Plant diversity,herbivory and resistance of a plant community to invasion in Mediterranean annual communities

Several components of the diversity of plant communities, such as species richness, species composition, number of functional groups and functional composition, have been shown to directly affect the performance of exotic species. Exotics can also be affected by herbivores of the native plant community. However, these two possible mechanisms limiting invasion have never been investigated together. The aim of this study was to investigate the relationships between plant diversity, herbivory and performance of two annual exotics, Conyza bonariensis and C. canadensis, in Mediterranean annual communities. We wanted to test whether herbivory of these exotics was influenced either by species richness, functional-group richness or functional-group composition. We also studied the relationship between herbivory on the exotic species and their performance. Herbivory increased with increasing species and functional-group richness for both Conyza species. These patterns are interpreted as reflecting a greater number of available herbivore niches in a richer, more complex, plant community. The identities of functional groups also affected Conyza herbivory, which decreased in the presence of Asteraceae or Fabaceae and increased in the presence of Poaceae. Increasing herbivory had consequences for vegetative and demographic parameters of both invasive species: survival, final biomass and net fecundity decreased with increasing herbivory, leading to a loss of reproductive capacity. We conclude that communities characterised by a high number of grass species instead of Asteraceae or Fabaceae may be more resistant to invasion by the two Conyza species, in part due to predation by native herbivores.     

Biodiversity influences invasion success of a facultative epiphytic seaweed in a marine forest

The biotic resistance hypothesis predicts that more diverse communities should have greater resistance to invasions than species-poor communities. However for facultative and obligate epiphytic invaders a high native species richness, abundance and community complexity might provide more resources for the invader to thrive to. We conducted surveys across space and time to test for the influence of native algal species abundance and richness on the abundance of the invasive facultative epiphytic filamentous alga Lophocladia lallemandii in a Mediterranean Cystoseira balearica seaweed forest. By removing different functional groups of algae, we also tested whether these relationships were dependent on the complexity and abundance of the native algal community. When invasion was first detected, Lophocladia abundance was positively related to species richness, but the correlation became negative after two years of invasion. Similarly, a negative relationship was also observed across sites. The removal experiment revealed that more complex native communities were more heavily invaded, where also a positive relationship was found between native algal richness and Lophocladia, independently of the native algal abundance. Our observational and experimental data show that, at early stages of invasion, species-rich seaweed forests are not more resistant to invasion than species-poor communities. Higher richness of native algal species may increase resource availability (i.e. substrate) for invader establishment, thus facilitating invasion. After the initial invasion stage, native species richness decreases with time since invasion, suggesting negative impacts of invasive species on native biodiversity.     

Effects of belowground resource use comlementarity on invasion of constructed grassland plant communities

At two field sites that differed in fertility, we investigated how species richness, functional group diversity, and species composition of constructed plant communities influenced invasion. Grassland communities were constructed to be either functionally diverse or functionally simple based on belowground resource use patterns of constituent species. Communities were also constructed with different numbers of species (two or five) to examine interactions between species richness, functional diversity and invasion resistance. We hypothesized that communities with more complementary belowground resource use (i.e., more species rich and more functionally diverse communities) would be less easily invaded than communities with greater degrees of belowground resource use overlap. Two contrasting invasive species were introduced: an early-season, shallow rooting annual grass, Bromus hordeaceus (soft chess), and a late-season, deep rooting annual forb, Centaurea solstitialis (yellow starthistle). Invader responses to species richness and functional diversity treatments differed between sites. In general, the more similar the patterns of belowground resource use between residents of the plant community and the invader, the poorer the invader’s performance. Complementarity or overlap of resource use among species in the constructed communities appeared to affect invader success less than complementarity or overlap of resource use between the invader and the species present in the community.     

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。     

Elevated success of multispecies bacterial invasions impacts community composition during ecological succession

Successful microbial invasions are determined by a species’ ability to occupy a niche in the new habitat whilst resisting competitive exclusion by the resident community. Despite the recognised importance of biotic factors in determining the invasiveness of microbial communities, the success and impact of multiple concurrent invaders on the resident community has not been examined. Simultaneous invasions might have synergistic effects, for example if resident species need to exhibit divergent phenotypes to compete with the invasive populations. We used three phylogenetically diverse bacterial species to invade two compositionally distinct communities in a controlled, naturalised in vitro system. By initiating the invader introductions at different stages of succession, we could disentangle the relative importance of resident community structure, invader diversity and time pre‐invasion. Our results indicate that multiple invaders increase overall invasion success, but do not alter the successional trajectory of the whole community.     

Species composition,functional and phylogenetic distances correlate with success of invasive Chromolaena odorata in an experimental test

Biotic resistance may influence invasion success; however, the relative roles of species richness, functional or phylogenetic distance in predicting invasion success are not fully understood. We used biomass fraction of Chromolaena odorata, an invasive species in tropical and subtropical areas, as a measure of ‘invasion success’ in a series of artificial communities varying in species richness. Communities were constructed using species from Mexico (native range) or China (non‐native range). We found strong evidence of biotic resistance: species richness and community biomass were negatively related with invasion success; invader biomass was greater in plant communities from China than from Mexico. Harvesting time had a greater effect on invasion success in plant communities from China than on those from Mexico. Functional and phylogenetic distances both correlated with invasion success and more functionally distant communities were more easily invaded. The effects of plant‐soil fungi and plant allelochemical interactions on invasion success were species‐specific.     

Is biotic resistance enhanced by natural variation in diversity?

Theories linking diversity to ecosystem function have been challenged by the widespread observation of more exotic species in more diverse native communities. Few studies have addressed the underlying processes by dissecting how biotic resistance to new invaders may be shaped by the same environmental influences that determine diversity and other community properties. In grasslands with heterogeneous soils, we added invaders and removed competitors to analyze the causes of invasion resistance. Abiotic resistance was measured using invader success in the absence of the resident community. Biotic resistance was measured as the reduction in invader success in the presence of the resident community. Invaders were most successful where biotic resistance was lowest and abiotic resistance was highest, confirming the dominant role of biotic resistance. Contrary to theory, though, biotic resistance was highest where both species richness and functional diversity were lowest. In the multivariate framework of a structural equation model, biotic resistance was independent of community diversity, and was highest where fertile soils led to high community biomass. Seed predation slightly augmented biotic resistance without qualitatively changing the results. Soil‐related genotypic variation in the invader also did not affect the results. We conclude that in natural systems, diversity may be correlated with invasibility and yet have little effect on biotic resistance to invasion. More generally, the environmental causes of variation in diversity should be considered when examining the potential functional consequences of diversity.     

Your worst enemy could be your best friend: predator contributions to invasion resistance and persistence of natives

Native predators are postulated to have an important role in biotic resistance of communities to invasion and community resilience. Effects of predators can be complex, and mechanisms by which predators affect invasion success and impact are understood for only a few well-studied communities. We tested experimentally whether a native predator limits an invasive species’ success and impact on a native competitor for a community of aquatic insect larvae in water-filled containers. The native mosquito Aedes triseriatus alone had no significant effect on abundance of the invasive mosquito Aedes albopictus. The native predatory midge Corethrella appendiculata, at low or high density, significantly reduced A. albopictus abundance. This effect was not caused by trait-mediated oviposition avoidance of containers with predators, but instead was a density-mediated effect caused by predator-induced mortality. The presence of this predator significantly reduced survivorship of the native species, but high predator density also significantly increased development rate of the native species when the invader was present, consistent with predator-mediated release from interspecific competition with the invader. Thus, a native predator can indirectly benefit its native prey when a superior competitor invades. This shows the importance of native predators as a component of biodiversity for both biotic resistance to invasion and resilience of a community perturbed by successful invasion.     

Arrival order among native plant functional groups does not affect invasibility of constructed dune communities

Different arrival order scenarios of native functional groups to a site may influence both resource use during development and final community structure. Arrival order may then indirectly influence community resistance to invasion. We present a mesocosm experiment of constructed coastal dune communities that monitored biotic and abiotic responses to different arrival orders of native functional groups. Constructed communities were compared with unplanted mesocosms. We then simulated a single invasion event by bitou (Chrysanthemoides monilifera ssp. rotundata), a dominant exotic shrub of coastal communities. We evaluated the hypothesis that plantings with simultaneous representation of grass, herb and shrub functional groups at the beginning of the experiment would more completely sequester resources and limit invasion than staggered plantings. Staggered plantings in turn would offer greater resource use and invasion resistance than unplanted mesocosms. Contrary to our expectations, there were few effects of arrival order on abiotic variables for the duration of the experiment and arrival order was unimportant in final community invasibility. All planted mesocosms supported significantly more invader germinants and significantly less invader abundance than unplanted mesocosms. Native functional group plantings may have a nurse effect during the invader germination and establishment phase and a competitive function during the invader juvenile and adult phase. Arrival order per se did not affect resource use and community invasibility in our mesocosm experiment. While grass, herb and shrub functional group plantings will not prevent invasion success in restored communities, they may limit final invader biomass.     

Species interactions contribute to the success of a global plant invader

Biological invasions are ubiquitous ecological phenomena that often impact native ecosystems. Some introduced species have evolved traits that enhance their ability to compete and dominate in recipient communities. However, it is still unknown if introduced species can evolve traits that may enhance their species interactions to fuel invasion success. We tested whether Centaurea solstitialis (yellow starthistle) from introduced populations have greater performance than native counterparts, and whether they generate more beneficial plant-soil interactions. We used common garden and plant-soil feedback experiments with soils and seeds from native Eurasian and introduced Californian populations. We found that performance of Centaurea did not differ among source genotypes, implying that the success of this invasive species is not due to evolutionary changes. However, Centaurea grew significantly larger in soils from introduced regions than from native regions, indicating a reduction in natural enemy pressure from native populations. We conclude that species interactions, not evolution, may contribute to Centaurea’s invasion success in introduced populations.     

Comparison of the effect of elevated CO2 on an invasive species (Centaurea solstitialis) in monoculture and community settings

The ongoing increase in atmospheric CO₂ concentration ([CO₂]) is likely to change the species composition of plant communities. To investigate whether growth of a highly invasive plant species, Centaurea solstitialis (yellow starthistle), was affected by elevated [CO₂], and whether the success of this species would increase under CO₂ enrichment, I grew the species in serpentine soil microcosms, both as a monoculture and as a component of a grassland community. Centaurea grown in monoculture responded strongly to [CO₂] enrichment of 350 mol mol^–1, increasing aboveground biomass production by 70%, inflorescence production by 74%, and midday photosynthesis by an average of 132%. When grown in competition with common serpentine grassland species, Centaurea responded to CO₂ enrichment with similar but nonsignificant increases (+69% aboveground biomass, +71% inflorescence production), while total aboveground biomass of the polyculture increased by 28%. Centaurea's positive CO₂ response in monoculture and parallel (but non-significant) response in polyculture provoke questions about possible consequences of increasing CO₂ for more typical California grasslands, where the invader already causes major problems.     

The relative importance of allelopathy in interference: the effects of an invasive weed on a native bunchgrass

The relative importance of allelopathy and resource competition in plant-plant interactions has been vigorously debated but seldom tested. We used activated carbon to manipulate the effects of root exudates of Centaurea maculosa, a noxious weed in much of western North America, on root elongation rates and growth of the native bunchgrass Festuca idahoensis in order to investigate the relative importance of allelopathy in the total interference of Centaurea. In root observation chambers, Festuca root elongation rates decreased to ᅢ% of the control, beginning 4 days before contacting Centaurea roots in silica sand. However, when activated carbon, which has a high affinity for adsorbing to organic compounds, was added to the sand the effects of Centaurea roots on Festuca root elongation were reduced. In other experiments, Festuca plants were 50% smaller when grown with Centaurea than with conspecifics in pure silica sand. However, Festuca grown with Centaurea in mixtures of sand and activated carbon were 85% larger than Festuca grown with Centaurea in silica sand without carbon. These results suggest that allelopathy accounts for a substantial proportion of the total interference of Centaurea on Festuca, shifting the balance of competition in favor of Centaurea. However, Centaurea outperformed Festuca even in the presence of activated carbon, demonstrating the importance of the combined roles of resource competition and allelopathy.     

Soil biota composition and the performance of a noxious weed across its invaded range

The success of invasive plant species is driven, in part, by feedback with soil ecosystems. Yet, how variation in belowground communities across latitudinal gradients affects invader distributions remains poorly understood. To determine the effect of soil communities on the performance of the noxious weed Cirsium arvense across its invaded range, we grew seedlings for 40 days in soils collected across a 699 km linear distance from both inside and outside established populations. We also described the mesofaunal and bacterial communities across all soil samples. We found that C. arvense typically performed better when grown in soils sourced from northern populations than from southern locations where it has a longer invasion history. We also found evidence that C. arvense performed best in soils sourced from outside invaded patches, although this was not consistent across all sites. The bacterial community showed a significant increase in the magnitude of compositional change in invaded sites at higher latitudes, while the mesofaunal community showed the opposite pattern. Bacterial community composition was significantly correlated with C. arvense performance, although mesofaunal community composition was not. Our results demonstrate that the interactions between an invasive plant and associated soil communities change across the invaded range, and the bacterial community in particular may affect variation in plant performance. Observed patterns may be caused by C.arvense presence and time since invasion allowing for an accumulation of species‐specific pathogens in southern soils, while the naïveté of northern soils to invasion results in a more responsive bacterial community. Although these interactions are difficult to predict, such effects could possibly facilitate the establishment of this exotic species to novel locations.     

Biodiversity and belowground interactions mediate community invasion resistance against a tall herb invader

Aims Species-rich plant communities are hypothesized to be more resistant against plant invasions because they use resources in a more efficient way. However, the relative contributions of aboveground competition and belowground interactions for invasion resistance are still poorly understood.Methods We compared the performance of Knautia arvensis transplants growing in plots differing in plant diversity both under full competition and with shoots of neighbors tied back to determine the relative strength of aboveground competition in suppressing this test invader without the confounding effect of shading. In addition, we assessed the effects of belowground competition and soil-borne pathogens on transplant performance.Important findings Both aboveground competition and plant species richness strongly and independently affected invader performance. Aboveground biomass, height, leaf mass per area and flowering of transplanted individuals of K. arvensis decreased with increasing species richness of the host community. Species-rich and species-poor communities both imposed equally strong aboveground competition on K. arvensis. However, belowground interactions (especially belowground root competition) had strong negative effects on transplant performance. In addition, the presence of grasses in a plant community further reduced the performance of K. arvensis. Our results suggest that belowground competition can render species-rich host communities more suppressive to newly arriving species, thus enhancing community invasion resistance.     

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.     

Physical habitat attribute mediates biotic resistance to non-indigenous species invasion

A soft-shelled non-indigenous clam, Nuttallia obscurata, has invaded coastal soft-sediment habitats of the northeastern Pacific. In a survey of 35 sites within the San Juan Islands, Washington, USA, Nuttallia was found almost exclusively in sandy substrates, higher in the intertidal than most native clams (>1 m above mean lower low water). The distinctive distribution of Nuttallia suggested that tidal height and sediment composition may be important physical factors that control its refuge availability, regulating its exposure to predation and ultimately the success of its invasion. I tethered Nuttallia for 24 h in the high intertidal where it is typically found and in the low intertidal at an elevation where it was never found. Clams restrained to the surface suffered high mortality from crab predation at both tidal heights, whereas control clams with unrestricted burrowing movement exhibited high mortality rates only in the low intertidal. In a second experiment, I transplanted sediment within and between the two intertidal heights to measure effects of tidal height and sediment type on survival and burial depth of Nuttallia. At both tidal heights all clams placed on mud-cobble substrate, naturally common in the low intertidal, suffered high mortality rates (>60% in 24 h). Nuttallia on loosely packed sand substrate, naturally found in the upper intertidal, survived much better, however, because they buried deeper than in the tightly packed mud. Caged control clams at both tidal heights suffered no mortality. Apparently native predators are mitigating community level impacts of an invader by excluding Nuttallia completely from some beaches with improper sediment characteristics or relegating it in others to a zone not often inhabited by native species, thereby reducing potential competitive interactions. These findings show that a physical habitat characteristic can mediate biotic resistance to an invader and thus control invasion success and community-level impacts. Generally, such physical-biological interactions may explain some of the reported site-to-site variability in invasion success, as well as the patchy distribution of many soft-sediment infaunal species.     

Species diversity reduces invasion success in pathogen‐regulated communities

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

Species,community, and ecosystem-level responses following the invasion of the red alga <Emphasis Type="Italic">Dasysiphonia japonica</Emphasis> to the western North Atlantic Ocean

Species invasions have been increasing in frequency worldwide, yet critical gaps remain in our understanding of how invaders affect community structure and ecosystem functioning, particularly during the initial stages of invasion. Even less is known about changes in the invader that may take place immediately following an invasion. This study examined the recent invasion of the red macroalga Dasysiphonia (formerly, Heterosiphonia) japonica to the western North Atlantic Ocean with the aim of filling in gaps in our understanding of the impacts that invasive seaweeds have at the species, community and ecosystem levels immediately following their establishment. Within 5 years of invasion, community composition had changed and biodiversity had decreased to nearly half of pre-invasion levels. In addition, the relative proportion of Dasysiphonia decreased by 35% over our four-year study from initially high levels shortly after establishment. We found evidence that functional traits of this initially aggressive invader changed over time, as it ultimately became a less aggressive, co-inhabiting member of the local algal community, particularly with respect to nutrient uptake and relative abundances, although native diversity remained low relative to pre-invasion levels. Using these realistic changes in community structure, including decreases in biodiversity, we also showed that nutrient uptake of algal assemblages changed over time, suggesting changes in the functional characteristics of invaded communities, with implications for ecosystem-level processes such as nutrient fluxes. This study provides rare empirical evidence about the successional stages occurring at the individual, community, and ecosystem levels during the first 5 years of an invasion.     

Invasion of an intact plant community: the role of population versus community level diversity

To improve the understanding of how native plant diversity influences invasion, we examined how population and community diversity may directly and indirectly be related to invasion in a natural field setting. Due to the large impact of the dominant C₄ grass species (Andropogon gerardii) on invasion resistance of tallgrass prairie, we hypothesized that genetic diversity and associated traits within a population of this species would be more strongly related to invasion than diversity or traits of the rest of the community. We added seeds of the exotic invasive C₄ grass, A. bladhii, to 1-m² plots in intact tallgrass prairie that varied in genetic diversity of A. gerardii and plant community diversity, but not species richness. We assessed relationships among genetic diversity and traits of A. gerardii, community diversity, community aggregated traits, resource availability, and early season establishment and late-season persistence of the invader using structural equation modeling (SEM). SEM models suggested that community diversity likely enhanced invasion indirectly through increasing community aggregated specific leaf area as a consequence of more favorable microclimatic conditions for seedling establishment. In contrast, neither population nor community diversity was directly or indirectly related to late season survival of invasive seedlings. Our research suggests that while much of diversity–invasion research has separately focused on the direct effects of genetic and species diversity, when taken together, we find that the role of both levels of diversity on invasion resistance may be more complex, whereby effects of diversity may be primarily indirect via traits and vary depending on the stage of invasion.     

Weak effects of the exotic invasive <Emphasis Type="Italic">Carpobrotus edulis</Emphasis> on the structure and composition of Portuguese sand-dune communities

Sand dune ecosystems have a high conservation value worldwide, but they are highly threatened by exotic plant invasion. We investigated the impacts of the exotic invasive species Carpobrotus edulis on the composition and structure (spatial pattern, total cover, species diversity and species co-occurrence) of native sand dune communities in the western coast of Portugal. We studied eight sites following a north-south gradient; in each site we established 8–10 transects of 25 contiguous quadrats of one square meter. C. edulis had a significantly clumped pattern in five of the study sites, which, however, was not related to the spatial pattern of native species. The effects of climate on the community structure variables were on average three times stronger than those of C. edulis. This species also had small effects on the floristic composition of native species. Our results indicate that the success and impacts of C. edulis are habitat-dependent and context-specific. They also provide evidence of a strong resilience to the impacts of invasion in the studied sand dune ecosystems: C. edulis did not reach large abundances or exert negative impacts on native communities to the extent expected. These ecosystems provide a unique opportunity to increase our understanding on the origin of impacts by invasive species, and on how particular communities resist the impacts of an invader.