Biotic resistance buffers the effects of nutrient enrichment on the success of a highly invasive aquatic plant

1. Biotic resistance theory predicts that the presence of a species‐rich native community is able to inhibit the establishment of invasive species. On the other hand, disturbed environments, for example, through eutrophication, tend to be more easily invaded.
2. We investigated the effect of biotic resistance on the establishment success of the aquatic grass Urochloa arrecta in nutrient‐enriched and non‐enriched mesocosms so as to test the hypothesis that the effects of biotic resistance on invader success depend on ecosystem trophic state.
3. Invader success, measured as plant biomass and length, increased with nutrient enrichment in treatments without biotic resistance, whereas there was no difference between the success of the invader in enriched or non‐enriched mesocosms in the presence of biotic resistance.
4. Our results suggest that eutrophication enhanced the resistance offered by the native community and thereby reduced the establishment of invaders. The results underscore the importance of native communities as biotic filters to invasion, especially in environments that are subjected to anthropogenic impacts such as nutrient enrichment.

     

Biological soil crusts determine the germination and growth of two exotic plants

In arid and semiarid ecosystems, the potential threats of exotic invasive species are enhanced due to increasing human activities. Biological soil crusts (BSCs), acting as arid ecosystem engineers, may play an important role in preventing the establishment of exotic invasive plants. Our goal was to examine whether BSCs could inhibit the establishment of probable exotic plant species originating from adjacent grasslands located along the southeast edge of the Tengger Desert. In our study, we investigated the effects of three BSC types (cyanobacteria, lichen, and moss crusts) under two disturbance conditions (intact and disturbed) on the establishment of two exotic plant species (Ceratoides latens and Setaria viridis) using indoor experiments. We found both negative and positive effects of BSCs on the establishment of the two exotic plant species. Compared with the disturbed BSCs, the germination percentages of C. latens and S. viridis were reduced by 54% to 87% and 89% to 93%, respectively, in intact BSCs. In contrast, BSCs significantly promoted the height growth and aboveground biomass of the two exotic plant species (p < .05) by enhancing the soil water and nutrient availability for the exotic plants. Our results confirm that BSCs strongly suppress the rapid expansion of exotic plant populations by inhibiting germination of seed with big size or appendages and have a weak inhibitory effect on exotic plant with small and smooth seeds. This may decrease the threat of propagation of exotic species. In the meantime, BSCs promote the growth of a few successful engraftment seedlings, which increased the beta diversity. Our work suggests that better understanding the two opposing effects of BSCs on the establishment of exotic plant species in different growth stages (germination and growth) is important for maintaining the health and stability of revegetated regions.     

Biotic resistance on the increase: native predators structure invasive zebra mussel populations

1. Abundant native predators, parasites and pathogens that switch to consuming a hyper‐successful exotic species may be able to control the invasive population. Native predators may, however, need time to adapt to feed effectively on an exotic resource. In this case, mortality on an exotic population from native predators could increase over time even without a numerical increase in the predator population. 2. We measured mortality of zebra mussels (Dreissena polymorpha) in the Hudson River both in controls open to predation and in exclosures that excluded large predators to estimate mortality of zebra mussels from large predators and other causes. 3. We found that predation by the blue crab (Callinectes sapidus), and perhaps other predators, causes high mortality on zebra mussels in the Hudson River estuary. This predation apparently led to increased mortality and altered population structure in the invader over time. 4. Long‐term data from the Hudson River suggest that components of the invaded ecosystem, like rotifers, are recovering through predator‐caused release from zebra mussel grazing. Increased mortality on hyper‐successful exotic populations over time may be a common phenomenon with both ecological and management implications.     

Allelopathy and exotic plant invasion

The primary hypothesis for the astonishing success of many exotics as community invaders relative to their importance in their native communities is that they have escaped the natural enemies that control their population growth – the `natural enemies hypothesis'. However, the frequent failure of introduced biocontrols, weak consumer effects on the growth and reproduction of some invaders, and the lack of consistent strong top-down regulation in many natural ecological systems indicate that other mechanisms must be involved in the success of some exotic plants. One mechanism may be the release by the invader of chemical compounds that have harmful effects on the members of the recipient plant community (i.e., allelopathy). Here, we provide an abbreviated compilation of evidence for allelopathy in general, present a detailed case study for Centaurea diffusa, an invasive Eurasian forb in western North America, and review general evidence for allelopathic effects of invasive plants in native communities. The primary rationale for considering allelopathy as a mechanism for the success of invaders is based on two premises. First, invaders often establish virtual monocultures where diverse communities once flourished, a phenomenon unusual in natural communities. Second, allelopathy may be more important in recipient than in origin communities because the former are more likely to be naïve to the chemicals possessed by newly arrived species. Indeed, results from experiments on C. diffusa suggest that this invader produces chemicals that long-term and familiar Eurasian neighbors have adapted to, but that C. diffusa's new North American neighbors have not. A large number of early studies demonstrated strong potential allelopathic effects of exotic invasive plants; however, most of this work rests on controversial methodology. Nevertheless, during the last 15 years, methodological approaches have improved. Allelopathic effects have been tested on native species, allelochemicals have been tested in varying resource conditions, models have been used to estimate comparisons of resource and allelopathic effects, and experimental techniques have been used to ameliorate chemical effects. We do not recommend allelopathy as a `unifying theory' for plant interactions, nor do we espouse the view that allelopathy is the dominant way that plants interact, but we argue that non-resource mechanisms should be returned to the discussion table as a potential mechanism for explaining the remarkable success of some invasive species. Ecologists should consider the possibility that resource and non-resource mechanisms may work simultaneously, but vary in their relative importance depending on the ecological context in which they are studied. One such context might be exotic plant invasion.     

浅议我国外来物种入侵问题及其防治对策

随着世界经济的发展和全球经济一体化进程的加快,外来物种入侵已成为影响国家安全的主要因素之一。我国外来入侵物种数量繁多,已成为遭受外来生物危害最严重的国家之一,入侵物种已对我国生态系统、环境和社会经济造成了严重的影响。近年来,各国都积极采取措施,制定外来物种入侵管理政策,加强外来物种入侵立法,确保国家及国际生物安全。然而,我国现有的针对外来物种的相关法律法规存在明显滞后。本文综述了我国外来物种入侵的现状及相关立法,指出外来物种入侵可在不同生态系之间发生,揭示了行政区划与生态系差异造成外来物种入侵管理存在"自然与经济的错位"、国内管理方面存在"生态系统与行政区域的错位"的"双错位现象",分析了我国外来物种的管理现状及今后的发展方向,以期探求我国在国际背景下的外来物种管理对策,提出制定《外来入侵物种法》及相关法律的国家体系、建立跨部门协作科学研究体系以及建设行政管理体系,增强抵御外来物种入侵的能力,实施以防为主的管理策略。     

Biotic resistance to invasion is ubiquitous across ecosystems of the United States

The biotic resistance hypothesis predicts that diverse native communities are more resistant to invasion. However, past studies vary in their support for this hypothesis due to an apparent contradiction between experimental studies, which support biotic resistance, and observational studies, which find that native and non‐native species richness are positively related at broad scales (small‐scale studies are more variable). Here, we present a novel analysis of the biotic resistance hypothesis using 24 456 observations of plant richness spanning four community types and seven ecoregions of the United States. Non‐native plant occurrence was negatively related to native plant richness across all community types and ecoregions, although the strength of biotic resistance varied across different ecological, anthropogenic and climatic contexts. Our results strongly support the biotic resistance hypothesis, thus reconciling differences between experimental and observational studies and providing evidence for the shared benefits between invasive species management and native biodiversity conservation.     

Biotic resistance to Chrysanthemoides monilifera ssp. monilifera in Tasmania

Boneseed, Chrysanthemoides monilifera ssp. monilifera (Asteraceae) is concentrated in and near cities and towns on the north and east coasts of Tasmania. Its absence from intervening rural and bushland areas cannot be attributed to environmental conditions or a lack of time for dispersal from introduction points. The hypothesis tested in the present paper is that the range of boneseed in Tasmania is limited by biotic resistance through herbivory. Cafeteria experiments and field observations showed that sheep (Ovis aries), cattle (Bos taurus), Tasmanian pademelons (Thylogale billardierii), Bennett's wallabies (Macropus rufogriseus), garden weevils (Phlyctinus callosus) and native invertebrates all consumed boneseed, while common brushtail possums (Trichosurus vulpecula) did not. A boneseed population subjected to sheep grazing for 168 days suffered high mortality, while an adjacent ungrazed population survived intact. A replicated exclosure experiment showed that 75 days of grazing by cattle reduced the size of boneseed plants. Observations of a population subject to Bennett's wallaby and Tasmanian pademelon grazing over 1 year and 2 months showed consistently high leaf damage to foliage within pademelon reach and a decline in population, with high mortality rates in the driest and coldest times. Leaf loss attributable to invertebrates did not prevent a nearby population without wallabies from growing. The distributions of the taxa were consistent with biotic resistance, with those demonstrating no severe effect on boneseed individuals widespread, while those with evidence of severe effects more common in rural areas than in urban areas. Boneseed seemed unlikely to survive for very long at normal stocking levels. Macropod grazing, particularly that of T. billardierii, may also inhibit the invasion of boneseed. Thus, the recent introduction of foxes to Tasmania may not only cause the extinction of species such as T. billardierii, but also may cause an expansion of the range of boneseed.     

Biotic resistance experienced by an invasive crustacean in a temperate estuary

Communities high in species diversity tend to be more successful in resisting invaders than those low in species diversity. It has been proposed that the biotic resistance offered by native predators, competitors and disease organisms plays a role. In Yaquina Bay, Oregon, we observed very little overlap in the distribution of the invasive European green crab, Carcinus maenas, and the larger red rock crab, Cancer productus. C. productus dominates the more saline, cooler lower estuary and C. maenas, the less saline, warmer upper estuary. Because caged C. maenas survive well in the lower estuary, we decided to test the hypothesis that C. productus prey on C. maenas and thus contribute to their exclusion from the more physically benign lower estuary. A laboratory species interaction experiment was designed to determine whether C. productus preys on smaller C. maenas at a higher rate than on smaller crabs of their own species. Crabs of both species were collected and sorted by weight into three size classes: small, medium and large. Small and medium crabs of both species were paired with C. maenas and C. productus of various sizes. When conspecifics were paired, mortality was less than 14%, even in the presence of larger crabs. Smaller C. productus survived well in the presence of larger C. maenas, but the reverse was not true. When small C. maenas (60–67 mm carapace width) were matched with medium and large C. productus, their mortality increased to 52% and 76%, respectively. A less dramatic pattern was observed for medium C. maenas (73–80 mm) in the presence of medium and large C. productus. Thus on the West Coast of North America, the more aggressive red rock crab, C. productus, has the potential to reduce the abundance of C. maenas in the more saline and cooler lower estuaries.     

From richer to poorer: successful invasion by freshwater fishes depends on species richness of donor and recipient basins

Evidence for the theory of biotic resistance is equivocal, with experiments often finding a negative relationship between invasion success and native species richness, and large‐scale comparative studies finding a positive relationship. Biotic resistance derives from local species interactions, yet global and regional studies often analyze data at coarse spatial grains. In addition, differences in competitive environments across regions may confound tests of biotic resistance based solely on native species richness of the invaded community. Using global and regional data sets for fishes in river and stream reaches, we ask two questions: (1) does a negative relationship exist between native and non‐native species richness and (2) do non‐native species originate from higher diversity systems. A negative relationship between native and non‐native species richness in local assemblages was found at the global scale, while regional patterns revealed the opposite trend. At both spatial scales, however, nearly all non‐native species originated from river basins with higher native species richness than the basin of the invaded community. Together, these findings imply that coevolved ecological interactions in species‐rich systems inhibit establishment of generalist non‐native species from less diverse communities. Consideration of both the ecological and evolutionary aspects of community assembly is critical to understanding invasion patterns. Distinct evolutionary histories in different regions strongly influence invasion of intact communities that are relatively unimpacted by human actions, and may explain the conflicting relationship between native and non‐native species richness found at different spatial scales.     

Specificity of herbivore‐induced responses in an invasive species,Alternanthera philoxeroides (alligator weed)

Herbivory‐induced responses in plants can both negatively affect subsequently colonizing herbivores and mitigate the effect of herbivory on the host. However, it is still less known whether plants exhibit specific responses to specialist and generalist herbivores in non‐secondary metabolite traits and how specificity to specialists and generalists differs between invasive and native plant populations. We exposed an invasive plant, Alternanthera philoxeroides, to Agasicles hygrophila (Coleoptera, Chrysomelidae; specialist), Spodoptera litura (Lepidoptera, Noctuidae; generalist), manual clipping, or application of exogenous jasmonic acid and examined both the specificity of elicitation in traits of fitness (e.g., aboveground biomass), morphology (e.g., root:shoot ratio), and chemistry (e.g., C/N ratio and lignin), and specificity of effect on the subsequent performance of A. hygrophila and S. litura. Then, we assessed variation of the specificity between invasive and native populations (USA and Argentina, respectively). The results showed S. litura induced higher branching intensity and specific leaf area but lower C/N ratio than A. hygrophila, whereas A. hygrophila induced higher trichome density than S. litura. The negative effect of induction on subsequent larval growth was greater for S. litura than for A. hygrophila. Invasive populations had a weaker response to S. litura than to A. hygrophila in triterpenoid saponins and C/N ratio, while native populations responded similarly to these two herbivores. The specific effect on the two herbivores feeding on induced plants did not vary between invasive and native populations. Overall, we demonstrate specificity of elicitation to specialist and generalist herbivores in non‐secondary metabolite traits, and that the generalist is more susceptible to induction than the specialist. Furthermore, chemical responses specific to specialist and generalist herbivores only exist in the invasive populations, consistent with an evolutionary change in specificity in the invasive populations.     

Responses of morphological and physiological traits to herbivory by snails of three invasive and native submerged plants

3种入侵和本地沉水植物形态和生理性状对螺类牧食的响应 沉水植物水盾草(Cabomba caroliniana)已成为中国太湖流域的优势入侵水生植物。与外来物种的原产地环境相比,引入地新环境中存在的专食性天敌较少。外来物种可能会逃避其原产地环境中的天敌牧食,又因为它们的适口性相对较差,从而导致在引入地外来物种通常比本地物种遭受的牧食者影响更低(天敌逃逸假说)。本研究的目的是比较水盾草与共生的本地沉水植物对本地牧食者的响应。我们进行了一个中宇宙尺度实验,研究了水盾草和两种共生的本地沉水植物黑藻(Hydrilla verticillata)和穗花狐尾藻(Myriophyllum spicatum)对两种本地广食性腹足纲螺类萝卜螺(Radix swinhoei)和环棱螺(Sinotaia quadrata)的牧食响应。记录了它们的形态性状指标(总生物量、冠根比和相对生长率)和生理性状指标(叶片总非结构性碳、木质素和纤维素)。研究结果表明,环棱螺对3种沉水植物性状指标的影响较少。随着本地广食性螺类萝卜螺数量的增加,黑藻和穗花狐尾藻大部分植物性状发生了改变,而水盾草的植物性状表现出相对稳定的趋势。水盾草对萝卜螺的牧食更具抵抗力,这与天敌逃逸假说的假设一致。这一发现说明牧食性螺类促进了水盾草的入侵,这可能会改变沉水植物群落中的物种组成。     

Relationship between native and exotic plant species at multiple savannah sites

We tested whether the recently proposed two‐part measure of degree of invasion (DI) of a community relating exotic proportion of cover to exotic proportion of richness can characterize patterns of plant invasions at multiple savannah sites in Southern Africa. Regression analysis was performed on transformed data to assess how this two‐part measure of DI compares to other metrics of community invasibility. The results indicate that at the plot level, the absolute cover of exotics was not significantly related to native cover for three sites out of four assessed (R² ≤ 0.17; P > 0.05). Also, at all four sites, no significant relationships were detected between native and exotic plant richness at both the 1‐m² and 400‐m² plot scales. By contrast, significant (P < 0.05) positive linear relationships were observed between exotic proportion of richness and exotic proportion of cover at all sites (R² was as high as 0.67 and 0.97 for two sites). Our results indicate that the new two‐part measure of DI is able to characterize patterns of plant invasions across plant communities in African savannahs.     

A null model of exotic plant diversity tested with exotic and native species-area relationships

At large spatial scales, exotic and native plant diversity exhibit a strong positive relationship. This may occur because exotic and native species respond similarly to processes that influence diversity over large geographical areas. To test this hypothesis, we compared exotic and native species–area relationships within six North American ecoregions. We predicted and found that within ecoregions the ratio of exotic to native species richness remains constant with increasing area. Furthermore, we predicted that areas with more native species than predicted by the species–area relationship would have proportionally more exotics as well. We did find that these exotic and native deviations were highly correlated, but areas that were good (or bad) for native plants were even better (or worse) for exotics. Similar processes appear to influence exotic and native plant diversity but the degree of this influence may differ with site quality.     

Biotic resistance in marine environments

Biological invasions depend in part on the resistance of native communities. Meta‐analyses of terrestrial experiments demonstrate that native primary producers and herbivores generally resist invasions of primary producers, and that resistance through competition strengthens with native producer diversity. To test the generality of these findings, we conducted a meta‐analysis of marine experiments. We found that native marine producers generally failed to resist producer invasions through competition unless the native community was diverse, and this diversity effect was weaker in marine than in terrestrial systems. In contrast, native consumers equally resisted invasive producers in both ecosystems. Most marine experiments, however, tested invasive consumers and these invasions were resisted more strongly than were producer invasions. Given these differences between ecosystems and between marine trophic levels, we used a model‐selection approach to assess if factors other than the resistance mechanism (i.e. competition vs. consumption) are more important for predicting marine biotic resistance. These results suggest that understanding marine biotic resistance depends on latitude, habitat and invader taxon, in addition to distinguishing between competition with and consumption by native species. By examining biotic resistance within and across ecosystems, our work provides a more complete understanding of the factors that underlie biological invasions.     

植物外来种入侵及其对生态系统的影响

对植物外来种的入侵及生态系统的影响进行综述与分析,植物入侵多种因子的影响,可分为外因和内因两类,植物外来种对生态系统的影响主要体现在生产力,土壤营养,水分,干扰体制,群落的结构和动态等方面,在管理外来种时,需对外来种的特性和影响因子进行系统的观察研究,采用机械法,化学方法和生物控制法等综合办法控制植物的入侵,引进植物引来种时,要对引进种的行为特性进行了调查研究,注意其安全性。     

Field experiments show contradictory short‐ and long‐term myrmecochorous plant impacts on seed‐dispersing ants

1. Some interactions previously described as mutualistic were revealed to be commensal or parasitic in subsequent investigations. Ant‐mediated seed dispersal has been described as a mutualism for more than a century; however, recent research suggests that it may be commensal or parasitic. Plants demonstrably benefit from ant‐mediated seed dispersal, although there is little evidence available to demonstrate that the interaction benefits long‐term ant fitness. 2. Field experiments were conducted in temperate North America focused on a key seed‐dispersing ant. All herbaceous plants were removed from a forest understorey for 13 years, and supplemented ant colonies with large elaiosome‐bearing seeds aiming to examine potential long‐ and short‐term myrmecochorous plant benefits for the ants. 3. If elaiosome‐bearing seeds benefit ants, suggesting a mutualistic relationship, it is expected that there would be greater worker and/or alate abundance and greater fat reserves (colony lipid content) with seed supplementation (short‐term) and in areas with high understorey herb abundance. 4. Short‐term seed supplementation of ant colonies did not result in an increase with respect to numbers or fat stores, although it did prompt the production of colony sexuals, which is a potential fitness benefit. In the long term, however, there was no positive effect on the ants and, instead, there were negative effects because the removal of elaiosome‐bearing plants corresponded with greater colony health. 5. The data obtained in the present study suggest that the ant–plant interaction ranged from occasionally beneficial to neutral to overall negative for the ant partner. Such results did not support considering the interaction as a mutualism. Collectively, the data suggest the need to reconsider the nature of the relationship between these ants and plants.     

海南岛外来植物入侵风险评价指标体系

基于海南岛生物入侵防控及外来物种管理的需求,对海南外来入侵植物的种类、分布情况和危害现状进行了调查。调查发现,目前海南受外来植物入侵影响较重,外来入侵植物共约160种,隶属于38个科。构建了适应于海南当地生态环境特点与经济社会发展的外来植物风险评估指标体系,该指标体系共分3级,包括5个一级指标,和相应的12个二级指标,42个三级指标,采用层次分析法确定了体系各级指标的权重。对海南50种常见的主要外来入侵植物进行了风险评估,并按照评估结果将外来入侵植物划分为不同风险等级,提出相应的管理策略。外来入侵生物风险评估体系可被作为管理部门制定早期预警和决策措施的重要工具,通过风险评估与风险管理,建立一套应对海南外来生物入侵的快速反应体系,为外来生物入侵的防范和生态安全提供保障。