Establishment Of Invaders Research Articles

Nutrient enrichment undermines invasion resistance to Spartina alterniflora in a saltmarsh: Insights from modern coexistence theory

Abstract The ability of native communities to maintain invasion resistance in the face of environmental changes is crucial for biodiversity conservation. However, previous studies have primarily focused on the inhibitive effects experienced by invaders, providing limited insights into the ultimate outcome of resistance. In this study, we integrated modern coexistence theory into biotic resistance research to investigate the impact of nutrient enrichment on the resistance of Phragmites australis marshes to Spartina alterniflora invasion in the Yangtze estuary saltmarshes of China. Our results demonstrated that under non‐enriched conditions, successful invasion resistance was facilitated by stable coexistence between native and invasive species in the field. This prediction of invasion resistance aligned with the distribution dynamics of the two species in the Yangtze estuary saltmarshes over the past two decades. However, nutrient enrichment was likely to lead to a fundamental shift in their coexistence and ultimately, the failure of resistance. Synthesis and applications. Integrating modern coexistence theory into biotic resistance studies advances the assessment of invasion resistance by shifting from quantifying relative strength to predicting explicit resistance outcomes. Ecosystem managers can draw explicit conclusions about the potential establishment and impact of invaders by analysing observational or experimental data within the framework of modern coexistence theory. This information aids in identifying the most efficient strategies for addressing invasive species.

Species invasiveness and community invasibility of North American freshwater fish fauna revealed via trait-based analysis

While biological invasions are recognized as a major threat to global biodiversity, determining non-native species’ abilities to establish in new areas (species invasiveness) and the vulnerability of those areas to invasions (community invasibility) is challenging. Here, we use trait-based analysis to profile invasive species and quantify the community invasibility for >1,800 North American freshwater fish communities. We show that, in addition to effects attributed to propagule pressure caused by human intervention, species with higher fecundity, longer lifespan and larger size tend to be more invasive. Community invasibility peaks when the functional distance among native species was high, leaving unoccupied functional space for the establishment of potential invaders. Our findings illustrate how the functional traits of non-native species determining their invasiveness, and the functional characteristics of the invaded community determining its invasibility, may be identified. Considering those two determinants together will enable better predictions of invasions.

Successful invasions to freshwater systems double with climate warming

AbstractAlthough invasive species are recognized as a major threat to freshwaters, little is known about how future climate warming will affect the success of invaders. Ecosystems are also often simultaneously impacted by multiple, potentially interacting, invading species and the management of species invasions can be incredibly difficult. Sensitive methods such as environmental DNA (eDNA) metabarcoding, may be key to the early detection and monitoring of invaders. Therefore, to quantify invader success, we performed an outdoor mesocosm experiment mimicking present and future temperatures (IPCC RPC 8.5) combined with a multispecies invasion of planktonic organisms collected from a region with 3–4°C higher mean temperature. 18S eDNA metabarcoding was used to track invaders and impacts on the established eukaryotic community over 22 weeks. We show that invasion success doubled with climate warming compared to present temperatures, implying a substantial increase in successful establishment of invaders in the future. In addition, after the invasion, species richness increased at warming, but not at present, climate conditions. Our quantification of the establishment of invaders in a climate warming scenario, using advanced eDNA methodologies, provides an important step in understanding and predicting how climate warming may affect the success of invasive species in the future, thereby allowing for more informed management recommendations in adapting society to climate change.

Disturbance and nutrient availability drive absinthe (Artemisia absinthium) invasion in a native rough fescue grassland

ABSTRACT Understanding plant invasion is a critical issue in the conservation of native grasslands. Diverse and productive plant communities may inhibit invader establishment via biotic resistance, whereas disturbances may facilitate invasion by weakening resistance. These factors may also interact, yet their combined effects on invasion are unclear. Absinthe (Artemisia absinthium) is an invasive herbaceous species of North American grasslands that can impact biodiversity and forage production, yet little is known about its invasion ecology. To fill this gap, we established an experiment manipulating disturbance via herbicide and fire, as well as resources via fertilizer addition, to explore mechanisms of absinthe invasion into native grassland. We transplanted absinthe seedlings into treated plots, then measured survival and growth along with changes in the plant community. Absinthe survival was approximately 65%, except in undisturbed plots with added fertilizer where it was reduced by 50%. High survival also corresponded with the abundance of other invaders suggesting potential facilitation among invaders. Absinthe growth, conversely, was higher in disturbed plots especially when fertilizer was added. Combined, these results indicate that productive communities may have greater biotic resistance to absinthe invasion but that, when disturbed, these communities are at the greatest risk.

Scale-dependent changes in species richness caused by invader competition

Ecologists have produced substantial literature about the impact of biological invasions on species diversity. However, due to scale-dependent factors, no consistent conclusion has been reached regarding whether biological invasion leads to a loss of species diversity. In this project, we incorporate an invasion scenario into community assembly theories through spatially explicit individual-based models. We explore how invader dominance shifts community richness at local scales and how local-scale extinctions translate to broader scales. To do so, native community dynamics are simulated according to the niche-neutral continuum theory and constructed on a heterogeneous landscape with a limited environmental gradient. We synthetically assume that the establishment of invaders is wholly affected by their niche or fitness difference from native populations. By analyzing richness changes across all sample sizes, we obtained the following results: (1) for randomly distributed habitat resources, the magnitude of richness loss linearly decreases with sample size when strong niche overlap occurs, but the magnitude is more nonlinearly shaped in communities assembled by niche departures; (2) spatially structured native populations could have greater resistance to invasion competition, and if on clustered habitats and the species suffering more niche overlap, then larger extinction events would occur at broader scales; and (3) these invasion modification trends could be linked with native population spatial structures and abundance evenness. The above findings are solely based on simulation experiments and implemented under several synthetic assumptions. This suggests theoretical linkages between invasion scale-dependent influences and mechanistic issues, such as the properties of the invader and invaded communities, which can inform invasive species management and restoration.

The dynamics of disease mediated invasions by hosts with immune reproductive tradeoff

The modern world involves both increasingly frequent introduction of novel invasive animals into new habitat ranges and novel epidemic-causing pathogens into new host populations. Both of these phenomena have been well studied. Less well explored, however, is how the success of species invasions may themselves be affected by the pathogens they bring with them. In this paper, we construct a simple, modified Susceptible-Infected-Recovered model for a vector-borne pathogen affecting two annually reproducing hosts. We consider an invasion scenario in which a susceptible native host species is invaded by a disease-resistant species carrying a vector-borne infection. We assume the presence of abundant, but previously disease-free, competent vectors. We find that the success of invasion is critically sensitive to the infectivity of the pathogen. The more the pathogen is able to spread, the more fit the invasive host is in competition with the more vulnerable native species; the pathogen acts as a ‘wingman pathogen,’ enhancing the probability of invader establishment. While not surprising, we provide a quantitative predictive framework for the long-term outcomes from these important coupled dynamics in a world in which compound invasions of hosts and pathogens are increasingly likely.

Multiple co-occurrent alien invaders constrain aquatic biodiversity in rivers.

A greater understanding and effective management of biological invasions is a priority for biodiversity conservation globally. Many freshwater ecosystems are experiencing the colonization and spread of multiple co-occurrent alien species. Here the implications of both the relative abundance and richness of alien invaders on aquatic macroinvertebrate taxonomic and functional richness, ecosystem quality, and functional redundancy are assessed using long-term data from rivers in England. Based on the most common aquatic invaders, results indicated that their richness, rather than abundance, was the most important factor negatively affecting aquatic macroinvertebrate biodiversity. However, the response of functional redundancy was negatively affected by invader abundance at the river basin scale. The response of communities varied as the number of invading taxa increased, with the most marked reductions following the colonization of the first few invaders. Results indicate that different facets of multiple biological invasions influence distinct aspects of aquatic biodiversity. Preventing the establishment of new invaders and limiting invader taxa richness within a community should therefore be a conservation priority. These findings will assist river scientists in understanding mechanisms driving changes in biodiversity and facilitate the testing of ecological theories while also ensuring environmental managers and regulators can prioritize conservation / management opportunities.

Urban evolution of invasive species

Urban areas can drive rapid evolution of species and are hotspots for entry and establishment of invasive species, but it is unclear how urban evolution is impacting the process and risk of invasion by non‐native species. Using a synthesis of current research as a basis, we contend that urban evolution of non‐native species can enhance invader establishment, spread, and ecological impact, which can facilitate invader success under future global change conditions. In addition, we highlight that ecological opportunity in urban areas is a potentially important but unexplored mechanism that could accelerate invader evolution. Urban evolution of invaders is a critical new area of research that could reveal novel pathways and mechanisms in the invasion process, and improve predictions and management of invasive species around the world.

Using Invasional Meltdown Theory to Understand Patterns of Invasive Richness and Abundance in Forests of the Northeastern USA

Invasional meltdown hypothesizes that invasive species facilitate the establishment of subsequent invaders, with cascading consequences for ecosystem structure and function, including the extirpation of native species. However, meltdown has rarely been tested empirically with large regional data sets, leading to somewhat equivocal support. Recently, the USDA Forest Service initiated sampling of nonnative plants within a subset of their Forest Inventory and Analysis (FIA) plots, presenting an ideal dataset to evaluate meltdown and its consequences across large geographies. We used FIA data from 963 plots across 11 northeastern states to test the predictions of invasional meltdown and to examine the mechanism through which invasion impacts native tree seedling richness. Remeasured plots showed a significant increase in the number and cover of invasive species over time. We provide support for invasional meltdown, finding a pattern of accelerating invasive species richness over 4–5 y in plots with higher initial invasive species richness. Also, we found that plots with higher levels of invasive abundance corresponded to decreased native tree species richness at one point in time and that the magnitude of the relationship appeared to be stronger after 4–5 y. Our results suggest that evidence of invasional meltdown is more clearly evident when examining invasive plant species richness over time; further, invader abundance (measured as cover) is a better predictor of impacts than invader richness, which supports growing evidence that invasive abundance is an important driver of ecological impact.

Darwin’s naturalization conundrum can be explained by spatial scale

Darwin proposed two seemingly contradictory hypotheses regarding factors influencing the outcome of biological invasions. He initially posited that nonnative species closely related to native species would be more likely to successfully establish, because they might share adaptations to the local environment (preadaptation hypothesis). However, based on observations that the majority of naturalized plant species in the United States belonged to nonnative genera, he concluded that the lack of competitive exclusion would facilitate the establishment of alien invaders phylogenetically distinct from the native flora (competition-relatedness hypothesis). To date, no consensus has been reached regarding these opposing hypotheses. Here, following Darwin, we use the flora of the United States to examine patterns of taxonomic and phylogenetic relatedness between native and nonnative taxa across thousands of nested locations ranging in size and extent, from local to regional scales. We find that the probability of observing the signature of environmental filtering over that of competition increases with spatial scale. Further, native and nonnative species tended to be less related in warm, humid environments. Our work provides an empirical assessment of the role of observation scale and climate in biological invasions and demonstrates that Darwin's two opposing hypotheses need not be mutually exclusive.

Gridlock and beltways: the genetic context of urban invasions.

The rapid expansion of urban land across the globe presents new and numerous opportunities for invasive species to spread and flourish. Ecologists historically rejected urban ecosystems as important environments for ecology and evolution research but are beginning to recognize the importance of these systems in shaping the biology of invasion. Urbanization can aid the introduction, establishment, and spread of invaders, and these processes have substantial consequences on native species and ecosystems. Therefore, it is valuable to understand how urban areas influence populations at all stages in the invasion process. Population genetic tools are essential to explore the driving forces of invasive species dispersal, connectivity, and adaptation within cities. In this review, we synthesize current research about the influence of urban landscapes on invasion genetics dynamics. We conclude that urban areas are not only points of entry for many invasive species, they also facilitate population establishment, are pools for genetic diversity, and provide corridors for further spread both within and out of cities. We recommend the continued use of genetic studies to inform invasive species management and to understand the underlying ecological and evolutionary processes governing successful invasion.

Propagule composition regulates the success of an invasive seaweed across a heterogeneous seascape

Abstract Propagule pressure is acknowledged as a key determinant of invasion success. Nonetheless, the role of morphological or physiological attributes of propagules (i.e. their quality) in regulating invader establishment has been little explored. In particular, no study has investigated how the presence of propagules differing in quality within an inoculum influences establishment across heterogeneous landscapes. We experimentally tested the hypothesis that the quality (+Fronds+Rhizoids; +Fronds−Rhizoids; −Fronds+Rhizoids) and the diversity (1, 2 and 3 fragment types) of vegetative fragments of the seaweed Caulerpa taxifolia determine their establishment success across seascapes consisting of bare sediments and patches of the seagrass Zostera muelleri exposed to different disturbance intensities (control, seagrass canopy clipping and total removal). After 6 weeks, seaweed biomass, stolon and frond length, frond and rhizoid number were generally greater in unvegetated habitats (bare sediments and total seagrass removal) than full or reduced seagrass canopies. The type and the diversity of types of fragments inoculated had significant effects on the final biomass and morphological features of C. taxifolia only in vegetated habitats. In control plots, inocula of fragments retaining both fronds and rhizoids achieved higher biomass, developed longer stolons and more fronds. In canopy clipping plots, mixed inocula of +Fronds+Rhizoids and −Fronds+Rhizoids fragments had the greatest biomass and stolon length. Synthesis. Assessing how propagules differing in quality perform in different habitats might be not sufficient to draw a comprehensive picture of invasion risk, as their establishment can be modulated by both negative and positive interactions among them. Propagule composition should be, therefore, considered as a further dimension of propagule pressure. Our results also suggest that the relevance of specific propagule traits for invader establishment decreases from intact to degraded habitats. Considering propagule size in terms of amount of competent propagules, rather than an absolute measure, would refine our ability of predicting invasion risk across habitats differing in biotic or abiotic conditions.

Current and projected future risks of freshwater fish invasions in China

Biological invasions are a primary threat to global biodiversity, supporting mounting calls for the development of early‐warning systems to manage existing and emerging invaders. Here, we evaluated the geographical pattern of invasion risks of currently established and potentially emerging nonnative freshwater fishes in China by jointly considering the threats of introduction and establishment under climate change. Introduction threats were estimated according to proxies of human activities and propagule pressure for two primary pathways (aquaculture or ornamental). Establishment threats for 51 current and 64 potential invaders (based on whether having established or not self‐sustaining populations) were assessed using an ensemble of species distribution models under current (1960–1990) and future [2041–2060 (2050s) and 2061–2080 (2070s)] climate scenarios. Geographical patterns of invasion risk were then assessed by overlaying the threats of introduction and establishment for each species group both in present‐day and in the future. We found that eastern China displayed the highest threat of introduction. By contrast, southeastern and northwestern regions were identified as the most suitable for the establishment of both current and potential invaders. Under a changing climate, 83 out of 115 species displayed an increase in habitat suitability, resulting in an overall increase of 4.8% by 2050s and 7.1% by 2070s in the extent of suitable habitat for nonnative freshwater fishes. Taken together, invasion risk was found to be highest in southeastern China and lowest in the Tibet Plateau. Our research highlights the importance of assessing invasion risk by integrating the threats associated with the introduction and establishment stages. In particular, our findings revealed convergent patterns of invasion risk between current and potential nonnative freshwater fishes under climate change. Geographic patterns in hotspots of existing and emerging invasions provide critical insights to guide the allocation of resources to monitor and control existing and emerging invasions in China.

Differences in the abilities of native and invasive amphipods to tolerate poor water quality and recolonise degraded habitats

It has been hypothesised that freshwater pollution may favour the establishment of tolerant invaders at the expense of less-tolerant residents. To test this, this study capitalises on the aftermath of a farm pollution incident, a slurry discharge to a small British stream, which eliminated the resident macroinvertebrate assemblage. Stream transplant bioassay experiments at the discharge site revealed a higher pollution tolerance of the invasive amphipod Crangonyx pseudogracilis and the isopod Asellus aquaticus than that of the native amphipod Gammarus duebeni celticus and the invasive Gammarus pulex. A. aquaticus recolonised the stream up to the discharge point within days, followed by C. pseudogracilis, then G. pulex, with G. duebeni celticus undetected after a month. Values of a macroinvertebrate-derived biotic index of organic water quality, the Average Score Per Taxon (ASPT), gradually increased up to the discharge point in the 30 days following the incident. In a larger-scale survey (72 river and 12 pond sites), inclusion/exclusion of Gammarus spp. had no effect on ASPT values, while inclusion of C. pseudogracilis increased values, despite an assumed equivalence of these amphipods in the index. Pollution could facilitate the establishment of tolerant invaders, and this has implications for biomonitoring.

Boater Knowledge and Behavior Regarding Aquatic Invasive Species at a Boat Wash Station

Overland movement of recreational boaters between waterbodies is a significant pathway for spreading aquatic invasive species (AIS). Boater awareness of AIS and proper boat cleaning procedures may prove beneficial in reducing the transport, introduction, and establishment of invaders. Our objectives were to observe the use and determine the efficacy of a public boat wash station, gain a better understanding of boater knowledge on AIS and boat cleaning procedures, and help guide future educational outreach and AIS management. We conducted a three-tier human subjects survey at Tenmile Lake, Oregon over a two-year period in which a boat wash station was installed, allowing comparison of boater behavior and attitudes before and after the station was operational. The survey consisted of an observational survey, a short form (quick answer) survey, and an in-depth survey in which boaters could elaborate on their knowledge of AIS and regulations. The comparison between boaters surveyed before and after the installation of a boat wash station indicated discrepancies between what boaters say they will do and how they behave. While 76% of boaters said they would use a boat wash station prior to its installation, only 39% of boaters were observed using the station after it was installed. Additionally, the surveys identified knowledge gaps of boater AIS awareness, with > 20% of boaters unable to name any AIS. Our research indicates that even with innovative tools and targeted outreach, knowledge of AIS is still limited among the boating community and other education and management techniques need exploration.

Ensemble forecasting of the global potential distribution of the invasive Chinese mitten crab, Eriocheir sinensis

Invasive alien species have posed substantial threats to freshwater ecosystems and are generally difficult and cost-intensive to eradicate once established. Therefore, it is of great importance to identify their potential distribution and take preventive actions. Species distribution modelling has been regarded as a powerful tool to identify areas that provide suitable environmental conditions for the establishment of invaders. In this study, an ensemble species distribution model was developed for the Chinese mitten crab (Eriocheir sinensis), by using 188 worldwide occurrence records and 11 environmental variables to predict its global potential distribution in freshwater habitats. The ensemble model showed high predictive performance and indicated that annual mean temperature was the most important environmental variable that limits the distribution of this invasive crustacean. The model successfully predicted the known distribution of Chinese mitten crab in its native range and invasive ranges and suggested that this invader has not yet fully realized its potential distribution in Europe and North America. The model also predicted some additional suitable areas where this crab has not yet been observed (Japan, part of South America, Australia, and New Zealand). Management strategies are proposed to control this invasive crab in light of these results.

Native granivores reduce the establishment of native grasses but not invasive Bromus tectorum

Seed predation can structure plant communities by imposing strong population controls on some species but not others. In this context, studies from various ecosystems report that native granivores selectively forage for seeds from native species over seeds from exotic invaders, which could disproportionately favor the establishment of invaders and facilitate their dominance in communities. However, few studies have connected selective foraging for native seeds to differential patterns of establishment among native and invasive species. Thus, the extent to which preferential foraging for native seeds favors the establishment of invasive plants is unclear. Here, we used experimental seed additions and exclosure treatments at five field sites distributed across ≈ 80,000 km2 of the Great Basin Desert, USA to compare the effects of rodent foraging on the establishment of less-preferred cheatgrass (Bromus tectorum—an annual species native to Eurasia that is exotic and highly invasive across the Great Basin) and four species of more-preferred native grasses that commonly co-occur with cheatgrass. Rodent foraging reduced the establishment of each native species by at least 80% but had no effect on the establishment of cheatgrass, and this finding was consistent across study sites. Our results suggest that selective foraging for native species may favor the establishment of cheatgrass over native grasses, potentially exacerbating one of the most extensive plant invasions in North America.

Does local isolation allow an invasive thistle to escape enemy pressure?

Enemy release often is invoked to explain the success of invasive plants: an invader benefits from reduced attack as it escapes specialized enemies through the invasion process. Enemy release typically is thought of as occurring at large geographic scales, but local-scale interactions may also be important for invader establishment and success. Furthermore, most tests of local enemy release have been conducted over a single year even though release may be a transient phenomenon, especially at small scales. In this study, we used a multi-year field experiment to investigate whether locally isolated populations of the noxious non-native weed Cirsium arvense benefit from reduced levels of aboveground damage, and whether any initial advantage is lost over subsequent growing seasons. Populations of C. arvense were grown in plots at set distances from established source populations for 4years. In the first year of the experiment, folivory significantly declined with host isolation, but damage from specialist stem gallers and seed predators did not. However, in subsequent years of observation, folivores began colonizing isolated C. arvense populations while stem gallers exhibited very slow colonization of more isolated plots; seed predation showed no pattern with distance in any year. Local enemy escape did not result in increased plant performance, which instead negatively correlated with degree of isolation. Nonetheless, our results stress the importance of multi-year observations in tests of enemy release, since the herbivory patterns initially observed often changed within subsequent years depending on the dispersal ability and biology of the causal organism involved.

Biodiversity influences invasion success of a facultative epiphytic seaweed in a marine forest

The biotic resistance hypothesis predicts that more diverse communities should have greater resistance to invasions than species-poor communities. However for facultative and obligate epiphytic invaders a high native species richness, abundance and community complexity might provide more resources for the invader to thrive to. We conducted surveys across space and time to test for the influence of native algal species abundance and richness on the abundance of the invasive facultative epiphytic filamentous alga Lophocladia lallemandii in a Mediterranean Cystoseira balearica seaweed forest. By removing different functional groups of algae, we also tested whether these relationships were dependent on the complexity and abundance of the native algal community. When invasion was first detected, Lophocladia abundance was positively related to species richness, but the correlation became negative after two years of invasion. Similarly, a negative relationship was also observed across sites. The removal experiment revealed that more complex native communities were more heavily invaded, where also a positive relationship was found between native algal richness and Lophocladia, independently of the native algal abundance. Our observational and experimental data show that, at early stages of invasion, species-rich seaweed forests are not more resistant to invasion than species-poor communities. Higher richness of native algal species may increase resource availability (i.e. substrate) for invader establishment, thus facilitating invasion. After the initial invasion stage, native species richness decreases with time since invasion, suggesting negative impacts of invasive species on native biodiversity.

Mechanisms of Invasion Resistance of Aquatic Plant Communities.

Invasive plant species are among the major threats to freshwater biodiversity. Few experimental studies have investigated whether native plant diversity can provide biotic resistance to invaders in freshwater ecosystems. At small spatial scales, invasion resistance may increase with plant species richness due to a better use of available resources, leaving less available for a potential invader (Complementarity effect) and/or the greater probability to have a highly competitive (or productive) native species in the community (Selection effect). In submerged aquatic plant communities, we tested the following hypotheses: (1) invader establishment success is greatest in the absence of a native plant community; (2) lower in plant communities with greater native species richness, due to complementary and/or selection effects; and (3) invader establishment success would be lowest in rooted plant communities, based on the limiting similarity theory as the invader is a rooted submerged species. In a greenhouse experiment, we established mesocosms planted with 0 (bare sediment), 1, 2, and 4 submerged plant species native to NW Europe and subjected these to the South African invader Lagarosiphon major (Ridl.) Moss. We used two rooted (Myriophyllum spicatum L., Potamogeton perfoliatus L.) and two non-rooted native species (Ceratophyllum demersum L., Utricularia vulgaris L.) representing two distinct functional groups considering their nutrient acquisition strategy which follows from their growth form, with, respectively, the sediment and water column as their main nutrient source. We found that the presence of native vegetation overall decreased the establishment success of an alien aquatic plant species. The strength of this observed biotic resistance increased with increasing species richness of the native community. Mainly due to a selection effect, the native biomass of mixed communities overyielded, and this further lowered the establishment success of the invader in our experiment. The strongest biotic resistance was caused by the two native plant species that were of the same functional group, i.e., functionally most similar to the invader. These results support the prediction of Elton’s biotic resistance hypothesis in aquatic ecosystems and indicate that both species richness and functional group identity can play an important role in decreasing establishment success of alien plant species.