first effect outweighed the second. Grazing reduced by 25% G. triacanthos survival and by 40% its biomass. Consumption was independent of plant size and neighbouring vegetation, but large plants had more resprouting capacity than small plants. Regardless of grazing, small plants had higher mortality than large plants. Intact neighbouring vegetation did not modify survival but decreased tree biomass which may affect middle or long-term tolerance and survival. Intensively managed rotational grazing with resting periods for pasture recovery may limit woody establishment, but this management strategy should be applied immediately after tree germination. The increasing intensification of production systems requires to evaluate the impact of management on invasion processes.
Myoporum is a genus of trees and shrubs native to the Northern Hemisphere that has been introduced to many parts of the world, mainly for ornamental purposes. We assessed the introduction history, distribution, and extent of naturalization/invasion for Myoporum species in South Africa. Information was collated to determine key events associated with the introduction, establishment, and nat-uralization of Myoporum in South Africa. Data were collated to determine the current distribution of the genus in South Africa. Twenty sites in the Western Cape were sampled to determine correlates of naturalization. Myoporum was first recorded in South Africa in 1934. Three species were confirmed to be present in South Africa: M. insulare, M. laetum and M. montanum (37 %, 25 % and 24 % of all iNaturalist records respectively). Most records are from the Western Cape (91 %) and small parts of the Eastern Cape; isolated populations occur in Gauteng and the Northern Cape. We could not confirm the presence M. petiolatum, M. tenuifolium or M. tetrandrum. Field surveys revealed widespread naturalization of M. insulare (46 % of all Research Grade observations in iNaturalist); this species was categorized code D1 in the introduction-naturalization-invasion continuum. Myoporum laetum (C3) and M. montanum (C2) are also widely naturalized but over smaller areas. Naturalized populations comprised predominantly juvenile M. insulare plants occurring in highly disturbed (transformed) habitats. Formal risk analyses for all Myoporum species in South Africa are needed as the basis for re-evaluation of their status in national legislation.
Climate change and invasive species impose severe threats to biodiversity, ecosystem, and economy; however, the impact on human well-being and livelihood is not much known. The interaction between these is complex and intensifying, and there is increasing evidence that climate change is amplifying the deleterious effects caused by invasive species. Worldwide, the damage resulting from invasive species accounts for 5% of the global economy and has an impact on a large number of sectors such as forestry, agriculture, aquaculture, trade, recreation, etc. Variations in climatic conditions are more likely to interrupt the existing populations of native as well as aquatic invasive species and also increase the susceptibility of the aquatic ecosystem by creating favourable conditions for invasive species as they are more adaptable to disturbances and varied environmental conditions. Climate change is anticipated to cause warmer water temperatures, minimize ice cover, change the pattern of streamflow, increase salinization, etc., which would modify the pathways through which invasive species infiltrate the aquatic bodies. In addition, climate change will transform the ecological effects of invasive species by increasing their predatory and competitive effect on indigenous species and by enhancing the harmfulness of certain diseases. The impact of invasive species is anticipated to be more deleterious as they proliferate both in numbers and degree; can considerably change the composition, chemistry, structure, and function of aquatic systems. However, a clear insight into how climate change upsets invasive species growth and a study of their combined effects on the ecosystems is still required. Further to minimize the compounding impact of climate change on the devastating effect of invasive species, various preventive and control measures are required to regulate the invasive species that presently possess moderate effects and are restricted by seasonally adverse conditions. The present chapter focuses on how climate change affects plant invasion in the aquatic system and their complex interactions. This chapter also discusses various methods used for the management and restoration of the invaded ecosystem.
This is an Open Access Journal / article distributed under the terms of the Creative Commons Attribution License (CC BY-NC-ND 3.0) which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. All rights reserved. To estimate the hepatoprotective effects of the methanolic seed extract of Solanum seaforthianum in Wistar albino rats treated with carbon tetrachloride(CCl4). Liver damage in rats treated with CCl4 0.7ml/kg/Bw, administered, on alternate intraperitonially days for ten days) was studied by assessing parameters such as serum glutamate oxaloacetate transaminase (SGOT), serum glutamate pyruvate transaminase (SGPT), alkaline phosphatase (ALP), acid phosphatase (ACP) and bilirubin(total and direct). The effect of co-administration of Solanum seaforthianum. (doses 100 and 200mg/kg p.o.) on the above parameters were investigated. These biochemical observations were supplemented by weight and histological examination of liver sections. Silymarin was used as positive control. Data were analyzed by one way ANOVA, followed by Tukey-Kramer. Administration of Solanum seaforthianum(doses 100 and 200 mg/kg p.o) and the Pharmacognostical studies made on the powdered leaves of Solanum seaforthianum like ash values, extractive value, loss on drying gave valuable information. The preliminary phytochemical investigation showed the presence of, alkaloids, glycosides, flavonoids, and steroids, carbohydrate, fatty acids and proteins and phenolic compounds. in methanol extract. Based on the results obtained from the present study, it can be concluded methanolic extract of Solanum seaforthianum was found to be more potent and effective hepatoprotective and antioxidant activity. The study suggests preventive action of ABSTRACT RESEARCH ARTICLE 54601 Solanum seaforthianum in carbon tetrachloride induced liver toxicity. Hepatic cell regeneration process was dose dependent.
Scenario planning is a useful technique for dealing with complex issues such as biological invasions in natural ecosystems. Invasions are driven by four primary driving forces: arrival and accumulation of propagules(seeds, spores, whole organisms and vegetative parts); disturbance regimes; fragmentation of natural landscapes; and changes in the availability of limiting factors. The primary driving forces are influenced by a range secondary driving forces which are mediated by people. Some of these forces can either facilitate or constrain invasions depending on circumstances but the direction in which they will act is not known. These are termed uncertainties and their future trajectories will shape the future of invasions. A scenario analysis prepared for South Africa showed clearly that gaining the political support to implement coherent policies, laws and regulations is one of the ways in which the biological invasions can be controlled. The ability to do this will depend significantly on the availability of the necessary resources which, in turn, depends very largely on sustained economic growth and the support of the general populace.
Although ecologists commonly talk about the impacts of nonindigenous species, little formal attention has been given to defining what we mean by impact, or connecting ecological theory with particular measures of impact. The resulting lack of generalizations regarding invasion impacts is more than an academic problem; we need to be able to distinguish invaders with minor effects from those with large effects in order to prioritize management efforts. This paper focuses on defining, evaluating, and comparing a variety of measures of impact drawn from empirical examples and theoretical reasoning. We begin by arguing that the total impact of an invader includes three fundamental dimensions: range, abundance, and the per-capita or per-biomass effect of the invader. Then we summarize previous approaches to measuring impact at different organizational levels, and suggest some new approaches. Reviewing mathematical models of impact, we argue that theoretical studies using community assembly models could act as a basis for better empirical studies and monitoring programs, as well as provide a clearer understanding of the relationship among different types of impact. We then discuss some of the particular challenges that come from the need to prioritize invasive species in a management or policy context. We end with recommendations about how the field of invasion biology might proceed in order to build a general framework for understanding and predicting impacts. In particular, we advocate studies designed to explore the correlations among different measures: Are the results of complex multivariate methods adequately captured by simple composite metrics such as species richness? How well are impacts on native populations correlated with impacts on ecosystem functions? Are there useful bioindicators for invasion impacts? To what extent does the impact of an invasive species depend on the system in which it is measured? Three approaches would provide new insights in this line of inquiry: (1) studies that measure impacts at multiple scales and multiple levels of organization, (2) studies that synthesize currently available data on different response variables, and (3) models designed to guide empirical work and explore generalities.
Much confusion exists in the English-language literature on plant invasions concerning the terms ‘naturalized’ and ‘invasive’ and their associated concepts. Several authors have used these terms in proposing schemes for conceptualizing the sequence of events from introduction to invasion, but often imprecisely, erroneously or in contradictory ways. This greatly complicates the formulation of robust generalizations in invasion ecology.
Based on an extensive and critical survey of the literature we defined a minimum set of key terms related to a graphic scheme which conceptualizes the naturalization/invasion process. Introduction means that the plant (or its propagule) has been transported by humans across a major geographical barrier. Naturalization starts when abiotic and biotic barriers to survival are surmounted and when various barriers to regular reproduction are overcome. Invasion further requires that introduced plants produce reproductive offspring in areas distant from sites of introduction (approximate scales: > 100 m over < 50 years for taxa spreading by seeds and other propagules; > 6 m/3 years for taxa spreading by roots, rhizomes, stolons or creeping stems). Taxa that can cope with the abiotic environment and biota in the general area may invade disturbed, seminatural communities. Invasion of successionally mature, undisturbed communities usually requires that the alien taxon overcomes a different category of barriers.
We propose that the term ‘invasive’ should be used without any inference to environmental or economic impact. Terms like ‘pests’ and ‘weeds’ are suitable labels for the 50–80% of invaders that have harmful effects. About 10% of invasive plants that change the character, condition, form, or nature of ecosystems over substantial areas may be termed ‘transformers’.
Natural areas, natural resources, and agricultural production systems have been damaged by introduced species, and are jeopardized by future invasions. A rating system was developed to prioritize research and control efforts for preventing species invasions and eradicating established exotic pests. Four rating criteria were the species potential (1) to be introduced; (2) to establish; (3) to cause damage; and (4) to be controlled. Each species was rated independently for each criterion, and these ratings summed to provide a total score. The rating system was developed with 24 exotic bird and mammal species with well-known invasion and pest histories. We then rated the 14 bird and mammal species on the California Department of Food and Agriculture most unwanted exotic species list, and 10 other species. The rating system provided surprising objectivity for assessing the threat of species invasion and pest status. Of the 14 ‘most unwanted species’, four were rated as a low threat, and 13 of the 34 other rated species were recommended for this list. Certainly, this list should be lengthened. A quick-response apparatus was also developed to provide information on perceived exotic species threats. It consisted of a data base of expert contacts and citations on exotic pest species damage, biology, ecology, and control technology.
New plant taxa from around the world continue to be imported into Australia and New Zealand. Many of these taxa have the potential to become agricultural or environmental weeds and this risk needs to be assessed before allowing their entry. A weed risk assessment system is described that uses information on a taxon's current weed status in other parts of the world, climate and environmental preferences, and biological attributes. The system is designed to be operated by quarantine personnel via a user-friendly computer interface.The model was tested against experts' scores for weediness for 370 taxa present in Australia, representing both weeds and useful taxa from agriculture, the environment, and other sectors. The model was judged on its ability to correctly ‘reject’ weeds, ‘accept’ non-weeds, and generate a low proportion of taxa which could not be decisively categorised, termed ‘evaluate’. More than 70% of the taxa were rejected or accepted. All taxa classified as serious weeds, and most minor weeds, were rejected or required further evaluation, while only 7% of non-weeds were rejected. The model was modified to New Zealand conditions and evaluated against the opinions of several groups of experts and against economic measures. The model produced a weediness score very similar to the mean of the experts scores. The latter were highly variable: agriculturalists tended to accept known weeds, conservationists tended to reject most adventive taxa, and only botanists produced scores similar to the model. The model scores also tended to be independent of economic value as measured in this study. The model could be adapted for use as a screening tool in any region of the world.
The development and application of an expert system is described for screening alien woody plants for their invasive potential in South African fynbos. The system is proposed for use by potential introducers to demonstrate low invasive risk before importing woody alien species for cultivation. Rules for the system were derived from empirical evidence by quantifying invasion windows and barriers that have limited the set of widespread woody invaders (trees and shrubs) in fynbos to fewer than 20, out of several hundred introduced species. The system first compares broad-scale environmental conditions (climate and soil) between the home environment of a species and fynbos. Features of the plant in its home environment (basic life history traits, population characteristics, regeneration biology, habitat preferences) are then assessed. Finally, an assessment is made of life history adaptations to the prevailing fire regime in fynbos (juvenile period, fire-survival capacity of adult plants, seed bank longevity). The reasoning is explicit and the steps leading to a conclusion (high risk/low risk) can be retraced.Besides the obvious application in identifying species with a high risk of invading, the system has considerable potential for modelling, and for teaching the concepts of biological invasions. The rules provide an explicit conceptualization of invasion processes in fynbos and identify multiple paths to invasive success (not all of which have been realized yet). The system can therefore be used in planning control operations (for optimal allocation of control effort to critical stages in invasion), and for predicting the outcome of changes (e.g. in fire frequency) on the dimensions of invasion windows, and for assessing what changes are needed to prevent or reduce the extent of invasion by a given taxon.Application of the system is demonstrated on Pinus and Banksia taxa and a selection of species from Californian chaparral.