Biological control as an invasion process: disturbance and propagule pressure affect the invasion success of Lythrum salicaria biological control agents

Understanding the mechanisms behind the successful colonization and establishment of introduced species is important for both preventing the invasion of unwanted species and improving release programs for biological control agents. However, it is often not possible to determine important introduction details, such as date, number of organisms, and introduction location when examining factors affecting invasion success. Here we use biological control introduction data to assess the role of propagule pressure, disturbance, and residence time on invasion success of four herbivorous insect species introduced for the control of the invasive wetland plant, Lythrum salicaria, in the Columbia River Estuary. Two sets of field surveys determined persistence at prior release sites, colonization of new sites, and abundance within colonized sites. We quantified propagule pressure in four ways to examine the effect of different measurements. These included three measurements of introduction size (proximity to introduction site, introduction size at a local scale, and introduction size at a regional scale) and one measure of introduction number (number of introduction events in a region). Disturbance was examined along a tidal inundation gradient (distance from river mouth) and as habitat (island or mainland). Statistical models and model averaging were used to determine which factors were driving invasion success. In this study we found: (1) sparse evidence for the positive influence of propagule pressure on invasion success; (2) disturbance can negatively affect the invasion success of herbivorous insects; (3) the effects of disturbance and propagule pressure are species specific and vary among invasion stages, and (4) not all measures of propagule pressure show the same results, therefore single measures and proxies should be used cautiously.     

Allee Effects, Propagule Pressure and the Probability of Establishment: Risk Analysis for Biological Invasions

Colonization is of longstanding interest in theoretical ecology and biogeography, and in the management of weeds and other invasive species, including insect pests and emerging infectious diseases. Due to accelerating invasion rates and widespread economic costs and environmental damages caused by invasive species, colonization theory has lately become a matter of considerable interest. Here we review the concept of propagule pressure to inquire if colonization theory might provide quantitative tools for risk assessment of biological invasions. By formalizing the concept of propagule pressure in terms of stochastic differential equation models of population growth, we seek a synthesis of invasion biology and theoretical population biology. We focus on two components of propagule pressure that affect the chance of invasion: (1) the number of individuals initially introduced, and (2) the rate of subsequent immigration. We also examine how Allee effects, which are expected to be common in newly introduced populations, may inhibit establishment of introduced propagules. We find that the establishment curve (i.e., the chance of invasion as a function of initial population size), can take a variety of shapes depending on immigration rate, carrying capacity, and the severity of Allee effects. Additionally, Allee effects can cause the stationary distribution of population sizes to be bimodal, which we suggest is a possible explanation for time lags commonly observed between the detection of an introduced population and widespread invasion of the landscape.     

The role of propagule pressure in explaining species invasions

Human-mediated species invasions are a significant component of current global environmental change. There is every indication that the rate at which locations are accumulating non-native species is accelerating as free trade and globalization advance. Thus, the need to incorporate predictive models in the assessment of invasion risk has become acute. However, finding elements of the invasion process that provide consistent explanatory power has proved elusive. Here, we propose propagule pressure as a key element to understanding why some introduced populations fail to establish whereas others succeed. In the process, we illustrate how the study of propagule pressure can provide an opportunity to tie together disparate research agendas within invasion ecology.     

Relationship between propagule pressure and colonization pressure in invasion ecology: a test with ships' ballast

Increasing empirical evidence indicates the number of released individuals (i.e. propagule pressure) and number of released species (i.e. colonization pressure) are key determinants of the number of species that successfully invade new habitats. In view of these relationships, and the possibility that ships transport whole communities of organisms, we collected 333 ballast water and sediment samples to investigate the relationship between propagule and colonization pressure for a variety of diverse taxonomic groups (diatoms, dinoflagellates and invertebrates). We also reviewed the scientific literature to compare the number of species transported by ships to those reported in nature. Here, we show that even though ships transport nearly entire local communities, a strong relationship between propagule and colonization pressure exists only for dinoflagellates. Our study provides evidence that colonization pressure of invertebrates and diatoms may fluctuate widely irrespective of propagule pressure. We suggest that the lack of correspondence is explained by reduced uptake of invertebrates into the transport vector and the sensitivity of invertebrates and diatoms to selective pressures during transportation. Selection during transportation is initially evident through decreases in propagule pressure, followed by decreased colonization pressure in the most sensitive taxa.     

Determinants of vertebrate invasion success in Europe and North America

Species that are frequently introduced to an exotic range have a high potential of becoming invasive. Besides propagule pressure, however, no other generally strong determinant of invasion success is known. Although evidence has accumulated that human affiliates (domesticates, pets, human commensals) also have high invasion success, existing studies do not distinguish whether this success can be completely explained by or is partly independent of propagule pressure. Here, we analyze both factors independently, propagule pressure and human affiliation. We also consider a third factor directly related to humans, hunting, and 17 traits on each species' population size and extent, diet, body size, and life history. Our dataset includes all 2362 freshwater fish, mammals, and birds native to Europe or North America. In contrast to most previous studies, we look at the complete invasion process consisting of (1) introduction, (2) establishment, and (3) spread. In this way, we not only consider which of the introduced species became invasive but also which species were introduced. Of the 20 factors tested, propagule pressure and human affiliation were the two strongest determinants of invasion success across all taxa and steps. This was true for multivariate analyses that account for intercorrelations among variables as well as univariate analyses, suggesting that human affiliation influenced invasion success independently of propagule pressure. Some factors affected the different steps of the invasion process antagonistically. For example, game species were much more likely to be introduced to an exotic continent than nonhunted species but tended to be less likely to establish themselves and spread. Such antagonistic effects show the importance of considering the complete invasion process.     

Residence time and human-mediated propagule pressure at work in the alien flora of Galapagos

Introduced species present the greatest threat to the unique terrestrial biodiversity of the Galapagos Islands. We assess the current status of plant invasion in Galapagos, predict the likelihood of future naturalizations and invasions from the existing introduced flora, and suggest measures to help limit future invasions. There has been a 1.46 fold increase in plant biodiversity in Galapagos due to alien plant naturalizations, reflecting a similar trend on islands elsewhere. There are 870 alien plant species recorded in the archipelago. Of evaluated species, 34% species have naturalized. Within this group are the invasive species (16% of evaluated) and the transformers (3.3% of evaluated). We show that, as expected, naturalized species have been present in the archipelago longer than non-naturalized species. We also find that a higher human-mediated propagule pressure is associated with a greater human population and with properties that have been settled longer. This, combined with the relatively recent introduction of most species, leads us to the conclusion that Galapagos is at an early stage of plant invasion. We predict that more species from the existing alien flora will find an opportunity to naturalize and invade as propagule pressure increases alongside rapid human population growth associated with immigration to serve the booming tourism industry. In order to reduce future invasion risk, we suggest reviewing inter-island quarantine measures and continuing community education efforts to reduce human-mediated propagule pressure.     

Propagule pressure hypothesis not supported by an 80‐year experiment on woody species invasion

Ecological filters and availability of propagules play key roles structuring natural communities. Propagule pressure has recently been suggested to be a fundamental factor explaining the success or failure of biological introductions. We tested this hypothesis with a remarkable data set on trees introduced to Isla Victoria, Nahuel Huapi National Park, Argentina. More than 130 species of woody plants, many known to be highly invasive elsewhere, were introduced to this island early in the 20th century, as part of an experiment to test their suitability as commercial forestry trees for this region. We obtained detailed data on three estimates of propagule pressure (number of introduced individuals, number of areas where introduced, and number of years during which the species was planted) for 18 exotic woody species. We matched these data with a survey of the species and number of individuals currently invading the island. None of the three estimates of propagule pressure predicted the current pattern of invasion. We suggest that other factors, such as biotic resistance, may be operating to determine the observed pattern of invasion, and that propagule pressure may play a relatively minor role in explaining at least some observed patterns of invasion success and failure.     

Propagule pressure and native community connectivity interact to influence invasion success in metacommunities

Mechanistic insights from invasion biology indicate that propagule pressure of exotic species and native community structure can independently influence establishment success. The role of native community connectivity via species dispersal and its potential interaction with propagule pressure on invasion success in metacommunities, however, remains unknown. Native community connectivity may increase biotic resistance to invasion by enhancing species richness and evenness, but the effects could depend upon the level of propagule pressure. In this study, a mesocosm experiment was used to evaluate the independent and combined effects of exotic propagule pressure and native community connectivity on invasion success. The effects of three levels of exotic Daphnia lumholtzi propagule pressure on establishment success, community structure and ecosystem attributes were evaluated in native zooplankton communities connected by species dispersal versus unconnected communities, and relative to a control without native species. Establishment of the exotic species exhibited a propagule dose‐dependent relationship with high levels of propagule pressure resulting in the greatest establishment success. Native community connectivity, however, effectively reduced establishment at the low level of propagule pressure and further augmented native species richness across propagule pressure treatments. Propagule pressure largely determined the negative impacts of the exotic species on native species richness, native biomass and edible producer biomass. The results highlight that native community connectivity can reduce invasion success at a low propagule dose and decrease extinction risk of native competitors, but high propagule pressure can overcome connectivity‐mediated biotic resistance to influence establishment and impact of the exotic species. Together, the results emphasize the importance of the interaction of propagule pressure and community connectivity as a regulator of invasion success, and argue for the maintenance of metacommunity connectivity to confer invasion resistance.     

The comparative importance of species traits and introduction characteristics in tropical plant invasions

Aim We used alien plant species introduced to a botanic garden to investigate the relative importance of species traits (leaf traits, dispersal syndrome) and introduction characteristics (propagule pressure, residence time and distance to forest) in explaining establishment success in surrounding tropical forest. We also used invasion scores from a weed risk assessment protocol as an independent measure of invasion risk and assessed differences in variables between high‐ and low‐risk species. Location East Usambara mountains, Tanzania. Methods Forest transect surveys identified species establishing in disturbed and intact forest. Leaf traits (specific leaf area and foliar nutrient concentrations) were measured from leaves sampled in high‐light environments. Results A leaf traits spectrum was apparent, but species succeeding or failing to establish in either disturbed or intact forest were not located in different parts of the spectrum. Species with high invasion risk did not differ in their location on the leaf trait spectrum compared with low‐risk species but were more likely to be bird/primate‐dispersed. For 15 species establishing in forest quadrats, median canopy cover of quadrats where seedlings were present was correlated with a species value along the leaf trait spectrum. Species establishing in disturbed forest were planted in twice as many plantations and were marginally more likely to be bird‐ or primate‐dispersed than species failing to become established in disturbed forest. Establishment in intact forest was more likely for species planted closer to forest edges. Main conclusions Leaf and dispersal traits appear less important in the colonization of tropical forest than introduction characteristics. It appears, given sufficient propagule pressure or proximity to forest, alien species are much more likely to establish independently of leaf traits or dispersal syndrome in continental tropical forests.     

Invasion Dynamics of a Fish-Free Landscape by Brown Trout (Salmo trutta)

Metapopulation dynamics over the course of an invasion are usually difficult to grasp because they require large and reliable data collection, often unavailable. The invasion of the fish-free freshwater ecosystems of the remote sub-Antarctic Kerguelen Islands following man-made introductions of brown trout (Salmo trutta) in the 1950''s is an exception to this rule. Benefiting from a full long term environmental research monitoring of the invasion, we built a Bayesian dynamic metapopulation model to analyze the invasion dynamics of 85 river systems over 51 years. The model accounted for patch size (river length and connections to lakes), alternative dispersal pathways between rivers, temporal trends in dynamics, and uncertainty in colonization date. The results show that the model correctly represents the observed pattern of invasion, especially if we assume a coastal dispersal pathway between patches. Landscape attributes such as patch size influenced the colonization function, but had no effect on propagule pressure. Independently from patch size and distance between patches, propagule pressure and colonization function were not constant through time. Propagule pressure increased over the course of colonization, whereas the colonization function decreased, conditional on propagule pressure. The resulting pattern of this antagonistic interplay is an initial rapid invasion phase followed by a strong decrease in the invasion rate. These temporal trends may be due to either adaptive processes or environmental gradients encountered along the colonization front. It was not possible to distinguish these two hypotheses. Because invasibility of Kerguelen Is. freshwater ecosystems is very high due to the lack of a pre-existing fish fauna and minimal human interference, our estimates of invasion dynamics represent a blueprint for the potential of brown trout invasiveness in pristine environments. Our conclusions shed light on the future of polar regions where, because of climate change, fish-free ecosystems become increasingly accessible to invasion by fish species.     

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

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.     

The role of preadaptation,propagule pressure and competition in the colonization of new habitats

To successfully colonize new habitats, organisms not only need to gain access to it, they also need to cope with the selective pressures imposed by the local biotic and abiotic conditions. The number of immigrants, the preadaptation to the local habitat and the presence of competitors are important factors determining the success of colonization. Here, using two experimental set-ups, we studied the effect of interspecific competition in combination with propagule pressure and preadaptation on the colonization success of new habitats. Our model system consisted of tomato plants (the novel habitat), the two-spotted spider mite Tetranychus urticae as our focal species and the red spider mite Tetranychus evansi as a competitor. Our results show that propagule pressure and preadaptation positively affect colonization success. More successful populations reach larger final population sizes either by having higher per capita growth rates (due to preadaptation effects) or by starting a population with a larger number of individuals. Although populations are more successful colonizing non-competitive environments than competitive ones, propagule pressure and preadaptation counteract the negative effects of competition, promoting colonization success. Our study shows the importance of propagule pressure and preadaptation for successful colonization of new habitats by providing the ability to cope with both the exigencies of new environments and the community context.     

Recruitment in species-rich grasslands: the effects of functional traits and propagule pressure

Aims The assembly of plant communities is a complex process which combines impacts from the species pool, dispersal and propagule pressure, niche requirements of colonizing species and the niche structure of the community. Recent theory development has incorporated all these aspects, e.g. in 'stochastic niche theory'. We investigated recruitment into a species-rich grassland community, using an experimental approach where we manipulated the trait composition of the community and examined the success of colonizing species entering with various propagule pressure. Specifically, we examined two predictions: (i) colonization success increases with increasing difference between traits of the colonizing species and the trait profile of the community and (ii) colonization success increases with increasing propagule pressure.Methods The examined communities were species-rich semi-natural grasslands located in southern Sweden. After a careful documentation of the composition of the plant communities at the experimental sites, we manipulated the trait profile of species-rich grassland plots based on the plant functional trait specific leaf area (SLA), which is correlated with several key life history functions. In addition to SLA, seed mass was also used to describe the trait profile of grassland plots. Seeds of 12 plant species from the regional species pool, varying in SLA and seed mass, were sown into plots using four different levels of propagule pressure. Recruitment was examined after 1 year. We also planted juvenile 'plug plants' of the same species which allowed us to examine survivorship and growth beyond the seedling stage.Important findings Overall we found very limited evidence for relationships between the traits of the colonizing species and the trait profile of the community and for recruitment after sowing these relationships were contrary to the prediction. Survival of plug plants after two seasons of growth was high irrespective of the trait profile of the community, but growth of plug plants was affected by the trait profile of the surrounding community. For four of the species there was a positive effect of increased propagule pressure on colonization. The results suggest that species assembly in species-rich grasslands is not strongly dependent on the niche structure of the community. However, the finding that colonization of only a third of the species responded positively to increased propagule pressure indicates that there might be niche-related effects that were not captured by our treatments. Overall, our results indicate that the factors determining colonization in this community are species specific. Some species are able to colonize irrespective of niche relationships, provided that the propagule pressure is sufficiently high to overcome stochastic mortality after seed arrival. For other species, however, we cannot exclude that niche assembly occurred, but we failed to identify the relevant niche factor.     

Parsing propagule pressure: Number,not size,of introductions drives colonization success in a novel environment

Predicting whether individuals will colonize a novel habitat is of fundamental ecological interest and is crucial to conservation efforts. A consistently supported predictor of colonization success is the number of individuals introduced, also called propagule pressure. Propagule pressure increases with the number of introductions and the number of individuals per introduction (the size of the introduction), but it is unresolved which process is a stronger driver of colonization success. Furthermore, their relative importance may depend upon the environment, with multiple introductions potentially enhancing colonization of fluctuating environments. To evaluate the relative importance of the number and size of introductions and its dependence upon environmental variability, we paired demographic simulations with a microcosm experiment. Using Tribolium flour beetles as a model system, we introduced a fixed number of individuals into replicated novel habitats of stable or fluctuating quality, varying the number of introductions through time and size of each introduction. We evaluated establishment probability and the size of extant populations through seven generations. We found that establishment probability generally increased with more, smaller introductions, but was not affected by biologically realistic fluctuations in environmental quality. Population size was not significantly affected by environmental variability in the simulations, but populations in the microcosms grew larger in a stable environment, especially with more introduction events. In general, the microcosm experiment yielded higher establishment probability and larger populations than the demographic simulations. We suggest that genetic mechanisms likely underlie these differences and thus deserve more attention in efforts to parse propagule pressure. Our results highlight the importance of preventing further introductions of undesirable species to invaded sites and suggest conservation efforts should focus on increasing the number of introductions or reintroductions of desirable species rather than increasing the size of those introduction events into harsh environments.     

Inferring process from pattern in plant invasions: a semimechanistic model incorporating propagule pressure and environmental factors

Propagule pressure is intuitively a key factor in biological invasions: increased availability of propagules increases the chances of establishment, persistence, naturalization, and invasion. The role of propagule pressure relative to disturbance and various environmental factors is, however, difficult to quantify. We explored the relative importance of factors driving invasions using detailed data on the distribution and percentage cover of alien tree species on South Africa's Agulhas Plain (2,160 km2). Classification trees based on geology, climate, land use, and topography adequately explained distribution but not abundance (canopy cover) of three widespread invasive species (Acacia cyclops, Acacia saligna, and Pinus pinaster). A semimechanistic model was then developed to quantify the roles of propagule pressure and environmental heterogeneity in structuring invasion patterns. The intensity of propagule pressure (approximated by the distance from putative invasion foci) was a much better predictor of canopy cover than any environmental factor that was considered. The influence of environmental factors was then assessed on the residuals of the first model to determine how propagule pressure interacts with environmental factors. The mediating effect of environmental factors was species specific. Models combining propagule pressure and environmental factors successfully predicted more than 70% of the variation in canopy cover for each species.     

Role of propagule pressure in colonization success: disentangling the relative importance of demographic,genetic and habitat effects

High propagule pressure is arguably the only consistent predictor of colonization success. More individuals enhance colonization success because they aid in overcoming demographic consequences of small population size (e.g. stochasticity and Allee effects). The number of founders can also have direct genetic effects: with fewer individuals, more inbreeding and thus inbreeding depression will occur, whereas more individuals typically harbour greater genetic variation. Thus, the demographic and genetic components of propagule pressure are interrelated, making it difficult to understand which mechanisms are most important in determining colonization success. We experimentally disentangled the demographic and genetic components of propagule pressure by manipulating the number of founders (fewer or more), and genetic background (inbred or outbred) of individuals released in a series of three complementary experiments. We used Bemisia whiteflies and released them onto either their natal host (benign) or a novel host (challenging). Our experiments revealed that having more founding individuals and those individuals being outbred both increased the number of adults produced, but that only genetic background consistently shaped net reproductive rate of experimental populations. Environment was also important and interacted with propagule size to determine the number of adults produced. Quality of the environment interacted also with genetic background to determine establishment success, with a more pronounced effect of inbreeding depression in harsh environments. This interaction did not hold for the net reproductive rate. These data show that the positive effect of propagule pressure on founding success can be driven as much by underlying genetic processes as by demographics. Genetic effects can be immediate and have sizable effects on fitness.     

Phylogeographic insights into the invasion history and secondary spread of the signal crayfish in Japan

Successful invasion by nonindigenous species is often attributed to high propagule pressure, yet some foreign species become widespread despite showing reduced genetic variation due to founder effects. The signal crayfish (Pacifastacus leniusculus) is one such example, where rapid spread across Japan in recent decades is believed to be the result of only three founding populations. To infer the history and explore the success of this remarkable crayfish invasion, we combined detailed phylogeographical and morphological analyses conducted in both the introduced and native ranges. We sequenced 16S mitochondrial DNA of signal crayfish from across the introduced range in Japan (537 samples, 20 sites) and the native range in western North America (700 samples, 50 sites). Because chela size is often related to aggressive behavior in crayfish, and hence, their invasion success, we also measured chela size of a subset of specimens in both introduced and native ranges. Genetic diversity of introduced signal crayfish populations was as high as that of the dominant phylogeographic group in the native range, suggesting high propagule pressure during invasion. More recently established crayfish populations in Japan that originated through secondary spread from one of the founding populations exhibit reduced genetic diversity relative to older populations, probably as a result of founder effects. However, these newer populations also show larger chela size, consistent with expectations of rapid adaptations or phenotypic responses during the invasion process. Introduced signal crayfish populations in Japan originate from multiple source populations from a wide geographic range in the native range of western North America. A combination of high genetic diversity, especially for older populations in the invasive range, and rapid adaptation to colonization, manifested as larger chela in recent invasions, likely contribute to invasion success of signal crayfish in Japan.     

Across islands and continents,mammals are more successful invaders than birds

Many invasive species cause ecological or economic damage, and the fraction of introduced species that become invasive is an important determinant of the overall costs caused by invaders. According to the widely quoted tens rule, about 10% of all introduced species establish themselves and about 10% of these established species become invasive. Global taxonomic differences in the fraction of species becoming invasive have not been described. In a global analysis of mammal and bird introductions, I show that both mammals and birds have a much higher invasion success than predicted by the tens rule, and that mammals have a significantly higher success than birds. Averaged across islands and continents, 79% of mammals and 50% of birds introduced have established themselves and 63% of mammals and 34% of birds established have become invasive. My analysis also does not support the hypothesis that islands are more susceptible to invaders than continents, as I did not find a significant relationship between invasion success and the size of the island or continent to which the species were introduced. The data set used in this study has a number of limitations, e.g. information on propagule pressure was not available at this global scale, so understanding the mechanisms behind the observed patterns has to be postponed to future studies.     

Decomposing propagule pressure: the effects of propagule size and propagule frequency on invasion success

Propagule pressure quantifies the inflow of individuals to a location and appears to be a key driver of invasion success. It is often defined as the average number of individuals introduced per time unit, or equivalently as the product of the average number of individuals introduced per introduction event (propagule size) and the frequency of introduction events (propagule frequency). Here we study how the influence of propagule size, frequency, and their product depends on the underlying ecological conditions. While previous studies have focused on introductions under environmental heterogeneity or a strong Allee effect, we examine a range of ecological scenarios that differ in the type of density dependence and in the sign of per capita growth rate. Our results indicate that the relative influence of propagule size and frequency depends mainly on the sign of per capita growth rate. Given a certain average number of individuals introduced per time unit, a high propagule frequency accelerates invasions under ecological scenarios with positive average per capita growth rate throughout the invasion process (‘easy’ scenarios). If per capita growth rate is negative throughout the invasion process (‘difficult’ scenarios) or if there is both an easy and a difficult stage (‘mixed scenarios’), a high propagule size leads to a faster invasion than a high propagule frequency. To explain this finding, we argue that for a fixed value of the product of propagule size and frequency, an increase in propagule size leads to an increase in demographic variance, which promotes invasion success in difficult and mixed but not in easy scenarios. However, we also show that in many of these cases, the product of propagule size and frequency still correlates more strongly with invasion success than either of the single components. Finally, we illustrate our approach with empirical examples from the literature.     

Hotspots of plant invasion predicted by propagule pressure and ecosystem characteristics

Aim Biological invasions pose a major conservation threat and are occurring at an unprecedented rate. Disproportionate levels of invasion across the landscape indicate that propagule pressure and ecosystem characteristics can mediate invasion success. However, most invasion predictions relate to species’ characteristics (invasiveness) and habitat requirements. Given myriad invaders and the inability to generalize from single‐species studies, more general predictions about invasion are required. We present a simple new method for characterizing and predicting landscape susceptibility to invasion that is not species‐specific. Location Corangamite catchment (13,340 km²), south‐east Australia. Methods Using spatially referenced data on the locations of non‐native plant species, we modelled their expected proportional cover as a function of a site’s environmental conditions and geographic location. Models were built as boosted regression trees (BRTs). Results On average, the BRTs explained 38% of variation in occupancy and abundance of all exotic species and exotic forbs. Variables indicating propagule pressure, human impacts, abiotic and community characteristics were rated as the top four most influential variables in each model. Presumably reflecting higher propagule pressure and resource availability, invasion was highest near edges of vegetation fragments and areas of human activity. Sites with high vegetation cover had higher probability of occupancy but lower proportional cover of invaders, the latter trend suggesting a form of biotic resistance. Invasion patterns varied little in time despite the data spanning 34 years. Main conclusions To our knowledge, this is the first multispecies model based on occupancy and abundance data used to predict invasion risk at the landscape scale. Our approach is flexible and can be applied in different biomes, at multiple scales and for different taxonomic groups. Quantifying general patterns and processes of plant invasion will increase understanding of invasion and community ecology. Predicting invasion risk enables spatial prioritization of weed surveillance and control.