Invasions: the perspective of diverse plant communities

Exotic plant invasions represent a threat to natural and managed ecosystems. Understanding of the mechanisms that determine why a given species may invade a given ecosystem, or why some biomes and regions seem more prone to invasions, is limited. One potential reason for this lack of progress may lie in how few studies have addressed invasion mechanisms from the point of view of the invaded community. On the other hand, the renewed debate about the relationship between ecological diversity and ecosystem stability offers the opportunity to revisit existing theory and empirical evidence, and to attempt to investigate which characteristics of plant communities, including their diversity, contribute to their invasibility. Empirical studies have shown both positive and negative relationships between species diversity of resident plant communities and their invasibility by external species. Rather than attempting to build a larger collection of case studies, research now needs to address the mechanisms underlying these relationships. Previous knowledge about the mechanisms favouring invasion needs to be coupled with community theory to form the basis of these new investigations. Modern community theory offers hypotheses and techniques to analyse the invasibility of communities depending on their diversity and other factors, such as species’ life histories and environmental variability. The body of knowledge accumulated in invasion ecology suggests that the role of disturbances, in interaction with fertility, and the importance of interactions with other trophic levels, are specific factors for consideration. In addition, it is essential for future studies to explicitly tease apart the effects of species richness per se from the effects of other components of ecological diversity, such as functional diversity (the number of functional groups) and trophic diversity (the number of interactions among trophic levels).     

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.     

Reductions in grassland species evenness increase dicot seedling invasion and spittle bug infestation

Previous experiments that tested whether diverse plant communities have lower invasibility have all varied species richness. We experimentally varied evenness of four grassland species (three grasses and one forb) by planting a field experiment in Texas, and monitored the number of unplanted dicot and monocot species that invaded plots for two growing seasons. By varying evenness, we eliminated any sampling effect in our diversity treatment, because all plots contained the same plant species. Experimentally reducing evenness led to a greater number of dicot invaders, which emerged in plots throughout the growing season, but had less of an effect on monocot invaders, which emerged in flushes when experimental plants were semi‐dormant. Frequency of Solidago canadensis (altissima) stems with spittle bugs significantly increased with reductions in evenness during the first year, apparently because the greater number of Solidago stems in high evenness plots diluted the spittle‐bug effect. These results support the view that higher diversity plant communities are more resistant to dicot invaders and insect herbivores.     

Control of plant species diversity and community invasibility by species immigration: seed richness versus seed density

Immigration rates of species into communities are widely understood to influence community diversity, which in turn is widely expected to influence the susceptibility of ecosystems to species invasion. For a given community, however, immigration processes may impact diversity by means of two separable components: the number of species represented in seed inputs and the density of seed per species. The independent effects of these components on plant species diversity and consequent rates of invasion are poorly understood. We constructed experimental plant communities through repeated seed additions to independently measure the effects of seed richness and seed density on the trajectory of species diversity during the development of annual plant communities. Because we sowed species not found in the immediate study area, we were able to assess the invasibility of the resulting communities by recording the rate of establishment of species from adjacent vegetation. Early in community development when species only weakly interacted, seed richness had a strong effect on community diversity whereas seed density had little effect. After the plants became established, the effect of seed richness on measured diversity strongly depended on seed density, and disappeared at the highest level of seed density. The ability of surrounding vegetation to invade the experimental communities was decreased by seed density but not by seed richness, primarily because the individual effects of a few sown species could explain the observed invasion rates. These results suggest that seed density is just as important as seed richness in the control of species diversity, and perhaps a more important determinant of community invasibility than seed richness in dynamic plant assemblages.     

Giant invasive Heracleum persicum: Friend or foe of plant diversity?

The impact of invasion on diversity varies widely and remains elusive. Despite the considerable attempts to understand mechanisms of biological invasion, it is largely unknown whether some communities’ characteristics promote biological invasion, or whether some inherent characteristics of invaders enable them to invade other communities. Our aims were to assess the impact of one of the massive plant invaders of Scandinavia on vascular plant species diversity, disentangle attributes of invasible and noninvasible communities, and evaluate the relationship between invasibility and genetic diversity of a dominant invader. We studied 56 pairs of Heracleum persicum Desf. ex Fisch.‐invaded and noninvaded plots from 12 locations in northern Norway. There was lower native cover, evenness, taxonomic diversity, native biomass, and species richness in the invaded plots than in the noninvaded plots. The invaded plots had nearly two native species fewer than the noninvaded plots on average. Within the invaded plots, cover of H. persicum had a strong negative effect on the native cover, evenness, and native biomass, and a positive association with the height of the native plants. Plant communities containing only native species appeared more invasible than those that included exotic species, particularly H. persicum. Genetic diversity of H. persicum was positively correlated with invasibility but not with community diversity. The invasion of a plant community by H. persicum exerts consistent negative pressure on vascular plant diversity. The lack of positive correlation between impacts and genetic diversity of H. persicum indicates that even a small founder population may cause high impact. We highlight community stability or saturation as an important determinant of invasibility. While the invasion by H. persicum may decrease susceptibility of a plant community to further invasion, it severely reduces the abundance of native species and makes them more vulnerable to competitive exclusion.     

Experimental demonstration that plant diversity reduces invasibility – evidence of a biological mechanism or a consequence of sampling effect?

Several recent studies have claimed to present experimental evidence from synthesised plant communities in which diversity was varied as a treatment that diversity reduces community invasibility by other plant species. This type of result contrasts from that of many observational studies which find diversity and invasibility to be positively correlated in nature, but some recent literature has claimed that these observational studies are confounded by extrinsic covarying factors while experimental studies are not. In this article I evaluate each of eight experiments from six recent publications in which the effect of varying plant diversity on the success of invasive species was investigated. In each case that invasibility was identified by the authors as being adversely affected by plant species richness, the result can be explained by factors that covaried with diversity in the experiment, most notably as a consequence of “sampling effect” (in which the most competitive species or species combination in the total species pool has a greater probability of occurring as species richness is increased), or through the incorrect use of statistical techniques. It is proposed that the apparent discrepancy between the results of many observational and experimental studies at least in grasslands is because: (1) in observational studies competitive dominant species are often associated with the most productive plots, and these dominants both reduce diversity through competitive exclusion of subordinates and competitively suppress invasive species; and (2) in recent experimental studies “sampling effect” results in the most competitive species (and therefore those most likely to suppress the invader) occurring with greater frequency as diversity is increased. Both observational and experimental studies therefore point to the role of competitive dominants in reducing invasibility, and in both situations species richness of the plant community need not be invoked as an explanation for the results.     

Genotypic diversity and environmental variability affect the invasibility of experimental plant populations

The world is experiencing increasing climatic variability, an ongoing loss of biodiversity and a growing spread of invasive species. Previous experimental studies demonstrated that the invasibility of plant populations is reduced with increasing resident genetic diversity and is promoted by environmental fluctuations, but their combined effect has so far not been considered. In a growth chamber experiment, we tested whether the genotypic diversity of experimental populations of Arabidopsis thaliana (1, 3 or 6 genotypes) and temperature fluctuations affect population invasion by Senecio vulgaris, and how these factors interact. We found that genotypic diversity tended to increase the invasion resistance of experimental plant populations in terms of the biomass ratio between the species, and that temperature fluctuations strongly favoured Arabidopsis (biomass: +49%) over Senecio (–28%). However, there were no interactions between environmental fluctuations and genotypic diversity. Nevertheless, the magnitude of net diversity effects and transgressive overyielding depended on temperature conditions, indicating that increased environmental variability can influence diversity mechanisms. Our study shows that, although genotypic diversity and environmental variability did not interact, these two factors independently affected the invasibility of plant populations.     

Productivity gradients cause positive diversity–invasibility relationships in microbial communities

Models predict that community invasibility generally declines with species diversity, a prediction confirmed by small‐scale experiments. Large‐scale observations and experiments, however, find that diverse communities tend to be more heavily invaded than simple communities. One hypothesis states that large‐scale environmental heterogeneity, which similarly influences native and invasive species, can cause a positive correlation between diversity and invasibility, overriding the local negative effects of diversity on invasibility. We tested this hypothesis using aquatic microbial communities consisting of protists and rotifers consuming bacteria and nanoflagellates. We constructed a productivity gradient to simulate large‐scale environmental heterogeneity, started communities with the same number of species along this gradient, and subjected equilibrial communities to invasion by non‐resident consumer species. Both invaders and most resident species increased their abundances with resource enrichment, resulting in a positive correlation between diversity and invasibility. Intraspecific interference competition within resident species and the positive effect of enrichment on the number of available resources probably accounted for the higher invasibility with enrichment. Our results provide direct experimental evidence that environmental heterogeneity in productivity can cause a positive diversity–invasibility relationship.     

Genetic diversity and invasibility: a test using a model system with a novel experimental design

Biological invasion is one aspect of ecosystem function that may be controlled by the biological diversity of the invaded community, and there have been a number of recent studies that investigated relationships between diversity and invasibility. Most experimental studies report that higher species or functional group diversity increases resistance to invasion, but the role of genetic diversity is unknown. We used a model organism, Arabidopsis thaliana (Brassicaceae), to investigate relationships between genotypic richness and community invasibility by creating communities with 1, 2, 4, and 8 genotypes of A. thaliana at constant low (417 plants m⁻²) and high (834 plants m⁻²) densities, that once established, were invaded with a congener, Arabidopsis suecica. To reduce the potential effects of methodological confounding related to “sampling effects,”“variance reduction effects,” or confounding of abundance with diversity, we (1) created random communities from a relatively large pool of functionally and phenotypically similar genotypes, (2) evaluated individual and community traits across richness treatments, and (3) analyzed similarity of communities within treatments (for “quasi- replication”) and between adjacent treatments (for “nestedness”). Genotypic richness had no effect on A. suecica demography (emergence, survivorship), size (biomass, rosette area), or reproductive potential (rates of bolting and fruiting or number and size of bolts). In contrast, the density of A. thaliana genotypes had strong effects on the size and reproductive potential of A. suecica, which suggests that characteristics of the recipient community other than genotypic richness (e.g. light) form the most important determinant of community invasibility. Individual- and community-level traits of community members (cover, biomass, survivorship) did not differ among richness treatments, and within- and between-treatment similarity was reduced (relative to other recent experiments) but not eliminated. We evaluate our results vis-a-vis recent analyses of diversity-invasibility experiments, and provide directions for future investigations of genetic diversity.     

Interactive effects of mycorrhizae and a root hemiparasite on plant community productivity and diversity

Plant communities can be affected both by arbuscular mycorrhizal fungi (AMF) and hemiparasitic plants. However, little is known about the interactive effects of these two biotic factors on the productivity and diversity of plant communities. To address this question, we set up a greenhouse study in which different AMF inocula and a hemiparasitic plant (Rhinanthus minor) were added to experimental grassland communities in a fully factorial design. In addition, single plants of each species in the grassland community were grown with the same treatments to distinguish direct AMF effects from indirect effects via plant competition. We found that AMF changed plant community structure by influencing the plant species differently. At the community level, AMF decreased the productivity by 15–24%, depending on the particular AMF treatment, mainly because two dominant species, Holcus lanatus and Plantago lanceolata, showed a negative mycorrhizal dependency. Concomitantly, plant diversity increased due to AMF inoculation and was highest in the treatment with a combination of two commercial AM strains. AMF had a positive effect on growth of the hemiparasite, and thereby induced a negative impact of the hemiparasite on host plant biomass which was not found in non-inoculated communities. However, the hemiparasite did not increase plant diversity. Our results highlight the importance of interactions with soil microbes for plant community structure and that these indirect effects can vary among AMF treatments. We conclude that mutualistic interactions with AMF, but not antagonistic interactions with a root hemiparasite, promote plant diversity in this grassland community. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Spatial heterogeneity of plant–soil feedbacks increases per capita reproductive biomass of species at an establishment disadvantage

Plant–soil feedbacks have been widely implicated as a driver of plant community diversity, and the coexistence prediction generated by a negative plant–soil feedback can be tested using the mutual invasibility criterion: if two populations are able to invade one another, this result is consistent with stable coexistence. We previously showed that two co-occurring Rumex species exhibit negative pairwise plant–soil feedbacks, predicting that plant–soil feedbacks could lead to their coexistence. However, whether plants are able to reproduce when at an establishment disadvantage (“invasibility”), or what drivers in the soil might correlate with this pattern, are unknown. To address these questions, we created experimental plots with heterogeneous and homogeneous soils using field-collected conditioned soils from each of these Rumex species. We then allowed resident plants of each species to establish and added invader seeds of the congener to evaluate invasibility. Rumex congeners were mutually invasible, in that both species were able to establish and reproduce in the other’s resident population. Invaders of both species had twice as much reproduction in heterogeneous compared to homogeneous soils; thus the spatial arrangement of plant–soil feedbacks may influence coexistence. Soil mixing had a non-additive effect on the soil bacterial and fungal communities, soil moisture, and phosphorous availability, suggesting that disturbance could dramatically alter soil legacy effects. Because the spatial arrangement of soil patches has coexistence implications, plant–soil feedback studies should move beyond studies of mean effects of single patch types, to consider how the spatial arrangement of patches in the field influences plant communities.     

Phylogenetic diversity is a better predictor of wetland community resistance to Alternanthera philoxeroides invasion than species richness

• Highly biodiversity communities have been shown to better resist plant invasions through complementarity effects. Species richness (SR) is a widely used biodiversity metric but lacks explanatory power when there are only a few species. Communities with low SR can have a wide variety of phylogenetic diversities (PD), which might allow for a better prediction of invasibility.
• We assessed the effect of diversity reduction of a wetland community assemblage typical of the Beijing area on biotic resistance to invasion of the exotic weed Alternanthera philoxeroides and compared the reduction in SR and PD in predicting community invasibility.
• The eight studied resident species performed similarly when grown alone and when grown in eight‐species communities together with the invasive A. philoxeroides. Variation partitioning showed that PD contributed more to variation in both A. philoxeroides traits and community indicators than SR. All A. philoxeroides traits and community indicators, except for evenness index, showed a linear relationship with PD. However, only stem length of A. philoxeroides differed between the one‐ and two‐species treatments, and the diversity index of the communities differed between the one‐ and two‐species treatments and between the one‐ and four‐species treatments.
• Our results showed that in natural or semi‐natural wetlands with relatively low SR, PD may be a better predictor of invasibility than SR. When designing management strategies for mitigating A. philoxeroides invasion, deliberately raising PD is expected to be more efficient than simply increasing species number.

     

Tree species diversity alters plant defense investment in an experimental forest plantation in southern Mexico

How plant species diversity affects traits conferring herbivore resistance (e.g., chemical defenses), as well as the mechanisms underlying such effects, has received little attention. One potential mechanism for the effect of diversity on plant defenses is that increased plant growth at high diversity could lead to reduced investment in defenses via growth–defense trade‐offs. We measured tree growth (diameter at breast height) and collected leaves to quantify total phenolics in 2.5‐year‐old plants of six tropical tree species (N = 597 plants) in a young experimental plantation in southern Mexico. Selected plants were classified as monocultures or as polycultures represented by mixtures of four of the six species examined. Tree species diversity had a significant negative effect on total phenolics, where polycultures exhibited a 13 percent lower mean concentration than monocultures. However, there was marked variation in the effects of diversity on defenses among tree species, with some species exhibiting strong reductions in phenolic levels in mixtures, whereas others were unresponsive. In addition, tree species diversity had no effect on growth, nor was the negative effect of diversity on chemical defenses mediated by a growth–defense trade‐off. These results demonstrate that tree diversity can alter investment in chemical defenses in long‐lived tree species but that such effect may not always be under strong control by plant endogenous resource allocation trade‐offs. Regardless of the underlying mechanism, these findings have important implications for predicting effects on consumers and ecosystem function.     

Fungal diversity increases soil fungistasis and resistance to microbial invasion by a non resident species

Biodiversity decline is a major concern for ecosystem functioning. Recent research efforts have been mostly focused on terrestrial plants, while, despite their importance in both natural and artificial ecosystems, little is known about soil microbial communities. This work aims at investigating the effects of fungal species richness on soil invasion by non resident microbes. Synthetic fungal communities with a species diversity ranging from 1 to 8 were assembled in laboratory microcosms and used in three factorial experiments to assess the effect of diversity on soil fungistasis, microbial invasion of soil amended with plant litter and of plant rhizosphere. The capability of different microbes to colonize environments characterized by different resident microbial communities was measured. The number of microbial species in the microcosms positively affected soil fungistasis that was also induced more rapidly in presence of synthetic communities with more species. Moreover, the increase of resident fungal diversity dramatically reduced the invasibility of both soil and plant rhizosphere. We found lower variability of soil fungistasis and invasibility in microcosms with higher species richness of microbial communities. Our study pointed out the existence of negative relationships between fungal diversity and soil invasibility by non resident microbes. Therefore, the loss of microbial species may adversely affect ecosystem functionality under specific environmental conditions.     

Host plant species effects on arbuscular mycorrhizal fungal communities in tallgrass prairie

Symbiotic associations between plants and arbuscular mycorrhizal (AM) fungi are ubiquitous in many herbaceous plant communities and can have large effects on these communities and ecosystem processes. The extent of species-specificity between these plant and fungal symbionts in nature is poorly known, yet reciprocal effects of the composition of plant and soil microbe communities is an important assumption of recent theoretical models of plant community structure. In grassland ecosystems, host plant species may have an important role in determining development and sporulation of AM fungi and patterns of fungal species composition and diversity. In this study, the effects of five different host plant species [Poa pratensis L., Sporobolus heterolepis (A. Gray) A. Gray, Panicum virgatum L., Baptisia bracteata Muhl. ex Ell., Solidago missouriensis Nutt.] on spore communities of AM fungi in tallgrass prairie were examined. Spore abundances and species composition of fungal communities of soil samples collected from patches within tallgrass prairie were significantly influenced by the host plant species that dominated the patch. The AM fungal spore community associated with B. bracteata showed the highest species diversity and the fungi associated with Pa. virgatum showed the lowest diversity. Results from sorghum trap cultures using soil collected from under different host plant species showed differential sporulations of AM fungal species. In addition, a greenhouse study was conducted in which different host plant species were grown in similar tallgrass prairie soil. After 4 months of growth, AM fungal species composition was significantly different beneath each host species. These results strongly suggest that AM fungi show some degree of host-specificity and are not randomly distributed in tallgrass prairie. The demonstration that host plant species composition influences AM fungal species composition provides support for current feedback models predicting strong regulatory effects of soil communities on plant community structure. Differential responses of AM fungi to host plant species may also play an important role in the regulation of species composition and diversity in AM fungal communities. Received: 29 January 1999 / Accepted: 20 October 1999     

Contrasting effects of resource availability and plant mortality on plant community invasion by Bromus tectorum L.

The positive effect of disturbance on plant community invasibility is one of the more consistent results in invasion ecology. It is generally attributed to a coincident increase in available resources (due to the disturbance) that allows non-resident plant species to establish (Davis MA, Grime JP Thompson K, J Ecol 88:528–534, 2000). However, most research addressing this issue has been in artificial or highly modified plant communities. Our goal in this study was to investigate the interactive effects of resource availability and plant mortality disturbance on the invasion of natural plant communities. We conducted a series of experiments that examined the response of Bromus tectorum L., a highly invasive annual grass, to experimentally created gradients of resource availability [nitrogen (N) and water] and resident plant species mortality. We found that B. tectorum biomass was co-limited by N and water. Biomass at the end of the growing season was a saturating function (i.e., increased to a maximum) of water, which determined maximum biomass, and N, which determined the rate at which maximum biomass was attained. Despite that fact that plant mortality increased N availability, it had a negative impact on invasion success. Plant mortality also decreased foliar cover, standing dead biomass, and soil cover by litter. In harsh environments, removing foliar and soil cover may increase germination and seedling stress by increasing soil temperatures and water loss. Across all treatments, B. tectorum success decreased with decreasing foliar cover and standing dead biomass. This, in combination with the strong limitation of B. tectorum biomass by water in this experiment, suggests that our plant mortality disturbance removed soil cover that may have otherwise aided B. tectorum invasion into this semi-arid plant community by reducing water stress.     

Interactive effects of resource enrichment and resident diversity on invasion of native grassland by <Emphasis Type="Italic">Lolium arundinaceum</Emphasis>

Resident diversity and resource enrichment are both recognized as potentially important determinants of community invasibility, but the effects of these biotic and abiotic factors on invasions are often investigated separately, and little work has been done to directly compare their relative effects or to examine their potential interactions. Here, we evaluate the individual and interactive effects of resident diversity and resource enrichment on plant community resistance to invasion. We factorially manipulated plant diversity and the enrichment of belowground (soil nitrogen) and aboveground (light) resources in low-fertility grassland communities invaded by Lolium arundinaceum, the most abundant invasive grass in eastern North America. Soil nitrogen enrichment enhanced L. arundinaceum performance, but increased resident diversity dampened this effect of nitrogen enrichment. Increased light availability (via clipping of aboveground vegetation) had a negligible effect on community invasibility. These results demonstrate that a community’s susceptibility to invasion can be contingent upon the type of resource pulse and the diversity of resident species. In order to assess the generality of these results, future studies that test the effects of resident diversity and resource enrichment against a range of invasive species and in other environmental contexts (e.g., sites differing in soil fertility and light regimes) are needed. Such studies may help to resolve conflicting interpretations of the diversity–invasibility relationship and provide direction for management strategies.     

Dominant species identity regulates invasibility of old-field plant communities

Dominant species are known to exert strong influence over community dynamics, although little work has addressed how they affect invasibility. In this study, we examined whether dominant species identity and abundance affected invasibility of old-field plant communities. To quantify invasibility, we added seeds of 19 plant species into plots dominated by one of four different herbaceous perennial species ( Andropogon virginicus , Bromus inermis , Centaurea maculosa , or Solidago canadensis ) . We found that, independent of species richness and abiotic variables, plots dominated by Andropogon were the least invasible, while Bromus and Centaurea plots had the highest invasibility. We examined several potential mechanisms by which these dominant species might influence invasibility, and found invasion to increase with decreasing litter biomass and increasing community species richness. The abundance of the dominant species was not a significant predictor of invasion. These results indicate that dominant species identity plays an important role in determining invasibility of plant communities, though exact mechanisms underlying these effects still need to be explored.     

Invasibility of experimental grassland communities: the role of earthworms, plant functional group identity and seed size

Invasions of natural communities by non‐indigenous species threaten native biodiversity and are currently rated as one of the most important global‐scale environmental problems. The mechanisms that make communities resistant to invasions and drive the establishment success of seedlings are essential both for management and for understanding community assembly and structure. Especially in grasslands, anecic earthworms are known to function as ecosystem engineers, however, their direct effects on plant community composition and on the invasibility of plant communities via plant seed burial, ingestion and digestion are poorly understood. In a greenhouse experiment we investigated the impact of Lumbricus terrestris, plant functional group identity and seed size of plant invader species and plant functional group of the established plant community on the number and biomass of plant invaders. We set up 120 microcosms comprising four plant community treatments, two earthworm treatments and three plant invader treatments containing three seed size classes. Earthworm performance was influenced by an interaction between plant functional group identity of the established plant community and that of invader species. The established plant community and invader seed size affected the number of invader plants significantly, while invader biomass was only affected by the established community. Since earthworm effects on the number and biomass of invader plants varied with seed size and plant functional group identity they probably play a key role in seedling establishment and plant community composition. Seeds and germinating seedlings in earthworm burrows may significantly contribute to earthworm nutrition, but this deserves further attention. Lumbricus terrestris likely behaves like a ‘farmer’ by collecting plant seeds which cannot directly be swallowed or digested. Presumably, these seeds are left in middens and become eatable after partial microbial decay. Increased earthworm numbers in more diverse plant communities likely contribute to the positive relationship between plant species diversity and resistance against invaders.     

Non-native grass invasion alters native plant composition in experimental communities

Invasions of non-native species are considered to have significant impacts on native species, but few studies have quantified the direct effects of invasions on native community structure and composition. Many studies on the effects of invasions fail to distinguish between (1) differential responses of native and non-native species to environmental conditions, and (2) direct impacts of invasions on native communities. In particular, invasions may alter community assembly following disturbance and prevent recolonization of native species. To determine if invasions directly impact native communities, we established 32 experimental plots (27.5 m²) and seeded them with 12 native species. Then, we added seed of a non-native invasive grass (Microstegium vimineum) to half of the plots and compared native plant community responses between control and invaded plots. Invasion reduced native biomass by 46, 64, and 58%, respectively, over three growing seasons. After the second year of the experiment, invaded plots had 43% lower species richness and 38% lower diversity as calculated from the Shannon index. Nonmetric multidimensional scaling ordination showed a significant divergence in composition between invaded and control plots. Further, there was a strong negative relationship between invader and native plant biomass, signifying that native plants are more strongly suppressed in densely invaded areas. Our results show that a non-native invasive plant inhibits native species establishment and growth following disturbance and that native species do not gain competitive dominance after multiple growing seasons. Thus, plant invaders can alter the structure of native plant communities and reduce the success of restoration efforts.