Invasibility Of Plant Communities Research Articles

Effects of Different Degrees of Xanthium spinosum Invasion on the Invasibility of Plant Communities in the Yili Grassland of Northwest China.

Studying the effects of different degrees of exotic plant invasion on native plants' community structure and plant diversity is essential for evaluating the harm caused to ecosystems by plant invasion. In this study, we investigated the effects of Xanthium spinosum, a widespread invasive species, on plant community species diversity and community stability in the Ili River Valley area of Xinjiang, China, under three invasion levels (no invasion and low, moderate, and heavy invasion), and the competitive advantage index, invasion intensity, and contribution of plant community species diversity to community stability and invasibility were determined for the prickly fungus under different degrees of invasion. The results show that there were significant differences (p < 0.05) in the species diversity and community stability of plant communities caused by different degrees of invasion of X. spinosum. The species diversity and stability of plant communities were negatively correlated with the community invasibility, competitive advantage, and invasion intensity of X. spinosum (p < 0.05); therefore, the competitive advantage and invasion intensity of X. spinosum increase with the increase of its invasion degree. On the contrary, community species diversity and stability decreased with the increase of its invasion degree, ultimately leading to differences in community invasibility under different invasion degrees. The Shannon-Wiener and Simpson's indices were the greatest contributors to community stability and invasibility, respectively. Moderate and heavy levels of invasion by X. spinosum reduced the diversity and stability of local plant communities, increased the invasibility of communities, and substantially affected the structures of plant communities. Therefore, the continued invasion by X. spinosum will have an immeasurable impact on the fragile ecosystems and diversity of indigenous species in Xinjiang. We recommend that this invasive species be controlled and eradicated at the early stages of invasion to prevent further harm.

Invasibility of Common Plant Community Types of the Middle Urals

This paper specifies the invasibility of common plant community types in the natural habitats of the Middle Urals. Invasibility was defined as the vulnerability of a community to alien plant species invasions, regardless of the conditions in which the community existed. We analyzed 749 vegetation relevés made in natural bogs, floodplains, rocky grasslands, meadows, and forest communities. We surveyed urban and non-urban habitats (30–40 km from the city). Invasibility was calculated in two different ways based on two parameters: the number and proportion of alien species in the relevé. These invasibility parameters are widely applicable and comparable, scale-independent, measurable, and reliable, based on data that do not require the destruction of ecosystems or biota and are based on available data. Based on the invasibility degree, we identified three community-type groups with two subgroups in one group. In ascending order of invasibility degree, the community types were arranged as follows: (1) bogs with and without trees; (2a) coniferous forests; grass communities in floodplains; (2b) deciduous forests, coniferous plantations, floodplain communities with woody plants; and (3) dry meadows and rocky grasslands. Obtained results of the assessment of different plant community invasibility may be used to understand patterns of alien plant distribution in local habitats and the reasons for the different vulnerability of communities to plant invasions.

Increased Leaf Bacterial Network Complexity along the Native Plant Diversity Gradient Facilitates Plant Invasion?

Understanding the mechanisms of biological invasion is critical to biodiversity protection. Previous studies have produced inconsistent relationships between native species richness and invasibility, referred to as the invasion paradox. Although facilitative interactions among species have been proposed to explain the non-negative diversity-invasibility relationship, little is known about the facilitation of plant-associated microbes in invasions. We established a two-year field biodiversity experiment with a native plant species richness gradient (1, 2, 4, or 8 species) and analyzed the effects of community structure and network complexity of leaf bacteria on invasion success. Our results indicated a positive relationship between invasibility and network complexity of leaf bacteria of the invader. Consistent with previous studies, we also found that native plant species richness increased the leaf bacterial diversity and network complexity. Moreover, the results of the leaf bacteria community assembly of the invader suggested that the complex bacteria community resulted from higher native diversity rather than higher invader biomass. We concluded that increased leaf bacterial network complexity along the native plant diversity gradient likely facilitated plant invasion. Our findings provided evidence of a potential mechanism by which microbes may affect the plant community invasibility, hopefully helping to explain the non-negative relationship between native diversity and invasibility.

Disturbance and indirect effects of climate warming support a plant invader in mountains

Climate warming and increased disturbance (resulting from intensified land use) are expected to enhance the invasibility of plant communities and the performance of exotic species also at high elevations, and thus pose additional threats to mountain ecosystems. The invasion success of introduced genotypes will also depend on their degree of pre‐adaption to high elevation climatic conditions, which may vary intra‐specifically across source populations. For populations currently spreading in the lowlands, climate warming might reduce the climatic distance to high‐elevation sites and thus remove a barrier to upwards spread.Here, we investigated the various facets of mountain invasions in a single, integrative experimental study. We applied a community transplant approach between high‐ and low‐elevation sites in the European Alps to address effects of climate warming and disturbance through land use on community invasibility and the performance of the exotic species Senecio inaequidens, a potential future plant invader in the Alps. Additionally, the transplant sites served as common gardens to test the influence of climatic pre‐adaptation to current (high site) and future (low site) climatic conditions on the performance of S. inaequidens in the transplanted communities. The 16 invasive central and western European S. inaequidens source population locations covered a wide geographic range, and thus a wide amplitude of climatic distances and presumed pre‐adaptation to our gardens.Our results attest to a strong effect of disturbance, which increased community invasibility, and promoted the performance of the exotic species. Contrary to our expectation, experimentally induced climate warming did not increase community invasibility. However, the performance of the S. inaequidens populations was positively related to their pre‐adaptation to the climatic conditions of our common gardens. Climate warming might thus promote the invasion of exotic species by reducing the climatic distance between mountain ranges and locations of potential source populations.

Competitive trait hierarchies of native communities and invasive propagule pressure consistently predict invasion success during grassland establishment

Invasive non-native plants challenge ecosystems restoration, and understanding the factors that determine the establishment of invasive plants is crucial to improve restoration outcomes. However, the drivers of invasibility of plant communities are not sufficiently clear, and combined effects are not understood. Therefore, we investigated the contribution of the main drivers of invasion success during early phases of restoration, i.e., biotic resistance, invasive propagule pressure, and environmental fluctuations. We compared the contribution of these drivers in a series of mesocosms experiments using designed grasslands as a model system, and Solidago gigantea as invasive model species. Two grassland communities were designed according to competitive trait hierarchies with different sowing patterns, reflecting variation in biotic resistance. We then manipulated invader propagule pressure and applied different scenarios of environmental fluctuation, i.e., flood, heat, and N fertilization. Invasive biomass was considered as proxy for invasion success, while native biomass represented restoration success. There were consistent effects of biotic resistance to S. gigantea invasion via competitive trait hierarchies in the three experiments. Communities dominated by species with high-competition traits were more resistant regardless of environmental fluctuation. Clumped seeding of the native community reduced invasibility, whereas high non-native propagule density increased invasion. The effects of environmental fluctuation were less consistent and context-dependent, thus playing a secondary role when compared to biotic drivers of invasion. Restoration initiatives on grasslands impacted by invasive plants should consider biotic resistance of the restored community as a key driver and the importance of controlling further arrivals of invasive species during community assembly.

A weed risk analytical screen to assist in the prioritisation of an invasive flora for containment

Prioritising weeds for control and deciding upon the type of control and its associated investment are fundamental to weed management planning. Risk analysis is central to this process, combining the activities of risk assessment, risk management and risk communication. Risk assessment methodology has a rich history, but management feasibility has typically been a secondary matter, dealt with separately or not at all. Determinants of management feasibility for weeds include the stage of invasion, weed biology, means of control and cost of weed control. Here, we describe a simple weed risk analytical screen that combines risk assessment with species traits that influence management feasibility. We consider stage of invasion, species biological/dispersal characteristics and plant community invasibility in a preliminary analysis of the risk posed by the non-native plant species on Christmas Island in the Indian Ocean. For each of 31 high-risk species considered to be ineradicable under existing funding constraints, we analyse the risk posed to two major plant communities: evergreen closed-canopy rainforest and semi-deciduous scrub forest. Weed risk ratings are combined with ratings for species-intrinsic feasibility of containment (based on a measure that combines time to reproduction with potential for long distance dispersal) to create preliminary rankings for containment specific to each community. These rankings will provide a key input for a more thorough analysis of containment feasibility – one that considers spatial distributions/landscape features, management aspects and the social environment. We propose a general non-symmetric relationship between weed risk and management feasibility, considering risk to be the dominant component of risk analysis. Therefore, in this analysis species are ranked according to their intrinsic containment feasibility within similar levels of risk to produce an initial prioritisation list for containment. Shade-tolerant weeds are of particular concern for the closed-canopy evergreen rainforest on Christmas Island, but a greater diversity of weeds is likely to invade the semi-deciduous scrub forest because of higher light availability. Nevertheless, future invasion of both communities will likely be conditioned by disturbance, both natural and anthropogenic. The plant communities of Christmas Island have undergone significant fragmentation because of clearing for phosphate mining and other purposes. With a substantial number of invasive plant species firmly established and having the potential to spread further, minimising future anthropogenic disturbance is paramount to reducing community invasibility and therefore conserving the island’s unique biodiversity.

Facilitation and the invasibility of plant communities

Abstract One of the most studied emergent functions of plant community diversity is the resistance of diverse communities to non‐native invasions. As emphasized in this Special Feature, facilitation among native species is a key mechanism by which biodiversity increases various functions, including resistance to invasion. However, when diverse assemblages facilitate non‐native species, diversity–invasibility resistance may be compromised. Here, I review the scientific literature on plant invasion in which facilitative interactions, either among native and non‐native plant species or among non‐native species, affect community invasibility. Native species can directly facilitate non‐native species, and also generate net indirect facilitative effects through suppressing species that compete with non‐native invaders, but examples of the latter are not common. Such direct and indirect facilitation among non‐native species contributes to ‘invasional meltdown’ that restructures native communities. In general, facilitative interactions between native and non‐native species increased with environmental stress, suggesting that community diversity might resist invasion more effectively in environmentally favourable sites, whereas in environmentally severe sites, facilitative interactions may contribute to invasibility. Synthesis. Native and non‐native species can facilitate each other in direct and indirect ways, with important consequences for the invasibility of communities. Facilitative interactions may alter the fundamental relationship between diversity and invasibility, particularly in environmentally severe habitats.

Effects of elevation and selective disturbance on soil climate and vegetation in big sagebrush communities

AbstractChanging climatic conditions prompt concerns about vegetation response to disturbance under future compared to past conditions. In this long‐term study, we examined soil climate and vegetation differences at lower, mid, and upper elevations in two separate locations in the Great Basin, USA. We hypothesized that soil climate and vegetation associations across the elevational gradient could help predict responses under future warming and drying. We measured soil water availability, soil temperatures, and vegetation cover in relation to fire and perennial herb removal at each elevation for 13–17 yr after treatment. Seasonal soil water availability increased, while soil temperature‐related variables decreased with increasing elevation. Soil water availability in spring was positively correlated with October through June precipitation (r2 = 0.53), while water availability in spring through fall was positively correlated with perennial plant cover (r2 = 0.65). Partition models separated low, mid, and high elevations by spring soil water availability and growing degree days when soil water was available (R2 = 0.93). Current soil climate and vegetation conditions at lower elevations could indicate future conditions at higher elevations under a warmer and drier climate. During the first years after removal of perennial grasses and forbs, there was an increase in soil water availability in spring at 13–30 cm soil depth that was associated with sagebrush establishment, particularly at upper elevations. In subsequent years, sagebrush continued to dominate even though little difference in soil water availability existed between disturbed and undisturbed plots. This indicates that quickly establishing sagebrush preempted resources and reduced perennial herb recovery. Resource preemption after disturbance will likely be a major driver of plant succession in the future as in the past. Species that establish best under future warmer and drier conditions are most likely to dominate after disturbance. A negative correlation (r2 = 0.34) between the standard deviation of annual spring soil water availability and perennial vegetation cover, which helps resist annual grass invasion, supports the hypothesis that greater resource fluctuation is associated with greater plant community invasibility. Current responses to fire and loss of native plant cover across elevational gradients can indicate future responses under a warmer and drier climate.

Functional Divergence Drives Invasibility of Plant Communities at the Edges of a Resource Availability Gradient

Invasive Alien Species (IAS) are a serious threat to biodiversity, severely affecting natural habitats and species assemblages. However, no consistent empirical evidence emerged on which functional traits or trait combination may foster community invasibility. Novel insights on the functional features promoting community invasibility may arise from the use of mechanistic traits, like those associated with drought resistance, which have been seldom included in trait-based studies. Here, we tested for the functional strategies of native and invasive assemblage (i.e., environmental filtering hypothesis vs. niche divergence), and we assessed how the functional space determined by native species could influence community invasibility at the edges of a resource availability gradient. Our results showed that invasive species pools need to have a certain degree of differentiation in order to persist in highly invaded communities, suggesting that functional niche divergence may foster community invasibility. In addition, resident native communities more susceptible to invasion are those which, on average, have higher resource acquisition capacity, and lower drought resistance coupled with an apparently reduced water-use efficiency. We advocate the use of a mechanistic perspective in future research to comprehensively understand invasion dynamics, providing also new insights on the factors underlying community invasibility in different ecosystems.

Twelve-year dynamics of alien and native understorey plants following variable retention harvesting in Nothofagus pumilio forests in Southern Patagonia

Disturbances have frequently been shown to increase the invasibility of plant communities. Harvesting, the most common and severe type of anthropogenic disturbance in forests, creates pulses of enhanced resource availability for alien plants and for native colonizers. However, it is unclear for how long initial changes in species composition of the understorey persist after harvesting. Using annually surveyed permanent plots (n = 72 plots across six stands), we analysed temporal changes of plant understorey composition of Patagonian Nothofagus pumilio forests during 12 years under three different conditions created by variable retention harvesting: retained forest patches (aggregates) of 30 m radius in a density of one patch ha−1 (AR), dispersed retention within the influence of the aggregate (DRI), retained dispersed single trees (DR); and primary unharvested forests as control (PF). Our results show that: (i) cover of native forest species in the understorey declined only very slowly in AR, but alien species cover increased strongly and became dominant nine years after harvesting (YAH); (ii) DR and DRI supported higher cover of alien species in the understorey than AR and PF, and alien species became dominant two to three YAH. Yet, they started to decline following a peak at eight YAH; (iii the cover of native forest species in the understorey was high in the beginning but alien species became dominant in DR and DRI after four and seven YAH, respectively. Tree regeneration significantly influenced understorey dynamics, i.e. (iv) in DR, a negative relationship with alien species cover, and a positive relationship with native colonizers from other habitats; (v) in DR and in DRI, this relationship was negative with alien species richness; (vi) in DRI, it was positive with native colonizers from associated environments (cover and richness). We conclude that alien plants invade Patagonian forests after harvesting, but decline with time when tree regeneration is established. Our results highlight the role of harvesting in facilitating plant invasions in forests, and how the spatio-temporal trajectories of such invasions are influenced by different levels of disturbance created by harvesting according to the position within the resultant matrix.

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.

Comparing traits of native and alien plants: Can we do better?

Abstract A frequent question in invasion ecology is whether the traits of alien plant species differ from natives. This question is increasingly addressed at the plant community scale to assess the role trait differences play in community assembly. Nevertheless, there remains considerable debate as to the value of native vs. alien trait comparisons. Recommendations are made to address seven conceptual and methodological limitations in current approaches used to assess trait differences between native and alien plants: account for spatial scale dependence, make an informed choice of traits, evaluate multiple trait dimensions, incorporate intraspecific trait variation, use appropriate null models, examine environmental gradients, and assess temporal variation in trait spectra. Tackling these limitations will dramatically improve the value of native vs. alien trait comparisons. Traits should be selected based on expectations of abiotic and/or biotic constraints. Phylogenetic distances can provide valuable insights but are no substitute for the measurement of individual traits when contrasting alien and native species. An ideal set of traits would include one or more leaf, stem, seed and root traits as well as flowering phenology and plant height at maturity. Comparisons of intraspecific trait variation among native and alien species within the same plant community are rare but could improve predictions of interspecific competition and the response of communities to plant invasions. The full plant community should be examined rather than selected species subsets and comparisons between native and alien species traits are better undertaken at a neighbourhood scale. The frequency distribution of alien and native species traits may change over time through competitive exclusion as well as contemporary evolution. Repeated surveys of species abundances and traits are required to understand the consequences of trait differences between aliens and natives for plant community assembly. Analyses of the role of species traits in plant invasions need to quantify differences in a priori identified traits within communities along suitable environmental gradients and test hypothetical trait patterns against appropriate null expectations. The infrequent application of such approaches may explain the limited generalisations regarding the role of trait differences between native and alien species in the invasion of plant communities. A plain language summary is available for this article.

Allelopathy and resource competition: the effects of Phragmites australis invasion in plant communities

BackgroundPhragmites australis, a ubiquitous wetland plant, has been considered one of the most invasive species in the world. Allelopathy appears to be one of the invasion mechanisms, however, the effects could be masked by resource competition among target plants. The difficulty of distinguishing allelopathy from resource competition among plants has hindered investigations of the role of phytotoxic allelochemicals in plant communities. This has been addressed via experiments conducted in both the greenhouse and laboratory by growing associated plants, Melaleuca ericifolia, Rumex conglomeratus, and model plant, Lactuca sativa at varying densities with the allelopathic plant, P. australis, its litter and leachate of P. australis litter. This study investigated the potential interacting influences of allelopathy and resource competition on plant growth–density relationships.ResultsIn greenhouse, the root exudates mediated effects showed the strongest growth inhibition of M. ericifolia at high density whereas litter mediated results revealed increased growth at medium density treatments compared to low and high density. Again, laboratory experiments related to seed germination and seedling growth of L. sativa and R. conglomeratus exhibited phytotoxicity decreased showing positive growth as plant density increased and vice versa. Overall, the differential effects were observed among experiments but maximum individual plant biomass and some other positive effects on plant traits such as root and shoot length, chlorophyll content occurred at an intermediate density. This was attributed to the sharing of the available phytotoxin among plants at high densities which is compatible to density-dependent phytotoxicity model.ConclusionsThe results demonstrated that plant–plant interference is the combined effect of allelopathy and resource competition with many other factors but this experimental design, target-neighbor mixed-culture in combination of plant grown at varying densities with varying level of phytotoxins, mono-culture, can successfully separate allelopathic effects from competition.

Recovery dynamics and invasibility of herbaceous plant communities after exposure to experimental climate extremes

Do climatic extremes increase the invasibility of plant communities, for example through the creation of gaps and the associated local surplus of available resources? To address this question, small experimental communities consisting of three forb species were first subjected to extreme drought and/or heat treatments in different seasons and species mortality and end-of-season biomass were examined. Then, the establishment of new species and their effects on the productivity of the community were recorded in two subsequent years without additional treatments.The immediate response to the experimentally induced extremes was similar in all three originally planted species, with drought treatments in summer and autumn, especially when combined with heat, inducing the greatest plant mortality. Recovery in terms of end-of-season aboveground biomass was species-specific however. The dominant species, the N-fixer Trifolium repens, recovered poorly from the drought and drought+heat treatments. Differences in community biomass between treatments and to the controls were no longer significant in the next year. Graminoid species, especially, successfully invaded the communities, possibly because of functional dissimilarity with the species already present. Invasibility in the year following the extreme events was increased in communities that had been exposed to both a heat wave and a drought, but the number of newly established species did not increase community productivity. The identities of invading species varied distinctly, but had no clear relation with the extreme events the communities had been exposed too.The induced climate extremes greatly affected the survival and productivity of the species and influenced the invasibility of the plant communities. However, none of the community properties seemed to be affected in the longer run, as the induced responses faded out after one or two years.

Are removal experiments effective tools for assessing plant community resistance and recovery from invasion?

AbstractRemoval experiments are useful tools for assessing two important aspects of plant community invasion: (1) resistance to invasion and (2) recovery after invasive species removal. We discuss the potential of such experiments based on a brief systematic review of the literature on community resistance, as measured by invasibility after removal of resident species (reduction of taxonomic/functional richness), and on community recovery, as measured by resident community response after invasive species removal. We found 62 research articles, most of them related to invasive species removal. Few studies used removals to test biotic resistance, despite the importance of resident removals for identifying community components that play key roles in the often‐controversial invasion resistance hypotheses. Furthermore, appropriate experimental controls were rarely used, which would allow separation of the effect of local species extinction from that of disturbance. We hope this review stimulates plant ecologists to adopt removal experiments for studying invasion processes.

Impacts of prairie grass species restoration on plant community invasibility and soil processes in abandoned agricultural fields

AbstractQuestionCan plant species with particular traits be used to address common problems encountered during agricultural land restoration, including invasion by undesirable species and altered soil conditions? How does the establishment of dominant old‐field and prairie grasses with different functional traits affect community invasibility and soil nitrogen (N) cycling in abandoned agricultural fields?LocationW.K. Kellogg Biological Station, Hickory Corners, Michigan, USA.MethodsWe established seedlings of several common old‐field and prairie grasses in a Trait Assessment Study to determine the magnitude of species differences in traits expected to influence community invasibility and soil processes. We also established monocultures of these species in a field experiment to determine how species with particular traits impacted community invasibility and soil processes 2 and 4 yr after restoration.ResultsThe prairie species consistently produced more shoot, root and litter biomass, and had more recalcitrant tissue than the old‐field species, and there were corresponding differences in invasibility and nutrient cycling. Soils under the three prairie grasses had significantly lower inorganic N after 2 yr and significantly lower potential N‐mineralization after 4 yr. The prairie plots also had significantly less invasion than the old‐field plots after 4 yr. Plots dominated by the old‐field species had higher biomass of non‐native species, and higher numbers of colonizing species than plots dominated by prairie species.ConclusionsRestoration using prairie grasses with particular effect traits can reduce N cycling and availability, and lead to a decrease in invasion by undesirable species from adjacent unrestored sites.

Ecological Characteristics That Enhance Broussonetia papyrifera’s Invasion in a Semideciduous Forest in Ghana

Broussonetia papyrifera (L.) Vent. (Moraceae) was introduced to Ghana in 1969 and has since become second only to Chromolaena odorata as an invasive species in Ghanaian forests. This study determined its ecological traits that enhance its invasion of plant communities. Fruiting and viability patterns were studied through monthly monitoring of 985 trees (≥6 cm gbh) in one forest site. The effect of light on its seed germination was tested in light-proof boxes. Means of propagation were determined by tracking the origin of newly regenerated plants on a newly cleared plot of land that B. papyrifera had occupied. It fruited twice a year with one season (January–March) producing more fruits than the other (July-August). Fruiting occurred in trees as small as 9 cm gbh but the percentage of individuals fruiting in each size class increased with tree size. There was a clear pattern of seed viability during the January–March fruiting period. The species did not appear to have a naturally high seed viability with germination always below 50% of each weekly collection. Seed germination was depressed in dark. These results suggest that the species may be competitive in disturbed forest sites and therefore its spread may be aided by forest degradation.

Resident plant diversity and introduced earthworms have contrasting effects on the success of invasive plants

Theoretical predictions and empirical studies suggest that resident species diversity is an important driver of community invasibility. Through trait-based processes, plants in communities with high resident species diversity occupy a wider range of ecological niches and are more productive than low diversity communities, potentially reducing the opportunities for invasion through niche preemption. In terrestrial plant communities, biotic ecosystem engineers such as earthworms can also affect invasibility by reducing leaf litter stocks and influencing soil conditions. In a greenhouse experiment, we simultaneously manipulated resident species diversity and earthworm presence to investigate independent and interactive effects of these two variables on the success of several invasive plants. Higher diversity of resident species was associated with lower biomass of invasives, with the effect mediated through resident species biomass. The presence of earthworms had a strong positive effect on the biomass of invasive species across all levels of resident species diversity and a weaker indirect negative effect via decreased soil moisture. Earthworms also weakened the positive correlation between resident species diversity and productivity. We did not observe any interactive effects of resident species biomass and earthworms on invasive species success. Partitioning the net biodiversity effect indicated that selection effects increased with resident species diversity whereas complementarity effects did not. Results suggest that managing for diverse forest communities may decrease the susceptibility of these communities to invasions. However, the presence of introduced earthworms in previously earthworm-free sites may undermine these efforts. Furthermore, future studies of plant community invasibility should account for the effects of introduced earthworms.

Species pool, species richness, density compensation effect, and invasibility of plant communities

The published data on the resistance of plant communities to invasions of alien species are discussed. It is shown that the variation in the number of such species on small areas of cenoses depends on the balance between their local richness and species pool size. This ratio has been used to predict the degree of invasibility of natural plant communities in Western Caucasus (alpine low-grass meadows and heaths, subalpine middle-grass meadows of high-altitude mountain massifs and ridges isolated or not by forest communities, and tree layer of mountain forests). The results demonstrate that the communities with high species richness, isolated, evolutionarily young, and disturbed in their previous history display a higher invasive potential.

Invasibility of a nutrient-poor pasture through resident and non-resident herbs is controlled by litter, gap size and propagule pressure.

Since inference concerning the relative effects of propagule pressure, biotic interactions, site conditions and species traits on the invasibility of plant communities is limited, we carried out a field experiment to study the role of these factors for absolute and relative seedling emergence in three resident and three non-resident confamilial herb species on a nutrient-poor temperate pasture. We set up a factorial field experiment with two levels each of the factors litter cover (0 and 400 g m−2), gap size (0.01 and 0.1 m2) and propagule pressure (5 and 50 seeds) and documented soil temperature, soil water content and relative light availability. Recruitment was recorded in spring and autumn 2010 and in spring 2011 to cover initial seedling emergence, establishment after summer drought and final establishment after the first winter. Litter alleviated temperature and moisture conditions and had positive effects on proportional and absolute seedling emergence during all phases of recruitment. Large gaps presented competition-free space with high light availability but showed higher temperature amplitudes and lower soil moisture. Proportional and absolute seedling recruitment was significantly higher in large than in small gaps. In contrast, propagule pressure facilitated absolute seedling emergence but had no effects on proportional emergence or the chance for successful colonisation. Despite significantly higher initial seedling emergence of resident than non-resident species, seed mass and other species-specific traits may be better predictors for idiosyncratic variation in seedling establishment than status. Our data support the fluctuating resource hypothesis and demonstrate that the reserve effect of seeds may facilitate seedling emergence. The direct comparison of propagule pressure with other environmental factors showed that propagule pressure affects absolute seedling abundance, which may be crucial for species that depend on other individuals for sexual reproduction. However, propagule batch size did not significantly affect the chance for successful colonisation of disturbed plots.