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PubMed2002

Competitive displacement among insects and arachnids.

Competitive displacement is the most severe outcome of interspecific competition. For the purposes of this review, we define this type of displacement as the removal of a formerly established species from a habitat as a result of direct or indirect competitive interactions with another species. We reviewed the literature for recent putative cases of competitive displacement among insects and arachnids and assessed the evidence for the role of interspecific competition in these displacements. We found evidence for mechanisms of both exploitation and interference competition operating in these cases of competitive displacement. Many of the cases that we identified involve the operation of more than one competitive mechanism, and many cases were mediated by other noncompetitive factors. Most, but not all, of these displacements occurred between closely related species. In the majority of cases, exotic species displaced native species or previously established exotic species, often in anthropogenically-altered habitats. The cases that we identified have occurred across a broad range of taxa and environments. Therefore we suggest that competitive displacement has the potential to be a widespread phenomenon, and the frequency of these displacement events may increase, given the ever-increasing degree of anthropogenic changes to the environment. A greater awareness of competitive displacement events should lead to more studies documenting the relative importance of key factors and developing hypotheses that explain observed patterns.

Animals↗
PubMed2001

Resource competition determines selection of B cell repertoires.

Previous experiments with mouse chimeras demonstrated that cellular competition for antigen-specific survival signals plays a crucial role in the maintenance of the naive B cell repertoire. Transgenic (Tg) B cell populations in these chimeras have a shortened lifespan and poor competitive abilities as compared to more diverse non-Tg populations in the same mice. We develop a mathematical model to investigate the mechanism of B cell competition. The model allows for various B cell clones, generated in the bone marrow, to go into the peripheral circulation, where they compete specifically for various ligands providing survival signals. In the model we also find the observed poor competitive abilities of the Tg repertoire. Investigating the nature of the competition in the model, we find that most of the competition is "intraspecific" occurring largely within the clone of truly Tg B cells, and within the repertoire of leaky Tg and non-Tg B cells. This is confirmed by analysing a simplified version of the model, which only allows for intraspecific competition, and resembles a simple ecological model with density-dependent death. The fact that our model accounts for the data, casts doubt on a previous interpretation of the same data arguing that more diverse repertoires outcompete repertoires of lower diversity. Here, we conclude that most of the data can be explained with intraspecific competition, and formulate an experimental prediction that allows one to distinguish between the previous interpretation of inter-specific competition between repertoires, and the current interpretation of intraspecific competition.

Animals↗
PubMed2004

Variation in the relative magnitude of intraspecific and interspecific competitive effects in novel versus familiar environments in two Drosophila species.

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

Animals↗
PubMed2002

Modeling competitive adsorption of molybdate, sulfate, selenate, and selenite using a Freundlich-type multi-component isotherm.

This study examined the interactions of MoO4(2-) + SO4(2-), MoO4(2-) + SeO4(2-), and MoO4(2-) + SeO3(2-) systems on gamma-Al2O3 to better understand the competitive adsorption of these anions in the natural environment. The Freundlich isotherms of anionic adsorption onto gamma-Al2O3 in single and binary solutes were also investigated to estimate the competition between these anions. Experimental results indicate that a higher concentration of competitive solute yields a higher efficiency of the competitive solute's prevention of MoO4(2-) adsorption. The most significant result was found in the MoO4(2-) + SeO3(2-) system. The Freundlich isotherm constant (n) increases with the competitive solute concentration. The suitability of a Freundlich-type isotherm, the Sheindorf-Rebuhn-Sheintuch (SRS) equation, and the modified SRS equation in representing the competitive adsorption of MoO4(2-), SO4(2-), SeO4(2-), and SeO3(2-) on gamma-Al2O3 surface, was also examined. Each set of isotherm data was found to conform to linear SRS expressions, allowing competition coefficients to be derived on a concentration basis for each binary-solute system. The competition coefficient aij and relative affinity coefficients alphaij can be seen as a way to quantify competitive interactions. The proposed SRS and modified SRS equations are simple mathematical expressions accounting for competitive interactions of anions present in a mixture for the range of concentrations over which each individual component exhibits Freundlich behavior.

Adsorption↗
PubMed1985

The population dynamics of competition between parasites.

A number of published studies of competition between parasite species are examined and compared. It is suggested that two general levels of interaction are discernible: these correspond to the two levels of competition recognized by workers studying free-living animals and plants: 'exploitation' and 'interference' competition. The former may be defined as the joint utilization of a host species by two or more parasite species, while the latter occurs when antagonistic mechanisms are utilized by one species either to reduce the survival or fecundity of a second species or to displace it from a preferred site of attachment. Data illustrating both levels of interaction are collated from a survey of the published literature and these suggest that interference competition invariably operates asymmetrically. The data are also used to estimate a number of population parameters which are important in determining the impact of competition at the population level. Theoretical models of host-parasite associations for both classes of competition are used to examine the expected patterns of population dynamics that will be exhibited by simple two-species communities of parasites that utilize the same host population. The analysis suggests that the most important factor allowing competing species of parasites to coexist is the statistical distribution of the parasites within the host population. A joint stable equilibrium should be possible if both species are aggregated in their distribution. The size of the parasite burdens at equilibrium is then determined by other life-history parameters such as pathogenicity, rates of resource utilization and antagonistic ability. Comparison of these theoretical expectations with a variety of sets of empirical data forms the basis for a discussion about the importance of competition in natural parasite populations. The models are used to assess quantitatively the potential for using competing parasite species as biological control agents for pathogens of economic or medical importance. The most important criterion for identifying a successful control agent is an ability to infect a high proportion of the host population. If such a parasite species also exhibits an intermediate level of pathology or an efficient ability to utilize shared common resources, antagonistic interactions between the parasite species contribute only secondarily to the success of the control. Competition in parasites is compared with competition in free-living animals and plants. The comparison suggests further experimental tests which may help to assess the importance of competition in determining the structure of more complex parasite-host communities.

Animals↗
PubMed1990

The biometrical genetics of competitive parameters in Drosophila melanogaster.

Despite the importance of competition as an evolutionary determinant in natural populations there have been few studies of the genetical control of competitive ability. Here, we report the results of a biometrical analysis of four continuously varying traits which, between them, describe the competitive interactions in mixed cultures of Drosophila melanogaster. The analysis involved the parental, F1, F2 and backcross generations (including all reciprocals) derived from crosses between two highly inbred lines isolated from the Texas population of D. melanogaster. The competitive performance of each genotype in monoculture and in duoculture with a phenotypically distinct tester were assessed using a yield-density regression analysis. Appropriate genetic models were fitted using a variance weighted least squares procedure and the resulting genetic components of the generation means used to define the genetical architecture of competition. Of the four competitive parameters investigated here the e-value, which describes the competitive performance of the indicator genotype at a fixed reference density, was found to be determined by simple additive genetic effects with no evidence of significant dominance. Conversely, competitive performance in monoculture (intra-genotypic competition) did display a significant net dominance component and the observed values in the F1 and parental generations indicated some degree of heterosis. Of the two competitive parameters determining performance in duoculture (inter-genotypic sensitivity and inter-genotypic pressure) the former was found to have a complex genetic determination involving not only additive and dominance components of the progeny's own genotype but also dominance components of the F1 maternal genotypes. There were also additive-dominance and dominance-dominance non-allelic interactions. Heterosis was evident, determined both by the progeny's own genotype and by one of the F1 maternal genotypes.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2005

Dynamics of multiple infection and within-host competition in genetically diverse malaria infections.

Within-host competition between coinfecting parasite strains shapes the evolution of parasite phenotypes such as virulence and drug resistance. Although this evolution has a strong theoretical basis, within-host competition has rarely been studied experimentally, particularly in medically relevant pathogens with hosts that have pronounced specific and nonspecific immune responses against coinfecting strains. We investigated multiple infection in malaria, using two pairs of genetically distinct clones of the rodent malaria Plasmodium chabaudi in mice. Clones were inoculated into mice simultaneously or 3 or 11 days apart, and population sizes were tracked using immunofluorescence or quantitative polymerase chain reaction. In all experiments, at least one of the two clones suffered strong competitive suppression, probably through both resource- and immune-mediated (apparent) competition. Clones differed in intrinsic competitive ability, but prior residency was also an important determinant of competitive outcome. When clones infected mice first, they did not suffer from competition, but they did when infecting mice at the same time or after their competitor, more so the later they infected their host. Consequently, clones that are competitively inferior in head-to-head competition can be competitively superior if they infect hosts first. These results are discussed in the light of strain-specific immunity, drug resistance, and virulence evolution theory.

Animals↗
PubMed2006

Experimental and model analyses of the effects of competition on individual size variation in wood frog (Rana sylvatica) tadpoles.

1. Size variation is a ubiquitous feature of animal populations and is predicted to strongly influence species abundance and dynamics; however, the factors that determine size variation are not well understood. 2. In a mesocosm experiment, we found that the relationship between mean and variation in wood frog (Rana sylvatica) tadpole size is qualitatively different at different levels of competition created by manipulating resource supply rates or tadpole density. At low competition, relative size variation (as measured by the coefficient of variation) decreased as a function of mean size, while at high competition, relative size variation increased. Therefore, increased competition magnified differences in individual performance as measured by growth rate. 3. A model was developed to estimate the contribution of size-dependent factors (i.e. based on size alone) and size-independent factors (i.e. resulting from persistent inherent phenotypic differences other than size that affect growth) on the empirical patterns. 4. Model analysis of the low competition treatment indicated that size-dependent factors alone can describe the relationship between mean size and size variation. To fit the data, the size scaling exponent that describes the dependence of growth rate on size was determined. The estimated value, 0-83, is in the range of that derived from physiological studies. 5. At high competition, the model analysis indicated that individual differences in foraging ability, either size-based or due to inherent phenotypic differences (size-independent factors), were much more pronounced than at low competition. The model was used to quantify the changes in size-dependent or size-independent factors that underlie the effect of competition on size-variation. In contrast to results at low competition, parameters derived from physiological studies could not be used to describe the observed relationships. 6. Our experimental and model results elucidate the role of size-dependent and size-independent factors in the development of size variation, and highlight and quantify the context dependence of individual (intrapopulation) differences in competitive abilities.

Animals↗
PubMed1989

Quantitative comparison of the laboratory and field competitiveness of Rhizobium leguminosarum biovar phaseoli.

Rhizobium leguminosarum bv. phaseoli KIM5s outcompeted strain CE3 in bean (Phaseolus vulgaris L.) root nodulation when plants were grown at any of three field sites, each with a different soil type and indigenous population, or in the laboratory in a sterilized sand, a sterilized peat-vermiculite mixture, or a nonsterile field soil. A mathematical model describing nodulation competitiveness was empirically derived to evaluate the relative competitiveness of the two strains under these conditions. This model relates the proportional representation of the two strains in the inoculum to the proportional representation of nodules occupied by each strain or both strains and provides a measure of competitiveness, which is referred to as the competitiveness index. Statistical comparisons of competitiveness indices showed that the relative competitiveness of KIM5s and CE3 remained constant when the two strains were applied in a constant ratio over a range of inoculum concentrations, from 10(3) to 10(7) cells per seed, and when they were applied in various ratios to six P. vulgaris cultivars. Furthermore, the relative competitiveness of KIM5s and CE3 in the laboratory did not differ significantly from their relative competitiveness at the three field sites studied. Thus, a study of the basis for nodulation competitiveness of KIM5s and CE3 in the laboratory has the potential to provide an understanding of competitiveness both in the laboratory and in the field.

Models, Biological↗
PubMed1992

Competition among Rhizobium leguminosarum bv. phaseoli strains for nodulation of common bean.

Six effective Rhizobium leguminosarum bv. phaseoli strains were examined for nodulation competitiveness on common bean (Phaseolus vulgaris L.), using all possible two-strain combinations of inoculum. Nodule occupancy was determined with strain-specific fluorescent antibodies. The strains were divided into three groups according to their overall competitive abilities on pole bean cv. Kentucky Wonder and bush bean cv. Bountiful. Strains TAL 182 and TAL 1472 were highly competitive (greater than 70% nodule occupancy); strains KIM-5, Viking 1, and CIAT 899 were moderately competitive (approximately 50% nodule occupancy); and strain CIAT 632 was poorly competitive (less than 5% nodule occupancy). The competitiveness of the six strains was similar on the two host cultivars. The proportion of competing strains in the inoculum influenced the nodule occupancy of the highly competitive and moderately competitive strains, but not that of the poorly competitive strain. Two outstanding strains (TAL 182 and TAL 1472) were identified as ideal model strains for molecular and genetic studies on nodulation competitiveness.

Fabaceae↗
PubMed1998

Sperm competition in birds.

Sperm competition in birds occurs when a female is inseminated by more than one male during a single breeding cycle. Despite most birds being socially monogamous, sperm competition is widespread and results in frequent extra-pair paternity. Sperm competition is a fundamental part of sexual selection since it results in differential reproductive success among males. Male adaptations to sperm competition include relatively large testes, large sperm stores and long spermatozoa, mate guarding and frequent pair copulations. Females show no obvious morphological adaptations to sperm competition but, by controlling whether copulations are successful, they probably determine its frequency and extent. Despite this, the evolutionary benefits females acquire from extra-pair fertilizations are poorly understood. Experiments in which females are inseminated with equal numbers of spermatozoa from two males usually show last male sperm precedence. Understanding the mechanism of sperm competition requires understanding of why the last male to inseminate a female fertilizes a disproportionate number of eggs. The data from sperm competition studies on the domestic fowl, turkeys and zebra finches are consistent only with a passive sperm loss model of sperm competition. The mechanism is as follows: after insemination, spermatozoa enter the sperm storage tubules located in the oviduct, from which they are lost at a constant rate over days or weeks. All else being equal, the interval between two inseminations determines the probability of fertilization: the second of two inseminations fertilizes most eggs simply because, by the time fertilization occurs, fewer of these spermatozoa have been lost. Other factors also affect the outcome of sperm competition: the timing of insemination relative to oviposition, the differential fertilizing capacity of males and differences in the numbers of spermatozoa inseminated; as a consequence, last male sperm precedence is not automatic. On the basis of the mechanism of sperm competition, the optimal strategy for both males and females to maximize their likelihood of extra-pair fertilization is to copulate with an extra-pair partner as close as possible to the onset of oviposition.

Adaptation, Physiological↗
PubMedDate not supplied

Ultraviolet-B radiation and plant competition: experimental approaches and underlying mechanisms.

Under realistic stratospheric ozone depletion scenarios, ultraviolet-B radiation (280-320 nm) (UV-B) influences plant morphology and plant competitive interactions. Influence of UV-B on plant competition can be studied using a variety of experimental and analytical approaches including inverse yield-density models and allometric, neighborhood or size-structure analyses that provide links between plant and ecosystem responses. These approaches differ in their abilities to extract information regarding competitive interactions and their morphological underpinnings. Only a limited number of studies have been carried out to investigate UV-B effects on plant competition, and most of these have used the replacement series approach, which has received much criticism. Nonetheless, results to date indicate that slight differences in UV-B-induced morphological responses of species grown within associations can alter canopy structure thereby influencing photosynthetically active radiation (PAR) interception and relative competitive ability. Because the response of individuals of the same species is expected to be uniform, UV-B may influence intraspecific competition less than interspecific competition. Before we can make clear generalizations and predictions concerning the effects of this radiation on plant competition, an understanding is crucial of the mechanisms underlying UV-B-induced shifts in competitive interactions by assessing competition over time.

Ecology↗
PubMed2000

Additive and nonadditive effects of herbivory and competition on tree seedling mortality, growth, and allocation.

The interaction between simulated cotyledon herbivory and interspecific competition was studied in a greenhouse experiment using two species of trees, Acer rubrum and Quercus palustris, which commonly invade abandoned agricultural fields. Herbivory treatments were applied as a gradient of cotyledon removal for A. rubrum with 0, 25, 50, 75, and 100% of cotyledon tissue removed. Cotyledons from Q. palustris were clipped and removed (control, early, and late removal) to create a gradient of seed reserve availability. The competition treatment consisted of plugs of old-field vegetation that filled the pots with perennial cover. Mortality of seedlings was higher with competition. There was a significant interaction between herbivory and competition with the highest mortality occurring with competition at the highest intensity of herbivory in both species. Herbivory reduced biomass for Q. palustris only, while competition reduced biomass in both species. Neither species showed an interaction between herbivory and competition for growth. There was a significant interaction between herbivory and competition on allocation patterns for both species, with greater allocation to roots with competition at the highest intensity of herbivory. This study demonstrates the potential for cotyledon herbivory and competition to interact, altering the invasion of tree seedlings into abandoned agricultural land.

Journal Article↗
PubMed2004

Spermatozoal traits and sperm competition in Atlantic salmon: relative sperm velocity is the primary determinant of fertilization success.

Sperm competition occurs when sperm from more than one male compete for fertilizations. This form of post-copulatory sexual selection is recognized as a significant and widespread force in the evolution of male reproductive biology and as a key determinant of differential male reproductive success. Despite its importance, however, detailed mechanisms of sperm competition at the gamete level remain poorly understood. Here, we use natural variation in spermatozoal traits among wild Atlantic salmon (Salmo salar), a species naturally adapted to sperm competition, to examine how the relative influences of sperm (i) number, (ii) velocity, (iii) longevity, and (iv) total length determine sperm competition success. Atlantic salmon fertilize externally, and we were therefore able to conduct controlled in vitro fertilization competitions while concurrently measuring spermatozoal traits within the aqueous micro-environment to which salmon gametes are naturally adapted. Microsatellite DNA fingerprinting revealed that a male's relative sperm velocity was the primary determinant of sperm competition success. There was no significant relationship between fertilization success and either relative sperm number or total length; sperm longevity showed an inverse relationship with competition success. These relationships were consistent for two experimental repeats of the in vitro fertilization competitions. Our results therefore show, under the natural microenvironment for salmon gametes, that relative sperm velocity is a key spermatozoal component for sperm competition success. Atlantic salmon sperm can be considered to enter a competition analogous to a race in which the fastest sperm have the highest probability of success.

Animals↗
PubMed2004

Managing under managed community care: the experiences of clients, providers and managers in Ontario's competitive home care sector.

In 1996, a newly elected government in the Province of Ontario, Canada, introduced a managed competition environment into the home care sector through the establishment of a competitive contracting process for home care services. Through 65 in-depth, semi-structured interviews conducted between November 1999 and January 2001, we trace the implementation of this competitive contracting policy within Ontario's newly established managed community care environment and assess the effects of competitive contracting against two sets of goals: (1). quality of care goals that consider continuity of care of paramount importance in the provision of home care; and (2). the managed competition goal of increased efficiency. In assessing the implementation of this policy against these goals, we highlight the conflicts that can arise in pursuing different policy goals in response to different formulations of the policy problem that underpin them. We map stakeholder experiences with the competitive contracting policy onto relevant contracting and managed competition literatures. When measured against the goals of quality of care and efficiency, the findings presented here offer a mixed review of the experiences to date with the competitive contracting process introduced in Ontario's home care sector and suggest improvements for managing future competitive contracting processes.

Attitude of Health Personnel↗
PubMed2000

Parasite-mediated and direct competition in a two-host shared macroparasite system.

This paper investigates the local dynamical behaviour of a deterministic model describing two host species experiencing three forms of competition: direct competition, apparent competition mediated by macroparasites, and intra-specific (density-dependent) competition. The problem of algebraic intractability is sidestepped by adopting a geometric approach, in which an array of maps is constructed in parameter space, each structured by bifurcation surfaces which mark qualitative changes in system behaviour. The maps provide both a succinct and a comprehensive overview of the stability and feasibility structure of the system equilibria, from which can be deduced the possible modes of local dynamical behaviour. A detailed examination of these maps shows that (i) the system is highly sensitive to the effect of infection on fecundity with synchronous sustained cycles readily generated by Hopf bifurcations; (ii) for a broad range of parameter values, pertinent to actual biological systems, apparent competition mediated by macroparasites is sufficient, on its own, to explain host exclusion; (iii) direct competition reinforces parasite-mediated competition to expand the host exclusion region; and (iv) the condition for host exclusion can be expressed simply in a form which holds for both micro- and macroparasite models and which involves just two key indices, measuring tolerance to the infection and the strength of direct competition. The techniques used in this paper are not restricted to the analysis of host-parasite systems but can be applied to a wide range of nonlinear population models. They are therefore as relevant to the analysis of such general issues as exploitative competition and trophic interactions as they are to specific epidemiological problems.

Animals↗
PubMed2005

Gender differences in testosterone and cortisol response to competition.

This study examined intra-individual change in testosterone, cortisol, and hormone-behavior relationships in response to a rowing ergometer competition. Forty-six members (23 females) of a university crew team provided saliva samples before, 20- and 40-min post-competition, as well as baselines on a non-competition day. Behavioral assessments included measures of previous rowing experience, dominance, competitiveness, bonding with teammates, pre- and post-competition mental state and performance. Men's and women's endocrine responses to this competitive setting were more different than alike and varied by level of competitive experience, the specific phase of the competitive event, and the particular hormone measured. Inter-individual differences in testosterone and cortisol were differentially associated with social affiliation with teammates but rarely with dominance or competitiveness. Theoretically, the findings support the integration of features of the 'tend and befriend' model with the biosocial model of status, and suggest future research directions that may lead to clarification and refinement of those ideas.

Adolescent↗
PubMed2006

Salivary alpha-amylase response to competition: relation to gender, previous experience, and attitudes.

This study examined individual differences in salivary alpha-amylase response to competition in relation to gender, previous experience, behavior, attitudes, and performance. Participants were 42 (21 women) members of a collegiate crew team. Saliva samples were collected before, 20- and 40-min post-ergometer competition and at the same times on a non-competition day for comparison. Samples were assayed for salivary biomarkers of sympathetic nervous system (alpha-amylase) and hypothalamic-pituitary-adrenal axis (cortisol) activity. Behavioral assessments included self-reports of dominance, competitiveness, bonding with teammates, competition-related strategic thinking, and performance. On average, salivary alpha-amylase increased 156% in response to the ergometer competition. By comparison, cortisol increased 87% across the same time period. Salivary alpha-amylase was higher across the competition for varsity than for novice athletes, and was positively associated with performance and interest in team-bonding. Regression analyses revealed that alpha-amylase reactivity explained individual differences in dominance and team bonding above and beyond that associated with cortisol reactivity, and that joint inactivation in alpha-amylase and cortisol reactivity to competition (low-low) was associated with high perceived dominance. The findings are among the first to integrate salivary alpha-amylase into the study of competition and reveal that intra-individual change in alpha-amylase may be influenced by a confluence of factors that include contextual, behavioral, and psychological factors and processes.

Adolescent↗