Distinct Community States Research Articles

Primary succession of Bifidobacteria drives pathogen resistance in neonatal microbiota assembly

Human microbiota assembly commences at birth, seeded by both maternal and environmental microorganisms. Ecological theory postulates that primary colonizers dictate microbial community assembly outcomes, yet such microbial priority effects in the human gut remain underexplored. Here using longitudinal faecal metagenomics, we characterized neonatal microbiota assembly for a cohort of 1,288 neonates from the UK. We show that the pioneering neonatal gut microbiota can be stratified into one of three distinct community states, each dominated by a single microbial species and influenced by clinical and host factors, such as maternal age, ethnicity and parity. A community state dominated by Enterococcus faecalis displayed stochastic microbiota assembly with persistent high pathogen loads into infancy. In contrast, community states dominated by Bifidobacterium, specifically B. longum and particularly B. breve, exhibited a stable assembly trajectory and long-term pathogen colonization resistance, probably due to strain-specific functional adaptions to a breast milk-rich neonatal diet. Consistent with our human cohort observation, B. breve demonstrated priority effects and conferred pathogen colonization resistance in a germ-free mouse model. Our findings solidify the crucial role of Bifidobacteria as primary colonizers in shaping the microbiota assembly and functions in early life.

O-276 Implantation failure is not associated with seven different vaginal bacterial community-state types; however, a distinct signature, including Ureaplasma parvum, is strongly linked to treatment failure

Abstract Study question Is microbial diversity or vaginal bacterial composition associated with embryonic implantation in women undergoing frozen-thawed embryo transfer? Summary answer While global diversity measures of microbial composition do not correlate with positive implantation, specific taxa may serve as predictors of treatment outcomes. What is known already Clinical studies have indicated a reduced likelihood of embryo implantation in women diagnosed with microbial dysbiosis. The application of Next-Generation Sequencing for microbiota analysis has significantly advanced our understanding of vaginal microbiome composition. Recent studies have proposed distinct vaginal microbiome patterns as potential predictors of IVF treatment success or failure. However, the evidence regarding the biological basis of test algorithms (such as the independent association of distinct community state types with outcomes) and the reliability of diagnostic test performance (including replicability and external validity) in predicting implantation success or failure in women undergoing embryo transfer remains less robust. Study design, size, duration In a prospective, single-centre observational cohort study (05/2017-03/2019, NCT03507673, local ethical approval reference no.18-005), conducted at a university-affiliated centre for reproductive medicine in Germany, vaginal swab samples were collected from infertile, asymptomatic female patients (n = 260), aged 18-45, undergoing frozen-thawed embryo transfer. Patients, each contributing a single treatment cycle, received sequential oral estradiol and oral estradiol/dydrogesterone in an HRT regimen for FET. Vaginal swabs were taken from baseline visit before transfer day and stored at –80 °C. Participants/materials, setting, methods 16S rRNA gene amplicon sequencing was performed targeting V3/V4 hypervariable region using Illumina MiSeq platform. Raw data assembly, filtering and taxonomic assignment were performed using mothur against EzBioCloud database. Statistical analyses, and visualization were conducted using R. Community structure via hierarchical clustering, alpha and beta diversity, and indicator species via LefSe analysis were assessed. Main results and the role of chance Based on a hierarchical clustering using the 25 most abundant bacterial taxa, seven separate community state types (CSTs) being either dominated by species of the genus Lactobacillus (CST-A, CST-B, CST-C, CST-E, CST-G) or Gardnerella (CST-F) or presenting a variable community composition (CST-D) were identified. A community state type dominated by Lactobacillus fornicalis was found in a small fraction of women and enhanced resolution among the Gardnerella genus impacts CST definitions in this sample. Embryonic implantation had no significant association to either alpha diversity measures or CST distribution; likewise, the previously suggested ratio of relative abundances of Lactobacillus iners vs. Lactobacillus crispatus was not associated with implantation. Lactobacillus fornicalis is positively linked to implantation, while Lactobacillus iners and Ureaplasma parvum were linked to implantation failure: above 50% relative abundance of Lactobacillus fornicalis, was associated with 75% positive implantation. Conversely, just 0.5% relative abundance of Ureaplasma parvum was associated with a decrease in implantation rate to approximately 20%. LefSe and binary regression analysis identified a relatively rare (6% in our sample) microbiome signature, mostly from CST-C (Lactobacillus iners dominated) with a very low likelihood of implantation, with Ureaplasma parvum as the common denominator. Limitations, reasons for caution Studies on the vaginal microbiome in IVF face issues such as study design variability, small sample-sizes, limited control for temporal dynamics, and patient heterogeneity. Attempts at establishing causal relationships through interventional studies in IVF have been limited and inconclusive. Understanding the microbiome’s functional aspects is crucial for reproductive outcome elucidation. Wider implications of the findings Current study findings don’t confirm previously reported associations between common vaginal bacterial CSTs and outcome. Therefore, screening for vaginal microbiome composition in asymptomatic women with subfertility is not recommended outside research. Signatures/taxae with an independent and strong negative association with implantation are likely to be rare yet deserve further attention. Trial registration number NCT03507673

Role of Vaginal microbiota and Human Papillomavirus infection in cervical Oncogenesis: where do we stand?

Vaginal Microbiota has emerged as an important contributing factor for viral persistence, which can lead to the process of oncogenesis. The female reproductive system has a unique microbiome that helps keeping health maintenance and protects against infection. Contrary to other mucosal sites, the vaginal microbiota often exhibits limited diversity and contains few Lactobacilli species, which were categorized into five distinct community state types using high-throughput 16S rRNA gene sequencing. According to current research, Human Papillomavirus (HPV) acquisition, persistence, and cervical cancer development are influenced by increasing vaginal microbiome diversity and decreased Lactobacillus spp. This review explores the intricate relationship between vaginal microbiota and HPV in the context of cervical cancer development. It discusses the impact of microbiota diversity and dysbiosis on the acquisition, persistence, and clearance of HPV. The review also emphasizes the role of vaginal microbiota in maintaining a healthy female reproductive system and explores the potential of various interventions, such as probiotics and prebiotics, in restoring a healthy vaginal environment. Challenges and future directions for a healthy vaginal environment are also revealed in the article.

Urban wastewater bacterial communities assemble into seasonal steady states

BackgroundMicroorganisms in urban sanitary sewers exhibit community properties that suggest sewers are a novel ecosystem. Sewer microorganisms present both an opportunity as a control point for wastewater treatment and a risk to human health. If treatment processes are to be improved and health risks quantified, then it is necessary to understand microbial distributions and dynamics within this community. Here, we use 16S rRNA gene sequencing to characterize raw influent wastewater bacterial communities in a 5-year time series from two wastewater treatment plants in Milwaukee, WI; influent wastewater from 77 treatment plants across the USA; and wastewater in 12 Milwaukee residential sewers.ResultsIn Milwaukee, we find that in transit from residences to treatment plants, the human bacterial component of wastewater decreases in proportion and exhibits stochastic temporal variation. In contrast, the resident sewer community increases in abundance during transit and cycles seasonally according to changes in wastewater temperature. The result is a bacterial community that assembles into two distinct community states each year according to the extremes in wastewater temperature. Wastewater bacterial communities from other northern US cities follow temporal trends that mirror those in Milwaukee, but southern US cities have distinct community compositions and differ in their seasonal patterns.ConclusionsOur findings provide evidence that environmental conditions associated with seasonal change and climatic differences related to geography predictably structure the bacterial communities residing in below-ground sewer pipes.DoZVvrVoCyPBLkvDJ_Cd4MVideo abstract

The Pulmonary Microbiome after Lung Transplantation is Associated with Gastroesophageal Reflux Disease, Inflammation, and Allograft Dysfunction

<h3>Purpose</h3> The pulmonary microbiome modulates the immune milieu in the lung allograft and is in turn influenced by gastroesophageal reflux disease (GERD). Although GERD has been linked to a higher risk of chronic lung allograft dysfunction (CLAD), the underlying role of the allograft microbiome is unknown. We assessed the relationships between the post-transplant pulmonary microbiome, GERD, inflammation, and allograft dysfunction. <h3>Methods</h3> 24 GERD and 51 no-GERD patients, defined by pH-impedance probe, had bronchoalveolar lavage (BAL) obtained at 3, 6, 9, and 12 months post-transplant. A separate cohort of 18 patients who underwent Nissen fundoplication for GERD within the first year post-transplant had BAL obtained before and after surgery. BAL were analysed for microbial communities and inflammatory cytokines using 16S rRNA sequencing and immunoassays, respectively. <h3>Results</h3> Hierarchical clustering of all GERD and no-GERD samples by Bray-Curtis dissimilarity identified three distinct community state types (CST) (Figure A). CST1 was positively associated with GERD and enriched in <i>Prevotella</i> and <i>Veillonella</i>. CST2 was negatively associated with GERD and enriched in <i>Streptococcus</i>. CST3 was independent of GERD and enriched in <i>Pseudomonas</i> and <i>Staphylococcus</i>. Compared to other CSTs, CST3 was associated with a greater proportion of patients with BAL inflammation, concurrent acute decline in lung function at 3 months, and earlier CLAD onset. Patients with the greatest decrease in BAL inflammation post-Nissen also had decreases in bacterial load and density of <i>Prevotella</i> (Figure B). <h3>Conclusion</h3> Pulmonary microbiota after lung transplantation cluster into three CSTs which differ by their associations with GERD. CST3 may represent a GERD-independent state of microbial dysbiosis which promotes inflammation and allograft rejection. Anti-GERD surgery appears to decrease GERD-associated microbiota.

Vaginotypes of the human vaginal microbiome.

The human vaginal environment harbours a community of bacteria that plays an important role in maintaining vaginal health and in protecting this environment from various urogenital infections. This bacterial population, also known as vaginal microbiota, has been demonstrated to be dominated by members of the Lactobacillus genus. Several studies employing 16S rRNA gene-based amplicon sequencing have classified the vaginal microbiota into five distinct community state types (CSTs) or vaginotypes. To deepen our understanding of the vaginal microbiota we performed an in-depth meta-analysis of 1312 publicly available datasets concerning healthy vaginal microbiome information obtained by metagenomics sequencing. The analysis confirmed the predominance of taxa belonging to the Lactobacillus genus, followed by members of the genera Gardnerella, Vibrio and Atopobium. Moreover, the statistical robustness offered by this meta-analysis allowed us to disentangle the species-level composition of dominant and accessory taxa constituting each vaginotype and to revisit and refine the previously proposed CST classification. In addition, a functional characterization of the metagenomic datasets revealed particular genetic features associated with each assigned vaginotype.

The penile microbiota of Black South African men: relationship with human papillomavirus and HIV infection

BackgroundTo date, the microbiota of the human penis has been studied mostly in connection with circumcision, HIV risk and female partner bacterial vaginosis (BV). These studies have shown that male circumcision reduces penile anaerobic bacteria, that greater abundance of penile anaerobic bacteria is correlated with increased cytokine levels and greater risk of HIV infection, and that the penile microbiota is an important harbour for BV-associated bacteria. While circumcision has been shown to significantly reduce the risk of acquiring human papillomavirus (HPV) infection, the relationship of the penile microbiota with HPV is still unknown. In this study, we examined the penile microbiota of HPV-infected men as well as the impact of HIV status.ResultsThe penile skin microbiota of 238 men from Cape Town (South Africa) were profiled using Illumina sequencing of the V3-V4 hypervariable regions of the 16S rRNA gene. Corynebacterium and Prevotella were found to be the most abundant genera. Six distinct community state types (CSTs) were identified. CST-1, dominated by Corynebacterium, corresponded to less infections with high-risk HPV (HR-HPV) relative to CSTs 2–6. Men in CST-5 had greater relative abundances of Prevotella, Clostridiales, and Porphyromonas and a lower relative abundance of Corynebacterium. Moreover, they were significantly more likely to have HPV or HR-HPV infections than men in CST-1. Using a machine learning approach, we identified greater relative abundances of the anaerobic BV-associated bacteria (Prevotella, Peptinophilus, and Dialister) and lower relative abundance of Corynebacterium in HR-HPV-infected men compared to HR-HPV-uninfected men. No association was observed between HIV and CST, although the penile microbiota of HIV-infected men had greater relative abundances of Staphylococcus compared to HIV-uninfected men.ConclusionsWe found significant differences in the penile microbiota composition of men with and without HPV and HIV infections. HIV and HR-HPV infections were strongly associated with greater relative abundances of Staphylococcus and BV-associated bacterial taxa (notably Prevotella, Peptinophilus and Dialister), respectively. It is possible that these taxa could increase susceptibility to HIV and HR-HPV acquisition, in addition to creating conditions in which infections persist. Further longitudinal studies are required to establish causal relationships and to determine the extent of the effect.

Factors associated with the composition and diversity of the cervical microbiota of reproductive-age Black South African women: a retrospective cross-sectional study.

BackgroundLactobacillus spp. are common bacteria in the cervical and vaginal microbiota (CVM) and are thought to represent a “healthy” cervicovaginal state. Several studies have found an independent association between ethnicity/race and cervical and vaginal microbiota (CVM) composition. Women of sub-Saharan African descent appear to be significantly more likely to have non-Lactobacillus-dominated CVM compared to women of European descent. The factors contributing to these differences remain to be fully elucidated. The CVM of Black South African women and factors influencing their CVM remain understudied. In this study, we characterized the cervical microbiota of reproductive-age South African women and assessed the associations of these microbiota with participants’ metadata.MethodsThe cervical microbiota from cervical DNA of 62 reproductive-age women were profiled by Ion Torrent sequencing the V4 hypervariable region of the bacterial 16S ribosomal RNA (rRNA) gene and analyzed with the Quantitative Insights Into Microbial Ecology (QIIME), UPARSE, and metagenomeSeq tools. Associations between cervical microbiota and participants’ metadata were assessed using GraphPad Prism, R packages and an in-house script.ResultsThe cervical microbiota clustered into three distinct community state types (CSTs): Lactobacillus iners-dominated cervical microbiota (CST I (38.7%, 24/62)), unclassified Lactobacillus-dominated cervical microbiota (CST II (4.8%, 3/62)), and diverse cervical microbiota (CST III (56.5%, 35/62)) with an array of heterogeneous bacteria, predominantly the bacterial vaginosis (BV)-associated Gardnerella, Prevotella, Sneathia, and Shuttleworthia. CST III was associated with BV (p = 0.001). Women in CST I were more likely to be on hormonal contraception, especially progestin-based, compared to women in CST III (odds ratio: 5.2 (95% CI [1.6–17.2]); p = 0.005). Women on hormonal contraception had a significantly lower alpha (Shannon indices: 0.9 (0.2–1.9) versus 2.3 (0.6–2.3); p = 0.025) and beta (permutational multivariate analysis of variance (PERMANOVA) pseudo-F statistic =4.31, p = 0.019) diversity compared to non-users. There was no significant difference in the alpha (Shannon indices: 1.0 (0.3–2.2) versus 1.9 (0.3–2.2); p = 0.483) and beta (PERMANOVA pseudo-F statistic = 0.89, p = 0.373) diversity in women with versus without human papillomavirus infection.ConclusionsThe majority of Black women in our study had non-Lactobacillus-dominated cervical microbiota. Additional studies are needed to examine whether such microbiota represent abnormal, intermediate or variant states of health. Lastly, the association of hormonal contraception with L. iners dominance requires further in-depth research to confirm this association, determine its biological mechanism and whether it has a beneficial effect on the cervicovaginal health.

640: Does first-trimester clean catch urine reflect the microbiome of the vagina or lower urinary tract?

Assess if microbes in clean catch urine of pregnant women resemble those of the vagina or the lower urinary tract. This is a prospective IRB-approved cohort study of 29 pregnant women with a singleton pregnancy <14 weeks. Exclusion criteria included recurrent UTI, antibiotic use since conception, and urinary tract anomalies. During their 1st trimester visit, consented subjects provided paired clean catch urine and vaginal swab samples. Samples were assessed by an Expanded Quantitative Culture (EQC) method that - compared to the standard urine culture method - uses more types of media and atmospheric conditions with a longer incubation period. Colony forming units (CFU) were counted for each morphologically distinct colony type; representative colonies were purified for identification using MALDI-TOF Mass Spectroscopy. Species identification and CFU counts were combined for analysis using the Bray-Curtis dissimilarity index (BC). A BC value of 0.5 was used to sort paired vaginal and urine samples as similar (<0.5) or different (≥0.5). For descriptive purposes, samples were hierarchically clustered with complete-linkage agglomeration by their vaginal culture results. The majority of the subjects were multiparous (n=20) with a mean age of 29.6 years. EQC detected live microbes in both urine and vaginal samples of all subjects. The vaginal samples clustered into 4 distinct community state structures (Figure). Based on BC indices, paired urine and vaginal samples were dissimilar (BC≥0.5) for 72% (n=21) of subjects. Of the 8 subjects with similar urine and vaginal microbiomes (BC<0.5), Lactobacillus was dominant in 88% of vaginal samples (n=7) and 100% of urine samples (n=8). In contrast, of the 21 subjects with dissimilar microbiomes (BC>0.5), a minority (24%, n=5) had a Lactobacillus-dominant urine sample (p<0.01). The clean catch urine microbiome of pregnant women in their 1st trimester is most often distinct from their vaginal microbiome, evidence that their lower urinary tract contains a unique microbiome. Women with dissimilar urine and vaginal microbiomes were less likely to have Lactobacillus-dominant urine. Given these findings, the vaginal microbiome should not be assumed to represent the lower urinary tract microbiome.

Quantifying the Adaptive Cycle

The adaptive cycle was proposed as a conceptual model to portray patterns of change in complex systems. Despite the model having potential for elucidating change across systems, it has been used mainly as a metaphor, describing system dynamics qualitatively. We use a quantitative approach for testing premises (reorganisation, conservatism, adaptation) in the adaptive cycle, using Baltic Sea phytoplankton communities as an example of such complex system dynamics. Phytoplankton organizes in recurring spring and summer blooms, a well-established paradigm in planktology and succession theory, with characteristic temporal trajectories during blooms that may be consistent with adaptive cycle phases. We used long-term (1994–2011) data and multivariate analysis of community structure to assess key components of the adaptive cycle. Specifically, we tested predictions about: reorganisation: spring and summer blooms comprise distinct community states; conservatism: community trajectories during individual adaptive cycles are conservative; and adaptation: phytoplankton species during blooms change in the long term. All predictions were supported by our analyses. Results suggest that traditional ecological paradigms such as phytoplankton successional models have potential for moving the adaptive cycle from a metaphor to a framework that can improve our understanding how complex systems organize and reorganize following collapse. Quantifying reorganization, conservatism and adaptation provides opportunities to cope with the intricacies and uncertainties associated with fast ecological change, driven by shifting system controls. Ultimately, combining traditional ecological paradigms with heuristics of complex system dynamics using quantitative approaches may help refine ecological theory and improve our understanding of the resilience of ecosystems.

Community divergence in a tropical forest following a severe cyclone

Cyclones are relatively infrequent, may cause massive and widespread disturbance to tropical regions, and are recognized as important determinants of the structure of tropical rainforest communities. Climate change scientists predict that the intensity of cyclones will increase in the future; understanding the long-term implications of these major disturbances for tropical forest composition and structure will be vital in anticipating and adapting to future changes and impacts. We established a long-term monitoring site in a rainforest area impacted by severe tropical Cyclone Larry which crossed the North Queensland coast of Australia in March 2006. We monitored recruitment, growth and mortality of nearly 17 000 seedlings in 90 quadrats across the study area for almost 5 years following the cyclone and measured the impact of variation in cyclone disturbance and debris load on community diversity, composition and dispersion as the forest recovered. We show that the level of structural disturbance sustained by the forest has a strong and immediate influence on community dynamics. Quadrats in severely disturbed areas, which were characterized by multiple treefalls and extensive canopy loss, had higher levels of diversity and variation in community assemblage than quadrats in areas characterized primarily by branch loss and defoliation. A rapid divergence in community composition between quadrats in the most- and least-severely disturbed areas resulted in the development of statistically distinct community states across relatively small scales. This provides further evidence that severe cyclones are important in maintaining species diversity in tropical forests.

Cultivated Vaginal Microbiomes Alter HIV-1 Infection and Antiretroviral Efficacy in Colonized Epithelial Multilayer Cultures

There is a pressing need for modeling of the symbiotic and at times dysbiotic relationship established between bacterial microbiomes and human mucosal surfaces. In particular clinical studies have indicated that the complex vaginal microbiome (VMB) contributes to the protection against sexually-transmitted pathogens including the life-threatening human immunodeficiency virus (HIV-1). The human microbiome project has substantially increased our understanding of the complex bacterial communities in the vagina however, as is the case for most microbiomes, very few of the community member species have been successfully cultivated in the laboratory limiting the types of studies that can be completed. A genetically controlled ex vivo model system is critically needed to study the complex interactions and associated molecular dialog. We present the first vaginal mucosal culture model that supports colonization by both healthy and dysbiotic VMB from vaginal swabs collected from routine gynecological patients. The immortalized vaginal epithelial cells used in the model and VMB cryopreservation methods provide the opportunity to reproducibly create replicates for lab-based evaluations of this important mucosal/bacterial community interface. The culture system also contains HIV-1 susceptible cells allowing us to study the impact of representative microbiomes on replication. Our results show that our culture system supports stable and reproducible colonization by VMB representing distinct community state types and that the selected representatives have significantly different effects on the replication of HIV-1. Further, we show the utility of the system to predict unwanted alterations in efficacy or bacterial community profiles following topical application of a front line antiretroviral.

O05.4 Association Between Chlamydia Trachomatis Genital Infection and the Vaginal Microbiome

Background Chlamydia trachomatis (CT) genital infection is one of the most widespread sexually transmitted infections (STIs). It is thought that certain types of vaginal microbiota are better able to prevent...

Identifying and evaluating causes of alternative community states in wetland plant communities

Ecological communities can vary greatly in species composition. Often this variation is discontinuous, in that abrupt changes in composition occur over small distances in space or short periods of time. A wide range of hypotheses from different subfields of ecology have been proposed to explain these patterns. I suggest a framework to quantitatively evaluate these hypotheses with observational data by characterizing 1) how community composition varies across sites in space, 2) how community composition varies through time, and 3) the possible drivers of this variation. I applied this approach to understand the community composition of producers in temporary and semipermanent wetlands in Michigan, USA. I identified several distinct community states which were variously dominated by particular plant functional groups (submerged, floating or emergent plants) or had no plants throughout a season. Evaluating possible hypotheses to explain this variation, I found that similar communities were not necessarily clustered near each other, suggesting that dispersal was not limited for these plants. Some sites exhibited a great deal of change in plant composition among years, shifting between two community states, but there was relatively little change at sites within a year. Moreover, these shifts did not occur in a particular order to suggest directional change or repeating cycles. Community composition was associated with several environmental variables such as pH, light and depth, and multivariate analyses suggested that species had complex, nonlinear responses to these possible drivers. Alternative stable states and interactions among multiple nonlinear drivers best explained the patterns observed in these wetlands. By formalizing initial data collection in other systems with the framework suggested here, we may gain insight into the causes of alternative community states beyond wetlands and the role of climate change and other anthropogenic forces in precipitating transitions between states.

Complex shifts between food web states in response to whole‐ecosystem manipulations

Food webs can respond in surprising and complex ways to temporary alterations in their species composition. When such a perturbation is reversed, food webs have been shown to either return to the pre‐perturbation community state or remain in the food web configuration that established during the perturbation. Here we report findings from a replicated whole‐lake experiment investigating food web responses to a perturbation and its consecutive reversal. We could identify three distinct community states in the food web that corresponded to the periods before, during and after the perturbation. Most importantly, we demonstrate the establishment of a distinct post‐perturbation food web configuration that differed from both the pre‐ and during‐perturbation communities in phytoplankton biomass and micro‐ and mesozooplankton species composition. We suggest that the pre‐ and post‐perturbation food web configurations may represent two alternative stable community states. We provide explanations for how each of the contrasting communities may be maintained through altered species interactions. These findings add to the discussion of how natural food webs react to environmental change and imply that the range of potential ecosystem dynamics in response to perturbations can be wider and more complex than is often recognized.

Thresholds and multiple community states in marine fouling communities: integrating natural history with management strategies

The epifaunal communities characteristic of the southern New England, USA, region between eastern Long Island Sound, CT, and Cape Cod, MA, comprise a complex system in which different mechanisms at certain threshold levels can cause the establishment of 4 distinct community states: (1) a diverse native community dominated by bryozoans and sponges most commonly found in open coast areas, (2) an invasive ascidian community characteristic of marinas and areas of coastal development, (3) a mussel-dominated community occurring after massive recruitment and (4) an ascidian community dominated by Diplosoma listerianum that occurs only in years following warm winters. Each of these states is fairly resilient, but the spatial extent and duration of each state can be highly variable. Transitions among the states occur if some set of threshold conditions are surpassed and reasonable predictions can be made based on knowledge of the natural history of the species within the system. Two sets of processes seem to control the resilience of each state and the thresh- olds beyond which a transition to a new state occurs: (1) fast and local processes such as within- population recruitment, predation or bioengineering by mussels and (2) slow regional processes such as climate change, coastal development or habitat restoration. Of these, coastal development and restoration efforts are under management control and could have large effects on these community states, in particular the native community. However, we may not be able to influence large-scale regional processes such as climate change that may favor non-native communities. Nevertheless, the different community states can be used as indicators of both local and regional management success and allow local management efforts to be put in the context of larger scale shifts in threshold condi- tions that affect regional community patterns.

PERTURBATIONS ALTER COMMUNITY CONVERGENCE, DIVERGENCE, AND FORMATION OF MULTIPLE COMMUNITY STATES

Environmental perturbations (e.g., disturbance, fertilization) commonly shift communities to a new mean state, but much less is known about their effects on the variability (dispersion) of communities around the mean, particularly when perturbations are combined. Community dispersion may increase or decrease (representing a divergence or convergence among communities) if changing environmental conditions alter species interactions or magnify small initial differences that develop during community assembly. We used data from an experimental study of disturbance and fertilization in a low-productivity grassland to test how these two perturbations affect patterns of species composition and abundance. We found that a one-time biomass reduction decreased community dispersion, which persisted over four growing seasons. Conversely, continuous fertilization increased community dispersion and, when combined with disturbance, led to the formation of three distinct community states. These results illustrate that perturbations can have differing effects on community dispersion. Attention to the variance in community responses to perturbations lends insight into how ecological interactions determine community structure, which may be missed when focusing only on mean responses. Furthermore, multiple perturbations may have complex effects on community dispersion, yielding convergence or divergence patterns that are difficult to predict based on analysis of single factors.