Abstract Garlic mustard is an invasive Eurasian biennial that has spread throughout the eastern United States and southern Canada. Populations of this plant vary in their susceptibility to Erysiphe cruciferarum, a causal agent of powdery mildew disease in Brassicaceous plants. We examined whether there were biogeographic patterns in the distribution of resistance in invasive North American and native European populations of this plant. We grew plants from 78 invasive and 20 native populations and screened them for powdery mildew resistance in the greenhouse. We found that populations were mostly monomorphic for either resistance or susceptibility but that some polymorphic populations were found from both continents. The proportion of populations showing resistance versus susceptibility was similar in both Europe and North America. Within continents, the spatial distribution of resistant and susceptible populations did not deviate significantly from random. We also examined whether the possession of the resistance trait alter intraspecific competitive dynamics. In two trials, we competed plants from resistant and susceptible populations in a target-neighbor design in the presence and absence of powdery mildew inoculum and examined the growth of the target plant. Target plants from resistant populations were overall larger than target plants from susceptible populations. Target plants were overall larger when grown in competition with susceptible neighbors. Further, resistant target plants showed a greater degree of release from competition when grown with a susceptible neighbor versus a resistant neighbor than the degree of release shown by susceptible target plants. This suggests a benefit of possessing the resistance trait with little apparent costs which should promote selection for this trait within plant populations.

Abstract Invasive species are a major driver of global biodiversity loss. However, we often lack a mechanistic understanding why some non-native species become invasive. Missing coevolutionary history between invasive predators and native prey can lead to a lack of predator avoidance by native prey and consequently higher consumption rates by invasive predators. We compared predation rates of four native European lady beetle species and the invasive lady beetle species Harmonia axyridis Pallas (Coleoptera: Coccinellidae), using the native pea aphid Acyrthosiphon pisum Harris (Hemiptera: Aphididae) for prey. Here, we also quantified initial dropping of A. pisum from host plants. In an additional plant-choice experiment, we tested for aphid avoidance of plants bearing lady beetle cues. Differences in predation were determined by predator body size. Initial dropping of aphids did not differ between the invasive lady beetle species H. axyridis and the remaining native lady beetle species. However, A. pisum showed no avoidance behavior towards H. axyridis cues, but to cues of the most voracious native species. Thus, relatively large body size and missing chemical cue avoidance by aphids can benefit the invasive H. axyridis.

Abstract Predicting changes in the abundance and distribution of introduced species over time is difficult, but clues regarding the underlying causes of these changes may come from long-term surveys. Resurveys conducted over large spatial scales, for example, can be used to discriminate between site-specific causes of decline and those that act at the population level. Here we used regional resurvey data to address changes in Argentine ant site occupancy in urban, agricultural, and natural environments in coastal southern California. We resurveyed 145 locations originally sampled 12–14 years ago and observed a slight decrease in Argentine ant site occupation, from 85 to 79% across all sites. At the majority of sites where apparent absences were recorded, however, the Argentine ant had merely retreated outside of the proscribed sampling area (a 25-m radius circle) and was still present within 80 m, on average, of the original sampling point. This finding thus suggests that the apparent absences observed most likely reflect contractions of the area occupied by polydomous supercolonies, possibly in response to local-scale changes in environmental conditions, as opposed to processes acting at larger spatial scales. We also conducted back-to-back annual resurveys (in 2018 and 2019) of all urban sampling points (n = 95) to quantify annual turnover in site occupancy. These resurveys revealed 2.4% site turnover with 97% (92/95) sites being classified as either presences (n = 85) or absences (n = 7) in both years. Our results support the findings of resurveys conducted in northern California, where Argentine ant distributions have slowly expanded over decadal time scales. Historical resurveys can provide insights into why populations of introduced species change over time and should incorporate an appreciation of how invader traits affect detectability.

Abstract Boom-bust dynamics of invasive species have long intrigued scientists and managers alike, but quantification of such dynamics, let alone their causes, is rare. We documented the decline of a previously prolific invasive mudsnail, Batillaria attramentaria, at Elkhorn Slough estuary in central California, USA. The mudsnail was the most abundant epibenthic invertebrate in the estuary, maintaining very high densities for many decades before declining heterogeneously throughout the estuary over the past decade, decreasing in density by three orders of magnitude at some sites. We used field and laboratory experiments to test several possible mechanisms for its demise. We show that the crab Pachygrapsus crassipes can prey heavily on Batillaria. We detected high dissolution rates of Batillaria shells, and we measured greater predation rates on tethered snails with dissolved versus intact shells. Warm water temperatures and high water levels coincided with the period of most dramatic Batillaria declines (2013–2015). Localized water impoundments appear to buffer environmental drivers of the decline because Batillaria remained abundant at sites with artificial tidal restriction, while the population crashed at one site after full tidal exchange was restored. We also investigated trematode parasite prevalence and molluscicide applications to the surrounding watershed as possible causes of mudsnail declines, but they had little explanatory power. Our findings illustrate the potential for population crashes even for long-established introduced species at pest levels of abundance, and demonstrate that such declines can exhibit spatial heterogeneity. Both of these results highlight the value of investigating population dynamics of invaders across multiple temporal and spatial scales.

There are thousands of case studies documenting invasive species’ impacts and these have led to the development of over 30 hypotheses that describe how invasions occur and their impacts manifest. The proliferation of invasion hypotheses over the decades has spurred several efforts to identify conceptual overlap and increase clarity of impact mechanism in the field. What is still lacking, however, is a comprehensive accounting of the evidence base on invasive species impacts, especially in regard to the biological scales, temporal scales, and taxonomic groups that receive research. Identifying the ‘known unknowns’ of empirical invasion impact research serves a critical function in the effort to evaluate support for existing hypotheses and generate novel hypotheses. We built a taxonomically and geographically comprehensive database of over 1500 research articles that report measures of invasive species’ ecological impacts published over the past 18 years (1999–2016). We found that, field-wide, published measures of invasive species’ impacts are highly skewed toward those measured at the population or community-level with scarce information on impacts at other biological scales (e.g., physiology, behavior). We also show that existing impact evidence stems most often from one-off studies of single invasive species. Yet, even for species that receive consistent attention, impacts have rarely been documented across more than one biological scale, beyond very short time periods, or in several ecosystems. In order to predict and anticipate how impacts manifest in a variety of temporal and biological contexts, the evidence base informing existing invasion hypotheses must become more integrative both within and across publications.

Abstract The incident command system (ICS) is a framework for organizing and directing on-site, tactical responses to a particular event or series of events. ICS provides a command structure for coordination, information flow, analysis, decision-making, communications, and implementation in an authoritative and standardized manner. The ICS framework has been used for emergency response in a variety of situations where the environment, human health, or other resources are at risk, including wildfires, natural disasters, terrorist attacks, oil and chemical spills, infectious disease outbreaks, and invasive species. This paper outlines the key components of ICS, as well as major elements for building ICS capacity. It concludes with a list of considerations for applying ICS in the invasive species context.

Short-distance dispersal enables introduced alien species to colonise and invade local habitats following their initial introduction, but is often poorly understood for many freshwater taxa. Knowledge gaps in range expansion of alien species can be overcome using predictive approaches such as individual based models (IBMs), especially if predictions can be improved through fitting to empirical data, but this can be challenging for models having multiple parameters. We therefore estimated the parameters of a model implemented in the RangeShifter IBM platform by approximate Bayesian computation (ABC) in order to predict the further invasion of a lowland river (Great Ouse, England) by a small-bodied invasive fish (bitterling Rhodeus sericeus). Prior estimates for parameters were obtained from the literature and expert opinion. Model fitting was conducted using a time-series (1983 to 2018) of sampling data at fixed locations and revealed that for 5 of 11 model parameters, the posterior distributions differed markedly from prior assumptions. In particular, sub-adult maximum emigration probability was substantially higher in the posteriors than priors. Simulations of bitterling range expansion predicted that following detection in 1984, their early expansion involved a relatively high population growth rate that stabilised after 5 years. The pattern of bitterling patch occupancy was sigmoidal, with 20% of the catchment occupied after 20 years, increasing to 80% after 30 years. Predictions were then for 95% occupancy after 69 years. The development of this IBM thus successfully simulated the range expansion dynamics of this small-bodied invasive fish, with ABC improving the simulation precision. This combined methodology also highlighted that sub-adult dispersal was more likely to contribute to the rapid colonisation rate than expert opinion suggested. These results emphasise the importance of time-series data for refining IBM parameters generally and increasing our understanding of dispersal behaviour and range expansion dynamics specifically.

House mice (Mus musculus) are a widespread invasive species on islands. Where they are the sole introduced mammal they can have particularly strong negative impacts on recipient ecosystems. House mice impacts have been documented on almost every component of the terrestrial ecosystem on Southern Ocean islands, including plants, invertebrates, birds and ecosystem function. We undertook a comprehensive study to determine the impacts of house mice on Antipodes Island, New Zealand. This study was done prior to mouse eradication to inform monitoring and restoration. We used invertebrate pitfall trapping on the main Antipodes Island and neighbouring mouse-free offshore islands together with mouse stomach contents and stable isotope analyses of mouse livers to examine dietary preferences. We identified directly impacted and consumed invertebrate Orders relative to their abundance and provided a comprehensive picture of resource flow and overlap in the invaded terrestrial ecosystem. The remote terrestrial ecosystem of Antipodes Island was tightly circumscribed with strong resource overlap. Mouse diet varied seasonally with resource availability, dominated by invertebrates and land birds in summer, and plants and seabirds in winter. Invertebrates that were preferentially preyed upon were Amphipoda, Lepidoptera and some species of Coleoptera. These patterns suggest the ecosystem is annually driven by a seasonal bottom-up resource pulse over summer, where mice are a selective predator, differentially preying on invertebrates relative to invertebrate abundance. Mice appear to be exhausting preferred prey as they systematically consume their way through the terrestrial ecosystem. Land bird diet also varied seasonally and some of these birds likely competed with mice for invertebrate prey. Eradication of mice from Antipodes Island should reduce the predation on invertebrates and reduce the effects of competition and predation on land birds. This should have flow-on effects to the abundance of invertebrates and endemic land bird sub-species of pipit and snipe.

Since 2007, the non-indigenous calanoid copepod Pseudodiaptomus marinus Sato, 1913 has been increasingly recorded in numerous European sites, spreading at an unexpectedly fast pace over a short time-span. This species presents specific biological and behavioural traits which make it of particular interest for ecological and applied research topics. On 29–30 January 2018, 29 scientists from nine European Countries established the EUROBUS (Towards a EURopean OBservatory of the non-indigenous calanoid copepod Pseudodiaptomus marinUS) Working Group (WG). This WG aimed at creating a European network of institutions and researchers working on the various aspects of the biology and ecology of P. marinus, with an open forum where sharing experience and know-how among WG participants. This brought to an updated distribution map of P. marinus in European waters, as well as to the identification of priority research lines and potential joint initiatives under the WGEUROBUS umbrella. This contribution, stemming from the experts participating at the WG, represents the manifesto of the current and future initiatives developed within WGEUROBUS.

Abstract We assessed the relative importance of human activity and environmental suitability as drivers of compositional dissimilarity of alien birds for 65 of the most populous cities of the Iberian Peninsula. We examined how these drivers relate to Zeta diversity (ζ) for alien Passeriformes and Psittaciformes. We performed the analysis using multiple orders of ζ, which provides insight on the role played by rare and common species in determining levels of dissimilarity. We also ran the analyses using the community of native Passeriformes as a phylogenetically close contrasting control. Our results showed that the proportion of urban area, a variable related to colonization and propagule pressure, had a strong influence on Psittaciformes but not on alien Passeriformes. This latter group showed to be primarily influenced by environmental factors, similarly to what was found for native Passeriformes. On other hand, human connectivity, as measured by distance through roads and railways seemed to play a significant role in shaping the compositional dissimilarity of alien Passeriformes, but not Psittaciformes. Regardless of the group analysed, the relative importance of the explanatory variables was similar for both rare and common species. Our findings highlight differences between the factors driving compositional dissimilarity for distinct groups of birds. While the emerging biogeography of Psittaciformes is mainly a reflection of distinctiveness in urban areas, alien Passeriformes are more strongly affected by the natural environment and thus their biogeography may increasingly resemble the one of their native counterparts.

Identifying and establishing the relative importance of different anthropogenic pathways of invasive non-native species (INNS) introduction is critical for effective management of their establishment and spread in the long-term. Angling has been identified as one of these pathways. An online survey of 680 British anglers was conducted to establish patterns of movement by British anglers abroad, and to establish their awareness and use of biosecurity practices. The survey revealed that 44% of British anglers travelled abroad for fishing, visiting 72 different countries. France was the most frequently visited country, accounting for one-third of all trips abroad. The estimated time taken to travel from Western Europe into Great Britain (GB) is within the time frame that INNS have been shown to survive on damp angling equipment. Without biosecurity, it is therefore highly likely that INNS could be unintentionally transported into GB on damp angling gear. Since the launch of the Check, Clean Dry biosecurity campaign in GB in 2011, the number of anglers cleaning their equipment after every trip has increased by 15%, and 80% of anglers now undertake some form of biosecurity. However, a significant proportion of the angling population is still not implementing sufficient, or the correct biosecurity measures to minimize the risk of INNS dispersal on damp angling equipment. With the increase in movement of anglers abroad for fishing, further work is required to establish the potential for INNS introduction through this pathway.

Abstract Forest understory plants in eastern North America experience heavy white-tailed deer browse and impacts associated with invasive species, including of non-native earthworms. Deer and earthworms are well-recognized ecosystem engineers due to their ability to alter understory plant dynamics. We examined interactive effects of earthworms and deer on four woodland ferns (Adiantum pedatum, Polystichum acrostichoides, Dryopteris marginalis, and Dennstaedtia punctilobula) using field and common garden experiments. In the field we planted ferns into fenced and deer-accessible plots in areas with and without earthworms. In a common garden, we assessed both fern responses to different earthworms and fern effects on earthworm survival and biomass. Venue was important in determining experimental outcomes. In the field, A. pedatum and P. acrostichoides showed higher survival and P. acrostichoides grew taller or produced more fronds in worm-invaded areas. Deer presence/absence was not a significant explanatory variable. In the common garden, earthworms had no discernible effect on ferns, but fern species identity varied in its effect on earthworm survival and leaf litter decomposition. Our findings indicate that, contrary to many other understory species, common ferns benefit or are unaffected by earthworms. That we obtained different and even contradictory results in different venues should encourage investigators to validate common garden findings in the field.

Biological invasions are occurring at increasing rates since the onset of the twentieth century. While ports and marinas have been identified as a major point-of-entry for the spread of marine non-indigenous species (NIS), their relationships with wild habitats however needs further scrutiny. We had the rare opportunity to monitor the real-time colonization dynamics of a newly-built marina by the notorious invasive kelp Undaria pinnatifida in the Bay-of-Morlaix, Brittany (France). Field surveys (> 20,000 individuals geo-localized) were combined with genetic analyses (10 microsatellite loci, N = 890 individuals) over 3 years (i.e., 6 generations in the study area). Regarding the colonization dynamics, a dramatic snow-ball effect was documented over time, with local density reaching up to 50 individuals per m after 2 years. Assignment tests showed that the primary colonizers came from neighboring populations established in natural rocky reefs. A shift towards a self-sustaining population was however observed the following year, with 44% of self-assignment. These processes are best explained by (i) life history traits, notably rapid growth and selfing, and (ii) natural dispersal within the marina combined with human-mediated dispersal—through leisure boating- over longer distances. Spill-over effects have been previously documented, and here also reported, to explain the expansion of U. pinnatifida from marinas to the wild. We showed that the on-going ocean sprawl also offers a perfect arena for spill-back events (i.e., spread from natural habitats to artificial structures), highlighting the need for careful surveillance of newly built infrastructures.

Genetic variability of invasive pests informs us of fundamental aspects of the invasion, which help leverage limited resources for more efficient pest management. This is particularly important for managing species with invasive populations fueled by recurrent introductions. We present the first phylogeographic study of the velvet longhorned beetle (Trichoferus campestris [Faldermann]), a highly polyphagous wood-boring insect native to Asia. It attacks many ecologically and economically important tree species and has repeatedly invaded Europe and North America in recent decades. Despite the apparent threat posed by this pest, little work has been attempted to characterize genetic variability of T. campestris. One hurdle is the lack of genetic data beyond the “universal” COI barcode. Here we designed primer sequences to amplify five additional mitochondrial protein-coding genes for T. campestris (COII, COIII, Cytb, ND2, and ND4). We included specimens collected from domestic locations in the Unites States (U.S.) as well as samples intercepted in solid wood packaging material (WPM) at U.S. ports of entry. Phylogenetic analyses recovered three mitochondrial lineages, all including both domestic collections and port-interceptions. Our results demonstrated high levels of haplotype diversity among invasive populations of T. campestris. Specimens collected from the same geographical vicinity can be placed into different lineages. Species delimitation analyses further suggest presence of multiple molecular operational taxonomic units, which could complicate regulatory policies. We evaluated mechanisms that underlie the increasing number of T. campestris individuals and populations detected in the U.S. Results support the hypothesis of independent introductions through WPM, but are inconsistent with secondary invasions through the bridgehead effect.

Non-native species have been introduced at escalating rates during the last decades, mainly due to the dispersion generated by the increasing trade and transport worldwide. Mollusks, the second largest metazoan phylum in terms of species richness, are no exception to this pattern, but, to date, a comprehensive synthesis of non-native mollusk species (NNMS) in South America was not available. For this purpose, an e-discussion group was formed with malacologists and taxonomists from South America, where we exchanged and analyzed bibliography, databases and information about NNMS, providing expert opinion to this assessment. The first list of non-native mollusk species for South America, considering terrestrial, freshwater and marine environments, includes 86 NNMS distributed in 152 ecoregions (terrestrial, freshwater and marine) of the 189 recognized for the South American continent. Information on their native region, vectors, first record for South America and distribution, are also provided. In the analysis of the distribution of the NNMS and the entry points of each species (e.g., ports, cargo and passenger airports, cities) and status of conservation of the ecoregions, four hot spots were recognized: Subtropical-Atlantic, Northern Andes, Central Andes and Southern Andes. This work, thus, sets the baseline on NNMS for South America, a key piece of information regarding the development of policies targeting the management of biological invasions and their socio-ecological impacts.

Orthodontium lineare, a neophyte of southern origin, was accidentally introduced by human activity to England in 1910. Subsequently it spread to mainland Europe and it continues to move eastward. In Poland, it was discovered in 1980–1981. The present paper shows dot distribution maps at 10-year intervals and discusses the dynamics of its invasion and ecological preferences using statistical methods. In Poland in 2018 it was known in 235 stands. 86.4% of them were located below 300 m a.s.l. and only 10.2% of the sites reached the lower mountain forest belt (maximum altitude 915 m). On every substrate and in all types of habitat, it commonly produced sporophytes (88.6% of the specimens investigated). Most commonly it occurred on rotting wood (38.5% of all records) and the base of tree trunks (33.9%). Rarely it grew on soil (podzolic and acid podzolic brown soils and humus on rocks; in total 19.0%). Most frequently it was found on the dead wood of Pinus sylvestris (31.3% of all records on rotting wood) and Picea abies (23.2%) and on the bases of P. sylvestris trunks (77.0%). Up to 2018 it was listed in 13 habitat types, including eight EU habitats (two priority), the most often invaded being semi-natural forest (25.8% of all records), boggy woodland (23.4%), mature managed forest developed from old plantations (17.7%), wooded dunes of the Atlantic region (9.7%) and Luzulo-Fagetum beech forest (6.9%). 33 accompanying taxa were found comprising 6 liverworts, 23 mosses and 4 lichens. Of these only 7 species were present in more than 5% of the samples.

Abstract Biological invasions are a growing threat to biodiversity. The black rat, one of the worst pest in the world, is responsible for extensive population decline of many autochthonous and endemic species, particularly in island ecosystems. A number of rat eradication campaigns have been conducted, however, such endeavors do not always result in a complete removal of the pest. This may be due to the occurrence of individuals resistant to common rodenticides and/or a re-invasion of the same environment from interconnected areas when appropriate eradication units are not defined before starting an eradication campaign. Our study is a multidisciplinary approach whereby genetic and epidemiological methods were used to provide background information for successful eradication of black rats. We investigated the occurrence of mutations in the VKORC1 gene known to confer resistance to rodenticides and evaluated the spread of zoonoses across three islands of the Pontine Archipelago, an Italian hotspot of endemic Mediterranean biodiversity and a possible mainland source of invasion. As part of an eradication campaign, we also assessed patterns of genetic diversity at 10 microsatellite loci in order to identify eradication units. We recorded a strong population structure and revealed at least two distinct eradication units. Some degree of admixture was recorded on Ponza, the largest island and likely the main source of rats invading the other two islands. We did not record the occurrence of rats resistant to anticoagulants, but we revealed transmission of vector-borne pathogens in commensal habitats of the Archipelago.

Abstract Rapid response strategies are necessary to effectively manage non-indigenous species. After detection, only few species persist and potentially cause harm. To help prioritize resources, we employed a multispecies, geographically explicit approach, focusing on non-indigenous aquarium fish establishment in the USA. We modeled casual (i.e. temporary) establishment and persistence separately, to identify which species should be prioritized after detection. To facilitate the usability of quantitative models, we converted our results into simple “rules of thumb”, wherein each factor’s contribution represents a multiplier. Finally, from a fundamental perspective, separating casual and persistent establishment improved our understanding of the earlier stages of invasions. We identified five species ranking highest for rapid response, if detected in California, New Mexico and Texas. These states, along with Florida and Hawaii, should take precedence in management funding, being those that currently host more persistent species and where more new establishments are forecasted. Expectedly, the important factors differed considerably between sub-stages, with species traits and propagule pressure being most relevant for casual establishment, and the environment being more predictive of persistence. Notably, propagule pressure had no effect on persistence, suggesting that it would not help target eradication for aquarium fish. Our model allows comparisons for > 1000 species across locations to target rapid responses after detection, and can provide guidance for species not currently traded. Our analysis re-evaluates “risky” species in terms of persistence, suggesting that many species which were flagged in the literature actually pose low risk. Conversely, we identify species that, if detected, warrant rapid response.

A key challenge in invasion biology is identifying characteristics that allow some species to be repeatedly successful at invading novel environments. Invasions can often be disproportionately driven by a single sex, with differences in behavioural mechanisms between the sexes potentially underlying sex-biased invasiveness. Here, we took an animal personality approach to study the behaviour of two repeatedly successful congeneric invasive species, the western mosquitofish, Gambusia affinis, and the eastern mosquitofish, Gambusia holbrooki. In each species, we investigated whether males and females shared common personality traits (i.e. behavioural types and behavioural syndromes), with the aim of identifying possible behavioural mechanisms that could help explain why mosquitofish invasions are often characterised by sex-biased founder populations. We found sex-dependent personality, although sex differences varied between species. Male G. affinis were bolder and less social than female G. affinis, whereas we found no behavioural type differences between the sexes in G. holbrooki. We also found a consistent correlation between boldness and exploration in both sexes within G. affinis, but this correlation was weak in G. holbrooki. Finally, exploration was also correlated with sociability in male G. affinis, but not in females. Our results suggest that behavioural tendencies may diverge, both among species and between the sexes, because of adaptation experienced during different invasion pathways. Broadly, identifying the behavioural mechanisms that predict an individual’s ‘invasiveness’ may be difficult to tease apart between species because each invasion is characterised by different abiotic and biotic interactions that likely require different suites of behaviours. Future studies are needed to elucidate whether, in fact, personality variation between the sexes can mediate the occurrence of sex-biased invasions.

Abstract The free-floating American duckweed, Lemna minuta, is an invasive species now widespread in Europe. Yet, its impact on freshwater ecosystems has been poorly investigated. In this study, the effects of the presence of this invasive duckweed on water quality, and aquatic plant and invertebrate communities were evaluated in sites in Central Italy. Water chemical and physical factors and community descriptors were analyzed to identify these effects. Surveys were carried out across 17 paired aquatic sites. Site pairs were similar in microclimate, hydrogeology and water quality, but differed in relation to the presence/absence of L. minuta floating mats. In sites with mats, light and dissolved oxygen in water were negatively correlated with increasing mat coverage and thickness. The limited light and hypoxic conditions under mats inhibited plant growth and had a selective impact on the invertebrate community. Sites with L. minuta had aquatic communities with a lower plant taxa richness and a contrasting composition, compared with those in sites without. At sites with mats some plants were unaffected, but the majority of plant taxa documented at sites without Lemna were no present at sites with Lemna or were very rare (macroalgae, submerged rhizophytes). As for invertebrates, hypoxic-tolerant taxa dominated under mats (Ostracoda, Copepoda, Isopoda), whilst those more sensitive to oxygen depletion, or obligate herbivores, or those with a winged stage or swimming on water surface, were rare or absent (Ephemeroptera, Amphipoda, Chironomus, Notonecta). Lemna minuta mats presence was associated with alterations in the underlying aquatic ecosystem, severely threatening the conservation of these habitats. Active management strategies, including spread-prevention techniques, or mechanical removal combined with biological control, are required to conserve these habitats.

Abstract Since the 1960s conservation efforts have focused on recovering island biodiversity by eradicating invasive rodents. These eradication campaigns have led to considerable conservation gains, particularly for nesting seabirds. However, eradications are complex and lengthy endeavors and are even more challenging when humans are co-inhabitants of the targeted island. Furthermore, the method of eradication matters and recent proposals to consider genetic technologies for rodent eradication require specific scrutiny. One such technology is the potential use of a gene drive for biasing offspring sex ratios in invasive house mice, Mus musculus, that would spread and prevent the production of one sex, allowing die-off from lack of reproduction and natural attrition. Practitioners can gain insight into the potential for adoption of this technology from examining stakeholder engagement. This paper uses scenario analysis to address the eradication of rodents on inhabited and uninhabited islands, by specifically comparing the traditional approach of using rodenticides with sex-biasing gene drives. Concurrently the International Union for Conservation of Nature is assessing the risks and value of gene drives in general for conservation. Hence, we make the case that the ethical challenges with the use of gene drive sex-biasing techniques and the effectiveness of this tool will rely as much on its public acceptance and its democratic use as the actual science used to construct the technology.

Abstract Correlative species distribution modelling is a widely used method to predict potential species ranges but can suffer from limitations in integrating species’ fundamental niches. Therefore, they might underestimate suitable ranges, but including physiological information can improve accuracy of predictions and help identify mechanisms of e.g. range limitation. However, approaches using both, results from correlative as well as physiological investigations are rare, especially in research on seaweeds. Here, we provide results from both approaches to predict the suitable habitat range of Capreolia implexa (Rhodophyta) in its native range (Australia and New Zealand) and invaded range (Chile) under present and future climate scenarios (year 2100, rcp 2.6 and rcp 8.5). We used the Maxent modelling technique and physiological knowledge from a temperature tolerance experiment (2–20 °C) for thermal niche estimation. Results from both approaches suggest larger suitable habitat ranges under present day conditions for both regions than currently occupied. Abiotic range limitation in the native range led to underestimation of the suitable temperature range by Maxent (here lower temperature limit = 8.3 °C). Predictions based on the laboratory temperature tolerance experiment suggest additional suitable habitat in colder regions (here lower temperature limit = 6.6 ± 0.4 °C). Under future climate conditions, both native and invaded ranges should shift southward, which will lead to an overall loss of suitable habitat in the native range. Like that, rcp 8.5 conditions should reduce the native range to 50% of the present-day extent. We demonstrate the limitation of correlative SDM modelling for species that live on continental margins and that physiological experiments can help to identify species’ niches beyond correlative analyses, providing valuable information for range projections. Furthermore, we provide valuable insights relevant for both invasion management and conservation. Graphic abstract

Abstract Removing invasive rodents from islands has many ecological and social benefits. However, eradications fail more frequently on tropical than on temperate islands, and causes for these failures are not yet well understood. We addressed two major plausible reasons for eradication failure, testing whether actively reproducing females and pre-weaned pups may not consume bait during an experimental eradication of Rattus exulans on a tropical island (22 ha). We tested four hypotheses, namely that: (1) lactating female rats take longer than non-lactating conspecifics to consume rodent bait, (2) pre-weaned rats orphaned following bait application survive when their mother dies, (3) surviving pre-weaned rats are not exposed to rodent bait and (4) lactating females have different natural diets than non-lactating females. Our experimental eradication was successful, despite conditions that are typically associated with eradication failures: rat density was high (95% CI 65–153 rats/ha), rats were breeding and juveniles of all age-classes were present, land crabs were abundant, bait was only widely available for two nights after bait application (i.e. > 90% of bait disappeared within 36 h; bait was undetectable after 60 h), and alternative foods (e.g. coconut) were abundantly available. We found no evidence to support hypotheses that rat breeding and diet specialisation limit bait uptake because all rats consumed bait. Rat eradication was achieved with a relatively low bait application rate (32 kg/ha; 16 kg/ha per application) and a short (7 day) interval between the two bait applications. We recommend further experimental eradications targeting R. exulans and other invasive Rattus and Mus species on larger islands, but conclude that with comprehensive bait coverage eradications on tropical islands can be successful despite abundant natural food and a high density of reproductively active rats.

The ability of federal agencies to carry out actions or programs is based on their legal authorities. Efforts to improve federal capacities for the early detection of and rapid response to invasive species (EDRR) require careful delineation of legal authorities, regulations, and policies that would enable or limit EDRR. Building on information provided by federal agencies and an inspection of the US Code and the Code of Federal Regulations, we review and identify relevant authorities to determine federal legal capacities, gaps, and inconsistencies to address EDRR. The EDRR process can be examined in the context of four categories, including (1) explicit invasive species authorities, (2) emergency authorities that could be triggered during a crisis or serve as models for enhanced invasive species EDRR authorities, (3) supporting authorities that could be used under agency discretion, and (4) constraining authorities and legal requirements. Although the Plant Protection Act and the Animal Health Protection Act are comprehensive authorities that address the detection of and response to organisms that threaten plant and livestock health, there is no single authority that encompasses EDRR for all invasive species. Rather, there is a patchwork of authorities that unevenly addresses various aspects of EDRR. In addition to gaps in authority, EDRR efforts could be constrained by environmental compliance, as well as subnational governance and private rights. Although some of these gaps could be closed through legislation, others need to be addressed using the discretionary power of federal agencies and their ability to establish cooperation mechanisms with private and subnational entities.

Abstract With a view toward creating a national Early Detection and Rapid Response Program (EDRR) program, the United States National Invasive Species Council Management Plan for 2016–2018 calls for a series of assessments of federal EDRR capacities, including the evaluation of “relevant federal information systems to provide the data and other information necessary for risk analyses/horizon scanning, rapid specimen identification, and rapid response planning.” This paper is a response to that directive. We provide an overview of information management needs for enacting EDRR and discuss challenges to meeting these needs. We then review the history of relevant US policy directives for advancing invasive species information systems and provide an overview of federal invasive species information system capacities, including current gaps and inconsistencies. We conclude with a summary of key principles and needs for establishing a national invasive species information framework. Our findings are consistent with earlier studies and, thus, emphasize the need to act on long-recognized needs. As a supplement to this paper, we have cataloged federal invasive species databases and information tools identified through this work.

Abstract The 2016–2018National Invasive Species Council (NISC) Management Plan and Executive Order 13751 call for US federal agencies to foster technology development and application to address invasive species and their impacts. This paper complements and draws on an Innovation Summit, review of advanced biotechnologies applicable to invasive species management, and a survey of federal agencies that respond to these high-level directives. We provide an assessment of federal government capacities for the early detection of and rapid response to invasive species (EDRR) through advances in technology application; examples of emerging technologies for the detection, identification, reporting, and response to invasive species; and guidance for fostering further advancements in applicable technologies. Throughout the paper, we provide examples of how federal agencies are applying technologies to improve programmatic effectiveness and cost-efficiencies. We also highlight the outstanding technology-related needs identified by federal agencies to overcome barriers to enacting EDRR. Examples include improvements in research facility infrastructure, data mobilization across a wide range of invasive species parameters (from genetic to landscape scales), promotion of and support for filling key gaps in technological capacity (e.g., portable, field-ready devices with automated capacities), and greater investments in technology prizes and challenge competitions.

Abstract The invasive species issue is inherently a matter of risk; what is the risk that an invasive species will adversely impact valued assets? The early detection of and rapid response to invasive species (EDRR) requires that an assessment of risk is conducted as rapidly as possible. We define risk screening as rapid characterization of the types and degree of risks posed by a population of non-native species in a particular spatio-temporal context. Risk screening is used to evaluate the degree to which various response measures are warranted and justifiable. In this paper, we evaluate the US government’s risk screening programs with a view towards advancing national EDRR capacity. Our survey-based findings, consistent with prior analyses, indicate that risk evaluation by federal agencies has largely been a reactive, ad hoc process, and there is a need to improve information sharing, risk evaluation tools, and staff capacity for risk screening. We provide an overview of the US Department of Agriculture’s Tiered Weed Risk Evaluation and US Fish and Wildlife Service’s Ecological Risk Screening Summaries, two relatively new approaches to invasive species risk screening that hold promise as the basis for future work. We emphasize the need for a clearinghouse of risk evaluation protocols, tools, completed assessments and associated information; development of performance metrics and standardized protocols for risk screening; as well as support for complementary, science-based tools to facilitate and validate risk screening.

This paper responds to national policy directives intended to improve the US government’s capacity to protect national security from the adverse impacts of invasive species. It is the final, synthesizing contribution to a Special Issue of Biological Invasions comprising 12 papers that collectively inform the development and implementation of a national program for the early detection of and rapid response to invasive species (EDRR). The blueprint sets forth policies, goals, and actions to be taken by relevant Executive Branch agencies and components of the Executive Office of the President to develop a national EDRR program, appropriations permitting. It is designed to function as guidance for advancing federal policy through Presidential, Secretarial, and/or Congressional directives. Those committed to protecting national security, the economy, and the well-being of American people are forewarned that our ability to establish a national EDRR program is undermined by the diminishment of the federal workforce; institutional structures, policies, and programs; and directly applicable leadership mechanisms, including the National Invasive Species Council, Invasive Species Advisory Committee, and their managerial Secretariat.

Abstract Globalization necessitates that we address the negative externalities of international trade and transport, including biological invasion. The US government defines invasive species to mean, “with regard to a particular ecosystem, a non-native organism whose introduction causes, or is likely to cause, economic or environmental harm, or harm to human, animal, or plant health.” Here we address the role of early detection of and rapid response to invasive species (EDRR) in minimizing the impact of invasive species on US interests. We provide a review of EDRR’s usage as a federal policy and planning term, introduce a new conceptual framework for EDRR, and assess US federal capacities for enacting well-coordinated EDRR. Developing a national EDRR program is a worthwhile goal; our assessment nonetheless indicates that the federal government and its partners need to overcome substantial conceptual, institutional, and operational challenges that include establishing clear and consistent terminology use, strategically identifying and communicating agency functions, improving interagency budgeting, facilitating the application of emerging technologies and other resources to support EDRR, and making information relevant to EDRR preparedness and implementation more readily accessible. This paper is the first in a special issue of Biological Invasions that includes 12 complementary papers intended to inform the development and implementation of a national EDRR program.

Federal agency programs and associated actions are contingent on having the legal authority to act. There is no single authority established to direct the early detection of and rapid response to invasive species (EDRR). Rather, a patchwork of authorities unevenly addresses various aspects of the suite of EDRR measures. To support the development of national EDRR capacity, it is essential to delineate the federal legal statutes, regulations, and policies that enable or limit invasive species EDRR. Here we set forth general principles and a checklist of actions that agencies can refer to when they construct a more comprehensive EDRR legal and policy framework for addressing invasive species. This guidance is intended to complement the review and analysis of federal authorities contained elsewhere in this issue (Burgos-Rodríguez and Burgiel in Biol Invasions.  https://doi.org/10.1007/s10530-019-02148-w, 2019, this issue).

The early detection of and rapid response to invasive species (EDRR) depends on accurate and rapid identification of non-native species. The 2016–2018 National Invasive Species Council Management Plan called for an assessment of US government (federal) capacity to report on the identity of non-native organisms intercepted through early detection programs. This paper serves as the response to that action item. Here we summarize survey-based findings and make recommendations for improving the federal government’s capacity to identify non-native species authoritatively in a timely manner. We conclude with recommendations to improve accurate identification within the context of EDRR by increasing coordination, maintaining taxonomic expertise, creating an identification tools clearinghouse, developing and using taxonomic standards for naming and identification protocols, expanding the content of DNA and DNA Barcode libraries, ensuring long-term sustainability of biological collections, and engaging and empowering citizens and citizen science groups.

A watch list is a list of invasive species to be prioritized for surveillance, reporting, and other possible responses in order to reduce the risk of impact to valued assets. Watch lists are basic, yet multi-functional tools for the early detection of and rapid response to invasive species. There is, however, a need to substantially improve watch list standardization, accessibility, and associated communication strategies. We provide guidance to achieve these aims, including an overview of guiding principles and a list of questions to consider when one develops, communicates, and applies invasive species watch lists. Our guidance is intended to support invasive species watch lists development and application globally.

In the original publication, incorrect version of Supplementary material (Appendix S2) has been published. The corrected Supplementary material is provided in this correction.

The vector mosquitoes Aedes aegypti (L.), native to Africa, and Aedes albopictus (Skuse), native to Asia, are widespread invasives whose spatial distributions frequently overlap. Predictive models of their distributions are typically correlative rather than mechanistic, and based on only abiotic variables describing putative environmental requirements despite extensive evidence of competitive interactions leading to displacements. Here we review putative roles of competition contributing to distribution changes where the two species meet. The strongest evidence for competitive displacements comes from multiple examples of habitat segregation where the two species co-occur and massive reductions in the range and abundance of A. aegypti attributable to A. albopictus invasions in the southeastern U.S.A. and Bermuda (U.K). We summarize evidence to support the primacy of asymmetric reproductive interference, or satyrization, and larval resource competition, both favoring A. albopictus, as displacement mechanisms. Where evidence of satyrization or interspecific resource competition is weak, differences in local environments or alternative ecologies or behaviors of these Aedes spp. may explain local variation in the outcomes of invasions. Predictive distribution modeling for both these major disease vectors needs to incorporate species interactions between them as an important process that is likely to limit their realized niches and future distributions. Experimental tests of satyrization and resource competition are needed across the broad ranges of these species, as are models that incorporate both reproductive interference and resource competition to evaluate interaction strengths and mechanisms. These vectors exemplify how fundamental principles of community ecology may influence distributions of invasive species.

Ballast water moved by transoceanic vessels has been recognized globally as a predominant vector for the introduction of aquatic nonindigenous species (NIS). In contrast, domestic ships operating within confined geographic areas have been viewed as low risk for invasions, and are exempt from regulation in consequence. We examined if the St. Lawrence River could serve as a source of NIS for the Laurentian Great Lakes by surveying ballast water carried by domestic vessels and comparing biological composition in predominant St. Lawrence River-Great Lakes port-pairs in order to determine the likelihood that NIS could be transported to, and survive in, the Great Lakes. Thirteen potential invaders were sampled from ballast water, while 26 taxa sampled from St. Lawrence River ports are not reported from the Great Lakes. The majority of NIS recorded in samples are marine species with low potential for survival in the Great Lakes, however two euryhaline species (copepod Oithona similis, and amphipod Gammarus palustris) and two taxa reported from brackish waters (copepod Microsetella norvegica and decapod Cancer irroratus) may pose a risk for invasion. In addition, four marine NIS were collected in freshwater samples indicating that at least a subset of marine species have potential as new invaders to the Great Lakes. Based on results from this study, the ports of Montreal, Sorel, Tracy and Trois Rivières appear to pose the highest risk for new ballast-mediated NIS from the St. Lawrence River to the Great Lakes.

The distributions of many species are not at equilibrium with their environment. This includes spreading non-native species and species undergoing range shifts in response to climate change. The habitat associations of these species may change during range expansion as less favourable climatic conditions at expanding range margins constrain species to use only the most favourable habitats, violating the species distribution model assumption of stationarity. Alternatively, changes in habitat associations could result from density-dependent habitat selection; at range margins, population densities are initially low so species can exhibit density-independent selection of the most favourable habitats, while in the range core, where population densities are higher, species spread into less favourable habitat. We investigate if the habitat preferences of the non-native common waxbill Estrilda astrild changed as they spread in three directions (north, east and south-east) in the Iberian Peninsula. There are different degrees of climatic suitability and colonization speed across range expansion axes, allowing us to separate the effects of climate from residence time. In contrast to previous studies we find a stronger effect of residence time than climate in influencing the prevalence of common waxbills. As well as a strong additive effect of residence time, there were some changes in habitat associations, which were consistent with density-dependent habitat selection. The combination of broader habitat associations and higher prevalence in areas that have been colonised for longer means that species distribution models constructed early in the invasion process are likely to underestimate species' potential distribution.

The non-native kelp, Undaria pinnatifida, is considered one of the world's worst invasive species. The northeast Atlantic is a hotspot of Undaria invasion, yet there is limited knowledge on its invasion dynamics. In the UK its distribution is strongly associated with artificial structures, primarily marina and harbour pontoons, with relatively few records of Undaria on natural substrates. Here, the southwest UK is used as a case region, to explicitly link Undaria distribution-abundance patterns in artificial marina habitats with those in natural rocky reef habitats. Using a mixture of in situ recording and video survey techniques, Undaria was found at all thirteen marina sites surveyed; but in only 17 of 35 rocky reef sites, all of which were in 2 of the 5 larger systems surveyed (Plymouth Sound and Torbay). The distribution-abundance patterns of Undaria at reef sites were analysed using zero-inflated models. The probability of finding Undaria on rocky reef increased with increasing proximity to marinas with high abundances of Undaria. Total propagule pressure from marinas also increased the probability of occurrence, and was positively related to Undaria abundance and cover at reef sites. Increases in the cover of native kelps, Laminaria spp., and wave exposure at reef sites were linked to a reduced probability of Undaria occurrence, and lower abundance and cover. Identifying high risk areas, natural boundaries and factors affecting the spread and abundance of non-native species in natural habitats is key to future management prioritisation. Where Undaria is confined to artificial substrates management may be deemed a low priority. However, the results of this study suggest that controlling the abundance and propagule pressure in artificial habitats may limit, to some extent, the spillover of Undaria into natural rocky reef habitats, where it has the potential to interact with and influence native communities.

A diverse and abundant fouling community dominated by Lessepsian non-indigenous species was identified on a 13.5-m-long steel buoy stranded on the Israeli coast but originating from Port Said, at the Mediterranean entrance of the Suez Canal, Egypt. The molluscan community was sampled quantitatively by scraping. Three quarters of the individuals and more than half of the species were non-indigenous. Among the latter, a mytilid bivalve, Gregariella cf. ehrenbergi, was first recorded in the Mediterranean Sea on the basis of these samples, suggesting that the full consideration of all potential vectors can contribute to non-indigenous species detection. Large floating objects in coastal waters, such as buoys, are particularly suitable for colonization by Lessepsian species because hard substrates, and artificial ones in particular, are highly susceptible to the establishment of non-indigenous species. Moreover, their size and persistence enable the development of abundant and mature fouling communities that can disseminate propagules as eggs and larvae over long distances and for extended periods if detached. This report highlights the potential for large rafting debris as a vector of the spread of non-indigenous biota within the Mediterranean Sea.

Invasive alien species have the potential to alter biodiversity and ecosystem processes. In freshwaters, detritus decomposition is a major ecosystem service but it remains uncertain whether invasive alien decapods process detritus differently to natives. This study examined leaf litter processing, and cascading effects on biofilms, by the European native white clawed crayfish (Austropotamobius pallipes) compared to two invasive alien decapod species: the American signal crayfish (Pacifastacus leniusculus) and the Chinese mitten crab (Eriocheir sinensis). Invasive alien decapods were responsible for higher leaf litter decomposition than the native. In comparison with native crayfish, invasive alien crab and crayfish showed higher rates of litter consumption, increased production of smaller leaf fragments, fine particulate organic matter (FPOM) and dissolved organic carbon. Nutrients (ammonia and soluble reactive phosphorous) derived from excretion (measured separately in the absence of biofilms) varied among decapod species, being lower for P. leniusculus. However, nutrient concentrations did not vary among species in the detritivory experiments with biofilm, implying nutrients were utilised for biofilm production and respiration as no differences in biomass were evident among decapod treatments. These results show invasive alien decapods have the potential to increase the magnitude of detrital processing to FPOM in rivers, but indirect impacts on primary producers due to nutrient release are uncertain based on this experimental context.

Biological invasions can have various impacts on the diversity of important microbial mutualists such as mycorrhizal fungi, but few studies have tested whether the effects of invasions on mycorrhizal diversity are consistent across spatial gradients. Furthermore, few of these studies have taken place in tropical ecosystems that experience an inordinate rate of invasions into native habitats. Here, we examined the effects of plant invasions dominated by non-native tree species on the diversity of arbuscular mycorrhizal (AM) fungi in Hawaii. To test the hypothesis that invasions result in consistent changes in AM fungal diversity across spatial gradients relative to native forest habitats, we sampled soil in paired native and invaded sites from three watersheds and used amplicon sequencing to characterize AM fungal communities. Whether our analyses considered phylogenetic relatedness or not, we found that invasions consistently increased the richness of AM fungi. However, AM fungal species composition was not related to invasion status of the vegetation nor local environment, but stratified by watershed. Our results suggest that while invasions can lead to an overall increase in the diversity of microbial mutualists, the effects of plant host identity or geographic structuring potentially outweigh those of invasive species in determining the community membership of AM fungi. Thus, host specificity and spatial factors such as dispersal need to be taken into consideration when examining the effects of biological invasions on symbiotic microbes.

Intraspecific hybridization between diverged populations can enhance fitness via various genetic mechanisms. The benefits of such admixture have been proposed to be particularly relevant in biological invasions, when invasive populations originating from different source populations are found sympatrically. However, it remains poorly understood if admixture is an important contributor to plant invasive success and how admixture effects compare between invasive and native ranges. Here, we used experimental crosses in Lythrum salicaria, a species with well-established history of multiple introductions to Eastern North America, to quantify and compare admixture effects in native European and invasive North American populations. We observed heterosis in between-population crosses both in native and invasive ranges. However, invasive-range heterosis was restricted to crosses between two different Eastern and Western invasion fronts, whereas heterosis was absent in geographically distant crosses within a single large invasion front. Our results suggest that multiple introductions have led to already-admixed invasion fronts, such that experimental crosses do not further increase performance, but that contact between different invasion fronts further enhances fitness after admixture. Thus, intra-continental movement of invasive plants in their introduced range has the potential to boost invasiveness even in well-established and successfully spreading invasive species.

Hemigrapsus sanguineus, the Asian shore crab, has rapidly replaced Carcinus maenas, the green crab, as the most abundant crab on rocky shores in the northwest Atlantic since its introduction to the United States (USA) in 1988. The northern edge of this progressing invasion is the Gulf of Maine, where Asian shore crabs are only abundant in the south. We compared H. sanguineus population densities to those from published 2005 surveys and quantified genetic variation using the cytochrome c oxidase subunit I gene. We found that the range of H. sanguineus had extended northward since 2005, that population density had increased substantially (at least 10-fold at all sites), and that Asian shore crabs had become the dominant intertidal crab species in New Hampshire and southern Maine. Despite the significant increase in population density of H. sanguineus, populations only increased by a factor of 14 in Maine compared to 70 in southern New England, possibly due to cooler temperatures in the Gulf of Maine. Genetically, populations were predominantly composed of a single haplotype of Japanese, Korean, or Taiwanese origin, although an additional seven haplotypes were found. Six of these haplotypes were of Asian origin, while two are newly described. Large increases in population sizes of genetically diverse individuals in Maine will likely have a large ecological impact, causing a reduction in populations of mussels, barnacles, snails, and other crabs, similar to what has occurred at southern sites with large populations of this invasive crab species.

Invasive species remain one of the greatest threats to global biodiversity. Their control would be enhanced through the development of more effective and sustainable pest management strategies. Recently, a novel form of genetic pest management (GPM) has been developed in which the mating behaviour of insect pests is exploited to introduce genetically engineered DNA sequences into wild conspecific populations. These 'transgenes' work in one or more ways to reduce the damage caused by a particular pest, for example reducing its density, or its ability to vector disease. Although currently being developed for use against economically important insect pests, these technologies would be highly appropriate for application against invasive species that threaten biodiversity. Importantly, these technologies have begun to advance in scope beyond insects to vertebrates, which include some of the world's worst invasives. Here we review the current state of this rapidly progressing field and, using an established set of eradication criteria, discuss the characteristics which make GPM technologies suitable for application against invasive pests.

Harbors and marinas are well known gateways for species introductions in marine environments but little work has been done to ascertain relationships between species diversity, harbor type, and geographic distance to uncover patterns of secondary spread. Here, we sampled ascidians from 32 harbors along ca. 300 km of the NW Mediterranean coast and investigated patterns of distribution and spread related to harbor type (marina, fishing, commercial) and geographic location using multivariate techniques. In total, 28 ascidians were identified at the species level and another 9 at the genus level based on morphology and genetic barcoding. Eight species were assigned to introduced forms, 15 were given native status and 5 were classified as cryptogenic. Aplidium accarense was reported for the first time in the Mediterranean Sea and was especially abundant in 23 of the harbors. Introduced and cryptogenic species were abundant in most of the surveyed harbors, while native forms were rare and restricted to a few harbors. Significant differences in the distribution of ascidians according to harbor type and latitudinal position were observed. These differences were due to the distribution of introduced species. We obtained a significant correlation between geographic distance and ascidian composition, indicating that closely located harbors shared more ascidian species among them. This study showed that harbors act as dispersal strongholds for introduced species, with native species only appearing sporadically, and that harbor type and geographic location should also be considered when developing management plans to constrain the spread of non-indigenous species in highly urbanized coastlines.

When a species is introduced into a new location, it may escape, at least temporarily, from its natural enemies. In field surveys, we found that when the exotic, invasive mosquito, Aedes albopictus, invades new sites, it initially experiences reduced infection by its gut parasite, Ascogregarina taiwanensis. To determine the effect of this escape from parasitism on the competitive ability of A. albopictus, we performed a laboratory competition experiment in which infected and uninfected A. albopictus larvae were reared in microcosms alone and in competition with larvae of the native mosquito, Ochlerotatus triseriatus. We analyzed the effect of parasitism by A. taiwanensis on A. albopictus performance when subjected to intra- and interspecific competition across a range of larval densities, as well as the effect of A. albopictus parasitism by A. taiwanensis on the competitive impact of A. albopictus on O. triseriatus. At a density of 30 O. triseriatus larvae, O. triseriatus survivorship was significantly reduced by the addition of 30 unifected A. albopictus, but not by addition of 30 infected A. albopictus, and not by addition of 15 A. albopictus whether infected or uninfected. Although estimated finite rate of population increase (λ') showed similar trends, and was significantly affected by treatments, no pairwise differences in rate of increase were significant. Infection by A. taiwanensis also significantly prolonged A. albopictus female development time and reduced the intraspecific competitive effect of increased density of A. albopictus, but did not affect A. albopictus survivorship, mass, or estimated finite rate of population increase. Thus, when A. albopictus escapes from this parasite as it colonizes new sites, this escape may give it a small, but significant, added competitive advantage over O. triseriatus, which may facilitate range expansion of A. albopictus and enhance A. albopictus's initial impact on resident species.

Interactions between invasive species can have important consequences for the speed and impact of biological invasions. Containers occupied by the invasive mosquito, Aedes albopictus Skuse, may be sensitive to invasive plants whose leaves fall into this larval habitat. To examine the potential for interactions between invasive leaf species and larval A. albopictus, we conducted a field survey of leaf material found with A. albopictus in containers in Palm Beach County, Florida and measured density dependent responses of A. albopictus larvae to two invasive and one native leaf species in laboratory experiments. We found increased diversity of leaf species, particularly invasive species, in areas further from the urbanized coast, and a significant positive association between the presence of Schinus terebinthifolious (Brazilian pepper) and the abundance of A. albopictus. In laboratory experiments, we determined that larval growth and survivorship were significantly affected by both larval density and leaf species which, in turn, resulted in higher population performance on the most abundant invasive species (Brazilian pepper) relative to the most abundant native species, Quercus virginiana (live oak). These results suggest invasive leaf species can alleviate density dependent reductions in population performance in A. albopictus, and may contribute to its invasion success and potential to spread infectious disease.

Understanding the migration patterns of invasive organisms is of paramount importance to predict and prevent their further spread. Previous attempts at reconstructing the entire history of the sudden oak death (SOD) epidemic in California were limited by: (1) incomplete sampling; (2) the inability to include infestations caused by a single genotype of the pathogen; (3) collapsing of non-spatially contiguous yet genetically similar samples into large meta-samples that confounded the coalescent analyses. Here, we employ an intensive sampling coverage of 832 isolates of Phytopthora ramorum (the causative agent of SOD) from 60 California forests, genotyped at nine microsatellite loci, to reconstruct its invasion. By using age of infestation as a constraint on coalescent analyses, by dividing genetically indistinguishable meta-populations into highly-resolved sets of spatially contiguous populations, and by using Bruvo genetic distances for most analyses, we reconstruct the entire history of the epidemic and convincingly show infected nursery plants are the original source for the entire California epidemic. Results indicate that multiple human-mediated introductions occurred in most counties and that further disease sources were represented by large wild infestations. The study also identifies minor introductions, some of them relatively recent, linked to infected ornamental plants. Finally, using archival isolates collected soon after the discovery of the pathogen in California, we corroborate that the epidemic is likely to have resulted form 3 to 4 core founder individuals evolved from a single genotype. This is probably the most complete reconstruction ever completed for an invasion by an exotic forest pathogen, and the approach here described may be useful for the reconstruction of invasions by any clonally reproducing organism with a relatively limited natural dispersal range.

Hosts of soybean rust (Phakopsora pachyrhizi) are sensitive to low temperatures, limiting this obligate parasite in the United States to overwintering sites in a restricted area along the Gulf Coast. This temperature sensitivity of soybean rust hosts allowed us to study spatial spread of epidemic invasions over similar territory for seven sequential years, 2005-2011. The epidemic front expanded slowly from early April through July, with the majority of expansion occurring from August through November. There was a 7.4-fold range of final epidemic extent (0.4 to 3.0 million km2) from the year of smallest final disease extent (2011) to that of the largest (2007). The final epidemic area of each year was regressed against epidemic areas recorded at one-week intervals to determine the association of final epidemic extent with current epidemic extent. Coefficients of determination for these regressions varied between 0.44 to 0.62 during April and May. The correlation coefficients varied between 0.70 and 0.96 from early June through October, and then increased monotonically to 1.0 by year's end. Thus, the spatial extent of disease when the epidemics began rapid expansion may have been a crucial contributor to subsequent spread of soybean rust. Our analyses used presence/absence data at the county level to evaluate the spread of the epidemic front only; the subsequent local intensification of disease could be strongly influenced by other factors, including weather.

Human disturbance modifies selection regimes, depressing native species fitness and enabling the establishment of non-indigenous species with suitable traits. A major impediment to test the effect of disturbance on invasion success is the lack of long-term data on the history of invasions. Here, we overcome this problem and reconstruct the effect of disturbance on the invasion of the bivalve Anadara transversa from sediment cores in the Adriatic Sea. We show that (1) the onset of major eutrophication in the 1970s shifted communities towards species tolerating hypoxia, and (2) A. transversa was introduced in the 1970s but failed to reach reproductive size until the late 1990s because of metal contamination, resulting in an establishment and detection lag of ~25 years. Subfossil assemblages enabled us to (1) disentangle the distinct stages of invasion, (2) quantify time-lags and (3) finely reconstruct the interaction between environmental factors and the invasion process, showing that while disturbance does promote invasions, a synergism of multiple disturbances can shift selection regimes beyond tolerance limits and induce significant time lags in establishment. The quantification of these time lags enabled us to reject the hypothesis that aquaculture was an initial vector of introduction, making shipping the most probable source.

The bryozoan Schizoporella japonica Ortmann (1890) was first recorded in European waters in 2010 and has since been reported from further locations in Great Britain (GB) and Norway. This paper provides a new earliest European record for the species from 2009, a first record from Ireland and presence and absence records from a total of 231 marinas and harbours across GB, Ireland, the Isle of Man, France and Portugal. This species is typically associated with human activity, including commercial and recreational vessels, aquaculture equipment, and both wave and tidal energy devices. It has also been observed in the natural environment, fouling rocks and boulders. The species has an extensive but widely discontinuous distribution in GB and Ireland. Although found frequently in marinas and harbours in Scotland, it inhabits only a few sites in England, Wales and Ireland, interspersed with wide gaps that are well documented as genuine absences. This appears to be a rare example of a southward-spreading invasion in GB and Ireland. The species has been reported from the Isle of Man and Norway but has not been found in France or Portugal. In the future we expect S. japonica to spread into suitable sections of the English, Welsh and Irish coasts, and further within Europe. The species' capability for long-distance saltatory spread and potential for negative impact on native ecosystems and economic activity suggests that S. japonica should now be considered invasive in GB and Ireland. As such, it is recommended that biosecurity procedures alongside effective surveillance and monitoring should be prioritised for regions outside the species' current distribution.

Invasive, non-native species represent a major threat to biodiversity worldwide. The African amphibian Xenopus laevis is widely regarded as an invasive species and a threat to local faunas. Populations originating at the Western Cape, South Africa, have been introduced on four continents, mostly in areas with a similar Mediterranean climate. Some introduced populations are also established in cooler environments where persistence for many decades suggests a capacity for long-term adaptation. In these cases, recent climate warming might enhance invasion ability, favouring range expansion, population growth and negative effects on native faunas. In the cool temperate UK, populations have been established for about 50 years in Wales and for an unknown period, probably >20 years, in England (Lincolnshire). Our field studies over 30 and 10 years, respectively, show that in favourable conditions there may be good recruitment, fast individual growth rates and large body size; maximum longevity exceeds 23 years. Nevertheless, areas of distribution remained limited, with numbers <500 in each population. In 2010, only a single individual was captured at each locality and further searching failed to record any others in repeated sampling up to 2014. We conclude that both populations are now extinct. The winters of 2009-2010 and 2010-2011 experienced extreme cold and drought (December 2010 was the coldest in 120 years and the third driest in 100 years). The extinction of X. laevis in these areas indicates that even relatively long-established alien species remain vulnerable to rare extreme weather conditions.

Watersports equipment can act as a vector for the introduction and spread of invasive non native species (INNS) in freshwater environments. To support advice given to recreational water users under the UK Government's Check Clean Dry biosecurity campaign and ensure its effectiveness at killing a range of aquatic INNS, we conducted a survival experiment on seven INNS which pose a high risk to UK freshwaters. The efficacy of exposure to hot water (45 °C, 15 min) was tested as a method by which waters users could 'clean' their equipment and was compared to drying and a control group (no treatment). Hot water had caused 99 % mortality across all species 1 h after treatment and was more effective than drying at all time points (1 h: χ2 = 117.24, p < 0.001; 1 day χ2 = 95.68, p < 0.001; 8 days χ2 = 12.16, p < 0.001 and 16 days χ2 = 7.58, p < 0.001). Drying caused significantly higher mortality than the control (no action) from day 4 (χ2 = 8.49, p < 0.01) onwards. In the absence of hot water or drying, 6/7 of these species survived for 16 days, highlighting the importance of good biosecurity practice to reduce the risk of accidental spread. In an additional experiment the minimum lethal temperature and exposure time in hot water to cause 100 % mortality in American signal crayfish (Pacifastacus leniusculus), was determined to be 5 min at 40 °C. Hot water provides a simple, rapid and effective method to clean equipment. We recommend that it is advocated in future biosecurity awareness campaigns.

Despite considerable efforts devoted to investigate the community assembly processes driving plant invasions, few general conclusions have been drawn so far. Three main processes, generally acting as successive filters, are thought to be of prime importance. The invader has to disperse (1st filter) into a suitable environment (2nd filter) and succeed in establishing in recipient communities through competitive interactions (3rd filter) using two strategies: competition avoidance by the use of different resources (resource opportunity), or competitive exclusion of native species. Surprisingly, despite the general consensus on the importance of investigating these three processes and their interplay, they are usually studied independently. Here we aim to analyse these three filters together, by including them all: abiotic environment, dispersal and biotic interactions, into models of invasive species distributions. We first propose a suite of indices (based on species functional dissimilarities) supposed to reflect the two competitive strategies (resource opportunity and competition exclusion). Then, we use a set of generalised linear models to explain the distribution of seven herbaceous invaders in natural communities (using a large vegetation database for the French Alps containing 5,000 community-plots). Finally, we measure the relative importance of competitive interaction indices, identify the type of coexistence mechanism involved and how this varies along environmental gradients. Adding competition indices significantly improved model's performance, but neither resource opportunity nor competitive exclusion were common strategies among the seven species. Overall, we show that combining environmental, dispersal and biotic information to model invasions has excellent potential for improving our understanding of invader success.

Non-indigenous ascidians are of particular concern to aquaculture industry and, paradoxically, the activities associated with it represent an important way to translocate these species worldwide. In 2012 a non-indigenous ascidian was found covering the oyster crops in the Ebro Delta (Western Mediterranean). We have identified the ascidian genetically and morphologically as Didemnum vexillum Kott, 2002. This finding indicates that the species is currently expanding its distribution in the Mediterranean Sea, as it has recently been found in the eastern basin (Venice, Adriatic Sea). Introduced populations of D. vexillum are found in temperate and cold waters worldwide, and a successful establishment in the Mediterranean implies a remarkable capacity of adaptability to warm, subtropical conditions. We assessed the life cycle (growth and reproduction) of the ascidian at the studied site. The species has a marked seasonal cycle, with regression in the warmest months and reappearance during winter. In spring D. vexillum reaches its maximum abundance, followed by a peak in reproduction just before regression. This cycle is reversed with respect to the one observed in colder waters, highlighting a plastic biological cycle of this invader and an hitherto unknown ability to establish itself in warm waters. We also analysed the genetic structure of the population of the Ebro Delta and the one established in the Lagoon of Venice using COI sequence data. The low genetic diversity in our samples (three haplotypes) was consistent with what is observed in the introduced populations worldwide. It is likely that the ascidian was introduced with oyster stock from bivalve cultures in the Atlantic French coasts, where the same three haplotypes have been reported. The high boating activity in the Ebro Delta makes further human-mediated transport of the species highly likely, and nearby fishing grounds can be severely affected if invaded. It is urgent to implement measures to prevent the continuous expansion of this ascidian pest in the Mediterranean.

The mosquito Aedes japonicus (Diptera: Culicidae) has spread rapidly through North America since its introduction in the 1990s. The mechanisms underlying its establishment in container communities occupied by competitors Aedes triseriatus and Aedes albopictus are unclear. Possibilities include (A) temporal separation of A. japonicus from other Aedes, (B) oviposition avoidance by A. japonicus of sites containing heterospecific Aedes larvae, and (C) non-additive competitive effects in assemblages of multiple Aedes. Containers sampled throughout the summer in an oak-hickory forest near Eureka, MO showed peak abundance for A. japonicus occurring significantly earlier in the season than either of the other Aedes species. Despite this, A. japonicus co-occurred with one other Aedes species in 53 % of samples when present, and co-occurred with both other Aedes in 18 % of samples. In a field oviposition experiment, A. japonicus laid significantly more eggs in forest edge containers than in forest interior containers, but did not avoid containers with low or high densities of larvae of A. triseriatus, A. albopictus, or both, compared to containers without larvae. Interspecific competitive effects (measured as decrease in the index of performance, λ') of A. triseriatus or A. albopictus alone on A. japonicus larvae were not evident at the densities used, but the effect of both Aedes combined was significantly negative and super-additive of effects of individual interspecific competitors. Thus, neither oviposition avoidance of competitors nor non-additive competitive effects contribute to the invasion success of A. japonicus in North America. Distinct seasonal phenology may reduce competitive interactions with resident Aedes.