Competition and coexistence in submerged aquatic plant communities: the effects of species interactions versus abiotic factors

SUMMARY. 1. Aquatic plant biomass and species composition were studied at three sites in Long Lake, Alberta, Canada, to examine the role of biotic and abiotic factors in determining species diversity.
2. Results of controlled in situ experiments to test for interspecific competition showed that biomass of plants in four different mixed communities did not increase in response to selected species removals ( P >0.1, n =16).
3. In contrast, biomass of the dominant taxa (Myriophyllum exalbescens, Ceratophyllum demersum and Chara sp.) were correlated ( P <0.05) with abiotic factors (i.e. distance from shore, water depth, sediment exchangeable phosphorus concentration and/or sediment organic content).
4. These results suggest that interspecific competition between naturally coexisting species of submerged aquatic plants is slight and that spatial heterogeneity or differential utilization of abiotic resources promotes species diversity in submerged plant communities.     

Experimental test of the Invasional Meltdown Hypothesis: an exotic herbivore facilitates an exotic plant,but the plant does not reciprocally facilitate the herbivore

相似文献   

Soil Organism and Plant Introductions in Restoration of Species-Rich Grassland Communities

Soil organisms can strongly affect competitive interactions and successional replacements of grassland plant species. However, introduction of whole soil communities as management strategy in grassland restoration has received little experimental testing. In a 5-year field experiment at a topsoil-removed ex-arable site ( receptor site ), we tested effects of (1) spreading hay and soil, independently or combined, and (2) transplanting intact turfs on plant and soil nematode community development. Material for the treatments was obtained from later successional, species-rich grassland ( donor site ). Spreading hay affected plant community composition, whereas spreading soil did not have additional effects. Plant species composition of transplanted turfs became less similar to that in the donor site. Moreover, most plants did not expand into the receiving plots. Soil spreading and turf transplantation did not affect soil nematode community composition. Unfavorable soil conditions (e.g., low organic matter content and seasonal fluctuations in water level) at the receptor site may have limited plant and nematode survival in the turfs and may have precluded successful establishment outside the turfs. We conclude that introduction of later successional soil organisms into a topsoil-removed soil did not facilitate the establishment of later successional plants, probably because of the "mismatch" in abiotic soil conditions between the donor and the receptor site. Further research should focus on the required conditions for establishment of soil organisms at restoration sites in order to make use of their contribution to grassland restoration. We propose that introduction of organisms from "intermediate" stages will be more effective as management strategy than introduction of organisms from "target" stages.     

High diversity and low specificity of fungi associated with seedless epiphytic plants

Epiphytes, which grow on other plants for support, make up a large portion of Earth's plant diversity. Like other plants, their surfaces and interiors are colonized by diverse assemblages of fungi that can benefit their hosts by increasing tolerance for abiotic stressors and resistance to disease or harm them as pathogens. Fungal communities associated with epiphytic plants and the processes that structure these communities are poorly known. To address this, we sampled seven epiphytic seedless plant taxa in a Costa Rican rainforest and examined the effects of host identity and microhabitat on external and endophytic fungal communities. We found low host specificity for both external and endophytic fungi and weak differentiation between epiphytic and neighboring epilithic plant hosts. High turnover in fungi within and between hosts and habitats reveals that epiphytic plant-associated fungal communities are highly diverse and suggests that they are structured by stochastic processes.     

Do riparian plant community characteristics differ between <Emphasis Type="Italic">Tamarix</Emphasis> (L.) invaded and non-invaded sites on the upper Verde River,Arizona?

Invasion by Tamarix (L.) can severely alter riparian areas of the western U.S., which are globally rare ecosystems. The upper Verde River, Arizona, is a relatively free-flowing river and has abundant native riparian vegetation. Tamarix is present on the upper Verde but is a minor component of the vegetation (8% of stems). This study sought to determine whether riparian vegetation characteristics differed between sites where Tamarix was present and sites where Tamarix was absent during the invasion of the upper Verde. We hypothesized that herbaceous understory and woody plant communities would differ between Tamarix present and absent sites. Our hypothesis was generally confirmed, the two types of sites were different. Tamarix present sites had greater abundance of all vegetation, native understory species, graminoids, and native trees, and a positive association with perennial native wetland plant species. Tamarix absent sites had greater abundance of exotic plants and upland adapted plants and an association with greater abiotic cover and litter. These results are contrary to other reports of Tamarix association with depauperate riparian plant communities, and suggest that Tamarix invasion of a watershed with a relatively natural flow regime and a robust native plant community follows similar establishment patterns as the native riparian plant community.     

Can model species be used to advance the field of invasion ecology?

Hypotheses for explaining plant invasions have focused on a variety of factors that may influence invasion success, including propagule pressure, interactions of the introduced species with the biotic, abiotic, or disturbance properties of the new ecosystem, or the genetic characteristics of the invader itself. Evaluating the relative importance of these factors has been difficult because for most invaders key information about the introduced population or the introduction event is not available. We propose that natural experiments using model species is an important tool to test multiple invasion hypotheses at the same time, providing a complementary approach to meta-analysis and literature review. By focusing on a single candidate species, Pinus contorta, we explore several attributes that we propose constitute a good model, including: (a) intentional and relatively well documented introduction into a wide range of environments and countries across the world during the past century, where invasion success or failure has already occurred, (b) conspicuous growth form that simplifies assessment of growth rates, and comparisons across native and introduced ecosystems around the world, and, (c) documented and replicated variability of introduction intensity, genetic characteristics of the introduced populations, contrasting biotic communities present at sites of introduction, and abiotic conditions within and across introduced ecosystems. We propose that identifying model species with these characteristics will provide opportunities to disentangle the relative importance of different mechanisms hypothesized to influence invasion success, and thereby advance the field of invasion ecology.     

Impacts and interactions of multiple human perturbations in a California salt marsh

Multiple disturbances to ecosystems can influence community structure by modifying resistance to and recovery from invasion by non-native species. Predicting how invasibility responds to multiple anthropogenic impacts is particularly challenging due to the variety of potential stressors and complex responses. Using manipulative field experiments, we examined the relative impact of perturbations that primarily change abiotic or biotic factors to promote invasion in coastal salt marsh plant communities. Specifically we test the hypotheses that nitrogen enrichment and human trampling facilitate invasion of upland weeds into salt marsh, and that the ability of salt marsh communities to resist and/or recover from invasion is modified by hydrological conditions. Nitrogen enrichment affected invasion of non-native upland plants at only one of six sites, and increased aboveground native marsh biomass at only two sites. Percent cover of native marsh plants declined with trampling at all sites, but recovered earlier at tidally flushed sites than at tidally restricted sites. Synergistic interactions between trampling and restricting tidal flow resulted in significantly higher cover of non-native upland plants in trampled plots at tidally restricted sites. Percent cover of non-native plants recovered to pre-trampling levels in fully tidal sites, but remained higher in tidally restricted sites after 22 months. Thus, perturbations that reduce biotic resistance interact with perturbations that alter abiotic conditions to promote invasion. This suggests that to effectively conserve or restore native biodiversity in altered systems, one must consider impacts of multiple human disturbances, and the interactions between them. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The potential role of public gardens as sentinels of plant invasion

Public gardens can help prevent detrimental effects of plant invasions by collecting and sharing data on taxa spreading from cultivation early in the invasion process, thereby acting as sentinels of plant invasion. Existing initiatives have called for public gardens to adopt measures preventing plant invasion, but it is unclear what actions individual gardens are implementing, as there is no formal mechanism for communicating their progress. This study used internal lists of escaping taxa from seven public gardens in the Midwestern United States and Canada to demonstrate how public gardens can collectively contribute data that is critical to assessing potential invasiveness. It also reveals methodological differences in how gardens develop their lists of escaping plants, leading to recommendations for standardization. Data pooled across gardens yielded 769 species spreading from cultivation at one or more gardens. Eight woody species were listed by all gardens despite not consistently being recognized as invasive by states and provinces containing the gardens; some species recorded by multiple gardens did not appear on any invasive lists. While it may be premature to call taxa escaping from cultivation at a few public gardens “invasive” or even “potentially invasive”, these plants should be monitored and evaluated with this information shared to facilitate stronger conclusions about risk. Thus, public gardens have a unique expertise in assisting invasive plant efforts as sentinels, particularly if challenges related to methodological inconsistencies and data sharing are suitably addressed, which is herein recommended through the adoption of a set of standardized guidelines.

     

Nonnative plant shifts functional groups of arthropods following drought

Nonnative plants alter the composition of native plant communities, with concomitant effects on arthropods. However, plant invasions may not be the only disturbance affecting native communities, and multiple disturbances can have compounding effects. We assessed the effects of invasion and drought on plant and arthropod communities by comparing grasslands dominated by nonnative Old World bluestem grasses (OWBs, Dichanthium annulatum) to grasslands dominated by native plants during a period of decreasing drought severity (2011–2013). Native plant communities had more species of plants and arthropods (/m²) than areas dominated by OWBs during extreme drought, but richness was comparable as drought severity decreased. Abundance of arthropods was greater in native plant communities than in OWB communities during extreme drought, but OWB communities had more arthropods during moderate and non-drought conditions. We observed a shift in the arthropod community from one dominated by detritivores to one dominated by herbivores following plant invasion; the magnitude of this shift increased as drought severity decreased. Both plant communities were dominated by nonnative arthropods. A nonnative leafhopper (Balclutha rubrostriata) and native mites (Mochlozetidae) dominated OWB communities as drought severity decreased, and OWBs may serve as refugia for both taxa. Nonnative woodlice (Armadillidium vulgare) dominated native plant communities during extreme and non-drought conditions and abundance of this species may be associated with an increase in plant litter and available nutrients. Given the importance of arthropods for ecosystem services, incorporating arthropod data into conservation studies may demonstrate how changes in arthropod diversity alter ecosystem function where nonnative plants are dominant.     

Plant immunization

Plant immunization is the process of activating natural defense system present in plant induced by biotic or abiotic factors. Plants are pre-treated with inducing agents stimulate plant defense responses that form chemical or physical barriers that are used against the pathogen invasion. Inducers used usually give the signals to rouse the plant defense genes ultimately resulting into induced systemic resistance. In many plant-pathogen interactions, R-Avr gene interactions results in localized acquired resistance or hypersensitive response and at distal ends of plant, a broad spectrum resistance is induced known as systemic acquired resistance (SAR). Various biotic or abiotic factors induce systemic resistance in plants that is phenotypically similar to pathogen-induced systemic acquired resistance (SAR). Some of the biotic or abiotic determinants induce systemic resistance in plants through salicylic acid (SA) dependent SAR pathway, others require jasmonic acid (JA) or ethylene. Host plant remains in induced condition for a period of time, and upon challenge inoculation, resistance responses are accelerated and enhanced. Induced systemic resistance (ISR) is effective under field conditions and offers a natural mechanism for biological control of plant disease.     

Entangled fates of holobiont genomes during invasion: nested bacterial and host diversities in Caulerpa taxifolia

Successful prevention and mitigation of biological invasions requires retracing the initial steps of introduction, as well as understanding key elements enhancing the adaptability of invasive species. We studied the genetic diversity of the green alga Caulerpa taxifolia and its associated bacterial communities in several areas around the world. The striking congruence of α and β diversity of the algal genome and endophytic communities reveals a tight association, supporting the holobiont concept as best describing the unit of spreading and invasion. Both genomic compartments support the hypotheses of a unique accidental introduction in the Mediterranean and of multiple invasion events in southern Australia. In addition to helping with tracing the origin of invasion, bacterial communities exhibit metabolic functions that can potentially enhance adaptability and competitiveness of the consortium they form with their host. We thus hypothesize that low genetic diversities of both host and symbiont communities may contribute to the recent regression in the Mediterranean, in contrast with the persistence of highly diverse assemblages in southern Australia. This study supports the importance of scaling up from the host to the holobiont for a comprehensive understanding of invasions.     

Effects of an introduced mustard,Thlaspi arvense,on soil fungal communities in subalpine meadows

The subalpine meadows of the Rocky Mountains, USA, are at the advancing front of global change; however, little is known about the sensitivities of high-elevation soil fungal communities to ongoing ecological changes. Soil fungi are sensitive to abiotic and biotic environmental stressors, including climate change, soil disturbance, and the presence of introduced, non-native plants. Invasive plants in the Brassicaceae (mustard family) are known to alter fungal community structure, suppress arbuscular mycorrhizal fungi, and change their relationship with native plant hosts in forest ecosystems, but these phenomena have not been studied in the subalpine zone where non-native mustard plants are becoming established. Here, we investigated whether the presence of the introduced mustard plant, Thlaspi arvense, is associated with distinct properties of the whole fungal and arbuscular mycorrhizal fungal communities in subalpine meadow ecosystems. We observed clear differences in the composition, relative abundance of core taxa, and mean taxon relatedness of soil fungal communities in plots with T. arvense relative to those with only native vegetation. A suite of novel fungi were associated with T. arvense, and overall patterns of AMF phylogenetic diversity were drastically reduced in association with its presence. Our results suggest that T. arvense introduction impacts the soil fungal community, with potential implications for native plant communities and soil nutrient cycling in high elevation meadows of the Rocky Mountains.     

Woody Plant Invasion in Relictual Grasslands

Invasion by alien plants is one of the greatest and most widely distributed causes of man-made changes in ecosystems. One of its most conspicuous variants is the invasion of natural grasslands by exotic trees, which not only means the addition of new taxa to the native biota, but also the introduction of completely new life-forms. In Argentina, the Pampean plain is one of the most highly altered regions and its natural vegetation is only partially conserved in the coastal dunes, swampy lowlands, mountainous areas, riparian habitats, field edges, roadside verges and railway tracksides. Nevertheless, all these habitats are undergoing severe invasions by woody aliens that jeopardize their conservation. We present a preliminary survey of the alien woody plants affecting the last remaining natural grasslands of the Argentine Pampas, the history of their colonization and the phase of the invasion process in which they presently occur. We construct an index of invasion severity to evaluate which woody exotic species should be considered invasive and therefore targeted for control, and to set priorities for the application of management practices according to the seriousness of the threat posed to biodiversity conservation.     

Impacts of alien plant invasion on native plant communities are mediated by functional identity of resident species,not resource availability

Alien plant invasion and nutrient enrichment as a result of anthropogenic landscape modification seriously threaten native plant community diversity. It is poorly understood, however, whether these two disturbances interact with the functional identity of recipient native plants to drive community change. We performed a mesocosm experiment to examine whether the interactive effects of invasion by a stoloniferous turf‐grass Stenotaphrum secundatum and nutrient enrichment vary across different plant growth forms of an endangered coastal plant community. Communities contained 18 species (drawn without replacement from a pool of 31 species) with either runner, tufted or woody growth forms. Species were well‐established and reproductively mature prior to S. secundatum introduction. Species growth (% cover), reproductive output, soil temperature and light availability were monitored for two growing seasons. Invasion and nutrient enrichment (two levels: ‘natural control’ and ‘enriched’) had no effect on species richness, community composition, reproductive output, soil temperature or light penetration. There was no interactive effect of nutrients and invasion on community productivity (i.e. final biomass), such that invasion caused a reduction in community biomass at both natural and enriched nutrient levels. This was driven only by reduced biomass of functionally‐similar native runner species, which share similar root morphologies and nutrient‐acquisition strategies with S. secundatum. Our study indicates that impacts of invasion are dependent upon the functional identity of species within recipient communities, not the availability of resources. This shows that management cannot buffer invader effects by manipulating resource availability. Revegetation strategies should target functionally‐similar natives for replacement following invader control.     

外来植物紫茎泽兰入侵对土壤理化性质及丛枝菌根真菌(AMF)群落的影响

为了揭示外来植物紫茎泽兰入侵对入侵地土壤丛枝菌根真菌(AMF)群落及相关肥力的影响,比较测定了紫茎泽兰不同入侵程度土壤理化性质、AMF侵染率及AMF群落的差异。结果表明,紫茎泽兰入侵降低了土壤pH,使土壤中有机碳、全氮和速效钾含量分别增加83.0%,106.9%和111.0%;尽管对全磷含量没有显著影响,但有机磷含量呈升高的趋势,而速效磷呈降低的趋势。紫茎泽兰入侵降低了本地植物的AMF侵染率;随着入侵程度的加深,土壤中以膨胀无梗囊霉(Acauospora dilatata)为优势种的AMF群落结构逐渐转变为以近明球囊霉(Glomus claroideum )为优势种的结构,紫茎泽兰可在其根周选择培育近明球囊霉,而对其它AMF种,特别是对膨胀无梗囊霉则存在抑制作用;基于各AMF种多度的聚类分析表明,形成紫茎泽兰单优群落土壤中各AMF种多度与未入侵的本地植物群落及入侵程度较轻的紫茎泽兰与本地植物群落之间存在明显分歧。综合分析推断认为,紫茎法兰入侵改变了入侵地土壤理化性状,抑制AMF对土著植物的侵染,改变AMF群落,并在其根周选择培育近明球囊霉,这可能是紫茎泽兰入侵及扩张的重要途径之一。     

Alien plants associate with widespread generalist arbuscular mycorrhizal fungal taxa: evidence from a continental‐scale study using massively parallel 454 sequencing

Aim The biogeography of arbuscular mycorrhizal (AM) fungi is poorly understood, and consequently the potential of AM fungi to determine plant distribution has been largely overlooked. We aimed to describe AM fungal communities associating with a single host‐plant species across a wide geographical area, including the plant’s native, invasive and experimentally introduced ranges. We hypothesized that an alien AM plant associates primarily with the geographically widespread generalist AM fungal taxa present in a novel range. Location Europe, China. Methods We transplanted the palm Trachycarpus fortunei into nine European sites where it does not occur as a native species, into one site where it is naturalized (Switzerland), and into one glasshouse site. We harvested plant roots after two seasons. In addition, we sampled palms at three sites in the plant’s native range (China). Roots were subjected to DNA extraction, polymerase chain reaction (PCR) and 454 sequencing of AM fungal sequences. We analysed fungal communities with non‐metric multidimensional scaling (NMDS) ordination and cluster analysis and studied the frequency of geographically widespread fungal taxa with log‐linear analysis. We compared fungal communities in the roots of the palm with those in resident plants at one site in the introduced range (Estonia) where natural AM fungal communities had previously been studied. Results We recorded a total of 73 AM fungal taxa. AM fungal communities in the native and introduced ranges differed from one another, while those in the invasive range contained taxa present in both other ranges. Geographically widespread AM fungal taxa were over‐represented in palm roots in all regions, but especially in the introduced range. At the Estonian site, the palm was colonized by the same community of widespread AM fungal taxa as associate with resident habitat‐generalist plants; by contrast, resident forest‐specialist plants were colonized by a diverse community of widespread and other AM fungal taxa. Main conclusions AM fungal communities in the native, invasive and experimentally introduced ranges varied in taxonomic composition and richness, but they shared a pool of geographically widespread, non‐host‐specific taxa that might support the invasion of a generalist alien plant. Our dataset provides the first geographical overview of AM taxon distributions obtained using a single host‐plant species.     

Twenty‐five years of plant community dynamics and invasion in New Zealand tussock grasslands

Understanding how plant communities respond to plant invasions is important both for understanding community structure and for predicting future ecosystem change. In a system undergoing intense plant invasion for 25 years, we investigated patterns of community change at a regional scale. Specifically, we sought to quantify how tussock grassland plant community structure had changed and whether changes were related to increases in plant invasion. Frequency data for all vascular plants were recorded on 124, permanent transects in tussock grasslands across the lower eastern South Island of New Zealand measured three times over a period of 25 years. Multivariate analyses of species richness were used to describe spatial and temporal patterns in the vegetation. Linear mixed‐effects models were used to relate temporal changes in community structure to the level and rate of invasion of three dominant invasive species in the genus Hieracium while accounting for relationships with other biotic and abiotic variables. There was a strong compositional gradient from exotic‐ to native‐dominated plant communities that correlated with increasing elevation. Over the 25 years, small‐scale species richness significantly decreased and then increased again; however, these changes differed in different plant communities. Exotic species frequency consistently increased on some transects and consistently declined on others. Species richness changes were correlated with the level of Hieracium invasion and abiotic factors, although the relationship with Hieracium changed from negative to positive over time. Compositional changes were not related to measured predictors. Our results suggest that observed broad‐scale fluctuations in species richness and community composition dynamics were not driven by Hieracium invasion. Given the relatively minor changes in community composition over time, we conclude that there is no evidence for widespread degradation of these grasslands over the last 25 years. However, because of continuing weed invasion, particularly at lower elevations, impacts may emerge in the longer term.     

Resistance of plant communities to invasion by tall fescue: An experimental study combining species diversity,functional traits and nutrient levels

The grass Festuca arundinacea is often planted for slope stabilisation in South Korea, and is spreading widely beyond the introduction sites. This study used a functional group approach to examine the resistance of plant combinations to invasion by F. arundinacea based on the limiting similarity and diversity-resistance hypotheses, and to elucidate the process of colonisation. The study simulated the environment of construction sites and surrounding areas that might be encountered by expanding populations of F. arundinacea. The role of nutrient condition in the ranking of functional group competitive ability was also examined. Twelve native plant species were categorised into three functional groups using combinations of functional traits. Pairwise (one-to-one competition), multiple (four different neighbouring species) and monoculture experimental settings were designed using two nutrient levels. The Relative Competition Index was used to interpret the competitive effect of neighbouring species on F. arundinacea. Species of the same functional group as F. arundinacea were unable to resist invasion, but annual plants with niche preemption ability could outcompete it. Competitive relationships between native plants and F. arundinacea were explained partially by functional group identity but were inconsistent with the limiting similarity hypothesis and the diversity-interaction. Unforeseen interactions within the artificial communities also produced unexpected effects. In designing artificial plant communities, it is necessary to consider functional traits that reflect the species characteristic of particular periods and indirect effects that modify the interaction between other species.     

Abiotic factors control invasion by Argentine ants at the community scale

1. A prominent and unresolved question in ecology concerns why communities differ in their susceptibility to invasion. While studies often emphasize biotic resistance, it is less widely appreciated how the physical environment affects community vulnerability to invasion. 2. In this study we performed field experiments to test how abiotic variation directly and indirectly influences the extent to which Linepithema humile Mayr (Argentine ants) invade seasonally dry environments in southern California. 3. In controlled and replicated experiments involving drip irrigation, we demonstrate (i) that elevated levels of soil moisture increased both the abundance of Argentine ants and their ability to invade native ant communities and (ii) that cessation of irrigation caused declines in the abundance of Argentine ants and led to their withdrawal from previously occupied areas. 4. Because drip irrigation stimulated plant growth, in an additional experiment we manipulated both soil moisture and plant cover to assess the direct vs. indirect effects of added water on the abundance of L. humile. 5. Local abundance of Argentine ants increased in irrigated plots but was 38% higher in irrigated plots with plants compared to irrigated plots where plant growth was suppressed. The results of this experiment thus argue for a direct role of soil moisture in influencing Argentine ant abundance but suggest that that the indirect effects of added water may also be important. 6. Our study illustrates more generally that fine-scale variation in the physical environment can control whether communities become invaded by non-native species and suggests that an understanding of community susceptibility to invasion will be improved by a better appreciation of interactions between the biotic and abiotic environment.     

Comparative analysis of biological and ecological differentiation of Anemone palmata L. (Ranunculaceae) in the western Mediterranean (France and Spain): an assessment of rarity and population persistence

The western Mediterranean geophyte Anemone palmata L. is one of the most endangered angiosperm taxa in France. The biological and ecological characteristics of French populations are contrasted with those of populations from near the centre of the species distribution in east Spain. A Correspondence Analysis discriminates the French and Spanish populations according to substrate and the composition of the plant communities where they grow. The karyological study reveals that east Spanish plants are autotetraploid, whilst the French are all diploid. In addition, morphological differences were registered, Spanish plants generally being more vigorous. French populations consist mostly of senile plants, without or with very low rates of sexual reproduction, whereas the populations from Spain include all age groups with an important percentage of juvenile plants. The main factors which could affect the French populations are habitat destruction and modification of plant communities due to land-abandonment. Conservation measures should be supported by immediate protection of the French localities and habitats to prevent the total decline of this rare species.  © 2002 The Linnean Society of London, Botanical Journal of the Linnean Society , 2002, 140 , 95–114.