Mycorrhizal species identity affects plant community structure and invasion: a microcosm study

Previous studies have shown that arbuscular mycorrhizal fungi (AMF) can mediate plant interactions, thereby affecting plant community structure. Little is known, however, about whether the presence of different AMF species leads to differences in plant community structure or invasion success by introduced species. To investigate the effects of AMF species on community structure and invasion, we created replicate microcosms containing soil inoculated with one of three different AMF species (Glomus spurcum Pfeiffer, Walker & Bloss, Scutellospora erythropa (Koske & Walker) Walker & Sanders, or Scutellospora verrucosa (Koske & Walker) Walker & Sanders) or a mixture of all three AMF species. Seeds of seven naturally co‐occurring plant species (Ageratum conyzoides L., Cyperus compressus L., Chamaecrista nictitans (L.), Crotalaria incana L., Hyptis pectinata (L.) Poit., Sida rhombifolia L., Melinis repens (Willd.) Zizka) in Hawai‘i were sown equally into these microcosms, which were placed on outdoor benches. Plant community development was monitored over a season. Mid‐way through the experiment, an invader (Bidens pilosa L.) was added to the established communities to determine whether mycorrhizal species identity affected invasion success. Final aboveground and belowground phytomass were used to assess plant community differences among treatments. Although the identity of the dominant plant species (Melinis repens) remained the same in all treatments, community dominance, community productivity, plant species richness, Shannon index of diversity, and invasion success all varied with AMF species identity. Invasion success was not inversely related to species richness or diversity. Instead, increased richness, diversity, and invasion success all appeared to be related to decreased dominance by M. repens in the presence of certain AMF species. These results indicate that the composition of the AMF community belowground can influence the structure of the plant community aboveground, and may play a role in facilitating or repelling invasion.     

Beetle community responses to grey willow (Salix cinerea) invasion within three New Zealand wetlands

Abstract

We investigated the effects of invasion by introduced grey willow (Salix cinerea) on beetle communities within four wetland vegetation types: native vegetation, native vegetation following grey willow removal, native vegetation undergoing grey willow invasion and dense grey willow-dominated vegetation. In total, 1505 beetles from 90 species were collected using modified Malaise traps. Native wetland vegetation had significantly lower beetle species richness than willow-dominated vegetation and was dominated by herbivores, whereas detritivores characterised willow-dominated vegetation. Beetle abundance was highest in the willow-dominated vegetation and mostly comprised detritivores. In contrast, beetle abundance was lowest in native wetland vegetation, but had even proportions of herbivores and detritivores. Native wetland vegetation had a high proportion of native beetles present. As grey willows invaded, introduced beetles became more common. The beetle community composition differed significantly between grey willow-dominated vegetation and native wetland vegetation. These compositional differences were mainly due to the increasing complexity of vegetation structure following grey willow invasion. The beetle communities within restored native wetland vegetation were most similar to those within the native wetland vegetation. From a conservation perspective, these results are encouraging and suggest that, although grey willows dramatically alter the composition of beetle communities present, these communities can be restored to a beetle fauna that is similar to those found within native wetland through the removal of the willows.     

Environmental Productivity and Biodiversity Effects on Invertebrate Community Invasibility

Productivity influences the availability of resources for colonizing species. Biodiversity may also influence invasibility of communities because of more complete use of resource types with increasing species richness. We hypothesized that communities with higher environmental productivity and lower species richness should be more invasible by a competitor than those where productivity is low or where richness is high. We experimentally examined the invasion resistance of herbivorous meiofauna of Jamaican rock pools by a competitor crustacean (Ostracoda: Potamocypris sp. (Brady)) by contrasting three levels of nutrient input and four levels of species richness. Although relative abundance (dominance) of the invasive was largely unaffected by resource availability, increasing resources did increase the success rate of establishment. Effects of species richness on dominance were more pronounced with a trend towards the lowest species richness treatment of 2 resident species being more invasible than those with 4, 6, or 7 species. These results can be attributed to a ‘sampling effect associated with the introduction of Alona davidii (Richard) into the higher biodiversity treatments. Alona dominated the communities where it established and precluded dominance by the introduced ostracod. Our experimental study supports the idea that niche availability and community interactions define community invasibility and does not support the application of a neutral community model for local food web management where predictions of exotic species impacts are needed.     

Species composition,functional and phylogenetic distances correlate with success of invasive Chromolaena odorata in an experimental test

Biotic resistance may influence invasion success; however, the relative roles of species richness, functional or phylogenetic distance in predicting invasion success are not fully understood. We used biomass fraction of Chromolaena odorata, an invasive species in tropical and subtropical areas, as a measure of ‘invasion success’ in a series of artificial communities varying in species richness. Communities were constructed using species from Mexico (native range) or China (non‐native range). We found strong evidence of biotic resistance: species richness and community biomass were negatively related with invasion success; invader biomass was greater in plant communities from China than from Mexico. Harvesting time had a greater effect on invasion success in plant communities from China than on those from Mexico. Functional and phylogenetic distances both correlated with invasion success and more functionally distant communities were more easily invaded. The effects of plant‐soil fungi and plant allelochemical interactions on invasion success were species‐specific.     

Contrasting community responses to fertilization and the role of the competitive ability of dominant species

Question: Do contrasting biotic contexts in nutrient‐poor grasslands affect the predictability of invasion by exploitative species following fertilization? Location: French Alps. Methods: We examined community responses after 2 years of nutrient addition for two nutrient‐poor European calcareous grasslands, a mesoxeric community dominated by the short bunchgrass Bromus erectus and a mesic community dominated by the tall rhizomatous grass Brachypodium rupestre. We also performed reciprocal transplantations of these two dominant slow‐growing species and Arrhenatherum elatius, a tall fast‐growing grass that dominates nutrient‐rich communities and is likely to invade nutrient‐poor communities after fertilization. Transplants were grown with or without neighbors, in order to measure their individual responses (without neighbors) and competition intensity (by comparing performances with and without neighbors using the Relative Neighbor Effect index – RNE) during one growing season in all three communities. Results: In the Bromus community, fertilization induced a strong increase in fast‐growing grasses (including A. elatius). Competition intensity was low for the three transplanted grasses, but strongly increased with resource addition, to reach values observed in the Arrhenatherum community. In the Brachypodium community, no change in competition intensity with fertilization was detected, because of the high mortality of the two “non‐resident” species, irrespective of the presence of neighbors. Conclusions: Community responses to nutrient improvement are context‐dependent and vary as a function of the biotic environment. Soil processes are proposed as the main drivers of community resistance to the invasion of fast‐growing species in the mesic, nutrient‐poor grassland dominated by the large conservative competitor B. rupestre.     

Twenty‐five years of plant community dynamics and invasion in New Zealand tussock grasslands

Understanding how plant communities respond to plant invasions is important both for understanding community structure and for predicting future ecosystem change. In a system undergoing intense plant invasion for 25 years, we investigated patterns of community change at a regional scale. Specifically, we sought to quantify how tussock grassland plant community structure had changed and whether changes were related to increases in plant invasion. Frequency data for all vascular plants were recorded on 124, permanent transects in tussock grasslands across the lower eastern South Island of New Zealand measured three times over a period of 25 years. Multivariate analyses of species richness were used to describe spatial and temporal patterns in the vegetation. Linear mixed‐effects models were used to relate temporal changes in community structure to the level and rate of invasion of three dominant invasive species in the genus Hieracium while accounting for relationships with other biotic and abiotic variables. There was a strong compositional gradient from exotic‐ to native‐dominated plant communities that correlated with increasing elevation. Over the 25 years, small‐scale species richness significantly decreased and then increased again; however, these changes differed in different plant communities. Exotic species frequency consistently increased on some transects and consistently declined on others. Species richness changes were correlated with the level of Hieracium invasion and abiotic factors, although the relationship with Hieracium changed from negative to positive over time. Compositional changes were not related to measured predictors. Our results suggest that observed broad‐scale fluctuations in species richness and community composition dynamics were not driven by Hieracium invasion. Given the relatively minor changes in community composition over time, we conclude that there is no evidence for widespread degradation of these grasslands over the last 25 years. However, because of continuing weed invasion, particularly at lower elevations, impacts may emerge in the longer term.     

Community dominance patterns, not colonizer genetic diversity, drive colonization success in a test using grassland species

Successful colonization and/or invasion depend on characteristics of the invaded community and of the colonizer itself. Although many studies have documented a negative relationship between invasibility and biodiversity, the importance of community evenness is rarely examined and thus poorly understood. However, colonizer characteristics, including population genetic diversity, can also be important determinants of colonization success. We conducted a greenhouse experiment to assess the relative importance of community evenness and colonizer population genetic diversity using the weed Arabidopsis thaliana. We added seeds of A. thaliana (varying genetic diversity while keeping propagule pressure constant) to four types of constructed plant communities: those dominated by legumes, grasses or forbs, or with equal abundances of all three functional groups. We selected community members from a large pool of species to avoid the confounding effects of species identity. We also assessed the success of multiple seedbank colonizers to assess generality in the effects of our evenness treatments. Equal-abundance communities were no better at suppressing colonization than communities dominated by a single functional group. Forb-dominated communities suppressed A. thaliana colonization better than grass-dominated communities and suppressed seedbank colonizers better than legume-dominated communities. Equal-abundance communities were similar to forb-dominated ones in their eventual composition and in their invasibility, suggesting that forbs drove colonizer suppression in that treatment rather than high evenness itself. Most of our forbs grew quickly, yielding productive forb-dominated communities; this points to the importance of growth and colonization phenology in our system. A. thaliana genetic diversity did not affect colonization success, perhaps because strong interspecific competition substantially limited A. thaliana seedling emergence.     

Genetic diversity is positively associated with fine‐scale momentary abundance of an invasive ant

Many introduced species become invasive despite genetic bottlenecks that should, in theory, decrease the chances of invasion success. By contrast, population genetic bottlenecks have been hypothesized to increase the invasion success of unicolonial ants by increasing the genetic similarity between descendent populations, thus promoting co‐operation. We investigated these alternate hypotheses in the unicolonial yellow crazy ant, Anoplolepis gracilipes, which has invaded Arnhem Land in Australia's Northern Territory. We used momentary abundance as a surrogate measure of invasion success, and investigated the relationship between A. gracilipes genetic diversity and its abundance, and the effect of its abundance on species diversity and community structure. We also investigated whether selected habitat characteristics contributed to differences in A. gracilipes abundance, for which we found no evidence. Our results revealed a significant positive association between A. gracilipes genetic diversity and abundance. Invaded communities were less diverse and differed in structure from uninvaded communities, and these effects were stronger as A. gracilipes abundance increased. These results contradict the hypothesis that genetic bottlenecks may promote unicoloniality. However, our A. gracilipes study population has diverged since its introduction, which may have obscured evidence of the bottleneck that would likely have occurred on arrival. The relative importance of genetic diversity to invasion success may be context dependent, and the role of genetic diversity may be more obvious in the absence of highly favorable novel ecological conditions.     

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

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

Soil pathogen communities associated with native and non‐native Phragmites australis populations in freshwater wetlands

Soil pathogens are believed to be major contributors to negative plant–soil feedbacks that regulate plant community dynamics and plant invasions. While the theoretical basis for pathogen regulation of plant communities is well established within the plant–soil feedback framework, direct experimental evidence for pathogen community responses to plants has been limited, often relying largely on indirect evidence based on above‐ground plant responses. As a result, specific soil pathogen responses accompanying above‐ground plant community dynamics are largely unknown. Here, we examine the oomycete pathogens in soils conditioned by established populations of native noninvasive and non‐native invasive haplotypes of Phragmites australis (European common reed). Our aim was to assess whether populations of invasive plants harbor unique communities of pathogens that differ from those associated with noninvasive populations and whether the distribution of taxa within these communities may help to explain invasive success. We compared the composition and abundance of pathogenic and saprobic oomycete species over a 2‐year period. Despite a diversity of oomycete taxa detected in soils from both native and non‐native populations, pathogen communities from both invaded and noninvaded soils were dominated by species of Pythium. Pathogen species that contributed the most to the differences observed between invaded and noninvaded soils were distributed between invaded and noninvaded soils. However, the specific taxa in invaded soils responsible for community differences were distinct from those in noninvaded soils that contributed to community differences. Our results indicate that, despite the phylogenetic relatedness of native and non‐native P. australis haplotypes, pathogen communities associated with the dominant non‐native haplotype are distinct from those of the rare native haplotype. Pathogen taxa that dominate either noninvaded or invaded soils suggest different potential mechanisms of invasion facilitation. These findings are consistent with the hypothesis that non‐native plant species that dominate landscapes may “cultivate” a different soil pathogen community to their rhizosphere than those of rarer native species.     

外来植物紫茎泽兰入侵对土壤理化性质及丛枝菌根真菌(AMF)群落的影响

为了揭示外来植物紫茎泽兰入侵对入侵地土壤丛枝菌根真菌(AMF)群落及相关肥力的影响,比较测定了紫茎泽兰不同入侵程度土壤理化性质、AMF侵染率及AMF群落的差异。结果表明,紫茎泽兰入侵降低了土壤pH,使土壤中有机碳、全氮和速效钾含量分别增加83.0%,106.9%和111.0%;尽管对全磷含量没有显著影响,但有机磷含量呈升高的趋势,而速效磷呈降低的趋势。紫茎泽兰入侵降低了本地植物的AMF侵染率;随着入侵程度的加深,土壤中以膨胀无梗囊霉(Acauospora dilatata)为优势种的AMF群落结构逐渐转变为以近明球囊霉(Glomus claroideum )为优势种的结构,紫茎泽兰可在其根周选择培育近明球囊霉,而对其它AMF种,特别是对膨胀无梗囊霉则存在抑制作用;基于各AMF种多度的聚类分析表明,形成紫茎泽兰单优群落土壤中各AMF种多度与未入侵的本地植物群落及入侵程度较轻的紫茎泽兰与本地植物群落之间存在明显分歧。综合分析推断认为,紫茎法兰入侵改变了入侵地土壤理化性状,抑制AMF对土著植物的侵染,改变AMF群落,并在其根周选择培育近明球囊霉,这可能是紫茎泽兰入侵及扩张的重要途径之一。     

The Role of Light and Soil Moisture in Plant Community Resistance to Invasion by Yellow Starthistle (Centaurea solstitialis)

To resist establishment by an invasive plant, a community may require one or more species functionally similar to the invader in their resource acquisition pattern. In this study, communities consisting of native winter annual forbs, non‐native annual grasses, native perennials, or a combination of the two native communities were established with and without Centaurea solstitialis to determine the effect of soil moisture and light availability on plant community invasion resistance. The annual plant communities were unable to resist invasion by C. solstitialis. In the native winter annual forb community, senescence in late spring increased light penetration (>75%) to the soil surface, allowing seeded C. solstitialis to quickly establish and dominate the plots. In addition, native annual forbs utilized only shallow soil moisture, whereas C. solstitialis used shallow and deep soil moisture. In communities containing native perennials, only Elymus glaucus established well and eventually dominated the plots. During the first 2 years of establishment, water use pattern of perennial communities was similar to native annual forbs and resistance to invasion was associated with reduced light availability during the critical stages of C. solstitialis establishment. In later years, however, water use pattern of perennial grass communities was similar or greater than C. solstitialis‐dominated plots. These results show that Central Valley grasslands that include E. glaucus resist C. solstitialis invasion by a combination of light suppression and soil water competition. Spatiotemporal resource utilization patterns, and not just functional similarity, should be considered when developing restoration strategies to resist invasion by many non‐native species.     

Disentangling the effects of plant species invasion and urban development on arthropod community composition

Urban development and species invasion are two major global threats to biodiversity. These threats often co‐occur, as developed areas are more prone to species invasion. However, few empirical studies have tested if both factors affect biodiversity in similar ways. Here we study the individual and combined effects of urban development and plant invasion on the composition of arthropod communities. We assessed 36 paired invaded and non‐invaded sample plots, invaded by the plant Antigonon leptopus, with half of these pairs located in natural and the other half in developed land‐use types on the Caribbean island of St. Eustatius. We used several taxonomic and functional variables to describe community composition and diversity. Our results show that both urban development and A. leptopus invasion affected community composition, albeit in different ways. Development significantly increased species richness and exponential Shannon diversity, while invasion had no effect on these variables. However, invasion significantly increased arthropod abundance and caused biotic homogenization. Specifically, uninvaded arthropod communities were distinctly different in species composition between developed and natural sites, while they became undistinguishable after A. leptopus invasion. Moreover, functional variables were significantly affected by species invasion, but not by urban development. Invaded communities had higher community‐weighted mean body size and the feeding guild composition of invaded arthropod communities was characterized by the exceptional numbers of nectarivores, herbivores, and detritivores. With the exception of species richness and exponential Shannon diversity, invasion influenced four out of six response variables to a greater degree than urban development did. Hence, we can conclude that species invasion is not just a passenger of urban development but also a driver of change.     

Long‐term vegetation changes in experimentally grazed and ungrazed back‐barrier marshes in the Wadden Sea

Abstract. Vegetation succession in three back‐barrier salt marshes in the Wadden Sea was studied using a data set comprising 25 years of vegetation development recorded at permanent quadrats. The effect of livestock grazing on succession was assessed by comparing quadrats where grazing was experimentally prevented or imposed. We studied changes at the species level as well as at the level of the plant community. Special attention is given to effects on plant species richness and community characteristics that are relevant for lagomorphs (hares and rabbits) and geese. Inundation frequency and grazing were most important in explaining the variation in species abundance data. The three marshes studied overlap in the occurrence of different plant communities and the observed patterns were consistent between them. Clear differences in frequency and abundance of plant species were observed related to grazing. Most plant species had a greater incidence in grazed treatments. Species richness increased with elevation, and was 1.5 to 2 × higher in the grazed salt marsh. Grazing negatively influenced Atriplex portulacoides and Elymus athericus, whereas Puccinellia maritima and Festuca rubra showed a positive response. The communities dominated by Elymus athericus, Artemisia maritima and Atriplex portulacoides were restricted to the ungrazed marsh. Communities dominated by Puccinellia maritima, Juncus gerardi and Festuca rubra predominantly occurred at grazed sites. As small vertebrate herbivores prefer these plants and communities for foraging, livestock grazing thus facilitates for them.     

Comparison of predator–prey interactions with and without intraguild predation by manipulation of the nitrogen source

Theory predicts that intraguild predation leads to different community dynamics than the trophic cascades of a linear food chain. However, experimental comparisons of these two food‐web modules are rare. Mixotrophic plankton species combine photoautotrophic and heterotrophic nutrition by grazing upon other phytoplankton species. We found that the mixotrophic chrysophyte Ochromonas can grow autotrophically on ammonium, but not on nitrate. This offered a unique opportunity to compare predator–prey interactions in the presence and absence of intraguild predation, without changing the species composition of the community. With ammonium as nitrogen source, Ochromonas can compete with its autotrophic prey for nitrogen and therefore acts as intraguild predator. With nitrate, Ochromonas acts solely as predator, and is not in competition with its prey for nitrogen. We parameterized a simple intraguild predation model based on chemostat experiments with monocultures of Ochromonas and the toxic cyanobacterium Microcystis. Subsequently, we tested the model predictions by inoculating Ochromonas into the Microcystis monocultures, and vice versa. The results showed that Microcystis was a better competitor for ammonium than Ochromonas. In agreement with theoretical predictions, Microcystis was much more strongly suppressed by intraguild predation on ammonium than by top–down predation on nitrate. Yet, Microcystis persisted at very low population densities, because the type III functional response of Ochromonas implied that the grazing pressure upon Microcystis became low when Microcystis was rare. Our results provide experimental support for intraguild predation theory, and indicate that intraguild predation may enable biological control of microbial pest species.     

Plant-soil feedbacks do not explain invasion success of Acacia species in introduced range populations in Australia

Legumes, especially acacias, are considered amongst the most successful invaders globally. However there is still very little known about the role of soil microbial communities in their invasion success in novel ranges. We examined the role of the soil microbial community in the invasion success of four Acacia species (A. cyclops, A. longifolia, A. melanoxylon and A. saligna) and a close relative Paraserianthes lophantha, introduced into novel regions within Australia using a “black-box” approach. Seed and soil material were collected from multiple populations within each species’ native and introduced range within Australia and used in a plant-soil feedback experiment to assess the effect of the soil microbial community on plant growth and nodulation. We found no effect, either positive or negative, of soil origin on species’ performance, however there was a significant interaction between species and seed origin. Seed origin had a significant effect on the biomass of two species, A. cyclops and A. saligna. A. cyclops plants from the native range performed better across all soils than plants from the introduced range. The opposite trend was observed for A. saligna, with plants from the introduced range performing better overall than plants from the native range. Seed or soil origin did not have a significant effect on the presence and number of nodules suggesting that rhizobia do not constrain the invasion success of these legumes. Our results suggest that plant-soil feedbacks are unlikely to have played a significant role in the invasion success of these five species introduced into novel regions within Australia. This may be due to the widespread occurrence of acacias and their associated soil microbial communities throughout the Australian continent.     

Effects of belowground resource use comlementarity on invasion of constructed grassland plant communities

At two field sites that differed in fertility, we investigated how species richness, functional group diversity, and species composition of constructed plant communities influenced invasion. Grassland communities were constructed to be either functionally diverse or functionally simple based on belowground resource use patterns of constituent species. Communities were also constructed with different numbers of species (two or five) to examine interactions between species richness, functional diversity and invasion resistance. We hypothesized that communities with more complementary belowground resource use (i.e., more species rich and more functionally diverse communities) would be less easily invaded than communities with greater degrees of belowground resource use overlap. Two contrasting invasive species were introduced: an early-season, shallow rooting annual grass, Bromus hordeaceus (soft chess), and a late-season, deep rooting annual forb, Centaurea solstitialis (yellow starthistle). Invader responses to species richness and functional diversity treatments differed between sites. In general, the more similar the patterns of belowground resource use between residents of the plant community and the invader, the poorer the invader’s performance. Complementarity or overlap of resource use among species in the constructed communities appeared to affect invader success less than complementarity or overlap of resource use between the invader and the species present in the community.     

Increased chemical resistance explains low herbivore colonization of introduced seaweed

The success of introduced species is often attributed to release from co-evolved enemies in the new range and a subsequent decreased allocation to defense (EICA), but these hypotheses have rarely been evaluated for systems with low host-specificity of enemies. Here, we compare herbivore utilization of the brown seaweed, Fucus evanescens, and its coexisting competitors both in its native and new ranges, to test certain predictions derived from these hypotheses in a system dominated by generalist herbivores. While F. evanescens was shown to be a preferred host in its native range, invading populations supported a less diverse herbivore fauna and it was less preferred in laboratory choice experiments with important herbivores, when compared to co-occurring seaweeds. These results are consistent with the enemy release hypothesis, despite the fact that the herbivore communities in both regions were mainly composed of generalist species. However, in contrast to the prediction of EICA, analysis of anti-grazing compounds indicated a higher allocation to defense in introduced compared to native F. evanescens. The results suggest that the invader is subjected to less intense enemy control in the new range, but that this is due to an increased allocation to defense rather than release from specialized herbivores. This indicates that increased resistance to herbivory might be an important strategy for invasion success in systems dominated by generalist herbivores.     

Elevated success of multispecies bacterial invasions impacts community composition during ecological succession

Successful microbial invasions are determined by a species’ ability to occupy a niche in the new habitat whilst resisting competitive exclusion by the resident community. Despite the recognised importance of biotic factors in determining the invasiveness of microbial communities, the success and impact of multiple concurrent invaders on the resident community has not been examined. Simultaneous invasions might have synergistic effects, for example if resident species need to exhibit divergent phenotypes to compete with the invasive populations. We used three phylogenetically diverse bacterial species to invade two compositionally distinct communities in a controlled, naturalised in vitro system. By initiating the invader introductions at different stages of succession, we could disentangle the relative importance of resident community structure, invader diversity and time pre‐invasion. Our results indicate that multiple invaders increase overall invasion success, but do not alter the successional trajectory of the whole community.     

Abundance of introduced species at home predicts abundance away in herbaceous communities

Many ecosystems worldwide are dominated by introduced plant species, leading to loss of biodiversity and ecosystem function. A common but rarely tested assumption is that these plants are more abundant in introduced vs. native communities, because ecological or evolutionary-based shifts in populations underlie invasion success. Here, data for 26 herbaceous species at 39 sites, within eight countries, revealed that species abundances were similar at native (home) and introduced (away) sites - grass species were generally abundant home and away, while forbs were low in abundance, but more abundant at home. Sites with six or more of these species had similar community abundance hierarchies, suggesting that suites of introduced species are assembling similarly on different continents. Overall, we found that substantial changes to populations are not necessarily a pre-condition for invasion success and that increases in species abundance are unusual. Instead, abundance at home predicts abundance away, a potentially useful additional criterion for biosecurity programmes.