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Despite the paramount role of plant diversity for ecosystem functioning, biogeochemical cycles, and human welfare, knowledge of its global distribution is still incomplete, hampering basic research and biodiversity conservation.Here, we used machine learning (random forests, extreme gradient boosting, and neural networks) and conventional statistical methods (generalized linear models and generalized additive models) to test environment-related hypotheses of broad-scale vascular plant diversity gradients and to model and predict species richness and phylogenetic richness worldwide. To this end, we used 830 regional plant inventories including c. 300 000 species and predictors of past and present environmental conditions.Machine learning showed a superior performance, explaining up to 80.9% of species richness and 83.3% of phylogenetic richness, illustrating the great potential of such techniques for disentangling complex and interacting associations between the environment and plant diversity. Current climate and environmental heterogeneity emerged as the primary drivers, while past environmental conditions left only small but detectable imprints on plant diversity.Finally, we combined predictions from multiple modeling techniques (ensemble predictions) to reveal global patterns and centers of plant diversity at multiple resolutions down to 7774 km2 . Our predictive maps provide accurate estimates of global plant diversity available at grain sizes relevant for conservation and macroecology. published

Despite the paramount role of plant diversity for ecosystem functioning, biogeochemical cycles, and human welfare, knowledge of its global distribution is incomplete, hampering basic research and biodiversity conservation. Here, we used machine learning (random forests, extreme gradient boosting, neural networks) and conventional statistical methods (generalised linear models, generalised additive models) to model species richness and phylogenetic richness of vascular plants worldwide based on 830 regional plant inventories including c. 300,000 species and predictors of past and present environmental conditions. Machine learning showed an outstanding performance, explaining up to 80.9% of species richness and 83.3% of phylogenetic richness. Current climate and environmental heterogeneity emerged as the primary drivers, while past environmental conditions left only small but detectable imprints on plant diversity. Finally, we combined predictions from multiple modelling techniques (ensemble predictions) to reveal global patterns and centres of plant diversity at multiple resolutions down to 7,774 km2. Our predictive maps provide the most accurate estimates of global plant diversity available to date at grain sizes relevant for conservation and macroecology.

Sustainable cities depend on urban forests. City trees improve our health, clean the air, store CO2, and cool local temperatures. Comparatively less is known about urban forests as ecosystems, particularly their spatial composition, nativity statuses, biodiversity, and tree health. Here, we assembled and standardized a new dataset of N=5,132,890 trees from 63 of the largest US cities with detailed information on location, health, nativity status, and species. We further designed new tools to analyze the ecosystem structure of urban forests, including spatial clustering and abundance of native trees, and validate these tools in comparison to past methods. We show that city trees are significantly clustered by species in 93% of cities, potentially increasing pest vulnerability (even in cities with biodiverse urban forests). Further, non-native species significantly homogenize urban forests across cities, while native trees comprise 0.44%-85.6% (median=45.6%) of city tree populations. Native trees are less frequent in drier cities, and indeed climate significantly shapes both nativity and biodiversity in urban forests. Parks are more biodiverse than urban settings. Compared to past work which focused primarily on canopy cover and species richness, we show the importance of analyzing spatial composition and nativity statuses in urban forests (and we created new datasets and tools to do so). This dataset could be analyzed in combination with citizen-science datasets on bird, insect, or plant biodiversity; social and demographic data; or data on the physical environment. Urban forests offer a rare opportunity to intentionally design biodiverse, heterogenous, rich ecosystems.

Sustainable cities depend on urban forests. City trees-pillars of urban forests-improve our health, clean the air, store COsub2/sub, and cool local temperatures. Comparatively less is known about city tree communities as ecosystems, particularly regarding spatial composition, species diversity, tree health, and the abundance of introduced species. Here, we assembled and standardized a new dataset ofiN/i= 5,660,237 trees from 63 of the largest US cities with detailed information on location, health, species, and whether a species is introduced or naturally occurring (i.e., "native"). We further designed new tools to analyze spatial clustering and the abundance of introduced species. We show that trees significantly cluster by species in 98% of cities, potentially increasing pest vulnerability (even in species-diverse cities). Further, introduced species significantly homogenize tree communities across cities, while naturally occurring trees (i.e., "native" trees) comprise 0.51-87.4% (median = 45.6%) of city tree populations. Introduced species are more common in drier cities, and climate also shapes tree species diversity across urban forests. Parks have greater tree species diversity than urban settings. Compared to past work which focused on canopy cover and species richness, we show the importance of analyzing spatial composition and introduced species in urban ecosystems (and we develop new tools and datasets to do so). Future work could analyze city trees alongside sociodemographic variables or bird, insect, and plant diversity (e.g., from citizen-science initiatives). With these tools, we may evaluate existing city trees in new, nuanced ways and design future plantings to maximize resistance to pests and climate change. We depend on city trees.Trees in towns and cities provide critical services to humans, animals and other living things. They help prevent climate change by capturing and storing carbon dioxide; they provide food and shelter to other species, they scrub the air of microscopic pollutants, cool local temperatures, and improve the mental and physical health of those who have access to them. In general, naturally occurring (so called native) plant species support richer local ecosystems – such as bird and butterfly communities – than plants that have been introduced from other areas. However, relatively little is known about which species of trees are found in towns and cities or how these species are distributed. Here, McCoy, Goulet-Scott et al. assembled a dataset of 5.6 million city trees from 63 cities in the United States. This dataset contained rich data on the exact location, species, and health of individual city trees – including park trees, those in urban forests, and trees that line city streets. In nearly all of the cities, the same tree species were found clustered next to each other, even in cities that had many different species of tree overall. This tendency of tree species to flock together may make these communities more vulnerable to disease and pest outbreaks. Trees in more developed environments, like those that line streets, were much less species diverse than trees spread across parks. Cities with wetter, cooler climates tended to have higher percentages of native tree species compared to cities with drier, hotter climates. Younger cities also had a greater percentage of native tree species than older cities, which may reflect increased awareness of the importance of native tree species among urban planners in more recent years. The cities that had planted non-native tree species tended to select the same species, which contributed to tree communities in different cities looking more alike. McCoy, Goulet-Scott et al. provide easy-to-use tools academics and urban foresters can use to assess how diverse tree communities in individual cities are. This work may help local decision-makers to select and plant trees that build resilience against climate change, pest and disease outbreaks, and maximize the health benefits trees provide all city dwellers.

This study documented the vascular flora of Crawford and Cherokee counties in southeastern Kansas. Each county experienced limited collecting in recent decades. A Pittsburg State University (PSU) student (Earl Sam Gibson) summarized the flora for Crawford County in 1963, but an extensive survey had never been done for Cherokee County. Fieldwork for the present study occurred mostly in 2014 and 2015. Based on approximately 6450 newly collected specimens, vouchers at PSU (including those of Gibson), and taxa documented by Biota of North America (BONAP) (Kartesz 2019), a total of 1420 unique taxa (species, subspecies and varieties) are reported for both counties combined, including 148 families, 591 genera, and 111 non-nominal infraspecific taxa. Of these, 1177 (82.9%) are native and 243 (17.1%) are non-native. A total of 44 state and 233 county records are documented. Included were 15 species of hybrid origin and 22 that are adventive from other parts of North America. Significant among the state records is the first published adventive occurrence of the genus Pinus in Kansas. Cherokee County has 1281 taxa (including 81 county records), whereas Crawford County includes 1139 taxa (and 152 county records). An important finding of our study, along with those of other recent floristic studies, is that vascular plant distributions at the state and county levels in the U.S.A. are less well documented than probably assumed. This is especially true for weedy taxa occurring in highly agricultural and industrial counties, as well as taxa with small, inconspicuous flowers and inflorescences, and counties relatively distant from universities. Undocumented occurrences of such taxa may adversely affect interpretations and distribution patterns of abundance, as for example when modeling plant distributions.

International audience; Our ability to predict the identity of future invasive alien species is largely based upon knowledge of prior invasion history. Emerging alien species-those never encountered as aliens before-therefore pose a significant challenge to biosecurity interventions worldwide. Understanding their temporal trends, origins, and the drivers of their spread is pivotal to improving prevention and risk assessment tools. Here, we use a database of 45,984 first records of 16,019 established alien species to investigate the temporal dynamics of occurrences of emerging alien species worldwide. Even after many centuries of invasions the rate of emergence of new alien species is still high: Onequarter of first records during 2000-2005 were of species that had not been previously recorded anywhere as alien, though with large variation across taxa. Model results show that the high proportion of emerging alien species cannot be solely explained by increases in well-known drivers such as the amount of imported commodities from historically important source regions. Instead, these dynamics reflect the incorporation of new regions into the pool of potential alien species, likely as a consequence of expanding trade networks and environmental change. This process compensates for the depletion of the historically important source species pool through successive invasions. We estimate that 1-16% of all species on Earth, depending on the taxonomic group, qualify as potential alien species. These results suggest that there remains a high proportion of emerging alien species we have yet to encounter, with future impacts that are difficult to predict.

This study compares four models for predicting the potential distribution of non-indigenous weed species in the conterminous U.S. The comparison focused on evaluating modeling tools and protocols as currently used for weed risk assessment or for predicting the potential distribution of invasive weeds. We used six weed species (three highly invasive and three less invasive non-indigenous species) that have been established in the U.S. for more than 75 years. The experiment involved providing non-U. S. location data to users familiar with one of the four evaluated techniques, who then developed predictive models that were applied to the United States without knowing the identity of the species or its U.S. distribution. We compared a simple GIS climate matching technique known as Proto3, a simple climate matching tool CLIMEX Match Climates, the correlative model MaxEnt, and a process model known as the Thornley Transport Resistance (TTR) model. Two experienced users ran each modeling tool except TTR, which had one user. Models were trained with global species distribution data excluding any U.S. data, and then were evaluated using the current known U.S. distribution. The influence of weed species identity and modeling tool on prevalence and sensitivity effects was compared using a generalized linear mixed model. Each modeling tool itself had a low statistical significance, while weed species alone accounted for 69.1 and 48.5% of the variance for prevalence and sensitivity, respectively. These results suggest that simple modeling tools might perform as well as complex ones in the case of predicting potential distribution for a weed not yet present in the United States. Considerations of model accuracy should also be balanced with those of reproducibility and ease of use. More important than the choice of modeling tool is the construction of robust protocols and testing both new and experienced users under blind test conditions that approximate operational conditions.

This dataset provides the Global Naturalized Alien Flora (GloNAF) database, ver-sion 1.2. Glo NAF represents a data compendium on th e occurrence and identit y of naturalizedalien vascular plant taxa across geographic regions (e.g. countries, states, provinces, districts,islands) around the globe. The dataset includes 13,939 taxa and covers 1,029 regions (including381 islands). The dataset is based on 210 data sources. For each ta x on-b y-region combination, wepr ovide information on whether the tax on is consider ed to be naturalized in the specific region(i.e. has established self-sustaining popula tions in the wild). Non-native taxa are marked as“alien”, when it is not clear whether they are naturalized. To facilitate alignment with other plantdatabases, we pro v ide f or each taxon the name as given in the original data source and the stan-dardized taxon and family names used by The Plant List Version 1.1 (http://www.theplantlist.org/). We pro vide an ESRI shapefile including polygons f or each region and informa tion on whetherit is an island or a mainland region, the country and the Taxonomic Databases Working Group(TDWG) regions it is part of (TDWG levels 1–4). We also provide several variables that can beused to filter the data according to quality and completeness of alien taxon lists, which varyamong the combinations of regions and da ta sources. A pre vious version of the GloNAF dataset(version 1.1) has already been used in several studies on, for example, historical spatial flows oftaxa between continents and geographical patterns and determinants of naturalization across dif-ferent taxonomic groups. We intend the updated and expanded GloNAF version presented hereto be a global resource useful for studying plant inv asions and changes in biodiversity from regio-nal to global scales. We release these data into the public domain under a Crea ti ve CommonsZer o license waiver (https://creati v ecommons.org/share-y our -work/public-domain/cc0/). Wheny ou use the da ta in your publication, we request that y ou cite this da ta paper. If GloN AF is amajor part of the data analyzed in your study, you should consider inviting the GloNAF coreteam (see Metadata S1: Originators in the Overall project description) as collaborators. If youplan to use the GloNAF dataset, we encourage y ou to contact the GloNAF core team to checkwhether there have been recent updates of the dataset, and whether similar analyses are already ongoing.

Aim Biogeographers have long known that plant species do not fully encompass their fundamental niche. Nonetheless, in practice, species distribution modelling assumes that plant distributions represent a reasonable approximation of their environmental tolerance. For ecological forecasting, projections of habitat loss due to climate change assume that many species will be unable to tolerate climate conditions outside of those found within their current distributional ranges. We aim to test how well occurrences in the native range approximate the climatic conditions in which plant species can survive. Location Continental USA. Methods We compared the climatic conditions between occurrences in the US native versus US non-native ranges using 144 non-invasive plant species. We quantified differences in January minimum temperature, July maximum temperature and annual precipitation as indicators of climatic tolerance. We also compared modelled potential distributions throughout the US based on native and total ranges to test how expanded climatic tolerance translates into predicted geographical range. Results Most species (86%) had non-native occurrences in climates outside those described by their native distributions. For the 80 species with lower minimum temperatures at non-native occurrences, the median expansion of minimum temperature tolerance was −2.9 °C. Similarly, for the 90 species with lower precipitation at non-native occurrences, the median expansion of minimum annual precipitation was −23 cm. Broader climatic conditions at non-native occurrences expanded the modelled potential geographical range by a median of 35%, with smaller range species showing larger expansions of potential geographical range. Main conclusions Our results show that plants' native ranges strongly underestimate climatic tolerance, leading species distribution models to underpredict potential range. The climatic tolerance of species with narrow native ranges appears most prone to underestimation. These findings suggest that many plants will be able to persist in situ with climate change for far longer than projected by species distribution models.

Using the recently built Global Naturalized Alien Flora (GloNAF) database, containing data on the distribution of naturalized alien plants in 483 mainland and 361 island regions of the world, we describe patterns in diversity and geographic distribution of naturalized and invasive plant species, taxonomic, phylogenetic and life-history structure of the global naturalized flora as well 204 Preslia 89: 203–274, 2017 as levels of naturalization and their determinants. The mainland regions with the highest numbers of naturalized aliens are some Australian states (with New South Wales being the richest on this continent) and several North American regions (of which California with 1753 naturalized plant species represents the world’s richest region in terms of naturalized alien vascular plants). England, Japan, New Zealand and the Hawaiian archipelago harbour most naturalized plants among islands or island groups. These regions also form the main hotspots of the regional levels of naturalization, measured as the percentage of naturalized aliens in the total flora of the region. Such hotspots of relative naturalized species richness appear on both the western and eastern coasts of North America, in north-western Europe, South Africa, south-eastern Australia, New Zealand, and India. High levels of island invasions by naturalized plants are concentrated in the Pacific, but also occur on individual islands across all oceans. The numbers of naturalized species are closely correlated with those of native species, with a stronger correlation and steeper increase for islands than mainland regions, indicating a greater vulnerability of islands to invasion by species that become successfully naturalized. South Africa, India, California, Cuba, Florida, Queensland and Japan have the highest numbers of invasive species. Regions in temperate and tropical zonobiomes harbour in total 9036 and 6774 naturalized species, respectively, followed by 3280 species naturalized in the Mediterranean zonobiome, 3057 in the subtropical zonobiome and 321 in the Arctic. The NewWorld is richer in naturalized alien plants, with 9905 species compared to 7923 recorded in the Old World. While isolation is the key factor driving the level of naturalization on islands, zonobiomes differing in climatic regimes, and socioeconomy represented by per capita GDP, are central for mainland regions. The 11 most widely distributed species each occur in regions covering about one third of the globe or more in terms of the number of regions where they are naturalized and at least 35% of the Earth’s land surface in terms of those regions’ areas, with the most widely distributed species Sonchus oleraceus occuring in 48% of the regions that cover 42% of the world area. Other widely distributed species are Ricinus communis, Oxalis corniculata, Portulaca oleracea, Eleusine indica, Chenopodium album, Capsella bursa-pastoris, Stellaria media, Bidens pilosa, Datura stramonium and Echinochloa crus-galli. Using the occurrence as invasive rather than only naturalized yields a different ranking, with Lantana camara (120 regions out of 349 for which data on invasive status are known), Calotropis procera (118), Eichhornia crassipes (113), Sonchus oleraceus (108) and Leucaena leucocephala (103) on top. As to the life-history spectra, islands harbour more naturalized woody species (34.4%) thanmainland regions (29.5%), and fewer annual herbs (18.7% compared to 22.3%). Ranking families by their absolute numbers of naturalized species reveals that Compositae (1343 species), Poaceae (1267) and Leguminosae (1189) contribute most to the global naturalized alien flora. Some families are disproportionally represented by naturalized aliens on islands (Arecaceae, Araceae, Acanthaceae, Amaryllidaceae, Asparagaceae, Convolvulaceae, Rubiaceae, Malvaceae), and much fewer so on mainland (e.g. Brassicaceae, Caryophyllaceae, Boraginaceae). Relating the numbers of naturalized species in a family to its total global richness shows that some of the large species-rich families are over-represented among naturalized aliens (e.g. Poaceae, Leguminosae, Rosaceae, Amaranthaceae, Pinaceae), some under-represented (e.g. Euphorbiaceae, Rubiaceae), whereas the one richest in naturalized species, Compositae, reaches a value expected from its global species richness. Significant phylogenetic signal indicates that families with an increased potential of their species to naturalize are not distributed randomly on the evolutionary tree. Solanum (112 species), Euphorbia (108) and Carex (106) are the genera richest in terms of naturalized species; over-represented on islands are Cotoneaster, Juncus, Eucalyptus, Salix, Hypericum, Geranium and Persicaria, while those relatively richer in naturalized species on the mainland are Atriplex, Opuntia, Oenothera, Artemisia, Vicia, Galium and Rosa. The data presented in this paper also point to where information is lacking and set priorities for future data collection. The GloNAF database has potential for designing concerted action to fill such data gaps, and provide a basis for allocating resources most efficiently towards better understanding and management of plant invasions worldwide. published

Aim The percentage of alien species found in a given habitat depends on the habitat vulnerability to invasion (invasibility) and the number of species introduced (propagule pressure). However, the global pool of alien species suited to a given habitat also varies. Here, we identify donor habitats of invasive alien plant species originating from Europe, examine the match between habitats they occupy in Europe and recipient areas and test whether donor habitats of invasive plants tend to be vulnerable or resistant to invasions. Location Europe (source area), North America and the World (recipient areas). Methods Native European vascular plants invasive in North America and other parts of the World were identified for 35 European natural habitats. Percentages of species invasive outside Europe, of the total number of native species occurring in each European habitat, were used to compare these habitats as donors for invasion. Habitat preferences of European species in their recipient areas were compared with those in Europe. Results European alluvial forests, alder carrs and coastal sand-dunes harbour the highest percentages of native species that are invasive outside Europe. Outside their native range, European species tend to invade habitats that are similar to their donor habitats in Europe, but species of alluvial and coastal habitats also frequently invade other habitats. European habitats that are important donors of invasive species globally experience the highest levels of invasion by alien species from other regions; this relationship was, however, not confirmed for invasions to North America if considered separately. Main conclusions Some European habitats are more important donors of invasive plants than others. Therefore, the level of invasion of different habitats is affected also by the differences in the number of invasive species provided by various donor habitats. At a global scale, more important donor habitats are also likely to be more invaded.

The number of invasive exotic plant species establishing in the United States is continuing to rise. When prevention of exotic species from entering into a country fails at the national level and the species establishes, reproduces, spreads, and becomes invasive, the most successful action at a local level is early detection followed by eradication. We have developed a simple geographic information system (GIS) analysis for developing watch lists for early detection of invasive exotic plants that relies upon currently available species distribution data coupled with environmental data to aid in describing coarse-scale potential distributions. This GIS analysis tool develops environmental envelopes for species based upon the known distribution of a species thought to be invasive and represents the first approximation of its potential habitat while the necessary data are collected to perform more in-depth analyses. To validate this method we looked at a time series of species distributions for 66 species in Pacific Northwest and northern Rocky Mountain counties. The time series analysis presented here did select counties that the invasive exotic weeds invaded in subsequent years, showing that this technique could be useful in developing watch lists for the spread of particular exotic species. We applied this same habitat-matching model based upon bioclimatic envelopes to 100 invasive exotics with various levels of known distributions within continental U.S. counties. For species with climatically limited distributions, county watch lists describe county-specific vulnerability to invasion. Species with matching habitats in a county would be added to that county's list. These watch lists can influence management decisions for early warning, control prioritization, and targeted research to determine specific locations within vulnerable counties. This tool provides useful information for rapid assessment of the potential distribution based upon climate envelopes of current distributions for new invasive exotic species.

The factors that promote invasive behavior in introduced plant species occur across many scales of biological and ecological organization. Factors that act at relatively small scales, for example, the evolution of biological traits associated with invasiveness, scale up to shape species distributions among different climates and habitats, as well as other characteristics linked to invasion, such as attractiveness for cultivation (and by extension propagule pressure). To identify drivers of invasion it is therefore necessary to disentangle the contribution of multiple factors that are interdependent. To this end, we formulated a conceptual model describing the process of invasion of central European species into North America based on a sequence of "drivers." We then used confirmatory path analysis to test whether the conceptual model is supported by a statistical model inferred from a comprehensive database containing 466 species. The path analysis revealed that naturalization of central European plants in North America, in terms of the number of North American regions invaded, most strongly depends on residence time in the invaded range and the number of habitats occupied by species in their native range. In addition to the confirmatory path analysis, we identified the effects of various biological traits on several important drivers of the conceptualized invasion process. The data supported a model that included indirect effects of biological traits on invasion via their effect on the number of native range habitats occupied and cultivation in the native range. For example, persistent seed banks and longer flowering periods are positively correlated with number of native habitats, while a stress-tolerant life strategy is negatively correlated with native range cultivation. However, the importance of the biological traits is nearly an order of magnitude less than that of the larger scale drivers and highly dependent on the invasion stage (traits were associated only with native range drivers). This suggests that future research should explicitly link biological traits to the different stages of invasion, and that a failure to consider residence time or characteristics of the native range may seriously overestimate the role of biological traits, which, in turn, may result in spurious predictions of plant invasiveness.

Detailed knowledge of patterns of native species richness, an important component of biodiversity, and non-native species invasions is often lacking even though this knowledge is essential to conservation efforts. However, we cannot afford to wait for complete information on the distribution and abundance of native and harmful invasive species. Using information from counties well surveyed for plants across the USA, we developed models to fill data gaps in poorly surveyed areas by estimating the density (number of species km -2 ) of native and non-native plant species. Here, we show that native plant species density is non-random, predictable, and is the best predictor of non-native plant species density. We found that eastern agricultural sites and coastal areas are among the most invaded in terms of non-native plant species densities, and that the central USA appears to have the greatest ratio of non-native to native species. These large-scale models could also be applied to smaller spatial scales or other taxa to set priorities for conservation and invasion mitigation, prevention, and control efforts.

We quantified broad-scale patterns of species richness and species density (mean # species/km2) for native and non-indigenous plants, birds, and fishes in the continental USA and Hawaii. We hypothesized that the species density of native and non-indigenous taxa would generally decrease in northern latitudes and higher elevations following declines in potential evapotranspiration, mean temperature, and precipitation. County data on plants (n = 3004 counties) and birds (n=3074 counties), and drainage (6 HUC) data on fishes (n = 328 drainages) showed that the densities of native and non-indigenous species were strongly positively correlated for plant species (r = 0.86, P < 0.0001), bird species (r = 0.93, P

We undertook a floristic study of Highland Heights Community Park and undeveloped adjacent property, Cuyahoga County, Ohio, locating 403 taxa of vascular plants belonging to 249 genera and 87 families. The study site is a large suburban park containing a mixture of wetland and non-wetland communities. A wet meadow within the site supports a diverse flora and is currently the only known locality in Ohio for Solidago puberula. Also this meadow represents the sole location in Cuyahoga County for Rhynchospora capitellata and Hypericum gentianoides.

Fourteen vascular plants taxa not previously documented for Pennsylvania are reported with vouchers for each county of occurrence. These include 1 lycophyte, 2 monocots and 11 dicots. Nine species are native to the United States while 5 species have been introduced to the United States.

Observations from islands, small-scale experiments, and mathematical models have generally supported the paradigm that habitats of low plant diversity are more vulnerable to plant invasions than areas of high plant diversity. We summarize two independent data sets to show exactly the opposite pattern at multiple spatial scales. More significant, and alarming, is that hotspots of native plant diversity have been far more heavily invaded than areas of low plant diversity in most parts of the United States when considered at larger spatial scales. Our findings suggest that we cannot expect such hotspots to repel invasions, and that the threat of invasion is significant and predictably greatest in these areas.

All around the globe, humans have greatly altered the abiotic and biotic environment with ever-increasing speed. One defining feature of the Anthropocene epoch(1,2) is the erosion of biogeographical barriers by human-mediated dispersal of species into new regions, where they can naturalize and cause ecological, economic and social damage(3). So far, no comprehensive analysis of the global accumulation and exchange of alien plant species between continents has been performed, primarily because of a lack of data. Here we bridge this knowledge gap by using a unique global database on the occurrences of naturalized alien plant species in 481 mainland and 362 island regions. In total, 13,168 plant species, corresponding to 3.9% of the extant global vascular flora, or approximately the size of the native European flora, have become naturalized somewhere on the globe as a result of human activity. North America has accumulated the largest number of naturalized species, whereas the Pacific Islands show the fastest increase in species numbers with respect to their land area. Continents in the Northern Hemisphere have been the major donors of naturalized alien species to all other continents. Our results quantify for the first time the extent of plant naturalizations worldwide, and illustrate the urgent need for globally integrated efforts to control, manage and understand the spread of alien species.

We find ourselves in general agreement with many of Harrison's remarks especially since we both find our data present a ' strong case that at county to state scales, exotic plant invasions have led to few native plant extinctions' (emphasis added, Harrison 2007: 000). Where we differ appears related to the breadth of scales to which our conclusions may apply. In addition to the Myth of Plant Species Saturation (henceforth, the Myth paper ) in question, we have published several complementary papers which may alleviate some of Harrison's concerns.

Plant species assemblages, communities or regional floras might be termed 'saturated' when additional immigrant species are unsuccessful at establishing due to competitive exclusion or other inter-specific interactions, or when the immigration of species is off-set by extirpation of species. This is clearly not the case for state, regional or national floras in the USA where colonization (i.e. invasion by exotic species) exceeds extirpation by roughly a 24 to 1 margin. We report an alarming temporal trend in plant invasions in the Pacific Northwest over the past 100 years whereby counties highest in native species richness appear increasingly invaded over time. Despite the possibility of some increased awareness and reporting of native and exotic plant species in recent decades, historical records show a significant, consistent long-term increase in exotic species (number and frequency) at county, state and regional scales in the Pacific Northwest. Here, as in other regions of the country, colonization rates by exotic species are high and extirpation rates are negligible. The rates of species accumulation in space in multi-scale vegetation plots may provide some clues to the mechanisms of the invasion process from local to national scales.

A tabular summary, by family, of the 56,431 computerized names of those native and naturalized plant taxa included in the "Synonymized checklist of the vascular flora of the United States, Canada and Greenland" (including Alaska, Hawaii, Puerto Rico and the U.S. Virgin Islands) indicates a flora of 21,588 species in 2,921 genera in 254 families. There are also 740 named hybrids and, under current concepts, 2,491 subspecies and 8,008 varieties in our flora. There are 20,429 pertinent or "manual level" synonyms, with the number of synonyms exceeding the number of accepted names in many families (especially those families of high agricultural, horticultural or other economic importance). THE VASCULAR FLORA of North America represents the second most diversified group of higher organisms of the continental biota, being surpassed in numbers of species only by the enormous class Insecta. Yet, despite the desires and efforts of American taxonomists over the past century or more, there has previously been no biologically realistic enumeration of the many taxa of Tracheophyta in this vast and diverse area. However, through a cooperative effort between the Biota of Northi America Program and the North Carolina Botanical Garden, some 56,000 pertinent names from the extensive taxonomic data files of the senior author have been computerized, reviewed, evaluated, and appropriately synonymized by over 250 botanical specialists to produce a relatively complete and biologically valid checklist of our flora. The numerical comparison presented in Table 1 is a summary, by plant family, of the detailed data in the "Synonymized checklist of the vascular flora of the United States, Canada and Greenland" (including Alaska, Hawaii, Puerto Rico and the U.S. Virgin Islands) by Kartesz and Kartesz (1980). The 254 plant families listed in Table 1 essentially follow the concepts of Crabbe, Jermy and Mickel (1975) for the Pteridophyta, Little (1979) for the Gymnospermae, and the convenient conservative treatment of Windler and Windler (1973) for the Angiospermae. The treatment of the 2,921 genera, 21,588 species, 2,491 subspecies, 8,008 varieties, and 740 named hybrids generally represents the current status of these taxa in the eyes of the many I Received for publication 11 January 1980; revision accepted 4 February 1980. specialists who generously provided critical reviews of their particular groups. It should be noted that the two categories, subspecies and variety, are not mutually exclusive as tabulated here and that the synonymy concerned is generally given only to the functional or "manual" level. Therefore, the 20,429 synonyms (for all categories or ranks) likely represent a practical minimum number of synonyms for our flora, although the number might be reduced by 5 or 10% and still be functional. On the other hand, depending on the degree to which purely historic and monographic synonymy might be included, the list of synonyms in the checklist could easily be increased to 100,000 or more with little increase in general functional value. Clearly, we have long ago passed the point prophesied by de Candolle ". . . when botanists will have made, unmade, often remade, raised or lowered, and redefined several hundred thousand groups from classes to mere varieties, and when synonyms will have become much more numerous than accepted groups" (Weatherby, 1949). Until there was an appropriate and critical biological assessment and evaluation of such extensive synonymy by cooperating specialists, a realistic enumeration of the valid and accepted plant taxa of the area treated was impossible. With the Kartesz and Kartesz checklist we have the first accurate approximation of the total diversity of our flora, critically synonymized, which will provide a realistic and, hopefully, a somewhat standardized and stabilized reference base for many aspects of taxonomic, floristic and ecological work. Indeed, the checklist has already been of critical value toward the preparation of an "Atlas" and the planning for a "Guide" which could be further