The impact of interactions on invasion and colonization resistance in microbial communities

In human microbiota, the prevention or promotion of invasions can be crucial to human health. Invasion outcomes, in turn, are impacted by the composition of resident communities and interactions of resident members with the invader. Here we study how interactions influence invasion outcomes in microbial communities, when interactions are primarily mediated by chemicals that are released into or consumed from the environment. We use a previously developed dynamic model which explicitly includes species abundances and the concentrations of chemicals that mediate species interaction. Using this model, we assessed how species interactions impact invasion by simulating a new species being introduced into an existing resident community. We classified invasion outcomes as resistance, augmentation, displacement, or disruption depending on whether the richness of the resident community was maintained or decreased and whether the invader was maintained in the community or went extinct. We found that as the number of invaders introduced into the resident community increased, disruption rather than augmentation became more prevalent. With more facilitation of the invader by the resident community, resistance outcomes were replaced by displacement and augmentation. By contrast, with more facilitation among residents, displacement outcomes shifted to resistance. When facilitation of the resident community by the invader was eliminated, the majority of augmentation outcomes turned into displacement, while when inhibition of residents by invaders was eliminated, invasion outcomes were largely unaffected. Our results suggest that a better understanding of interactions within resident communities and between residents and invaders is crucial to predicting the success of invasions into microbial communities.     

Siderophores drive invasion dynamics in bacterial communities through their dual role as public good versus public bad

Microbial invasions can compromise ecosystem services and spur dysbiosis and disease in hosts. Nevertheless, the mechanisms determining invasion outcomes often remain unclear. Here, we examine the role of iron-scavenging siderophores in driving invasions of Pseudomonas aeruginosa into resident communities of environmental pseudomonads. Siderophores can be ‘public goods’ by delivering iron to individuals possessing matching receptors; but they can also be ‘public bads’ by withholding iron from competitors lacking these receptors. Accordingly, siderophores should either promote or impede invasion, depending on their effects on invader and resident growth. Using supernatant feeding and invasion assays, we show that invasion success indeed increased when the invader could use its siderophores to inhibit (public bad) rather than stimulate (public good) resident growth. Conversely, invasion success decreased the more the invader was inhibited by the residents’ siderophores. Our findings identify siderophores as a major driver of invasion dynamics in bacterial communities under iron-limited conditions.     

Species diversity reduces invasion success in pathogen‐regulated communities

The loss of natural enemies is thought to explain why certain invasive species are so spectacularly successful in their introduced range. However, if losing natural enemies leads to unregulated population growth, this implies that native species are themselves normally subject to natural enemy regulation. One possible widespread mechanism of natural enemy regulation is negative soil feedbacks, in which resident species growing on home soils are disadvantaged because of a build‐up of species‐specific soil pathogens. Here we construct simple models in which pathogens cause resident species to suffer reduced competitive ability on home soils and consider the consequences of such pathogen regulation for potential invading species. We show that the probability of successful invasion and its timescale depend strongly on the competitive ability of the invader on resident soils, but are unaffected by whether or not the invader also suffers reduced competitive ability on home soils (i.e. pathogen regulation). This is because, at the start of an invasion, the invader is rare and hence mostly encounters resident soils. However, the lack of pathogen regulation does allow the invader to achieve an unusually high population density. We also show that increasing resident species diversity in a pathogen‐regulated community increases invasion resistance by reducing the frequency of home‐site encounters. Diverse communities are more resistant to invasion than monocultures of the component species: they preclude a greater range of potential invaders, slow the timescale of invasion and reduce invader population size. Thus, widespread pathogen regulation of resident species is a potential explanation for the empirical observation that diverse communities are more invasion resistant.     

Traits linked with species invasiveness and community invasibility vary with time,stage and indicator of invasion in a long‐term grassland experiment

Much uncertainty remains about traits linked with successful invasion – the establishment and spread of non‐resident species into existing communities. Using a 20‐year experiment, where 50 non‐resident (but mostly native) grassland plant species were sown into savannah plots, we ask how traits linked with invasion depend on invasion stage (establishment, spread), indicator of invasion success (occupancy, relative abundance), time, environmental conditions, propagule rain, and traits of invaders and invaded communities. Trait data for 164 taxa showed that invader occupancy was primarily associated with traits of invaders, traits of recipient communities, and invader‐community interactions. Invader abundance was more strongly associated with community traits (e.g. proportion legume) and trait differences between invaders and the most similar resident species. Annuals and invaders with high‐specific leaf area were only successful early in stand development, whereas invaders with conservative carbon capture strategies persisted long‐term. Our results indicate that invasion is context‐dependent and long‐term experiments are required to comprehensively understand invasions.     

Facilitation promotes invasions in plant‐associated microbial communities

While several studies have established a positive correlation between community diversity and invasion resistance, it is less clear how species interactions within resident communities shape this process. Here, we experimentally tested how antagonistic and facilitative pairwise interactions within resident model microbial communities predict invasion by the plant–pathogenic bacterium Ralstonia solanacearum. We found that facilitative resident community interactions promoted and antagonistic interactions suppressed invasions both in the lab and in the tomato plant rhizosphere. Crucially, pairwise interactions reliably explained observed invasion outcomes also in multispecies communities, and mechanistically, this was linked to direct inhibition of the invader by antagonistic communities (antibiosis), and to a lesser degree by resource competition between members of the resident community and the invader. Together, our findings suggest that the type and strength of pairwise interactions can reliably predict the outcome of invasions in more complex multispecies communities.     

A meta-analysis of biotic resistance to exotic plant invasions

Biotic resistance describes the ability of resident species in a community to reduce the success of exotic invasions. Although resistance is a well‐accepted phenomenon, less clear are the processes that contribute most to it, and whether those processes are strong enough to completely repel invaders. Current perceptions of strong, competition‐driven biotic resistance stem from classic ecological theory, Elton's formulation of ecological resistance, and the general acceptance of the enemies‐release hypothesis. We conducted a meta‐analysis of the plant invasions literature to quantify the contribution of resident competitors, diversity, herbivores and soil fungal communities to biotic resistance. Results indicated large negative effects of all factors except fungal communities on invader establishment and performance. Contrary to predictions derived from the natural enemies hypothesis, resident herbivores reduced invasion success as effectively as resident competitors. Although biotic resistance significantly reduced the establishment of individual invaders, we found little evidence that species interactions completely repelled invasions. We conclude that ecological interactions rarely enable communities to resist invasion, but instead constrain the abundance of invasive species once they have successfully established.     

Non-random recruitment of invader species in experimental grasslands

To assess potential effects of seed limitation, characteristics of invader species and characteristics of established plant communities on recruitment success, we conducted a split-plot experiment factorially combining three weeding treatments corresponding to increasing successional age (regular weeding相似文献   

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.     

Species diversity, invasion, and alternative community states in sequentially assembled communities

The relationship between resident species diversity and invasion is generally negative in experimental studies but takes various forms in observational studies of natural communities. We hypothesized that stochastic species colonization, which applies to natural communities but not to experimental communities generally assembled through simultaneous species introduction, may lead to nonnegative diversity-invasion relationships via incurring priority effects. To test this hypothesis, we manipulated both resident species diversity and colonization history in sequentially assembled communities of bacterivorous protist species. We found that, despite a significant effect of assembly history on invader abundance, invader abundance decreased with diversity. This result was largely driven by positive selection effects associated with the dominant influence of an invasion-resistant species, which shared the most similar resource use pattern with the invader, and by the overall weak priority effects observed for the resident communities. Increasing species diversity, however, significantly strengthened priority effects, providing the first experimental support for the idea that larger species pools promote alternative community states. We suggest that elucidating mechanisms regulating the strength of priority effects may help in understanding variation in diversity-invasion relationships among natural communities.     

Spatial interplay of plant competition and consumer foraging mediate plant coexistence and drive the invasion ratchet

Indirect effects may play an important role in structuring plant communities. Using a spatially explicit model of consumer foraging and plant competition, we demonstrate how the relationship between the spatial area over which plants compete and the spatial scale of consumer behaviour can determine the outcome of competition when one plant species provides a refuge for mobile consumers (i.e. refuge-mediated apparent competition). Once an initial population of the invader is established, complete invasion may be inevitable because of an ever-advancing invasion front ratchets forward driven by a feeding front of mobile consumers. Because the spatial extent of apparent competition determines the area available for colonization, consumers may also dictate the rate at which an invasion occurs. We find that, as long as refuge-mediated apparent competition is sufficiently localized, invasion is possible even in systems characterized by low overall levels of consumer pressure. Moreover, we show that a stable equilibrium can result in which both resident and invading plants coexist, suggesting that spatial heterogeneity created by refuge-mediated apparent competition may be important in mediating coexistence in plant communities. The spatial interplay of consumer behaviour and plant competition may be an underappreciated mechanism affecting the composition, diversity and spatial pattern of plant communities.     

Intraspecific genotypic richness and relatedness predict the invasibility of microbial communities

Biological invasions can lead to extinction events in resident communities and compromise ecosystem functioning. We tested the effect of two widespread biodiversity measurements, genotypic richness and genotypic dissimilarity on community invasibility. We manipulated the genetic structure of bacterial communities (Pseudomonas fluorescens) and submitted them to invasion by Serratia liquefaciens. We show that the two diversity measures impact on invasibility via distinct and additive mechanisms. Genotypic dissimilarity of the resident communities linearly increased productivity and in parallel decreased invasion success, indicating that high dissimilarity prevents invasion through niche pre-emption. By contrast, genotypic richness exerted a hump-shaped effect on invasion and was linked to the production of toxins antagonistic to the invader. This effect peaked at intermediate richness, suggesting that high richness levels may increase invasibility. Invasibility could be well predicted by the combination of these two mechanisms, documenting that both genotypic richness and dissimilarity need to be considered, if we are to understand the biotic properties determining the susceptibility of ecosystems to biological invasions.     

Consumer diversity mediates invasion dynamics at multiple trophic levels

Theory and recent experiments, mostly focused on plants, indicate that biodiversity can reduce invasion success, but diversity effects on mobile animal invasion have received little attention. We tested effects of mobile crustacean grazer diversity (species richness) on the establishment of invaders at multiple trophic levels in flow-through seagrass mesocosms. On average, increasing diversity of resident grazers reduced population growth and biomass of experimentally introduced grazers. This increase in invasion resistance was concurrent with reductions in food and habitat availability and increases in resident density, paralleling previous results with plants. In many cases, mixtures of resident species resisted invasion better than did any single resident species, arguing that interactions among residents, rather than a sampling mechanism, explained diversity effects on invasion. Higher grazer diversity also generally reduced biomass of naturally recruiting invertebrates and algae and shifted epiphytic community dominance from algae to sessile invertebrates. Exploitation competition, then, appears to contribute to the diversity effect on invasion in both plant and animal systems. Our results further suggest that resident competitive advantage may also be at work in multi-trophic level systems. Thus, negative effects of local diversity on invasion appear general, and trophically mediated processes can also strongly influence invader success and identity in multi-trophic level systems.     

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.     

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.     

Mutualism as a constraint on invasion success for legumes and rhizobia

Because hereditary symbiont transmission is normally absent in the mutualism of legume plants and root‐nodule bacteria (rhizobia), dispersing plants may often arrive at new habitats where mutualist partners are too rare to provide full benefits. Factors governing invasion success were explored by analysing a system of two coupled pairwise competition models: a legume invader competing with a resident non‐mutualistic plant, and a rhizobial population competing with a resident population of nonsymbiotic bacteria. The non‐linear dependence of benefits on partner abundance in this mutualism creates the possibility of two alternative population size equilibria, so that a threshold density can exist for invasion. If legumes and rhizobia exceed a critical population size, both species achieve rapid population growth, while if initial densities of both species are below their respective thresholds, they remain rare and are thus vulnerable to extinction in the presence of competitors. Overall, the results indicate that legumes may often fail at colonization attempts within habitats where mutualist partners are scarce. Data on legume prevalence in island floras and rates of geographical spread by legume weeds are consistent with this inference. Predictive insights about invasiveness may emerge from comparative research on key traits identified by the model, especially the shape of the function determining the number of nodules formed at low rhizobial density.     

Differential influence of Pomphorhynchus laevis (Acanthocephala) on the behaviour of native and invader gammarid species.

Although various species of acanthocephalan parasites can increase the vulnerability of their amphipod intermediate hosts to predation, particularly by altering their photophobic behaviour, their influence on the structure of amphipod communities and the success of invader species has so far received little attention. We compared the prevalence and behavioural influence of a fish acanthocephalan parasite, Pomphorhynchus laevis, in two species of amphipods, Gammarus pulex and Gammarus roeseli in sympatry in the river Ouche (Burgundy, eastern France). There, G. pulex is a resident species, whereas G. roeseli is a recent coloniser. Both uninfected G. pulex and G. roeseli were strongly photophobic, although less so in the invading species. However, there was no significant difference in reaction to light between infected and uninfected G. roeseli, whereas infected G. pulex were strongly photophilic. We discuss our results in relation to the parasite's ability to manipulate invading host species, the possibility that resistant individuals have been selected during the invasion process, and the role that acanthocephalan parasites can play in shaping the structure of amphipod communities.     

Ecological Invasion,Roughened Fronts,and a Competitor’s Extreme Advance: Integrating Stochastic Spatial-Growth Models

Both community ecology and conservation biology seek further understanding of factors governing the advance of an invasive species. We model biological invasion as an individual-based, stochastic process on a two-dimensional landscape. An ecologically superior invader and a resident species compete for space preemptively. Our general model includes the basic contact process and a variant of the Eden model as special cases. We employ the concept of a “roughened” front to quantify effects of discreteness and stochasticity on invasion; we emphasize the probability distribution of the front-runner’s relative position. That is, we analyze the location of the most advanced invader as the extreme deviation about the front’s mean position. We find that a class of models with different assumptions about neighborhood interactions exhibits universal characteristics. That is, key features of the invasion dynamics span a class of models, independently of locally detailed demographic rules. Our results integrate theories of invasive spatial growth and generate novel hypotheses linking habitat or landscape size (length of the invading front) to invasion velocity, and to the relative position of the most advanced invader.     

Mycorrhizal suppression and phosphorus addition influence the stability of plant community composition and function in a temperate steppe

Nutrient enrichment can reduce ecosystem stability, typically measured as temporal stability of a single function, e.g. plant productivity. Moreover, nutrient enrichment can alter plant–soil interactions (e.g. mycorrhizal symbiosis) that determine plant community composition and productivity. Thus, it is likely that nutrient enrichment and interactions between plants and their soil communities co-determine the stability in plant community composition and productivity. Yet our understanding as to how nutrient enrichment affects multiple facets of ecosystem stability, such as functional and compositional stability, and the role of above–belowground interactions are still lacking. We tested how mycorrhizal suppression and phosphorus (P) addition influenced multiple facets of ecosystem stability in a three-year field study in a temperate steppe. Here we focused on the functional and compositional stability of plant community; functional stability is the temporal community variance in primary productivity; compositional stability is represented by compositional resistance, turnover, species extinction and invasion. Community variance was partitioned into population variance defined as community productivity weighted average of the species temporal variance in performance, and species synchrony defined as the degree of temporal positive covariation among species. Compared to treatments with mycorrhizal suppression, the intact AM fungal communities reduced community variance in primary productivity by reducing species synchrony at high levels of P addition. Species synchrony and population variance were linearly associated with community variance with the intact AM fungal communities, while these relationships were decoupled or weakened by mycorrhizal suppression. The intact AM fungal communities promoted the compositional resistance of plant communities by reducing compositional turnover, but this effect was suppressed by P addition. P addition increased the number of species extinctions and thus promoted compositional turnover. Our study shows P addition and AM fungal communities can jointly and independently modify the various components of ecosystem stability in terms of plant community productivity and composition.     

Marine biogeography and ecology: invasions and introductions

Although biogeography and ecology had previously been considered distinct disciplines, this outlook began to change in the early 1990s. Several people expressed interest in creating a link that would help ecologists become more aware of external influences on communities and help biogeographers realize that distribution patterns had their genesis at the community level. They proposed an interdisciplinary approach called macroecology. This concept has been aided by the advent of phylogeography, for a better knowledge of genetic relationships has had great interdisciplinary value. Two areas of research that should obviously benefit from a macroecological approach are: (1) the question of local vs. regional diversity and (2) the question of whether invader species pose a threat to biodiversity. The two questions are related, because both deal with the vulnerability of ecosystems to penetration by invading species. Biogeographers, who have studied the broad oceanic patterns of dispersal and colonization, tend to regard isolated communities as being open to invasion from areas with greater biodiversity. It became evident that many wide‐ranging species were produced in centres of origin, and that the location of communities with respect to such centres had a direct effect on the level of species diversity. Ecologists, in earlier years, thought that a community could become saturated with species and would thereafter be self‐sustaining. But recent research has shown that saturation is probably never achieved and that the assembly of communities and their maintenance is more or less dependent on the invasion of species from elsewhere. The study of invasions that take place in coastal areas, usually the result of ship traffic and/or aquaculture imports, has special importance due to numerous opinions expressed by scientists and policy‐makers that such invasions are a major threat to biodiversity. However, none of the studies so far conducted has identified the extinction of a single, native marine species due to the influence of an exotic invader. Furthermore, fossil evidence of historical invasions does not indicate that invasive species have caused native extinctions or reductions in biodiversity.     

Connectance determines invasion success via trophic interactions in model food webs

Human induced global change has greatly altered the structure and composition of food webs through the invasion of non‐native species and the extinction of native species. Much attention has been paid to the effects of species deletions on food web structure and stability. However, recent empirical evidence suggests that for most taxa local species richness has increased as successful invasions outpace extinctions at this scale. This pattern suggests that food webs, which represent feeding interactions at the local scale, may be increasing in species richness. Knowledge of how food web structure relates to invasive species establishment and the effect of successful invaders on subsequent food web structure remains an unknown but potentially important aspect of global change. Here we explore the effect of food web topology on invasion success in model food webs to develop hypotheses about how the distribution of biodiversity across trophic levels affects the success of invasion at each trophic level. Our results suggest a connectance (C) based framework for predicting invasion success in food webs due to the way that C constrains the number of species at each trophic level and thus the number of potential predators and prey for an invader at a given trophic level. We use the relationship between C and the proportion of species at each trophic level in 14 well studied food webs to make the following predictions; 1) the success of basal invaders will increase as C increases due to the decrease in herbivores in high C webs, 2) herbivore invasion success will decrease as C increases due to the decrease in the proportion of basal species and increase in intermediate species and omnivores in high C webs. 3) Top predator invasion success will increase as C increases due to the increase in intermediate prey species. However, it is not clear how the relative influence of trophic structure compares to empirically known predictors of invasion success such as invader traits, propagule pressure, and resource availability.