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Abstract: Aim: Darwin's naturalization hypothesis states that dissimilarity to native species may benefit alien species establishment due to empty niches and reduced competition. We here add a new dimension to large‐scale tests of community invasibility, investigating the role that previously established alien species play in facilitating or hindering new invasions in plant communities. Location: Permanent grasslands across France (including mainland and Corsica), as a receding ecosystem of great conservation importance. Methods: Focusing on 121 alien plant species occurring in 7,215 vegetation plots, we quantified biotic similarity between new invaders and resident alien species (i.e., alien species with longer residence times) based on phylogenetic and trait distances. Additionally, we calculated distances to native species for each alien species and plot. Using multispecies distribution models, we analysed the influence of these biotic similarity measures and additional covariates on establishment success (presence/absence) of new invaders. Results: We found that biotic similarity to resident alien species consistently increased establishment success of more recently introduced species. Phylogenetic relatedness to previous invaders had an equally strong positive effect as relatedness to native species. Conversely, trait similarity to natives hindered alien establishment as predicted by Darwin's naturalization hypothesis. These results highlight that various mechanisms may act simultaneously to determine alien establishment success. Main conclusions: Our results suggest that, with greater similarity among alien species, invasion success increases. Such a pattern may arise either due to actual facilitation among invaders or as a result of weaker competitive interactions among invaders than between native and alien species, leading to an indirect facilitative effect. Alternatively, recent environmental changes (e.g., eutrophication, climate change) may have added new environmental filters. Determining how initial invasions might pave the road for subsequent invasions is crucial for effective multispecies management decisions and contributes a new aspect to our understanding of community assembly. [ABSTRACT FROM AUTHOR]

In the absence of empirical evidence, invasive species are often assumed to have negative impacts because of their conspicuously high abundance. The giant African land snail Achatina ( Lissachatina) fulica is one such invader where its impact in natural ecosystems remains completely untested. On Christmas Island (Indian Ocean), A. fulica has become established across large tracts of rainforest following the impacts of invasive yellow crazy ant ( Anoplolepis gracilipes) in mutualism with non-native scale insects. Yellow crazy ants facilitate the secondary invasion of A. fulica by extirpating native red land crabs ( Gecarcoidea natalis) that are normally effective predators of A. fulica. We used a multifaceted approach to investigate some potential impacts of abundant A. fulica in invaded rainforest. Over the course of a wet season, diel activity transects showed that A. fulica consumed detrital material almost exclusively. However, stable isotope analysis did not confidently identify A. fulica as a predominantly detritivorous species. We found no statistically significant treatment effects of A. fulica exclusion on standing leaf litter and seedling recruitment processes during a 6-month manipulative field study. However, litter cover and biomass did remain slightly higher where A. fulica were excluded, albeit with overlapping confidence intervals with control plots. Our study constitutes the first empirical test for impact of A. fulica in a natural ecosystem and suggests that for Christmas Island rainforest, this species is not a damaging invader. Other studies will need to assess the impacts of A. fulica in other natural areas before these findings could be considered broadly applicable. [ABSTRACT FROM AUTHOR]

Positive interactions between exotic species may increase ecosystem-level impacts and potentially facilitate the entry and spread of other exotic species. Invader-facilitated invasion success-'secondary invasion'-is a key conceptual aspect of the well-known invasional meltdown hypothesis, but remains poorly defined and empirically underexplored. Drawing from heuristic models and published empirical studies, we explore this form of 'secondary invasion' and discuss the phenomenon within the recognized conceptual framework of the determinants of invasion success. The term 'secondary invasion' has been used haphazardly in the literature to refer to multiple invasion phenomena, most of which have other more accepted titles. Our usage of the term secondary invasion is akin to 'invader-facilitated invasion,' which we define as the phenomenon in which the invasion success of one exotic species is contingent on the presence, influence, and impacts of one or more other exotic species. We present case studies of secondary invasion whereby primary invaders facilitate the entry or establishment of exotic species into communities where they were previously excluded from becoming invasive. Our synthesis, discussion, and conceptual framework of this type of secondary invasion provides a useful reference to better explain how invasive species can alter key properties of recipient ecosystems that can ultimately determine the invasion success of other species. This study increases our appreciation for complex interactions following invasion and highlights the impacts of invasive species themselves as possible determinants of invasion success. We anticipate that highlighting 'secondary invasion' in this way will enable studies reporting similar phenomena to be identified and linked through consistent terminology. [ABSTRACT FROM AUTHOR]

We present the EU LIFE PonDerat project, which is aimed at restoring the natural ecosystems of the Pontine Archipelago, a group of islands located off the western coast of Italy. The spread of invasive species is a major environmental threat on these islands, which are rich in rare habitats and endemic species and are important sites for the conservation of Mediterranean biodiversity. The project focuses on the conservation of species and habitats that are protected by EU laws but are currently threatened by introduced plants and animals. The main targets of the control measures are black rats, feral goats, mouflons and invasive plants of the genusCarpobrotus. Conservation measures focus on the shearwatersPuffinus yelkouanandCalonectris diomedea, which are endemic to the Mediterranean and are listed in Annex I of the European Bird Directive. Conservation measures also focus on island habitats of great biogeographical value, which are listed in Annex I of the European Habitats Directive and are seriously threatened by introduced herbivores and by invasive plants. The main outcome expected from this project is that the restoration of ecosystem functions will enhance the breeding success and population size of shearwaters and lead to the recovery of target habitats. [ABSTRACT FROM PUBLISHER]

Interactions between invasive species can be difficult to predict and can result in unanticipated impacts of significance for native fauna. Here we show that introduced European rabbits ( Oryctolagus cuniculus) create habitat that enables invasive redback spiders ( Latrodectus hasselti Thorell, 1870) to establish and prey upon the nationally endangered, endemic Cromwell chafer beetle ( Prodontria lewisii Broun, 1904). We examined the spatial relationship between rabbit holes and redback spider occurrence, recorded all prey caught in redback spider webs over a 4-month period, and tested the role rabbit holes play in providing habitat for redback spiders experimentally, by filling in rabbit holes in areas used by spiders and monitoring subsequent occupation of the areas over four months. Redback spiders predominately resided in old rabbit holes, with the highest densities of spiders coinciding with high densities of rabbit holes. Cromwell chafer beetles were commonly caught in webs. Filling in rabbit holes eliminated the presence of redback spiders at all treated sites and reduced prey capture in those areas. Conservation management to protect Cromwell chafer beetles should focus on eliminating rabbits and their holes from beetle habitat. [ABSTRACT FROM AUTHOR]

Invasive ecosystem engineers, such as dreissenid mussels, may facilitate subsequent invaders. Despite their potential ecological importance, interactions between dreissenid mussels and crayfish in the Laurentian Great Lakes have received little research attention. Invasive rusty crayfish ( Orconectes rusticus ) have recently spread within the Great Lakes, and we hypothesized that food resources provided by invasive dreissenid mussels may have enhanced this spread. Dreissenid mussels may also benefit native crayfish such as the virile crayfish ( O. virilis ), but the distribution of virile species has not increased in the Great Lakes in recent years. We tested the interactive effects of dreissenid mussels and crayfish density on the growth, survival and activity of sympatric rusty and virile crayfish using a mesocosm experiment. We found that dreissenid mussels increased growth and activity of rusty crayfish while high crayfish densities negatively affected rusty crayfish growth. Dreissenid mussels did not affect growth or activity of virile crayfish, but high crayfish densities negatively affected their survival. The different responses of rusty and virile crayfish to mussels may be due to a greater ability of invasive crayfish to exploit mussel-associated food resources and/or to behavioral interactions between these species in sympatry. Our results suggest that dreissenid mussels may facilitate the establishment, spread, and potential impacts of invasive crayfish where these taxa co-occur. [ABSTRACT FROM AUTHOR]

Invasive plants can have both positive and negative impacts on ecosystem services (ES), with decisions on control often being characterised by conflicts over loss of their perceived positive impacts on individual ES following removal. We present an analytical framework to aid in reducing such conflicts by allocating control effort to both minimise negative impacts and to maximise positive impacts on multiple ES. We used spatial models to map the negative impacts of invasive conifers on biodiversity, perceived landscape quality, and water yield and their positive impacts on erosion protection and carbon storage across a major catchment in the South Island of New Zealand. We tested the effect of distribution type (i.e. hotspot vs Gaussian) on trade-offs among these ES. We also tested whether using a non-linear function optimisation algorithm to fit variable weights to individual ES significantly reduced trade-offs. We show that an optimised multiple-ES approach could considerably reduce conflicts around invasive tree management arising from their contrasting impacts on different ecosystem services (i.e. by reducing trade-offs between ES), but cannot remove such conflict altogether. Our results are consistent with studies showing that ES with a hot-spot type distribution are the most vulnerable to trade-offs in multi-ES prioritisation, and hence will be the most likely to cause conflict in invasive tree control decisions. Our approach also shows that giving higher priority to ES with hot-spot distributions could reduce conflicts (by reducing tradeoffs). We argue that, when ES data are available, including estimates of ES impacts should be among the 'due diligence' requirements for developing invasive tree control strategies. [ABSTRACT FROM AUTHOR]

Non-native species often acquire novel interspecific interactions, which are central to several hypotheses of invasion success, including biotic resistance and invasional meltdown. However, the outcome of these interactions is not often linked with the demographic evidence based on the full life cycle of the species. The Philippine Ground Orchid ( Spathoglottis plicata) has invaded Puerto Rico and has acquired both negative and positive interspecific interactions involving the native weevil Stethobaris polita and the invasive red fire ant Solenopsis invicta, respectively. We studied a population in the Rio Abajo Forest, and asked how these interactions affect population demography by using a combination of field, experimental and modelling approaches. Stage-structured matrix population models based on four years of field observations showed that the population of S. plicata is growing at a rate (λ) of 1.05 under natural conditions. When we modified fecundity values based on experimental exclusion of weevils and ants, the control treatment showed a similar λ. Excluding weevils increased λ to 1.20, whereas the exclusion of ants decreased λ to 1.03. When we incorporate demographic and environmental stochasticity in our models, exclusion of invasive red fire ants significantly reduces the orchid abundance over time. Although weevils offer some biotic resistance to S. plicata, these effects do not prevent orchid population growth and expansion. On the other hand, invasive red fire ants have a positive effect on the invasive orchid's λ, partially supporting the invasional meltdown hypothesis. This study presents a method that allows one to combine opposing mechanisms of species interactions within the same quantitative framework, and the results highlight the importance of considering acquired plant-animal interactions and stochastic processes when evaluating the population growth rates and dynamics of invasive plants. [ABSTRACT FROM AUTHOR]

Aim Typically, non-indigenous species have been studied in relation to either the invaded (recipient) community or the donor community. However, we still lack a broad understanding of the mechanisms underlying the establishment of non-indigenous species that combines both perspectives. Since the opening of the Suez Canal, hundreds of species have invaded the Mediterranean from the Red Sea, forming a unique system in which the entire species pool (donor, non-indigenous and recipient) is known. Focusing on species ecological traits, we assess the drivers underlying the establishment of non-indigenous species from the onset of the invasion to the present. Location The Mediterranean and Red Seas. Methods We compiled traits for shallow-water hard-bottom-associated fishes in the Mediterranean (recipient community) and the Red Sea (donor community). We compared the multivariate community trait structure of non-indigenous species with the donor and recipient assemblages, using a novel method to objectively assign trait-weights, thereby increasing the robustness of the results. Patterns were contrasted with two types of null models which provide distinct insights into the ecological processes. Results Non-indigenous fishes are very diverse ecologically, substantially increasing the total community trait diversity of the Mediterranean. Trait similarity between non-indigenous and indigenous Mediterranean species was lower than expected, indicating that non-indigenous fishes tend to occupy relatively vacant niches within the Mediterranean. However, we further found that over time non-indigenous species display increased trait similarity to indigenous Mediterranean species. This suggests that trait constraints associated with successful establishment are weakening. Main conclusions Non-indigenous species establish in relatively vacant ecological niches, opening the possibility for predicting establishment using ecological traits. However, the weakening of trait constraints through time suggests that forecasting future establishment may be difficult. Faced with an accelerating pace of invasion, it appears that the Mediterranean is going to be transformed into an extension of the Red Sea in terms of trait and species composition. [ABSTRACT FROM AUTHOR]

Aggregations of the Ponto-Caspian invasive zebra mussel (Dreissena polymorpha) constitute a suitable habitat for macroinvertebrates, considerably increasing their abundance and providing effective antipredator protection. Thus, the overall effect of a mussel bed on particular predator species may vary from positive to negative, depending on both prey density increase and predator ability to prey in a structurally complex habitat. Alien Ponto-Caspian goby fish are likely to be facilitated when introduced into new areas by zebra mussels, provided that they are capable of utilizing mussel beds as habitat and feeding grounds. We ran laboratory experiments to find which prey (chironomid larvae) densities (from ca. 500 to 2,000 individuals m−2) in a mussel bed make it a more beneficial feeding ground for the racer goby Babka gymnotrachelus (RG) and western tubenose goby Proterorhinus semilunaris (WTG) compared to sandy and stone substrata (containing the basic prey density of 500 ind. m−2). Moreover, we checked how food availability affects habitat selection by fish. Mussel beds became more suitable for fish than alternative mineral substrata when food abundance was at least two times higher (1,000 vs. 500 ind. m−2), regardless of fish size and species. WTG was associated with mussel beds regardless of its size and prey density, whereas RG switched to this habitat when it became a better feeding ground than alternative substrata. Larger RG exhibited a stronger affinity for mussels than small individuals. WTG fed more efficiently from a mussel bed at high food abundances than RG. A literature review has shown that increasing chironomid density, which in our study was sufficient to make a mussel habitat an attractive feeding ground for the gobies, is commonly observed in mussel beds in the field. Therefore, we conclude that zebra mussels may positively affect the alien goby species and are likely to facilitate their establishment in novel areas, contributing to an invasional meltdown in the Ponto-Caspian invasive community.

Although the co-occurrence of nonnative vertebrates is a ubiquitous global phenomenon, the study of interactions between invaders is poorly represented in the literature. Limited understanding of the interactions between co-occurring vertebrates can be problematic for predicting how the removal of only one invasive—a common management scenario—will affect native communities. We suggest a trophic food web framework for predicting the effects of single-species management on native biodiversity. We used a literature search and meta-analysis to assess current understanding of how the removal of one invasive vertebrate affects native biodiversity relative to when two invasives are present. The majority of studies focused on the removal of carnivores, mainly within aquatic systems, which highlights a critical knowledge gap in our understanding of co-occurring invasive vertebrates. We found that removal of one invasive vertebrate caused a significant negative effect on native species compared to when two invasive vertebrates were present. These unexpected results could arise because of the positioning and hierarchy of the co-occurring invasives in the food web (e.g., carnivore–carnivore or carnivore–herbivore). We consider that there are important knowledge gaps to determinate the effects of multiple co-existing invaders on native ecosystems, and this information could be precious for management.

When non-native species appear in a new ecosystem, it may sufficiently affect native communities i.a. through interactions with native as well as other alien species. The „invasional meltdown“ hypothesis states that the presence of non-indigenous species facilitates the introduction and establishment of the other non-native species. A series of laboratory experiments was conducted to test the occurence of the invasional meltdown phenomenon within the invasive Ponto-Caspian community. Preferences of an alien Ponto-Caspian racer goby, Babka gymnotrachelus, towards amphipod preys of the same origin (Dikerogammarus villosus, Pontogammarus robustoides) or a prey species native to the resident community (Gammarus fossarum) were verified. Additionally, we tested the profitability of native and invasive prey for fish in a growth experiment. Accordingly with the invasional meltdown hypothesis, we hypothesized that the racer goby would prefer Ponto-Caspian amphipods and that prey would be more beneficial than native ones, which should result in higher growth rate. We found that the racer goby always selected the native G. fossarum, regardless of the prey mobility, presence of shelters and waterborne chemical cues. Thus, the fish choice was based on the assessment of prey quality during a direct contact with amphipods. The growth experiment indicated that the racer goby grew better on the native gammarid compared to the Ponto-Caspian species. The outcome of our study does not support the invasional meltdown hypothesis within the goby – gammarids predator prey system, as the non-native prey species were neither prefered nor more profitable food for the alien predator. Thus, we found no evidence for facilitation of establishement between the studied taxa.

The entry of secondary invaders into, or their expansion within, native communities is contingent on the changes wrought by other (primary) invaders. When primary invaders have altered more than one property of the recipient community, standard descriptive and modeling approaches only provide a best guess of the mechanism permitting the secondary invasion. In rainforest on Christmas Island, we conducted a manipulative field experiment to determine the mechanism of invasion success for a community of land snails dominated by non-native species. The invasion of rainforest by the yellow crazy ant ( Anoplolepis gracilipes) has facilitated these land snails, either by creating enemy-free space and/or increased habitat and resources (in the form of leaf litter) through the removal of the native omnivorous-detritivorous red land crab ( Gecarcoidea natalis). We manipulated predator densities (high and low) and leaf litter (high and low) in replicated blocks of four treatment combinations at two sites. Over the course of one wet season (five months), we found that plots with high leaf litter biomass contained significantly more snails than those with low biomass, regardless of whether those plots had high or low predation pressure, at both the site where land crabs have always been abundant, and at the site where they have been absent for many years prior to the experiment. Each site was dominated by small snail species (<2 mm length), and through handling size and predation experiments we demonstrated that red crabs tend not to handle and eat snails of that size. These results suggest that secondary invasion by this community of non-native land snails is facilitated most strongly by habitat and resource augmentation, an indirect consequence of red land crab removal, and that the creation of enemy-free space is not important. By using a full-factorial experimental approach, we have confidently determined-rather than inferred-the mechanism by which primary invaders indirectly facilitate a community of secondary invaders. [ABSTRACT FROM AUTHOR]

Biological invasions are a key threat to freshwater biodiversity, and identifying determinants of invasion success is a global conservation priority. The establishment of introduced species is predicted to be hindered by pre-existing, functionally similar invasive species. Over a five-year period we, however, find that in the River Lee (UK), recently introduced non-native virile crayfish (Orconectes virilis) increased in range and abundance, despite the presence of established alien signal crayfish (Pacifastacus leniusculus). In regions of sympatry, virile crayfish had a detrimental effect on signal crayfish abundance but not vice versa. Competition experiments revealed that virile crayfish were more aggressive than signal crayfish and outcompeted them for shelter. Together, these results provide early evidence for the potential over-invasion of signal crayfish by competitively dominant virile crayfish. Based on our results and the limited distribution of virile crayfish in Europe, we recommend that efforts to contain them within the Lee catchment be implemented immediately. [ABSTRACT FROM AUTHOR]

The quantification of invader impacts remains a major hurdle to understanding and managing invasions. Here, we demonstrate a method for quantifying the community-level impact of multiple plant invaders by applying Parker et al.'s (1999) equation (impact = range × local abundance × per capita effect or per unit effect) using data from 620 survey plots from 31 grasslands across west-central Montana, USA. In testing for interactive effects of multiple invaders on native plant abundance (percent cover), we found no evidence for invasional meltdown or synergistic interactions for the 25 exotics tested. While much concern exists regarding impact thresholds, we also found little evidence for nonlinear relationships between invader abundance and impacts. These results suggest that management actions that reduce invader abundance should reduce invader impacts monotonically in this system. Eleven of 25 invaders had significant per unit impacts (negative local-scale relationships between invader and native cover). In decomposing the components of impact, we found that local invader abundance had a significant influence on the likelihood of impact, but range (number of plots occupied) did not. This analysis helped to differentiate measures of invasiveness (local abundance and range) from impact to distinguish high-impact invaders from invaders that exhibit negligible impacts, even when widespread. Distinguishing between high- and low-impact invaders should help refine trait-based prediction of problem species. Despite the unique information derived from evaluation of per unit effects of invaders, invasiveness scores based on range and local abundance produced similar rankings to impact scores that incorporated estimates of per unit effects. Hence, information on range and local abundance alone was sufficient to identify problematic plant invaders at the regional scale. In comparing empirical data on invader impacts to the state noxious weed list, we found that the noxious weed list captured 45% of the high-impact invaders but missed 55% and assigned the lowest risk category to the highest-impact invader. While such subjective weed lists help to guide invasive species management, empirical data are needed to develop more comprehensive rankings of ecological impacts. Using weed lists to classify invaders for testing invasion theory is not well supported. [ABSTRACT FROM AUTHOR]

One explanation for why invasive species are successful is that they escape natural enemies from their native range or experience lower attack from natural enemies in the introduced range relative to native species (i.e., the enemy-release hypothesis). However, little is known about how invasive plants interact with co-introduced herbivores or natural enemies of the introduced herbivores. We focus on Phragmites australis, a wetland grass native to Europe (EU) and North America (NA). Within the past 100-150 years, invasive European genotypes of P. australis and several species of specialist Lipara gall flies have spread within NA. On both continents we surveyed P. australis patches for Lipara infestation (proportion of stems infested) and Lipara mortality from natural enemies. Our objectives were to assess evidence for enemy-release in the invaded (NA) versus native (EU) range and whether Lipara infestation or mortality differed between invasive and native P. australis genotypes in NA. Enemy-release varied regionally; Lipara were absent throughout most of NA, supporting enemy-release of Phragmites. However, where Lipara were present, the proportion of invasive P. australis stems infested with Lipara was higher in the introduced (11 %) than native range (<1 %). This difference may be explained by the absence of Lipara parasitoids in our NA survey, strongly supporting enemy-release of Lipara. In NA, native P. australis genotypes exhibited higher Lipara infestation (32 %) than invasive genotypes (11 %), largely driven by L. rufitarsis. We attribute genotypic differences in infestation to a combination of Lipara exhibiting 34 % greater performance (gall diameter) and suffering four times less vertebrate predation on native than invasive genotypes. Our study suggests that complex interactions can result from the co-introduction of plants and their herbivores, and that a multitrophic perspective is required for investigating how biotic interactions influence invasion success. [ABSTRACT FROM AUTHOR]

When an invasive predator encounters native and invasive prey, two scenarios are possible: the predator may benefit from the presence of naïve native prey or choose prey from its region of origin, reflecting their common evolutionary history., To determine interactions between an invasive predator and native and invasive prey, we used the Ponto-Caspian racer goby Babka gymnotrachelus as predator and gammarids as prey: native Gammarus fossarum and Ponto-Caspian Dikerogammarus villosus and Pontogammarus robustoides. We hypothesised that prey origin would affect fish preferences and growth rate and conducted a series of laboratory experiments on fish predation and growth and estimated profitability of prey of different origin., The goby preferred native prey to the Ponto-Caspian gammarids, irrespective of prey motility, the presence of shelters or waterborne chemical cues. Moreover, fish grew better when fed native prey., Thus, we suggest that fish selectivity was based on the assessment of prey quality during direct contact with gammarids. A diet consisting of Ponto-Caspian gammarids did not facilitate an invader originating from the same region, which benefited more from the presence of a local prey species., Ponto-Caspian gammarids and gobies are successful invaders in inland waters, usually main rivers. The gobies, in contrast to the invasive gammarids, enter smaller tributaries that serve as refugia for native gammarids. We show that the gobies may benefit from the presence of native prey species in such locations. [ABSTRACT FROM AUTHOR]

Mutualism between invaders may alter a key characteristic of the recipient community, leading to the entry or in situ release of other exotic species. We considered whether mutualism between invasive yellow crazy ant Anoplolepis gracilipes and exotic honeydew-producing scale insects indirectly facilitated land snails (exotic and native) via the removal of a native omnivore, the red land crab Gecarcoidea natalis. In plateau rainforest on Christmas Island, Indian Ocean, the land snail community was surveyed at 28 sites representing four forest states that differed in the density of red crabs, the abundance of yellow crazy ants and management history. One-way ANOVAs and multivariate analyses were used to determine differences in land snail species abundance and composition between forest states. Sample-based rarefaction was used to determine differences in species richness. The removal of the red land crab by supercolonies of yellow crazy ants was associated with a significant increase in the abundance of both invasive (14 species) and native (four species) land snails. Compositional differences in the land snail community were driven most strongly by the significantly greater abundance of a few common species in forest states devoid of red crabs. In forest where the crab population had recovered following management for ants, the land snail assemblage did not differ from intact, uninvaded forest. The land snail community was dominated by exotic species that can coexist alongside red crabs in rainforest uninvaded by exotic ants and scale insects. However, the ant-scale mutualism significantly increased land snail abundance and altered their composition indirectly though the alteration of the recipient community. We suggest these constitute 'population-release' secondary invasion in which the impacts of previously successful invaders facilitate a significant increase in abundance of other exotic species already established at low density within the community. Understanding facilitative interactions between invaders and indirect consequences of impacts will provide invaluable insights for conservation in heavily invaded ecosystems. [ABSTRACT FROM AUTHOR]

With accelerating rates of invasion being documented in many ecosystems, communities of interacting invasive species are becoming increasingly common. Opposing theories predict that invaders can either hinder or promote one another's success. Additionally, evidence suggests that co-occurring invaders can interact to amplify or mitigate one another's impacts on ecosystems. However, there has not been a quantitative review on interactions among multiple invasive animals. Here I use a meta-analysis approach to show that, across a global scale, the mean interaction among invaders was to reduce one another's performance. This pattern was consistent when considering interactions between marine animals but interactions were neutral overall in terrestrial and freshwater ecosystems. Crucially, individual studies showed that neutral interactions were the most common interaction type. Further, I demonstrate that the combined ecological impacts of multiple invaders were frequently the sum of their independent effects (additive) but the mean effect was non-additive and less than predicted (antagonistic). In both meta-analyses, the disparity between the most frequent and mean interaction type indicates that case studies of multiple invasions commonly have different outcomes to global trends. These results will help predict how co-occurring invasive animals interact and assist in developing management strategies for problematic invaders in our changing world. [ABSTRACT FROM AUTHOR]

Ecosystems with multiple exotic species may be affected by facilitative invader interactions, which could lead to additional invasions (invasional meltdown hypothesis). Experiments show that one-way facilitation favours exotic species and observational studies suggest that reciprocal facilitation among exotic species may lead to an invasional meltdown. We conducted a mesocosm experiment to determine whether reciprocal facilitation occurs in wetland communities., We established communities with native wetland plants and aquatic snails. Communities were assigned to treatments: control (only natives), exotic snail ( Pomacea maculata) invasion, exotic plant ( Alternanthera philoxeroides) invasion, sequential invasion (snails then plants or plants then snails) or simultaneous invasion (snails and plants)., Pomacea maculata preferentially consumed native plants, so A. philoxeroides comprised a larger percentage of plant mass and native plant mass was lowest in sequential (snail then plant) invasion treatments., Even though P. maculata may indirectly facilitate A. philoxeroides, A. philoxeroides did not reciprocally facilitate P. maculata. Rather, ecosystems invaded by multiple exotic species may be affected by one-way facilitation or reflect exotic species' common responses to abiotic factors or common paths of introduction. [ABSTRACT FROM AUTHOR]

Humans have effectively transported thousands of species around the globe and, with accelerated trade; the rate of introductions has increased over time. Aquatic ecosystems seem at particular risk from invasive species because of threats to biodiversity and human needs for water resources. Here, we review some known aspects of aquatic invasive species (AIS) and explore several new questions. We describe impacts of AIS, factors limiting their dispersal, and the role that humans play in transporting AIS. We also review the characteristics of species that should be the greatest threat for future invasions, including those that pave the way for invasions by other species ('invasional meltdown'). Susceptible aquatic communities, such as reservoirs, may serve as stepping stones for invasions of new landscapes. Some microbes disperse long distance, infect new hosts and grow in the external aquatic medium, a process that has consequences for human health. We also discuss the interaction between species invasions and other human impacts (climate change, landscape conversion), as well as the possible connection of invasions with regime shifts in lakes. Since many invaders become permanent features of the environment, we discuss how humans live with invasive species, and conclude with questions for future research. [ABSTRACT FROM AUTHOR]

The movement of species is one of the most pervasive forms of global change, and few ecosystems remain uninvaded by nonnative species. Studying species interactions is crucial for understanding their distribution and abundance, particularly for nonnative species because interactions may influence the probability of invasion and consequent ecological impact. Interactions among nonnatives are relatively understudied, though the likelihood of nonnative species co-occurrence is high. We quantify and describe the types of interactions among nonnative plants and determine what factors affect interaction outcomes for ecosystems globally. We reviewed 65 studies comprising 201 observations and recorded the interaction type, traits of the interacting species, and study characteristics. We conducted a census of interaction types and a meta-analysis of experiments that tested nonnative competition intensity. Both methods showed that negative and neutral interactions prevailed, and a number of studies reported that the removal of a dominant nonnative led to competitive release of other nonnatives. Positive interactions were less frequently reported and positive mean effect sizes were rare, but the plant characteristics nitrogen fixation, life cycle (annual or perennial), and functional group significantly influenced positive interactions. Positive interactions were three times more frequent when a neighboring nonnative was a nitrogen fixer and 3.5 times lower when a neighboring nonnative was an annual. Woody plants were two or four times more likely to have positive interactions relative to grasses or herbs, respectively. The prevalence of negative interactions suggests that managers should prepare for reinvasion of sites when treating dominant nonnatives. Though positive interactions were infrequent, managers may be able to anticipate positive interactions among nonnatives based upon traits of the co-occurring invaders. Predicting positive nonnative interactions is an important tool for determining habitat susceptibility to a particular invasion and for prioritizing management of nonnatives with a higher likelihood of positive interactions. [ABSTRACT FROM AUTHOR]

The outcome of multiple invasions from a common origin may lead to facilitative interactions because the invaders have co-evolved under similar environmental conditions. This outcome is often referred to as invasional meltdown, with a resultant increase in invasive species and a decline in native species richness and abundance., This study seeks to assess the full scope of the threat posed by a high-risk group of 23 freshwater invaders originating from the Ponto-Caspian region (south-east Europe) across Great Britain. Ponto-Caspian invaders constitute a group of special concern because they have recently caused a large-scale invasion into western Europe., According to a literature review, 76% of reported interactions between Ponto-Caspian invaders are positive (mostly provision of food and commensalism) or neutral (habitat partitioning). Negative interactions (mostly predation) were rare, thus highlighting the ability of Ponto-Caspian invaders to coexist., At least 14 out of the 23 Ponto-Caspian organisms investigated are well-established in the Rhine estuary and Dutch ports. Four of them ( Hemimysis anomala, Dikerogammarus villosus, D. haemobaphes and Hypania invalida) have recently established in Great Britain. Regression models suggest the rest are under a critical risk of being transported, with four species predicted to have arrived already to Great Britain: Echinogammarus ischnus, Jaera istri, Limnomysis benedeni and D. bispinosus., According to species distribution models, the cumulative risk of invasion of multiple Ponto-Caspian species, thus invasional meltdown, is highest in the south-east of England and decreases to the north and west., Great Britain might be on the brink of invasional meltdown, and as a consequence, confronting the problem of Ponto-Caspian invasive species is a vital element for national biosecurity., Synthesis and applications. The predictive models and maps developed in this study provide a means for an evidence-based prioritization of species and habitats for the management of existing and future invasions of Ponto-Caspian species. This integrated approach can be easily applied to risk assess other groups of species and habitats. [ABSTRACT FROM AUTHOR]

Banana passionfruit (Passiflora tripartita var. mollissima) is a noxious vine that is invasive in forest patches in coastal regions throughout New Zealand. We investigated the dispersal mechanisms that facilitate its spread in the Marlborough Sounds. To find out which animals act as dispersers, we monitored tagged fruits in the field. Fruits were removed quickly after ripening. Significantly fewer fruits were wholly removed from off-road locations than locations on road edges, but removal was high in both cases (70% and 93% respectively) indicating likely dispersal by both humans and wild animals. We found no evidence of dispersal by birds, but infrared cameras documented possums and rats consuming fruits in the field. To investigate the effect of fruit handling on germination, we compared germination success between hand-cleaned seeds, fleshy seeds and intact fruits in the field and glasshouse. Seeds germinated readily in all treatments, with no significant difference between treatments, so seeds do not require frugivore handling to germinate. In addition, we measured the germination of seeds extracted from 1.5 kg of feral pig faeces collected from Sus scrofa at Te Weuweu Bay and from faeces from captive possums (Trichosurus vulpecula) and Norway rats (Rattus norvegicus). Seeds extracted from pig and possum droppings readily germinated, with final germination success not significantly different from hand-cleaned or in-flesh seeds. None of the few intact seeds excreted by Norway rats germinated. Banana passionfruit has formed invasive mutualisms for both pollination (with introduced bees) and dispersal (with introduced mammals), exacerbating its spread. Weed management should combine direct weed control with limiting the spread of banana passionfruit, by managing feral pigs and possums. [ABSTRACT FROM AUTHOR]

Loss of complex natural microhabitats due to human activity is a major cause of decreased biodiversity but its effects on biological invasion are not well understood. The effects of physical environmental factors, especially the type of agricultural canals, on the invasive freshwater snail Pomacea canaliculata were studied at 33 sites in the Chikugogawa River basin, Kyushu, Japan. Differences among sites in the local fauna and vegetation were also monitored. Structural equation modeling with a model selection procedure revealed that canals with a concrete lining had more snails. The effect was indirect in that the concrete lining reduced animal species richness and increased the invasive waterweed Egeria densa, which may serve as a refuge, protecting the snails from predation. A tethering experiment conducted simultaneously indicated high predation pressure on the snails: over 20 % of the tethered snails were lost within a day. Thus, human impacts may increase biological invasion by reducing biotic resistance and increasing the risk of invasional meltdown. [ABSTRACT FROM AUTHOR]

Mutualisms structure ecosystems and mediate their functioning. They also enhance invasions of many alien species. Invasions disrupt native mutualisms, often leading to population declines, reduced biodiversity, and altered ecosystem functioning. Focusing on three main types of mutualisms (pollination, seed dispersal, and plant-microbial symbioses) and drawing on examples from different ecosystems and from species- and community-level studies, we review the key mechanisms whereby such positive interactions mediate invasions and are in turn influenced by invasions. High interaction generalization is "the norm" in most systems, allowing alien species to infiltrate recipient communities. We identify traits that influence invasiveness (e.g., selfing capacity in plants, animal behavioral traits) or invasibility (e.g., partner choice in mycorrhizas/rhizobia) through mutualistic interactions. Mutualistic disruptions due to invasions are pervasive, and subsequent cascading effects are also widespread. Ecological networks provide a useful framework for predicting tipping points for community collapse in response to invasions and other synergistic drivers of global change. [ABSTRACT FROM AUTHOR]

Facilitation among species may promote non-native plant invasions through alteration of environmental conditions, enemies or mutualists. However, the role of non-trophic indirect facilitation in invasions has rarely been examined., We used a long-term field experiment to test for indirect facilitation by invasions of Microstegium vimineum (stiltgrass) on a secondary invasion of Alliaria petiolata (garlic mustard) by introducing Alliaria seed into replicated plots previously invaded experimentally by Microstegium., Alliaria more readily colonized control plots without Microstegium but produced almost seven times more biomass and nearly four times as many siliques per plant in Microstegium-invaded plots. Improved performance of Alliaria in Microstegium-invaded plots compared to control plots overwhelmed differences in total number of plants such that, on average, invaded plots contained 327% greater total Alliaria biomass and 234% more total siliques compared to control plots., The facilitation of Alliaria in Microstegium-invaded plots was associated with an 85% reduction in the biomass of resident species at the peak of the growing season and significantly greater light availability in Microstegium-invaded than control plots early in the growing season., Synthesis. Our results demonstrate that an initial plant invasion associated with suppression of resident species and increased resource availability can facilitate a secondary plant invasion. Such positive interactions among species with similar habitat requirements, but offset phenologies, may exacerbate invasions and their impacts on native ecosystems. [ABSTRACT FROM AUTHOR]

DNA identification of mammal species occurring in the diet of a predator is potentially a useful approach to remotely monitor the distribution of multiple species. This is important in Ireland, where it has been shown that the combined presence of the introduced bank vole and greater white-toothed shrew impact the distribution of the indigenous small mammals, the wood mouse and pygmy shrew. Direct monitoring of these species and their interactions requires trapping, a labour-intensive and costly approach. In this study, we applied an indirect method by genetically testing the presence of small mammals in pine marten scats collected during the National Pine Marten Survey (2005-2007) to map their distribution. We also included additional scats to investigate if less common prey items, the red squirrel and grey squirrel, could also be detected. This study demonstrates that all target species were genetically detected from pine marten scats. This strategy could be implemented as a monitoring programme for indigenous and introduced mammal species. [ABSTRACT FROM AUTHOR]

Ecological interactions among invasive species can affect not only the success of the invaders, but also their impact on ecosystems in the invaded range. In Australia, both dung beetles (subfamily Scarabaeinae) and cane toads ( Rhinella marina) were introduced for biocontrol: the beetles to break down bovine faeces piles (cowpats) that otherwise accumulate and reduce pasture productivity, and the cane toad to consume scarab beetles that eat sugarcane and thus reduce sugar production. The dung beetles have been a success, whereas the toads have been a failure. Our experimental studies show that as well as impacting native fauna directly, cane toads reduce the rate of cowpat breakdown by consuming dung beetles. In the laboratory, dehydrated toads actively sought out cowpats based on scent cues, and in field enclosures, the presence of a cane toad significantly reduced rates of cowpat decomposition. Although toads have benefited from agricultural activities, their spread across Australia likely has reduced the effectiveness of one of the most successful biocontrol programmes ever conducted in that continent. [ABSTRACT FROM AUTHOR]

Biological invasions depend in part on the resistance of native communities. Meta-analyses of terrestrial experiments demonstrate that native primary producers and herbivores generally resist invasions of primary producers, and that resistance through competition strengthens with native producer diversity. To test the generality of these findings, we conducted a meta-analysis of marine experiments. We found that native marine producers generally failed to resist producer invasions through competition unless the native community was diverse, and this diversity effect was weaker in marine than in terrestrial systems. In contrast, native consumers equally resisted invasive producers in both ecosystems. Most marine experiments, however, tested invasive consumers and these invasions were resisted more strongly than were producer invasions. Given these differences between ecosystems and between marine trophic levels, we used a model-selection approach to assess if factors other than the resistance mechanism (i.e. competition vs. consumption) are more important for predicting marine biotic resistance. These results suggest that understanding marine biotic resistance depends on latitude, habitat and invader taxon, in addition to distinguishing between competition with and consumption by native species. By examining biotic resistance within and across ecosystems, our work provides a more complete understanding of the factors that underlie biological invasions. [ABSTRACT FROM AUTHOR]

Abstract: Though biological invasion studies have proliferated in recent decades, a consistent emphasis remains on the study of single-species invasions. Here, we juxtapose the number of invasive plants reported as co-occurring within conservation habitats in one of the most comprehensive global conservation management databases (The Nature Conservancy’s Conservation Projects) with the number of published studies that address impacts of co-occurring invasive plants. We reviewed 86 conservation projects and 153 peer-reviewed publications and found that only one-third of studies mentioned co-occurring invaders, although over two-thirds of habitats were multiply invaded, indicating researchers are more likely to study single invaders, even though conservation managers are more often faced with multiple invaders in a given habitat. Of those studies focused on multiple invasives, the majority did not attempt to differentiate impacts caused by species when found alone or with other invaders and instead either treated co-occurring invaders together as a single, undifferentiated group or compared impacts between invasive plant monocultures. Less than 6% of all studies analyzed invader interactions. The high prevalence of co-occurring invasive plants should encourage more research on multiple invaders, which may better inform prioritization of which species to manage. Specifically, we suggest research on how effects of multiple invaders differ from those of single invaders, what types of interactions (facilitative, competitive, neutral) are most commonly found between invaders, and what effects interactions might have on the overall impact (additive or non-additive) of the individual invader. Though we acknowledge the challenge of studying multiple invaders, there is a critical need to address these questions to make invasion research more relevant to conservation programs. [Copyright &y& Elsevier]

Abstract: In North America, many species of European earthworms have been introduced to northern forests. Facilitative or competitive interactions between these earthworm species may result in non-additive effects on native plant and animal species. We investigated the combined versus individual effects of the litter-dwelling earthworm Dendrobaena octaedra Savigny, 1826 and the deep-burrowing species Lumbricus terrestris L., 1758 on microarthropod assemblages from boreal forest soil by conducting a mesocosm experiment. Soil cores from earthworm-free areas of northern Alberta, Canada, were inoculated with D. octaedra alone, L. terrestris alone, both worm species together, or no earthworms. After 4.5 months, microarthropods were extracted from the soil, counted, and identified to higher taxa. Oribatid mites were further identified to family and genus. Abundance of microarthropods was significantly lower in the treatment containing both species than in the no earthworm treatment and the L. terrestris treatment. Oribatida and Prostigmata/Astigmata differed significantly among treatments and were lowest in the treatment containing both earthworm species, followed by the D. octaedra treatment, although post-hoc pairwise comparisons were not significant. Within the Oribatida, composition differed between the control and L. terrestris treatments as compared to the D. octaedra and both-species treatments, with Suctobelbella and Tectocepheus in particular having higher abundances in the control treatment. Effects of the two earthworm species on microarthropods were neither synergistic nor antagonistic. Our results indicate that earthworms can have strong effects on microarthropod assemblages in boreal forest soils. Future research should examine whether these changes have cascading effects on nutrient cycling, microbial communities, or plant growth. [Copyright &y& Elsevier]

Invasive species represent a major threat with both direct and indirect effects on natural ecosystems, including effects on established and coevolved relationships. In a series of experiments, we examined how the interaction between two native species, a unionid mussel ( Unio pictorum) and the European bitterling ( Rhodeus amarus), a fish that parasitises unionids, was affected by the non-native zebra mussel ( Dreissena polymorpha). The zebra mussel fouls hard substrates, including shells of living unionids, and its presence is often associated with a decrease in population density of native unionid mussels. Bitterling lay their eggs into live unionids and the embryos develop inside their gills. Using a range of zebra mussel densities, we demonstrated that zebra mussel fouling had a negative effect on the number of bitterling eggs inside the mussel host, with abundances of 5-10 zebra mussels (shell size 15-25 mm) per unionid critical for bitterling ability to utilise the host. In a further experiment, we found that bitterling did not discriminate between unfouled unionids and those fouled with five zebra mussels. Most ovipositions into fouled hosts, however, were unsuccessful as eggs failed to reach the unionid gills. We discuss implications of such unsuccessful ovipositions for bitterling recruitment and population dynamics. [ABSTRACT FROM AUTHOR]

Empirical support for 'invasional meltdown', where the presence of one invading species facilitates another and compounds negative impacts on indigenous species, is equivocal with few convincing studies. In Ireland, the bank vole was introduced 80 years ago and now occupies a third of the island. The greater white-toothed shrew arrived more recently within the invasive range of the bank vole. We surveyed the abundance of both invasive species and two indigenous species, the wood mouse and pygmy shrew, throughout their respective ranges. The negative effects of invasive on indigenous species were strong and cumulative bringing about species replacement. The greater white-toothed shrew, the second invader, had a positive and synergistic effect on the abundance of the bank vole, the first invader, but a negative and compounding effect on the abundance of the wood mouse and occurrence of the pygmy shrew. The gradual replacement of the wood mouse by the bank vole decreased with distance from the point of the bank vole's introduction whilst no pygmy shrews were captured where both invasive species were present. Such interactions may not be unique to invasions but characteristic of all multispecies communities. Small mammals are central in terrestrial food webs and compositional changes to this community in Ireland are likely to reverberate throughout the ecosystem. Vegetation composition and structure, invertebrate communities and the productivity of avian and mammalian predators are likely to be affected. Control of these invasive species may only be effected through landscape and habitat management. [ABSTRACT FROM AUTHOR]

Abstract: We evaluated the mechanisms that determine the increasing abundance of Pinus halepensis in mountain pampean grasslands in Argentina that is associated with the continuous presence of feral horses. We hypothesized that direct and indirect effects of feral horse grazing on grassland may affect the establishment of P. halepensis. We conducted a field experiment to evaluate this hypothesis, studying the response of seedling emergence, survival and growth to herbaceous vegetation defoliation and soil disturbance in sites with contrasting grazing histories. We also evaluated the composition and structure of plant communities of each experimental site. Direct effects such as heavily defoliated perennial grasses and indirect effects such as reduced specific and functional richness, diversity and evenness enhanced the emergence, survival and early growth of the seedlings. High bare ground percentage and low grass biomass also positively affected seedling establishment. The experimental evidence demonstrates that the combination of invasional meltdown hypothesis and weakened biotic resistance are the mechanisms involved in P. halepensis invasion. [Copyright &y& Elsevier]

It would be an exaggeration to argue that most invasions produce ecosystem impacts, and the term should be reserved for cases in which many species in an ecosystem are affected. However, certain facts suggest that true ecosystem impacts are more common than is normally assumed. First, the term 'ecosystem impact' has often been reserved for cases in which the nutrient regime or nutrient cycling is affected, whereas physical structural and other changes in ecosystems ought to be included. Second, as with all ecology, very few systems and species have been studied relative to all those that exist, so it is likely that many ecosystem impacts remain to be detected. Third, there are many types of impacts, many are idiosyncratic, many are subtle, and many are indirect, so it is likely that many impacts have simply not been recognized even in studied systems. Finally, the frequency of the lag phenomenon in invasions implies that at least some existing non-native species that are currently having little or no impact will eventually have much greater ones. These facts suggest that, even if it would be an overstatement to say that most invasions cause ecosystem impacts, it would not be more of an overstatement than the common assertion that very few introduced species have any significant impact. [ABSTRACT FROM AUTHOR]

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Recent research on invasive ants suggests that their success may be facilitated by increased resources at introduced locations stemming from the emergence of novel trophic interactions with abundant honeydew-producing Hemiptera. Moreover, those Hemiptera may themselves often be introduced or invasive. To test the importance of mutualisms for invasive species, we conducted a study in the southeastern United States of factors hypothesized to affect the abundance of an invasive ant native to South America, Solenopsis invicta. The study was conducted within grazing pastures, where S. invicta can be extremely abundant while also exhibiting substantial variability in abundance. A path analysis showed that the abundance of S. invicta was strongly and positively affected by the abundance of an invasive honeydew-producing mealybug native to Asia, Antonina graminis, and by the mealybugs' host grasses because of their strong positive effect on mealybug abundance. Abundance of the mealybug was primarily attributable to an invasive host grass native to Africa, Cynodon dactylon. The abundance of S. invicta was also positively affected by the abundance of other arthropods that they are likely to consume, and those arthropods were positively affected by the abundance of both the A. graminis host grasses and other plants. Thus the study shows that the distribution and abundance of different plant species could have important effects on the abundance of S. invicta through their effect on the ants' food resources. The results are also consistent with the hypothesis that the emergence of novel trophic interactions among invasive species can promote the abundance of invasive ants. [ABSTRACT FROM AUTHOR]

We consider the possibility of an extensive invasional meltdown occurring in central North America involving eleven Eurasian species. The scenario begins with the potential co-facilitation between the European earthworm Lumbricus terrestris and European buckthorn, Rhamnus cathartica. Once introduced, European buckthorn has served as the overwintering host for two important invasive crop pests, oat crown rust, Puccinea coronata and the soybean aphid, Aphis glycines. The spread of R. cathartica itself may have been aided by seed dispersal by the European starling, Sturnus vulgaris, and the presence of L. terrestris has likely facilitated the invasion of Bipalium adventitium, an Asian predatory flatworm that specializes on earthworms. Beyond this, the soybean aphid is consumed by a number of introduced species, including the lady beetle Harmonia axyridis, the ground beetle Agonum muelleri and the parasitoid Aphelinus certus. We hypothesize that the presence of soybean aphid increases regional abundances of these species. We discuss both the evidence for this multi-species invasional meltdown scenario and potential implications of meltdown dynamics for invasive species management. The particular management issues that we discuss are: (1) opportunities for managing multiple invasive species simultaneously by targeting facilitator species, and (2) implications of meltdown dynamics for biological control introductions against the soybean aphid. [ABSTRACT FROM AUTHOR]

In Mauritius, many of the worst invasive plant species have fleshy fruits and rely on animals for dispersal. The introduced red-whiskered bulbul ( Pycnonotus jocosus) feeds on many fleshy-fruited species, and often moves from invaded and degraded habitats into higher quality native forests, thus potentially acting as a mediator of continued plant invasion into these areas. Furthermore, gut passage may influence seed germination. To investigate this, we fed fleshy fruits of two invasive plant species, Ligustrum robustum and Clidemia hirta, to red-whiskered bulbuls. Gut passage times of seeds were recorded. Gut-passed seeds were sown and their germination rate and germination success compared with that of hand-cleaned seeds, as well as that of seeds in whole fruits. Gut passage and hand-cleaning had significant positive effects on germination of both species. Gut-passed seeds of both C. hirta and L. robustum germinated faster than hand-cleaned seeds. However, for L. robustum, this was only true when compared with hand-cleaned seeds with intact endocarp; when compared with hand-cleaned seeds without endocarp, there was no difference. For overall germination success, there was a positive effect of gut passage for C. hirta, but not for L. robustum. For both C. hirta and L. robustum, no seeds in intact fruits geminated, suggesting that removal of pulp is essential for germination. Our results suggest that, first, the initial invasion of native forests in Mauritius may not have happened so rapidly without efficient avian seed dispersers like the red-whiskered bulbul. Second, the bulbul is likely to be a major factor in the continued re-invasion of C. hirta and L. robustum into weeded and restored conservation management areas. [ABSTRACT FROM AUTHOR]