Differential effect of inter- and intraspecific competition on the performance of invasive and native Taraxacum species

Inter- and intraspecific competitive abilities are significant determinants of invasive success and the ecological impact of non-native plants. We tested two major hypotheses on the competitive ability of invasive species using invasive (Taraxacum officinale) and native (T. platycarpum) dandelions: differential interspecific competitive ability between invasive and native species and the kin recognition of invasive species. We collected seeds from two field sites where the two dandelion species occurred nearby. Plants were grown alone, with kin (plants from the same maternal genotype) or strangers (plants from different populations) of the same species, or with different species in a growth chamber, and the performance at the early developmental stage between species and treatments was compared. The invasive dandelions outcompeted the native dandelions when competing against each other, although no difference between species was detected without competition or with intraspecific competition. Populations of native species responded to interspecific competition differently. The effect of kinship on plant performance differed between the tested populations in both species. A population produced more biomass than the other populations when grown with a stranger, and this trend was manifested more in native species. Our results support the hypothesis that invasive plants have better competitive ability than native plants, which potentially contributes to the establishment and the range expansion of T. officinale in the introduced range. Although kin recognition is expected to evolve in invasive species, the competitive ability of populations rather than kinship seems to affect plant growth of invasive T. officinale under intraspecific competition.     

Growth rates of rhizosphere microorganisms depend on competitive abilities of plants and N supply

Abstract

Plant‐microbial interactions under N‐limiting conditions are governed by competitive abilities of plants for N. Our study aimed to examine how two plant species of strawberry, Fragaria vesca L. (native species) and Duchesnea indica (Andrews) Focke (an invasive plant in central Europe), growing in intra‐specific and inter‐specific competition alter the functions of rhizosphere microorganisms in dependence on N availability. By intra‐specific competition at low N level, a 2.4‐fold slower microbial‐specific growth rate was observed under D. indica characterized by smaller root biomass and lower N content in roots compared with F. vesca. By inter‐specific competition of both plants at low N level, microbial growth rates were similar to those for D. indica indicating that plants with stronger competitive abilities for N controls microbial community in the rhizosphere. Since a high N level smoothed the differences between plant species in root and microbial biomass as well as in microbial growth rates under both intra‐specific and inter‐specific competition, we conclude that competitive abilities of plant species were crucial for microbial growth in the rhizosphere only under N imitation.     

Competitive effects and responses of the invasive grass Eragrostis plana in Río de la Plata grasslands

The ability of an invasive species to establish is mostly determined by its biotic interactions with native species from the recipient community. Here, we evaluate the competitive effects and responses of the invasive Eragrostis plana when interacting with native species, in order to identify possible mechanisms driving invasion in Río de la Plata grasslands. A pairwise competition experiment was performed consisting of treatments that varied in the identity of neighbour plant species: (i) control (no interaction); (ii) intraspecific interaction; (iii) interspecific interaction between native and invasive species; and (iv) interspecific interaction between two co‐occurring native species. Data analysis was separated into the effect of E. plana on the performance of three native perennial grasses (target species: Aristida laevis, Eragrostis neesii and Paspalum notatum) and the response of E. plana to natives (target species: E. plana). Separately for each target species, components of plant performance were compared between neighbouring species treatments. We found that the strength of competitive interactions depended on both target and neighbour species identity. Regarding natives, interspecific competition was stronger than intraspecific. Native species showed distinctive responses to whether the neighbour was the invasive or a co‐occurring native (Eragrostis lugens). Competition between E. plana and native species was stronger than between co‐occurring natives. We demonstrated E. plana had a greater negative effect on native's species performance than the native congener E. lugens. Regarding E. plana, intraspecific competition was stronger than interspecific, and its response was positive or neutral when interacting with natives, suggesting its high tolerance to grow in competition with neighbours. We conclude E. plana's negative effects on native species performance, and its positive or neutral responses to neighbouring native plants demonstrate its strong competitive ability in the recipient community. This may explain its invasion success in southern Brazil and in the encompassing Río de la Plata grasslands. Abstract in Spanish is available with online material.     

Effects of nitrogen availability on competition between Bromus tectorum and Bouteloua gracilis

Exotic plant invasions are a serious concern for land managers and conservationists. There is evidence that increased nitrogen availability favors exotic species and decreased nitrogen availability favors non-weedy native species. This study was conducted to test the effect of nitrogen availability on competition between two grass species with contrasting life histories, cheatgrass (Bromus tectorum), a North American exotic, and blue grama (Bouteloua gracilis), a North American native. We investigated the effects of nitrogen availability and competition on aboveground biomass, belowground biomass, height, and % nitrogen tissue concentrations by growing the two species in the greenhouse under five levels of nitrogen and six levels of competition. Nitrogen availability affected competition between Bromus tectorum and Bouteloua gracilis. At the lowest level of N availability, neither species was affected by competition. As N availability increased, aboveground biomass gain of Bromus was more negatively affected by intraspecific competition relative to interspecific competition while the opposite occurred for Bouteloua. At the competition level at which each species gained the most aboveground biomass, Bromus had a linear response to increasing N availability while the response of Bouteloua was asymptotic. Our results do provide some support for the theory that fast growing exotic species have a rapid response to nutrient enrichment while native non-weedy species do not, and that low N levels can reduce competitive pressure from the exotic on the native.     

施氮对空心莲子草(Alternanthera philoxeroides)和莲子草(Alternanthera sessilis)种间关系的影响

自然界的氮素释放总是呈现出空间和时间上的异质性,但关于异质性氮释放对于入侵植物和本地植物种间关系影响的研究相对较少。将入侵植物空心莲子草(Alternanthera philoxeroides)和同属本地植物莲子草(Alternanthera sessilis)分别进行单种种植(12株,无种间竞争)和混种种植(每种6株,有种间竞争),模拟大气氮湿沉降设置由两种不同施氮总量(15g N m~(-2)a~(-1)和30g N m~(-2)a~(-1))和两种不同施氮频率(每5天1次和每15天1次)交叉组成的4种施氮处理,并以不施氮为对照。施氮总量的增加显著促进了两种植物的生长,但对两种植物的种间竞争关系没有显著影响。施氮频率对两种植物的生长以及种间竞争关系都没有显著影响。两种植物在面对竞争时表现出不同的生物量分配策略,空心莲子草将更多的生物量分配到茎,而莲子草将更多的生物量分配到根。在全球变化的背景下,大气氮湿沉降可能会改变两种植物的种群结构和动态,但可能对这两种植物的种间关系影响较小。     

Effects of Competition and Temporal Variation on the Evolutionary Potential of Two Native Bunchgrass Species

The capacity of restored plant populations to adapt to new environmental challenges depends on within‐population genetic variation. We examined how much genetic and environmentally based variation for fitness‐associated traits exists within populations of two native grasses commonly used for restoration in California. We were also interested in understanding how phenotypic expression of genetic variation for these traits varies with growth environment. Thirty maternal families of Elymus glaucus (Blue wild rye) and Nassella pulchra (Purple needlegrass) were sampled from both coastal and interior populations and reciprocally transplanted into three replicated common gardens with and without interspecific competition at each site. Reproductive output of families differed both among years and with competition treatments. Phenotypic expression of genetic variation in culm production differed among populations and was very low when families were grown with interspecific competition. Without interspecific competition, the degree of genetic determination peaked in year two in both species (8.4 and 15.1% in E. glaucus and N. pulchra, respectively). Significant genetic differences in reproduction and phenotypic plasticity were found among N. pulchra subpopulations sampled less than 3 km apart, further highlighting the importance of thoroughly sampling available genetic variation in populations used for restoration. The variable and generally low expression of genetic variation indicates that rates of adaptation in restored populations of these native grasses may vary temporally and may be especially slow within competitive environments.     

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

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

Allochthonous prey subsidies provide an asymmetric growth benefit to invasive bluegills over native cyprinids under the competitive conditions in a pond

Asymmetry in the competition abilities between invasive and native consumers can potentially influence the colonization success by invasive species. We tested whether a subsidy of allochthonous prey enhanced an asymmetric competition between invasive bluegill (Lepomis macrochirus) and two native cyprinid fish, that is, stone moroko (Pseudorasbora parva) and tamoroko (Gnathopogon elongatus elongatus). A field experiment was conducted using enclosures wherein the strength of interspecific competition and the presence/absence of allochthonous prey were manipulated. The experiment revealed that allochthonous prey alleviated the limitation of fish growths caused by a severe competition for aquatic prey resources. However, the importance of allochthonous prey differed considerably between invasive bluegill and the two native cyprinids. Individual bluegills grew faster when the allochthonous prey was supplied, whereas no difference in growth was observed in the two cyprinids whether or not allochthonous prey was supplied. Interestingly, the importance of allochthonous prey on the total amount of bluegill growth varied depending on the numerical abundance of native cyprinid competitors, and this importance increased when the native cyprinids were abundant. These findings indicated that allochthonous prey provides an asymmetric growth benefit to invasive bluegills over the two native cyprinids by alleviating asymmetrically the competition strength in a Japanese pond, especially under the conditions of severe interspecific resource competition and a limitation in the utilization of in situ prey resources.     

Competition between Native Hawaiian Plants and the Invasive Grass Megathyrsus maximus: Implications of Functional Diversity for Ecological Restoration

Biodiversity loss is a global crisis, due primarily to habitat destruction and widespread nonnative invasions. Invasive grasses are particularly problematic in many tropical ecosystems, where they possess traits that promote their persistence and can drastically alter native plant communities. We explored the ecophysiological basis for restoring native Hawaiian dryland ecosystems currently dominated by the nonnative invasive grass Megathyrsus maximus (guinea grass) in a garden experiment. Three native species—Myoporum sandwicense (naio; canopy tree), Dodonaea viscosa (aalii; shrub), and Plumbago zeylanica (iliee; groundcover)—were grown with M. maximus at three levels of native functional diversity (one, two, or three species) while holding overall plant density constant. We tested which individual and functional combinations of native species were more productive and best suppressed M. maximus growth and reproduction. Megathyrsus maximus had 39–94% higher maximum photosynthetic rates (A_max) than native species and increasing native functional diversity did not affect M. maximus A_max. Aboveground, belowground, and total biomass of M. maximus varied with functional diversity, although intraspecific competition reduced growth as much as interspecific competition. Reproductive tiller production by M. maximus decreased significantly when planted with any of the native species and with increasing native functional diversity. These results indicate that high native functional diversity in an ecological restoration setting may aid in the control of a dominant invasive grass and the reintroduction of diverse native species. Recommendations for restoring degraded nonnative grasslands in Hawaii and throughout the tropics include selection of native species that are ecophysiologically competitive and have high functional diversity.     

Demonstration of pollinator‐mediated competition between two native Impatiens species,Impatiens noli‐tangere and I. textori (Balsaminaceae)

Plant–plant interspecific competition via pollinators occurs when the flowering seasons of two or more plant species overlap and the pollinator fauna is shared. Negative sexual interactions between species (reproductive interference) through improper heterospecific pollen transfer have recently been reported between native and invasive species demonstrating pollination‐driven competition. We focused on two native Impatiens species (I. noli‐tangere and I. textori) found in Japan and examined whether pollinator‐mediated plant competition occurs between them. We demonstrate that I. noli‐tangere and I. textori share the same pollination niche (i.e., flowering season, pollinator fauna, and position of pollen on the pollinator's body). In addition, heterospecific pollen grains were deposited on most stigmas of both I. noli‐tangere and I. textori flowers that were situated within 2 m of flowers of the other species resulting in depressed fruit set. Further, by hand‐pollination experiments, we show that when as few as 10% of the pollen grains are heterospecific, fruit set is decreased to less than half in both species. These results show that intensive pollinator‐mediated competition occurs between I. noli‐tangere and I. textori. This study suggests that intensive pollinator‐mediated competition occurs in the wild even when interacting species are both native and not invasive.     

Relationships between Methylobacteria and Glyphosate with Native and Invasive Plant Species: Implications for Restoration

After removing invasive plants, whether by herbicides or other means, typical restoration design focuses on rebuilding native plant communities while disregarding soil microbial communities. However, microbial–plant interactions are known to influence the relative success of native versus invasive plants. Therefore, the abundance and composition of soil microorganisms may affect restoration efforts. We assessed the effect of herbicide treatment on phytosymbiotic pink‐pigmented facultative methylotrophic (PPFM) bacteria and the potential consequences of native and invasive species establishment post‐herbicide treatment in the lab and in a coastal sage scrub (CSS)/grassland restoration site. Lab tests showed that 4% glyphosate reduced PPFM abundance. PPFM addition to seeds increased seedling length of a native plant (Artemisia californica) but not an invasive plant (Hirschfeldia incana). At the restoration site, methanol addition (a PPFM substrate) improved native bunchgrass (Nassella pulchra) germination and size by 35% over controls. In a separate multispecies field experiment, PPFM addition stimulated the germination of N. pulchra, but not that of three invasive species. Neither PPFM nor methanol addition strongly affected the growth of any plant species. Overall, these results are consistent with the hypothesis that PPFMs have a greater benefit to native than invasive species. Together, these experiments suggest that methanol or PPFM addition could be useful in improving CSS/grassland restorations. Future work should test PPFM effects on additional species and determine how these results vary under different environmental conditions.     

Competitive context alters plant-soil feedback in an experimental woodland community

Recent findings on feedback between plants and soil microbial communities have improved our understanding of mechanisms underlying the success and consequences of invasions. However, additional studies to test for feedback in the presence and absence of interspecific competition, which may alter the strength or direction of feedbacks, are needed. We tested for soil microbial feedback in communities of the invasive grass Microstegium vimineum and commonly co-occurring native plant species. To incorporate competitive context, we used a factorial design with three plant treatments (M. vimineum alone, M. vimineum with the native plant community, and the native community without M. vimineum) and two soil inoculum treatments (experimentally invaded and uninvaded soil). When competing with M. vimineum, native communities were 27% more productive in invaded than uninvaded soil. In contrast, soil type did not significantly affect M. vimineum biomass or fecundity. At the community level, these results indicate a net negative soil microbial feedback when native plants and M. vimineum are grown in competitive mixture, but not when they are grown separately. Since positive, not negative, feedback is associated with dominance and invasion, our findings do not support plant–soil feedback as a driver of invasion in this species. Our results do show that the importance of soil feedback can change with competitive context. Such context-dependency implies that soil feedback may change when competitive interactions between natives and invading species shift as invasions progress.     

Soil conditioning and plant–soil feedbacks affect competitive relationships between native and invasive grasses

Understanding how competition from invasive species and soil conditions individually and interactively affect native performance will increase knowledge of invasion dynamics and can be used to improve the success of restoration plans. This study, conducted in Reno NV, USA, uses a two-phase plant?Csoil feedback experiment coupled with a target-neighbor competition design to examine the individual and interactive effects of both soil conditions and invasive neighbors on native performance. Study species include invasive species (Bromus tectorum and Agropyron cristatum) and native species (Elymus elymoides and Pseudoroegneria spicata). Results indicate that both plant performance and competitive interactions were influenced by species-specific soil conditioning. Specifically, invasive B. tectorum generated a larger competitive effect on natives than invasive A. cristatum; however, only A. cristatum conditioned soil in a manner that increased competitive effects of conspecifics on natives. Native P. spicata was relatively unaffected by soil conditioning and conversely, E. elymoides was strongly affected by soil conditioning. Few previous studies have examined soil conditioning and the interaction of soil conditioning and neighbor effects that both are potentially important mechanisms in structuring plant communities and influencing plant invasion.     

Novel weapons and invasion: biogeographic differences in the competitive effects of Centaurea maculosa and its root exudate (±)-catechin

Recent studies suggest that the invasive success of Centaurea maculosa may be related to its stronger allelopathic effects on native North American species than on related European species, one component of the “novel weapons” hypothesis. Other research indicates that C. maculosa plants from the invasive range in North America have evolved to be larger and better competitors than conspecifics from the native range in Europe, a component of the “evolution of increased competitive ability” hypothesis. These hypotheses are not mutually exclusive, but this evidence sets the stage for comparing the relative importance of evolved competitive ability to inherent competitive traits. In a competition experiment with a large number of C. maculosa populations, we found no difference in the competitive effects of C. maculosa plants from North America and Europe on other species. However, both North American and European C. maculosa were much better competitors against plants native to North America than congeners native to Romania, collected in areas where C. maculosa is also native. These results are consistent with the novel weapons hypothesis. But, in a second experiment using just one population from North America and Europe, and where North American and European species were collected from a broader range of sites, competitive interactions were weaker overall, and the competitive effects of C. maculosa were slightly stronger against European species than against North American species. Also consistent with the novel weapons hypothesis, (±)-catechin had stronger effects on native North American species than on native European species in two experiments. Our results suggest that the regional composition of the plant communities being invaded by C. maculosa may be more important for invasive success than the evolution of increased size and competitive ability. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Differential responses to fertilization and competition among invasive,noninvasive alien,and native Bidens species

Comparative studies of invasive, noninvasive alien, and native congenic plant species can identify plant traits that drive invasiveness. In particular, functional traits associated with rapid growth rate and high fecundity likely facilitate invasive success. As such traits often exhibit high phenotypic plasticity, characterizing plastic responses to anthropogenic environmental changes such as eutrophication and disturbance is important for predicting the invasive success of alien plant species in the future. Here, we compared trait expression and phenotypic plasticity at the species level among invasive, noninvasive alien, and native Bidens species. Plants were grown under nutrient addition and competition treatments, and their functional, morphological, and seed traits were examined. Invasive B. frondosa exhibited higher phenotypic plasticity in most measured traits than did the alien noninvasive B. pilosa or native B. bipinnata. However, differential plastic responses to environmental treatments rarely altered the rank of trait values among the three Bidens species, except for the number of inflorescences. The achene size of B. frondosa was larger, but its pappus length was shorter than that of B. pilosa. Two species demonstrated opposite plastic responses of pappus length to fertilization. These results suggest that the plasticity of functional traits does not significantly contribute to the invasive success of B. frondosa. The dispersal efficiency of B. frondosa is expected to be lower than that of B. pilosa, suggesting that long‐distance dispersal is likely not a critical factor in determining invasive success.     

Effects of Competition and Life History Stage on the Expression of Local Adaptation in Two Native Bunchgrasses

Concerns about the use of genetically appropriate material in restoration often focus on questions of local adaptation. Many reciprocal transplant studies have demonstrated local adaptation in native plant species, but very few have examined how interspecific competition affects the expression of adaptive variation. Our study examined regional scales of adaptation between foothill and coastal populations of two California native bunchgrasses (Elymus glaucus and Nassella pulchra). By combining competitive manipulations with reciprocal transplants, we examined the importance of the vegetation at a site as a selective factor in the process of local adaptation. By monitoring survival and reproduction of reciprocally transplanted populations over the course of 3 years, we also studied the effect of life history stage on the expression of local adaptation. For most of the fitness components we measured, local adaptation was detected and interspecific competition consistently amplified its expression. Expression of local adaptation was especially apparent in the more inbreeding species E. glaucus and suggests that with weaker gene flow, selection may be more effective in creating ecotypes within this species. Local adaptation was detected at all life history stages but was most strongly expressed in traits associated with adult reproduction and the viability of seeds produced by the transplants. Taken together, our results indicate that the importance of local adaptation will become more apparent in the later stages of a restoration project as the plants at a site begin to reproduce and as they experience greater interspecific competition from the maturing vegetation at the site.     

Within plant interspecific competition does not limit the highly invasive thrips,Frankliniella occidentalis in Florida

1. Species invasions are often linked to reductions in biodiversity, and competitive superiority is often cited as the main reason for the success of an invasive species. Although invaded ecosystems are often examined, few have studied areas in which an invasive species has failed to successfully invade. 2. The western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), is a damaging pest and tospovirus vector that has invaded most of the world, and competitive superiority is considered one of the main reasons for this species' success. 3. However, a recent study demonstrated that competition between larval F. occidentalis and a native thrips species may be limiting F. occidentalis abundance in much of the eastern United States. Frankliniella occidentalis also has a limited abundance in central and southern Florida, which is dominated by the endemic F. bispinosa (Morgan). The potential for interspecific competition to limit F. occidentalis abundance in Florida was assessed. 4. The effects of competition between F. occidentalis and F. bispinosa on adult reproduction on a common host (Capsicum annuum L.) were quantified, using a response surface experimental design and a combination of linear and non‐linear competition models. 5. Evidence of symmetric competition between these thrips species was found, but contrary to expectations, F. occidentalis reproduced more in dense interspecific populations than F. bispinosa. These results suggest that, unlike most of the eastern US, interspecific competition is not important in limiting F. occidentalis abundance in central and southern Florida.     

Using phenological niche separation to improve management in a Northern Glaciated Plains grassland

Many of the remaining patches of untilled (native) prairie in the Northern Glaciated Plains of North America are heavily invaded by the cool‐season grasses, Bromus inermis and Poa pratensis. However, the native vegetation in these patches contains many warm‐season species. This difference in phenology can be used to benefit restoration. We conducted an experiment to examine the efficacy of restoration treatments (mowing and prescribed fire) applied early in the growing season for consecutive years to decrease cool‐season invasive plant biomass without impacting the native warm‐season species. Our treatments were successful at significantly decreasing invasive cool‐season plant biomass and increasing native warm‐season plant biomass. No differences between treatments (mowing and prescribed fire) were found. Results suggest that incorporating differences in phenology between target and nontarget species into management may increase restoration success.     

Competition increasingly dominates the responsiveness of juvenile beech and spruce to elevated CO2 and/or O3 concentrations throughout two subsequent growing seasons

Saplings of Fagus sylvatica and Picea abies were grown in mono‐ and mixed cultures in a 2‐year phytotron study under all four combinations of ambient and elevated ozone (O₃) and carbon dioxide (CO₂) concentrations. The hypotheses tested were (1) that the competitiveness of beech rather than spruce is negatively affected by the exposure to enhanced O₃ concentrations, (2) spruce benefits from the increase of resource availability (elevated CO₂) in the mixed culture and (3) that the responsiveness of plants to CO₂ and O₃ depends on the type of competition (i.e. intra vs. interspecific). Beech displayed a competitive disadvantage when growing in mixture with spruce: after two growing seasons under interspecific competition, beech showed significant reductions in leaf gas exchange, biomass development and crown volume as compared with beech plants growing in monoculture. In competition with spruce, beech appeared to be nitrogen (N)‐limited, whereas spruce tended to benefit in terms of its plant N status. The responsiveness of the juvenile trees to the atmospheric treatments differed between species and was dominated by the type of competition: spruce growth benefited from elevated CO₂ concentrations, while beech growth suffered from the enhanced O₃ regime. In general, interspecific competition enhanced these atmospheric treatment effects, supporting our hypotheses. Significant differences in root : shoot biomass ratio between the type of competition under both elevated O₃ and CO₂ were not caused by readjustments of biomass partitioning, but were dependent on tree size. Our study stresses that competition is an important factor driving plant development, and suggests that the knowledge about responses of plants to elevated CO₂ and/or O₃, acquired from plants growing in monoculture, may not be transferred to plants grown under interspecific competition as typically found in the field.