Spatial Segregation of Congeneric Invaders in Central Pennsylvania, USA

Carduus acanthoides and Carduus nutans (plumeless and musk thistles) are among the most noxious weeds in the United States of America, presenting a serious challenge in cropping and pasture systems. Unfortunately, a lack of detailed spatial distribution information hampers both our ability to understand the factors affecting their invasive success, and the effectiveness of monitoring and management efforts. To examine patterns of distribution and co-occurrence at a local level, we sampled a 5000 km² area of central Pennsylvania that cut a transect across known areas of C. acanthoides and C. nutans infestation. A number of potential environmental explanatory variables were recorded and analyzed to examine whether they correlated with observed species distribution patterns. Patterns of forest density and spatial aggregation of the thistles were the primary covariates that significantly impacted both species’ distributions. The survey established that the frequency of sightings for each species diminished as the ranges converged, with only brief overlap: the two species are strongly negatively correlated in space. Understanding environmental correlates of infestation and the pattern of spatial dissociation of these two invasive species is an important step towards an improved understanding of the mechanisms underlying their invasive potential, and hence towards effective weed control.     

Biogeographical contrasts to assess local and regional patterns of invasion: a case study with two reciprocally introduced exotic maple trees

Quantitative comparisons of distribution and abundance of exotic species in their native and non‐native ranges represent a first step when studying invaders. However, this approach is rarely applied 2 particularly to tree species. Using biogeographical contrasts coupled with regional dispersal surveys, we assessed whether two exotic maple tree species, Acer negundo and Acer platanoides, can be classified as invasive in the non‐native regions surveyed. We also examined the importance of biogeography in determining the degree of invasion by exotic species using this reciprocal approach. Local‐scale surveys were conducted in a total of 34 forests to compare density, relative abundance, age structure of native and introduced populations, and whether the two introduced maple species negatively affected native tree species density. Regional‐scale surveys of a total of 136 forests were then conducted to assess distribution in the introduced regions. Introduced populations of A. negundo were denser than populations measured in their native range and negatively related to native tree species density. Age structure did not differ between regions for this species. At the regional scale, this species has invaded most of the riparian corridors sampled in France. Conversely, the density of A. platanoides introduced populations was similar to that of native populations and was not related to native tree species density. Although seedling recruitment was higher away than at home, this species has invaded only 9% of the forests sampled in southern Ontario, Canada. Although reported invasive, these two exotic maple species differed in their relative demographic parameters and regional spread. Acer negundo is currently invasive in southern France while A. platanoides is not aggressively invasive in southern Ontario. Importantly, this study effectively demonstrates that biogeography through structured contrasts provide a direct means to infer invasion of exotic species.     

Suitable areas for the invasion expansion of Xylocopa bees in South America

The introduction of exotic species into native ecosystems can be a cause for concern when those species are invasive. Invasive species cause ecological problems and have socio-cultural impacts on human health and the economy; for example, invasive bees may negatively impact their introduced ecosystem by spreading diseases or outcompeting native pollinators. Xylocopa spp. bees are diverse and distributed throughout the Neotropics. However, Xylocopa augusti (Lepeletier, 1841) and Xylocopa splendidula (Lepeletier, 1841) are not native to Mediterranean Chile. This study aimed to evaluate the invasive potential of these exotic species and predict the potential macroecological effects of their invasions. We also aimed to pinpoint possible distributions for these species throughout South America. We correlated biogeographic occurrence data with climatic variables for each species to model their potential distribution in both current and future scenarios. The models provide strong evidence that both species are changing their distributions: their ranges are expanding towards western South America, particularly Bolivia, Chile and Peru. We demonstrate an increase in niche overlap between these species and show there are new geographic areas vulnerable to the establishment of these invasive bees under current and future climate conditions. These data suggest that these bees may adapt their geographic distribution as the climate changes and pose a threat to native pollinators in new geographic areas.     

Influence of insects and fungal pathogens on individual and population parameters of <Emphasis Type="Italic">Cirsium arvense</Emphasis> in its native and introduced ranges

Introduced weeds are hypothesized to be invasive in their exotic ranges due to release from natural enemies. Cirsium arvense (Californian, Canada, or creeping thistle) is a weed of Eurasian origin that was inadvertently introduced to New Zealand (NZ), where it is presently one of the worst invasive weeds. We tested the ‘enemy release hypothesis’ (ERH) by establishing natural enemy exclusion plots in both the native (Europe) and introduced (NZ) ranges of C. arvense. We followed the development and fate of individually labelled shoots and recorded recruitment of new shoots into the population over two years. Natural enemy exclusion had minimal impact on shoot height and relative growth rate in either range. However, natural enemies did have a significant effect on shoot population growth and development in the native range, supporting the ERH. In year one, exclusion of insect herbivores increased mean population growth by 2.1–3.6 shoots m⁻², and in year two exclusion of pathogens increased mean population growth by 2.7–4.1 shoots m⁻². Exclusion of insect herbivores in the native range also increased the probability of shoots developing from the budding to the reproductive growth stage by 4.0× in the first year, and 13.4× in the second year; but exclusion of pathogens had no effect on shoot development in either year. In accordance with the ERH, exclusion of insect herbivores and pathogens did not benefit shoot development or population growth in the introduced range. In either range, we found no evidence for an additive benefit of dual exclusion of insects and pathogens, and in no case was there an interaction between insect and pathogen exclusion. This study further demonstrates the value of conducting manipulative experiments in the native and introduced ranges of an invasive plant to elucidate invasion mechanisms.     

Exotic thistles increase native ant abundance through the maintenance of enhanced aphid populations

Exotic species change the structure and composition of invaded communities in multiple ways, but the sign of their impact on native species is still controversial. We evaluated the effects of the thistles Carduus thoermeri and Onopordum acanthium—two of the most abundant exotic plant species in disturbed areas of the Patagonian steppe—on the native tending ant assemblage. Exotic thistles showed an increased number of plants with aphids and had greater aphid density than native plants. Since native tending ants were present only in plants with aphids, their abundance was higher in infested thistles than in native plants. Path analyses confirmed that ant activity depended more on aphid density than on thistle traits. Our results suggest that the presence of exotic thistles in disturbed areas of NW Patagonia indirectly benefit the native ant assemblage through the maintenance of an increased aphid population. This illustrates how the impact of exotic on native species can depend on the ecological context.     

Seed predators and the evolutionarily stable flowering strategy in the invasive plant, Carduus nutans

In plants where reproduction is fatal, seed-feeding insects may have a major impact on the evolutionarily stable reproductive strategy by altering fecundity schedules in a size-dependent manner. We explored this in Carduus nutans, a facultative biennial native to Europe, using two years of data from the South of France. An integral projection model based on detailed statistical models of the demography of Carduus nutans and characteristics of herbivore attack showed that seed predators select for smaller flowering size. An elasticity analysis showed that changes in the slope relating seed herbivore attack rates to plant total receptacle area had a large effect on lifetime reproductive success relative to most other plant demographic rates. Together, these two results indicate that in the absence of seed predators, as is the case in the exotic range of this invasive species, flowering size could evolve to be larger. Further analysis also showed that subsequent introduction of different species of seed-feeders as biocontrol agents could lead to different evolutionary outcomes dependent on the ecological characteristics of the seed-feeders, allowing the direction and magnitude of evolutionary change in flowering size to be predicted based on what seed predators have been introduced where and when. Such data would allow us to distinguish between the effect of seed predators and other hypotheses for size increase in the invasive habitat.     

What makes a weed a weed: life history traits of native and exotic plants in the USA

I compared ten life history traits (vegetative reproduction, breeding system, compatibility, pollination system, shade tolerance, habitat, life span, life form, morphology, and toxicity) from two existing databases for the 19,960 plant species that occur in the USA. I used two-way tests of independence to determine if there were significant life history traits that distinguish weeds from non-weeds, exotic weeds from native weeds, and invasive exotic weeds from non-invasive exotic weeds. Life span was the most significant life history trait for weeds in general; weeds were more likely to be annuals and biennials and less likely to perennials than non-weeds. In addition, vegetative reproduction, breeding system, compatibility, shade tolerance, and life form were related to life span. Annual and biennial weeds (whether native, exotic, or exotic invasives) were more likely to be wetland adapted, armed, and toxic than annual or biennial non-weeds. Perennial weeds (whether native, exotic, or exotic invasives) were less likely to be forbs or subshrubs, and more likely to be wetland adapted, toxic, shade intolerant, grasses, vines and trees than perennial non-weeds. Exotic annual and perennial weeds were less likely to be wetland species than native weeds, but more likely to be wetland species than non-weeds. Invasive exotic weeds, in contrast, were less likely to be forbs and more likely to be perennial, monoecious, self-incompatible, and trees and than non-invasive exotics.     

Classical Biological Control of Nodding and Plumeless Thistles

Nodding (musk) thistle (Carduus thoermeri Weinmann in the Carduus nutans L. group) and plumeless thistle (Carduus acanthoides L.) are introduced noxious weeds of Eurasian origin. Both weeds are problematic in pastures, rangelands, and croplands and along state highways in many parts of the United States. The success of both species of thistles is largely due to their prolific seed production, seed longevity, competitive ability, and lack of natural enemies. Classical biological control of nodding thistle in Virginia has been achieved with three exotic thistle herbivores, Rhinocyllus conicus Froelich (Coleoptera: Curculionidae), Trichosirocalus horridus (Panzer) (Coleoptera: Curculionidae), and Cassida rubiginosa Müller (Coleoptera: Chrysomelidae). T. horridus also effectively controls plumeless thistle. These insect herbivores complement each other. Nodding thistle biological control is achieved in about 5–6 years in Virginia, Missouri, and Montana. In addition, a rust fungus (Puccinia carduorum Jacky) (Uredinales: Pucciniaceae) has been introduced and established for control of nodding thistle in Virginia. Development and reproduction of the three thistle herbivores are not adversely affected by the rust. The rust hastens plant senescence and reduces seed production. Control of plumeless thistle with R. conicus and T. horridus takes approximately twice as long as control of nodding thistle.     

Facilitative interactions between an exotic mammal and native and exotic plants: hog deer (<Emphasis Type="Italic">Axis porcinus</Emphasis>) as seed dispersers in south-eastern Australia

Endozoochory by exotic mammalian herbivores could modify vegetation composition by facilitating the dispersal and establishment of exotic and native plant species. We examined the potential for endozoochoric dispersal of native and exotic plants by exotic hog deer (Axis porcinus) in south-eastern Australia. We quantified the germinable seed content of hog deer faecal pellets collected in five vegetation types within a 10,500-ha study area that was representative of their Australian range. Twenty exotic and 22 native species germinated from hog deer faecal pellets and significantly more native species germinated compared to exotic species. Seedlings of the encroaching native shrub Acacia longifolia var. sophorae emerged, but no native trees emerged and the percentage of grasses that germinated was low (11%). The species composition of germinants was similar among the five vegetation types. We estimated that the hog deer population in our study area could potentially disperse >130,000 viable seeds daily. Our study shows how an exotic mammal can disperse seeds from both native and invasive plants and highlights the need for endozoochory to be considered more widely in studies assessing the impacts of exotic mammals on plant communities.     

外来杂草入侵的化学机制

由外来杂草入侵引发的严重生态和经济问题已倍受关注,外来杂草在新生境成功入侵,除了具备一些基本的生物生态学特征外,还应具备一些特有的入侵机制,阐明外来杂草的各种入侵机制可以为入侵杂草的预测和控制提供科学依据。外来杂草只有在新生境中与原产地生物种间的相互作用中取得优势,才能定植并扩增种群而成功入侵．在这些外来杂草和原产地生物种间的相互作用关系中,化学关系是不可忽视的方面．目前研究已经证实：植物的化感作用在外来杂草成功入侵中发挥着重要的作用．事实上,植物也可以通过合成和释放特定的化学物质防御或抑制新生境的动物、植物和微生物．外来杂草入侵的化学机制涉及到植物化学生态学的各个方面。因此,外来杂草的化学生态学特征应作为入侵种预测的重要指标之一,外来杂草入侵的化学机制应是今后重要的研究方向。     

Mycorrhizal associations of an invasive tree are enhanced by both genetic and environmental mechanisms

Biotic interactions involving exotic plants in their introduced ranges may differ from those of co‐occurring plant species and from interactions in their native ranges. When interactions are less negative, or more positive compared to native plant species, this may increase invasion success, and differences among ranges may cause changes in exotic plant traits. Here, we investigated arbuscular mycorrhizae (AM) associated with Triadica sebifera seedlings from populations in native (China) and introduced ranges (US) and with seedlings from US and China species within three co‐occurring genera (Liquidambar, Ulmus, Celtis) grown in multiple common gardens in both ranges. No general pattern of higher or lower AM colonization was found in the introduced range for China and US Celtis, Liquidambar, or Ulmus species. However, AM colonization was significantly higher for Triadica than for other genera, particularly in the introduced range, suggesting AM may improve Triadica's invasion success. Triadica AM colonization was higher in US than China gardens, decreased with increasing soil nitrogen in China, but was independent of soil nitrogen in the US. This might reflect a different effect of soil fertility on this mutualism among ranges. Introduced Triadica populations had higher AM colonization than native populations, particularly in US gardens, implying a possible advantage from greater AM association in the introduced range. This is the first field study demonstrating post‐introduction changes in mycorrhizal colonization of an invasive species. It indicates that there are ecological and evolutionary components to the effect of positive interactions on plant invasions.     

Integrating Management Techniques to Restore Sites Invaded by Mile‐a‐Minute Weed,Persicaria perfoliata

Efforts to suppress an invasive weed are often undertaken with the goal of facilitating the recovery of a diverse native plant community. In some cases, however, reduction in the abundance of the target weed results in an increase in other exotic weeds. Mile‐a‐minute weed (Persicaria perfoliata (L.) H. Gross (Polygonaceae)) is an annual vine from Asia that has invaded the eastern United States, where it can form dense monocultures. The host‐specific Asian weevil Rhinoncomimus latipes Korotyaev (Coleoptera: Curculionidae) was first released in the United States in 2004 as part of a classical biological control program. At three sites invaded by mile‐a‐minute weed, biological control was integrated with pre‐emergent herbicide use and two densities of native plantings. After 2 years, native plant cover differed significantly and was greater than 80% in the plots with plantings and pre‐emergent herbicide but less than 30% in the planting treatments without herbicide. Where mile‐a‐minute cover decreased at the two sites with the greatest pressure from exotic plants, plots were dominated by another exotic weed, Microstegium vimineum (Trin.) A. Camus, Japanese stiltgrass. The combination of biocontrol, pre‐emergent herbicide, and revegetation with native plants suppressed mile‐a‐minute weed, prevented invasion by Japanese stiltgrass, and increased the abundance of native plants. The selection of the management strategies used to control mile‐a‐minute weed determined the extent of recovery of the native plant community.     

Do invasive freshwater fish species grow better when they are invasive?

A frequent assumption in invasion ecology is that invasive species have enhanced growth rates in their invasive ranges. However, invasions frequently occur in sub‐tropical and tropical environments where growth could be higher simply due to climatic conditions rather than novel habitat. In this study, a meta‐analysis of growth rates (length‐at‐age data) was completed for six invasive freshwater fish species: common carp Cyprinus carpio, largemouth bass Micropterus salmoides, brown trout Salmo trutta, brown bullhead Ictalurus nebulosus, flathead catfish Pylodictis olivaris and northern snakehead Channa argus. Significant effects of climate on growth were observed for all species except common carp, and following normalization of growth for climate effects, a range of growth responses between native and invasive populations were revealed. Two species (brown trout, flathead catfish) showed significantly increased growth rates in invasive compared to native ranges, but two species (common carp, largemouth bass) showed significantly faster growth in native ranges, and two other species (northern snakehead, brown bullhead) showed no difference in growth rates. No species showed both significantly enhanced growth rates and initial sizes in invasive compared to native ranges. Using the comparative method, countergradient growth variations were apparent for all species within their native ranges and for all but one species in invasive ranges. Invasive populations of freshwater fish do not always grow faster when invasive and future studies need to consider growth covariates (e.g. climate and countergradient growth) prior to comparing life‐history differences between invasive and native populations.     

Trophic cascades in an invaded ecosystem: native keystone predators facilitate a dominant invader in an estuarine community

Determining how various factors contribute to the invasibility of systems is essential for both understanding community formation and informing management of natural areas. Research demonstrating that predators can provide biotic resistance to invasions by consuming invasive species has led to the presence of healthy predator populations being associated with reduced invasion potential of ecosystems. However, predators structure communities in many ways and their presence could also potentially facilitate invasions if they decrease populations of native species that compete with or consume an invader. We considered these two impacts of predators on invasion by analyzing the effects of two keystone predators (Pisaster spp. and Enhydra lutris nereis) on two foundation species (a native mussel Mytilus californianus and the invasive exotic bryozoan Watersipora subtorquata, a putative competitor for space with Mytilus californianus). Both native predators were found to facilitate the invasion of the exotic bryozoan, and the rate of invasion was highest when both predators were present. Facilitation of W. subtorquata occurred via indirect mechanisms that partly involved the removal of a competitor (mussels) via predation. These results illustrate that although predators can provide biotic resistance to invasion, healthy predator populations do not always confer this advantage and in fact may facilitate invasions. Therefore, implementation of management actions to enhance populations of top predators could also potentially increase the invasibility of some ecosystems.     

Removal of invasive shrubs reduces exotic earthworm populations

Invasive species are a leading threat to native ecosystems, and research regarding their effective control is at the forefront of applied ecology. Exotic facilitation has been credited with advancing the success of several aggressive invasive species. Here, we suggest using the knowledge of exotic facilitations to control invasive earthworm populations. In northern hardwood forests, the invasive shrubs Rhamnus cathartica (buckthorn) and Lonicera x bella (honeysuckle) produce high quality leaf litter, and their abundance is positively correlated with exotic earthworms, which increase nutrient cycling rates. We performed an invasive plant removal experiment in two northern hardwood forest stands, one dominated by buckthorn and the other by honeysuckle. Removal of invasive shrubs reduced exotic earthworm populations by roughly 50% for the following 3 years. By targeting invasive species that are part of positive feedback loops, land managers can multiply the positive effects of invasive species removal.     

Dispersal and recruitment limitation in native versus exotic tree species: life‐history strategies and Janzen‐Connell effects

Life‐history traits of invasive exotic plants are typically considered to be exceptional vis‐à‐vis native species. In particular, hyper‐fecundity and long range dispersal are regarded as invasive traits, but direct comparisons with native species are needed to identify the life‐history stages behind invasiveness. Until recently, this task was particularly problematic in forests as tree fecundity and dispersal were difficult to characterize in closed stands. We used inverse modelling to parameterize fecundity, seed dispersal and seedling dispersion functions for two exotic and eight native tree species in closed‐canopy forests in Connecticut, USA. Interannual variation in seed production was dramatic for all species, with complete seed crop failures in at least one year for six native species. However, the average per capita seed production of the exotic Ailanthus altissima was extraordinary: > 40 times higher than the next highest species. Seed production of the shade tolerant exotic Acer platanoides was average, but much higher than the native shade tolerant species, and the density of its established seedlings (≥ 3 years) was higher than any other species. Overall, the data supported a model in which adults of native and exotic species must reach a minimum size before seed production occurred. Once reached, the relationship between tree diameter and seed production was fairly flat for seven species, including both exotics. Seed dispersal was highly localized and usually showed a steep decline with increasing distance from parent trees: only Ailanthus altissima and Fraxinus americana had mean dispersal distances > 10 m. Janzen‐Connell patterns were clearly evident for both native and exotic species, as the mode and mean dispersion distance of seedlings were further from potential parent trees than seeds. The comparable intensity of Janzen‐Connell effects between native and exotic species suggests that the enemy escape hypothesis alone cannot explain the invasiveness of these exotics. Our study confirms the general importance of colonization processes in invasions, yet demonstrates how invasiveness can occur via divergent colonization strategies. Dispersal limitation of Acer platanoides and recruitment limitation of Ailanthus altissima will likely constitute some limit on their invasiveness in closed‐canopy forests.     

Population Ecology of Hybrid Mesquite (Prosopis Species) in Western Australia: How Does it Differ from Native Range Invasions and What are the Implications for Impacts and Management?

Beneficial exotic trees and shrubs have been widely spread throughout semiarid and arid regions of the world. These trees and shrubs can however cause severe negative impacts. Mesquite (Prosopis species), native to the New World, is one example which continues to be promoted despite causing serious impacts both in its native and introduced ranges. We describe the population structure of the largest population of fire-tolerant hybrid mesquite (P. velutina × P. glandulosa var. glandulosa × P. pallida) in Australia, which was intentionally established in the 1930s. We compare it with invasive populations within its native range, and consider the implications for managing exotic mesquite invasions. We found relatively high juvenile densities at all levels of canopy cover (<30% to 90–100%), and low mortality rates for both juveniles and adults (<2%/y), which suggests that populations are still in an early phase of invasion. Exotic populations differed from native range populations in being more dense (average 4,859 adults/ha), having a sizable sapling (seedling and juvenile) bank that can remain quiescent under canopy cover (average 10,914 seedlings and juveniles/ha), failing to act as nurse plants for native shrubs (<8 native shrubs/ha), and almost totally excluding the herbaceous (grass) layer (average 0.3% cover). Our results suggest that ecosystem impacts in the introduced range are likely to be even worse, and management even more difficult, than has already been reported for invasions within its native range. The lack of feasible means for managing highly invasive, broad-scale mesquite populations need to be addressed, and needs to be considered explicitly when promoting mesquite as a beneficial plant.     

Effects of generalist herbivory on resistance and resource allocation by the invasive plant,Phytolacca americana

Successful invasions by exotic plants are often attributed to a loss of co‐evolved specialists and a re‐allocation of resources from defense to growth and reproduction. However, invasive plants are rarely completely released from insect herbivory because they are frequently attacked by generalists in their introduced ranges. The novel generalist community may also affect the invasive plant's defensive strategies and resource allocation. Here, we tested this hypothesis using American pokeweed (Phytolacca americana L.), a species that has become invasive in China, which is native to North America. We examined resistance, tolerance, growth and reproduction of plant populations from both China and the USA when plants were exposed to natural generalist herbivores in China. We found that leaf damage was greater for invasive populations than for native populations, indicating that plants from invasive ranges had lower resistance to herbivory than those from native ranges. A regression of the percentage of leaf damage against mass showed that there was no significant difference in tolerance between invasive and native populations, even though the shoot, root, fruit and total mass were larger for invasive populations than for native populations. These results suggest that generalist herbivores are important drivers mediating the defensive strategies and resource allocation of the invasive American pokeweed.     

The roles of leporid species that have been translocated: a review of their ecosystem effects as native and exotic species

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Different climatic envelopes among invasive populations may lead to underestimations of current and future biological invasions

Aim We explore the impact of calibrating ecological niche models (ENMs) using (1) native range (NR) data versus (2) entire range (ER) data (native and invasive) on projections of current and future distributions of three Hieracium species. Location H. aurantiacum, H. murorum and H. pilosella are native to Europe and invasive in Australia, New Zealand and North America. Methods Differences among the native and invasive realized climatic niches of each species were quantified. Eight ENMs in BIOMOD were calibrated with (1) NR and (2) ER data. Current European, North American and Australian distributions were projected. Future Australian distributions were modelled using four climate change scenarios for 2030. Results The invasive climatic niche of H. murorum is primarily a subset of that expressed in its native range. Invasive populations of H. aurantiacum and H. pilosella occupy different climatic niches to those realized in their native ranges. Furthermore, geographically separate invasive populations of these two species have distinct climatic niches. ENMs calibrated on the realized niche of native regions projected smaller distributions than models incorporating data from species’ entire ranges, and failed to correctly predict many known invasive populations. Under future climate scenarios, projected distributions decreased by similar percentages, regardless of the data used to calibrate ENMs; however, the overall sizes of projected distributions varied substantially. Main conclusions This study provides quantitative evidence that invasive populations of Hieracium species can occur in areas with different climatic conditions than experienced in their native ranges. For these, and similar species, calibration of ENMs based on NR data only will misrepresent their potential invasive distribution. These errors will propagate when estimating climate change impacts. Thus, incorporating data from species’ entire distributions may result in a more thorough assessment of current and future ranges, and provides a closer approximation of the elusive fundamental niche.