Exotic weevil invasion increases floral herbivore community density,function, and impact on a native plant

Consumer communities are being re‐arranged through unprecedented rates of human‐mediated invasions and extinctions. Such changes in consumer diversity potentially alter community function and impact on resource populations. Although insect herbivore invasions are increasingly common, the influence of such species additions on native resident herbivore guilds, along with their individual and combined effects on native plant resources, are rarely investigated. Here, we used site‐to‐site and plant‐to‐plant variation in herbivore composition to examine how the addition of an invasive exotic weevil, Rhinocyllus conicus, combines with a guild of native floral herbivores (tephritid flies, pyralid moths) to influence two key components of herbivore community function – aggregate herbivore densities and cumulative levels of seed destruction – on a native thistle, Cirsium canescens. Invasion of a site by R. conicus more than doubled aggregate herbivore density, resulting in increased levels of seed destruction and a halving of seed production by the native thistle. Further, herbivore function was significantly higher on individual plants attacked by R. conicus, compared to plants attacked only by native herbivores. Insect densities and levels of seed destruction on plants attacked by multiple herbivore taxa never exceeded those observed for plants attacked by R. conicus alone, suggesting that increases in herbivore community function with invasion resulted from the inclusion of a functionally dominant insect rather than any complementarity effects. Some evidence for interference between insects emerged, with a trend towards reduced moth and weevil densities in two and three taxon mixtures compared with plants attacked by each taxon alone. However, density compensation was limited so that, overall, the addition of a novel herbivore to the floral guild was associated with a significant increase in herbivore community function and impact on seed production. The results suggest that invasion of a functionally dominant herbivore into an unsaturated recipient community can augment function within a resource guild.     

Local population density and group composition influence the signal‐preference relationship in Enchenopa treehoppers (Hemiptera: Membracidae)

Many animals exhibit social plasticity – changes in phenotype or behaviour in response to experience with conspecifics that change how evolutionary processes like sexual selection play out. Here, we asked whether social plasticity arising from variation in local population density in male advertisement signals and female mate preferences influences the form of sexual selection. We manipulated local density and determined whether this changed how the distribution of male signals overlapped with female preferences – the signal preference relationship. We specifically look at the shape of female mate preference functions, which, when compared to signal distributions, provide hypotheses about the form of sexual selection. We used Enchenopa binotata treehoppers, a group of plant‐feeding insects that exhibit natural variation in local densities across individual host plants, populations, species and years. We measured male signal frequency and female preference functions across the density treatments. We found that male signals varied across local social groups, but not according to local density. By contrast, female preferences varied with local density – favouring higher signal frequencies in denser environments. Thus, local density changes the signal–preference relationship and, consequently, the expected form of sexual selection. We found no influence of sex ratio on the signal–preference relationship. Our findings suggest that plasticity arising from variation in local group density and composition can alter the form of sexual selection with potentially important consequences both for the maintenance of variation and for speciation.     

Neotyphodium endophyte transmission to Lolium multiflorum seeds depends on the host plant fitness

Frequency and distribution of symbiosis in nature depend both on the direct symbiont effect on the host fitness and on its efficiency to spread within host populations (transmission). For vertically transmitted Neotyphodium fungi, the attention has been centered on the endophyte effect on host grass plants but little is known about the controls of transmission. Environmental and genetic factors have been suggested as important controls of transmission efficiency. We studied the effect of these two factors on the transmission efficiency of the Neotyphodium endophyte in Lolium multiflorum plants. Plant genotype of a host population naturally endophyte-infected (95%) was manipulated by conducting controlled crosses with genetically distant plant populations. The resulting progeny was subjected to two types of factors, resource shortage and oxidative stress induced by an herbicide. Irrespective of plant genotype, high resource level increased seed yield per plant by 26-fold, spike-to-seed transmission by 12%, and plant-to-seed transmission by 10% (not significant). Although herbicide effects could be mediated indirectly by changes in plant density or directly by oxidative stress, neither plant fitness nor transmission efficiency was affected. An interesting pattern between transmission efficiency and seed yield per plant was revealed when plants (from both experiments) were plotted together. Low yielding plants, that is plants that grew under low resource level at high plant density, showed high transmission failures whereas high yielding plants, that is plants growing at low density with and without herbicide treatment, showed high transmission rates. Transmission failures may be a consequence of the endophyte cost for host plants growing under restrictive conditions, suggesting that lower transmission efficiency may partially explain previous evidence showing lower endophyte infection frequency for grasses under stressful conditions. Host plants could be penalizing the endophyte through a competition-like mechanism, instead of depressing their own fitness.     

EFFECTS OF POSTDISPERSAL SEED PREDATION ON SPATIAL INEQUALITY AND SIZE VARIABILITY IN AN ANNUAL PLANT,ERODIUM CICUTARIUM (GERANIACEAE)

By decreasing seed density, ants introduced into flats of uniformly sown seeds of Erodium cicutarium (Geraniaceae) created differences in the neighbor-free area available to individual plants. The changes in spatial patterns brought about by the ants were greater when a higher proportion of seeds was removed but were independent of initial seed density. These spatial changes and differences in seed density were examined for their effects on plant size and reproduction. Gini values were calculated to determine inequalities. As the inequality in space among individual plants increased, the variation in final biomass increased. The number of individuals reproducing was constant among treatments, and yet seed production per plant was significantly greater for populations in which the spatial pattern was influenced by seed predation. The decrease in density and changed spatial pattern, due to previous seed predation, resulted in a few individuals having much more space than others and consequently producing many more seeds. The increase in reproductive effort per flat was much greater than could be explained by the changing density alone. Our experiment demonstrates that spatial inequality, such as that generated by seed predators, can be more important than density in generating size inequalities in plant populations. This result can profoundly alter the competitive interactions between plants and determine which plants produce seed for the next generation.     

EFFECT OF SEED DIMORPHISM ON THE DENSITY-DEPENDENT DYNAMICS OF EXPERIMENTAL POPULATIONS OF ATRIPLEX TRIANGULARIS (CHENOPODIACEAE)

The importance of seed size and density in determining individual plant performance and plant population dynamics in experimental populations of the halophyte Atriplex triangularis was studied. Two distinct seed morphs—large, light seeds and small, dark seeds—are produced by individual A. triangularis plants. Experimental populations consisting of seed size monocultures (large or small seeds) and seed size mixtures were established at three different densities, and the time of germination, plant size, plant survivorship, and plant fecundity were monitored. Marked variation in time of germination was observed among treatments and between seed sizes, but germination within any given treatment occurred over a five- to ten-day period. Large seeds produced larger plants than small seeds did, and this dichotomy was maintained over the course of the entire experiment. Germination date and seed size interacted such that larger plants grew from seeds which germinated earlier than those which germinated later, regardless of seed size. Germination date had a more pronounced effect than seed size did on plant mortality in high density populations. At high density, large seed monocultures experienced greater mortality than small seed monocultures did, but in seed size mixtures, the mortality was evenly distributed between plants from the two seed sizes. Regardless of density conditions and parentage, large and small seeds were produced in equal proportion by the plants. Total seed production, however, was dramatically affected by plant density, and to a lesser degree by germination date. Although seed size effects alone did not appear to affect directly final plant biomass and fecundity, effects of seed size early in ontogeny may have contributed to differences in fecundity.     

Population ecology of wild sunflowers: effects of seed density and post-dispersal vertebrate seed predators

Assessing the effects of seed density on the population dynamics of wild plant species with crop relatives will be vital in determining the potential effects of introducing traits into wild populations as a result of crop-to-wild gene flow. We examined experimental sunflower (Helianthus annuus) patches in eastern Kansas to determine the effects of seed density and predation on seedling recruitment and seed production in the next generation. High seed density treatment plots had significantly more seedlings and adult plants than did low seed density treatment plots. Overwinter vertebrate seed predator exclusion treatments resulted in increases in plant density compared to plots in which vertebrates were not excluded. Control patches (no seeds added) contained virtually no plants. Head production and estimated total seed production for a patch were not statistically different among treatments (excluding control plots). Although initial seed density and vertebrate post-dispersal seed predation do appear to have effects on seedling recruitment, neither appear to be limiting seed production of competing adult plants. Therefore, variation in seed densities (over the range examined) may have limited effects on local population dynamics. It is important to note that the choice of seed densities may affect the results obtained: the seed densities used in this study may, in retrospect, be higher than in the small roadside populations typical in eastern Kansas, yet other natural sites have much larger densities. Further, the effects of increased seed density at a local site may have other important effects such as altering metapopulation dynamics through increased long-distance dispersal or increased local seed bank size.     

Indirect interactions between browsers and seed predators affect the seed bank dynamics of a chaparral shrub

Interactions between herbivores and seed predators may have long-term consequences for plant populations that rely on persistent seed banks for recovery after unpredictable fires. We assessed the effects of browsing by deer and seed predation by rodents, ants and birds on the densities of seeds entering the seed bank of Ceanothus cuneatus var. rigidus, a maritime chaparral shrub in coastal California. Ceanothus produced many more seeds when protected from browsers in long-term experimental exclosures than did browsed plants, but the seed densities in the soil beneath browsed and unbrowsed Ceanothus were the same at the start of an intensive one-year study. The density of seeds in the soil initially increased in both treatments following summer seed drop: while densities returned to pre-drop levels within a few weeks under browsed plants, soil seed densities remained high for 5–8 months beneath unbrowsed plants. Rodent abundance (especially deer mice) was higher near unbrowsed plants than >30 m away, and rodents removed Ceanothus seeds from dishes in the experimental plots. At least in the short term, rodent density and rates of seed removal were inversely related to the intensity of browsing. Our data have management implications for maintaining viable Ceanothus populations by regulating the intensity of browsing and the timing, intensity and frequency of fires.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

Weeds in fields with contrasting conventional and genetically modified herbicide-tolerant crops. I. Effects on abundance and diversity

We compared the seedbanks, seed rains, plant densities and biomasses of weeds under two contrasting systems of management in beet, maize and spring oilseed rape. Weed seedbank and plant density were measured at the same locations in two subsequent seasons. About 60 fields were sown with each crop. Each field was split, one half being sown with a conventional variety managed according to the farmer's normal practice, the other half being sown with a genetically modified herbicide-tolerant (GMHT) variety, with weeds controlled by a broad-spectrum herbicide. In beet and rape, plant densities shortly after sowing were higher in the GMHT treatment. Following weed control in conventional beet, plant densities were approximately one-fifth of those in GMHT beet. In both beet and rape, this effect was reversed after the first application of broad-spectrum herbicide, so that late-season plant densities were lower in the GMHT treatments. Biomass and seed rain in GMHT crops were between one-third and one-sixth of those in conventional treatments. The effects of differing weed-seed returns in these two crops persisted in the seedbank: densities following the GMHT treatment were about 20% lower than those following the conventional treatment. The effect of growing maize was quite different. Weed density was higher throughout the season in the GMHT treatment. Late-season biomass was 82% higher and seed rain was 87% higher than in the conventional treatment. The difference was not subsequently detectable in the seedbank because the total seed return was low after both treatments. In all three crops, weed diversity was little affected by the treatment, except for transient effects immediately following herbicide application.     

The influence of plant density on the responses of Sinapis alba to CO2 and windspeed

Plants in nature live in populations of variable density, a characteristic which may influence individual plant responses to the environment. We investigated how the responses of Sinapis alba plants to different wind speeds and CO₂ concentrations could be modified by plant density. In our wind-density experiment the expectation that mechanical and physiological effects of wind will be ameliorated by growing in high density, as a result of positive plant interactions, was realised. Although individual plants were smaller at higher densities, the effect of increasing windspeed was much less than at lower plant densities. A similar reduced sensitivity of individual plant growth under high densities was also observed under CO₂ enrichment. When measured as a population or stand response, there was no effect of density on the CO₂ responses, with all stands showing very similar increases in total biomass with CO₂ enrichment. In the wind speed experiment, total biomass per stand increased significantly with density, although there was no effect of density on the wind speed response. Specific leaf area decreased with increasing wind speed and this response was significantly affected by the density at which the plants grew.     

Effects of simulated herbivory on tillering and reproduction in an annual ryegrass,Lolium remotum

This study examined the overall impact of simulated herbivory on tillering and reproductive performance of an annual ryegrass,Lolium remotum. The interaction between herbivore damage and intraspecific competition and the effect of the timing of damage were also studied. The experimental plants were sown at two densities and were randomly assigned to eight different damage treatments consisting of artificial leaf area removal by clipping with scissors or removal of one-third of the ripening seeds. The treatments were executed at two flowering stages. The pattern of tiller development differed significantly among treatments and between densities. At the lower density, earlier treatments delayed tiller development more than the same treatments executed later. At the higher density, all treatments delayed tiller development. The density effect was significant for all reproductive traits measured. The reproductive output of plants grown at the higher density was lower and the negative treatment effects were stronger than at the lower density. The treatment effect was significant for seed dry weight per plant and individual seed weight but not for number of seeds per plant. There were no statistically significant interaction effects between the damage treatments and density, suggesting that the plants responded to the damage similarly, irrespective of the density. The plants did not totally compensate for losses due to damage at either density, even though they slightly increased their resource allocation to sexual reproduction at the higher density.     

Genetic differences between populations and habitats in Lamium purpureum plastic response to conspecific density

Plants from Open and Shade habitats in two natural populations (Vršac and Avala) were grown in two densities (High and Low). As expected, density had significant effect on most of measured traits and that effect was concordant with Shade avoidance syndrome predictions. Genetic differences between populations both in mean trait values and in plastic responses to density were also detected. Number of leaves and flowers showed plasticity in Avala population only, while shoot weight was plastic in both populations but with greater plasticity in Avala population. Differences between habitats for plant height and number of internodes were present in Vršac population only. Habitat difference in response to density was revealed for seed weight and it was due to lack of response in plants originated from Shade habitat in Vršac population. This study showed that not only populations, but also subpopulations occupying different habitats can differ genetically in their plastic response to density, and that between-habitat differences can be population-specific. The text was submitted by the author in English.     

Plant proteome changes under abiotic stress--contribution of proteomics studies to understanding plant stress response

Plant acclimation to stress is associated with profound changes in proteome composition. Since proteins are directly involved in plant stress response, proteomics studies can significantly contribute to unravel the possible relationships between protein abundance and plant stress acclimation. In this review, proteomics studies dealing with plant response to a broad range of abiotic stress factors--cold, heat, drought, waterlogging, salinity, ozone treatment, hypoxia and anoxia, herbicide treatments, inadequate or excessive light conditions, disbalances in mineral nutrition, enhanced concentrations of heavy metals, radioactivity and mechanical wounding are discussed. Most studies have been carried out on model plants Arabidopsis thaliana and rice due to large protein sequence databases available; however, the variety of plant species used for proteomics analyses is rapidly increasing. Protein response pathways shared by different plant species under various stress conditions (glycolytic pathway, enzymes of ascorbate-glutathione cycle, accumulation of LEA proteins) as well as pathways unique to a given stress are discussed. Results from proteomics studies are interpreted with respect to physiological factors determining plant stress response. In conclusion, examples of application of proteomics studies in search for protein markers underlying phenotypic variation in physiological parameters associated with plant stress tolerance are given.     

Two heteropteran predators in relation to weed management in herbicide-tolerant corn

Deployment of genetically modified (GM), herbicide-tolerant corn may alter weed flora abundance and composition and may affect pests and their natural enemies. Among on-plant predators, Orius spp. are the prevalent group in Spain and were selected to study the impact of glyphosate use on predators. We also studied Nabis sp. which is commonly recorded on corn in the study area. For this, a 4-year study was conducted in NE Spain. Three different herbicide regimes were compared: two glyphosate (a broad-spectrum herbicide) treatments per season, no herbicide treatment, and one pre-emergence conventional treatment with selective herbicides against broadleaf and grassy weeds. Density of main arthropod herbivores and the above two predatory groups was recorded on plants. Differences between herbicide regimes were observed in the two functional groups studied, herbivores and heteropteran predators. The comparison of glyphosate-treated and untreated plots showed significant differences for both functional groups, but the differences between glyphosate-treated and conventionally treated plots for the two functional groups were lower. For Orius spp., annual density per plot was significantly correlated with annual density of leafhoppers and to a lesser extent, with aphids. Nabis sp. densities were never different between glyphosate-treated and conventionally treated plots, and Nabis sp. density showed no relation to any of the herbivores tested. We concluded that no significant changes in heteropteran predator densities may be expected from moderate alterations in weeds arising from the deployment of herbicide-tolerant corn varieties and that leafhoppers are probably the herbivore prey that most influences Orius spp. densities in corn in our study area.     

Genetic differences between populations and habitats in Lamium purpureum plastic response to conspecific density

Plants from Open and Shade habitats in two natural populations (Vrsac and Avala) were grown in two densities (High and Low). As expected, density had significant effect on most of measured traits and that effect was concordant with Shade avoidance syndrome predictions. Genetic differences between populations both in mean trait values and in plastic responses to density were also detected. Number of leaves and flowers showed plasticity in Avala population only, while shoot weight was plastic in both populations but with greater plasticity in Avala population. Differences between habitats for plant height and number of internodes were present in Vrsac population only. Habitat difference in response to density was revealed for seed weight and it was due to lack of response in plants originated from Shade habitat in Vrsac population. This study showed that not only populations, but also subpopulations occupying different habitats can differ genetically in their plastic response to density, and that between habitat differences can be population-specific.     

Seed vigor and plant competitiveness resulting from seeds of Eupatorium adenophorum in a persistent soil seed bank

Seeds in a persistent soil seed bank (PSSB) provide an effective way to maintain plant population and community stability. Seeds that persist in soil incur physiological costs of maintaining viability and vigor, thus, the growth capability of resulting plants may be reduced. However, a lot of functional roles of the PSSB have been deduced from seed germination capability, and little consideration has been given to interspecific and intraspecific competitive ability of the resulting plants. Eupatorium adenophorum was used as the study species to compare germination of different artificially aged PSSB seeds and competition at different densities between resulting plants of aged and freshly produced seeds. Seed burial caused decreases in survival rates but not germination speed. During the 175-day growth period, the individual biomass, average height, basal stem diameter and leaf number of plants from aged PSSB seeds were little lower than that of plants germinated from freshly produced seeds. However, the differences were not significant at any densities. Thus, (1) although seeds stored in soil exhibited a very high death rate, they maintained a high vigor for germination, and (2) resulting plants from PSSB seeds exhibited good competiveness to plants from new seeds of the same population. The results further confirm the significance of PSSB in maintaining stability of plant populations and communities.     

A Population Study to Aid the Selection of Improved Dried Pea (Pisum sativum) Crop Plants

The effect of genotype and plant density, over the range from100 to 277 plants m^–2, on plant to plant variation inprecision sown microplots has been assessed for three ‘leafless’(afafstst) pea (Pisum sativum) lines. This range of plantingdensities did not significantly affect the total above groundbiological yield per unit area of two of the genotypes (BS5and BS4) whereas the biological yield of the third (BS151) declinedat densities above 156 plants m^–2. The differences weredue to changes in seed yield. The effect of planting densityon the variation between plants for biological yield withinthe microplots differed between the genotypes. The distributionpattern of BS4 and BS5 changed from normal to skewed with increaseddensity, while the distribution for BS151 remained skewed atall planting densities. The differences between the three genotypes in the proportionof biological yield partitioned into seed yield (harvest index)on a unit area basis was due almost entirely to the differencesin structure of the plant populations. The maximum level ofpartitioning by individual plants was similar for all threelines. The difference between this maximum for an individualand the crop harvest index therefore represents the area forimprovement of crop harvest index through breeding. It is suggestedthat improvements in dried pea yields will come, therefore,by selecting plants which form more uniform populations withregard to plant size and to the proportion of plant biomasspartitioned into seed (plant harvest index). Pisum sativum, leafless pea, population, genetic variation, distribution patterns, harvest index     

Impact of plant and aphid stress history on infestation in arugula plants

Plants can activate inducible defence mechanisms against pests, pathogens, or chemical elicitors, such as ozone, mediated by reactive oxygen species (ROS), particularly hydrogen peroxide (H₂O₂). An unfavourable balance between ROS production and the plant antioxidant capacity seems to be responsible for the resulting susceptibility of the plant to insect attack. Arugula plants [Eruca sativa Mill. (Brassicaceae)] and green peach aphids, Myzus persicae (Sulzer) (Hemiptera: Aphididae), were used in this study to test the hypothesis that the growth of an aphid population depends on both plant and insect stress history. We investigated the impact of density and duration of a previous aphid infestation, and the time lag before re‐infestation, on aphid population growth. In a second experiment, we assessed the effect on aphid population growth of previous ozone exposure of arugula plants in open top chambers receiving a continuous O₃ fumigation of 100–120 p.p.b., 90 min per day during 3 days. A third experiment was conducted to study the effect of aphid density during a previous infestation on the population growth on an uninfested host. Both previous herbivory and ozone changed the oxidative status of plant tissues and facilitated aphid population growth, which increased with the duration and density of a previous infestation by aphids. Colonization success also depended on the aphids' own history. Aphids coming from high‐density populations and/or longer infestation periods produced larger populations on an (initially) uninfested plant. Pest outbreaks in a polluted environment might be expected to be modulated by the hosts' spatial‐temporal heterogeneity related to the ozone exposure and previous herbivory.     

Density-dependence in single-species populations of plants

The general form of yield-density relationships in plant populations is discussed with reference to reciprocal equations and the $\text{3}\text{2}$ power law, which describes the concomitant changes in plant weight and density during self-thinning. A model to describe the pattern of mortality in high density populations is also discussed with particular reference to the nature of intraspecific competition within plant populations.A reparameterized version of a reciprocal equation proposed by Bleasdale & Nelder is used to describe the relationship between individual plant weight and surviving plant density. The biological interpretation of the parameters is discussed in relation to the dry matter production of isolated plants, the density at which mutual interference between neighbours becomes appreciable, and the efficiency of resource utilization at high densities.The reparameterized equation is then used together with an equation which describes mortality during self-thinning as the basis for a new model to describe the relation between total plant yield and sowing density. The law of allometry is used in conjunction with the model to describe the relationship between the weight of a plant part and density, and this then forms the basis for a model of the population dynamics of annual plants with effectively discrete generations. Finally the dynamical behaviour of plant populations is discussed. It is concluded that most plant populations will show neighbourhood stability with exponential or perhaps oscillatory damping towards an equilibrium.     

Seedling disease in an annual legume: consequences for seedling mortality, plant size, and population seed production

The effect of seed and seedling mortality on plant population dynamics depends on the degree to which the growth and reproduction of surviving individuals can compensate for the deaths that occur. To explore this issue, we sowed seeds of the annual Kummerowia stipulacea at three densities in sunken pots in the field, which contained either field soil, microwaved field soil, or microwaved field soil augmented with oospores of three Pythium species. High sowing density reduced seedling establishment and seedling size, but these effects were independent of the soil treatment. In the oospore-augmented soil, seed and seedling survival was low. The surviving plants were initially smaller but, at maturity, average plant size was greatest in the oospore-augmented soil, compared to the other treatments. Total population seed production was unaffected by soil treatment, suggesting that the effect of disease was limited to the seedling stage, with surviving plants released from intraspecific competition. To test the hypothesis that the surviving plants in the oospore-augmented soil were more disease-resistant, seeds from each of the sowing density-soil type treatments were sown in a growth chamber inoculation study. No evidence for selection for resistance was found. A second inoculation experiment revealed that oospore inoculum reduced plant numbers and mass regardless of whether field or microwaved soil was used, suggesting that results from the field experiment were not dependent on the use of microwaved soil. The findings of this study indicate that the ecological effects of disease on individual plants and on plant populations are not necessarily equivalent. Received: 13 January 1999 / Accepted: 21 September 1999     

DENSITY-MEDIATED SEED AND STOLON PRODUCTION IN VIOLA (VIOLACEAE)

Seed and stolon production and spatial distribution were studied in two populations of Viola blanda and two populations of Viola rostrata in West Virginia. Mean individual plant biomass and the proportion of the population producing seeds and/or stolons both decreased with density. Both species possessed a characteristic “threshold” weight required before the onset of seed production; a similar minimum threshold weight was also reached before stolons were produced. It is suggested that competition at higher densities results in fewer individuals reaching the threshold weight for seed or stolon production. Thus, the density response appears as a reduction in the proportion of plants producing seeds or stolons.