Effects of Extreme Weather Events on Plant Productivity and Tissue Die-Back are Modified by Community Composition

Extreme weather events are expected to increase in frequency and magnitude due to climate change. Their effects on vegetation are widely unknown. Here, experimental grassland and heath communities in Central Europe were exposed either to a simulated single drought or to a prolonged heavy rainfall event. The magnitude of manipulations imitated the local 100-year weather extreme according to extreme value statistics. Overall productivity of both plant communities remained stable in the face of drought and heavy rainfall, despite significant effects on tissue die-back. Grassland communities were more resistant against the extreme weather events than heath communities. Furthermore, effects of extreme weather events on community tissue die-back were modified by functional diversity, even though conclusiveness in this part is limited by the fact that only one species composition was available per diversity level within this case study. More diverse grassland communities exhibited less tissue die-back than less complex grassland communities. On the other side, more diverse heath communities were more vulnerable to extreme weather events compared to less complex heath communities. Furthermore, legumes did not effectively contribute to the buffering against extreme weather events in both vegetation types. Tissue die-back proved a strong stress response in plant communities exposed to 100-year extreme weather events, even though one important ecosystem function, namely productivity, remained surprisingly stable in this experiment. Theories and concepts on biodiversity and ecosystem functioning (insurance hypothesis, redundancy hypothesis) may have to be revisited when extreme weather conditions are considered.     

Short-term invasibility patterns in burnt and unburnt experimental Mediterranean grassland communities of varying diversities

This paper reports the findings of a short-term natural invasibility field study in constructed Mediterranean herbaceous communities of varying diversities, under a fire treatment. Three components of invasibility, i.e. species richness, density and biomass of invaders, have been monitored in burnt and unburnt experimental plots with resident diversity ranging from monocultures to 18-species mixtures. In general, species richness, density and biomass of invaders decreased significantly with the increase of resident species richness. Furthermore, the density and biomass of invading species were significantly influenced by the species composition of resident communities. Although aboveground biomass, leaf area index, canopy height and percent bare ground of the resident communities explained a significant part of the variation in the success of invading species, these covariates did not fully explain the effects of resident species richness. Fire mainly influenced invasibility via soil nutrient levels. The effect of fire on observed invasibility patterns seems to be less important than the effects of resident species richness. Our results demonstrate the importance of species richness and composition in controlling the initial stages of plant invasions in Mediterranean grasslands but that there was a lack of interaction with the effects of fire disturbance.     

Extreme weather events and plant–plant interactions: shifts between competition and facilitation among grassland species in the face of drought and heavy rainfall

Biotic interactions play an important role in ecosystem function and structure in the face of global climate change. We tested how plant–plant interactions, namely competition and facilitation among grassland species, respond to extreme drought and heavy rainfall events. We also examined how the functional composition (grasses, forbs, legumes) of grassland communities influenced the competition intensity for grass species when facing extreme events. We exposed experimental grassland communities of different functional compositions to either an extreme single drought event or to a prolonged heavy rainfall event. Relative neighbour effect, relative crowding and interaction strength were calculated for five widespread European grassland species to quantify competition. Single climatic extremes caused species specific shifts in plant–plant interactions from facilitation to competition or vice versa but the nature of the shifts varied depending on the community composition. Facilitation by neighbouring plants was observed for Arrhenatherum elatius when subjected to drought. Contrarily, the facilitative effect of neighbours on Lotus corniculatus was transformed into competition. Heavy rainfall increased the competitive effect of neighbours on Holcus lanatus and Lotus corniculatus in communities composed of three functional groups. Competitive pressure on Geranium pratense and Plantago lanceolata was not affected by extreme weather events. Neither heavy rainfall nor extreme drought altered the overall productivity of the grassland communities. The complementary responses in competition intensity experienced by grassland species under drought suggest biotic interactions as one stabilizing mechanism for overall community performance. Understanding competitive dynamics under fluctuating resources is important for assessing plant community shifts and degree of stability of ecosystem functions.     

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.     

Beyond gradual warming: extreme weather events alter flower phenology of European grassland and heath species

Shifts in the phenology of plant and animal species or in the migratory arrival of birds are seen as ‘fingerprints’ of global warming. However, even if such responses have been documented in large continent‐wide datasets of the northern hemisphere, all studies to date correlate the phenological pattern of various taxa with gradual climatic trends. Here, we report a previously unobserved phenomenon: severe drought and heavy rain events caused phenological shifts in plants of the same magnitude as one decade of gradual warming. We present data from two vegetation periods in an experimental setting containing the first evidence of shifted phenological response of 10 grassland and heath species to simulated 100‐year extreme weather events in Central Europe. Averaged over all species, 32 days of drought significantly advanced the mid‐flowering date by 4 days. The flowering length was significantly extended by 4 days. Heavy rainfall (170 mm over 14 days) had no significant effect on the mid‐flowering date. However, heavy rainfall reduced the flowering length by several days. Observed shifts were species‐specific, (e.g. drought advanced the mid‐flowering date for Holcus lanatus by 1.5 days and delayed the mid‐flowering date for Calluna vulgaris by 5.7 days, heavy rain advanced mid‐flowering date of Lotus corniculatus by 26.6 days and shortened the flowering length of the same species by 36.9 days). Interestingly, the phenological response of individual species was modified by community composition. For example, the mid‐flowering date of C. vulgaris was delayed after drought by 9.3 days in communities composed of grasses and dwarf shrubs compared with communities composed of dwarf shrubs only. This indicates that responses to extreme events are context specific. Additionally, the phenological response of experimental communities to extreme weather events can be modified by the functional diversity of a stand. Future studies on phenological response patterns related to climate change would profit from explicitly addressing the role of extreme weather events.     

Soil biotic processes remain remarkably stable after 100-year extreme weather events in experimental grassland and heath

Climate change will increase the recurrence of extreme weather events such as drought and heavy rainfall. Evidence suggests that extreme weather events pose threats to ecosystem functioning, particularly to nutrient cycling and biomass production. These ecosystem functions depend strongly on below-ground biotic processes, including the activity and interactions among plants, soil fauna, and micro-organisms. Here, experimental grassland and heath communities of three phytodiversity levels were exposed either to a simulated single drought or to a heavy rainfall event. Both weather manipulations were repeated for two consecutive years. The magnitude of manipulations imitated the local 100-year extreme weather event. Heavy rainfall events increased below-ground plant biomass and stimulated soil enzyme activities as well as decomposition rates for both plant communities. In contrast, extreme drought did not reduce below-ground plant biomass and root length, soil enzyme activities, and cellulose decomposition rate. The low responsiveness of the measured ecosystem properties in face of the applied weather manipulations rendered the detection of significant interactions between weather events and phytodiversity impossible. Our data indicate on the one hand the close interaction between below ground plant parameters and microbial turnover processes in soil; on the other hand it shows that the plant–soil system can buffer against extreme drought events, at last for the period of investigation.     

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.     

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.     

Impacts of extreme climatic events on competition during grassland invasions

Phytometers of five C3 and five C4 species were transplanted into three different grasslands to study the effects of extreme climatic events on community invasibility and competition. Single extreme heating (eight hours at 52.5 °C) and rainfall (the equivalent of 100 mm) events in factorial combinations were superimposed on the grassland communities. A novel technique involving portable computer‐controlled chambers was used to create the heating events. In order to generate predictions of response to the extreme climatic events, the 10 phytometer species were categorized on the basis of 12 key plant functional traits. Using principal component analysis, two functional types (FTs) were identified as most likely to be advantaged (FT1, fast‐growing C4 annuals) and disadvantaged (FT2, slower‐growing C3 perennials) by an extreme climatic event. Competition between the resident vegetation and FT1 plus other C4 phytometers was consistently more intense within the exclusively C3 community compared to the dry C3/C4 community or moist C3/C4 community. The single extreme heating event had the greatest impact on competition, lowering the intensity of competition between the phytometers and resident vegetation. Our results indicate that competition is highly important in limiting the invasion of C3 grasslands by C4 species. The FT1 and FT2 responses confirmed predictions based on plant functional traits, whether growing as phytometers or as part of the resident vegetation. Future increases in climatic variability and the incidence of extreme climatic events are expected to suppress C3 competitive dominance and promote invasion of C4 species, in particular, the FT1 species.     

Diversity reduces invasibility in experimental plant communities: the role of plant species

Several studies have presented experimental evidence that diversity reduces invasibility in grassland communities. The interpretation of these results has been disputed recently and it was proposed that sampling effects were responsible for the observed decrease of invasibility with diversity. The experiments performed to date were not designed to adequately separate sampling from diversity effects. Using the establishment of native plant species in experimental plant communities as a model of invasibility, we show that the number of invaders decreased with increasing diversity. When the presence of particular species is included, their effects are dominant. Centaurea jacea showed a strong effect at low diversity, whereas Leucanthemum vulgare showed a very strong negative impact at each diversity level. The negative effect of the latter might be related to root‐feeding nematodes that showed far higher abundance in plots with Leucanthemum. However, diversity remained a significant factor in determining the number of invading species and the numbers of an abundant invader.     

植物群落的生物多样性及其可入侵性关系的实验研究

 生物入侵已经成为一个普遍性的环境问题,并为许多学者所关注。尽管一些理论研究和观察表明生物多样性丰富的群落不容易受到外来种的入侵,但后来有些实验研究并没能证实两者的负相关性,多样性 可入侵性假说仍然是入侵生态学领域争论比较多的一个焦点。人为构建不同物种多样性和物种功能群多样性（C3 禾本科植物、C4植物、非禾本科草本植物和豆科植物）梯度的小尺度群落,把其它影响可入侵性的外在因子和多样性效应隔离开来,研究入侵种喜旱莲子草(Alternanthera philoxeroides)在不同群落里的入侵过程来验证多样性 可入侵性及其相关假说。研究结果显示,物种功能群丰富的群落可入侵程度较低,功能群数目相同而物种多样性不同的群落可入侵性没有显著性差异,功能群特征不同的群落也表现出可入侵性的差异,生活史周期短的单一物种群落和有着生物固氮功能的豆科植物群落可入侵程度较高,与喜旱莲子草属于同一功能群且有着相似生态位的土著种莲子草(A. sessilis)对入侵的抵抗力最强。实验结果表明,物种多样性和群落可入侵性并没有很显著的负相关,而是与物种特性基础上的物种功能群多样性呈负相关,群落中留给入侵种生态位的机会很可能是决定群落可入侵性的一个关键因子。     

Effects of extreme weather events and legume presence on mycorrhization of Plantago lanceolata and Holcus lanatus in the field

Little is known about direct and indirect effects of extreme weather events on arbuscular mycorrhizal fungi (AMF) under field conditions. In a field experiment, we investigated the response of mycorrhization to drought and heavy rain in grassland communities. We quantified AMF biomass in soil, mycorrhization of roots of the grass Holcus lanatus and the forb Plantago lanceolata, as well as plant performance. Plants were grown in four‐species communities with or without a legume. We hypothesised that drought increases and heavy rain decreases mycorrhization, and that higher mycorrhization will be linked to improved stress resistance and higher biomass production. Soil AMF biomass increased under both weather extremes. Heavy rain generally benefitted plants and increased arbuscules in P. lanceolata. Drought neither reduced plant performance nor root mycorrhization. Arbuscules increased in H. lanatus several weeks after drought, and in P. lanceolata several weeks after heavy rain spells. These long‐lasting effects of weather events on mycorrhization highlight the indirect influence of climate on AMF via their host plant. Legume presence increased plant community biomass, but had only minor effects on mycorrhization. Arbuscule colonisation was negatively correlated with senescence during the dry summer. Mycorrhization and biomass production in P. lanceolata were positively related. However, increased mycorrhization was related to less biomass in the grass. AMF mycelium in soil might generally increase under extreme events, root colonisation, however, is host species specific. This might amplify community shifts in grassland under climate change by further increasing stress resistance of species that already benefit from changed precipitation.     

Sensitivity of three grassland communities to simulated extreme temperature and rainfall events

Three grassland communities in New Zealand with differing climates and proportions of C3 and C4 species were subjected to one‐off extreme heating (eight hours at 52.5°C) and rainfall (the equivalent of 100 mm) events. A novel experimental technique using portable computer‐controlled chambers simulated the extreme heating events. The productive, moist C3/C4 community was the most sensitive to the extreme events in terms of short‐term community composition compared with a dry C3/C4 community or an exclusively C3 community. An extreme heating event caused the greatest change to plant community species abundance by favouring the expansion of C4 species relative to C3 species, shifting C4 species abundance from 43% up to 84% at the productive, moist site. This was observed both in the presence and absence of added water. In the absence of C4 species, heating reduced community productivity by over 60%. The short‐term shifts in the abundance of C3 and C4 species in response to the single extreme climatic events did not have persistent effects on community structure or on soil nitrogen one year later. There was no consistent relationship between diversity and stability of biomass production of these plant communities, and species functional identity was the most effective explanation for the observed shifts in biomass production. The presence of C4 species resulted in an increased stability of productivity after extreme climatic events, but resulted in greater overall shifts in community composition. The presence of C4 species may buffer grassland community productivity against an increased frequency of extreme heating events associated with future global climate change.     

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.     

Different ways to die in a changing world: Consequences of climate change for tree species performance and survival through an ecophysiological perspective

Anthropogenic activities such as uncontrolled deforestation and increasing greenhouse gas emissions are responsible for triggering a series of environmental imbalances that affect the Earth's complex climate dynamics. As a consequence of these changes, several climate models forecast an intensification of extreme weather events over the upcoming decades, including heat waves and increasingly severe drought and flood episodes. The occurrence of such extreme weather will prompt profound changes in several plant communities, resulting in massive forest dieback events that can trigger a massive loss of biodiversity in several biomes worldwide. Despite the gravity of the situation, our knowledge regarding how extreme weather events can undermine the performance, survival, and distribution of forest species remains very fragmented. Therefore, the present review aimed to provide a broad and integrated perspective of the main biochemical, physiological, and morpho‐anatomical disorders that may compromise the performance and survival of forest species exposed to climate change factors, particularly drought, flooding, and global warming. In addition, we also discuss the controversial effects of high CO₂ concentrations in enhancing plant growth and reducing the deleterious effects of some extreme climatic events. We conclude with a discussion about the possible effects that the factors associated with the climate change might have on species distribution and forest composition.     

Stronger Short-Term Effects of Mowing Than Extreme Summer Weather on a Subalpine Grassland

Mowing is known to favor plant diversity and influence ecosystem functioning in semi-natural grasslands. This effect could be influenced by climate variability, especially in regions with harsh climate, such as subalpine zones. In particular, short-term extreme weather fluctuations may induce rapid plant responses, affecting in turn the response to mowing. We tested the effects of concomitant summer weather manipulation and mowing on a subalpine grassland in the Central French Alps for two consecutive years. We addressed two questions: (1) How is a subalpine grassland affected by extreme summer weather? (2) Does extreme summer weather alter mowing effects on the grassland plant diversity and functioning? We used a multi-level, integrative approach assessing the responses of six abundant plant species, as well as effects on plant community structure, biomass production, and litter decomposition rates. Extreme summer weather was simulated by increasing summer temperature by 1.1°C, and decreasing summer rainfall by 80%—resulting in a 30% decrease in total annual precipitation. In addition, a heat-wave event was simulated during the first year of the experiment. This weather manipulation was combined with a late-summer mowing treatment (mown vs. unmown). Extreme summer weather mainly increased leaf senescence and decreased plant vegetative growth. Leaf litter decomposition was slowed, but only for species characterized by the fastest rates of litter decomposition. Mowing increased plant diversity by restricting the dominant grass species, thereby favoring subordinates. In the short term, this subalpine grassland was rather resistant to extreme summer weather, whereas mowing cessation remained the main factor affecting its biodiversity.     

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.     

Species evenness and invasion resistance of experimental grassland communities

Concern for biodiversity loss coupled with the accelerated rate of biological invasions has provoked much interest in assessing how native plant species diversity affects invasibility. Although experimental studies extensively document the effects of species richness on invader performance, the role of species evenness in such studies is rarely examined. Species evenness warrants more attention because the relative abundances of species can account for substantially more of the variance in plant community diversity and tend to change more rapidly and more frequently in response to disturbances than the absolute numbers of species. In this study, we experimentally manipulated species evenness within native prairie grassland mesocosms. We assessed how evenness affected primary productivity, light availability and the resistance of native communities to invasion. The primary productivity of native communities increased significantly with species evenness, and this increase in productivity was accompanied by significant decreases in light availability. However, evenness had no effect on native community resistance to invasion by three common exotic invasive species. In this study, niche complementarity provides a potential mechanism for the effects of evenness on productivity and light availability, but these effects apparently were not strong enough to alter the invasibility of the experimental communities. Our results suggest that species evenness enhances community productivity but provides no benefit to invasion resistance in otherwise functionally diverse communities.     

Fluctuating resource availability increases invasibility in microbial microcosms

The fluctuating resource hypothesis (FRH) proposes that fluctuations in resource supply can temporally reduce competitive pressure from resident species, thereby providing ephemeral opportunities for invading species. Although FRH has the potential to integrate many existing hypotheses regarding mechanisms of community invasibility, previous tests and evaluations of FRH were based on single trophic level, did not take the timing effect into account, and had difficulties in distinguishing the effects of resource pulses from other simultaneous processes. Here we test FRH in multi‐trophic aquatic microcosms by creating resource pulses, by controlling resource quantity, propagule supply and pulse recurrence frequency, and by manipulating the timing of pulses relative to the timing of the arrival of new species (i.e. invaders) to local communities. The novelty of our work lies in that we directly manipulate resource pulse timing relative to invader introduction events and thus demonstrate the importance of this timing effect for community invasibility. Our study supports FRH in general: invasion success was positively related to resource pulses, and invaders had strong performance in treatments receiving coincident pulses, although not all invaders gained more benefit when resources were supplied at large‐magnitude than supplied at continuous rates. Since many ecosystems worldwide are experiencing high rates of anthropogenic nutrient input and increasing rates of precipitation, these ecosystems are potentially more fragile and susceptible to invasion. More experiments across multiple ecosystem types are needed to help formulate a general theory of community invasibility.     

Productivity, herbivory, and species traits rather than diversity influence invasibility of experimental phytoplankton communities

Biological invasions are a major threat to natural biodiversity; hence, understanding the mechanisms underlying invasibility (i.e., the susceptibility of a community to invasions by new species) is crucial. Invasibility of a resident community may be affected by a complex but hitherto hardly understood interplay of (1) productivity of the habitat, (2) diversity, (3) herbivory, and (4) the characteristics of both invasive and resident species. Using experimental phytoplankton microcosms, we investigated the effect of nutrient supply and species diversity on the invasibility of resident communities for two functionally different invaders in the presence or absence of an herbivore. With increasing nutrient supply, increased herbivore abundance indicated enhanced phytoplankton biomass production, and the invasion success of both invaders showed a unimodal pattern. At low nutrient supply (i.e., low influence of herbivory), the invasibility depended mainly on the competitive abilities of the invaders, whereas at high nutrient supply, the susceptibility to herbivory dominated. This resulted in different optimum nutrient levels for invasion success of the two species due to their individual functional traits. To test the effect of diversity on invasibility, a species richness gradient was generated by random selection from a resident species pool at an intermediate nutrient level. Invasibility was not affected by species richness; instead, it was driven by the functional traits of the resident and/or invasive species mediated by herbivore density. Overall, herbivory was the driving factor for invasibility of phytoplankton communities, which implies that other factors affecting the intensity of herbivory (e.g., productivity or edibility of primary producers) indirectly influence invasions.