Mapping the potential ranges of major plant invaders in South Africa, Lesotho and Swaziland using climatic suitability

A global assessment of the potential distribution of naturalized and planted populations of the ornamental alien tree Schinus molle

The Peruvian Peppertree (Schinus molle L.) is an evergreen tree native to semiarid environments of Peru and Bolivia in South America. This tree has been introduced and widely planted for ornamental and forestry purposes in several semiarid regions of the world because its seedlings are easily established and have a high survival rate; it also grows quickly, and it is tolerant of dry climates. We compared the global and regional niches of naturalized and planted populations of S. molle in order to examine the invasive stages and potential distribution of this species in four regions of the world. This work provides a novel approach for understanding the invasion dynamics of S. molle in these areas and elucidates the ecological processes that bring about such invasions. Most naturalized and planted populations were found to be in equilibrium with the environment. In its native range as well as in Australia and South Africa the models of the coverage area of habitat suitability for natural populations were the highest, whereas the coverage area of planted populations was lower. For planted populations in Australia and South Africa, a large percentage of predicted presences fell within sink populations. The invasion stages of S. molle vary across regions in its adventive range; this result may be attributable to residence time as well as climatic and anthropic factors that have contributed to the spread of populations.

Setting the scene for achievable post-2020 convention on biological diversity targets: A review of the impacts of invasive alien species on ecosystem services in Africa

Invasive alien species (IAS) are known to pose a serious threat to biodiversity, and reduce the ability of ecosystems to provide benefits to humans. In recognition of this threat and to address the impacts of IAS, Parties to the Convention on Biological Diversity (CBD) adopted Aichi Biodiversity Target 9, which is dedicated to the control or eradication of priority IAS and the management of their introduction pathways by 2020. The achievement of Target 9 relies strongly on the commitment and ability of Parties to set ambitious national or regional targets and achive them, the availability of information and the requisite expertise on invasion biology. Now that the global community is gearing for the post-2020 Biodiversity Framework, it is time to take stock and identify opportunities to improve the performance of the African region beyond 2020. We approached this task by reviewing information on the impacts of IAS on ecosystem services in Africa, as a large proportion of Africans directly rely on ecosystem services, particularly in rural areas. Furthermore, we assessed the expertise on IAS in Africa. Our data sources were National Reports of African countries to the CBD, as well as peer-reviewed scientific literature. National Reports under the CBD provide information on measures taken to implement the Convention at country level, as well as progress towards the achievement of set targets. We found National Reports for 48 (about 90%) countries of which 73% provided feedback on IAS indicating commitment to fight IAS. However, there were few studies within peer-reviewed scientific literature looking at impacts of IAS on ecosystem services in Africa and almost half of the authors were non-Africans. This alludes to limited scientific expertise to inform and support IAS management on the continent. Both the National Reports and scientific literature showed that provisioning services were the most negatively affected by IAS. Also, more than 100 species were listed as problematic. More efforts and resources are needed to document IAS impacts across different realms (e.g. marine, terrestrial and freshwater) and for sub-regional bodies so that more integrated strategies and approaches can be developed. This information is also needed to support the development and implementation of national legislative and regulatory initiatives, as well as to report on international obligations such as the Aichi Biodiversity Targets.

Ghosts from the past: even comprehensive sampling of the native range may not be enough to unravel the introduction history of invasive species-the case of Acacia dealbata invasions in South Africa

PREMISE OF THE STUDY

Knowledge about the introduction history (source(s), number and size of introduction events) of an invasive species is a crucial prerequisite to understand invasion success and to facilitate effective and sustainable management approaches, especially for effective biological control. We investigated the introduction history of the Australian legume tree Acacia dealbata in South Africa. Results of this study will not only provide critical information for the management of this species in South Africa, but will also broaden our overall knowledge on the invasion ecology of this globally important invasive tree.

METHODS

We used nuclear microsatellite markers to compare the genetic diversity and structure between 42 native Australian and 18 invasive South African populations and to test different and competing introduction scenarios using Approximate Bayesian Computation analyses.

KEY RESULTS

Australian populations were characterized by two distinct genetic clusters, while South African populations lacked any clear genetic structure and showed significantly lower levels of genetic diversity compared to native range populations. South African populations were also genetically divergent from native populations and the most likely introduction scenario indicated an unknown source population.

CONCLUSIONS

Although we cannot definitely prove the cause of the observed genetic novelty/diversification in South African Acacia dealbata populations, it cannot be attributable to insufficient sampling of native populations. Our study highlights the complexity of unravelling the introduction histories of commercially important alien species.

Modelling the current fractional cover of an invasive alien plant and drivers of its invasion in a dryland ecosystem

The development of spatially differentiated management strategies against invasive alien plant species requires a detailed understanding of their current distribution and of the level of invasion across the invaded range. The objectives of this study were to estimate the current fractional cover gradient of invasive trees of the genus Prosopis in the Afar Region, Ethiopia, and to identify drivers of its invasion. We used seventeen explanatory variables describing Landsat 8 image reflectance, topography, climate and landscape structures to model the current cover of Prosopis across the invaded range using the random forest (RF) algorithm. Validation of the RF algorithm confirmed high model performance with an accuracy of 92% and a Kappa-coefficient of 0.8. We found that, within 35 years after its introduction, Prosopis has invaded approximately 1.17 million ha at different cover levels in the Afar Region (12.3% of the surface). Normalized difference vegetation index (NDVI) and elevation showed the highest explanatory power among the 17 variables, in terms of both the invader's overall distribution as well as areas with high cover. Villages and linear landscape structures (rivers and roads) were found to be more important drivers of future Prosopis invasion than environmental variables, such as climate and topography, suggesting that Prosopis is likely to continue spreading and increasing in abundance in the case study area if left uncontrolled. We discuss how information on the fractional cover and the drivers of invasion can help in developing spatially-explicit management recommendations against a target invasive plant species.

Historicising perceptions and the national management framework for invasive alien plants in South Africa

This article offers a historical framework for understanding changes to human perceptions and efforts to manage invasive alien plants and weeds in South Africa from the mid-nineteenth century until the present. The article argues that South African legislation and policy for managing invasive alien plants and weeds has historically been limited because people have held contradictory values about plants, many private land owners have lacked resources and have not been compelled to follow government legislation and because policy has reflected the interests of a small group of farmers or scientific experts who have had limited influence on most private land owners and traditional land users. Successful control efforts often relied on technical expertise that was applied controversially or could be implemented on government land without extensive public consultation or social conflict. The creation of a national framework for invasive alien plants through the Working for Water Programme in 1995 and National Environmental Management of Biodiversity Act (no. 10) of 2004 (NEMBA) has increased public awareness, but the Programme and NEMBA remain limited by many of the same institutional and social constraints that experts and institutions faced in the past. In conclusion, the article draws on history to provide insights to contemporary challenges.

Even well-studied groups of alien species might be poorly inventoried: Australian Acacia species in South Africa as a case study

Understanding the status and extent of spread of alien plants is crucial for effective management. We explore this issue using Australian Acacia species (wattles) in South Africa (a global hotspot for wattle introductions and tree invasions). The last detailed inventory of wattles in South Africa was based on data collated forty years ago. This paper aimed to determine: 1) how many Australian Acacia species have been introduced to South Africa; 2) which species are still present; and 3) the status of naturalised taxa that might be viable targets for eradication. All herbaria in South Africa with specimens of introduced Australian Acacia species were visited and locality records were compared with records from literature sources, various databases, and expert knowledge. For taxa not already known to be widespread invaders, field surveys were conducted to determine whether plants are still present, and detailed surveys were undertaken of all naturalised populations. To confirm the putative identities of the naturalised taxa, we also sequenced one nuclear and one chloroplast gene. We found evidence that 141 Australian Acacia species have been introduced to South Africa (approximately double the estimate from previous work), but we could only confirm the current presence of 33 species. Fifteen wattle species are invasive (13 are in category E and two in category D2 in the Unified Framework for Biological Invasions); five have naturalised (C3); and 13 are present but there was no evidence that they had produced reproductive offspring (B2 or C1). DNA barcoding provided strong support for only 23 taxa (including two species not previously recorded from South Africa), the current name ascribed was not supported for three species and, for a further three species, there was no voucher specimen on GenBank against which their identity could be checked. Given the omissions and errors found during this systematic re-evaluation of historical records, it is clear that analyses of the type conducted here are crucial if the status of even well-studied groups of alien taxa is to be accurately determined.

Codling Moth (Lepidoptera: Tortricidae) Establishment in China: Stages of Invasion and Potential Future Distribution

Codling moth (Cydia pomonella L.) is an internal feeding pest of apples and can cause substantial economic losses to fruit growers due to larval feeding which in turn degrades fruit quality and can result in complete crop loss if left uncontrolled. Although this pest originally developed in central Asia, it was not known to occur in China until 1953. For the first three decades the spread of codling moth within China was slow. Within the last three decades, addition of new commercial apple orchards and improved transportation, this pest has spread to over 131 counties in seven provinces in China. We developed regional (China) and global ecological niche models using MaxEnt to identify areas at highest potential risk of codling moth establishment and spread. Our objectives were to 1) predict the potential distribution of codling moth in China, 2) identify the important environmental factors associated with codling moth distribution in China, and 3) identify the different stages of invasion of codling moth in China. Human footprint, annual temperature range, precipitation of wettest quarter, and degree days ≥10 °C were the most important predictors associated with codling moth distribution. Our analysis identified areas where codling moth has the potential to establish, and mapped the different stages of invasion (i.e., potential for population stabilization, colonization, adaptation, and sink) of codling moth in China. Our results can be used in effective monitoring and management to stem the spread of codling moth in China.

Dynamic models in research and management of biological invasions

Invasive species are increasing in number, extent and impact worldwide. Effective invasion management has thus become a core socio-ecological challenge. To tackle this challenge, integrating spatial-temporal dynamics of invasion processes with modelling approaches is a promising approach. The inclusion of dynamic processes in such modelling frameworks (i.e. dynamic or hybrid models, here defined as models that integrate both dynamic and static approaches) adds an explicit temporal dimension to the study and management of invasions, enabling the prediction of invasions and optimisation of multi-scale management and governance. However, the extent to which dynamic approaches have been used for that purpose is under-investigated. Based on a literature review, we examined the extent to which dynamic modelling has been used to address invasions worldwide. We then evaluated how the use of dynamic modelling has evolved through time in the scope of invasive species management. The results suggest that modelling, in particular dynamic modelling, has been increasingly applied to biological invasions, especially to support management decisions at local scales. Also, the combination of dynamic and static modelling approaches (hybrid models with a spatially explicit output) can be especially effective, not only to support management at early invasion stages (from prevention to early detection), but also to improve the monitoring of invasion processes and impact assessment. Further development and testing of such hybrid models may well be regarded as a priority for future research aiming to improve the management of invasions across scales.

Impacts of invasive plants on animal diversity in South Africa: A synthesis

Background: Increasing numbers of invasive alien plant (IAP) species are establishing around the globe and can have negative effects on resident animal species function and diversity. These impacts depend on a variety of factors, including the extent of invasion, the region and the taxonomic group affected. These context dependencies make extrapolations of IAP impacts on resident biota from region to region a substantial challenge. Objectives: Here, we synthesised data from studies that have examined the effects of IAPs on animal diversity in South Africa. Our focus is on ectothermic organisms (reptiles, amphibians and invertebrates). Method: We sourced relevant articles using keywords relating to (1) the effects of IAPs on species diversity (abundance, richness and composition), (2) the IAP and (3) the native ectotherm. We extracted the taxonomic and spatial coverage of IAPs and affected native species and assessed the extent of information given on potential mechanisms driving IAP impacts. Results: Across the 42 studies, IAPs had a decreasing or neutral effect on native animal abundance and richness and significantly changed species composition. This review highlighted the paucity of studies and the research deficits in taxonomic and geographic coverage and in the mechanisms underlying IAP impacts on ectotherms. Conclusion: By assessing the status of knowledge regarding the impacts of IAPs on resident animal species in South Africa, this study identifies information gaps and research priorities at the country level with a view to informing monitoring and conservation efforts, such as alien plant removal and control programmes, and ensuring that endemic terrestrial animal diversity is maintained.

Changes in the composition and distribution of alien plants in South Africa: An update from the Southern African Plant Invaders Atlas

Background: Data on alien species status and occurrence are essential variables for the monitoring and reporting of biological invasions. The Southern African Plant Invaders Atlas (SAPIA) Project has, over the past 23 years, atlassed alien plants growing outside of cultivation. Objectives: To document changes in the alien plant taxa recorded in SAPIA, assess trends in invasive distributions and explore effects of management and regulations.Method: The numbers of alien plant taxa recorded were compared between May 2006 and May 2016, and changes in the extent of invasions at a quarter-degree squares (qds) scale were compared between 2000 and 2016. The effectiveness of regulations and interventions was assessed in terms of the relative change in the extent of invasions. Results: As of May 2016, SAPIA had records for 773 alien plant taxa, an increase of 172 since 2006. Between 2000 and 2016, the number of qds occupied by alien plants increased by ~50%, due both to ongoing sampling and to spread. Successful classical biological control programmes have reduced the rate of spread of some taxa and in a few cases have led to range contractions. However, other interventions had no detectable effect at a qds scale. Conclusions: South Africa has a growing number of invasive alien plant species across an increasing area. More taxa should be listed under national regulations, but ultimately more needs to be done to ensure that management is strategic and effective. SAPIA is a valuable tool for monitoring alien plant status and should be developed further so that invasions can be accurately tracked over time.

Exotic trees modify the thermal landscape and food resources for lizard communities

Increasing numbers of invasive plant species are establishing around the globe, and these species frequently form dense stands that alter habitat structure in critical ways. Nevertheless, little is known about the mechanisms underlying the impacts of invasive alien plant species on native fauna. We first ask whether alien pine trees impact lizard species richness in the Cape Floristic Region of South Africa, a world-renowned biodiversity hotspot, by examining differences in lizard species richness, abundance, and diversity between native mountain fynbos and exotic pine tree-dominated habitats. We then examine two mutually non-exclusive processes: (i) changes in the thermal quality of the habitat and (ii) changes in the availability of food resources, to explain differences in lizard assemblages among habitat types. Lizard richness, abundance, and diversity were greater in fynbos habitat than in fynbos heavily invaded by pine and in pine plantations. The thermal quality of the environment and food resources was consistently higher in native fynbos than in pine forests, but these responses were more varied when comparisons were made along an invasion gradient and among seasons. Our results suggest that management strategies must consider spatially and temporally detailed measurements of thermal regimes and resources to assess the impacts of invasive vegetation on reptile diversity.

Paradigm of plant invasion: multifaceted review on sustainable management

A cascade of reviews and growing body of literature exists on forest invasion ecology, its mechanism or causes; however, no review addressed the sustainable management of invasive plants of forest in totality. Henceforth, the present paper aims to provide a critical review on the management of invasive species particularly in the context of forest plants. Plant invasion in forest is now increasingly being recognized as a global problem, and various continents are adversely affected, although to a differential scale. Quest for the ecological mechanism lying behind the success of invasive species over native species of forest has drawn the attention of researches worldwide particularly in the context of diversity-stability relationship. Transport, colonization, establishment, and landscape spread may be different steps in success of invasive plants in forest, and each and every step is checked through several ecological attributes. Further, several ecological attribute and hypothesis (enemy release, novel weapon, empty niche, evolution of increased competitive ability, etc.) were proposed pertaining to success of invasive plant species in forest ecosystems. However, a single theory will not be able to account for invasion success among all environments as it may vary spatially and temporally. Therefore, in order to formulate a sustainable management plan for invasive plants of forest, it is necessary to develop a synoptic view of the dynamic processes involved in the invasion process. Moreover, invasive species of forest can act synergistically with other elements of global change, including land-use change, climate change, increased concentrations of atmospheric carbon dioxide, and nitrogen deposition. Henceforth, a unified framework for biological invasions that reconciles and integrates the key features of the most commonly used invasion frameworks into a single conceptual model that can be applied to all human-mediated invasions.

Global invasion of Lantana camara: has the climatic niche been conserved across continents?

Lantana camara, a native plant from tropical America, is considered one of the most harmful invasive species worldwide. Several studies have identified potentially invasible areas under scenarios of global change, on the assumption that niche is conserved during the invasion process. Recent studies, however, suggest that many invasive plants do not conserve their niches. Using Principal Components Analyses (PCA), we tested the hypothesis of niche conservatism for L. camara by comparing its native niche in South America with its expressed niche in Africa, Australia and India. Using MaxEnt, the estimated niche for the native region was projected onto each invaded region to generate potential distributions there. Our results demonstrate that while L. camara occupied subsets of its original native niche in Africa and Australia, in India its niche shifted significantly. There, 34% of the occurrences were detected in warmer habitats nonexistent in its native range. The estimated niche for India was also projected onto Africa and Australia to identify other vulnerable areas predicted from the observed niche shift detected in India. As a result, new potentially invasible areas were identified in central Africa and southern Australia. Our findings do not support the hypothesis of niche conservatism for the invasion of L. camara. The mechanisms that allow this species to expand its niche need to be investigated in order to improve our capacity to predict long-term geographic changes in the face of global climatic changes.

Prosopis: a global assessment of the biogeography, benefits, impacts and management of one of the world's worst woody invasive plant taxa

Invasive species cause ecological, economic and social impacts and are key drivers of global change. This is the case for the genus Prosopis (mesquite; Fabaceae) where several taxa are among the world's most damaging invasive species. Many contentious issues ('conflicts of interest') surround these taxa, and management interventions have not yet sustainably reduced the negative impacts. There is an urgent need to better understand the factors that drive invasions and shape management actions, and to compare the effectiveness of different management approaches. This paper presents a global review of Prosopis, focusing on its distribution, impacts, benefits and approaches to management. Prosopis was found to occur in a 129 countries globally and many more countries are climatically suitable. All areas with naturalized or invasive Prosopis species at present are suitable for more taxa and many Asian and Mediterranean countries with no records of Prosopis are bioclimatically suitable. Several Prosopis species have substantial impacts on biodiversity, ecosystem services, and local and regional economies in their native and even more so in their invasive ranges; others provide multiple benefits to local communities. Management efforts are underway in only a small part of the invaded range. Countries where more research has been done are more likely to implement formal management than those where little published research is available. Management strategies differ among countries; developed nations use mainly mechanical and chemical control whereas developing nations tend to apply control through utilization approaches. A range of countries are also using biological control. Key gaps in knowledge and promising options for management are highlighted.

Three centuries of managing introduced conifers in South Africa: Benefits, impacts, changing perceptions and conflict resolution

Alien conifers, mainly pines, have been planted in South Africa for a range of purposes for over 300 years. Formal plantations cover 660,000 ha of the country, and invasive stands of varying density occur on a further 2.9 million ha. These trees have brought many benefits but have also caused unintended problems. The management of alien conifers has evolved in response to emerging problems such as excessive water use by plantations of conifers, changing values and markets, and the realities of a new ecological order brought about by invasive alien conifers. This paper reviews the history of conifer introductions to South Africa, the benefits and impacts with which they are associated, and the ongoing and evolving research that has been conducted to inform their management. The South African approach has included taking courageous steps to address the problem of highly invasive species that are also an important commercial crop. These interventions have not, however, had the desired effect of both retaining benefits from formal plantations while simultaneously reversing the trend of growing impacts associated with self-sown invasive stands. We suggest that different approaches need to be considered, including the systematic phasing out of commercial forestry in zones where it delivers low returns, and the introduction of more effective, focussed and integrated, region-specific approaches to the management of invasive stands of conifers. These steps would deliver much improved economic outcomes by protecting valuable ecosystem services, but will require political commitment to policies that could be unpopular in certain sectors of society.

The prioritisation of invasive alien plant control projects using a multi-criteria decision model informed by stakeholder input and spatial data

Invasions by alien plants are a significant threat to the biodiversity and functioning of ecosystems and the services they provide. The South African Working for Water program was established to address this problem. It needs to formulate objective and transparent priorities for clearing in the face of multiple and sometimes conflicting demands. This study used the analytic hierarchy process (a multi-criteria decision support technique) to develop and rank criteria for prioritising alien plant control operations in the Western Cape, South Africa. Stakeholder workshops were held to identify a goal and criteria and to conduct pair-wise comparisons to weight the criteria with respect to invasive alien plant control. The combination of stakeholder input (to develop decision models) with data-driven model solutions enabled us to include many alternatives (water catchments), that would otherwise not have been feasible. The most important criteria included the capacity to maintain gains made through control operations, the potential to enhance water resources and conserve biodiversity, and threats from priority invasive alien plant species. We selected spatial datasets and used them to generate weights that could be used to objectively compare alternatives with respect to agreed criteria. The analysis showed that there are many high priority catchments which are not receiving any funding and low priority catchments which are receiving substantial allocations. Clearly, there is a need for realigning priorities, including directing sufficient funds to the highest priority catchments to provide effective control. This approach provided a tractable, consensus-based solution that can be used to direct clearing operations.

Sensitivity analysis of CLIMEX parameters in modelling potential distribution of Lantana camara L

A process-based niche model of L. camara L. (lantana), a highly invasive shrub species, was developed to estimate its potential distribution using CLIMEX. Model development was carried out using its native and invasive distribution and validation was carried out with the extensive Australian distribution. A good fit was observed, with 86.7% of herbarium specimens collected in Australia occurring within the suitable and highly suitable categories. A sensitivity analysis was conducted to identify the model parameters that had the most influence on lantana distribution. The changes in suitability were assessed by mapping the regions where the distribution changed with each parameter alteration. This allowed an assessment of where, within Australia, the modification of each parameter was having the most impact, particularly in terms of the suitable and highly suitable locations. The sensitivity of various parameters was also evaluated by calculating the changes in area within the suitable and highly suitable categories. The limiting low temperature (DV0), limiting high temperature (DV3) and limiting low soil moisture (SM0) showed highest sensitivity to change. The other model parameters were relatively insensitive to change. Highly sensitive parameters require extensive research and data collection to be fitted accurately in species distribution models. The results from this study can inform more cost effective development of species distribution models for lantana. Such models form an integral part of the management of invasive species and the results can be used to streamline data collection requirements for potential distribution modelling.

Spatial prediction of habitat overlap of introduced and native thistles to identify potential areas of nontarget activity of biological control agents

Nontarget feeding of Rhinocyllus conicus Fröelich and Trichosirocalus horridus (Panzer) on native North American thistles in the genus Cirsium has been documented. Some species of these native thistles have shown greater infestation levels of R. conicus in populations that are in close proximity to the target plant species, Carduus nutans L. In 2005 a study was initiated to identify areas of potential nontarget feeding by R. conicus and T. horridus on thistle species by predicting habitats of two known introduced hosts [C. nutans and Cirsium vulgare (Savi) Tenore] and two native species [Cirsium carolinianum (Walter) Fernald and Schubert and C. discolor (Muhlenberg ex Willdenow) Sprengel] using Mahalanobis distance (D(2)). Cumulative frequency graphs showed that the D(2) models for all four plant species effectively identified site conditions that contribute to the presence of the respective species. Poisson regression showed an association between D(2) values and plant counts at field-test sites for C. nutans and C. carolinianum. However, negative binomial regression detected no association between D(2) values and plant counts for C. discolor or C. vulgare. Chi-square analysis indicated associations between both weevil species and sites where C. vulgare and Carduus nutans were found, but not between the weevil and native thistle species. Habitats of C. nutans and Cirsium carolinianum overlapped in ≈12% of the study area. Data-based habitat models may provide a powerful tool for land managers and scientists to monitor native plant populations for nontarget feeding by introduced biological control agents.

Temporal biodiversity change in transformed landscapes: a southern African perspective

Landscape transformation by humans is virtually ubiquitous, with several suggestions being made that the world's biomes should now be classified according to the extent and nature of this transformation. Even those areas that are thought to have a relatively limited human footprint have experienced substantial biodiversity change. This is true of both marine and terrestrial systems of southern Africa, a region of high biodiversity and including several large conservation areas. Global change drivers have had substantial effects across many levels of the biological hierarchy as is demonstrated in this review, which focuses on terrestrial systems. Interactions among drivers, such as between climate change and invasion, and between changing fire regimes and invasion, are complicating attribution of change effects and management thereof. Likewise CO(2) fertilization is having a much larger impact on terrestrial systems than perhaps commonly acknowledged. Temporal changes in biodiversity, and the seeming failure of institutional attempts to address them, underline a growing polarization of world views, which is hampering efforts to address urgent conservation needs.