Community Invasibility Research Articles

Co-occurring invasive plant interactions do not predict the impacts of invasion in experimental tallgrass prairie communities

Co-occurring invasive plants are an understudied yet common phenomenon, and likely to become even more frequent with climate change-driven range shifts. Invasive–invasive interactions can be competitive, neutral, or facilitative; symmetric or asymmetric; and their impact on natives can be additive or non-additive. Non-additive impacts, wherein co-occurring invasives either reduce or enhance one another’s effects, have heightened implications for invasive management and native restoration. We investigated interactions among three invasive species—the legume Lotus corniculatus, the forb Cirsium arvense, and the grass Phalaris arundinacea—that co-occur in North American prairie grassland, and impacts on native prairie communities. Employing greenhouse microcosms, we observed invasive–invasive interactions by growing invasive species alone, in pairs, and in three-way combination. To assess impacts of co-occurring invasives, native prairie communities were grown with each single invader, all pairs of invaders, or all three invaders together. For insight into mechanisms of species interactions, measurements of above- and below-ground resources were made. We predicted that the invasive legume would facilitate the other invasive species via nitrogen enrichment, promoting non-additive impacts on native communities. We observed that the legume promoted yields of the other invasive species and was associated with higher soil ammonium content. However, we also observed strongly asymmetric competition between co-occurring invasive species. Impacts of invaders on functionally diverse native communities were similar among invasion treatments and less than expected based on invasive impacts in monoculture. These results warrant future tests of functional group identity and diversity as predictors of co-occurring invasive species interactions and impacts.

Human rhinovirus spatial-temporal epidemiology in rural coastal Kenya, 2015-2016, observed through outpatient surveillance.

Background: Human rhinovirus (HRV) is the predominant cause of upper respiratory tract infections, resulting in a significant public health burden. The virus circulates as many different types (168), each generating strong homologous, but weak heterotypic, immunity. The influence of these features on transmission patterns of HRV in the community is understudied. Methods: Nasopharyngeal swabs were collected from patients with symptoms of acute respiratory infection (ARI) at nine out-patient facilities across a Health and Demographic Surveillance System between December 2015 and November 2016. HRV was diagnosed by real-time RT-PCR, and the VP4/VP2 genomic region of the positive samples sequenced. Phylogenetic analysis was used to determine the HRV types. Classification models and G-test statistic were used to investigate HRV type spatial distribution. Demographic characteristics and clinical features of ARI were also compared. Results: Of 5,744 NPS samples collected, HRV was detected in 1057 (18.4%), of which 817 (77.3%) were successfully sequenced. HRV species A, B and C were identified in 360 (44.1%), 67 (8.2%) and 390 (47.7%) samples, respectively. In total, 87 types were determined: 39, 10 and 38 occurred within species A, B and C, respectively. HRV types presented heterogeneous temporal patterns of persistence. Spatially, identical types occurred over a wide distance at similar times, but there was statistically significant evidence for clustering of types between health facilities in close proximity or linked by major road networks. Conclusion: This study records a high prevalence of HRV in out-patient presentations exhibiting high type diversity. Patterns of occurrence suggest frequent and independent community invasion of different types. Temporal differences of persistence between types may reflect variation in type-specific population immunity. Spatial patterns suggest either rapid spread or multiple invasions of the same type, but evidence of similar types amongst close health facilities, or along road systems, indicate type partitioning structured by local spread.

Human rhinovirus spatial-temporal epidemiology in rural coastal Kenya, 2015-2016, observed through outpatient surveillance

Background: Human rhinovirus (HRV) is the predominant cause of upper respiratory tract infections, resulting in a significant public health burden. The virus circulates as many different types (168), each generating strong homologous, but weak heterotypic, immunity. The influence of these features on transmission patterns of HRV in the community is understudied. Methods: Nasopharyngeal swabs were collected from patients with symptoms of acute respiratory infection (ARI) at nine out-patient facilities across a Health and Demographic Surveillance System between December 2015 and November 2016. HRV was diagnosed by real-time RT-PCR, and the VP4/VP2 genomic region of the positive samples sequenced. Phylogenetic analysis was used to determine the HRV types. Classification models and G-test statistic were used to investigate HRV type spatial distribution. Demographic characteristics and clinical features of ARI were also compared. Results: Of 5,744 NPS samples collected, HRV was detected in 1057 (18.4%), of which 817 (77.3%) were successfully sequenced. HRV species A, B and C were identified in 360 (44.1%), 67 (8.2%) and 390 (47.7%) samples, respectively. In total, 87 types were determined: 39, 10 and 38 occurred within species A, B and C, respectively. HRV types presented heterogeneous temporal patterns of persistence. Spatially, identical types occurred over a wide distance at similar times, but there was statistically significant evidence for clustering of types between health facilities in close proximity or linked by major road networks. Conclusion: This study records a high prevalence of HRV in out-patient presentations exhibiting high type diversity. Patterns of occurrence suggest frequent and independent community invasion of different types. Temporal differences of persistence between types may reflect variation in type-specific population immunity. Spatial patterns suggest either rapid spread or multiple invasions of the same type, but evidence of similar types amongst close health facilities, or along road systems, indicate type partitioning structured by local spread.

Plant-soil feedbacks promote coexistence and resilience in multi-species communities.

Both ecological theory and empirical evidence suggest that negative frequency dependent feedbacks structure plant communities, but integration of these findings has been limited. Here we develop a generic model of frequency dependent feedback to analyze coexistence and invasibility in random theoretical and real communities for which frequency dependence through plant-soil feedbacks (PSFs) was determined empirically. We investigated community stability and invasibility by means of mechanistic analysis of invasion conditions and numerical simulations. We found that communities fall along a spectrum of coexistence types ranging from strict pair-wise negative feedback to strict intransitive networks. Intermediate community structures characterized by partial intransitivity may feature “keystone competitors” which disproportionately influence community stability. Real communities were characterized by stronger negative feedback and higher robustness to species loss than randomly assembled communities. Partial intransitivity became increasingly likely in more diverse communities. The results presented here theoretically explain why more diverse communities are characterized by stronger negative frequency dependent feedbacks, a pattern previously encountered in observational studies. Natural communities are more likely to be maintained by strict negative plant-soil feedback than expected by chance, but our results also show that community stability often depends on partial intransitivity. These results suggest that plant-soil feedbacks can facilitate coexistence in multi-species communities, but that these feedbacks may also initiate cascading effects on community diversity following from single-species loss.

Urinary tract infections, immunity, and vaccination.

Urinary tract infections (UTI) are considered one of the main causes of morbidity worldwide, and uropathogenic Escherichia coli (UPEC) is the etiological agent associated with these infections. The high morbidity produced by the UTI and the limitation of antibiotic treatments promotes the search for new alternatives against these infections. The knowledge that has been generated regarding the immune response in the urinary tract is important for the development of effective strategies in the UTI prevention, treatment, and control. Molecular biology and bioinformatic tools have allowed the construction of fusion proteins as biomolecules for the development of a viable vaccine against UTI. The fimbrial adhesins (FimH, CsgA, and PapG) of UPEC are virulence factors that contribute to the adhesion, invasion, and formation of intracellular bacterial communities. The generation of recombinant proteins from fimbrial adhesins as a single molecule is obtained by fusion technology. A few in vivo and in vitro studies have shown that fusion proteins provide an efficient immune response and protection against UTI produced by UPEC. Intranasal immunization of immunogenic molecules has generated a response in the urinary tract mucosa compared with other routes of immunization. The objective of this review was to propose a vaccine designed against UTI caused by UPEC, describing the general scenario of the infection, the mechanism of pathogenicity of bacteria, and the immune response of the host.

The unknown followers: Discovery of a new species of Sycobia Walker (Hymenoptera: Epichrysomallinae) associated with Ficus benjamina L. (Moraceae) in the Neotropical region

Biotic invasion in mutualistic communities is of particular interest due to the possible establishment of new relationships with native species. Ficus species are widely cultivated as ornamental plants, and they host specific communities of chalcid wasps that are strictly associated with the fig inflorescences. Some introduced fig species are capable of establishing new relationships with the local fig wasps, and fig wasp species may also be concomitantly introduced with their host plants. Ficusbenjamina L. is widely cultivated across the world, but the associated fig wasps are not reported outside of the species native range. We describe for the first time a non-pollinating fig wasp associated with F.benjamina inflorescences outside its native distribution. Sycobiahodites Farache & Rasplus, sp. n. is the third known species of the genus and was recorded in populations of F.benjamina introduced in the Neotropical region throughout several localities in Argentina, Brazil and Colombia. Sycobia is a gall-inducing non-pollinating fig wasp genus associated with fig trees in the Oriental and Australasian regions. This species competes with pollinators for oviposition sites and may hinder the future establishment of the native pollinator of F.benjamina, Eupristinakoningsbergeri Grandi, 1916 in the New World. However, the occurrence of a gall inducing species in this host plant may open ecological opportunities for the establishment of species belonging to other trophic levels such as cleptoparasite and parasitoid wasps.

Resource stoichiometry shapes community invasion resistance via productivity-mediated species identity effects.

Diversity-invasion resistance relationships are often variable and sensitive to environmental conditions such as resource availability. Resource stoichiometry, the relative concentration of different elements in the environment, has been shown to have strong effects on the physiology and interactions between different species. Yet, its role for diversity-invasion resistance relationships is still poorly understood. Here, we explored how the ratio of nitrogen (N) and phosphorus affects the productivity and invasion resistance of constructed microbial communities by a plant pathogenic bacterium, Ralstonia solanacearum. We found that resource stoichiometry and species identity effects affected the invasion resistance of communities. Both high N concentration and resident community diversity constrained invasions, and two resident species, in particular, had strong negative effects on the relative density of the invader and the resident community productivity. While resource stoichiometry did not affect the mean productivity of the resident community, it favoured the growth of two species that strongly constrained invasions turning the slope of productivity-invasion resistance relationship more negative. Together our findings suggest that alterations in resource stoichiometry can change the community resistance to invasions by having disproportionate effects on species growth, potentially explaining changes in microbial community composition under eutrophication.

Twenty-five years of conferences on the Ecology and Management of Alien Plant invasions: the history of EMAPi 1992–2017

During the first 25 years, EMAPi conferences were attended by 1280 participants from 77 countries in five continents and produced 1474 presentations, including 44 keynotes on a broad range of aspects of plant invasion ecology. The series was established in Loughborough, UK, in 1992 and with its 14th conference in Lisbon, Portugal in 2017 it has become the longest held series of regular meetings in plant invasion ecology with the number of participants in the last decade ranging between 150 and 250 per meeting. Of the 14 events so far, eight were held in Europe, three in North America and the remaining three in Australia, Africa and South America, respectively. The EMAPi series continues to be maintained by an informal board consisting of the organizers of previous meetings. Over time, there has been a shift in research focus from the ecology of species invasiveness and community invasibility, analyses of distribution and spread dynamics towards presentations dealing with management, control, risk assessment and impact. The latter topics, although well represented even at the beginning of the EMAPi conferences, increased their proportion among the total number of papers delivered, from about a quarter to more than half. Ten conferences yielded proceedings and up to 2008 these were published as books with Backhuys Publishers, followed by two issues in the journals Biological Invasions and NeoBiota. By forming a long-term meeting platform, addressing a broad range of topics, reflecting development in plant invasion ecology, achieving truly global geographical coverage and bringing together researchers and practitioners into close contact, EMAPi has become an excellent venue for initiating collaborations among invasion ecologists worldwide.

Influence of Invasive Plants Parthenocissus vitacea and Vinca minor on Biodiversity Indices of Forest Communities

The distribution and phytocenotic association of Parthenocissus vitacea and Vinca minor on the territory of the Bryansk oblast has been studied. An assessment of the distribution of invasive species is performed on a grid basis. Vinca minor is registered in 19 cells on the territory of the oblast, which is 4.9% of the total number of cells and 11.9% of the surveyed cells; Parthenocissus vitacea is registered in 50 cells, which is 12.8% of the total number of cells and 31.2% of the surveyed cells. The invasive status of Parthenocissus vitacea and Vinca minor is category 3: an adventive species whose invasions into natural and seminatural habitats in the region at the moment are rare and, as a rule, confined to the outskirts of populated areas, abandoned parks, and old plantings. The indices of successful invasion in forest communities is the formation of syntaxonomically valid variants of the communities of Corylo avellanae-Pinetum sylvestris Bulokhov et Solomeshch 2003 with depleted cenoflora. Invasive species change the phytosphere and reliably decrease the indices of α and β diversity, the structural indices of communities. The invasion of Parthenocissus vitacea and Vinca minor is successful due to active vegetative reproduction, allelopathic effects on native species, and decreasing illumination of the ground cover. The introduction of invasive species into complex pine forests blocks demutational changes of endogenous succession during the restoration of broad-leaved forests: they simplify the structure of communities and reduce the intensity of the mosaic. Parthenocissus vitacea and Vinca minor can exhibit signs of an edificator, a community builder, and a violent and transformer type. There has been only one case of a radical transformation of the natural community, registered on the territory of the Lyubin Khutor nature monument (Novozybkovskii district, Bryansk oblast).

Human rhinovirus spatial-temporal epidemiology in rural coastal Kenya, 2015-2016, observed through outpatient surveillance.

Background: Human rhinovirus (HRV) is the predominant cause of upper respiratory tract infections, resulting in a significant public health burden. The virus circulates as many different types (~160), each generating strong homologous, but weak heterotypic, immunity. The influence of these features on transmission patterns of HRV in the community is understudied. Methods: Nasopharyngeal swabs were collected from patients with symptoms of acute respiratory infection (ARI) at nine out-patient facilities across a Health and Demographic Surveillance System between December 2015 and November 2016. HRV was diagnosed by real-time RT-PCR, and the VP4/VP2 genomic region of the positive samples sequenced. Phylogenetic analysis was used to determine the HRV types. Classification models and G-test statistic were used to investigate HRV type spatial distribution. Demographic characteristics and clinical features of ARI were also compared. Results: Of 5,744 NPS samples collected, HRV was detected in 1057 (18.4%), of which 817 (77.3%) were successfully sequenced. HRV species A, B and C were identified in 360 (44.1%), 67 (8.2%) and 390 (47.7%) samples, respectively. In total, 87 types were determined: 39, 10 and 38 occurred within species A, B and C, respectively. HRV types presented heterogeneous temporal patterns of persistence. Spatially, identical types occurred over a wide distance at similar times, but there was statistically significant evidence for clustering of types between health facilities in close proximity or linked by major road networks. Conclusion: This study records a high prevalence of HRV in out-patient presentations exhibiting high type diversity. Patterns of occurrence suggest frequent and independent community invasion of different types. Temporal differences of persistence between types may reflect variation in type-specific population immunity. Spatial patterns suggest either rapid spread or multiple invasions of the same type, but evidence of similar types amongst close health facilities, or along road systems, indicate type partitioning structured by local spread.

Limiting similarity, biotic resistance, nutrient supply, or enemies? What accounts for the invasion success of an exotic legume?

Models for community invasion can inform grassland management if patterns involving the environment, other species, and invader success are evident. We evaluated unplanned invasion by Lespedeza cuneata into an experiment on a reconstructed tallgrass prairie. Our goal was to determine whether the pattern of invasion by this exotic legume was consistent with either the limiting similarity hypothesis (LSH), which predicts poorer establishment of exotic species in communities with related species, or the biotic resistance hypothesis, which predicts a negative association between species richness and invasion. We also examined how the abiotic environment and hemiparasite abundance affect spread of L. cuneata. From 2006 to 2015 we manipulated fertilizer and presence of the hemiparasitic plant, Pedicularis canadensis, in 1-m2 plots, and manipulated shade for the first 5 years. We found greater support for the LSH. Biomasses of legumes and other forbs in 2008 were negatively associated with change in L. cuneata biomass over the next 7 years but species richness was not a significant effect. Lespedeza cuneata cover in 2015 was lower in fertilized plots relative to unfertilized plots and was negatively associated with a composite variable that described the number and combined cover of legume species and number of exotic species currently in the plots. Shading for five seasons reduced L. cuneata growth, but only significantly in fertilized plots from which the hemiparasite had been removed. Other analysis suggested that L. cuneata increased more as the hemiparasite abundance increased. Prairie management that increases the kinds and abundances of native legume species and increases soil fertility may slow the spread of L. cuneata, but we caution against adding P. canadensis to large areas without greater knowledge of site-specific effects.

Resistance is futile: lack of predator switching and a preference for native prey predict the success of an invasive prey species.

Invasive species continue to severely impact biodiversity, yet predicting the success or failure of introduced species has remained elusive. In particular, the relationship between community invasibility and native species diversity remains obscure. Here, we apply two traditional ecological concepts that inform prey population stability and hence invasibility. We first show that the native predatory crustacean Gammarus duebeni celticus exhibited similar type II (destabilizing) functional responses (FRs) towards native mayfly prey and invasive amphipod prey, when these prey species were presented separately. However, when the two prey species were presented simultaneously, the predator did not exhibit prey switching, instead consuming disproportionately more native prey than expected from the relative abundance of native and invasive species. These consumptive propensities foster reductions of native prey, while simultaneously limiting biotic resistance against the invasive species by the native predator. Since our theoretical considerations and laboratory results match known field invasion patterns, we advocate the increased consideration of FR and prey switching studies to understand and predict the success of invasive species.

Stochastic processes govern invasion success in microbial communities when the invader is phylogenetically close to resident bacteria.

Despite recent efforts in identifying the determinants of invasion in microbial communities, experimental observations across different ecosystems are inconclusive. While relationships between resident community diversity and invasion success are often noted, community diversity says little about community assembly processes. Community assembly processes may provide a more inclusive framework to explain-and potentially prevent or facilitate-invasion. Here we let replicate nitrite-oxidizing bacterial guilds assemble under different conditions from a natural source community and study their compositional patterns to infer the relative importance of the assembly processes. Then, an invader strain from that same guild was introduced at one of three propagule pressures. We found no significant correlation between community diversity and invasion success. Instead, we observed that the effect of selection on invasion success was surpassed by the effect of drift, as inferred from the substantial influence of propagule pressure on invasion success. This dominance of drift can probably be generalized to other invasion cases with high phylogenetic similarity between invader and resident community members. In these situations, our results suggest that attempting to modulate the invasibility of a community by altering its diversity is futile because stochastic processes determine the invasion outcome. Increasing or reducing propagule pressure is then deemed the most efficient avenue to enhance or limit invasion success.

Effects of initial disturbances and grazing regime on native grassland invasion by Eragrostis plana in southern Brazil

Extensive invasion of the Campos grasslands of southern Brazil by the South African grass, Eragrostis plana, is a serious conservation problem due to its low palatability and ability to quickly spread. Here, we experimentally evaluated the effects of grazing management and initial disturbance on invasibility of native grassland community by E. plana. The experiment, a split-plot complete block design with three replications, was established in 2004 in non-invaded native grassland. Grazing management treatments were assessed in main plots (continuous grazing, rotational grazing, and grazing exclusion). The initial disturbance treatments (light grazing, heavy grazing, and heavy grazing plus soil scarification) were applied in subplots before the sowing of standard amounts of E. plana seeds. The initial disturbances heavy grazing and heavy grazing plus soil scarification simulated the introduction of exotic forage species. We monitor plant species composition and diversity along six years after the experiment starting. The effects of grazing management and initial disturbance on community invasibility were significant and showed interaction. Heavy grazing plus soil scarification increased the rate of E. plana invasion and produced more pronounced changes in species composition than the other treatments. Plots under grazing exclusion, rotational or continuous grazing without soil scarification had low levels of invasion (<10% cover). But exclusion also changed community composition and decreased diversity, while rotational and continuous grazing maintained community structure. The invasion can be largely prevented on undisturbed grassland by employing either rotational or continuous grazing management. If the livestock producers want to introduce another forage species in their natural grasslands and, at the same time, do not run the risk of opening space for E. plana should avoid the use of soil scarification.

Seed Rain and Disturbance Impact Recruitment of Invasive Plants in Upland Forest

AbstractA critical question in invasion biology involves the relative importance of propagule rain and community invasibility. For plant invasions, invasibility is often related to disturbance, but few studies of forest invaders have simultaneously investigated both canopy and ground-level disturbance. We investigated the relative importance of seed rain, canopy disturbance, and soil disturbance in a mature forest in Maryland on the recruitment of four invasive species: wine raspberry (Rubus phoenicolasiusMaxim.), Japanese barberry (Berberis thunbergiiDC), multiflora rose (Rosa multifloraThunb.), and Japanese stiltgrass [Microstegium vimineum(Trin.) A. Camus]. Using complete censuses of a 9-ha plot at two points in time (2011–12 and 2014), we mapped new recruits, and related their locations to canopy and soil disturbance, as well as to a seed rain index based on locations of reproducing plants and seed-dispersal kernels.We found that propagule rain, as measured by the seed rain index, was a significant predictor of recruitment forB. thunbergii,R. phoenicolasius, andM. vimineum. ForR. multiflora, seed sources were not located, precluding assessment of propagule rain, but recruitment was linked to canopy disturbance, as was recruitment ofM. vimineum. However, because reproduction ofR. phoenicolasiusand, in some years, ofB. thunbergiiis higher in treefall gaps, these gaps experience higher propagule rain, with the result that recruitment is indirectly associated with these gaps. Ground-layer disturbance was an important predictor of recruitment only forB. thunbergii. Our findings reveal that the importance of propagule rain is the most consistent driver of recruitment, but canopy or ground-layer disturbance promotes recruitment of some invasive plant species.

Intraspecific variation in indirect plant-soil feedbacks influences a wetland plant invasion.

Plant-soil feedbacks (PSFs) influence plant competition via direct interactions with pathogens and mutualists or indirectly via apparent competition/mutualisms (i.e., spillover to co-occurring plants) and soil legacy effects. It is currently unknown how intraspecific variation in PSFs interacts with the environment (e.g., nutrient availability) to influence competition between native and invasive plants. We conducted a fully crossed multi-factor greenhouse experiment to determine the effects of Phragmites australis rhizosphere soil biota, interspecific competition, and nutrient availability on biomass of replicate populations from one native and two invasive lineages of common reed (P.australis) and a single lineage of native smooth cordgrass (Spartina alterniflora). Harmful soil biota consistently dominated PSFs involving all three P.australis lineages, reducing biomass by 10%. Indirect PSFs (i.e., soil biota spillover) from the two invasive P.australis lineages reduced S.alterniflora biomass by 7%, whereas PSFs from the native P.australis lineage increased S.alterniflora biomass by 6%. Interestingly, interspecific competition and PSFs interacted to weaken their respective impacts on S.alterniflora, whereas they exerted synergistic negative effects on P.australis. Phragmitesaustralis soil biota decreased S.alterniflora biomass when grown alone (i.e., a soil legacy), but increased S.alterniflora biomass when grown with P.australis, suggesting that P.australis recruits harmful generalist soil biota or facilitates S.alterniflora via spillover (i.e., apparent mutualism). Soil biota also reduced interspecific competition impacts on S.alterniflora, although it remained competitively inferior to P.australis across all treatments. Competitive interactions and responses to nutrients did not differ among P.australis lineages, indicating that interspecific competition and nutrient deposition may not be key drivers of P.australis invasion in North America. Although soil biota, interspecific competition, and nutrient availability appear to have no direct impact on the success of invasive P.australis lineages in North America, intraspecific lineage variation in indirect spillover and soil legacies from P.australis occur and may have important implications for co-occurring native species and restoration of invaded habitats. Our study integrates multiple factors linked to plant invasions, highlighting that indirect interactions are likely commonplace in influencing plant community dynamics and invasion success and impacts.

Drivers of invasive tree and shrub natural regeneration in temperate forests

We assessed drivers of ecological success along resource availability gradients for three invasive woody species: Prunus serotina Ehrh., Quercus rubra L. and Robinia pseudoacacia L. We aimed to check how much of invasion success, measured by invader biomass, is explained by propagule pressure and plant community invasibility. Using 3 years of observations from 372 study plots (100 m2 each) in temperate forests of Wielkopolski National Park (Poland) we investigated the hierarchy of predictors and partial dependencies using the random forest method. Our study indicated that propagule pressure explained more variance in success of invaders than invasibility—describing availability of resources and competitors in understory vegetation. We also found different responses of seedlings and saplings, connected with dependence on stored carbohydrates, which decreased seedling responses to resource availability gradients. However, resource availability (light and leaf litter predictors) had greater influence than predictors describing understory vegetation. Based on importance and response strength the species studied may be arranged by decreasing requirements for soil fertility and acidity: P. serotina < Q. rubra < R. pseudoacacia, whereas for light requirements and competition vulnerability the order is: P. serotina > Q. rubra > R. pseudoacacia. However, low light requirements of R. pseudoacacia may be biased by high proportion of sprouts supplied by parental trees. Results provide guidelines for effective management of invasive woody species in forest ecosystems and describe complex interactions between factors studied on ecological success of invaders.

Solidago canadensis invasion affects soil N-fixing bacterial communities in heterogeneous landscapes in urban ecosystems in East China

Soil nitrogen-fixing bacterial communities (SNB) can increase the level of available soil N via biological N-fixation to facilitate successful invasion of several invasive plant species (IPS). Meanwhile, landscape heterogeneity can greatly enhance regional invasibility and increase the chances of successful invasion of IPS. Thus, it is important to understand the soil micro-ecological mechanisms driving the successful invasion of IPS in heterogeneous landscapes. This study performed cross-site comparisons, via metagenomics, to comprehensively analyze the effects of Solidago canadensis invasion on SNB in heterogeneous landscapes in urban ecosystems. Rhizospheric soil samples of S. canadensis were obtained from nine urban ecosystems [Three replicate quadrats (including uninvaded sites and invaded sites) for each type of urban ecosystem]. S. canadensis invasion did not significantly affect soil physicochemical properties, the taxonomic diversity of plant communities, or the diversity and richness of SNB. However, some SNB taxa (i.e., f_Micromonosporaceae, f_Oscillatoriaceae, and f_Bacillaceae) changed significantly with S. canadensis invasion. Thus, S. canadensis invasion may alter the community structure, rather than the diversity and richness of SNB, to facilitate its invasion process. Of the nine urban ecosystems, the diversity and richness of SNB was highest in farmland wasteland. Accordingly, the community invasibility of farmland wasteland may be higher than that of the other types of urban ecosystem. In brief, landscape heterogeneity, rather than S. canadensis invasion, was the strongest controlling factor for the diversity and richness of SNB. One possible reason may be the differences in soil electrical conductivity and the taxonomic diversity of plant communities in the nine urban ecosystems, which can cause notable shifts in the diversity and richness of SNB.

It takes one to know one: Similarity to resident alien species increases establishment success of new invaders

AbstractAimDarwin's naturalization hypothesis states that dissimilarity to native species may benefit alien species establishment due to empty niches and reduced competition. We here add a new dimension to large‐scale tests of community invasibility, investigating the role that previously established alien species play in facilitating or hindering new invasions in plant communities.LocationPermanent grasslands across France (including mainland and Corsica), as a receding ecosystem of great conservation importance.MethodsFocusing on 121 alien plant species occurring in 7,215 vegetation plots, we quantified biotic similarity between new invaders and resident alien species (i.e., alien species with longer residence times) based on phylogenetic and trait distances. Additionally, we calculated distances to native species for each alien species and plot. Using multispecies distribution models, we analysed the influence of these biotic similarity measures and additional covariates on establishment success (presence/absence) of new invaders.ResultsWe found that biotic similarity to resident alien species consistently increased establishment success of more recently introduced species. Phylogenetic relatedness to previous invaders had an equally strong positive effect as relatedness to native species. Conversely, trait similarity to natives hindered alien establishment as predicted by Darwin's naturalization hypothesis. These results highlight that various mechanisms may act simultaneously to determine alien establishment success.Main conclusionsOur results suggest that, with greater similarity among alien species, invasion success increases. Such a pattern may arise either due to actual facilitation among invaders or as a result of weaker competitive interactions among invaders than between native and alien species, leading to an indirect facilitative effect. Alternatively, recent environmental changes (e.g., eutrophication, climate change) may have added new environmental filters. Determining how initial invasions might pave the road for subsequent invasions is crucial for effective multispecies management decisions and contributes a new aspect to our understanding of community assembly.

Structuring evidence for invasional meltdown: broad support but with biases and gaps

Negative interactions have been suggested as a major barrier for species arriving in a new habitat. More recently, positive interactions drew attention from community assembly theory and invasion science. The invasional meltdown hypothesis (IMH) introduced the idea that positive interactions among non-native species could facilitate one another’s invasion, even increasing their impact upon the native community. Many studies have addressed IMH, but with contrasting results, reflecting various types of evidence on a multitude of scales. Here we use the hierarchy-of-hypotheses (HoH) approach to differentiate key aspects of IMH, organizing and linking empirical studies to sub-hypotheses of IMH. We also assess the level of empirical support for each sub-hypothesis based on the evidence reported in the studies. We identified 150 studies addressing IMH. The majority of studies support IMH, but the evidence comes from studies with different aims and questions. Supporting studies at the community or ecosystem level are currently rare. Evidence is scarce for marine habitats and vertebrates. Few sub-hypotheses are questioned by more than 50% of the evaluated studies, indicating that non-native species do not affect each other’s survival, growth, reproduction, abundance, density or biomass in reciprocal A ↔ B interactions. With the HoH for IMH presented here, we can monitor progress in empirical tests and evidences of IMH. For instance, more tests at the community and ecosystem level are needed, as these are necessary to address the core of this hypothesis.