Disturbance-mediated competition: the interacting roles of inundation regime and mechanical and herbicidal control in determining native and invasive plant abundance

Disturbance is a key component of many successful plant invasions. However, interactions among natural and anthropogenic disturbances and effects of these interacting disturbances on invasive plants and desired vegetation are rarely examined. We investigated the effect of anthropogenic disturbance (herbicidal and mechanical) along a natural inundation gradient (20–282 days) on the biomass and resource allocation of the invasive wetland plant, alligator weed (Alternanthera philoxeroides), and two co-occurring competitor plants, the introduced grass, kikuyu (Pennisetum clandestinum), and the native grass, couch (Cynodon dactylon), over a 2-year period. In the absence of additional disturbance, kikuyu biomass was negatively affected, alligator weed biomass was positively affected, and couch biomass was not affected by inundation disturbance. In addition, kikuyu was not affected by the selective removal of alligator weed, while couch increased in wetter habitats where kikuyu was absent due to inundation stress. This suggests that kikuyu is a superior competitor in drier habitats and inundation facilitates the invasion of alligator weed, while couch is an inferior competitor to both kikuyu and alligator weed and is therefore suppressed across its entire niche by these two introduced species. Mowing alone had no effect on the biomass of the species, suggesting the plants are equally tolerant of shoot removal. Selective herbicide reduced alligator weed biomass by 97.5% and the combination of selective herbicide and mowing reduced the biomass of alligator weed significantly more than herbicide alone, by 98.6% compared with un-manipulated controls. To predict community change and prevent sequential exotic plant invasions after weed removal, it is necessary to consider the interacting effects of disturbance and the niche space of invasive species in the local propagule pool.     

Disturbance Promotes Non-Indigenous Bacterial Invasion in Soil Microcosms: Analysis of the Roles of Resource Availability and Community Structure

Background

Invasion-biology is largely based on non-experimental observation of larger organisms. Here, we apply an experimental approach to the subject. By using microbial-based microcosm-experiments, invasion-biology can be placed on firmer experimental, and hence, less anecdotal ground. A better understanding of the mechanisms that govern invasion-success of bacteria in soil communities will provide knowledge on the factors that hinder successful establishment of bacteria artificially inoculated into soil, e.g. for remediation purposes. Further, it will yield valuable information on general principles of invasion biology in other domains of life.

Methodology/Principal Findings

Here, we studied invasion and establishment success of GFP-tagged Pseudomonas fluorescens DSM 50090 in laboratory microcosms during a 42-day period. We used soil heating to create a disturbance gradient, and hypothesized that increased disturbance would facilitate invasion; our experiments confirmed this hypothesis. We suggest that the key factors associated with the heating disturbance that explain the enhanced invasion success are increased carbon substrate availability and reduced diversity, and thus, competition- and predation-release. In a second experiment we therefore separated the effects of increased carbon availability and decreased diversity. Here, we demonstrated that the effect of the indigenous soil community on bacterial invasion was stronger than that of resource availability. In particular, introduced bacteria established better in a long term perspective at lower diversity and predation pressure.

Conclusion

We propose increased use of microbial systems, for experimental study of invasion scenarios. They offer a simple and cost-efficient way to study and understand biological invasion. Consequently such systems can help us to better predict the mechanisms controlling changes in stability of communities and ecosystems. This is becoming increasingly relevant since anthropogenic disturbance causes increasing global change, which promotes invasion. Moreover, a thorough understanding of factors controlling invasion and establishment of artificially amended micro-organisms will mean a major step forward for soil-remediation microbiology.     

Effects of mean intensity and temporal variability of disturbance on the invasion of <Emphasis Type="Italic">Caulerpa racemosa</Emphasis> var. <Emphasis Type="Italic">cylindracea</Emphasis> (Caulerpales) in rock pools

Disturbance is a key factor influencing the invasibility of habitats and assemblages. This relationship was extensively studied in terrestrial systems, but it was scarcely tested in the marine environment. We investigated experimentally the interactive effects of changes in the intensity and temporal variability of mechanical disturbance by boulders on invasion dynamics of the green alga Caulerpa racemosa var. cylindracea in littoral rock pools. We tested the hypothesis that the success of invasion of C. racemosa would be (1) greater under large than under low intensity of disturbance, (2) greater under large than under low temporal variability of disturbance and that (3) interactive effects could also occur, with variability of disturbance magnifying the effects of intensity. C. racemosa was virtually absent in pools maintained under high intensity of disturbance, independently of temporal variability. High intensity of disturbance was also associated with lower density and length of fronds and thinner diameter of the stolons of the alga. The total number of native taxa and the abundance of encrusting coralline algae increased under high intensity of disturbance. Differently, turf-forming algae were positively affected by temporal variability of disturbance, while canopy-forming algae did not respond to experimental treatments. Our results suggest a direct negative effect of the most severe experimental conditions on the spread of C. racemosa in rock pools. This likely overwhelmed likely concomitant positive and negative effects mediated by resident organisms. The results of this study help anticipating invasion dynamics of C. racemosa in rock pools under climate change scenarios, in which both intensity and temporal variability of extreme meteorological events are predicted to increase.     

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.     

Are boreal ecosystems susceptible to alien plant invasion? Evidence from protected areas

Although biological invasion by alien species is a major contributor to loss of indigenous biological diversity, few studies have examined the susceptibility of the boreal biome to invasion. Based on studies of other ecosystems, we hypothesized that alien plants will be restricted to disturbed areas near human activity and will not be found in natural areas of boreal ecosystems in Gros Morne National Park (Canada), a protected area experiencing a wide range of disturbance regimes. The distribution of alien plants in the region was evaluated using surveys, and study sites were established in naturally and anthropogenically disturbed habitats that had been invaded. Within study sites, randomization tests evaluated the importance of disturbance to alien plant invasion by examining changes in environmental conditions and species abundance within various disturbance regimes, while the importance of site characteristics limiting the distribution of alien plants were examined using Canonical Correspondence Analysis. Consistent with studies in a variety of biomes, areas of high disturbance and human activity had the greatest abundance of resources and the highest percentage of alien species. However, contrary to our hypothesis, natural areas of boreal ecosystems were found susceptible to alien plant invasion. Vegetation types vulnerable to invasion include forests, riparian areas, fens, and alpine meadows. Natural disturbance occurring in these vegetation types caused increases in bare ground and/or light availability facilitating alien plant invasion. Although high soil pH was associated with alien plants in these areas, disturbance was not found to cause changes in soil pH, suggesting susceptibility to invasion is pre-determined by bedrock geology or other factors influencing soil pH. Moose (Alces alces), a non-native herbivore, acts as the primary conduit for alien plant invasion in GMNP by dispersing propagules and creating or prolonging disturbance by trampling and browsing vegetation. The recurrent nature of disturbance within the boreal biome and its interaction with site conditions and herbivores enables alien plants to persist away from areas of high human activity. Managers of natural lands should monitor such interactions to decrease the invasion potential of alien plants.     

Ovipositional host choice by an invader accelerates displacement of its indigenous competitor

While competition for resources leading to invasion success is well recognized, avoidance of competition is much less so. Changes in behaviour that lead to avoidance are usually displayed by the weaker competitor. In our case though it was the stronger competitor that changed its behaviour by choosing a host for oviposition that was unacceptable to its competitor and avoiding one that was mutually acceptable; this accelerated the displacement of the competitor. We showed this by enclosing both invader (Middle East Asia Minor 1, B biotype) and indigenous (Australia, AN biotype) members of Bemisia tabaci species complex (sap sucking insects known as whiteflies) into field cages with either two plants that were a mutually acceptable oviposition host or one host that was acceptable to both and the other acceptable to the invader only. When only the mutually acceptable host was available, invader and indigenous females oviposited equally across the two plants. However, when given the choice, adult invaders still distributed themselves evenly across both hosts, but shifted their oviposition away from the mutually acceptable host and instead laid mostly on the host poorly utilized by the indigenous competitor. This indicates that the invader can change ovipositional choice to escape into competition free space.     

Invasion resistance and persistence: established plants win,even with disturbance and high propagule pressure

Disturbances and propagule pressure are key mechanisms in plant community resistance to invasion, as well as persistence of invasions. Few studies, however, have experimentally tested the interaction of these two mechanisms. We initiated a study in a southwestern ponderosa pine (Pinus ponderosa Laws.)/bunch grass system to determine the susceptibility of remnant native plant communities to cheatgrass (Bromus tectorum L.) invasion, and persistence of cheatgrass in invaded areas. We used a 2 × 2 factorial design consisting of two levels of aboveground biomass removal and two levels of reciprocal seeding. We seeded cheatgrass seeds in native plots and a native seed mixture in cheatgrass plots. Two biomass removal disturbances and sowing seeds over 3 years did not reverse cheatgrass dominance in invaded plots or native grass dominance in non-invaded native plots. Our results suggest that two factors dictated the persistence of the resident communities. First, bottlebrush squirreltail (Elymus elymoides (Raf.) Swezey) was the dominant native herbaceous species on the study site. This species is typically a poor competitor with cheatgrass as a seedling, but is a strong competitor when mature. Second, differences in pretreatment levels of plant-available soil nitrogen and phosphorus may have favored the dominant species in each community. Annual species typically require higher levels of plant-available soil nutrients than perennial plants. This trend was observed in the annual cheatgrass community and perennial native community. Our study shows that established plants and soil properties can buffer the influences of disturbance and elevated propagule pressure on cheatgrass invasion.     

模拟酸雨对入侵植物豚草与伴生种鬼针草竞争关系的影响

植物种间相互作用对物种的生存至关重要。对于入侵植物而言, 更强的种间竞争能力是其成功入侵的重要机制。然而, 环境条件的变化可能会改变种间关系并影响植物入侵的最终结果。该研究主要探索当今严重的环境问题之一——酸雨对中国入侵植物豚草(Ambrosia artemisiifolia)和伴生本地种鬼针草(Bidens bipinnata)种间相互作用的影响, 进一步了解环境扰动对生物入侵的影响。该研究于2021年3月在北京师范大学室外条件下对豚草和鬼针草开展de Wit替代竞争实验, 并施加不同浓度的酸雨模拟溶液(pH = 3、4、5、7), 记录植株在第24、34、45天的株高以及生长季结束时的株高和地上生物量, 通过计算相对邻株效应指数(RNE)并绘制取代系列图表来评估两者的竞争关系。主要结果如下: 单一种植时, 中浓度酸雨(pH = 4)促进了豚草和鬼针草的早期生长, 而高浓度酸雨(pH = 3)显著抑制了两者的早期生长, 但不影响生长季结束时的株高。混合种植时, 高浓度酸雨(pH = 3)显著降低了豚草在第34、45天时的株高, 但在生长季结束时对豚草无影响, 而鬼针草在生长季结束时株高显著降低。鬼针草相对豚草的RNE在任何酸浓度下都不显著。而豚草相对鬼针草的RNE在无酸雨条件下显著, 酸雨处理时在鬼针草比例较低时显著。取代系列实验图表显示, 低浓度酸雨(pH = 5)增加了在低豚草比例下的鬼针草的竞争优势, 而高浓度酸雨增强了豚草的竞争优势。该研究表明酸雨显著影响了豚草和鬼针草的生长及种间竞争关系, 高浓度酸雨增加了入侵植物豚草的竞争优势。     

Demographic Stochasticity, Environmental Variability, and Windows of Invasion Risk for Bythotrephes Longimanus in North America

Biological invasions are a leading threat to freshwater biodiversity worldwide. A central unanswered question of invasion ecology is why some introduced populations establish while most fail. Answering this question will allow resource managers to increase the specificity and effectiveness of control efforts and policy. We studied the establishment of spiny water flea (Bythotrephes longimanus) in the United States and Canada by modeling introduction failure caused by demographic stochasticity, environmental variation, and seasonal environmental forcing. We compared predicted establishment rates with observed invasions of inland lakes in Ontario, Canada. Our findings suggest that environmental forcing can cause “windows” of invasion opportunity so that timing of introductions might be a greater determinant of population establishment than demographic stochasticity and random environmental variation. We expect this phenomenon to be exhibited by species representing a wide range of life histories. For spiny water flea in North America, a large window of invasion opportunity opens around the fourth week of May, persists through the summer, and closes with decreasing water temperatures in autumn. These results show how timing of introductions with respect to seasonally forced environmental drivers can be a key determinant of establishment success. By focusing on introductions during windows of invasion opportunity, resource managers can more effectively control invasion rates.     

Management of indigenous plant-microbe symbioses aids restoration of desertified ecosystems

Disturbance of natural plant communities is the first visible indication of a desertification process, but damage to physical, chemical, and biological soil properties is known to occur simultaneously. Such soil degradation limits reestablishment of the natural plant cover. In particular, desertification causes disturbance of plant-microbe symbioses which are a critical ecological factor in helping further plant growth in degraded ecosystems. Here we demonstrate, in two long-term experiments in a desertified Mediterranean ecosystem, that inoculation with indigenous arbuscular mycorrhizal fungi and with rhizobial nitrogen-fixing bacteria not only enhanced the establishment of key plant species but also increased soil fertility and quality. The dual symbiosis increased the soil nitrogen (N) content, organic matter, and hydrostable soil aggregates and enhanced N transfer from N-fixing to nonfixing species associated within the natural succession. We conclude that the introduction of target indigenous species of plants associated with a managed community of microbial symbionts is a successful biotechnological tool to aid the recovery of desertified ecosystems.     

Management of Indigenous Plant-Microbe Symbioses Aids Restoration of Desertified Ecosystems

Disturbance of natural plant communities is the first visible indication of a desertification process, but damage to physical, chemical, and biological soil properties is known to occur simultaneously. Such soil degradation limits reestablishment of the natural plant cover. In particular, desertification causes disturbance of plant-microbe symbioses which are a critical ecological factor in helping further plant growth in degraded ecosystems. Here we demonstrate, in two long-term experiments in a desertified Mediterranean ecosystem, that inoculation with indigenous arbuscular mycorrhizal fungi and with rhizobial nitrogen-fixing bacteria not only enhanced the establishment of key plant species but also increased soil fertility and quality. The dual symbiosis increased the soil nitrogen (N) content, organic matter, and hydrostable soil aggregates and enhanced N transfer from N-fixing to nonfixing species associated within the natural succession. We conclude that the introduction of target indigenous species of plants associated with a managed community of microbial symbionts is a successful biotechnological tool to aid the recovery of desertified ecosystems.     

The importance of disturbance by fire and other abiotic and biotic factors in driving cheatgrass invasion varies based on invasion stage

Disturbances create fluctuations in resource availability that alter abiotic and biotic constraints. Exotic invader response may be due to multiple factors related to disturbance regimes and complex interactions between other small- and large-scale abiotic and biotic processes that may vary across invasion stages. We explore how cheatgrass responds to both frequency and season of prescribed burning for a 10-year period in ponderosa pine forested stands. To understand interactions of fire disturbance, other abiotic factors, biotic resistance, and propagule pressure, we use long-term data from different spatial scales representing different invasion stages (local establishment or spread and broader scale extent/impact) to model cheatgrass dynamics. We found that after 10 years, cheatgrass cover increased with fall burning regardless of burn frequency (1 burn vs. 3 burns). There was no evidence that cheatgrass invasion is decreasing through time even in areas burned only once. Factors important for explaining local fine-scale cheatgrass establishment and spread, and broader scale extent/impact varied. The spatial extent of the first burns facilitated fine-scale cheatgrass establishment while bare soil cover constrained establishment. Biotic resistance, in the form of native annual forb cover, constrained fine-scale cheatgrass spread. Initial cheatgrass abundance in 2002, a factor related to propagule pressure, was key for explaining the broader scale extent/impact of cheatgrass by 2012. Biotic resistance, in the form of native perennial grass cover, constrained extent/impact but only when initial cheatgrass abundance was low. Our findings regarding factors affecting invasion dynamics may be useful to consider for future restoration and conservation efforts in burned ponderosa pine forests.     

Predicting introduction, establishment and potential impacts of smallmouth bass

Aim The introduction of non‐indigenous species has resulted in wide‐ranging ecological and economic impacts. Predictive modelling of the introduction and establishment of non‐indigenous species is imperative to identify areas at high risk of invasion to effectively manage non‐indigenous species and conserve native populations. Smallmouth bass (Micropterus dolomieu), a warm water fish species native to central North America has negatively impacted native fish communities, including cyprinids and salmonid populations, as a result of intentional introductions. We predicted the introduction risk; species establishment based on habitat suitability; identified lakes at high risk of invasion; and finally assessed the consequential impacts on native salmon, trout and cyprinid populations. Location Ontario and British Columbia, Canada. Methods Classification tree and logistic regression models were developed and validated to predict the introduction and establishment of smallmouth bass for thousands of lakes. Results Densely human populated areas and larger lake surface areas successfully identify lakes associated with the introduction of smallmouth bass (introduction model) in British Columbia. Climate, lake morphology and water chemistry variables were the driving environmental parameters to define suitable smallmouth bass habitat (establishment model). A combination of the introduction and establishment model identified 138 lakes that are currently at risk in British Columbia to the introduction and establishment of smallmouth bass. Of these 138 high‐risk lakes, 95% of them contain at least one species of salmon, trout or cyprinid, thereby increasing the potential impact of an invasion by smallmouth bass. Main conclusions Our framework can be applied to other terrestrial and aquatic species to obtain a better understanding of the potential risk posed by a non‐indigenous species to an ecosystem. Furthermore, our methodology can be used to focus management efforts on areas at higher risk (e.g. number of potential releases, more favourable habitats) to control future introductions of non‐indigenous species, thereby conserving native populations.     

外来种入侵的不确定性动态模拟

外来种的入侵性和生境的可入侵性是研究生物入侵机制的两个关键因素。总结多数学者的研究成果,认为外来种的拓殖率(迁移率或繁殖率)、死亡率及在新生境中的竞争力是衡量外来种能否入侵成功的内在实质。从全新的角度出发,将引入的外来种看作是对原有生态系统的一种干扰,并在此基础上结合Tilman的多物种共存模型提出了外来种干扰模型。模拟发现外来种在入侵时具有明显的不确定性。拓殖率小的外来种在新的生态系统中由于不能适应环境无法成功定殖,被排斥在系统之外;相反,拓殖率足够大的话,外来种由于自身优势具有很强的入侵性,在几年或几十年的时间内就会成功地在新的环境中建立种群并拓殖入侵,影响了当地物种的生存及原有生态系统的稳定。研究还发现,物种入侵也存在一定的不确定性,入侵不一定会一直持续下去,或许在百年或几百年的时间内入侵种会突然灭绝,原生态系统又恢复到原来的水平。     

Does disturbance,competition or resource limitation underlie Hieracium lepidulum invasion in New Zealand? Mechanisms of establishment and persistence,and functional differentiation among invasive and native species

The processes underlying plant invasions have been the subject of much ecological research. Understanding mechanisms of plant invasions are difficult to elucidate from observations, yet are crucial for ecological management of invasions. Hieracium lepidulum, an asteraceous invader in New Zealand, is a species for which several explanatory mechanisms can be raised. Alternative mechanisms, including competitive dominance, disturbance of resident vegetation allowing competitive release or nutrient resource limitation reducing competition with the invader are raised to explain invasion. We tested these hypotheses in two field experiments which manipulated competitive, disturbance and nutrient environments in pre‐invasion and post‐invasion vegetation. H. lepidulum and resident responses to environmental treatments were measured to allow interpretation of underlying mechanisms of establishment and persistence. We found that H. lepidulum differed in functional response profile from native species. We also found that other exotic invaders at the sites were functionally different to H. lepidulum in their responses. These data support the hypothesis that different invaders use different invasion mechanisms from one another. These data also suggest that functional differentiation between invaders and native resident vegetation may be an important contributing factor allowing invasion. H. lepidulum appeared to have little direct competitive effect on post‐invasion vegetation, suggesting that competition was not a dominant mechanism maintaining its persistence. There was weak support for disturbance allowing initial establishment of H. lepidulum in pre‐invasion vegetation, but disturbance did not lead to invader dominance. Strong support for nutrient limitation of resident species was provided by the rapid competitive responses with added nutrients despite presence of H. lepidulum. Rapid competitive suppression of H. lepidulum once nutrient limitation was alleviated suggests that nutrient limitation may be an important process allowing the invader to dominate. Possible roles of historical site degradation and/or invader‐induced soil chemical/microbial changes in nutrient availability are discussed.     

Controls over invasion of Bromus tectorum: The importance of climate,soil, disturbance and seed availability

Question: Predicting the future abundance and distribution of invasive plants requires knowing how they respond to environmental conditions. In arid and semi‐arid ecosystems where water is a limiting resource, environmental conditions and disturbance patterns influence invasions by altering acquisition and utilization of water over space and time. We ask: 1. How do variations in climatic and soil properties influence temporal soil water dynamics? 2. How does this variation affect the establishment of Bromus tectorum (cheatgrass), a cool‐season annual grass that has successfully colonized much of the U.S. Great Basin? Location: Short‐grass Steppe in northeastern Colorado, USA; Arid Lands Ecology reserve in southeastern Washington, USA; and the Patagonian steppe of the Chubut province in Argentina. Methods: We utilized a soil water model to simulate seasonal soil water dynamics in multiple combinations of climatic and soil properties. In addition, we utilized a gap dynamics model to simulate the impact of disturbance regime and seed availability on competition between B. tectorum and native plants. Results: Our results suggest that climate is very important, but that soil properties do not significantly influence the probability of observing conditions suitable for B. tectorum establishment. Results of the plant competition model indicate that frequent disturbance causes more Bromus tectorum in invaded areas and higher seed availability causes faster invasion. Conclusions: These results imply a general framework for understanding Bromus tectorum invasion in which climatic conditions dictate which areas are susceptible to invasion, disturbance regime dictates the severity of invasion and seed availability dictates the speed of invasion.     

Dynamics of Foliage and Thatch Populations of Introduced Pseudomonas fluorescens and Streptomyces sp. on a Fairway Turf

Populations of bacteria (Pseudomonas fluorescens A506 and Streptomyces sp. strain 93) applied to a creeping bentgrass/annual bluegrass fairway turf were followed over time on leaves and thatch. While introduced populations remained at detectable levels over a period of 11–25 days, they usually declined gradually and did not increase after their application to turf. Streptomyces rapidly disappeared from leaves while P. fluorescens was able to maintain similar population sizes on both thatch and leaves, after an initial decline of about 1-log unit, showing that it was actively colonizing the foliage despite loss of biomass from lawn mowing. Throughout these experiments, populations of indigenous microbes on foliage and thatch remained stable, about 10⁶ and 10⁸ cfu g⁻¹ for fungi and bacteria, respectively, and were not affected by the application of bacterial antagonists. Niche-clearing with hydrogen peroxide, which temporarily reduced the population size of indigenous microorganisms two-fold, caused population size of P. fluorescens to increase approximately ten-fold within 24 h, while it declined by about one log unit on untreated turf. It is concluded that the indigenous microflora competes with introduced bacterial antagonists and interferes with their establishment and persistence on turf. Additional studies with P. fluorescens revealed that its population size was inoculum dose-dependent and that solid top dressing was slightly more efficient than spraying liquid suspension in establishing the antagonist. It was possible to maintain P. fluorescens populations above 10⁵ cfu/g of thatch and leaves for 2 weeks or more with both top-dressing or spraying with about 1 to 3 × 10¹⁰ cfu m⁻².     

Prediction and error in multi‐stage models for spread of aquatic non‐indigenous species

Aim Predictions of spread of non‐indigenous species allow for greater efficiency in managing invasions by targeting areas for preventative measures. The invasion sequence is a useful concept in predictions of spread, as it allows us to test hypotheses about the transport and establishment of propagules in novel habitats. Our aims are twofold: (1) to develop and validate multi‐stage invasion models for the introduced fishhook waterflea, Cercopagis pengoi, and (2) to assess how variability in the transport patterns of the propagules influences the accuracy and spatial extent for predictions of spread. Location New York State, USA. Methods We developed a two‐stage model for the spread of C. pengoi. First, we developed a stochastic gravity model for dispersal based on surveys of recreational boat traffic in New York State as a proxy for propagule pressure. We then modelled the probability of establishment based on predicted levels of propagule pressure and measures of lakes’ physicochemistry. In addition, we used Monte Carlo simulations based on the gravity model to propagate variability in boater traffic through the establishment model to assess how uncertainty in dispersal influenced predictions of spread. Results The amount recreationalists were willing to spend, lake area and population size of the city nearest to the destination lake were significant factors affecting boater traffic. In turn, boater traffic, lake area, specific conductance and turbidity were significant predictors of establishment. The inclusion of stochastic dispersal reduced the rate of false positives (i.e. incorrect prediction of an invasion) in detecting invasions at the upper 95% prediction interval for the probability of establishment. Main conclusions Combinations of measures of propagule pressure, habitat suitability and stochastic dispersal allow for the most accurate predictions of spread. Further, multi‐stage spread models may overestimate the extent of spread if stochasticity in early stages of the models is not considered.     

Exotic plant invasions in an endemic‐rich habitat: The spread of an introduced Australian grass,Agrostis avenacea J. F. Gmel., in California vernal pools

Abstract The present abundance and historical spread of an exotic grass, Agrostis avenacea, is documented for California, USA, and for the vernal pools of San Diego County in particular. Agrostis avenacea is native to Australia where it is a common grass of ephemeral and fluctuating wetlands. California vernal pools, by reason of their extreme variability and high endemism, have been thought to be relatively resistant to invasion. The recent expansion of A. avenacea in San Diego County demonstrates, however, that the alleged resistance is probably a function of a relatively small pool of invaders and a low probability of targeted dispersal. Although A. avenacea is now abundant in pools with little current disturbance, human disturbance appears to have been a major factor that facilitated its initial establishment. This specific instance adds to the mounting evidence that there are probably few, if any, habitats immune to invasion.