INTRA- AND INTERCLONAL COMPETITION IN THE CLEISTOGAMOUS GRASS AMPHIBROMUS SCABRIVALVIS

The relationship of differences in life history traits among genotypes to competitive ability is not well known for most clonal plants. It has been hypothesized that genetically identical clones will compete more intensively than genetically distinct clones. The perennial grass Amphibromus scabrivalvis, which produces basal corms and cleistogamous seeds enclosed by leaf sheaths, exhibits pronounced clonal growth via rhizome and ramet production. In a controlled greenhouse experiment, clones of four genotypes of this species were grown under three regimes: alone in the absence of competition, paired with a clone of the same genotype (intraclonal competition), and paired with a clone of a different genotype (interclonal competition). There were differences in some biomass measures and in ramet and corm production among the four genotypes grown in the absence of competition. All genotypes showed a significant reduction in total biomass under both intra- and interclonal conditions, indicating that competition had occurred. For three of four genotypes, biomass allocation to corm increased under competition, while allocation to cleistogamous seeds was constant or increased slightly. Although some genotypes in specific interclonal combinations were less affected by competition than in intraclonal combinations, there was no support for the contention that the effects of competition were more intense for genetically identical clones.     

Evolutionary significance of genotypic variation in developmental reaction norms for a perennial grass under competitive stress

The developmental reaction norm (DRN) represents the set of ontogenetic trajectories that can be produced by a genotype exposed to different environmental conditions. Genetic variation in the DRN for growth traits and in the patterns of biomass allocation is critical to phenotypic evolution in heterogeneous environments. The DRN and patterns of biomass allocation were investigated in 11 clones of the caespitose, corm-forming, perennial grass Phleum pratense in relation to competitive stress imparted by Lolium perenne in a 16 week glasshouse experiment. A separate experiment assessed the ability of basal buds flanking a corm to sprout and the relationship of corm mass to sprout mass for the same clones. Corm fresh mass varied among clones and was significantly correlated with the dry mass of the tillers that sprouted from basal buds. In the competition experiment, clones in competitive environments varied significantly from those in non-competetive environments in terms of their DRNs for number of tillers and shoot dry mass. Thus, selection of DRNs would favour different genotypes in the two environments and at different times. Significant negative genetic correlations were detected for tiller number and mean tiller mass in the noncompetitive, but not the competitive, environment. Biomass allocation to stem bases was significantly greater for clones under competitive stress. Allocation to storage tissues such as corms may be adaptive if it enhances persistence in the competitive field environments typically occupied by caespitose grasses. Root and shoot allocation showed a significant clone by competition interaction. For P. pratense, genotypic variation in growth trajectories plays an important role in determining variation in individual performance, a condition necessary for the continued evolution of the DRN.     

Competition among native and invasive Phragmites australis populations: An experimental test of the effects of invasion status,genome size,and ploidy level

Among the traits whose relevance for plant invasions has recently been suggested are genome size (the amount of nuclear DNA) and ploidy level. So far, research on the role of genome size in invasiveness has been mostly based on indirect evidence by comparing species with different genome sizes, but how karyological traits influence competition at the intraspecific level remains unknown. We addressed these questions in a common‐garden experiment evaluating the outcome of direct intraspecific competition among 20 populations of Phragmites australis, represented by clones collected in North America and Europe, and differing in their status (native and invasive), genome size (small and large), and ploidy levels (tetraploid, hexaploid, or octoploid). Each clone was planted in competition with one of the others in all possible combinations with three replicates in 45‐L pots. Upon harvest, the identity of 21 shoots sampled per pot was revealed by flow cytometry and DNA analysis. Differences in performance were examined using relative proportions of shoots of each clone, ratios of their aboveground biomass, and relative yield total (RYT). The performance of the clones in competition primarily depended on the clone status (native vs. invasive). Measured in terms of shoot number or aboveground biomass, the strongest signal observed was that North American native clones always lost in competition to the other two groups. In addition, North American native clones were suppressed by European natives to a similar degree as by North American invasives. North American invasive clones had the largest average shoot biomass, but only by a limited, nonsignificant difference due to genome size. There was no effect of ploidy on competition. Since the North American invaders of European origin are able to outcompete the native North American clones, we suggest that their high competitiveness acts as an important driver in the early stages of their invasion.     

Response of Solidago canadensis clones to competition

Summary Transplants of ten Solidago canadensis clones were grown under high and low competition in the field to determine whether clones differed in survival, growth, and reproduction under natural conditions. Transplants had higher probability of survival and flowering and were larger in all measures of size when competition was experimentally reduced. Clones differed in almost all these measures of success, but only when variance among transplants within clones was reduced by excluding transplants that experienced heavy herbivore damage. Differences among clones were more apparent under low competition than under high competition, despite higher coefficients of variation within clones under low competition. Adjusting transplant size for initial size (parent ramet rhizome mass) did not change these results, although clones did differ in parent rhizome mass. All of these results suggest that there is little potential for selection to discriminate among these clones. Despite the strong differences in transplant performance between the competition treatments and among clones, the clones did not differ in competitive ability-almost none of the clone x competition interactions were significant. In addition, the measures of success of each clone were usually positively correlated between the high and low competition treatments, suggesting there were no tradeoffs between performance under high and low competition for these clones.     

Effects of clonal variation of the host plant,interspecific competition,and climate on the population size of a folivorous thrips

Summary The effects of clonal variation, interspecific competition, and climate upon the population size of Apterothrips secticornis was assessed by a series of observations and experimental manipulations. Three clones of the host plant, Erigeron glaucus, consistently supported different numbers of thrips during monthly censuses. When rosettes of the three clones were transplanted to a common garden, relative numbers of thrips on the clones remained the same as those observed where the clones grew in situ. The presence or absence of other hervivores had no effect on thrips numbers in the common garden. Plume moth caterpillars and thrips were observed to co-occur less often than expected in the field but this was caused by differences in habitat selection by these two species rather than being the result of interspecific competition. Populations of thrips were affected by climate, but analyses suggest that the host clone was a more important factor.     

Intraspecific variation among clones of a naïve rare grass affects competition with a nonnative,invasive forb

Intraspecific variation can have a major impact on plant community composition yet there is little information available on the extent that such variation by an already established species affects interspecific interactions of an invading species. The current research examined the competitiveness of clones of a globally rare but locally common native grass, Calamagrostis porteri ssp. insperata to invasion by Alliaria petiolata, a non‐native invasive species. A greenhouse experiment was conducted twice over consecutive years in which 15 clones from three populations of Calamagrostis were paired with rosettes of Alliaria in pots containing native forest soil previously uninvaded by Alliaria. Both species showed a negative response to the presence of the other species, although Alliaria more so than Calamagrostis. Moreover, the effect of Calamagrostis depended upon population, and, to a lesser extent, the individual clone paired with Alliaria. Competitive effects were stronger in the first experiment compared with when the experiment was repeated in the second year. The influence of Calamagrostis clones on the outcome of the experiment varied among populations and among clones, but also between years. Clones from one of the three populations were more influential than clones from the other two populations. Only one of 15 clones, both from the same population, was influential in both experiments. This research supports a growing literature indicating that intraspecific variability among clones of a dominant species can affect interspecific interactions and that such variability in a native species can affect performance of an invading species.     

种间竞争梯度对白车轴草与其病原菌白车轴草单孢莠菌相互作用的影响

白车轴草（Trifolium repens)在与其病原菌白车轴草单孢锈菌（Uromyces trifolii-repentis)的长期相互作用中分化形成了抗病型（Resistance)无性系和易感型（Susceptibility)无性系。该研究工作旨在了解：1）在种间竞争不断增强的环境梯度中,抗型无性系和易感型无性系的生长表现有何区别？2）在同样的实验条件下,分别对抗病型无性系和易感型无性系进行接种感染后,两者的生长表现又有何区别？在一严重感病的白车轴草自然种群中,分别标定了17个抗病型无性系和14个易感型无性系,从各元性系的匍匐茎上分别剪取长5cm,具两个叶两个腋芽的小段作实验材料。接种孢子采自同一植物种群植株的病叶。实验在一个处在次生演替阶段的草本植物群落上进行,在该群落的种间竞争梯度上选定3个分别具有弱、中等和强种间竞争的生境（Habitat)建立实验站,每站设置等量的实验和对照组。结果显示：1）随着环境中种间竞争强度的增加,无论接种与否,抗病型和易感型白车轴草的叶生长量均凝减;2）在弱或强种间竞争环境条件下,无论接种与否,易感型无性系的生长均好于抗抗病型无性系;但在中等间竞争强度的环境条件下,对照实验中易感型无性系的生长显著好于抗病型无性系,而接种实验中的结果相反。这一研究结果表明,种间竞争强度可能是影响给定生境中寄主植物抗病型和易感型遗传型比率的重要因子之一,这一影响还将随着病原菌存在而否而发生改变。     

Effect of arbuscular mycorrhiza on inter- and intraspecific competition of two grassland species

We were interested in the role of arbuscular mycorrhiza (AM) in the competition between plants of different sizes. A pot experiment of factorial design was established, in which AM root colonization and competition were used as treatments. Five-week-old Prunella vulgaris seedlings were chosen as target plants (i.e. plants whose response to competition was studied) and the following (13 replicates of each) were used as neighbours: (1) a large, 10-week-old P. vulgaris, (2) two P. vulgaris seedlings, and (3) a large, 10-week-old Fragaria vesca. In the experiment where small neighbours were grown together with small target plants, competition did not reduce target plant weight significantly, compared to the other two treatments. The competitive effects of large neighbours were significant, regardless of species (both older neighbours reduced the weights of target plants similarly), but there was a clear difference between intra- and interspecific competition when plants were mycorrhizal. In intraspecific competition with a large neighbour, the target plant shoot weight was reduced 24% when inoculated with AM. Thus, AM amplified rather than balanced intraspecific competition. In interspecific competition with old F. vesca, the shoot weights of target plants were 22% greater when inoculated with AM than when non-mycorrhizal. The results showed that, for given soil condition, AM might increase species diversity by increasing competitive intraspecific suppression and decreasing the interspecific suppression of small plants by larger neighbours.     

Stoichiometric differences in food quality: impacts on genetic diversity and the coexistence of aquatic herbivores in a <Emphasis Type="Italic">Daphnia</Emphasis> hybrid complex

The maintenance of genetic and species diversity in an assemblage of genotypes (clones) in the Daphnia pulex species complex (Cladocera: Anomopoda) in response to variation in the carbon:phosphorus ratio (quantity and quality) of the green alga, Scenedesmus acutus, was examined in a 90-day microcosm competition experiment. Results indicated that mixed assemblages of seven distinct genotypes (representing clonal lineages of D. pulex, D. pulicaria and interspecific hybrids) showed rapid loss of genetic diversity in all treatments (2 × 2 factorial design, high vs. low quantity, and high vs. low quality). However, the erosion of diversity (measured as the effective number of clones) was slowest under the poorest food conditions (i.e., low quantity, low quality) and by the conclusion of the experiment (90 days) had resulted in the (low, low) treatment having significantly greater genetic diversity than the other three treatments. In addition, significant genotype (clone) × (food) environment interactions were observed, with a different predominant species/clone found under low food quality versus high food quality (no significant differences were detected for the two food quantities). A clone of D. pulex displaced the other clones under low food quality conditions, while a clone of D. pulicaria displaced the other clones in the high food quality treatments. Subsequent life-history experiments were not sufficient to predict the outcome of competitive interactions among members of this clonal assemblage. Our results suggest that genetic diversity among herbivore species such as Daphnia may be impacted not only by differences in food quantity but also by those in food quality and could be important in the overall maintenance of genetic diversity in natural populations. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Resource competition between genetically varied and genetically uniform populations of Daphnia pulex (Leydig): does sexual reproduction confer a short‐term ecological advantage?

Small competitive advantages may suffice to compensate for a large disadvantage in intrinsic growth capacity. This well‐known principle from ecology has recently been applied to the enduring question of how sexual reproduction can persist in the face of invasion by female‐only parthenogens. Small competitive advantages resulting directly from sexual reproduction are predicted to cancel a two‐fold disadvantage in intrinsic growth capacity caused by males (which do not themselves produce offspring) comprising half the sexual population. In this paper we test the principal assumption of this theory, that the genetic variation produced by sexual reproduction confers a competitive advantage over self‐identical asexual invaders. We set up competition between a diverse clonal assembly of Daphnia pulex and genetically uniform populations from single clones. At young ages, the population comprising genetically varied Daphnia had significantly higher birth rates in competition with populations of genetically uniform Daphnia than in competition with itself, indicating competitive release and a Lotka–Volterra competition coefficient α₁₂ < 1. No such difference was apparent under conditions of greater food stress, possibly due to individuals channelling more energy into survival, or for old‐aged populations, possibly as a result of reduced selective pressures for high reproduction in old females. Mean birth rates differed between the clones at all ages in the presence of competition, providing evidence of variation in life history traits between clones. A Lotka–Volterra model predicted empirical estimates of α₁₂ = 0.896 (genetically uniform on varied) and α₂₁ = 1.010 (varied on uniform), which permits immediate coexistence of a sexual population of D. pulex even with an asexual lineage having twice the intrinsic growth capacity. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 85 , 111–123.     

A comparative approach to study inhibition of grazing and lipid composition of a toxic and non-toxic clone of Chrysochromulina polylepis (Prymnesiophyceae)

Since the massive bloom in 1988 in the North Sea, the prymnesiophyte flagellate Chrysochromulina polylepis Manton et Parke has been known for its ichtyotoxicity. Laboratory experiments using two different clones of C. polylepis were conducted in a comparative approach. Both clones were nearly similar in size and shape, but differed in their toxicity, as demonstrated by the Artemia bioassay. In order to study the effects of toxic C. polylepis on protozooplankton grazers, grazing experiments were performed with the heterotrophic dinoflagellate Oxyrrhis marina Dujardin as grazer. A first experiment was carried out in order to follow batch culture growth and initial grazing of O. marina when fed toxic or non-toxic clones of C. polylepis. Ingestion of the toxic clone was 27% of ingestion when fed with the non-toxic clone. When O. marina was fed with the toxic clone, vacuoles within O. marina contained fewer food particles per cell and the cells attained slower division rate (58% of the division rate estimated for the non-toxic clone). A second experiment was conducted to determine the grazing and growth response of O. marina as a function of algal food concentration. Profound differences in ingestion, clearance, division and gross growth efficiency of O. marina when fed the two clones of C. polylepis again were apparent. However, even at algal concentrations of 400×10³ ml⁻¹, O. marina is not killed by the presence or by the ingestion of toxic C. polylepis, indicating that the toxin deters grazers. In addition to grazing experiments, lipid classes and fatty acids of both algal clones were analysed and compared in order to follow the hypothesis that toxicity of C. polylepis is caused by liposaccharides, lipids, or fatty acids. However, the chemical composition with respect to lipid classes and fatty acids of both clones were quite similar, making an involvement of these substances in the toxicity towards Artemia and O. marina unlikely.     

An analysis of the costs and benefits of physiological integration between ramets in the clonal perennial herb Glechoma hederacea

Summary The costs and benefits, measured in terms of dry weight, of physiological integration between clonal ramets, were analysed in two experiments conducted on the clonal herb Glechoma hederacea. Firstly, integration between consecutively-produced ramets was examined in an experiment in which stolons grew from one set of growing conditions (either unshaded or shaded and either nutrient-rich or nutrient-poor) into conditions in which light or nutrient level was altered. Comparisons were made between the dry weight of the parts of the clones produced before and after growing conditions were changed, and the dry weights of the corresponding part of control clones subjected to constant growing conditions. In a second experiment, integration between two distinct parts of G. hederacea clones was investigated. In this experiment clones were grown from two connected parent ramets and the parts of the clone produced by each parent ramet were subjected independently to either nutrient-rich or nutrient-poor conditions. Ramets in resource-rich conditions provided considerable physiological support to those in resource-poor conditions. This was measured as a dry weight gain compared with the weight of the corresponding part of the control clones growing in resource-poor conditions. However, when stolons grew from resource-poor conditions into resource-rich conditions, there was no similar evidence of the resourcepoor ramtes receiving support from resource-rich ramets. Physiological integration did not result in dry weight gains when this would have necessitated basipetal translocation of resources.Because of the predominantly acropedal direction of movement of translocates in G. hederacea, the structure of the clone was important in determining the effectiveness of integration between ramets. Where physiological integration was effective, the cost to the supporting ramets in terms of dry weight was insignificant. Physiological integration allows clones to maintain a presence in less favourable sites with insignificant cost to ramets in favourable sites, thereby reducing the probability of invasion by other plants, and providing the potential for rapid clonal growth if conditions improve. Integrated support of ramets in unfavourable conditions also enables the clone to grow through unfavourable sites, thus increasing the probability of encountering more favourable conditions by wider foraging.     

Interspecific competition: A mechanism for rodent succession after fire in wet heathland

Abstract A competitive interaction between two species of sympatric native rodents was demonstrated experimentally in Myall Lakes National Park (NSW, Australia). The previously documented replacement of Pseudomys gracilicaudatus by Rattus lutreolus in areas of wet heath in this region implied that competition may be the mechanism facilitating this succession. We present experimental evidence to support this suggestion. Removal of the larger R. lutreolus caused a significant increase in abundance of P. gracilicaudatus on five experimental sites in comparison to five unmanipulated control sites. The sites used in this experiment were chosen from three ages of wet heath regenerating after fire. These three ages represent key stages in the replacement of P. gracilicaudatus by R. lutreolus and the occurrence of competition in each of these ages indicates that competition plays an important role in this succession. In the initial stages of the experiment when R. lutreolus were removed they were replaced by new R. lutreolus individuals. This has been interpreted as evidence of strong intraspecific competition. After removal of R. lutreolus, P. gracilicaudatus expanded its habitat range into microhabitats that had formerly been occupied by R. lutreolus and reduced its range in microhabitats previously occupied by P. gracilicaudatus. This leads us to believe that P. gracilicaudatus was occupying inferior microhabitat before the removal of R. lutreolus, which had excluded it from more preferable microhabitat.     

Different patterns of biomass allocation of mature and sapling host tree in response to liana competition in the southern temperate rainforest

Lianas can negatively affect their host tree. The evidence comes from studies in tropical forests, where lianas decrease the growth rate and reproduction of their host tree. This is primarily a consequence of water and nutrient competition, two limiting factors in tropical forests. In contrast, for some areas of southern temperate rainforests the competition for these resources could be less severe, because of the high rainfall and fertile soils. But so far, no study has determined the effect of liana competition over their host tree in southern temperate rainforests. The aim of this study was to evaluate, in field conditions, the effect of liana Cissus striata (Ruiz & Pavon) competition over the growth rate of mature Nothofagus obliqua (Bidr Egefam) host tree. In an experimental approach, we determined whether above‐ and/or below‐ground competition is more important in this interaction. We also looked for compensatory strategies that would allow to trees to respond to liana competition. In field conditions, we found that infested trees have a decrease in their relative growth rate of 26% and a reduction of the leaf area index (LAI) of 20% compared with control trees. In the greenhouse experiment, we found that saplings were water stressed and that there was light competition. Neither competition for water nor light had a significant effect on the growth rate of infested saplings. This could be explained because saplings showed compensatory strategies in response to competition. These strategies were based in the biomass distribution between organs (leaf area, slenderness index) and within leaves (LMA). In conclusion, we found that C. striata has a negative effect over the growth of mature and sapling N. obliqua host trees. This was a consequence of above‐ground and below‐ground competition, but we cannot disentangle which type of competition is more important. Trees respond to liana competition, mature host trees change the canopy architecture and saplings allocate resource between and within organs, which allows them to optimize resource capture.     

Direct and indirect effects of plant genetic variation on enemy impact

Abstract.

• 1 The Tritrophic and Enemy Impact concepts predict that natural enemy impact varies: (a) among plant genotypes and (b) may depend on the abundance of heterospecific herbivores, respectively. I tested these predictions using three herbivore species on potted, cloned genotypes of Salik sericea Marshall in a common garden experiment.
• 2 Densities of the leaf miner (Phyllonorycter salicifoliella (Chambers)) and two leaf galling sawflies (Phyllocolpa nigrita (Marlatt) and Phyllocolpa eleanorae Smith and Fritz) varied significantly among willow clones, indicating genetic variation in resistance.
• 3 Survival and natural enemy impact caused by egg and larval parasitoids and/or unknown predators differed significantly among willow clones for each of the three herbivore species, indicating genetic variation in survival and enemy impact.
• 4 Survival of Phyllonorycter was negatively density-dependent among clones.
• 5 Survival of Phyllonorycter and Phyllocolpa eleanorae were positively correlated with densities of heterospecific herbivores among clones and parasitism of these species were negatively correlated with densities of the same heterospecific herbivores among clones.
• 6 At least for Phyllonorycter this positive correlation may suggest either facilitation of survival between herbivore species, which do not share natural enemies, or an apparent interaction caused by host plant genetic variation.
• 7 Among clones, egg parasitism of Phyllocolpa eleanorae was weakly positively correlated with density of Phyllocolpa nigrita. Since these species share the same Trichogramma egg parasitoid, this interaction could support the hypothesis of apparent competition.

     

Genetics and the environment in interspecific competition: a study using the sibling species Drosophila melanogaster and Drosophila simulans

The outcome of interspecific competition of two closely related species may depend upon genetic variation in the two species and the environment in which the experiment is carried out. Interspecific competition in the two sibling species, Drosophila melanogaster and D. simulans, is usually investigated using longterm laboratory stocks that often have mutant markers that distinguish them. To examine competition in flies that genetically more closely resemble flies in nature, we utilized freshly caught wildtype isofemale lines of the two species collected at the same site in San Carlos, Mexico. Under ordinary laboratory conditions, D. melanogaster always won in competition. However, in hotter and drier conditions, D. simulans competed much more effectively. In these environmental conditions, there were genetic differences in competitive ability among lines with the outcome of competition primarily dependent upon the line of D. melanogaster used but in some cases also influenced by the line of D. simulans used. Differences in the measures of productivity and developmental time did not explain the differences in competitive ability among lines. This suggests that the outcome of competition was not due to differences in major fitness components among the isofemale lines but to some other attribute(s) that influenced competitive ability. When lines of flies were combined, the outcome of competition was generally consistent with competitive outcomes between pairs of lines. In several cases, the combination of lines performed better than the best of the constituent lines, suggesting that competitive ability was combined heterotically and that the total amount of genetic variation was important in the outcome of interspecific competition.     

Environmental correlates of the intrinsic rate of natural increase in primates

Morphological and life history traits of two clones of the cladoceran Daphnia pulex were measured in the presence and absence of size-selective insect predators, the midge larva Chaoborus flavicans, which preys on small Daphnia, and the water bug Notonecta glauca, which preys on large Daphnia. The aim was to detect predator-induced phenotypic changes, particularly the effect of simultaneous exposure to both types of predators. Other work has shown that in the presence of Chaoborus americanus, Daphnia pulex produce a socalled neck spine which may carry several teeth. The morphological modifications are supposed to serve as an anti-predator device. Furthermore, females exposed to Chaoborus often delay their maturation; this has been interpreted as a cost that balances the benefits of the neck teeth. In this investigation, females of both clones produced fewer but larger offspring than control animals when reared in the presence of Chaoborus flavicans. The offspring showed the typical neck spine and delayed first reproduction. In the presence of Notonecta glauca, one of the clones produced more and smaller offspring, and maturation occurred at earlier instars. The other clone also produced more offspring than the control but there was no size difference. When both predators were present, in most cases the reactions of the daphnids were similar to those in the Notonecta experiment. The response to Chaoborus appeared to be suppressed. The observed modifications are interpreted as evolved strategies that reduce the impact of size-selective predation. They are consistent with predictions of life-history theory.     

Intraspecific heritable variation in life-history traits can alter the outcome of interspecific competition among insect herbivores

Competition is one of the most important biotic factors determining the structure of ecological communities. In this study, we show that there is variation in competitive ability between two clones of the pea aphid, Acyrthosiphon pisum, both of which out-compete a clone of the vetch aphid, Megoura viciae, in the laboratory. We tested whether this variation in competitive ability would alter the outcome of interspecific competition in the field. While one pea aphid clone followed the pattern set in the laboratory, out-competing the Megoura viciae clone, another showed the reverse effect with Megoura viciae dominating. These differences appear to be the result of variation in early population growth rate between the pea aphid clones, rather than predation, although predation did lead to the eventual extinction of colonies. We also questioned whether intra- and interspecific differences in predator escape behaviour could affect the outcome of competition in the field. All three clones responded similarly to the presence of foraging hoverfly larvae (Episyrphus balteatus), but the Megoura viciae clone dropped from the plant significantly less often in response to the presence of a foraging two-spot ladybird (Adalia bipunctata). This work provides evidence that intraspecific variation in competitive ability can alter the outcome of interspecific competitive interactions in nature and suggests that species–specific behavioural traits may have the potential to modify the outcome of these interactions.     

Predicting the outcome of competition along a nitrogen gradient – a case study with Erodium cicutarium and Geranium pusillum

In order to understand the effect of a nitrogen gradient on the intraspecific and interspecific competition between plants, the two species Erodium cicutarium and Geranium pusillum were grown in a response-surface competion experiment at three different densities along a nitrogen gradient consisting of four different nitrogen levels. Seed set data were estimated from biomass measurements and fitted to a generalized hyperbolic competition model, and the probabilities of different ecological scenarios were calculated. The model predicted that E. cicutarium is competitively superior along most of the nitrogen gradient. This prediction was contrary to our prior expectations which were based on the occurance of the two species in natural habitats.     

Effects of neighbouring vegetation on eucalypt seedlings at two sites subject to different levels of abiotic stress

Abstract In spite of numerous studies on the effect of nutrient levels and/or standing crop on the intensity of resource competition the debate has not been resolved. Field studies that have used natural productivity gradients have generally supported the argument that competitive intensity and resource availability are positively correlated, whereas studies that have used artificial resource gradients have generally refuted the same argument. Here we report the results from study in which both approaches were used within the same system. We studied two species of eucalypt that occupy contrasting parts of the same landscape: Eucalyptus camaldulensis, found mostly along creek lines and in valleys with deep alluvial soils, and Eucalyptus microcarpa, found on hillsides and ridges with shallow soils. We studied the response of seedlings of the two species to the combined effects of competition and manipulated nutrient levels in a glasshouse experiment, and also investigated their responses to removal of neighbouring plants in the field. Eucalyptus microcarpa was less responsive to increased resource availability, which is consistent with one of the principal assumptions of Grime’s C‐S‐R model. In the glasshouse experiment both species of eucalypt responded in a qualitatively similar fashion to the combined effects of resource availability and competition: release from competition resulted in increased growth, but only in pots that received additional resources. In the field we found that neighbouring vegetation could severely limit the establishment of E. camaldulensis but the removal of neighbouring vegetation did not affect the performance of E. microcarpa seedlings. Eucalyptus camaldulensis seedlings suffered high levels of damage from herbivores. Our results thus generally support the predications of the C‐S‐R model, however, they indicate that the effects of competition and herbivory may be heavily confounded.