Invasive Alien Tree Species Research Articles

Verticillium Species as an Ecofriendly Alternative to Manage the Invasive Tree Ailanthus altissima (Mill.) Swingle

Environmental pollution, unintended harm to beneficial organisms, and the development of herbicide resistance among weeds are the main consequences of the massive and consistent use of chemical herbicides in recent decades. The growing need for alternative solutions has been reinforced by restrictive policies, leading to a search for natural herbicidal candidates. Mycoherbicides, formulations containing plant pathogenic fungi, are viewed as promising substitutes for chemical herbicides. In the case of Ailanthus altissima (Mill.) Swingle, one of the worst invasive alien tree species in the world, Verticillium-based mycoherbicides offer a viable method for control, inducing a lethal wilt disease and leading plants to death within a few years. The demonstrated significant effectiveness enables addressing challenges posed by other—conventional—approaches. The current analysis matches key internal (strengths and weaknesses) and external factors (opportunities and threats) of Verticillium Nees isolates as environmentally-friendly control agents against the invasive A. altissima, by listing each singularly and then crossing them among the categories, drawing from the collaborative efforts of American, Austrian, and Italian research teams.

Metabarcoding and Metabolomics Reveal the Effect of the Invasive Alien Tree Miconia calvescens DC. on Soil Diversity on the Tropical Island of Mo'orea (French Polynesia).

Miconia calvescens is a dominant invasive alien tree species that threatens several endemic plants in French Polynesia (South Pacific). While most analyses have been performed at the scale of plant communities, the effects on the rhizosphere have not been described so far. However, this compartment can be involved in plant fitness through inhibitory activities, nutritive exchanges, and communication with other organisms. In particular, it was not known whether M. calvescens forms specific associations with soil organisms or has a specific chemical composition of secondary metabolites. To tackle these issues, the rhizosphere of six plant species was sampled on the tropical island of Mo'orea in French Polynesia at both the seedling and tree stages. The diversity of soil organisms (bacteria, microeukaryotes, and metazoa) and of secondary metabolites was studied using high-throughput technologies (metabarcoding and metabolomics, respectively). We found that trees had higher effects on soil diversity than seedlings. Moreover, M. calvescens showed a specific association with microeukaryotes of the Cryptomycota family at the tree stage. This family was positively correlated with the terpenoids found in the soil. Many terpenoids were also found within the roots of M. calvescens, suggesting that these molecules were probably produced by the plant and favored the presence of Cryptomycota. Both terpenoids and Cryptomycota were thus specific chemicals and biomarkers of M. calvescens. Additional studies must be performed in the future to better understand if they contribute to the success of this invasive tree.

Further pathogenicity testing of Verticillium nonalfalfae, a biocontrol agent against the invasive Tree of Heaven (Ailanthus altissima), on non-target tree species in Europe

Verticillium nonalfalfae is an effective biological control agent against the highly invasive Tree of Heaven (Ailanthus altissima) in Europe, and previous research on ten economically and ecologically important tree species occurring in Austria have so far not revealed undesired non-target effects. In this study, another nine tree species including five native, two non-native as well as two invasive alien tree species were tested for susceptibility to the particular strain of V. nonalfalfae (Vert56) used for biological control of A. altissima. Stem inoculations on potted seedlings revealed that this strain of V. nonalfalfae is generally host-adapted to A. altissima. It induced chlorosis, necrosis and wilting already within two weeks post inoculation on A. altissima and resulted in almost completely defoliated or dead seedlings at the end of the vegetation period. Apart from two species (Quercus rubra and Sorbus aucuparia), that suffered from other abiotic/biotic agents, no mortality was observed on all other tree species tested; however, symptoms caused by other abiotic factors were also found on Prunus avium and Ulmus glabra. All tested tree species exhibited vascular discolorations and the fungus could be re-isolated at varying frequencies (6—100%) from inoculated seedlings of all non-target tree species, although five of these species exhibited no external symptoms. Results confirmed high susceptibility (S) of A. altissima to V. nonalfalfae, whereas Acer platanoides, Castanea sativa, Q. rubra, S. aucuparia and U. glabra were considered as tolerant (T), and A. negundo, P. avium, P. serotina and Q. petraea were rated as possible resistant (PR) due to the low rates of re-isolation.

Potential allelopathic effects of alien Acacia melanoxylon and indigenous Olea capensis subsp. macrocarpa on germination of Acacia melanoxylon

The invasive alien tree species Acacia melanoxylon R. Br has been known to invade some forest ecosystems in Europe through allelopathy (amongst other mechanisms) which allows a competitive advantage over indigenous species. Acacia melanoxylon is widespread in Southern Cape Afrotemperate Forest, South Africa, and may establish successfully in these forests in certain contexts. In a nursery trial we compared the allelopathic potential of phyllode or leaf leachates of Acacia melanoxylon and a common overstorey indigenous species, Olea capensis L. macrocarpa C.H. Wright on the germination of seeds of Acacia melanoxylon. We compared the germination of Acacia melanoxylon seeds among three treatments, namely Acacia melanoxylon leachate, Olea capensis macrocarpa leachate, and no leachate. Stain-testing of seeds showed high viability (90%). The average germination of Acacia melanoxylon was 67% and there was a marginally significant difference in the number of germinants between the three treatments (H= 6.16; df= 2; P= 0.04). Pairwise multiple comparisons of ranks revealed that germination was marginally higher under Acacia melanoxylon leachate treatment than the control (Z = 1.97; df = 2; P = 0.14), and marginally higher under Olea capensis macrocarpa leachate than the control (Z = 0.12; df = 2; P = 0.12). This suggests that germination of Acacia melanoxylon is improved, or at the least, unaffected, by allelopathic substances of itself or of a common Afrotemperate Forest overstorey species. We concluded that the lack of sensitivity of Acacia melanoxylon germination to allelopathic substances of itself or a common indigenous overstorey species likely contribute to its successful establishment in Southern Cape Afrotemperate Forest.

Effects of sika deer (Cervus nippon) on the spread of invasive alien tree species in a warm temperate evergreen forest

Effects of sika deer (Cervus nippon) on the spread of invasive alien tree species in a warm temperate evergreen forest

Chemical Compounds, Antitumor and Antimicrobial Activities of Dry Ethanol Extracts from Koelreuteria paniculata Laxm.

Koelreuteria paniculata Laxm. is used in traditional medicine and has various established biological activities, however, the species is considered to be a potentially invasive alien tree species for Bulgarian flora. However, there is still much to be studied about the phytochemical and biological characteristics of the species. The present study aimed to determine the chemical composition of the ethanol extracts of aerial plant parts, by GC-MS analysis, and to thereby evaluate their in vitro antitumor and antibacterial properties. All three extracts were tested against the HT-29 and PC3 tumor cell lines using the MTT assay. Fifty-six components were identified from leaf, flower, and stem bark extracts, and over 10% were the following constituents: pyrogallol, α-terpinyl acetate, neryl acetate, and α-terpinyl isobutanoate. The oxygenated monoterpenes predominated in the extracts, followed by the oxygenated aliphatics and phenylpropanoids. Significant antiproliferative activity on the HT-29 cell line (IC50–21.44 µg/mL and 23.63 µg/mL, respectively) was found for the flower and leaf extracts. Antibacterial activity was established for the following bacteria strains: Bacillus subtilis ATCC 6633, Bacillus cereus NCTC 10320, Escherichia coli ATCC 8739, Pseudomonas aeruginosa ATCC 6027, and Proteus vulgaris ATCC 6380. The stem bark and flower extracts showed better antimicrobial potential. K. paniculata could be considered as a potential source of biologically active substances with antitumor and antibacterial properties.

Opportunities and challenges in using remote sensing for invasive tree species management, and in the identification of restoration sites in tropical montane grasslands

Tropical montane grasslands (TMG) support biodiverse and endemic taxa and provide vital ecosystem services to downstream communities. Nevertheless, invasive alien tree species across the world have threatened tropical grasslands and grassland endemic species. In India, TMG in the Shola Sky Islands of the Western Ghats have been reduced due to exotic tree invasions (Acacias, Pines, and Eucalyptus species). The loss of grassland habitat has, in turn, reduced the range sizes of species endemic to grasslands (plants, birds, amphibians, and mammals), driving some populations to local extinction. Grassland conversion to exotic trees has also impacted ecosystem services in the Western Ghats. Conserving existing grassland and restoring invaded habitat is critical to reverse these losses. This research focused on identifying grassland restoration sites using satellite images with a high spatial resolution (RapidEye). We used an object-oriented Random Forest classification to map the area for grassland restoration. We identified an area of 254 sq. km. as suitable for grassland restoration and an area of 362 sq. km. for grassland conservation and preventing invasion by exotic tree species. For restoration, we recommend careful removal of young and isolated exotic trees at the invasion front and restoring grasslands, instead of removing dense stands of mature exotic trees. Although our limited data indicate that areas with low fire frequency tend to be invaded, and areas invaded by exotic trees tend to burn at higher intensities, we recommend a broader investigation of these patterns to critically examine a potential role for the use of fire in invasive species management. We assume that removing exotic tree species in the identified restoration sites and restoring the grassland will help recover lost habitat and ensure the viability of indigenous and endemic species and increase streamflow.

Outstanding performance of an invasive alien tree Bischofia javanica relative to native tree species and implications for management of insular primary forests.

Invasive alien tree species can exert severe impacts, especially in insular biodiversity hotspots, but have been inadequately studied. Knowledge of the life history and population trends of an invasive alien tree species is essential for appropriate ecosystem management. The invasive tree Bischofia javanica has overwhelmed native trees on Haha-jima Island in the Ogasawara Islands, Japan. We explored forest community dynamics 2 years after a typhoon damaged the Sekimon primary forests on Haha-jima Island, and predicted the rate of population increase of B. javanica using a logistic model from forest dynamics data for 19 years. During the 2 years after the typhoon, only B. javanica increased in population size, whereas populations of native tree species decreased. Stem diameter growth of B. javanica was more rapid than that of other tree species, including native pioneer trees. Among the understory stems below canopy trees of other species, B. javanica grew most rapidly and B. javanica canopy trees decreased growth of the dominant native Ardisia sieboldii. These competitive advantages were indicated to be the main mechanism by which B. javanica replaces native trees. The logistic model predicted that B. javanica would reach 30% of the total basal area between 2017 (in the eastern plot adjacent to a former B. javanica plantation) and 2057 (in the western plot distant from the plantation site), which is a maximum percentage allowing to eradicate under the present guideline of the National Forest. The results suggest immediate removal of B. javanica is required to preserve native biodiversity in these forests.

The hydrological performance of Prosopis juliflora (Sw.) growth as an invasive alien tree species in the semiarid tropics of northeastern Brazil

The fast-growing tree Prosopis juliflora was introduced into the drought-prone region of Caatinga in Brazil northeast some decades ago, forming now extensive populations that threaten native biodiversity and disrupt the physiognomy of the local flora. Semiarid tropical forests, such as those that occur in the Caatinga, experience periods of seasonal drought which interrupt cambial activity in woody plants, leading to the formation of annual growth rings that can be used to analyze tree age, growth variability over time and its interactions with climate. We produce a tree-ring width chronology of P. juliflora, in order to establish how this plant is linked to local climate variability, which in turn supports its role as an invasive species in the Caatinga forest. Statistical analyses of the potential effects of climatic variables on this chronology is related directly to the wet season (March–July) and torrential downpours occurring during the dry season (January–February). Moreover, the chronology is inversely related to mean temperature and insolation, which could potentially be linked to rapid loss of moisture in soils due to high temperatures. These findings provide important insights into the growth dynamics of this invasive species, which should contribute to the decision-making of managers and environmental agencies on the development of effective exotic forest management practices in the Caatinga region.

Exploring patterns, drivers and structure of plant community composition in alien Robinia pseudoacacia secondary woodlands

Abstract: Invasive alien tree species can strongly impact biodiversity and future projections predict their spread over natural, semi-natural and human habitats. However, little is known about plant communities that form during the first stages of invasion. We investigated the composition of plant communities in alien Robinia pseudoacacia L. secondary forests growing on grasslands and cultivated areas abandoned during the last 35-40 years in north-eastern Italy to understand whether these formations could cause floristic homogenization of plant communities composition. On the basis of a cluster analysis, plant communities were assigned to seven syntaxonomic categories and split into four groups characterized by the occurrence of 20 species indicative of (a) nitrogen-rich, (b) true forest and (c) open habitat conditions. RDA analysis enabled main stand and environmental variables filtering these communities to be identified and β-diversity components were partitioned through the SDR (Similarity - richness Difference - species Replacement) simplex approach. Plant composition patterns were significantly associated to variability in elevation, stand vertical structure, shrub cover, mean tree diameter and height, and basal area. Shrub cover discriminates between plant communities associated with open or shaded conditions. The partition of β-diversity components revealed that replacement is the prominent process structuring plant communities in these secondary forests. Our study showed that secondary Robinia forests growing on abandoned lands may host compositionally heterogeneous plant communities, thus contributing to regional biodiversity.

Five major invasive alien tree species in European Union forest habitat types of the Alpine and Continental biogeographical regions

Invasive alien species are a threat to protected areas, species, and habitats worldwide. At the European level, the control of invasive alien species is a major goal of the EU Biodiversity Strategy, and a cornerstone, in this sense, is the Regulation (EU) no.1143/2014 on invasive alien species. In this study, we focus on the Alpine and Continental biogeographical regions of Europe (defined in art. 1 of the Habitats Directive; 92/43/EEC), and aim to give an overview of the main protected forest habitat types threatened by invasive alien species, and focus on five major invasive alien tree species (Acer negundo, Ailanthus altissima, Prunus serotina, Quercus rubra, and Robinia pseudoacacia). We considered the assessment reports under Article 17 of the Habitats Directive for the period 2007–2012, and six documents on the interpretation of habitats, to identify those most threatened by invasive alien species and in particular by the five invasive alien trees. We also assessed selected examples of impacts by these alien tree species on different species groups within Natura 2000 sites to highlight potential effects on biodiversity. Riparian forest habitats and oak dominated woodlands are among the most prone to invasion within the two biogeographical regions. A. altissima and R. pseudoacacia are the most reported invasive alien trees in these protected forest habitat types. Furthermore, examples of the impacts of these tree species have indicated potential detrimental effects on forest habitats structure and functions, and on single species listed in the Habitats and Birds Directives. In conclusion, we emphasise the need for effective management and systematic monitoring of these five invasive alien tree species within Natura 2000 sites.

Episodic Canopy Structural Transformations and Biological Invasion in a Hawaiian Forest.

The remaining native forests on the Hawaiian Islands have been recognized as threatened by changing climate, increasing insect outbreak, new deadly pathogens, and growing populations of canopy structure-altering invasive species. The objective of this study was to assess long-term, net changes to upper canopy structure in sub-montane forests on the eastern slope of Mauna Kea volcano, Hawai‘i, in the context of continuing climate events, insect outbreaks, and biological invasion. We used high-resolution multi-temporal Light Detection and Ranging (LiDAR) data to quantify near-decadal net changes in forest canopy height and gap distributions at a critical transition between alien invaded lowland and native sub-montane forest at the end of a recent drought and host-specific insect (Scotorythra paludicola) outbreak. We found that sub-montane forests have experienced a net loss in average canopy height, and therefore structure and aboveground carbon stock. Additionally, where invasive alien tree species co-dominate with native trees, the upper canopy structure became more homogeneous. Tracking the loss of forest canopy height and spatial variation with airborne LiDAR is a cost-effective way to monitor forest canopy health, and to track and quantify ecological impacts of invasive species through space and time.

Knowledge, perceptions and willingness to control designated invasive tree species in urban household gardens in South Africa

Many biological invasions result in negative impacts on the environment and human livelihoods, but simultaneously some also provide benefits that are valued differently by various stakeholders. To inform policy and management of invasive species it is important to assess landowners’ and broader society’s knowledge and perceptions of invasive species, something which is lacking in many contexts, especially in urban settings. In this study we interviewed 153 householders living in a medium-sized South African town who had declared invasive alien trees in their gardens. Less than half of the respondents could identify the invasive tree on their property and only one-third knew that it was an invasive alien species. There was a positive association between income and education levels with exposure to media about invasive alien species and respondents’ ability to identify the species and name any other invasive alien tree species. Knowledge levels were unequal across species. Amongst those who knew the tree was an invasive alien species, reasons why they retained it in their gardens included that it would be costly or too much effort to remove, they liked the tree, that it was not causing any harm and that the property was rented and so its removal was not their responsibility. However, the majority of people (83 %) were willing to have it removed from their garden if done for free by appropriate agencies, which is promising for compliance with new regulations on invasive species implemented at the end of 2014 in South Africa. The results also highlight the need for targeted and appropriate education and awareness programs amongst urban householders on invasive alien species, relevant legislation and their obligations.

Restoration of invaded Cape Floristic Region riparian systems leads to a recovery in foliage-active arthropod alpha- and beta-diversity

The Cape Floristic Region of South Africa is a global biodiversity hotspot threatened by invasive alien plants (IAPs). We assessed the effect of plant invasions, and their subsequent clearing, on riparian arthropod diversity. Foliage-active arthropod communities were collected from two native and one invasive alien tree species. Alpha- and beta-diversity of their associated arthropod communities were compared between near pristine, Acacia-invaded and restored sites. Arthropod alpha-diversity at near pristine sites was higher than at restored sites, and was lowest at invaded sites. This was true for most arthropod taxonomic groups associated with all native tree species and suggests a general trend towards recovery in arthropod alpha-diversity after IAP removal. Overall, arthropod species turnover among sites was significantly influenced by plant invasions with communities at near pristine sites having higher turnover than those at restored and invaded sites. This pattern was not evident at the level of individual tree species. Although arthropod community composition was significantly influenced by plant invasions, only a few significant differences in arthropod community composition could be detected between restored and near pristine sites for all tree species and arthropod taxonomic groups. Assemblage composition on each tree species generally differed between sites with similar degrees of plant invasion indicating a strong turnover of arthropod communities across the landscape. Results further suggest that both arthropod alpha- and beta-diversity can recover after IAP removal, given sufficient time, but catchment signatures must be acknowledged when monitoring restoration recovery.

ESPÉCIES EXÓTICAS INVASORAS ARBÓREAS NO PARQUE DA BARREIRINHA EM CURITIBA: REGISTRO E IMPLICAÇÕES

http://dx.doi.org/10.5902/1980509818451A maioria das Unidades de Conservação (UCs) de Curitiba (PR, Brasil) está contaminada por espécies arbóreas exóticas invasoras (AEIs). Mas não existem estudos que evidenciem seu impacto e manejo apropriado. Os objetivos dessa pesquisa foram avaliar a participação das AEIs e as implicações e manejo da AEI de maior importância estrutural na composição florística-estrutural do Parque da Barreirinha, um remanescente de Floresta com Araucária. Foram avaliadas quatro unidades amostrais, com área total de 1600 m² representando aproximadamente 4% da área total do maciço vegetal. Foram amostrados 354 indivíduos arbóreos, com DAP ≥ 15 cm, e calculados, para cada espécie amostrada, os parâmetros de frequência relativa, densidade relativa, dominância relativa e valor de importância. Foram identificadas 29 famílias, 43 gêneros e 57 espécies, sendo três AEIs. A sustentabilidade e a integridade desta UC podem estar comprometidas devido à presença de Pittosporum undulatum Vent. (pau-incenso), espécie de maior valor de importância estrutural, alta densidade e frequência. Pelas características botânico-ecológicas do Pittosporum undulatum, a opção de remoção de indivíduos adultos deve ser considerada com muita cautela.

Black Wattle (Acacia mearnsii De Wild.) in Rwanda’s Forestry: Implications for Nature Conservation

Invasive species pose a serious threat to the conservation of biodiversity. Forestry is the most important driver of the introduction and spread of invasive woody plant species after horticulture. Nearly 450 invasive tree species have been recorded worldwide, amounting to more than half the world’s total invasive woody flora. Australian-native Acacia mearnsii De Wild. is one of world’s top invasive alien tree species related to forestry. The negative impacts of alien species spilling over into conservation areas because of forestry are increasingly being recognized, and we examined the context and timing of this Australian species’ arrival in Rwanda. The role of A. mearnsii in Rwanda’s colonial and postcolonial forestry and the species’ threat to biodiversity conservation are also investigated. The results suggest that A. mearnsii (a) was introduced into Rwanda between 1903 and 1918, with the earliest formal record dating back to 1941; (b) was a major forestry tree species until the late 1960s when its use in forestry declined significantly; and (c) demonstrates a comparatively higher occurrence and potential of regeneration within the understory of Eucalyptus plantations within the Congo-Nile ridge buttress; Bufundu and Bushiru; and Budaha, Ndiza, and Buberuka agro-bioclimatic zones. More recently, A. mearnsii has invaded national parks and forest reserves above 1,600 m of altitude. The most significant case was recorded at Nyungwe National Park (NNP), East Africa’s largest montane rainforest. A study of biomass and spatial patterns of this species in specific ecosystems, such as NNP, is strongly recommended in order to uphold effective conservation practices.

Developing an effective glyphosate application technique to control Bischofia javanica Blume, an invasive alien tree species in the Ogasawara Islands

Bischofia javanica is a non-native tree species in Japan's Ogasawara Islands, where it threatens native tree species due to its rapid propagation and growth. An effective method is needed to limit the expansion of B. javanica populations and to conserve the natural forest ecosystem of the islands. For this purpose, we examined the effectiveness of a new application technique for the herbicide glyphosate on B. javanica. In this method, glyphosate solution is directly injected into holes drilled in the stem, and each hole is then plugged with a cork stopper to prevent loss of the solution, namely the drill-and-plug method. We also developed an allometric regression model linking stem diameter with total aboveground biomass (AGB) to estimate the necessary herbicide dosage. Our results suggest that between 0.1 and 0.5 g kg−1 (active ingredient per unit AGB) is required to control most B. javanica trees. Verification of the drill-and-plug method using the minimum dosage (0.1 g kg−1) showed that most of the herbicide-treated trees were killed. These results suggest that the drill-and-plug method can help control the B. javanica invasion of the Ogasawara Islands.

Different factors affect the local distribution, persistence and spread of alien tree species in floodplain forests

Floodplain forests are characterized by high and increasing levels of invasions by plant species, but the factors that drive their spread are insufficiently understood. Using data from 708 plots surveyed twice (1998, 2008) supplemented with further data (management, stand age, distance to dispersal corridors, type of ecosystems invaded) we analyzed the factors which shape the local distribution, growth, persistence and spread of three invasive alien tree species (Acer negundo, Ailanthus altissima, Robinia pseudoacacia) in the National Park Donau-Auen in Austria. Using Generalized Linear Models (GLMs), we found that the distribution of the study species per plot is contingent on stand age (R. pseudoacacia), type of floodplain forest (A. negundo, A. altissima) and distance to the next water body (A. negundo). For all study species, colonization of new plots between both surveys is driven by short distance spread from already established invasion foci. Moreover, recipient habitats (softwood vs. hardwood floodplain forests) modify invasion success in species-specific ways. The probability of occurrence and colonization of plots located in softwood floodplain forests is higher for A. negundo, whereas A. altissima more frequently colonizes hardwood floodplain forests. Persistence of R. pseudoacacia decreases with stand age, whereas its growth rate is significantly higher in plots located in zones where management is allowed than in those which are not managed. Persistence and growth of the other two study species were not related to any explanatory variables analyzed. The on-going spread of the study species in the National Park Donau-Auen suggests that their local distribution is in disequilibrium, i.e. not all suitable habitat patches have yet been colonized. This implies that increased management efforts are necessary to reverse the spread of alien tree species in the study area and to maintain the high conservation value of this iconic area which protects one of the largest floodplain forests in Europe.

Valuation of pollinator forage services provided by Eucalyptus cladocalyx

We assess the monetary value of forage provisioning services for honeybees as provided by an alien tree species in the Western Cape province of South Africa. Although Eucalyptus cladocalyx is not an officially declared invader, it is cleared on a regular basis along with other invasive Eucalyptus species such as Eucalyptus camaldulensis, and Eucalyptus conferruminata (which have been prioritised for eradication in South Africa). We present some of the trade-offs associated with the clearing of E. cladocalyx by means of a practical example that illustrates a situation where the benefits of the species to certain stakeholders could support the containment of the species in demarcated areas, while allowing clearing outside such areas.Given the absence of market prices for such forage provisioning services, the replacement cost is used to present the value of the loss in forage as provided by E. cladocalyx if the alien tree species is cleared along with invasive alien tree species. Two replacement scenarios formed the basis for our calculations. The first scenario was an artificial diet as replacement for the forage provisioning service, which yielded a direct cost estimate of US$7.5m per year. The second was based on a Fynbos cultivation/restoration initiative aimed at substituting the forage provisioning service of E. cladocalyx, which yielded a direct cost of US$20.2m per year. These figures provide estimates of the potential additional cost burden on the beekeeping industry if E. cladocalyx is completely eradicated from the Western Cape. The cost estimates should be balanced against the negative impacts of E. cladocalyx on ecosystem services in order to make an informed decision with regard to appropriate management strategies for this species. The findings therefore serve as useful inputs to balance trade-offs for alien species that are considered as beneficial to some, but harmful to other.

Costs, benefits and management options for an invasive alien tree species: The case of mesquite in the Northern Cape, South Africa

Mesquite (Prosopis species) were introduced to South Africa to provide fodder and shade for livestock, but some have become invasive, impacting on water and grazing resources. Mesquite’s net economic effects are unclear and their unequal distribution leads to conflict. We estimated the value of mesquite invasions in the Northern Cape Province for different scenarios, differentiating between productive floodplains and upland areas. The estimated net economic value of mesquite in 2009, covering 1.47 million ha, was US$3.5–15.3 million. The value will become negative within 4–22 years, assuming annual rates of spread of 30 and 15%, respectively. The estimated 30-year present value (3% discount rate) of the benefits of control in the floodplains exceeded that of costs but the opposite was true in the uplands. Control efforts should therefore focus on floodplains while preventing spread from uplands into cleared or uninvaded floodplains. More efficient control methods are needed as estimated control costs (>US$9.5 million yr−1) exceed financial capabilities of Public Works programmes. Control in the floodplains was not economically justifiable using an 8% discount rate, because this substantially discounted future costs. We conclude that more effective control methods, such as biological control, are needed to prevent substantial economic losses.