After grazing exclusion,is there any modification of strategy for two guerrilla species: <Emphasis Type="Italic">Elymus repens</Emphasis> (L.) Gould and <Emphasis Type="Italic">Agrostis stolonifera</Emphasis> (L.)?

Elymus repens (L.) Gould and Agrostis stolonifera (L.), are competitive grasses with guerrilla strategy that invade grasslands with a low stocking rate. In this work, we tested the hypotheses that grazing exclusion facilitates vegetative development of rhizomes and stolons of these clonal grasses and that such change is a key mechanism for their abundance in set-aside grasslands. The competitive capacities of these two guerrilla species were characterised by samples in plant community (species richness and biomass) and on the level of individual species (morphometric measurements on stolons and rhizomes) during a growing season. Compared to grasslands where grazing was excluded for three years, species richness was higher in grazed site and the plant community structure differed. Indeed, with grazing exclusion, a shift from annual species with a diversified growth-form to perennial species with a tall tussock and graminoid growth-form was monitored. In ungrazed situation, Elymus repens and Agrostis stolonifera were the dominant grasses, and the standing biomass for the lowland community showed a significant increase compared to the grazed site. Vegetative development increased competitive capacities of these two guerrilla species and led by phenomenon of competitive exclusion to the disappearance of annuals species. With grazing cessation, Elymus repens was found to increase the size of aerial traits (shoot length and the number of leaves per shoot) and this may both be propitious for achieving dominance within plant communities and also maintaining its competitive local advantage. By contrast, Agrostis stolonifera showed an increase in a root trait, i.e. rhizome length, in the fenced site, which provide good ability for spatial propagation and then to explore adjacent patches. We concluded that Elymus repens presented a morphological capacity to change its colonising strategy from a guerrilla strategy to a phalanx strategy, by morphological variability of aerial organs, when it was submitted to competitive stress and environment modifications. Agrostis stolonifera showed a capacity to escape aerial competition resulting from grazing cessation, than to increase underground propagation capacity. The present study highlighted the capacities of Elymus repens to respond in an adaptative way to competitive pressure.     

Enhancing the diversity of breeding invertebrates within field margins of intensively managed grassland: Effects of alternative management practices

Severe declines in biodiversity have been well documented for many taxonomic groups due to intensification of agricultural practices. Establishment and appropriate management of arable field margins can improve the diversity and abundance of invertebrate groups; however, there is much less research on field margins within grassland systems. Three grassland field margin treatments (fencing off the existing vegetation “fenced”; fencing with rotavation and natural regeneration “rotavated” and; fencing with rotavation and seeding “seeded”) were compared to a grazed control in the adjacent intensively managed pasture. Invertebrates were sampled using emergence traps to investigate species breeding and overwintering within the margins. Using a manipulation experiment, we tested whether the removal of grazing pressure and nutrient inputs would increase the abundance and richness of breeding invertebrates within grassland field margins. We also tested whether field margin establishment treatments, with their different vegetation communities, would change the abundance and richness of breeding invertebrates in the field margins. Exclusion of grazing and nutrient inputs led to increased abundance and richness in nearly all invertebrate groups that we sampled. However, there were more complex effects of field margin establishment treatment on the abundance and richness of invertebrate taxa. Each of the three establishment treatments supported a distinct invertebrate community. The removal of grazing from grassland field margins provided a greater range of overwintering/breeding habitat for invertebrates. We demonstrate the capacity of field margin establishment to increase the abundance and richness in nearly all invertebrate groups in study plots that were located on previously more depauperate areas of intensively managed grassland. These results from grassland field margins provide evidence to support practical actions that can inform Greening (Pillar 1) and agri‐environment measures (Pillar 2) of the Common Agricultural Policy (CAP). Before implementing specific management regimes, the conservation aims of agri‐environment measures should be clarified by defining the target species or taxonomic groups.     

Small-scale distribution of species richness in a grassland (Bílé Karpaty Mts., Czech Republic)

Variation in the number of species was studied in a subthermophilous grassland at a scale of 0.05 ×0.05 m during a 5-year period. The observed variance of species richness (VSR) was compared with a null model based on random distribution of species over a set of squares. It was found that distribution of species richness had more values than, expected around the mean and less values at the “shoulders”. Both tails fell within the predicted limits. Application of the procedures removing spatial dependence (random shifts, rotation/reflection method byPalmer & van der Maarel 1995) and environmental heterogeneity (patch model byWatkins & Wilson 1992) did not change the observed pattern. Using simulations in which the number of clumps and clumping intensity were manipulated it was found that the effect of the clumped spatial pattern, on VSR results in a wide range of variances. Both variance excess and variance deficit were found more frequently than expected under the null model. To test the effect of the limitation to the number of individuals per square, a null model was developed which included that observed number of plant shoots per square, the observed distribution of the number of shoots belonging to individual species per square and the observed spatial distribution of the shoots. The observed VSR was still lower than that produced by the null model. Therefore, it is concluded that at a scale of 0.05×0.05 m plant species combine in a non-random way in the studied grassland. It is suggested that the shape of left and right “shoulders” of the species richness distribution may be caused by different factors, such as positive and negative covariance between species, respectively. Their simultaneous impact can generate the observed pattern in species richness.     

Differences in species richness and life-history traits between grazed and abandoned grasslands in southern Sweden

Disturbance has profound effects on plant community composition. This paper deals with the influence of grazing on species richness and proportions of life‐history attributes of grassland vegetation at six spatial scales (0.001–1000 m²) in two provinces of southern Sweden. The study comprised 33 dry grassland sites, including 22 grazed and 11 abandoned localities, and 28 sites of coastal brackish meadows, divided into five management types (from “heavily grazed” to “abandoned since long time”).
In general grazed sites were species‐richer than abandoned sites, especially at small plot sizes. However, there was a steeper increase in species number towards larger plot sizes in the abandoned sites. Heavy grazing in the coastal meadows resulted in a comparatively low number of species, corroborating the intermediate disturbance hypothesis.
The analysis of life‐history traits indicated the importance of taxonomic group, canopy structure, height, regenerative strategy and, in particular, life form. Leaf anatomy and seed dispersal seemed to be less important. The responses to grazing as regards species traits differed somewhat between grassland types. Grazed sites generally had high proportions of legumes, therophytes, species with basal position of leaves and with regeneration by means of a persistent seed bank. Abandonment of grazing favoured monocots, geophytes, species with vegetative regeneration and (partly) leafy canopy structure. Some differences between grazed and abandoned sites were confined to either the smallest or largest plot sizes, indicating different responses of matrix and interstitial species. Various positive associations (attribute syndromes) or negative associations between individual traits were identified. There was, for example, a positive link between the attributes “geophytes” and “ability of vegetative regeneration”. The recognition of such links is important to avoid misinterpreting certain attributes as functional adaptations to grazing while they are only positively correlated to other attributes of larger significance.     

Vegetation indicators for grazing activities on grassland to be implemented in outcome-oriented agri-environmental payment schemes in North-East Germany

In this study a new field of action for outcome-oriented payment schemes (payments by results) is introduced. That is the promotion of extensive summer grazing activities of livestock. Combining this objective with methods of outcome-oriented rewarding means to tie the payment to vegetation criteria indicating regular grazing activities instead of regularly controlling the presence of livestock on the field. In comparison to action-oriented concepts, the main advantage of this approach is that only one inspection walk is required. The paper describes and evaluates the first attempt to determine suitable indicator species for such a scheme. We pre-tested the approach with existing vegetation data only. A database of relevés, plant species and agriculturally used grassland communities of the regional district Doberan, Mecklenburg-Western Pomerania (Germany) was utilised. Grazing activities can be identified by the existence of a specific number of vascular species indicating grazing activities on a particular grassland field. We utilised the indicator function of grassland communities for management conditions to resolve the deficiency of data base relevés for providing no data about the management regime. The pasture community Lolio perennis-Cynosuretum cristati was used as a reference system to pre-test the suitability of different combinations of potential indicator species for separating pasture from non-pasture grassland. A tentative quantitative selection criterion for pasture grassland was proposed. That is the number of indicator species a relevé/grassland field has to hold to be identified as pasture grassland. Database relevés are not sufficient for calibrating such a concept, though. It is necessary to adjust the indicator species list to other drivers like use characteristics, nutrient availability, and soil moisture level. Here additional fieldwork is needed. As a first evidence for the validity of the concept, the suitability of the indicator list was measured by testing the correct identification of pasture grassland in a long-term grassland experiment built by the treatments “grazing” and “mowing”. ANOVA tests on the precision of the indicator species showed significant results. In the discussion a modification of the inventory method of grassland fields was recommended. The precision of the developed concept could be further enhanced by introducing the species dominance as a second factor deciding about the correct identification of pasture use.     

Effects of grazing disturbance on plant diversity,community structure and direction of succession in an alpine meadow on Tibet Plateau,China

To elucidate the effects of grazing intensity and grazing time on plant diversity and community structure, as well as the successional differentiation in an alpine meadow, a controlled grazing trial, with six grazing intensities on an alpine meadow was conducted in the eastern Qilian Mountain region for four years. Using species accumulation curves, RDA ordination and variance decomposition, we analyzed the changes in proportion of dominant species, richness, abundance, as well as the life forms of plant communities under grazing disturbance. Both the grazing intensity and grazing time had a significant effect on these dominant species, richness, abundance, as well as the life forms in the plant community (P < 0.01). More detailed results showed that: (1) The richness and abundance of plant species were highest in the light grazing plot, and these increased as time passed. In the heavy grazing plot, the abundance of plant species decreased as time passed, but the richness of these species did not change significantly. (2) The abundance of Gramineae and Umbelliferae were negatively and significantly correlated with the duration of grazing treatments, whereas Plantaginaceae and Geraniaceae were positively and significantly correlated with the grazing intensity. Over time, the abundance of bunch-type plants decreased and other life forms of plants have increased. With the increase in grazing intensity, the plants' abundance with the rosette type did not change, but other life forms of plants decreased. The results of variance decomposition indicated that grazing disturbance has had greater effects on life forms and plant populations, followed by the changes in the dominant species and their abundance, with lesser effects on the richness of the species. Grazing intensity had a greater effect than the duration of the grazing treatment had. The results of PCA showed that the climax community in both the lowest and the highest grazing intensity plots had changed over time. In the sample plots with light grazing intensity, the plant community changed to an Elymus nutans + Poa crymophila community, but later changed to a Melilotoides ruthenicus + Kobresia humilis community under heavy grazing.     

Effects of grazing disturbance on plant diversity,community structure and direction of succession in an alpine meadow on Tibet Plateau,China

To elucidate the effects of grazing intensity and grazing time on plant diversity and community structure, as well as the successional differentiation in an alpine meadow, a controlled grazing trial, with six grazing intensities on an alpine meadow was conducted in the eastern Qilian Mountain region for four years. Using species accumulation curves, RDA ordination and variance decomposition, we analyzed the changes in proportion of dominant species, richness, abundance, as well as the life forms of plant communities under grazing disturbance. Both the grazing intensity and grazing time had a significant effect on these dominant species, richness, abundance, as well as the life forms in the plant community (P < 0.01). More detailed results showed that: (1) The richness and abundance of plant species were highest in the light grazing plot, and these increased as time passed. In the heavy grazing plot, the abundance of plant species decreased as time passed, but the richness of these species did not change significantly. (2) The abundance of Gramineae and Umbelliferae were negatively and significantly correlated with the duration of grazing treatments, whereas Plantaginaceae and Geraniaceae were positively and significantly correlated with the grazing intensity. Over time, the abundance of bunch-type plants decreased and other life forms of plants have increased. With the increase in grazing intensity, the plants' abundance with the rosette type did not change, but other life forms of plants decreased. The results of variance decomposition indicated that grazing disturbance has had greater effects on life forms and plant populations, followed by the changes in the dominant species and their abundance, with lesser effects on the richness of the species. Grazing intensity had a greater effect than the duration of the grazing treatment had. The results of PCA showed that the climax community in both the lowest and the highest grazing intensity plots had changed over time. In the sample plots with light grazing intensity, the plant community changed to an Elymus nutans + Poa crymophila community, but later changed to a Melilotoides ruthenicus + Kobresia humilis community under heavy grazing.     

Synthetic sheep urine alters fungal community structure in an upland grassland soil

Agricultural improvement (fertilisation, liming, intensification of grazing) of acidic upland pastures results in loss of indigenous flora and notable changes in microbial community structure. Such practices have recently raised concerns regarding the possible impacts on natural ecosystem biodiversity and functioning. The effects of synthetic sheep urine (SSU) and plant species on fungal community structure in upland grassland microcosms were investigated. Plant species typical of agriculturally unimproved (Agrostis capillaris) and improved (Lolium perenne) pastures were treated with low, medium or high concentrations of SSU, with harvests carried out 10 d and 50 d after SSU application. Root biomass was negatively affected by SSU addition whereas shoot biomass did not display any significant change. Fungal richness (number of operational taxonomic units) was negatively correlated with SSU concentration (p < 0.001), and also with time (p < 0.001).Multi-dimensional scaling plots revealed significant changes in fungal community composition, depending on concentration of SSU and plant species type, while canonical correspondence analysis also emphasised the importance of interacting environmental variables. In addition, SIMPER analyses supported the finding that shifts in fungal community composition under different SSU and plant treatments had occurred. Overall, while SSU appeared to be influential in determining fungal community structure, community changes were largely driven by interacting environmental factors. This study contributes to our understanding of the potential implications of intensified farming, in particular increased pressure from grazing animals, on fungal community structure in semi-natural grassland systems.     

高寒草地植物群落关键种对不同放牧家畜组合放牧的响应

如何通过合理的利用方式提高高寒草地管理水平,实现其可持续利用一直是草地生态学领域的研究热点。为明确不同放牧家畜组合下高寒草地植物群落关键种的演替规律及其驱动因素,基于中等放牧强度设置了不同放牧家畜组合放牧样地（牦牛单牧、藏羊单牧、牦牛藏羊1 ： 2混牧、牦牛藏羊1 ： 4混牧、牦牛藏羊1 ： 6混牧）以及围封样地,并在连续放牧处理7年后系统分析了不同放牧家畜组合下植物群落特征与土壤理化性质变化,以期确定最优放牧组合。结果表明：（1）不同放牧家畜组合均会显著降低高寒草地植物盖度,但对其物种丰富度与多样性指数的影响并不显著。（2）牦牛藏羊1 ： 2混牧下植物群落特征与禁牧处理下植物群落特征较为相似,且牦牛藏羊1 ： 2混牧下的植物群落稳定性最强、组织水平最高。（3）牦牛单牧与1 ： 2混牧下关键种为矮生嵩草;藏羊单牧下关键种为天山针茅;1 ： 4混牧下关键种为星毛委陵菜;1 ： 6混牧与围封下关键种为赖草。（4）围封能够显著降低土壤容重,藏羊单牧则会显著增加土壤容重与土壤速效氮、磷含量。（5）土壤速效氮、容重与含水量是驱动不同放牧家畜组合下植物群落关键种演替的重要理化因子。综上所述,中等放牧强度下,牦牛藏羊1 ： 2混牧是青藏高原高寒草地较为理想的利用方式。此外,相较于单纯增加草地物种丰富度或多样性,建立植物群落物种之间的有效关联是提高青藏高原高寒草地管理水平的另一关键途径。     

Notes on syntaxonomy of cultural forest communities

It is suggested that communities of cultivated forests can be incorporated into the system of “natural” wood communities either as facies, variants or subassociations, if they differ below the association level; if they differ on the association level, a new association is suggested, with the prefix “culti-” before the planted dominant tree species. Most of our planted forest communities differ on the level of association or less; the communities ofRobinia are the exception. The authors agree withJurko (1963) in classifying them as individual associations, alliances, order and class.     

Restoration of grazing management and its effect on vegetation in an upland grassland

Question: How are plant species and functional group composition, and potential sward height affected by implementation of different grazing regimes on previously abandoned semi-natural grassland? Location: The Jizerské mountains, northern Czech Republic. Methods: We established a randomized block experiment with the following treatments: unmanaged control (U), intensive (IG) and extensive (EG) continuous grazing, first cut followed by intensive (ICG) and first cut followed by extensive (ECG) continuous grazing for the rest of the growing season. The percentage cover of all vascular plant species was recorded in 40 permanent plots. Results: Total plant species richness increased in all managed treatments, whereas species number was reduced in U at the end of the experiment. Tall forbs (Aegopodium podagraria, Galium album, Anthriscus sylvestris, Cirsium arvense) as well as tall grasses (Elytrigia repens and Alopecurus pratensis) were more abundant in U. Species associated with both grazing treatments (IG, EG) were Dactylis glomerata, Festuca rubra agg. and Phleum pratense. Agrostis capillaris, Taraxacum spp., Trifolium repens, Ranunculus repens and Cirsium vulgare were promoted by ECG and ICG. Abundance of tall grasses and tall forbs reflected the intensity of management in the order U>EG, ECG>IG, ICG. Prostrate forbs, on the other hand, increased their cover with increasing intensity: ICG>IG>ECG>EG. Conclusions: Plant species composition of semi-natural grasslands is affected by the defoliation regime. Continuous grazing on abandoned grassland alters the sward structure towards a permanent pasture with short, light-sensitive grasses and prostrate forbs. To maintain or enhance plant species richness in semi-natural grasslands, understanding the effects of different grazing regimes on plant species composition is necessary.     

New names inPhanerogamae 6

94 new nomenclatural combinations are proposed, resulting mostly from the study of taxonomic problems on the generic level. The reasons for the acceptance of the generic rank are given in the case ofAcetosa, Pulegium and of some sections ofOrobanche s. l. andSilene s. l. The taxonomic rank ofGentiana blepharophora Bordzi?. from the Caucasus has been revised and the taxon is classified as a subspecies ofGentianopsis ciliata. Nomenclatural problems of not numbered “species” inHoffmann's “Flora Deutschlands” from 1800 are demonstrated on the example ofArmeria elongata.     

Moderate grazing promotes the root biomass in Kobresia meadow on the northern Qinghai–Tibet Plateau

Grazing is an important modulator of both plant productivity and biodiversity in grassland community, yet how to determine a suitable grazing intensity in alpine grassland is still controversy. Here, we explore the effects of different grazing intensities on plant biomass and species composition, both at community level and functional group level, and examines the productivity–species richness relationship under four grazing patterns: no grazing (CK), light grazing (LG), moderate grazing, (MG) and heavy grazing (HG), attempt to determine a suitable grazing intensity in alpine grassland. The results were as follows. The total aboveground biomass (AGB) reduced with increasing grazing intensity, and the response of plant functional groups was different. AGB of both sedges and legumes increased from MG to HG, while the AGB of forbs reduced sharply and the grass AGB remained steady. There was a significant positive relationship between productivity and species richness both at community level and functional group level. In contrast, the belowground biomass (BGB) showed a unimodal relationship from CK to HG, peaking in MG (8,297.72 ± 621.29 g/m²). Interestingly, the grassland community tends to allocate more root biomass to the upper soil layer under increasing grazing intensities. Our results suggesting that moderate levels of disturbance may be the optimal grassland management strategy for alpine meadow in terms of root production.     

A probability distribution model of small -scale species richness in plant communities

We devised a probability distribution model that best expressed species richness per quadrat in grassland communities, and clarified the mechanism by which the mean richness per quadrat was always larger than the variance among quadrats. Our model will aid in the understanding of community structures, and allow comparisons among different communities. The model was constructed based on relatively simple theoretical assumptions about the mechanisms in play in target communities. We assumed in the model that the number of species occurring in an actual quadrat, j, is the sum of “the fundamental number of species”, k (constant), and “a fluctuating number of species”, i (a Poisson variate with the mean of μ); that is, j = k + i, where i, j and k are non-negative integers. The probability that j species occur in a quadrat is given by a Poisson-like distribution (extended Poisson), with two parameters k and μ. The mean species richness in the probability distribution is expressed by λ (= k + μ), and the variance is λ − k. The proposed model afforded a good fit for the observed frequency distribution of species richness per quadrat. If even one species is common among many quadrats, the mean number of species per quadrat is greater than the variance. The greater the number of common species among quadrats is, the larger is the value of k, and then the more pronounced is the difference between the mean and the variance (although the variance does not change). We fitted the model to 55 datasets collected by ourselves from grasslands in various locations (Tibet, Inner Mongolia, Slovakia, or Japan), with varying quadrat size (0.25, 0.0625, or 0.01 m²), and under differing management status (various stocking densities).     

Anthoxanthum odoratum andA. alpinum: Life history parameters at two different altitudes

The closely related species, diploidAnthoxanthum alpinum Á. Löve etD. Löve and tetraploidAnthoxanthum odoratum L., have distinct distributions:A. alpinum is an arctic-alpine species, whileA. odoratum is a species of lower altitudes and more southerly latitudes. However, some authors suggest that both species are potentially able to maintain their populations in the distribution area of the other species. Reciprocal transplant experiments in two mountain localities (one withA. alpinum, the other withA. odoratum) were carried out to study the survival and vegetative and generative reproduction of tussocks of both species in the area of distribution of the other species. The mortality of tussocks ofA. alpinum was higher in the “alien” locality, the life span of tillers was greatly reduced and the size of tussocks decreased considerably during the experiment compared to its “native” locality. These results suggest that there was a lack of favourable microsites forA. alpinum in the “alien” locality probably caused by the presence of strong competitors.A. odoratum flowered far less in the “alien” locality (both on tussocks and tillers) compared to the “native” one. These results suggest that environmental conditions are not suitable for the flowering ofA. odoratum in the “alien” locality. We conclude thatA. alpinum could have been limited in extending its distribution into the area ofA. odoratum by the presence of strong competitors, whileA. odoratum could have been limited by its ability to reproduce by seed.     

内蒙古荒漠草原主要草食动物食性及其营养生态位

为探讨荒漠草原主要食草动物(绵羊、达乌尔黄鼠和亚洲小车蝗)的食性及其种间生态位变化与草原退化的关系,以内蒙古苏尼特右旗荒漠草原为研究对象,于2003年7-8月运用饱和链烷技术研究小针茅群落不同放牧强度下绵羊、达乌尔黄鼠和亚洲小车蝗的食性及其生态位变化.结果表明:随牧压的增大,群落中小针茅数量显著减少,猪毛菜比例随着增加;绵羊、达乌尔黄鼠、亚洲小车蝗3种食草动物的主要食物均来源于猪毛菜,且采食比例随牧压增大而递增;3种食草动物对牧草的选择性指数随牧压的变化而变化,双齿葱和多根葱优先被采食;三者生态位重叠程度很高且生态位宽度随牧压的增加而变窄.重度放牧改变了小针茅群落的植物组成,使其演替为猪毛菜群落,3种食草动物对草地资源存在激烈的竞争,因此,控制鼠、虫的种群密度,防止鼠、虫害的发生对保证放牧家畜的食物安全具有十分重要的现实意义.     

Effects of grazing intensity in grasslands of the Espinal of central Chile

Question: How is grazing intensity associated with species and morpho‐functional traits (MFTs) composition, productivity and richness of annual dominated grasslands? Have native and exotic species similar associations to this gradient? Location: Anthropogenic grassland in the Espinal vegetation in the sub‐humid area of the mediterranean type climate region of Chile (35°58’ S, 72°17’ W). Methods: Data were obtained from a long‐term (eight years) experiment with six stocking rates (1 to 3.5 sheep/ha). Detrended Correspondence Analysis (DCA) and regression analysis were used to determinate the relationship between grazing intensity and biomass, richness, abundance and traits of the species. Results: The first DCA axis was related to grazing intensity and explained most of the floristic variation (69.3%); the abundance of some non‐native species, e.g. Vulpia megalura were highly correlated with this axis. In the DCA for MFTs the first axis explained 87% of the variance and was also related to grazing intensity; the abundance of small size plants and shallow roots increased with grazing intensity. The relative abundance of grasses and composites, but not of legumes, changed with stocking rate: as grazing intensity increased composites became the predominant species to the detriment of grasses. The above‐ground biomass measured in exclusion cages declined with increasing grazing pressure. The richness of exotic species was greater compared to native ones at low stocking rates, but they converge to similar values at higher stocking rates. However, the relative abundance of exotic species was greater than 75% in all stocking rates. Conclusions: Grazing intensification has large effects in the structure of grassland in central Chile. With grazing intensities greater than 1 sheep/ha species characteristics change; evolving in a few years (6–8) towards a similar community regardless of the stocking rate. The overgrazed community has more native than exotic species richness, possibly due to greater defence traits against herbivory of this group of species.     

TheSitona [Col.: Curculionidae] species occurring onMedicago and their natural enemies in the Mediterranean region

TwentySitona species were found in the Mediterranean region during surveys on 17 volunteer and/or cultivated species ofMedicago. The distribution of the weevils was analyzed using the 3 concepts of abundance, constancy and dominance. EightSitona species were consistently recorded on lucerne and medics; of these, 3 were “constants” and only 2,S. humeralis Stephens andS. lineatus Linnaeus, also “dominants” in the sites investigated. Sixteen species of natural enemies were identified (table 4), including 3 egg predators, 3 egg parasitoids, 1 entomogenous fungus, 2 nematodes and 5 parasitoids of adult weevils. The effect of each organism on theSitona population numbers was slight, not exceeding in most cases an average mortality of 15%. At high host densities however, the braconidMicroctonus aethiopoides Loan which is the preponderant parasitoid, caused estimated mortalities up to 80% during a single weevil generation. Noticeable changes in the relative importance of the natural enemies were observed between the areas surveyed, with onlyM. aethiopoides and the mymarid egg parasitoidPatasson lameerei Debauche occurring widely. Moroccan, French and Greek strains of these 2 parasitoids were forwarded to Australia for the biological control ofS. humeralis there.     

A study of the interactions between the internal larval parasites ofRhyacionia buoliana [Lepidoptera: Olethreutidae]

The interactions of 6 species of internal parasites ofRhyacionia buoliana Schiff. in a locality in northern Germany are discussed. The occurrence of super and multiparasitism is analyzed in relation to varying host densities and different rates of total parasitism. Multiparasitism betweenOrgilus obscurator Nees and 3 other species,Temelucha interruptor Grav.,Pristomerus sp. nearorbilatis Holmgr. and biting cleptoparasitic habits. Interspecific competition occurred in the form of direct physical attack between the first instar larvae. The coexistence of the internal larval parasites ofR. buoliana provides a good example of a system of “counter-balanced competition”.O. obscurator, although inferior at the level of intrinsic competition, was the dominant species at the level of extrinsic competition.     

Genetic diversity and invasibility: a test using a model system with a novel experimental design

Biological invasion is one aspect of ecosystem function that may be controlled by the biological diversity of the invaded community, and there have been a number of recent studies that investigated relationships between diversity and invasibility. Most experimental studies report that higher species or functional group diversity increases resistance to invasion, but the role of genetic diversity is unknown. We used a model organism, Arabidopsis thaliana (Brassicaceae), to investigate relationships between genotypic richness and community invasibility by creating communities with 1, 2, 4, and 8 genotypes of A. thaliana at constant low (417 plants m⁻²) and high (834 plants m⁻²) densities, that once established, were invaded with a congener, Arabidopsis suecica. To reduce the potential effects of methodological confounding related to “sampling effects,”“variance reduction effects,” or confounding of abundance with diversity, we (1) created random communities from a relatively large pool of functionally and phenotypically similar genotypes, (2) evaluated individual and community traits across richness treatments, and (3) analyzed similarity of communities within treatments (for “quasi- replication”) and between adjacent treatments (for “nestedness”). Genotypic richness had no effect on A. suecica demography (emergence, survivorship), size (biomass, rosette area), or reproductive potential (rates of bolting and fruiting or number and size of bolts). In contrast, the density of A. thaliana genotypes had strong effects on the size and reproductive potential of A. suecica, which suggests that characteristics of the recipient community other than genotypic richness (e.g. light) form the most important determinant of community invasibility. Individual- and community-level traits of community members (cover, biomass, survivorship) did not differ among richness treatments, and within- and between-treatment similarity was reduced (relative to other recent experiments) but not eliminated. We evaluate our results vis-a-vis recent analyses of diversity-invasibility experiments, and provide directions for future investigations of genetic diversity.