Browsing by Author "Richardson D.M."
Now showing 1 - 16 of 16
Results Per Page
Sort Options
- ItemA reference framework for the restoration of riparian vegetation in the Western Cape, South Africa, degraded by invasive Australian Acacias(2004) Prins N.; Holmes P.M.; Richardson D.M.Riparian vegetation, which normally differs structurally and compositionally from surrounding vegetation, has been degraded in many parts of the fynbos biome by several species of invasive Australian Acacia. Systematic clearing of dense stands of these alien trees was initiated in 1995, and information is urgently needed to guide the restoration of riparian habitats. A problem is that degradation of these communities is so advanced and widespread that in many cases managers do not know what species to use in restoration, or what kinds of target communities to aim for. This study's aim was to provide baseline information on riparian plant community structure and composition from non-transformed habitats. Species and environmental data were recorded from 76 sites located along the headwater systems of six rivers in the southwestern part of the Western Cape province. Analysis of the data, applying multivariate classification (TWINSPAN) and ordination (Detrended Correspondence Analysis) techniques, identified four prospective plant communities: 1) a Nivenia corymbosa-Brachylaena neriifolia Community; 2) a Leucadendron salicifolium-Berzelia lanuginosa Community; 3) a Cliffortia ruscifolia- Metrosideros angustifolia Community; and 4) a Kiggelaria africana-Brabejum stellatifolium Community. These formed a continuum with only the Leucadendron and Kiggelaria communities separating in ordination space. Soil pH differed between the latter two communities, reflecting different geology. It was found that many riparian specialist species are relatively widespread. For the study area, it is concluded that where information on the historical composition of riparian communities is lacking, target communities for restoration can be defined from pristine communities with similar geology, and secondly, altitude. In all cases the target community will comprise a large proportion of widespread, predominantly resprouting, riparian species.
- ItemConservation biogeography - foundations, concepts and challenges(2010) Richardson D.M.; Whittaker R.J.Conservation biogeography involves the application of biogeographical principles, theories, and analyses to problems regarding biodiversity conservation. The field was formally defined in 2005, and considerable research has been conducted in the ensuing 5 years.This editorial sets the context for 16 contributions in a special issue of Diversity and Distributions on developments and challenges in conservation biogeography. Papers are grouped into the following main themes: species distribution modelling; data requirements; approaches for assigning conservation priorities; approaches for integrating information from numerous disparate sources; special challenges involving invasive species; and the crucial issue of determining how elements of biodiversity are likely to respond to rapid climate change. One paper provides a synthesis of requirements for a robust conservation biogeography for freshwater ecosystems.Conservation biogeography is well poised to make a significant contribution to the process of providing policy makers with objectively formulated scenarios and options for the effective management of biodiversity. The editorial, and the papers in the special issue, deliberate on many of the exciting developments in play in the field, and the many complex challenges that lie ahead. © 2010 Blackwell Publishing Ltd.
- Item
- ItemFifty years of invasion ecology - The legacy of Charles Elton(2008) Richardson D.M.; Pysek P.The publication, in 1958, of Charles Elton's book The ecology of invasions by animals and plants launched the systematic study of biological invasions. Invasion ecology has grown to become an important multi-disciplinary subfield of ecology with growing links to many other disciplines. This paper examines the citation history of Elton's book using the Web of Science. We also examine Elton's influence in shaping the current research agenda in invasion ecology, for which we use the 28 papers in a special issue of Diversity and Distributions (Volume 14: 2) as a representative sample. After 50 years, Elton's book remains the most cited single source in the field (> 1500 citations), and is cited more often every year (> 100 times) than any other invasion-related publication, including influential papers in journals. Most citations to Elton's book refer to particular topics/concepts covered in the book, rather than citing it as a general reference about invasions. The shift in the distribution of topics/concepts cited with reference to Elton over time follows the same trend as for biogeography and ecology in general (increasing emphasis on analytical studies, multi-scale analyses, multi-disciplinary studies, etc.). Some topics emphasized by Elton are still the focus of current research (dispersal and spread of invasive organisms, impact on biodiversity, role of disturbance and enemy release) but several prominent themes in modern studies were not addressed by Elton. The emergence of new themes can be attributed to a general change in approach and emphasis underpinning research questions in conservation biogeography and applied ecology over the last half century (risk analysis, multi-scale comparisons, propagule pressure, experimental approaches) and to the recent emergence and increasing availability of large data sets on the distribution of introduced species and to the emergence of key technologies (e.g. geographic information systems, modelling techniques, including niche-based modelling, and molecular methods). Half a century after its publication, Charles Elton's book on invasions remains influential, but massive changes in the status of invasions and other environmental issues worldwide, together with advances in technology, are reshaping the game rules and priorities of invasion ecology. © 2008 The Authors.
- ItemIs phylogenetic relatedness to native species important for the establishment of reptiles introduced to California and Florida?(2011) Van Wilgen N.J.; Richardson D.M.Aim Charles Darwin posited that introduced species with close relatives were less likely to succeed because of fiercer competition resulting from their similarity to residents. There is much debate about the generality of this rule, and recent studies on plant and fish introductions have been inconclusive. Information on phylogenetic relatedness is potentially valuable for explaining invasion outcomes and could form part of screening protocols for minimizing future invasions. We provide the first test of this hypothesis for terrestrial vertebrates using two new molecular phylogenies for native and introduced reptiles for two regions with the best data on introduction histories.Location California and Florida, USA.Methods We performed an ordination of ecological traits to confirm that ecologically similar species are indeed closely related phylogenetically. We then inferred molecular phylogenies for introduced and native reptiles using sequence data for two nuclear and three mitochondrial genes. Using these phylogenies, we computed two distance metrics: the mean phylogenetic distance (MPD) between each introduced species and all native species in each region (which indicates the potential interactions between introduced species and all native species in the community) and the distance of each introduced species to its nearest native relative - NN (indicating the degree of similarity and associated likelihood of competition between each introduced species and its closest evolutionary analogue). These metrics were compared for introduced species that established and those that failed.Results We demonstrate that phylogenetically related species do share similar ecological functions. Furthermore, successfully introduced species are more distantly related to natives (for NN and MPD) than failed species, although variation is high.Main conclusions The evolutionary history of a region has value for explaining and predicting the outcome of human-driven introductions of reptiles. Phylogenetic metrics are thus useful inputs to multi-factor risk assessments, which are increasingly required for screening introduced species. © 2010 Blackwell Publishing Ltd.
- ItemManaging biological invasions: Charting courses to desirable futures in the Cape Floristic Region(2011) Roura-Pascual N.; Richardson D.M.; Chapman R.A.; Hichert T.; Krug R.M.Invasive species alter the functioning of natural ecosystems, creating "novel ecosystems" comprising species occurring in combinations with no analogs within a given biome. This poses major challenges for managers who cannot rely exclusively on previous experiences. Multiple factors that drive invasion and which interact in complex ways demand innovative management approaches. We show the utility of scenario planning in considering options for management in a region with substantial problems with invasive alien plants: South Africa's Cape Floristic Region. The approach allows us to identify the driving forces that shape the status and trajectories of major woody invasive plants and to identify sensible strategies by considering a set of scenarios based on the main uncertainties that encapsulate the linkages between the various components of the management of biological invasions. Attitudes of landowners and management capacity are shown to be the crucial uncertainties influencing the spread of major invasive species; axes based on these factors define our scenarios. Mapping current management projects onto scenario axes highlighted key differences among areas. These insights can assist in directing particular management units toward more desirable futures. Our study highlights the need to link social, political and legal constraints with ecological processes to assure the effectiveness of management operations in controlling biological invasions. © 2010 Springer-Verlag.
- ItemNative and naturalized range size in Pinus: Relative importance of biogeography, introduction effort and species traits(2011-10-13) Proches S.; Wilson J.R.U.; Richardson D.M.; Rejmanek M.Aim Pine trees (genus Pinus) represent an ancient lineage, naturally occurring almost exclusively in the Northern Hemisphere, but introduced and widely naturalized in both hemispheres. As large trees of interest to forestry, they attract much attention and their distribution is well documented in both indigenous and naturalized ranges. This creates an opportunity to analyse the relationship between indigenous and naturalized range sizes in the context of different levels of human usage, biological traits and the characteristics of the environments of origin. Location Global. Methods We combined and expanded pre-existing data sets for pine species distributions and pine species traits, and used a variety of regression techniques (including generalized additive models and zero-inflated Poisson models) to assess which variables explained naturalized and indigenous range sizes. Results Indigenous and naturalized range sizes are positively correlated but there are many notable exceptions. Some species have large indigenous ranges but small or no naturalized ranges, whereas others have small indigenous ranges, but have naturalized in many regions. Indigenous range is correlated to factors such as seed size (-), age at first reproduction (-), and latitude (+, supporting Rapoport's rule), but also to the extent of coverage of species in the forestry literature (+). Naturalized range size is strongly influenced by the extent of coverage of species in the forestry literature (+), a proxy for propagule pressure. Naturalization was also influenced by average elevation in the indigenous range (-) and age at first reproduction (-). Main conclusions The macroecological and evolutionary pressures facing plant groups are not directly transferable between indigenous and naturalized ranges. In particular, there are strong biases in species naturalization and expansion in invasive ranges that are unrelated to factors determining indigenous range size. At least for Pinus, a new set of macroecological patterns are emerging which are profoundly influenced by humans. © 2011 Blackwell Publishing Ltd.
- ItemPhylogeographic consequences of different introduction histories of invasive Australian Acacia species and Paraserianthes lophantha (Fabaceae) in South Africa(2011) Le Roux J.J.; Brown G.K.; Byrne M.; Ndlovu J.; Richardson D.M.; Thompson G.D.; Wilson J.R.U.Aim The genetic makeup and evolutionary potential of alien species can be profoundly influenced by their introduction history, but without detailed historical records, it can be difficult to ascertain the strength of this historical contingency. We explore how the known introduction histories combined with phylogeographic patterns in the native range have affected the genetic diversity in the invasive range for five Australian trees introduced to South Africa (Acacia cyclops, Acacia mearnsii, Acacia pycnantha, Acacia saligna and Paraserianthes lophantha). Location Western Australia, South Australia, Victoria and New South Wales (native and invasive ranges), and South Africa and the Hawaiian Islands (invasive range). Methods DNA sequence data were generated for all study species for either the external transcribed spacer (ETS) or plastid rpl32-trnL(UAG) gene regions (combined total of 180 DNA sequences). Using statistical parsimony networks and genetic diversity indices, we compared genetic structure and variation in native and invasive ranges. Results Australian acacia species tend to have high genetic diversity at the population level in their native ranges, often showing high intra-specific divergence. In most instances, these species have similar levels of population genetic diversity in their adventive ranges in South Africa, but lack structure. For A. cyclops, A. saligna and P. lophantha, we found evidence for intra-specific hybridization between mixed genetic entities in the invasive range, arguably as a result of the structured native range being broadly sampled prior to introduction. Main conclusions Invasive species that have been extensively used in forestry often have complex introduction histories resulting in equally complex genetic signatures in the invasive range. Our results show that extreme caution should be taken when using indirect inferences (molecular genetic data) of introduction histories in the absence of detailed introduction records. © 2011 Blackwell Publishing Ltd.
- ItemPlant invasions: Theoretical and practical challenges(2010) Richardson D.M.; Daehler C.C.; Leishman M.R.; Pauchard A.; Pysek P.[No abstract available]
- ItemPredicting the subspecific identity of invasive species using distribution models: Acacia saligna as an example(2011) Thompson G.D.; Robertson M.P.; Webber B.L.; Richardson D.M.; Le Roux J.J.; Wilson J.R.U.Aim To explore whether the subspecific genetic entities of Acacia saligna occupy different bioclimatic niches in their native and introduced ranges and whether these niches are predictable using species distribution models (SDMs). Location Australia, South Africa and the Mediterranean Basin. Methods Species distribution models were developed in MAXENT using six climatic variables to calculate the climatic suitability of the ranges of A. saligna. We assessed (1) the subspecific niche differences identified by SDMs using measures of niche overlap and model performance; (2) the ability of SDMs to predict the most likely subspecific genetic entities present in South Africa based on comparisons to genetic data; and (3) the ability of SDMs to predict the most likely subspecific genetic entities present in the Mediterranean Basin. All model projections were assessed for sensitivity and modelled prevalence as indicators of model fit and predictability. Results The SDMs identified different subspecific bioclimatic niches in the native range. Sensitivity and modelled prevalence show that none of the models correctly predicted the full range of A. saligna in South Africa or the Mediterranean Basin. Models also show that the South African niche is different to that in the native range. Main conclusions Subspecies of A. saligna occupy quantifiably distinct bioclimatic niches in their native ranges, implying that they should occupy distinct niches in their invasive ranges. However, projections to the introduced range did not correspond with known occurrences. Our SDMs are unable to predict the full introduced niche of A. saligna at a species or subspecies level in either South Africa or the Mediterranean Basin. Range limits in the native and introduced ranges may be determined by additional factors not used in the SDMs developed in this study. © 2011 Blackwell Publishing Ltd.
- ItemRelative roles of climatic suitability and anthropogenic influence in determining the pattern of spread in a global invader(2011) Roura-Pascual N.; Hui C.; Ikeda T.; Leday G.; Richardson D.M.; Carpintero S.; Espadaler X.; Gomez C.; Guenard B.; Hartley S.; Krushelnycky P.; Lester P.J.; McGeoch M.A.; Menke S.B.; Pedersen J.S.; Pitt J.P.W.; Reyes J.; Sanders N.J.; Suarez A.V.; Touyama Y.; Ward D.; Ward P.S.; Worner S.P.Because invasive species threaten the integrity of natural ecosystems, a major goal in ecology is to develop predictive models to determine which species may become widespread and where they may invade. Indeed, considerable progress has been made in understanding the factors that influence the local pattern of spread for specific invaders and the factors that are correlated with the number of introduced species that have become established in a given region. However, few studies have examined the relative importance of multiple drivers of invasion success for widespread species at global scales. Here, we use a dataset of >5,000 presence/absence records to examine the interplay between climatic suitability, biotic resistance by native taxa, human-aided dispersal, and human modification of habitats, in shaping the distribution of one of the world's most notorious invasive species, the Argentine ant (Linepithema humile). Climatic suitability and the extent of human modification of habitats are primarily responsible for the distribution of this global invader. However, we also found some evidence for biotic resistance by native communities. Somewhat surprisingly, and despite the often cited importance of propagule pressure as a crucial driver of invasions, metrics of the magnitude of international traded commodities among countries were not related to global distribution patterns. Together, our analyses on the global-scale distribution of this invasive species provide strong evidence for the interplay of biotic and abiotic determinants of spread and also highlight the challenges of limiting the spread and subsequent impact of highly invasive species.
- ItemRiparian vegetation: Degradation, alien plant invasions, and restoration prospects(2007) Richardson D.M.; Holmes P.M.; Esler K.J.; Galatowitsch S.M.; Stromberg J.C.; Kirkman S.P.; Pysek P.; Hobbs R.J.Rivers are conduits for materials and energy; this, the frequent and intense disturbances that these systems experience, and their narrow, linear nature, create problems for conservation of biodiversity and ecosystem functioning in the face of increasing human influence. In most parts of the world, riparian zones are highly modified. Changes caused by alien plants - or environmental changes that facilitate shifts in dominance creating novel ecosystems - are often important agents of perturbation in these systems. Many restoration projects are underway. Objective frameworks based on an understanding of biogeographical processes at different spatial scales (reach, segment, catchment), the specific relationships between invasive plants and resilience and ecosystem functioning, and realistic endpoints are needed to guide sustainable restoration initiatives. This paper examines the biogeography and the determinants of composition and structure of riparian vegetation in temperate and subtropical regions and conceptualizes the components of resilience in these systems. We consider changes to structure and functioning caused by, or associated with, alien plant invasions, in particular those that lead to breached abiotic- or biotic thresholds. These pose challenges when formulating restoration programmes. Pervasive and escalating human-mediated changes to multiple factors and at a range of scales in riparian environments demand innovative and pragmatic approaches to restoration. The application of a new framework accommodating such complexity is demonstrated with reference to a hypothetical riparian ecosystem under three scenarios: (1) system unaffected by invasive plants; (2) system initially uninvaded, but with flood-generated incursion of alien plants and escalating invasion-driven alteration; and (3) system affected by both invasions and engineering interventions. The scheme has been used to derive a decision-making framework for restoring riparian zones in South Africa and could guide similar initiatives in other parts of the world. © 2007 The Authors.
- ItemSpatially-explicit sensitivity analysis for conservation management: Exploring the influence of decisions in invasive alien plant management(2010) Roura-Pascual N.; Krug R.M.; Richardson D.M.; Hui C.Aim: Decision-support models have considerable potential for guiding management strategies when problems are complex. The robustness of such decision-making processes is rarely evaluated, and the influence of decision criteria (or factors) in management decisions is seldom considered. We present a framework for a spatially-explicit sensitivity analysis by using a scheme developed to provide objective guidelines, in the form of static priority maps, for managing woody invasive alien plants (IAPs). Location: The Cape Floristic Region, South Africa. Methods: The model included seven factors related to management history, fire risk, and the age, identity, density and spread of IAPs. Each factor had a weight associated that reflected its relative importance in prioritizing areas for clearing. We changed these factor weights using three approaches of sensitivity analysis and assessed the effect of these changes on the spatial structure of the resulting priority maps in three different management regions. Results: Different outcomes arose depending on the importance given to different factors. Priority maps were most sensitive to the fire-related factors, suggesting that fire is both a crucial driver of invasion in fynbos and an overriding determinant of management options. The factor 'area burnt recently' provided crucial information for the effective clearing of IAPs. The sensitivity of the model to changes in other factors was more context specific: levels of sensitivity were highly dependent on different features of the landscape, especially the spatial heterogeneity of particular factors. Main conclusions: By clarifying the importance of factors in shaping priority maps, the sensitivity analysis framework enabled us to identify the necessary factors to produce outcomes matching a pre-selected management strategy. This is important for cost-efficient management, as acquisition and curation of data is expensive. This spatially-explicit sensitivity analysis is, thus, recommended to evaluate the robustness and generality of selected management strategies, and validate the assumptions derived from decision-making protocols. © 2010 Blackwell Publishing Ltd.
- ItemThe biogeography of naturalization in alien plants(2006) Pysek P.; Richardson D.M.Aim: This paper reviews the main geographical determinants of naturalization in plants. Location: Global. Methods: Comparative studies of large data sets of alien floras are the main source of information on global patterns of naturalization. Results: Temperate mainland regions are more invaded than tropical mainland regions but there seems to be no difference in invasibility of temperate and tropical islands. Islands are more invaded than the mainland. The number of naturalized species in temperate regions decreases with latitude and their geographical ranges increase with latitude. The number of naturalized species on islands increases with temperature. Naturalized species contribute to floristic homogenization, but the phenomenon is scale-dependent. Main conclusions: Some robust patterns are evident from currently available data, but further research is needed on several aspects to advance our understanding of the biogeography of naturalization of alien plants. For example, measures of propagule pressure are needed to determine the invasibility of communities/ecosystems/regions. The patterns discussed in this paper are derived largely from numbers and proportions of naturalized species, and little is known about the proportion of introduced species that become naturalized. Further insights on naturalization rates, i.e. the proportion of aliens that successfully naturalize within regions, and on geographical and other determinants of its variation would provide us with better understanding of the invasion process. Comparative studies, and resulting generalizations, are almost exclusively based on numbers of species, but alien species differ in their impact on native biodiversity and ecosystem processes. © 2006 The Authors.
- ItemThe Roles of Climate, Phylogenetic Relatedness, Introduction Effort, and Reproductive Traits in the Establishment of Non-Native Reptiles and Amphibians(2012-02-15) Van Wilgen N.J.; Richardson D.M.We developed a method to predict the potential of non-native reptiles and amphibians (herpetofauna) to establish populations. This method may inform efforts to prevent the introduction of invasive non-native species. We used boosted regression trees to determine whether nine variables influence establishment success of introduced herpetofauna in California and Florida. We used an independent data set to assess model performance. Propagule pressure was the variable most strongly associated with establishment success. Species with short juvenile periods and species with phylogenetically more distant relatives in regional biotas were more likely to establish than species that start breeding later and those that have close relatives. Average climate match (the similarity of climate between native and non-native range) and life form were also important. Frogs and lizards were the taxonomic groups most likely to establish, whereas a much lower proportion of snakes and turtles established. We used results from our best model to compile a spreadsheet-based model for easy use and interpretation. Probability scores obtained from the spreadsheet model were strongly correlated with establishment success as were probabilities predicted for independent data by the boosted regression tree model. However, the error rate for predictions made with independent data was much higher than with cross validation using training data. This difference in predictive power does not preclude use of the model to assess the probability of establishment of herpetofauna because (1) the independent data had no information for two variables (meaning the full predictive capacity of the model could not be realized) and (2) the model structure is consistent with the recent literature on the primary determinants of establishment success for herpetofauna. It may still be difficult to predict the establishment probability of poorly studied taxa, but it is clear that non-native species (especially lizards and frogs) that mature early and come from environments similar to that of the introduction region have the highest probability of establishment. Resumen: Desarrollamos un método para predecir el potencial de reptiles y anfibios no nativos (herpetofauna) para establecer poblaciones. Este método puede proporcionar información para los esfuerzos de prevención de la introducción de especies no nativas invasoras. Utilizamos árboles de regresión y técnicas de remuestreo para determinar si nueve variables influyen en el establecimiento exitoso de herpetofauna introducida en California y Florida. Utilizamos un conjunto de datos independientes para evaluar el funcionamiento del modelo. Las especies con períodos juveniles cortos y especies con parientes más distantes filogenéticamente en las biotas regionales tuvieron mayor probabilidad de establecimiento que las especies que tardan en reproducirse. La concordancia climática promedio (la similitud en el clima entre el rango de distribución nativo y el no nativo) y la forma de vida también fueron importantes. Las ranas y lagartijas fueron los grupos taxonómicos con mayor probabilidad de establecimiento, mientras que serpientes y tortugas lo hicieron en mucho menor proporción. Utilizamos los resultados de nuestro mejor modelo para compilar un modelo basado en hoja de cálculo para facilitar su uso y la interpretación. Los valores de probabilidad obtenidos del modelo de hoja de cálculo se correlacionaron estrechamente con el éxito de establecimiento, del mismo modo que las probabilidades obtenidas de los árboles de regresión y técnicas de remuestreo basados en datos independientes. Sin embargo, la tasa de error para las predicciones a partir de datos independientes fue mucho más alta que con la validación cruzada. Esta diferencia en el poder predictivo no impide el uso del modelo para evaluar la probabilidad de establecimiento de herpetofauna porque los datos independientes no tenían información para dos variables (lo que significa que no se podía alcanzar la capacidad predictiva total del modelo) y que la estructura del modelo es consistente con la literatura reciente sobre los determinantes primarios del éxito de establecimiento de herpetofauna. Puede que aun sea difícil predecir la probabilidad del éxito de establecimiento de taxa poco estudiados, pero es claro que las especies no nativas (especialmente lagartijas y ranas) que maduran jóvenes y provienen de ambientes similares a los de la región de introducción tienen la mayor probabilidad de establecimiento. © 2012 Society for Conservation Biology.
- ItemTrees and shrubs as invasive alien species - a global review(2011) Richardson D.M.; Rejmanek M.Aim Woody plants were not widely considered to be important invasive alien species until fairly recently. Thousands of species of trees and shrubs have, however, been moved around the world. Many species have spread from planting sites, and some are now among the most widespread and damaging of invasive organisms. This article presents a global list of invasive alien trees and shrubs. It discusses taxonomic biases, geographical patterns, modes of dispersal, reasons for introductions and key issues regarding invasions of non-native woody plants around the world. Location Global. Methods An exhaustive survey was made of regional and national databases and the literature. Correspondence with botanists and ecologists and our own observations in many parts of the world expanded the list. Presence of invasive species was determined for each of 15 broad geographical regions. The main reasons for introduction and dissemination were determined for each species. Results The list comprises 622 species (357 trees, 265 shrubs in 29 plant orders, 78 families, 286 genera). Regions with the largest number of woody invasive alien species are: Australia (183); southern Africa (170); North America (163); Pacific Islands (147); and New Zealand (107). Species introduced for horticulture dominated the list (62% of species: 196 trees and 187 shrubs). The next most important reasons for introduction and dissemination were forestry (13%), food (10%) and agroforestry (7%). Three hundred and twenty-three species (52%) are currently known to be invasive in only one region, and another 126 (20%) occur in only two regions. Only 38 species (6%) are very widespread (invasive in six or more regions). Over 40% of invasive tree species and over 60% of invasive shrub species are bird dispersed. Main conclusions Only between 0.5% and 0.7% of the world's tree and shrub species are currently invasive outside their natural range, but woody plant invasions are rapidly increasing in importance around the world. The objectively compiled list of invasive species presented here provides a snapshot of the current dimensions of the phenomenon and will be useful for screening new introductions for invasive potential. © 2011 Blackwell Publishing Ltd.