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Van Wilgen, B.W. & Wilson, J.R. (Eds.) 2018. The status of biological invasions and their management in South Africa in 2017
... Parthenium hysterophorus was first documented in KwaZulu-Natal province, during the 1880s, however, it was not until the mid-1980s, after cyclone Demoina, that the weed became a prominent invader Strathie and McConnachie, 2019). Since then, P. hysterophorus invasions have rapidly proliferated within KwaZulu-Natal and spread northward, invading the Mpumalanga, North-West, and Limpopo provinces (Terblanche et al., 2016;Mao et al. 2021), with the most recent assessments suggesting the weed has increased its range by »500% in the last decade (Van Wilgen and Wilson, 2018). ...
Parthenium hysterophorus L. (Asteraceae) is a noxious, annual invasive herb prevalent in more than 50 countries worldwide. In South Africa, the weed is a highly damaging invasive species, particularly in savanna regions, threatening food security, native biodiversity, livelihoods and human health. Given the multitude of threats posed by P. hysterophorus, this study aimed to determine the influence of the invasion of P. hysterophorus and its associated seedbank dynamics at a small spatial scale. To assess this, two neighbouring savanna reserves, with differing stocking densities (i.e. over- and understocked), were selected and assessed seasonally over a four-year period. Overstocking exacerbated the invasion of P. hysterophorus, with greater densities, larger plants and increased seed production, at a plot level. This enhanced invasion was also reflected in changes to the site's above-ground plant community, with significantly lower species richness, evenness, and diversity. Parthenium hysterophorus comprised more than 80% of the overstocked reserve's seedbank, and was responsible for continued declines in the richness, evenness and diversity of species below-ground. Whereas P. hysterophorus at the understocked reserve, occupied a smaller proportion of the seedbank (∼70%), with richness, evenness and diversity all showing signs of recovery over the four-year period. Overall, this research highlighted the impacts of P. hysterophorus in South Africa, as well as the potential role grazing management has in limiting or facilitating P. hysterophorus invasions and their impacts within savannas.
Biological invasions pose a major threat to biodiversity and significant investment is required to prevent the introduction of alien species, contain introduced populations and mitigate associated impacts. The implementation of standardised long-term monitoring programmes enables the early identification of new alien species, the tracking of spread, the assessment of the effectiveness of management interventions, and an understanding of temporal and spatial trends. Globally, fouling species are known to cause economic losses through accumulation on vessel hulls and port infrastructure and via the disruption of aquaculture activities. Additionally, fouling taxa can have ecological impacts in recipient systems, most often driven by their dominance in biological interactions. In South Africa, this group accounts for the majority of marine alien taxa. Accordingly, this study tested an approach for monitoring fouling biota using open and caged PVC settlement plates deployed in marinas. After 16 weeks, plates were removed, biota identified, and relative abundance captured as percentage cover. This method proved to be effective, logistically simple and cost-efficient. Twenty-four taxa were recorded, of which 58% were invasive species. It is recommended that this monitoring approach be implemented around the South African coast as a first step to providing key information to inform management and research.
Melia azedarach L. (Meliaceae) is an invasive, rapidly growing tree, considered one of the most important transforming invasive alien plants in South Africa. Here we survey several localities across the city of Durban (eThekwini Municipality in the province of KwaZulu-Natal), with and without adult Melia trees, including indigenous trees, as well as other alien species. Notwithstanding significant differences between localities, we find that localities with adult Melia trees have substantially reduced tree species richness and marginally smaller tree diameter at breast height (values combining indigenous trees, Melia, and other alien trees). The composition of adult tree assemblages differs considerably from the composition of tree seedling assemblages at most localities, with Melia seedlings present at all localities, irrespective of the presence of adult trees. The differences between adult and seedling communities are more pronounced where adult Melia trees are absent, indicating rapid transformation due to incipient invasion. This also suggests that Melia dominance may cause long-lasting vegetation uniformity and that the transformation process caused by the alien trees at these localities is already somewhat stabilized. We discuss the implications of Melia presence and local dominance on the local ecosystems and suggest controlling this species at target localities of conservation relevance.
Biological invasions are a major threat to biodiversity and ecosystem functioning. Forest invasion by alien woody species can have cross-ecosystem effects. This is especially relevant in the case of stream-riparian forest meta-ecosystems as forest streams depend strongly on riparian vegetation for carbon, nutrients and energy. Forest invasion by woody species with dissimilar characteristics from native species may be particularly troublesome. The invasion of temperate deciduous broadleaf forests with low representation of nitrogen (N)-fixing species by N-fixers has the potential to induce ecosystem changes at the stream level. Although effects of tree invasion on stream ecosystems have been under assessed, knowledge of native and invasive tree characteristics allows prediction of invasion effects on streams. Here we present a conceptual model to predict the effects of forest invasion by alien N-fixing species on streams, using as a background the invasion of temperate deciduous broadleaf forests by leguminous Acacia species, which are among the most aggressive invaders worldwide. Effects are discussed using a trait-based approach to allow the model to be applied to other pairs of invaded ecosystem-invasive species, taking into account differences in species traits and environmental conditions. Anticipated effects of N-fixing species invasions include changes in water quality (increase in N concentration) and quantity (decrease in flow) and changes in litter input characteristics (altered diversity, seasonality, typology, quantity and quality). The magnitude of these changes will depend on the magnitude of differences in species traits, the extent and duration of the invasion and stream characteristics (e.g. basal nutrient concentration). The extensive literature on effects of nutrient enrichment of stream water, water scarcity and changes in litter input characteristics on aquatic communities and processes allows prediction of invasion effects on stream structure and function. The magnitude of invasion effects on aquatic communities and processes may, however, depend on interactions among different pathways (e.g. effects mediated by increases in stream nutrient concentration may contrast with those mediated by decreases in water availability or by decreases in litter nutritional quality). A review of the literature addressing effects of increasing cover of N-fixing species on streams suggests a wide application of the model, while it highlights the need to consider differences in the type of system and species when making generalizations. Changes induced by N-fixing species invasion on streams can jeopardize multiple ecosystem services (e.g. good quality water, hydroelectricity, leisure activities), with relevant social and economic consequences.
Extensive literature is available on the diversity and magnitude of impacts that alien species cause on recipient systems. Alien species may decrease or increase attributes of ecosystems (e.g. total biomass or species diversity), thus causing negative and positive environmental impacts. Alien species may also negatively or positively impact attributes linked to local human communities (e.g. the number of people involved in a given activity). Ethical and societal values contribute to define these environmental and socioeconomic impacts as deleterious or beneficial. Whilst most of the literature focuses on the deleterious effects of alien taxa, some recognise their beneficial impacts on ecosystems and human activities. Impact assessment frameworks show a similar tendency to evaluate mainly deleterious impacts: only relatively few, and not widely applied, frameworks incorporate the beneficial impacts of alien species. Here, we provide a summary of the frameworks assessing beneficial impacts and briefly discuss why they might have been less frequently cited and applied than frameworks assessing exclusively deleterious impacts. Then, we review arguments that invoke a greater consideration of positive and beneficial impacts caused by alien species across the invasion science literature. We collate and describe arguments from a set of 47 papers, grouping them in two categories (value-free and value-laden), which span from a theoretical, basic science perspective to an applied science perspective. We also provide example cases associated with each argument. We advocate that the development of transparent and evidence-based frameworks assessing positive and beneficial impacts might advance our scientific understanding of impact dynamics and better inform A peer-reviewed open-access journal NeoBiota Giovanni Vimercati et al. / NeoBiota 62: 525-545 (2020) 526 management and prioritisation decisions. We also advise that this development should be achieved by rec-ognising the underlying ethical and societal values of the frameworks and their intrinsic limitations. The evaluation of positive and beneficial impacts through impact assessment frameworks should not be seen as an attempt to outweigh or to discount deleterious impacts of alien taxa but rather as an opportunity to provide additional information for scientists, managers and policymakers.
Article impact statement: In an era of profound biodiversity crisis, invasion costs, invader impacts, and human agency should not be dismissed.
Globalization facilitated the spread of invasive alien species (IAS), undermining the stability of the world’s ecosystems. We investigated the metabolomic profiles of three IAS species: Chromolaena odorata (Asteraceae) Datura stramonium (Solanaceae), and Xanthium strumarium (Asteraceae), comparing metabolites of individual plants in their native habitats (USA), to their invasive counterparts growing in and around Kruger National Park (South Africa, ZA). Metabolomic samples were collected using RApid Metabolome Extraction and Storage (RAMES) technology, which immobilizes phytochemicals on glass fiber disks, reducing compound degradation, allowing long-term, storage and simplifying biochemical analysis. Metabolomic differences were analyzed using ultra-performance liquid chromatography-mass spectrometry (UPLC-MS) of samples eluted from RAMES disks. Partial Least Squares-Discriminant Analysis (PLS-DA) of metabolomes of individual plants allowed statistical separation of species, native and invasive populations of each species, and some populations on the same continent. Invasive populations of all species were more phytochemically diverse than their native counterparts, and their metabolomic profiles were statistically distinguishable from their native relatives. These data may elucidate the mechanisms of successful invasion and rapid adaptive evolution of IAS. Moreover, RAMES technology combined with PLS-DA statistical analysis may allow taxonomic identification of species and, possibly, populations within each species.
The NBA 2018 Synthesis report can be downloaded from the NBA website: http://nba.sanbi.org.za
Many studies in South Africa have examined the impacts of alien plants on ecosystems, but none have assessed the impact of guava ( Psidium guajava L.) invasion on soil properties. In this study, soils underneath guava‐invaded sites were assessed to determine if they had different soil physico‐chemical properties (pH, P, C, N, Na, K, Ca, Mg, moisture, penetration resistance, infiltration and water repellency) as compared to soils underneath uninvaded sites. Comparisons were made from three different sites over three autumn months. Results show that soil pH was significantly ( p < 0.005) higher underneath uninvaded than guava‐invaded sites. Soil P was three times higher underneath guava‐invaded as compared to invaded sites. The soils collected underneath guava‐invaded sites had a significantly ( p < 0.001) higher moisture content and were less compact but more repellent than soils from the uninvaded sites. Infiltration rate was significantly ( p < 0.001) higher in the uninvaded than the guava‐invaded sites. The study concludes that guava invasion alters some soil properties, thus creating favourable conditions for its growth and making it potentially more invasive. From a management standpoint, guava removal is encouraged; however, given guava's socio‐economic importance more research on cost and benefits is required.
A major challenge for the management of biological invasions is to ensure that data and information from basic inventories and ecological research are used alongside data from the monitoring and evaluation of interventions to trigger and improve policy and management responses. To address this issue, South Africa has committed to report on the status of biological invasions and their management every 3 years.
We propose a framework of indicators for reporting on biological invasions at a country level; assess the feasibility of the indicators using South Africa as a case study; and outline steps needed for indicator development.
We argue that a national status report on biological invasions should explicitly consider indicators for pathways, species, and sites, and should report on interventions in terms of inputs, outputs, and outcomes.
We propose 20 indicators based on data currently available, as well as existing international policy initiatives. For each indicator, we have developed a factsheet that includes different hierarchical metrics (considering data availability) and provide suggestions on assigning confidence levels. We also combine these indicators into four high‐level indicators to facilitate broader reporting and describe how forecasted indicators based on the concept of invasion debt could assist with scenario planning.
We found that many of the data required for these indicators are already available in South Africa, but they have been poorly collated to date. However, data for the indicators of most direct value to policy and planning—those dealing with the impact of biological invasions and the outcome of interventions—are scarce.
Policy implications . The framework of indicators developed here, for what we believe is the first ever national‐level report on the status of biological invasions and their management, will facilitate the inclusion of biological invasions in environmental reporting at national and international levels. By identifying knowledge gaps, a status report will also focus efforts on determining the size of a country's invasion debt and what can be done to reduce it.
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