Spatial arrangements affect suppression of invasive Alternanthera philoxeroides by native Hemarthria compressa

It has been hypothesized that differences in spatial arrangements change the relative frequency of intra- and interspecific encounters between plant species. Manipulating spatial arrangement may play a role in invasive plant suppression when native species are used as competitors against introduced species. In this study, a replacement series experiment was performed to investigate the effects of intraspecifically random and aggregated spatial arrangements on interactions between the native plant Hemarthria compressa and the invasive plant Alternanthera philoxeroides, to test the possibility and effectiveness of H. compressa in suppressing A. philoxeroides. When both species were planted in intraspecifically random spatial patterns, H. compressa had a competitive advantage over A. philoxeroides at relative densities of 2:2 and 3:1. However, aggregation increased the strength, and therefore the cost, of intraspecific competition in H. compressa, resulting in lower biomass production, which reduced its effectiveness as an interspecific competitor. As the relative density of H. compressa in mixtures decreased, plants allocated more biomass to belowground parts, but fewer interspecific encounters lowered its inhibitory effects on A. philoxeroides. The results not only confirm that the frequency of conspecific and heterospecific encounters can influence competitive outcomes, but also suggest that a reduction in the degree of spatial aggregation in H. compressa and an increase in its relative densities may be essential to increase the suppression of A. philoxeroides.     

The optimum Se application time for reducing Cd uptake by rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) and its mechanism

Background and aims

Intraspecific aggregation of plant individuals can promote species coexistence by delaying competitive exclusions. However, such impacts may differ among species with contrasting spatial architecture and rely on the spatial distribution of resources.

Methods

We grew a phalanx clonal plant Carex neurocarpa (with aggregated ramets) and a guerilla one Bolboschoenus planiculmis (with diffused ramets) in monocultures or in 1:1 mixtures with an even or a clustered distribution pattern of the two species in homogeneous or heterogeneous soils.

Results

After 16 months, shoot biomass and ramet number were greater in mixtures than in monocultures in C. neurocarpa, but smaller in B. planiculmis. However, the growth of neither C. neurocarpa nor B. planiculmis differed between even and clustered mixtures. Soil nutrient heterogeneity did not significantly affect the growth of either species, but increased relative yield of B. planiculmis and decreased that of C. neurocarpa.

Conclusions

The relative importance of intra- vs. interspecific competition depends on the spatial architecture of plants, and soil nutrient heterogeneity slows down competitive exclusion by decreasing differences in competitive ability between plants. However, our results do not support the idea that intraspecific aggregation of individuals alters competitive interactions between species.

     

Effects of spatial aggregation on competition,complementarity and resource use

Abstract The spatial distributions of most species are aggregated to varying degrees. A limited number of studies have examined the effects of spatial aggregation on interspecific and intraspecific interactions, generally finding that spatial aggregation can enhance coexistence between species by reducing the capacity for interspecific competition. Less well studied are the effects of spatial aggregation on complementarity (i.e. differences in resource use strategies) and resource use. Our primary hypothesis was that spatial aggregation reduces the complementarity between species owing to: (i) less interspecific interactions as a result of spatial separation; and (ii) less differences between species as a result of phenotypic plasticity. We further postulate that these negative effects of spatial aggregation on complementarity will reduce resource use by the community. Here we test these hypotheses in a pot experiment in which we applied three levels of spatial aggregation to three sets of two‐species mixtures of herbaceous perennial plant species from native grasslands of south‐eastern Australia. Both root and shoot biomass were significantly affected by spatial aggregation, although the nature of these affects depended upon the species involved, and the relative strengths of interspecific versus intraspecific competition. Complementarity between species in the distribution of their green leaves decreased significantly as spatial aggregation increased for one of the species mixtures, providing some evidence in support of our hypothesis that aggregation reduces complementarity through phenotypic plasticity. Spatial aggregation also altered light interception and use of soil moisture resources, although these effects were dependent on the species involved. We suggest that clear effects of spatial aggregation on complementarity and resource use may be obscured by the idiosyncratic way in which neighbour identity influences plant growth and hence plant size, limiting the ability to generalize, at the community level, any underlying effects of spatial pattern on ecological process.     

Fitness of Crop-Wild Hybrid Sunflower under Competitive Conditions: Implications for Crop-to-Wild Introgression

Understanding the likelihood and extent of introgression of novel alleles in hybrid zones requires comparison of lifetime fitness of parents and hybrid progeny. However, fitness differences among cross types can vary depending on biotic conditions, thereby influencing introgression patterns. Based on past work, we predicted that increased competition would enhance introgression between cultivated and wild sunflower (Helianthus annuus) by reducing fitness advantages of wild plants. To test this prediction, we established a factorial field experiment in Kansas, USA where we monitored the fitness of four cross types (Wild, F₁, F₂, and BC_w hybrids) under different levels of interspecific and intraspecific competition. Intraspecific manipulations consisted both of density of competitors and of frequency of crop-wild hybrids. We recorded emergence of overwintered seeds, survival to reproduction, and numbers of seeds produced per reproductive plant. We also calculated two compound fitness measures: seeds produced per emerged seedling and seeds produced per planted seed. Cross type and intraspecific competition affected emergence and survival to reproduction, respectively. Further, cross type interacted with competitive treatments to influence all other fitness traits. More intense competition treatments, especially related to density of intraspecific competitors, repeatedly reduced the fitness advantage of wild plants when considering seeds produced per reproductive plant and per emerged seedling, and F₂ plants often became indistinguishable from the wilds. Wild fitness remained superior when seedling emergence was also considered as part of fitness, but the fitness of F₂ hybrids relative to wild plants more than quadrupled with the addition of interspecific competitors and high densities of intraspecific competitors. Meanwhile, contrary to prediction, lower hybrid frequency reduced wild fitness advantage. These results emphasize the importance of taking a full life cycle perspective. Additionally, due to effects of exogenous selection, a given hybrid generation may be especially well-suited to hastening introgression under particular environmental conditions.     

Intraspecific variation and species coexistence

We use a two-species model of plant competition to explore the effect of intraspecific variation on community dynamics. The competitive ability ("performance") of each individual is assigned by an independent random draw from a species-specific probability distribution. If the density of individuals competing for open space is high (e.g., because fecundity is high), species with high maximum (or large variance in) performance are favored, while if density is low, species with high typical (e.g., mean) performance are favored. If there is an interspecific mean-variance performance trade-off, stable coexistence can occur across a limited range of intermediate densities, but the stabilizing effect of this trade-off appears to be weak. In the absence of this trade-off, one species is superior. In this case, intraspecific variation can blur interspecific differences (i.e., shift the dynamics toward what would be expected in the neutral case), but the strength of this effect diminishes as competitor density increases. If density is sufficiently high, the inferior species is driven to extinction just as rapidly as in the case where there is no overlap in performance between species. Intraspecific variation can facilitate coexistence, but this may be relatively unimportant in maintaining diversity in most real communities.     

AN EXPERIMENTAL STUDY OF COMPETITIVE PERFORMANCE IN BRASSICA RAPA (CRUCIFERAE)

Growth chamber experiments with rapid-cycling Brassica rapa were designed to estimate the signs and magnitudes of the genetic correlations for plant performance in each of three conditions: no-competition (isolated plants), intraspecific competition, and interspecific competition with Raphanus sativa. Biomass and flower number were highest in the no-competition treatment, intermediate under intraspecific competition, and lowest under interspecific competition. Significant among-family variation in biomass and flower number was found under each regime. The mean family performance (biomass or flower number) in the no-competition treatment was significantly positively correlated with the performance in only one of the competitive treatments (for biomass in the intraspecific treatment). For both biomass and flower number there was a significant positive correlation between family means in the intra- and interspecific regimes. These correlations were greater in magnitude than those for the comparison between no-competition and competition (intra- or interspecific) treatments. Our results suggest that the importance of traits affecting plant performance is environment-dependent; the performance of a family grown without competition was a poor predictor of performance with competition, while the performance of families grown under intra- and interspecific competition was positively correlated.     

Aggregation and species coexistence in fleas parasitic on small mammals

The aggregation model of coexistence states that species coexistence is facilitated if interspecific aggregation is reduced relative to intraspecific aggregation. We investigated the relationship between intraspecific and interspecific aggregation in 17 component communities (the flea assemblage of a host population) of fleas parasitic on small mammals and hypothesized that interspecific interactions should be reduced relative to intraspecific interactions, facilitating species coexistence. We predicted that the reduction of the level of interspecific aggregation in relation to the level of intraspecific aggregation would be positively correlated with total flea abundance and species richness of flea assemblages. We also expected that the higher degree of facilitation of flea coexistence would be affected by host parameters such as body mass, basal metabolic rate (BMR) and depth and complexity of burrows. Results of this study supported the aggregation model of coexistence and demonstrated that, in general, a) conspecific fleas were aggregated across their hosts; b) flea assemblages were not dominated by negative interspecific interactions; and c) the level of interspecific aggregation in flea assemblages was reduced in relation to the level of intraspecific aggregation. Intraspecific aggregation tended to be correlated positively to body mass, burrow complexity and mass-independent BMR of a host. Positive interspecific associations of fleas tended to occur more frequently in species-rich flea assemblages and/or in larger hosts possessing deep complex burrows. Intraspecific aggregation increased relative to interspecific aggregation when species richness of flea infracommunities (the flea assemblage of a host individual) and component communities increased. We conclude that the pattern of flea coexistence is related both to the structure of flea communities and affinities of host species.     

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.     

Competition and facilitation within and between a snail and a mayfly larva and the effect on the grazing process

We studied the competitive effects within and between two taxonomically distant freshwater herbivores, a snail and a mayfly, common in Swedish lakes, Lymnaea peregra and Cloeon dipterum, respectively, and their effect on grazing in a laboratory experiment. The experimental set-up consisted of 2-l aquaria, each containing a periphyton covered tile. Intra- and interspecific effects were tested by increasing the density of one species at a time in four different treatments, (1) snails (intraspecific treatment), (2) mayflies (intraspecific treatment), (3) mixed-snails (interspecific treatments, snails kept constant) and (4) mixed-mayflies (interspecific treatments, mayflies kept constant). Intraspecific competition affected both snails and mayflies negatively, i.e. increasing mortality with increasing con-specific density. Furthermore, there was a decrease in snail growth with increasing snail density. In the mixed-species treatments both species changed their microhabitat use indicating interspecific competition. Despite this, we also found a positive effect of mayfly density on snail growth, most likely due to indirect commensalism. No density-dependent effect of grazing on periphyton was found, probably due to interference competition between grazers. However, there was a significant difference in periphyton biomass, due to species composition of grazers. Irrespective of their densities, if they co-existed, the two grazer species decreased the periphyton biomass significantly compared with both single-species treatments. We considered this as a joint action of facilitation and interaction. Our results suggest that competition can be an important structuring factor in macroinvertebrate communities and that species composition can be significant for ecosystem processes within lentic environments.     

Do Cultivated Varieties of Native Plants Have the Ability to Outperform Their Wild Relatives?

Vast amounts of cultivars of native plants are annually introduced into the semi-natural range of their wild relatives for re-vegetation and restoration. As cultivars are often selected towards enhanced biomass production and might transfer these traits into wild relatives by hybridization, it is suggested that cultivars and the wild × cultivar hybrids are competitively superior to their wild relatives. The release of such varieties may therefore result in unintended changes in native vegetation. In this study we examined for two species frequently used in re-vegetation (Plantago lanceolata and Lotus corniculatus) whether cultivars and artificially generated intra-specific wild × cultivar hybrids may produce a higher vegetative and generative biomass than their wilds. For that purpose a competition experiment was conducted for two growing seasons in a common garden. Every plant type was growing (a.) alone, (b.) in pairwise combination with a similar plant type and (c.) in pairwise interaction with a different plant type. When competing with wilds cultivars of both species showed larger biomass production than their wilds in the first year only and hybrids showed larger biomass production than their wild relatives in both study years. As biomass production is an important factor determining fitness and competitive ability, we conclude that cultivars and hybrids are competitively superior their wild relatives. However, cultivars of both species experienced large fitness reductions (nearly complete mortality in L. corniculatus) due to local climatic conditions. We conclude that cultivars are good competitors only as long as they are not subjected to stressful environmental factors. As hybrids seemed to inherit both the ability to cope with the local climatic conditions from their wild parents as well as the enhanced competitive strength from their cultivars, we regard them as strong competitors and assume that they are able to outperform their wilds at least over the short-term.     

Aggregative Root Placement: A Feature During Interspecific Competition in Inland Sand-Dune Habitats

Segregation of roots is frequently observed in competing root systems. However, recently, intensified root growth in response to a neighbouring plant has been described in pot experiments [Gersani M, Brown J S, O'Brien E E, Maina G M and Abramsky Z 2001. J. Ecol. 89, 660–669]. This paper examines whether intense root growth towards a neighbour (aggregation) plays a role in competitive interactions between plant species from open nutrient-poor mid-European sand ecosystems. In a controlled field-competition experiment, root distribution patterns of intra- and interspecific pairs as well as single control plants of Corynephorus canescens, Festuca psammophila, Hieracium pilosella, Hypochoeris radicata and Conyza canadensis were investigated after one growing season. Under intraspecific competition plants tended to segregate their root systems, while under interspecific competition most species tended to aggregate roots towards their neighbours even at the expense of root development at the opposite competition-free side of the target. Preference of a root aggregation strategy over the occupation of competition-free soil in interspecific competition emphasizes the importance of contesting between individuals in relation to mere resource acquisition. It is suggested that in the presence of a competitor the plants might use root aggregation as a defensive reaction to maintain a strong competitive response and exclusive access to the resources of already occupied soil volumes.     

Individual flowering phenology shapes plant–pollinator interactions across ecological scales affecting plant reproduction

The balance of pollination competition and facilitation among co‐flowering plants and abiotic resource availability can modify plant species and individual reproduction. Floral resource succession and spatial heterogeneity modulate plant–pollinator interactions across ecological scales (individual plant, local assemblage, and interaction network of agroecological infrastructure across the farm). Intraspecific variation in flowering phenology can modulate the precise level of spatio‐temporal heterogeneity in floral resources, pollen donor density, and pollinator interactions that a plant individual is exposed to, thereby affecting reproduction. We tested how abiotic resources and multi‐scale plant–pollinator interactions affected individual plant seed set modulated by intraspecific variation in flowering phenology and spatio‐temporal floral heterogeneity arising from agroecological infrastructure. We transplanted two focal insect‐pollinated plant species (Cyanus segetum and Centaurea jacea, n = 288) into agroecological infrastructure (10 sown wildflower and six legume–grass strips) across a farm‐scale experiment (125 ha). We applied an individual‐based phenologically explicit approach to match precisely the flowering period of plant individuals to the concomitant level of spatio‐temporal heterogeneity in plant–pollinator interactions, potential pollen donors, floral resources, and abiotic conditions (temperature, water, and nitrogen). Individual plant attractiveness, assemblage floral density, and conspecific pollen donor density (C. jacea) improved seed set. Network linkage density increased focal species seed set and modified the effect of local assemblage richness and abundance on C. segetum. Mutual dependence on pollinators in networks increased C. segetum seed set, while C. jacea seed set was greatest where both specialization on pollinators and mutual dependence was high. Abiotic conditions were of little or no importance to seed set. Intra‐ and interspecific plant–pollinator interactions respond to spatio‐temporal heterogeneity arising from agroecological management affecting wild plant species reproduction. The interplay of pollinator interactions within and between ecological scales affecting seed set implies a co‐occurrence of pollinator‐mediated facilitative and competitive interactions among plant species and individuals.     

Plant traits and landscape simplification drive intraspecific trait diversity of Bombus terrestris in wildflower plantings

Wildflower plantings are an important mitigation tool within agri-environmental schemes to counter insect decline in resource-scarce agricultural landscapes. Effectiveness of wildflower plantings for insect conservation is typically studied at the community or species level. It is the individual, however, that is subject to changing abiotic and biotic conditions, not the species per se. Accordingly, functional traits of individuals, i.e., the intraspecific functional diversity within species, likely mediate responses to wildflower resources and landscape context. Here we focused on the ecologically and economically important wild insect pollinator Bombus terrestris to study its intraspecific functional diversity and plant-pollinator individual interactions in wildflower plantings. We found considerable trait variation among flower-visiting B. terrestris workers. Locally, this variation could be attributed to flowering plant traits, with larger workers visiting larger inflorescences and individuals with longer tongues preferentially feeding on zygomorphic but not radially symmetrical flowers. In addition, wildflower plantings with high floral abundance attracted individuals with larger pollen baskets. At the landscape scale, increasing proportion of arable land resulted in smaller B. terrestris individuals in wildflower plantings, and a decrease in the overall size diversity of workers. These findings highlight the so far little considered role of intraspecific variation in functional traits of wild pollinators, which can mediate the trait-matching between plants and pollinator individuals. Landscape simplification from agriculture threatens intraspecific pollinator diversity, with potential harmful effects for pollinator fitness and plant reproduction. Tailored wildflower plantings can thus serve as an important tool to increase intraspecific variation in simplified landscapes. When designing seed mixtures for these plantings, high complementarity in plant traits is key for promoting high intraspecific trait diversity of bumblebees and potentially of other associated insect species.     

Increased Plant Carbon Translocation Linked to Overyielding in Grassland Species Mixtures

Plant species richness and productivity often show a positive relationship, but the underlying mechanisms are not fully understood, especially at the plant species level. We examined how growing plants in species mixture influences intraspecific rates of short-term carbon (C-) translocation, and determined whether such short-term responses are reflected in biomass yields. We grew monocultures and mixtures of six common C3 grassland plant species in outdoor mesocosms, applied a ¹³C-CO₂ pulse in situ to trace assimilated C through plants, into the soil, and back to the atmosphere, and quantified species-specific biomass. Pulse derived ¹³C enrichment was highest in the legumes Lotus corniculatus and Trifolium repens, and relocation (i.e. transport from the leaves to other plant parts) of the recently assimilated ¹³C was most rapid in T. repens grown in 6-species mixtures. The grass Anthoxanthum odoratum also showed high levels of ¹³C enrichment in 6-species mixtures, while ¹³C enrichment was low in Lolium perenne, Plantago lanceolata and Achillea millefolium. Rates of C loss through respiration were highest in monocultures of T. repens and relatively low in species mixtures, while the proportion of ¹³C in the respired CO₂ was similar in monocultures and mixtures. The grass A. odoratum and legume T. repens were most promoted in 6-species mixtures, and together with L. corniculatus, caused the net biomass increase in 6-species mixtures. These plant species also had highest rates of ¹³C-label translocation, and for A. odoratum and T. repens this effect was greatest in plant individuals grown in species mixtures. Our study reveals that short-term plant C translocation can be accelerated in plant individuals of legume and C3 grass species when grown in mixtures, and that this is strongly positively related to overyielding. These results demonstrate a mechanistic coupling between changes in intraspecific plant carbon physiology and increased community level productivity in grassland systems.     

Variation in the relative magnitude of intraspecific and interspecific competitive effects in novel versus familiar environments in two<Emphasis Type="Italic">Drosophila</Emphasis> species

Models of competitor coevolution, especially the genetic feedback hypothesis, suggest that a negative correlation between intraspecific and interspecific competitive effects may be important in sustaining competitor coexistence, and can give rise to oscillatory dynamics with repeated reversals of competitive superiority. I reanalyzed previously published census data from an experiment in which populationsof Drosophila melanogaster andD. simulans underwent competitive coevolution in one familiar and two novel environments, to specifically look for any evidence of a negative relationship between intraspecific and interspecific competitive effects on population growth rates, and for any indication of short period cycling in the relative magnitude of intraspecific and interspecific competitive effects. While there was considerable variation in the relative magnitude of intraspecific and interspecific competitive effects over generations, among both populations and environments, there was no clear evidence supporting the genetic feedback hypothesis. Intraspecific and interspecific competitive effects on population growth rates were strongly positively correlated in novel environments, and uncorrelated in the familiar environment. Data from the familiar environment indicated that indices of competition of populations of the initially superior competitor,D. melanogaster, might be showing some cyclic behaviour, but I argue that this is likely to be transient, and not suggestive of sustained oscillatory dynamics predicted by the genetic feedback model. I discuss the results in the context of the importance of the genetic architecture of intraspecific and interspecific competitive abilities in determining the coevolutionary trajectory of competitive interactions.     

Using coloured roots to study root interaction and competition in intercropped legumes and non-legumes

Aims Root interactions between neighbour plants represent a fundamental aspect of the competitive dynamics in pure stand and mixed cropping systems. The comprehension of such phenomena places big methodological challenges, and still needs clarification. The objectives of this work were (i) to test if a species with coloured roots can be used to examine the interaction in a legume-non-legume intercropping system; (ii) to verify the importance of initial root growth on the successive root development of mixture component plants; (iii) to test if the root interaction in the shallow layers has consequences for deep root growth and (iv) to compare the effect of intraspecific and interspecific competition on root development and biomass growth.Methods A detailed study on root growth and interaction was carried out using rhizotron tubes where two legume species were grown in pure stands or were intercropped with red beet, a variety of Beta vulgaris L. with clear red roots. Within the rhizotrons, the three species were grown either without competitors, with two plants of the same species to measure intraspecific competition or with one legume and one red beet plant to study interspecific competition. The use of mixtures where one component has clearly coloured roots, together with several scalar measurements of root depth and proliferation, allowed the measurement of the root system of each species when grown in the mixtures.Important findings The use of rhizotron tubes coupled with species with coloured roots represented a valuable method to study the belowground interaction in mixed cropping systems. The initial root growth was a very important feature for the subsequent dominance of a species and it was not related to seed dimension. Initial root growth was also important because the root interactions in the shallower soil layers were found to influence the root growth in deeper soil. The root system of the red beet showed much faster and deeper growth than that of the legumes, and made red beet the dominant component in the mixtures while the legume root system was confined to the shallower soil layer. Intraspecific competition was well tolerated by the legumes, but it was limiting for the highly competitive red beet. The outcome of root interaction between neighbour plants was confirmed to be species-specific as it changed according to the intensity of the competitive effect/response of each species of the mixture: both legumes were slightly affected by the intraspecific and highly affected by interspecific competition while red beet was more affected by intraspecific competition but strongly dominant when intercropped with legumes.     

Fast seedling root growth leads to competitive superiority of invasive plants

The competitive superiority of invasive plants plays a key role in the process of plant invasions, enabling invasive plants to overcome the resistance of local plant communities. Fast aboveground growth and high densities lead to the competitive superiority of invasive species in the competition for light. However, little is understood of the role belowground root competition may play in invasion. We conducted an experiment to test the effect of root growth on the performance of an invasive shrub Cassia alata, a naturalized, non-invasive shrub Corchorus capsularis, and a native shrub Desmodium reticulatum. We compared seedling growth of the three species and their competitive ability in situ. The roots of the C. alata seedlings grew much faster than those of C. capsularis and D. reticulatum during the entire growth period although C. alata had shorter shoots than D. reticulatum. Furthermore, C. alata showed an apparent competition advantage compared to the other two species as evidenced by less biomass reduction in intraspecific competition and higher competitive effects in interspecific competition. Our study reveals that fast seedling root growth may be important in explaining the competitive advantages of invasive plants. Future studies should pay more attention to the belowground traits of invasive plants, the trade-off between shoot and root growth, and the role of root competition in affecting the population dynamics of invasive plants and the structures of invaded communities.     

Effects of hydrological regimes on competitive interactions of Schoenoplectus fluviatilis and two co-occurring wetland plants

We investigated the effects of different hydrological regimes (wet [flooded at a constant water depth for duration of the study], cycle [reflooded at eight weeks following natural drying], and wet–dry [initially flooded and allowed to naturally dry for duration of the study]) on the competitive ability of Schoenoplectus fluviatilis (Torr.) M. T. Strong with an annual, native wetland plant (Polygonum pensylvanicum L.) and a perennial, wetland plant (Schoenoplectus tabernaemontani [C. C. Gmel.] Palla). To assess competitive response of the plants, we used a greenhouse target-neighbor study with neighbor plants planted at varying densities (0 [control], 1, 10, and 15 plants pot⁻¹). Our results suggest that S. fluviatilis is competitively superior to S. tabernaemontani and P. pensylvanicum. S. tabernaemontani and P. pensylvanicum biomass declined by 90% and 75% in presence of S. fluviatilis, respectively. However, the competitive ability of S. fluviatilis was generally not enhanced by flooding regime. The competitive coefficients of S. fluviatilis were similar among the three hydrological regimes under intraspecific competition and interspecific competition with S. tabernaemontani, but for interspecific competition with P. pensylvanicum, the competitive coefficient for S. fluviatilis was higher for the cycle treatment compared to the wet–dry and dry treatments. Interestingly, S. tabernaemontani was a strong competitor against S. fluviatilis in the wet and cycle treatments, indicating that maintaining longer hydroperiods could be used as a management tool to encourage growth of S. tabernaemontani and reduce encroachment of S. fluviatilis.     

Competition among the adults of three grasshoppers (Orthop., Acrididae) on an alpine grassland

Abstract:  The intraspecific and interspecific competition among the adults of three common grasshoppers, Angaracris rhodopa (Fischer-Waldheim), Chorthippus dubius (Zubovsky) and Chorthippus fallax (Zubovsky) was investigated on Ganjia High Mountain Grassland in north-west China. The results indicated a strong intraspecific competition within species at high densities. The interspecific competition between the larger-sized species, A. rhodopa , and the smaller-sized species, C. dubius or C. fallax , was weak, while competition between C. dubius and C. fallax was strong. An asymmetric competitive interaction in mortality and fecundity associated with the diet and size was observed among the three species. Competition was strong between the two species that had a similar diet. The large-sized species had higher mortality rates and lower fecundity than the smaller-sized species. Intraspecific competition between the three species was mainly affected by food limitation, while interspecific competition was mainly associated with food limitation as well as the grasshoppers' adaptation to the environment.     

Shift in competitive ability mediated by soil biota in an invasive plant

Understanding the shifts in competitive ability and its driving forces is key to predict the future of plant invasion. Changes in the competition environment and soil biota are two selective forces that impose remarkable influences on competitive ability. By far, evidence of the interactive effects of competition environment and soil biota on competitive ability of invasive species is rare. Here, we investigated their interactive effects using an invasive perennial vine, Mikania micrantha. The competitive performance of seven M. micrantha populations varying in their conspecific and heterospecific abundance were monitored in a greenhouse experiment, by manipulating soil biota (live and sterilized) and competition conditions (competition‐free, intraspecific, and interspecific competition). Our results showed that with increasing conspecific abundance and decreasing heterospecific abundance, (1) M. micrantha increased intraspecific competition tolerance and intra‐ vs. interspecific competitive ability but decreased interspecific competition tolerance; (2) M. micrantha increased tolerance of the negative soil biota effect; and (3) interspecific competition tolerance of M. micrantha was increasingly suppressed by the presence of soil biota, but intraspecific competition tolerance was less affected. These results highlight the importance of the soil biota effect on the evolution of competitive ability during the invasion process. To better control M. micrantha invasion, our results imply that introduction of competition‐tolerant native plants that align with conservation priorities may be effective where M. micrantha populations are long‐established and inferior in inter‐ vs. intraspecific competitive ability, whereas eradication may be effective where populations are newly invaded and fast‐growing.