Although research on human-mediated exchanges of species has substantially intensified during the last centuries, we know surprisingly little about temporal dynamics of alien species accumulations across regions and taxa. Using a novel database of 45,813 first records of 16,926 established alien species, we show that the annual rate of first records worldwide has increased during the last 200 years, with 37% of all first records reported most recently (1970–2014). Inter-continental and inter-taxonomic variation can be largely attributed to the diaspora of European settlers in the nineteenth century and to the acceleration in trade in the twentieth century. For all taxonomic groups, the increase in numbers of alien species does not show any sign of saturation and most taxa even show increases in the rate of first records over time. This highlights that past efforts to mitigate invasions have not been effective enough to keep up with increasing globalization.

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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: One-quarter 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.

have generally supported a long-held ecological paradigm that, in natural areas, habitats of low plant diversity are more vulnerable to plant invasions than areas of high diversity (Elton 1958). This theory contends that, through the process of competitive exclusion (Grime 1973), species-rich areas are "immunized" against invasion by foreign plants through the preemption of resources by native species (Tilman 1999). One recent study by Kennedy et al. (2002) found that "diverse communities will probably require minimal maintenance and monitoring because they are generally effective at excluding undesirable invaders".It would be comforting to believe that areas with many plant species are less prone to invasion than those with fewer species. Botanical hotspots such as wetlands, riparian zones, Mediterranean environments, subtropical coastal areas, and tallgrass prairies might repel the frequent arrival of plants from other regions or countries. Mechanical, chemical, and biological control techniques might be limited to heavily invaded, species-poor areas, with little danger of compromising unique assemblages of native plant species.Recently, some ecologists have begun to question this perspective (Stohlgren et al. 1997(Stohlgren et al. , 1999Levine and D'Antonio 1999;Levine 2000). Casual observations have shown highly invasive plant species, including tamarisk (Tamarix spp.), Russian olive (Elaeagnus angustifolia), purple loostrife (Lythrum salicaria), and Chinese tallow (Sapium sebiferum), becoming widely established in species-rich riparian zones and wetlands. Until now, however, large, carefully collected data sets from natural landscapes have been unavailable. These are useful for comparing local, landscape, regional, and national patterns to those observed under carefully controlled conditions, such as heavily manipulated, small-scale, experimental plots protected from disturbance (Knops et al. 1997;Naeem et al. 2000;Kennedy et al. 2002). MethodsWe evaluated two large independent data sets on the distribution of native and non-native plant species. The first set was gathered from 316 large vegetation-monitoring plots in eight states, which are part of the USDA Forest Service 's Forest Health Monitoring Program (Cline et al. 1995). The plots are systematically spaced throughout the US (one every 63 942 ha), and the numbers vary by state: Colorado (33), Delaware (39), Michigan (71), Oregon (44), Pennsylvania (81), Virginia (15), Washington (12), and Wyoming (21). Each plot consists of four 168-m 2 subplots, with three 1-m 2 quadrats in each subplot. Between 1997 and 2001, all the plots were sampled once every summer. Data were collected on the presence and cover of native and non-native species in each quadrat and species presence in the subplots.The second data set was gathered over the past 20 years by the Biota of North America Program (www.bonap.org) at the University of North Carolina, Chapel Hill. Data were available for 44 of 50 US states, and were based on over 229 000 records of native and non-native p...

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.