Showing papers on "Propagule pressure published in 2016"

The global distribution of bamboos : assessing correlates of introduction and invasion

Abstract: There is a long history of species being moved around the world by humans. These introduced species can provide substantial benefits, but they can also have undesirable consequences. We explore the importance of human activities on the processes of species dissemination and potential invasions using the grass subfamily Bambusoideae ("bamboos"), a group that contains taxa that are widely utilised and that are often perceived as weedy. We (1) compiled an inventory of bamboo species and their current distributions; (2) determined which species have been introduced and become invasive outside their native ranges; and (3) explored correlates of introduction and invasion. Distribution data were collated from Kew's GrassBase, the Global Biodiversity Information Facility and other online herbarium information sources. Our list comprised 1662 species in 121 genera, of which 232 (14%) have been introduced beyond their native ranges. Twelve species were found to be invasive. A non-random selection of bamboos have been introduced and become invasive. Asiatic species in particular have been widely introduced. There was a clear over-representation of introduced species in the genera Bambusa and Phyllostachys which also contain most of the listed invasive species. The introduction of species also correlated with certain traits: taxa with larger culm dimensions were significantly more likely to have been moved to new areas; and those with many cultivars had a higher rate of dissemination and invasion. It is difficult to determine whether the patterns of introduction and invasion are due simply to differences in propagule pressure, or whether humans have deliberately selected inherently invasive taxa. In general, we suggest that human usage is a stronger driver of introductions and invasions in bamboos than in other taxa that have been well studied. It is likely that as bamboos are used more widely, the number and impact of invasions will increase unless environmental risks are carefully managed.

Defining invasiveness and invasibility in ecological networks

Abstract: The success of a biological invasion is context dependent, and yet two key concepts—the invasiveness of species and the invasibility of recipient ecosystems—are often defined and considered separately. We propose a framework that can elucidate the complex relationship between invasibility and invasiveness. It is based on trait-mediated interactions between species and depicts the response of an ecological network to the intrusion of an alien species, drawing on the concept of community saturation. Here, invasiveness of an introduced species with a particular trait is measured by its per capita population growth rate when the initial propagule pressure of the introduced species is very low. The invasibility of the recipient habitat or ecosystem is dependent on the structure of the resident ecological network and is defined as the total width of an opportunity niche in the trait space susceptible to invasion. Invasibility is thus a measure of network instability. We also correlate invasibility with the asymptotic stability of resident ecological network, measured by the leading eigenvalue of the interaction matrix that depicts trait-based interaction intensity multiplied by encounter rate (a pairwise product of propagule pressure of all members in a community). We further examine the relationship between invasibility and network architecture, including network connectance, nestedness and modularity. We exemplify this framework with a trait-based assembly model under perturbations in ways to emulate fluctuating resources and random trait composition in ecological networks. The maximum invasiveness of a potential invader (greatest intrinsic population growth rate) was found to be positively correlated with invasibility of the recipient ecological network. Additionally, ecosystems with high network modularity and high ecological stability tend to exhibit high invasibility. Where quantitative data are lacking we propose using a qualitative interaction matrix of the ecological network perceived by a potential invader so that the structural network stability and invasibility can be estimated from the literature or from expert opinion. This approach links network structure, invasiveness and invasibility in the context of trait-mediated interactions, such as the invasion of insects into mutualistic and antagonistic networks.

Modelling the introduction and spread of non-native species: international trade and climate change drive ragweed invasion

Abstract: Biological invasions are a major driver of global change, for which models can attribute causes, assess impacts and guide management. However, invasion models typically focus on spread from known introduction points or non-native distributions and ignore the transport processes by which species arrive. Here, we developed a simulation model to understand and describe plant invasion at a continental scale, integrating repeated transport through trade pathways, unintentional release events and the population dynamics and local anthropogenic dispersal that drive subsequent spread. We used the model to simulate the invasion of Europe by common ragweed (Ambrosia artemisiifolia), a globally invasive plant that causes serious harm as an aeroallergen and crop weed. Simulations starting in 1950 accurately reproduced ragweed's current distribution, including the presence of records in climatically unsuitable areas as a result of repeated introduction. Furthermore, the model outputs were strongly correlated with spatial and temporal patterns of ragweed pollen concentrations, which are fully independent of the calibration data. The model suggests that recent trends for warmer summers and increased volumes of international trade have accelerated the ragweed invasion. For the latter, long distance dispersal because of trade within the invaded continent is highlighted as a key invasion process, in addition to import from the native range. Biosecurity simulations, whereby transport through trade pathways is halted, showed that effective control is only achieved by early action targeting all relevant pathways. We conclude that invasion models would benefit from integrating introduction processes (transport and release) with spread dynamics, to better represent propagule pressure from native sources as well as mechanisms for long-distance dispersal within invaded continents. Ultimately, such integration may facilitate better prediction of spatial and temporal variation in invasion risk and provide useful guidance for management strategies to reduce the impacts of invasion.

Overseas seed dispersal by migratory birds.

Abstract: Long-distance dispersal (LDD) promotes the colonization of isolated and remote habitats, and thus it has been proposed as a mechanism for explaining the distributions of many species. Birds are key LDD vectors for many sessile organisms such as plants, yet LDD beyond local and regional scales has never been directly observed nor quantified. By sampling birds caught while in migratory flight by GPS-tracked wild falcons, we show that migratory birds transport seeds over hundreds of kilometres and mediate dispersal from mainland to oceanic islands. Up to 1.2% of birds that reached a small island of the Canary Archipelago (Alegranza) during their migration from Europe to Sub-Saharan Africa carried seeds in their guts. The billions of birds making seasonal migrations each year may then transport millions of seeds. None of the plant species transported by the birds occurs in Alegranza and most do not occur on nearby Canary Islands, providing a direct example of the importance of environmental filters in hampering successful colonization by immigrant species. The constant propagule pressure generated by these LDD events might, nevertheless, explain the colonization of some islands. Hence, migratory birds can mediate rapid range expansion or shifts of many plant taxa and determine their distribution.

Non-native species and invasion biology in a megadiverse country: scientometric analysis and ecological interactions in Brazil

Abstract: The number of invasions by non-native species and their negative impacts is increasing, especially in developing megadiverse countries. The current catalogue of hypotheses, concepts and management policies related to invasions was typically created and evaluated in temperate and economically rich regions. So what we know about invasion biology may not provide a general pattern applicable to developing megadiverse regions. All recent reviews have indicated a demand for more studies in tropical and megadiverse regions. Additionally, potential differences in the quality and availability of data may preclude any comparative analyses between developing and developed nations and hinder global strategies to combat invasions. Therefore, it is important to synthesize the existing knowledge on the subject in order to aid conservation activities in such regions. We conducted a systematic review of non-native species and invasions in Brazil and found geographical bias towards the most populated regions and biomes. Plants and fish were the most studied groups and were, for the most part, intentionally introduced, usually to meet some economic interest. We also noted the dominance of studies concerning terrestrial and freshwater ecosystems. Most studies did not address species interactions and, in the few cases where they did, the most frequently studied were between non-native and native species. There was a paucity of research beyond the level of species records and descriptive patterns. We highlight major vectors of introduction, the number of non-native species (proxy to colonization pressure) and papers (proxy to propagule pressure) related to each vector and the need for designed surveys and experiments focusing on specific invasion hypotheses and/or species interactions. We indicate major gaps and biases nationally that should be the focus for future studies, and that can serve as examples for other megadiverse countries.

Introduction bias affects relationships between the characteristics of ornamental alien plants and their naturalization success

Abstract: Aim Alien plants with certain characteristics may have been introduced earlier and more frequently than others. Such introduction bias may cause spurious associations between plant characteristics and naturalization (the establishment of self-sustaining populations in the wild). We aimed to disentangle direct and indirect (i.e. mediated by introduction history) effects of species characteristics on the naturalization success of alien plants introduced for horticulture. Location Germany (non-native range); rest of the world (native range). Methods We compiled a dataset of 435 alien plant species introduced in cultivation in Germany, including data on their year of introduction, the number of botanical gardens where they are planted, native range, biological traits and naturalization success. We used path analysis to estimate the direct effects of geographical origin and biological traits on naturalization, and their indirect effects mediated by year and/or frequency of introduction. Results We found significant direct positive effects of native range size and winter hardiness on naturalization. Alien species native to other parts of Europe and species with a large native range were brought to the country earlier than other species. In addition, woody species, winter-hardy species and tall species were planted more frequently than others. Because the number of botanical gardens where a species is planted increased naturalization success directly, and residence time did so indirectly through a significant positive association with the number of botanical gardens, most of the species characteristics had indirect effects on naturalization. Main conclusions Our approach allowed us to show that apparent effects of species characteristics on naturalization success can be at least partly indirect, due to introduction biases. This indicates that failure to recognize such introduction biases could impair our ability to explain the success of alien plant species.

Disentangling the four demographic dimensions of species invasiveness

Abstract: Funding was provided by the Australian Research Council (DE120102221 to J.A.C.) and the ARC Centre of Excellence for Environmental Decisions.

Drivers of plant invasion vary globally: evidence from pine invasions within six ecoregions

Abstract: Aim To determine biotic and abiotic controls on pine invasion globally within six ecoregions that include both introduced and native ranges. Locations Rio Negro province, Argentina; Aysen and Araucania regions, Chile; South Island (two ecoregions), New Zealand; Greater Yellowstone ecosystem, USA. Methods We quantified tree abundance and size across invasion fronts of the widespread invasive tree species Pinus contorta at each of the nine sites, encompassing both the native and introduced range. We also determined the relative importance of propagule pressure, abiotic characteristics and biotic factors for invasion success. Finally, key plant population metrics such as individual tree growth rates and reproductive effort were compared between native and introduced ranges. Results Pinus contorta density decreased with increasing distance from source population in all cases, but the importance and shape of this relationship differed among sites due, primarily to biotic factors. For example, areas dominated by native southern beech forest (Fuscospora cliffortioides or Nothofagus spp.) were not invaded, and this biotic resistance was not overcome by high propagule pressure. In contrast, shrublands were more highly invaded than grasslands, contradicting previous generalizations about pine invasions. Pinus contorta growth was faster, age to maturity was earlier and reproductive effort was higher in the introduced ranges compared with the native range, suggesting a demographic shift towards more rapid population growth in introduced regions. Climatic differences between the ranges may explain, at least in part, the observed pattern. Main conclusions We demonstrate that although biological invasions are driven by propagule pressure across different ecoregions, these processes interact strongly with biotic factors. Intriguingly, our results suggest that propagule pressure may become less important than biotic interactions as invasions proceed. Multi-region studies including both the native and introduced ranges provide unparalleled opportunities for understanding how these interactions change among regions as invasions proceed.

Combining trade data and niche modelling improves predictions of the origin and distribution of non-native European populations of a globally invasive species

Abstract: Aim Although propagule pressure and environmental constraints are among the most important factors determining invasion success, studies considering both factors simultaneously are scarce. Moreover, while recent evidence suggests that the environmental requirements of individuals from different geographical ranges may be different, the role of propagule origin in invasions has been largely overlooked. Our aim was to disentangle the relative role of niche requirements, propagule origin and propagule pressure on the distribution of an invasive bird species. Location Europe, Asia and Africa. Methods We used species distribution models, niche and deviance partitioning analyses to investigate the relative roles of propagule pressure (international trade), origin of individuals (Asian or African), and environmental constraints in determining the distribution of invasive ring-necked parakeets across 25 European countries. Results Differences between niches of native Asian and African parakeets were found, with the Asian niche matching the European niche more closely. In the invasive European range, distribution of parakeets was mainly explained by the pure effect of year of first importation (as a proxy of time since first introduction), the pure effect of geographical origin of propagules and the joint effect of environmental suitability and year of first importation, but not by overall propagule pressure. Only when taking into account the fraction of individuals whose native niche fitted better the European conditions – Asian parakeets – was the role of propagule pressure highlighted by models. Main conclusions While environmental-based predictions calibrated on native ranges can constitute a useful first-screening tool, incorporating information about propagule pressure and especially about the variability in its geographical origin may result in a much more thorough assessment of invasion risk. Trade data reveal as a valuable proxy of propagule origin and pressure that can be combined with niche modelling for predicting the fate of trade-mediated invasions in a variety of organisms.

Importance of geographic origin for invasion success: A case study of the North and Baltic Seas versus the Great Lakes–St. Lawrence River region

Abstract: Recently, several studies indicated that species from the Ponto-Caspian region may be evolutionarily predisposed to become nonindigenous species (NIS); however, origin of NIS established in different regions has rarely been compared to confirm these statements. More importantly, if species from certain area/s are proven to be better colonizers, management strategies to control transport vectors coming from those areas must be more stringent, as prevention of new introductions is a cheaper and more effective strategy than eradication or control of established NIS populations. To determine whether species evolved in certain areas have inherent advantages over other species in colonizing new habitats, we explored NIS established in the North and Baltic Seas and Great Lakes–St. Lawrence River regions—two areas intensively studied in concern to NIS, highly invaded by Ponto-Caspian species and with different salinity patterns (marine vs. freshwater). We compared observed numbers of NIS in these two regions to expected numbers of NIS from major donor regions. The expected numbers were calculated based on the available species pool from donor regions, frequency of shipping transit, and an environmental match between donor and recipient regions. A total of 281 NIS established in the North and Baltic Seas and 188 in the Great Lakes–St. Lawrence River. Ponto-Caspian taxa colonized both types of habitats, saltwater areas of the North and Baltic Seas and freshwater of the Great Lakes–St. Lawrence River, in much higher numbers than expected. Propagule pressure (i.e., number of introduced individuals or introduction effort) is of great importance for establishment success of NIS; however in our study, either shipping vector or environmental match between regions did not clarify the high numbers of Ponto-Caspian taxa in our study areas. Although we cannot exclude the influence of other transport vectors, our findings suggest that the origin of the species plays an important role for the predisposition of successful invaders.

Pine invasions in treeless environments: dispersal overruns microsite heterogeneity

Abstract: Understanding biological invasions patterns and mechanisms is highly needed for forecasting and managing these processes and their negative impacts. At small scales, ecological processes driving plant invasions are expected to produce a spatially explicit pattern driven by propagule pressure and local ground heterogeneity. Our aim was to determine the interplay between the intensity of seed rain, using distance to a mature plantation as a proxy, and microsite heterogeneity in the spreading of Pinus contorta in the treeless Patagonian steppe. Three one-hectare plots were located under different degrees of P. contorta invasion (Coyhaique Alto, 45° 30'S and 71° 42'W). We fitted three types of inhomogeneous Poisson models to each pine plot in an attempt for describing the observed pattern as accurately as possible: the "dispersal" models, "local ground heterogeneity" models, and "combined" models, using both types of covariates. To include the temporal axis in the invasion process, we analyzed both the pattern of young and old recruits and also of all recruits together. As hypothesized, the spatial patterns of recruited pines showed coarse scale heterogeneity. Early pine invasion spatial patterns in our Patagonian steppe site is not different from expectations of inhomogeneous Poisson processes taking into consideration a linear and negative dependency of pine recruit intensity on the distance to afforestations. Models including ground-cover predictors were able to describe the point pattern process only in a couple of cases but never better than dispersal models. This finding concurs with the idea that early invasions depend more on seed pressure than on the biotic and abiotic relationships seed and seedlings establish at the microsite scale. Our results show that without a timely and active management, P. contorta will invade the Patagonian steppe independently of the local ground-cover conditions.

The wildlife pet trade as a driver of introduction and establishment in alien birds in Taiwan

Abstract: The global trade in alien cage birds is flourishing and is considered to be one of the major routes by which species are entrained into the human-mediated invasion pathway. Here, we explore the likely influence of the wild bird trade on alien bird invasions in Taiwan. Specifically, we analyse the characteristics of alien bird species that have been successfully introduced and established at large in the wild. We use phylogenetic regression models to compare the traits of alien species recorded in the cage bird trade in Taiwan that have (or have not) subsequently been recorded at large in the wild, and the traits of species recorded in the wild that have (or have not) established (species identified in the Breeding Bird Survey in Taiwan). Alien species were more likely to be recorded as successfully introduced if they were commonly for sale in the Taiwanese pet bird trade, and possessed songs considered to be more attractive to people. Species that have been sold in the pet market for a longer period were also more likely to have been recorded in the wild. Establishment success was more likely for large-bodied bird species, but not strongly related to other predicted determinants of success, including proxies for propagule pressure and climate matching. We conclude that the pet trade influences bird invasions in Taiwan by determining which species are exposed to novel environments there, but which of those introduced species goes on to establish may depend more on their intrinsic life histories.

Alien and native plant richness and abundance respond to different environmental drivers across multiple gravel floodplain ecosystems

Abstract: Aim To assess whether native and alien plant cover and richness respond similarly to flow regime, propagule pressure, climate and floodplain characteristics in highly dynamic braided river ecosystems. Location Canterbury, New Zealand. Methods A regional, multiscale survey was conducted across 19 braided river floodplains in multiple catchments. We measured alien and native cover and richness across gradients of flow regime (flow magnitude, variability and high/low-flow events), propagule pressure (inferred from land cover), climate and local-scale floodplain substrate and topography. Boosted regression trees were used to determine the relative and absolute importance of these variables on plant cover and richness. Results The floodplain ecosystems were highly invaded with 154 alien species and only 31 natives. Alien cover was higher in rivers with larger maximum flows, in plots with fine substrate texture, and at higher local river bed elevations. Alien richness increased as the variability of winter flows increased and followed a hump-shaped relationship with river bed elevation. In contrast, native species richness and cover were both shaped primarily by climate and by land cover, higher in cooler and wetter areas with more adjacent native vegetation. Main conclusions Alien and native richness and cover were shaped by different variables, so managing the ecosystem (e.g. flow regime) to mitigate aliens would not necessarily promote natives. Promotion of natives will require considerations of propagule supply and whether extant native species are suited to low-elevation climatic conditions. Aliens were associated with predictors that approximate disturbance processes. Increased flow variability in winter could lead to an increase in the number of aliens; conversely, flow stabilization is likely to allow problematic invaders to increase in cover locally.

Propagule Pressure, Habitat Conditions and Clonal Integration Influence the Establishment and Growth of an Invasive Clonal Plant, Alternanthera philoxeroides.

Abstract: Many notorious invasive plants are clonal, spreading mainly by vegetative propagules. Propagule pressure (the number of propagules) may affect the establishment, growth and thus invasion success of these clonal plants, and such effects may also depend on habitat conditions. To understand how propagule pressure, habitat conditions and clonal integration affect the establishment and growth of the invasive clonal plants, an 8-week greenhouse with an invasive clonal plant, Alternanthera philoxeroides was conducted. High (five fragments) or low (one fragment) propagule pressure was established either in bare soil (open habitat) or dense native vegetation of Jussiaea repens (vegetative habitat), with the stolon connections either severed from or connected to the relatively older ramets. High propagule pressure greatly increased the establishment and growth of A. philoxeroides, especially when it grew in vegetative habitats. Surprisingly, high propagule pressure significantly reduced the growth of individual plants of A. philoxeroides in open habitats, whereas it did not affect the individual growth in vegetative habitats. A shift in the intraspecific interaction on A. philoxeroides from competition in open habitats to facilitation in vegetative habitats may be the main reason. Moreover, clonal integration significantly improved the growth of A. philoxeroides only in open habitats, especially with low propagule pressure, whereas it had no effects on the growth and competitive ability of A. philoxeroides in vegetative habitats, suggesting that clonal integration may be of most important for A. philoxeroides to explore new open space and spread. These findings suggest that propagule pressure may be crucial for the invasion success of A. philoxeroides, and such an effect also depends on habitat conditions.

Effect of shipping traffic on biofouling invasion success at population and community levels

Abstract: The relationship between shipping and invasion success in marine waters has rarely been demonstrated empirically. In commercial ports, greater shipping activity is expected to increase invasion success at both the community and population levels by altering the diversity of exotic species discharged (colonization pressure) and the number of introduction events (propagule number). This study sought large-scale correlations between metrics of shipping activities and exotic fouling species diversity at the community (species richness evaluated using standardized port surveys) and population (genetic diversity) levels. The richness of exotic fouling species was evaluated at the community level by sampling 15 commercial ports in marine temperate Canadian waters. At the population level, we investigated genetic diversity of the model fouling species Botryllus schlosseri in five commercial ports, by identifying 262 individuals at the mitochondrial cytochrome c oxidase subunit I gene and eight polymorphic nuclear microsatellites. Measures of community- and population-level richness were related to various measures of propagule number and colonization pressure related to ship ballast and hull biofouling. Patterns of exotic fouling diversity are consistent with the hypothesis that increased shipping arrivals increase establishment success at both the community and population levels. As expected for fouling exotic species, ballast water was a less important vector; introduction success was best correlated to the number of arrivals of non-merchant ships from close regions, suggesting that this vector is important for successful invasion of this group of organisms.

Intentionally introduced terrestrial invertebrates: patterns, risks, and options for management

Abstract: Our understanding and management of pathways of alien species introductions has improved significantly in the past few years. However, little attention has been paid in most parts of the world to the risks posed by the intentional introduction of alien terrestrial invertebrates which are not intended for use in biological control. We review the species and pathways involved in this intentional trade, and discuss key factors that mediate different aspects of risk. A total of 20 different intentions for the introduction of terrestrial invertebrates were identified. Uses and trade patterns have changed over time and further changes are likely in the future. In particular, invertebrates used in the pet trade, and as human food and animal feed are likely to increase in relevance. We assess priorities for future research and regulation based on the perceived “risk” of the uses including propagule pressure, security of captivity and ease of regulation. Regarding risk assessment, we examine three options: (a) using an existing generic protocol developed for a broad range of taxa; (b) developing a new protocol, possibly by adapting a protocol developed for other taxa; and (c) adopting the approach applied for biological control, i.e. structured experiments and observations. This review highlights the diversity of uses and associated threats of intentional terrestrial invertebrate introductions. It provides recommendations on how to tackle and prevent related issues and can therefore serve as a guideline for future work. We argue that the most suitable option for risk assessment might depend on the type or organism and the level of knowledge of the organism, as well as the intended use.

Strength in size not numbers: propagule size more important than number in sexually reproducing populations

Abstract: Propagule pressure has consistently been identified as a primary factor in invader success, and reducing it can be one of the most effective methods for preventing the establishment of non-native species. However, when policy is implemented to reduce propagule pressure it almost exclusively focuses on the size of individual introduction events (‘propagule size’), with little confirmation that controlling this single aspect of propagule pressure is the most effective strategy. The number of introduction events (‘propagule number’) can play as much, or more, of a role in invader success, yet only a small portion of propagule pressure research has studied the relative importance of size and number. We investigated the relative roles of propagule size and number in the establishment of a sexually reproducing species using a field mesocosm experiment that introduced Hemimysis anomala (a non-native mysid) across a range of propagule sizes and numbers. We found that single, large introductions had higher abundances and probabilities of survival than smaller, more frequent additions. This experiment illustrated that, for sexual reproducers, focusing on lowering propagule size can be the most effective method for reducing non-native establishment.

Introduction history, climatic suitability, native range size, species traits and their interactions explain establishment of Chinese woody species in Europe

Abstract: Aim A major challenge in ecology is to understand how multiple causal factors, which may interact, drive success of non-native plants in new ranges. In this study we addressed the role of introduction history, climatic suitability, native range size, species traits and their interactions in the establishment of Chinese woody species in Europe. Location China (native range), Europe (new range). Methods We tested whether establishment of 449 Chinese woody species in Europe was associated with residence time (time since earliest planting), planting frequency, climatic suitability, native range size and species traits. We also considered possible nonlinear effects and interactions among these variables. For the 38 species that have established in Europe, we further tested whether these variables and interactions explained their establishment in multiple European countries. Results Establishment of the 449 species in Europe was positively associated with residence time, planting frequency and climatic suitability. Except residence time, these factors were also positively associated with establishment of the 38 species in multiple countries. None of the traits tested had statistically significant main effects on establishment in Europe, but, for the established species, longer flowering period and having compound leaves were positively associated with establishment in multiple countries. The positive association between establishment in Europe and residence time was stronger for evergreen than for deciduous species. In addition, evergreens, unlike deciduous species, showed a positive association between establishment in Europe and fruiting duration. Moreover, establishment in multiple countries was positively associated with planting frequency for species with compound leaves but not for species with simple leaves, and the association between the establishment and fruiting duration changed from negative to moderately positive as climatic suitability increased. Main conclusions Introduction history and climatic suitability explain most of the variation in establishment, and modulate the role of species traits, such as leaf retention, leaf type and fruiting duration.

Species pools and environmental sorting control different aspects of plant diversity and functional trait composition in recovering grasslands

Abstract: Summary Succession theory argues that the taxonomic and functional structure of assembling communities is regulated by the interplay of local environmental conditions and regional factors such as available species pools or landscape configurations that moderate dispersal. Yet, the relative importance of these factors and whether they control different aspects of community structure, for example species diversity or the composition of plant traits, is not well understood. This limits predictions of spatial variation in successional outcomes. We explored patterns of species diversity and traits in recovering calcareous grasslands that varied in environmental conditions, age, past and present spatial landscape configuration, as well as in the propagule pressure they receive from historical grasslands that have large and functionally distinct species pools. Mixed-effects modelling and multimodel inference were used to dissect the leading drivers of species diversity and functional composition. Propagule pressure from the regional species pools of old grasslands during early stages of grassland re-assembly was an important determinant of community structure, with significant effects not only on species diversity but also on the composition of traits related to resource economics. Such species pool effects exceeded positive influences of environmental heterogeneity on species richness and confounded well-known diversity–environment and trait–environment relationships. As an orthogonal axis of community assembly, environmental sorting occurred along a gradient of soil texture that discriminated species according to their canopy height and – to a smaller extent – phenology. Additionally, high grassland perimeter–area ratios favoured specialist species richness and increased the proportion of species with predicted poor epizoochorous and wind dispersal. Synthesis. We provide empirical evidence that both species pool effects and local environmental sorting operate during community assembly of recovering calcareous grasslands, but that they influence different aspects of plant community structure. Further, we show that geographic variation in propagule pressure from large and functionally distinct source species pools can confound diversity–environment and trait–environment relationships. Therefore, besides accounting for environmental factors, considering the location of source species pools, as well as their compositional and functional characteristics, is a prerequisite for robust predictions of contingencies in taxonomic and functional community structure of successional habitats.

Testing alien plant distribution and habitat invasibility in mountain ecosystems: growth form matters.

Abstract: Most studies focused on understanding habitat invasibility use the current levels of invasion as a direct proxy of habitat invasibility. This has shown to be biased by the influence of propagule pressure and climate. We suggest that plant growth forms need to be considered as an extra factor, as habitat preferences might not be equal for all potential invaders. We test the influence of propagule pressure, climate and habitat characteristics on the current level of invasion and habitat invasibility, specifically addressing whether an analysis focused on growth forms evidence different patterns than the total pool of alien species. We used 499 floristic vegetation plots located in Cordoba Mountains. We used proportional alien richness of the total pool and for each growth form as response variables. We identified models that best explained current levels of invasion. We used the residuals of the models with propagule pressure and climate as the response variable. Then, we performed linear models to test the relationship between habitat characteristics and the residuals of the models. We found different drivers of current alien distribution patterns for the total pool and each growth form. Habitat invasibility was not equal when quantified for the total pool or growth forms. Shrublands and outcrops were recorded as less susceptible to woody invasion, while grasslands and native woodlands were resistant to the invasion of grasses and none habitat type was resistant to the invasion of forbs. We highlight that the current level of invasion and habitat invasibility are highly growth form dependent.

Pathways of alien invertebrate transfer to the Antarctic region

Abstract: Alien species pose an increasing threat to the biodiversity of the Antarctic region. Several alien species have established in Antarctic terrestrial communities, some representing novel functional groups such as pollinators and predators, with unknown impacts on ecosystem processes. We quantified the unintentional introduction of alien invertebrates to the Antarctic region over a 14-year period (2000–2013). To do this, probable pathways (Australian Antarctic cargo operations) and endpoints (research stations) for invertebrate introductions were searched. In addition, we undertook a stratified trapping programme targeting invertebrates on supply vessels in transit to the Antarctic region and also at cargo facilities in Australia during the 2012–2013 austral summer field season. Our results show that a diverse suite of invertebrate taxa were being introduced to the Antarctic region, with 1,376 individuals from at least 98 families observed or trapped during the sampling period. Many individuals were found alive. Diptera, Coleoptera and Lepidoptera were the most common taxa, comprising 74 % of the collection. At the family level, Phoridae (small flies) and Noctuidae (moths) were most commonly observed. Individuals from 38 different families were repeatedly introduced over the study period, sometimes in high numbers. Food and large cargo containers harboured the most individuals. These findings can assist in improving biosecurity protocols for logistic activities to Antarctica, thereby reducing the risk of invasions to the Antarctic region.

Propagule pressure cannot always overcome biotic resistance: the role of density‐dependent establishment in four invasive species

Abstract: Summary Successful invasions result from species functional traits interacting with the receiving community. Some have proposed that propagule pressure, or the size and number of introductions, can overcome high invasion resistance. However, few studies empirically investigate the relationship among functional traits, community composition, and propagule pressure. Here, we empirically examined how propagule pressure influenced the invasive success of four species in four resident communities (bare ground, grass-only, forb-only, and full resident community). The four species represented both small- and large-seeded grasses and forbs, and propagule sizes (the number of individuals in an introduction event) ranged from 16 to 160 000 seeds m−2. Invasion probability varied among species and communities and was strongly influenced by the propagule size. The large-seeded species had the highest invasion success in all communities, while the grasses did as well or better, respectively, in the grass-only and bare ground communities. However, contrary to existing assumptions, we found that the establishment probability of individual seeds decreased exponentially with increasing propagule size. Therefore, increased propagule pressure was subjected to severely diminishing returns on invasiveness. Thus, while propagule pressure played a role in determining invasion success, it was not always able to overcome communities with high invasion resistance, particularly for species that experienced strong density-dependent inhibition. The role of propagule pressure in invasion is more complex than previously thought and may present important trade-offs for bet-hedging colonisation strategies.

Unravelling the invasion pathways of the quagga mussel (Dreissena rostriformis) into Western Europe

Abstract: Genetic tools have been extremely useful to study the colonization history and dynamics of invasive species and infer source populations. In addition, understanding the distribution of genetic diversity of non-indigenous species is important to understand factors contributing to invasive success. Here, we used genetic markers to study the invasion history of the quagga mussel Dreissena rostriformis (Deshayes 1838). The invasion of North American and European watercourses by the quagga mussel is of major concern since this dreissenid mussel has widespread ecological and economic impacts in invaded regions. Through the use of mitochondrial (COI) and nuclear (microsatellites) markers and based on a large sampling, including both native and invasive ranges, we characterized the recent invasion of Western Europe by D. rostriformis. Scenario testing Bayesian analysis (approximate Bayesian computation methods) suggests that Western Europe was most probably invaded from the Pontic region via the southern corridor (Danube River, the Main-Danube Canal and the Main and Rhine rivers). Furthermore, pairwise F ST values suggest a second invasion to Western Europe from North America via trans-Atlantic shipping. The high genetic diversity and low differentiation among D. rostriformis populations suggest high propagule pressure and frequent exchanges between the Pontic region, Eastern North America and Western Europe. Our study concludes that multiple introductions and high propagule pressure have shaped the genetic composition of populations in Western Europe. The results provide valuable information for future management plans in order to control the spread of highly invasive aquatic species.

Black rat invasion of inland Sahel: insights from interviews and population genetics in south-western Niger

Abstract: Human population migrations, as well as long-distance trade activities, have been responsible for the spread of many invasive organisms. The black rat, Rattus rattus, has colonized most of the world following ship-mediated trade. Owing to its tight association with human infrastructures, this species has been able to survive in unfavourable environments, such as Sahelian Africa. In this work, we combined interview-based and population genetic surveys to investigate the processes underlying the ongoing invasion of southwestern Niger by black rats, with special emphasis on the capital city, Niamey. Our trapping and interview data are quite congruent, and all together point towards a patchy, but rather widespread, current distribution of R. rattus. Genetic data strongly suggest that road network development for truck-based commercial flow from/to international harbours located in neighbouring countries (Benin, Togo, and Nigeria) facilitates the passive dispersal of black rats over a long distance through unfavourable landscapes. Another potentially, more ancient, invasion route may be associated with boat transport along the Niger River. Human-mediated dispersal thus probably allows the foundation of persisting populations within highly anthropized areas while population dynamics may be more unstable in remote areas and mostly depends on propagule pressure.

Patterns of plant invasions at small spatial scale correspond with that at the whole country scale

Abstract: Invasions were studied at different spatial scales, however these scales were rarely compared. The aim of this study is to compare the rate of expansion of eight alien plant species within the borders of the city with expansion in the whole country and to analyse changes in distribution of these plant species in the last 30 years. We hypothesised that at the scale of the city, despite of stronger human impact, invasion processes will perform in a similar way as at the scale of whole country. Distribution changes of Bidens frondosa, Conyza canadensis, Echinocystis lobata, Helianthus tuberosus, Impatiens parviflora, Reynoutria japonica, Solidago canadensis and S. gigantea in 1 km squares grid was examined in the Warta River Valley in Poznan (W Poland) in 2013 and compared to data collected during 1980–1984. All species increased their distribution within the study area. E. lobata had the highest increment of occupied grid squares (from 0 in 1980’s to 28 in 2013), and S. gigantea had the lowest (from 8 to 9). Echinocystis lobata reached the highest invasion success, which results from species traits (hydrochory) and large propagule pressure from garden plots. Between species with different ecological traits residence time has relatively little influence on invasion success, however in cases of species with similar traits (S. canadensis and S. gigantea) residence time is a crucial factor determining invasion success. Although urban ecosystems are more vulnerable to biological invasions, this process occurs in a similar way as in the whole country. This similarity is connected with similar factors responsible for spread of invasive species: propagule pressure, time since introduction and spatial distributions of invasible habitats.

Accounting for residential propagule pressure improves prediction of urban plant invasion

Abstract: Plant invasions substantially impact the ecosystem services provided by forests in urbanizing regions. Knowing where invasion risk is greatest helps target early detection and eradication efforts, but developing an accurate predictive model of invasive species presence and spread on the basis of habitat suitability remains a challenge due to spatial variation in propagule pressure (the number of individuals released) which is likely conflated with suitability. In addition to neighborhood propagule pressure that originates with propagules dispersing from naturalized populations within invaded habitats, we expect residential propagule pressure arising from the widespread use of exotic plants in the yards of single‐family residences to be an important driver of invasions, and to notably improve the predictive accuracy of species distribution models (SDMs). To this end, we collected presence/absence data for a widespread forest invader, Ligustrum sinense (Chinese privet), from 400 stratified random plots located along an urban gradient across the Charlotte, North Carolina metropolitan area. We assessed the relative contribution of residential propagule pressure and neighborhood propagule pressure to improving the predictive performance of a probit SDM for Chinese privet that only contains environmental predictors. Our results indicate that, although the environment‐only model predicted the highest geographic area to be at risk of invasion by privet, it also had the highest rate of failure to accurately predict observed privet occurrences as indicated by the omission (incorrectly predicted absence) and commission (incorrectly predicted presence) error rates. Accounting for residential propagule pressure substantially improved model performance by reducing the omission error by nearly 50%, thereby improving upon the ability of the model to predict privet invasion in suboptimal habitat. Given that this increase in detection was accompanied by a decrease in the geographic area predicted at risk, we conclude that SDMs for invasive exotic shrubs and potentially for other synanthropic generalist plants may be highly inefficient when residential propagule pressure is not accounted for. Accounting for residential propagule pressure in models of invasive plants results in a more focused and accurate prediction of the area at risk, thus enabling decision makers to feasibly prioritize regional scale monitoring and control efforts.

Propagule quality mediates invasive plant establishment

Abstract: Propagule pressure is commonly considered a primary driver of invasive plant establishment and spread. However, the physical size or condition (i.e., quality) of propagules may also affect establishment, particularly under unfavorable habitat conditions such as low light environments. We used an outdoor mesocosm experiment to test the relative contribution of propagule size (number of individuals introduced) and quality (number of rhizome nodes) to the establishment and performance of the highly invasive cogongrass (Imperata cylindrica) under experimental sun and shade treatments. We found that the introduction of higher quality propagules (rhizome segments ≥3 nodes in length) significantly enhanced establishment across both light treatments, and increased final tiller count in the sun treatment. The sun treatment also enhanced rhizome growth, an effect that could increase spread rates and invasion success. Thus, while cogongrass is likely to establish in both sun and shade, introductions of large propagule sizes or large rhizomes in high light environments likely poses the greatest threat to native habitats. Our results demonstrate that propagule quality promoted both establishment and performance of a highly invasive grass species and suggest that propagule quality may play an important but underappreciated role in the invasion process.

Propagule pressure and environmental conditions interact to determine establishment success of an invasive plant species, glossy buckthorn (Frangula alnus), across five different wetland habitat types

Abstract: Many invasive plant species are able to establish within a wide range of community types. This establishment success depends on high propagule pressure and successful recruitment of seedlings following propagule dispersal into receptive environments. This study aimed to investigate interactions between propagule pressure and environmental resistance to seedling recruitment of the invasive shrub, glossy buckthorn (Frangula alnus Mill.), over a range of wetland habitat types. We measured propagule deposition using seed traps and recruitment success using sown plots, while characterizing vegetation and abiotic environmental conditions in five adjacent wetland habitat types. Drier habitats, which included Cedar Swamp, Shrub Carr, and String, had lower resistance to buckthorn establishment than the wetter Flark and Cattail Marsh. The drier habitats supported more woody species and provided more raised hummock surfaces essential for successful buckthorn recruitment and establishment. Propagule pressure was also higher in dry habitats that supported higher densities of adult glossy buckthorn, while long-distance dispersal into areas with low adult density was uncommon. Natural recruitment was highest in sites with intense propagule pressure, but experimental sowing of seeds demonstrated that buckthorn establishes in wet sites with higher resistance if propagule pressure is increased and seeds are deposited on hummocks. This study demonstrates the affinity of glossy buckthorn for drier wetland sites, and provides empirical evidence that environmental resistance can be overcome by higher propagule pressure.

Freshwater invasibility level depends on the population age structure of the invading mussel species

Abstract: Environmental conditions may affect invasibility and potentially prevent successful invasions of the freshwater bivalve Limnoperna fortunei Dunker, 1857. However, even though the larval stage may be considered an important species’ characteristic, the invasion processes of L. fortunei are mainly evaluated using only the adult stage. Therefore, the aim of this study was to identify what, and how, environmental filters might predict the likelihood of occurrence of each of the L. fortunei larval stages. Logistic regressions were applied using the larval stages of L. fortunei as a surrogate of population age structure and the environmental variables as the main filters that potentially predicted the likelihood of the occurrence of each larval stage of L. fortunei. The turbidity predicted the occurrence of the D-shaped larvae and straight-hinged veliger stages, while the umbonated-veliger and pediveliger stages were predicted by the pH and conductivity. Finally, the phytoplankton density (diatoms) predicted the occurrence of the umbonated-veliger, pediveliger and mainly the plantigrade stages. Our findings suggested that, during larvae development, from younger to older stages, the main environmental predictor of larvae occurrence shifted from abiotic to biotic variables, indicating that larval stages are indeed an important factor that helps in the evaluation of freshwater invasibility. These findings are particularly important for floodplain systems, where flood pulse dynamics may increase propagule pressure, leading to a successful spread among habitats. Then, even though environmental filters were important to predict successful invasions, we highlight that a successful invasion might depend on the population age structure of the invading species arriving in the new habitat. Thus, successful L. fortunei invasions in floodplain systems may depend on the interaction between invasiveness, invasibility and propagule pressure.