Drivers of the relative richness of naturalized and invasive plant species on Earth

Culturomics and iEcology provide novel opportunities to study human and social dimensions of alien species introductions

Invasive alien species negatively impact ecosystems, biodiversity, human societies, and economies. To prevent future invasions, it is crucial to understand both the ecological and the human and social factors determining whether a species is picked up, transported, and introduced beyond their native range. However, we often have little or no information on key human and social factors. Here, we explore how alien species introductions are shaped by a combination of ecological and human and social factors and highlight the potential of the emerging fields of conservation culturomics and iEcology for disentangling their relative importance. We argue that quantifying and assessing the relative importance of the human and social dimensions of alien species introductions can substantially improve our understanding of the invasion process.

Exploring the complex pre-adaptations of invasive plants to anthropogenic disturbance: a call for integration of archaeobotanical approaches

Pre-adaptation to anthropogenic disturbance is broadly considered key for plant invasion success. Nevertheless, empirical evidence remains scarce and fragmentary, given the multifaceted nature of anthropogenic disturbance itself and the complexity of other evolutionary forces shaping the (epi)-genomes of recent native and invasive plant populations. Here, we review and critically revisit the existing theory and empirical evidence in the field of evolutionary ecology and highlight novel integrative research avenues that work at the interface with archaeology to solve open questions. The approaches suggested so far focus on contemporary plant populations, although their genomes have rapidly changed since their initial introduction in response to numerous selective and stochastic forces. We elaborate that a role of pre-adaptation to anthropogenic disturbance in plant invasion success should thus additionally be validated based on the analyses of archaeobotanical remains. Such materials, in the light of detailed knowledge on past human societies could highlight fine-scale differences in the type and timing of past disturbances. We propose a combination of archaeobotanical, ancient DNA and morphometric analyses of plant macro- and microremains to assess past community composition, and species' functional traits to unravel the timing of adaptation processes, their drivers and their long-term consequences for invasive species. Although such methodologies have proven to be feasible for numerous crop plants, they have not been yet applied to wild invasive species, which opens a wide array of insights into their evolution.

Plant invasion and naturalization are influenced by genome size, ecology and economic use globally

Human factors and plant characteristics are important drivers of plant invasions, which threaten ecosystem integrity, biodiversity and human well-being. However, while previous studies often examined a limited number of factors or focused on a specific invasion stage (e.g., naturalization) for specific regions, a multi-factor and multi-stage analysis at the global scale is lacking. Here, we employ a multi-level framework to investigate the interplay between plant characteristics (genome size, Grime's adaptive CSR-strategies and native range size) and economic use and how these factors collectively affect plant naturalization and invasion success worldwide. While our findings derived from structural equation models highlight the substantial contribution of human assistance in both the naturalization and spread of invasive plants, we also uncovered the pivotal role of species' adaptive strategies among the factors studied, and the significantly varying influence of these factors across invasion stages. We further revealed that the effects of genome size on plant invasions were partially mediated by species adaptive strategies and native range size. Our study provides insights into the complex and dynamic process of plant invasions and identifies its key drivers worldwide.

Disentangling the role of environmental filtering and biotic resistance on alien invasions in a reservoir area

Large-scale water conservancy projects benefit human life but have modified the landscape and provided opportunities for alien plant invasions. Understanding the environmental (e.g., climate), human-related (e.g., population density, proximity to human activities), and biotic (e.g., native plant, community structure) factors driving invasions is essential in the management of alien plants and biodiversity conservation in areas with intense human pressure. To this end, we investigated the spatial patterns of alien plant species distribution in the Three Gorges Reservoir Area (TGRA) of China and distinguished the role of the external environment and community characteristics in determining the occurrence of alien plants with differing levels of known invasion impacts in China using random forest analyses and structural equation models. A total of 102 alien plant species belonging to 30 families and 67 genera were recorded, the majority being annual and biennial herbs (65.7%). The results showed a negative diversity-invasibility relationship and supported the biotic resistance hypothesis. Moreover, percentage coverage of native plants was found to interact with native species richness and had a predominant role in resisting alien plant species. We found alien dominance was mainly the result of disturbance (e.g., changes in hydrological regime), which drove native plant loss. Our results also demonstrated that disturbance and temperature were more important for the occurrence of malignant invaders than all alien plants. Overall, our study highlights the importance of restoring diverse and productive native communities in resistance to invasion.

Small genome size and variation in ploidy levels support the naturalization of vascular plants but constrain their invasive spread

Karyological characteristics are among the traits underpinning the invasion success of vascular plants. Using 11 049 species, we tested the effects of genome size and ploidy levels on plant naturalization (species forming self-sustaining populations where they are not native) and invasion (naturalized species spreading rapidly and having environmental impact). The probability that a species naturalized anywhere in the world decreased with increasing monoploid genome size (DNA content of a single chromosome set). Naturalized or invasive species with intermediate monoploid genomes were reported from many regions, but those with either small or large genomes occurred in fewer regions. By contrast, large holoploid genome sizes (DNA content of the unreplicated gametic nucleus) constrained naturalization but favoured invasion. We suggest that a small genome is an advantage during naturalization, being linked to traits favouring adaptation to local conditions, but for invasive spread, traits associated with a large holoploid genome, where the impact of polyploidy may act, facilitate long-distance dispersal and competition with other species.

Distribution Patterns and Determinants of Invasive Alien Plants in China

In recent years, invasive alien plants (IAPs) have caused serious ecological disasters and economic losses in China. This study combined three IAP species richness-related indices (species richness of IAPs, first records of IAPs, and the relative species richness of IAPs), as well as indices reflecting distribution and dispersal patterns (average similarity coefficient of IAPs) and invasiveness (average risk score of IAPs), to conduct an integrated regional-invasion risk assessment based on the principal component analysis (PCA) method. Partial least-squares (PLS) regression was conducted to explore the explanatory power of 12 environmental and anthropogenic factors on different invasion indices. The results indicated that coastal provinces and Yunnan had high IAP introduction risk, as well as high synthetic-risk scores. The dispersal of IAPs in mid-latitude provinces should be particularly prevented. For species richness of IAPs, more environmental factors with variable importance for the project (VIP) values higher than 1 were retained in the optimal model, reflecting the importance of environmental filtering on IAPs. Visitors were the most important predictor for first records of IAPs. Compared to species richness (R² = 79.5%), first records were difficult to predict (R² = 60.4%) and were influenced by anthropogenic factors. There was spatial distribution congruence of various families of IAPs. Generally, the correlations of the residuals of species richness were still significant, with 0.421 (p < 0.05) as the lowest Pearson correlation coefficient, which indicated that external factors could not fully explain the spatial distribution congruence. These findings could enrich the relevant research on IAP invasion mechanisms and provide suggestions for regional IAP detection and response.

Long-term trajectories of non-native vegetation on islands globally

Human-mediated changes in island vegetation are, among others, largely caused by the introduction and establishment of non-native species. However, data on past changes in non-native plant species abundance that predate historical documentation and censuses are scarce. Islands are among the few places where we can track human arrival in natural systems allowing us to reveal changes in vegetation dynamics with the arrival of non-native species. We matched fossil pollen data with botanical status information (native, non-native), and quantified the timing, trajectories and magnitude of non-native plant vegetational change on 29 islands over the past 5000 years. We recorded a proportional increase in pollen of non-native plant taxa within the last 1000 years. Individual island trajectories are context-dependent and linked to island settlement histories. Our data show that non-native plant introductions have a longer and more dynamic history than is generally recognized, with critical implications for biodiversity baselines and invasion biology.

Abiotic and Biotic Factors from the Past as Predictors of Alien Bird Richness and Temporal Beta-Diversity

The challenge of predicting the distribution of alien species has long been a focus of invasion ecology. Herein, we assessed biotic and abiotic factors from the 1980s as potential predictors of alien bird species patterns 20 years later in the state of New York. To assess the ability of each factor to predict future alien species patterns, we analysed the influence of biotic (native taxonomic, functional and phylogenetic diversity, and human population density) and abiotic (climate and land use) factors from the 1980s on the observed alien species richness patterns in the 2000s and the temporal change in the composition of the alien communities between the 1980s and the 2000s using both single-predictor and multivariate models. Alien species richness from the 1980s was a reliable predictor of the alien species richness and temporal beta-diversity patterns in the 2000s. Among abiotic factors, maximum temperature and agricultural land-uses constituted sufficient predictors of future alien species richness and better predictors than the native biotic factors. The performance of single-predictor models was generally weaker in predicting temporal alien beta-diversity; however, past alien species richness and maximum temperature again outperformed the other factors. Predictions and management decisions should focus on warm and agricultural areas, as well as areas with an already high number of established alien species.

Patterns of phylogenetic relatedness of non-native plants across the introduction-naturalization-invasion continuum in China

Human activities have caused the exchange of species among different parts of the world. When introduced species become naturalized and invasive, they may cause great negative impacts on the environment and human societies, and pose significant threats to biodiversity and ecosystem structure. Knowledge on phylogenetic relatedness between native and non-native species and among non-native species at different stages of species invasion may help for better understanding the drivers of species invasion. Here, I analyze a comprehensive data set including both native and non-native angiosperm species in China to determine phylogenetic relatedness of introduced species across a full invasion continuum (from introduction through naturalization to invasion). This study found that (1) introduced plants are a phylogenetically clustered subset of overall (i.e. native plus non-native) angiosperm flora, (2) naturalized plants are a phylogenetically clustered subset of introduced plants, and (3) invasive plants are a phylogenetically clustered subset of naturalized plants. These patterns hold regardless of spatial scales examined (i.e. national versus provincial scale) and whether basal- or tip-weighted metric of phylogenetic relatedness is considered. These findings are consistent with Darwin's preadaptation hypothesis.

The naturalized vascular flora of Malesia

Major regional gaps exist in the reporting and accessibility of naturalized plant species distribution data, especially within Southeast Asia. Here, we present the Malesian Naturalized Alien Flora database (MalNAF), the first standardized island-group level checklist of naturalized vascular plant species for the Malesian phytogeographical region. We used MalNAF to investigate the composition, origins, and habitat preferences of the naturalized flora. The naturalized vascular flora of Malesia consists of at least 1177 species. Richness is highest in the Philippines (539 spp.) and lowest in the Maluku Islands (87 spp.). But, the Lesser Sunda Islands had the highest naturalized species richness relative to native richness and Singapore has a higher naturalized plant species richness than would be expected given its size. When comparing the data for Malesia with a global dataset, we found that naturalized richness increased with area for islands but not for continental regions. Across the archipelago, 31 species are widespread, occurring in every island group, but the majority have a limited distribution of 2.4 ± 2.3 (mean ± SD) island groups per naturalized species. The naturalized plant species are representatives of 150 families, twenty of which are newly introduced to the region. Families richest in naturalized plant species in Malesia were Fabaceae (= Leguminosae) (160 spp.), Poaceae (= Gramineae) (138 spp.), and Asteraceae (= Compositae) (96 spp.). Most of these have a native range that includes tropical Asia, closely followed by those from Southern America (inclusive of the Caribbean, Central and South America), although at the island-group level, most have a higher proportion with a Southern American native range. Most naturalized species occur in anthropogenic habitats, but many are present in “natural” habitats with fewer species, such as Leucaena leucocephala, reported from specialized habitats like drylands. MalNAF provides a baseline for future studies of naturalized plant species distributions in the region.

Naturalization of an alien ancient fruit tree at a fine scale: Community structure and population dynamics of Cydonia oblonga in China

Naturalized plants play pivotal roles in local plant biodiversity and ecological functions; however, the drivers of naturalization remain poorly understood at a fine scale. Thus, understanding the processes of the development and dominance of alien plants in local natural habitats is of paramount importance. In the present study, we report for the first time the naturalization of Cydonia oblonga in China based on community structure and population dynamics at a fine scale. We conducted a comprehensive survey of the species through field community investigations, interviews, and a literature review. Cydonia oblonga is an ancient fruit tree with a long introduction history of over 4500 years worldwide and a cultivation history of over 2500 years in China. We analyzed C. oblonga community structure using the spatiotemporal substitution method and quantitatively analyzed population dynamics using a static life table, survivorship curve, and time series model to explore the naturalization processes. The following results were obtained. (i) The community comprised 31 coexisting vascular plant species (16 woody and 15 herbaceous species) belonging to 28 genera in 20 families. Rosaceae and Asteraceae were the two most dominant families. (ii) All individuals in the shrub layer as well as the C. oblonga population exhibited a roughly inverted J-shaped basal diameter distribution. A complete age structure was noted, and the survival curve was classified as Deevey type II. According to time series analysis, the population is estimated to increase in the future, specifically of medium and large individuals. (iii) Religious exchange, potent resource competitiveness, and similarity with the native habitat may be the major drivers of the introduction and successful naturalization of C. oblonga. These results suggest that alien species closely related to native ones are more likely to invade, naturalize, and dominate communities in local habitats.

Trends in the direction of global plant invasion biology research over the past two decades

Invasive plants are a growing ecological problem worldwide, but biases and patterns within invasive plant research may affect our understanding of invasive plant ecology. In this study, we analyzed 458 invasive plant papers sampled from the two journals dedicated entirely to the field of invasion biology, i.e., Biological Invasions and Neobiota. From these papers, we collected information on geographic coverage, climate, habitat, taxonomic coverage, plant functional type, and research topic to examine trends across a 21-year time period from 1999 to 2020. Our analysis found that invasive plant research was consistently biased toward temperate grassland and forest ecosystems particularly within the Americas, Europe, and Australia, and toward smaller, herbaceous invasive plant species (i.e., forbs, grasses, and shrubs), with an increase in interest in invasive nitrogen-fixing legumes over time. Our analysis also identified "hot" research topics in invasive plant research at specific time periods, such as a peak in the use of genetic analysis methods in 2014-2015 and a more recent focus on plant physiological and functional traits. While current models, concepts, and understanding of plant invasion ecology are still driven by such biases, this has been partially offset by recent increased research in understudied systems, as well as increasing awareness that plant invasion is heavily affected by their growth types, physiological traits, and soil interactions. As the field of invasion biology becomes ever increasingly important over time, focusing invasive plant research on understudied ecosystems and plant groups will allow us to develop a more holistic understanding of the ecology of invasive plants. In particular, given the outsized importance of the tropics to global biodiversity, the threats they face, and the dearth of studies, it is of critical importance that more invasive plant research is conducted within the tropics to develop a more globally representative understanding of invasive plant ecology.

Climate change may exacerbate the risk of invasiveness of non-native aquatic plants: the case of the Pannonian and Mediterranean regions of Croatia

Non-native aquatic plants are amongst the major threats to freshwater biodiversity and climate change is expected to facilitate their further spread and invasiveness. To date, in Croatia, no complete list of non-native extant and horizon aquatic plants has been compiled nor has a risk screening been performed. To address this knowledge gap, 10 extant and 14 horizon aquatic plant species were screened for their risk of invasiveness in the Pannonian and Mediterranean regions of Croatia under current and predicted (future) climate conditions. Overall, 90% and 60% of the extant species were classified as high risk for the Pannonian and Mediterranean regions, respectively, under both climate scenarios. Of the horizon species, 42% were classified as high risk under current conditions and, under climate change, this proportion increased to 78%. The ‘top invasive’ species (i.e. scored as very high risk) under both climate conditions and for both regions were extant Elodea nuttallii and horizon Lemna aequinoctialis. The horizon Hygrophila polysperma was very high risk for the Mediterranean Region under current climate conditions and for both regions under projected climate conditions. Azolla filiculoides, Elodea canadensis, Egeria densa and Utricularia gibba were also classified as high risk under current climate conditions and, after accounting for climate change, they became of very high risk in both regions. Further, Gymnocoronis spilanthoides and Lemna minuta were found to pose a very high risk under climate change only for the Pannonian Region. It is anticipated that the outcomes of this study will contribute to knowledge of the invasiveness of aquatic plants in different climatic regions and enable prioritisation measures for their control/eradication.

The role of phylogenetic relatedness on alien plant success depends on the stage of invasion

Darwin's naturalization hypothesis predicts successful alien invaders to be distantly related to native species, whereas his pre-adaptation hypothesis predicts the opposite. It has been suggested that depending on the invasion stage (that is, introduction, naturalization and invasiveness), both hypotheses, now known as Darwin's naturalization conundrum, could hold true. We tested this by analysing whether the likelihood of introduction for cultivation, as well as the subsequent stages of naturalization and spread (that is, becoming invasive) of species alien to Southern Africa are correlated with their phylogenetic distance to the native flora of this region. Although species are more likely to be introduced for cultivation if they are distantly related to the native flora, the probability of subsequent naturalization was higher for species closely related to the native flora. Furthermore, the probability of becoming invasive was higher for naturalized species distantly related to the native flora. These results were consistent across three different metrics of phylogenetic distance. Our study reveals that the relationship between phylogenetic distance to the native flora and the success of an alien species changes from one invasion stage to the other.

Assessing and Predicting the Distribution of Riparian Invasive Plants in Continental Portugal

The number of alien plant species is growing steadily across all world regions. These numbers tend to be exceptionally high in riparian ecosystems, often with substantial negative consequences for native species communities and ecosystem services provision. Here, we map the richness of invasive alien plant species in riparian ecosystems of continental Portugal, assess the relative importance of human and natural factors in shaping the uncovered patterns, and predict richness values along watercourses and at the municipal level for the whole study area. We found a higher richness of invasive alien plants in low altitudes and in downstream areas where human concentration is high. As time progresses, ongoing and increasing levels of socio-economic activity and globalization of plant trade will conceivably lead to a higher number of alien species becoming established. National and sub-national measures aiming to prevent and manage biological invasions in riparian ecosystems require coordinated efforts involving both local entities and those with responsibilities in the management of upstream catchment areas. These efforts must also be targeted to achieve future biodiversity protection goals as part of the EU Biodiversity Strategy for 2030.

Plant invasions in riparian areas of the Middle Danube Basin in Serbia

Riparian areas experience strong invasion pressures worldwide and represent important points of spread for invasive alien plants (IAPs) in the European mainland. The Danube Basin is a well-known point of high plant invasion levels. Given that the middle part of the Danube Basin is critically understudied and the general lack of data for Serbia, the study aimed to provide an insight into the spatial patterns of plant invasions in the riparian areas of Serbia (Middle Danube Basin area). A total of 250 field sites, distributed along 39 rivers (nine catchment areas) and six canal sections, were studied during a four-year period (2013–2016) for the presence and abundance of IAPs. At the landscape scale, we studied distribution patterns of IAPs, differences in invasion levels in different catchment areas and between rivers and canals. At the local scale, we investigated how the proximity to roads/railway lines, housing areas, different land-use types (primarily agriculture), and dominant vegetation on site related to invasion patterns. Of the 26 studied IAPs, those with a well-known weedy behavior, long history of cultivation and strong affinity for riparian areas prevailed in the study area. Riparian zones of the Danube catchment exhibited the highest invasion levels in terms of IAPs richness and abundance, followed by the catchment areas of the Timok, Sava and Zapadna Morava rivers. Surprisingly, the Danube-Tisa-Danube canal network had the lowest invasion level. At the local scale, agriculture in proximity of the field site and dominant vegetation on site were observed as significant predictors of the invasion level. On the other hand, proximity to roads/railway lines and housing areas was not related to the invasion level. Finally, our study provides the first systematic overview of IAPs’ distribution data for riparian areas of the Middle Danube Basin in Serbia, which could provide a basis for long-term monitoring of IAPs and development of future management plans.

Alien flora of Oman: invasion status, taxonomic composition, habitats, origin, and pathways of introduction

We present the first inventory and status assessment of the alien flora of Oman, mainly based on field data collected from 1998 to 2021. The study provides (i) a comprehensive account of alien vascular plant species occurring in the wild in Oman, with information on their taxonomic composition. For each species information is given on (ii) invasion status (casual, naturalized or invasive), biogeography, habitat and life-form characteristics, and pathways of introduction. Further, we (iii) explain the differences in the alien species composition in different parts of the country, and (iv) analyse the drivers of plant invasions in Oman. Out of the 111 alien species reported (7.7% of the total Oman vascular flora), 34 species are casuals and 77 naturalized; of the latter seven are considered invasive. The moderate number of alien plant species is likely a result of the country’s arid climate, with extremely high summer temperatures and low annual precipitation in most of its area, and the relatively long isolation of the country. The families richest in alien plant species are Fabaceae (17 species), Asteraceae (14 species) and Poaceae (12 species). More alien plants were found in northern Oman (82 species) than in southern Oman (60 species), and very few species are recorded from the central desert (7 species). The main habitats colonized were man-made habitats, either ruderal or agricultural. Most species alien to Oman are native to South America (49 species) or North America (43 species). This inventory provides a knowledge base for developing a national management strategy for alien vascular plants in Oman.

The global loss of floristic uniqueness

Regional species assemblages have been shaped by colonization, speciation and extinction over millions of years. Humans have altered biogeography by introducing species to new ranges. However, an analysis of how strongly naturalized plant species (i.e. alien plants that have established self-sustaining populations) affect the taxonomic and phylogenetic uniqueness of regional floras globally is still missing. Here, we present such an analysis with data from native and naturalized alien floras in 658 regions around the world. We find strong taxonomic and phylogenetic floristic homogenization overall, and that the natural decline in floristic similarity with increasing geographic distance is weakened by naturalized species. Floristic homogenization increases with climatic similarity, which emphasizes the importance of climate matching in plant naturalization. Moreover, floristic homogenization is greater between regions with current or past administrative relationships, indicating that being part of the same country as well as historical colonial ties facilitate floristic exchange, most likely due to more intensive trade and transport between such regions. Our findings show that naturalization of alien plants threatens taxonomic and phylogenetic uniqueness of regional floras globally. Unless more effective biosecurity measures are implemented, it is likely that with ongoing globalization, even the most distant regions will lose their floristic uniqueness.

Antigonon leptopus invasion is associated with plant community disassembly in a Caribbean island ecosystem

Invasions by non-native plant species are widely recognized as a major driver of biodiversity loss. Globally, (sub-)tropical islands form important components of biodiversity hotspots, while being particularly susceptible to invasions by plants in general and vines in particular. We studied the impact of the invasive vine A. leptopus on the diversity and structure of recipient plant communities on the northern Caribbean island St. Eustatius. We used a paired-plot design to study differences in species richness, evenness and community structure under A. leptopus-invaded and uninvaded conditions. Community structure was studied through species co-occurrence patterns. We found that in plots invaded by A. leptopus, species richness was 40–50% lower, and these plots also exhibited lower evenness. The magnitude of these negative impacts increased with increasing cover of A. leptopus. Invaded plots also showed higher degrees of homogeneity in species composition. Species co-occurrence patterns indicated that plant communities in uninvaded plots were characterized by segregation, whereas recipient plant communities in invaded plots exhibited random co-occurrence patterns. These observations suggest that invasion of A. leptopus is not only associated with reduced species richness and evenness of recipient communities in invaded sites, but also with a community disassembly process that may reduce diversity between sites. Given that A. leptopus is a successful invader of (sub-)tropical islands around the globe, these impacts on plant community structure highlight that this invasive species could be a particular conservation concern for these systems.

Ornamentals lead the way: global influences on plant invasions in the Caribbean

Understanding the historical factors associated with the invasion success of alien species in a region may help us to identify sources, vectors, and pathways that are more likely to originate new invaders. Here, we gather data for traits related to the history of introduction (e.g., continent of origin, reason for introduction, and date of introduction) of 616 alien plant species listed as invasive on 18 island groups across the Caribbean region. We used these data to evaluate how human activity has influenced plant invasions on Caribbean islands over time and whether invasion success could be driven by traits of the introduction process. We found that significantly more invasive plants (54%) were intentionally introduced for ornamental reasons than for any other purpose. Most invaders in the Caribbean are native to Asia, South America, and Africa and the cumulative number of invasive species in this region has been steadily increasing during the last 200 years, but since 1850, this trend has been led by species introduced as ornamentals. We also found a significant association between continent of origin and reason of introduction, with more invaders than expected being ornamentals from Asia and America, and forage species from Africa. Our results show that introduced ornamentals are successfully invading all major habitats across the Caribbean, exacerbating conservation issues and threatening native biodiversity. Armed with knowledge of origins and reasons for introductions, effective biosecurity actions as well as control and management strategies can be better targeted to address the problem of invasive species in the region.

Effects of land-use change and related pressures on alien and native subsets of island communities

Island species and habitats are particularly vulnerable to human disturbances, and anthropogenic changes are increasingly overwriting natural island biogeographic patterns. However, quantitative comparisons of how native and alien assemblages respond to human disturbances are scarce. Using data from 6,242 species of vertebrates, invertebrates and plants, from 7,718 sites on 81 islands, we model how land-use change, human population density and distance to the nearest road affect local assemblages of alien and native species on islands. We found that land-use change reduces both richness and abundance of native species, whereas the number and abundance of alien species are high in plantation forests and agricultural or urban sites. In contrast to the long-established pattern for native species (i.e., decline in species number with island isolation), more isolated islands have more alien species across most land uses than do less isolated islands. We show that alien species play a major role in the turnover of island assemblages: our models show that aliens outnumber natives among the species present at disturbed sites but absent from minimally-disturbed primary vegetation. Finally, we found a homogenization pattern for both native and alien assemblages across sites within most land uses. The declines of native species on islands in the face of human pressures, and the particular proneness to invasions of the more remote islands, highlight the need to reduce the intensity of human pressures on islands and to prevent the introduction and establishment of alien species.

The Convention on Biological Diversity (CBD)’s Post-2020 target on invasive alien species – what should it include and how should it be monitored?

The year 2020 and the next few years are critical for the development of the global biodiversity policy agenda until the mid-21st century, with countries agreeing to a Post-2020 Global Biodiversity Framework under the Convention on Biological Diversity (CBD). Reducing the substantial and still rising impacts of invasive alien species (IAS) on biodiversity will be essential if we are to meet the 2050 Vision where biodiversity is valued, conserved, and restored. A tentative target has been developed by the IUCN Invasive Species Specialist Group (ISSG), and formally submitted to the CBD for consideration in the discussion on the Post-2020 targets. Here, we present properties of this proposal that we regard as essential for an effective Post-2020 Framework. The target should explicitly consider the three main components of biological invasions, i.e. (i) pathways, (ii) species, and (iii) sites; the target should also be (iv) quantitative, (v) supplemented by a set of indicators that can be applied to track progress, and (vi) evaluated at medium- (2030) and long-term (2050) time horizons. We also present a proposed set of indicators to track progress. These properties and indicators are based on the increasing scientific understanding of biological invasions and effectiveness of responses. Achieving an ambitious action-oriented target so that the 2050 Vision can be achieved will require substantial effort and resources, and the cooperation of a wide range of stakeholders.

Scientists' warning on invasive alien species

Biological invasions are a global consequence of an increasingly connected world and the rise in human population size. The numbers of invasive alien species – the subset of alien species that spread widely in areas where they are not native, affecting the environment or human livelihoods – are increasing. Synergies with other global changes are exacerbating current invasions and facilitating new ones, thereby escalating the extent and impacts of invaders. Invasions have complex and often immense long‐term direct and indirect impacts. In many cases, such impacts become apparent or problematic only when invaders are well established and have large ranges. Invasive alien species break down biogeographic realms, affect native species richness and abundance, increase the risk of native species extinction, affect the genetic composition of native populations, change native animal behaviour, alter phylogenetic diversity across communities, and modify trophic networks. Many invasive alien species also change ecosystem functioning and the delivery of ecosystem services by altering nutrient and contaminant cycling, hydrology, habitat structure, and disturbance regimes. These biodiversity and ecosystem impacts are accelerating and will increase further in the future. Scientific evidence has identified policy strategies to reduce future invasions, but these strategies are often insufficiently implemented. For some nations, notably Australia and New Zealand, biosecurity has become a national priority. There have been long‐term successes, such as eradication of rats and cats on increasingly large islands and biological control of weeds across continental areas. However, in many countries, invasions receive little attention. Improved international cooperation is crucial to reduce the impacts of invasive alien species on biodiversity, ecosystem services, and human livelihoods. Countries can strengthen their biosecurity regulations to implement and enforce more effective management strategies that should also address other global changes that interact with invasions.

Economic use of plants is key to their naturalization success

Humans cultivate thousands of economic plants (i.e. plants with economic value) outside their native ranges. To analyze how this contributes to naturalization success, we combine global databases on economic uses and naturalization success of the world’s seed plants. Here we show that naturalization likelihood is 18 times higher for economic than non-economic plants. Naturalization success is highest for plants grown as animal food or for environmental uses (e.g. ornamentals), and increases with number of uses. Taxa from the Northern Hemisphere are disproportionately over-represented among economic plants, and economic plants from Asia have the greatest naturalization success. In regional naturalized floras, the percentage of economic plants exceeds the global percentage and increases towards the equator. Phylogenetic patterns in the naturalized flora partly result from phylogenetic patterns in the plants we cultivate. Our study illustrates that accounting for the intentional introduction of economic plants is key to unravelling drivers of plant naturalization.

Understanding why certain alien species become naturalized can shed light on biological invasion patterns. In this global analysis on thousands of taxa, van Kleunen and colleagues show that plant species of economic use are more likely to become naturalized, and that this underlies geographic patterns and phylogenetic signals in naturalization

Trait differentiation between native and introduced populations of the invasive plant Sonchus oleraceus L. (Asteraceae)

There is growing evidence that rapid adaptation to novel environments drives successful establishment and spread of invasive plant species. However, the mechanisms driving trait adaptation, such as selection pressure from novel climate niche envelopes, remain poorly tested at global scales. In this study, we investigated differences in 20 traits (relating to growth, resource acquisition, reproduction, phenology and defence) amongst 14 populations of the herbaceous plant Sonchus oleraceus L. (Asteraceae) across its native (Europe and North Africa) and introduced (Australia and New Zealand) ranges. We compared traits amongst populations grown under standard glasshouse conditions. Introduced S. oleraceus plants seemed to outperform native plants, i.e. possessing higher leaf and stem dry matter content, greater number of leaves and were taller at first flowering stage. Although introduced plants produced fewer seeds, they had a higher germination rate than native plants. We found strong evidence for adaptation along temperature and precipitation gradients for several traits (e.g. shoot height, biomass, leaf and stem dry matter contents increased with minimum temperatures, while germination rate decreased with annual precipitations and temperatures), which suggests that similar selective forces shape populations in both the native and invaded ranges. We detected significant shifts in the relationships (i.e. trade-offs) (i) between plant height and flowering time and (ii) between leaf-stem biomass and grain yield between native and introduced plants, indicating that invasion was associated with changes to life-history dynamics that may confer competitive advantages over native vegetation. Specifically, we found that, at first flowering, introduced plants tended to be taller than native ones and that investment in leaf and stem biomass was greater in introduced than in native plants for equivalent levels of grain yield. Our study has demonstrated that climatic conditions may drive rapid adaption to novel environments in invasive plant species.