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PubMed1981

The role of mRNA competition in regulating translation. I. Demonstration of competition in vivo.

SC-1 cells infected with reovirus continue to synthesize host proteins at a nearly normal rate for at least 12 h postinfection. The relative translational efficiencies of reovirus and host mRNAs were determined at various times during this interval. Analysis was performed by determining the sensitivity of individual mRNA translation rates to inhibition by low doses of cycloheximide, by measuring polysome sizes, and by quantitatively comparing mRNA concentration with protein synthesis rates. Treatment of infected cells with low doses of cycloheximide actually stimulates the synthesis of some of the major reovirus protein, whereas host protein synthesis is inhibited. An analysis of the polysomes encoding two reovirus proteins (microNS and sigma 3; synthesis of both is stimulated by cycloheximide) shows that these proteins are synthesized on polysomes smaller than those producing host proteins of comparable size. Reovirus mRNAs accumulate to unusually high levels during the first 8 h of infection, accounting for 30% to 45% by weight of the total mRNA in the cell. We interpret these results as indicating that viral and host mRNAs must compete for a message-discriminatory component prior to their binding to the 40 S ribosomal complex and that this component is limiting in reovirus-infected cells. Moreover, a hierarchy must exist among mRNAs in terms of their affinity for this component, host mRNAs in general having greater affinities than reovirus mRNAs.

Animals↗
PubMed2000

[Application of competition index in assessing intraspecific and interspecific spatial relations between manchurian ash and dahurian larch].

The competition among trees within a stand includes aboveground and underground parts. How to determine the zone of competition influence is important for predicting tree growth. We divide the zone into two parts: one part is defined as inside radius, the distance from nearest competitors to subject tree, in which, the competition influence is the sum of both aboveground and underground; the another part is defined as outside radius, the distance of competitors to subject tree, where the competition influence comes mostly from underground. Thus, the competition index, based upon the divided two radii, could be decomposed into different components. We used the data of manchurian ash(MA) (Fraxinus mandshurica) and dahurian larch(DL) (Larix gmelini) from pure and mixed stands to analyze the competition influence of aboveground and underground in inside and of underground in outside radius. The results show that for MA in pure stand, the competition effect was 59.06% from inside radius, and 40.94% from outside radius. For DL in pure stand, the competition effects in both radii were 71.31% and 28.69%, respectively. There was a stronger competition influence in inside than in outside radius. Underground competitions were the important issues for both species in pure stands. In a mixed stand of MA and DL, there were two species in both radii, and the effects of competition came from intraspecific and interspecific competitors. For subject tree of MA, the effect of interspecific competition was 49.55%, and that of intraspecific was 50.45%. For subject tree of DL, the effects were 81.11% and 18.89%, respectively. DL displayed a stronger interspecific competition than MA in the zone of competition influence, and was a main effect of competition on MA. For subject tree of MA, the effect of competition from inside radius was 25.33%, which was the sum of competition influence from aboveground and underground, and about 74.67% was from outside radius. For subject tree of DL, both in inside and outside radii were 55.37% and 44.64%, respectively. Comparing with the information from both species and two radii, the root-root competition from underground was an important component. DL grew well, and could reduce the impact of intraspecific competition for MA when both species mixed in a stand.

Ecosystem↗
PubMed1994

Population dynamics of sexual and resource competition.

Interspecific competition is usually thought of in terms of resource competition. Closely related species, however, may often compete also for a "sexual niche," a sexual resource. Both sexual and resource competition are likely to affect the dynamics of closely related sympatric species. Here, a combined model of sexual and resource competition is developed and analyzed in terms of phase planes. Population dynamics involving sexual competition alone are characterized by competitive exclusion and dependence on relative initial population size of the competing species. This dependence on initial population size is due to the nonlinear equilibrium isoclines of sexual competition, which contrast with the linear isoclines of the Lotka-Volterra competition equations. Dependence on initial population size is stronger when sexual competition is severe and/or when growth rates of the two species are low. Additional dynamic possibilities arise under combined sexual and resource competition. If the sexual competition is strong, competitive exclusion is the only outcome. But if sexual competition is weak, and if stable coexistence would occur without sexual competition, then both stable coexistence and competitive exclusion may occur simultaneously. A species that is relatively rare initially may be excluded, but when both species are initially of comparable abundance both will persist. Adding sexual competition to resource competition thus introduces additional uncertainty concerning the outcome of competition between two sympatric species.

Animals↗
PubMed1986

The intensity of competition versus its importance: an overlooked distinction and some implications.

The intensity of competition is a physiological concept, related directly to the well-being of individual organisms but only indirectly and conditionally to their fitness, and even more indirectly to the evolution of populations and the structure of communities. The importance of competition is primarily an ecological and evolutionary concept, related directly to the ecology and fitness of individuals but only indirectly to their physiological states. The intensity of competition is not necessarily correlated with the intensities of predation, disturbance, abiotic stress, or other ecological processes. The importance of competition is necessarily relative to the importances of other processes. Intensity refers primarily to the process of present competition, whereas importance refers primarily to the products of past competition. The distinction between the intensity and the importance of competition clarifies two long-standing ecological debates. Some ecologists have proposed that competition is greater in more stressful habitats, others the opposite, and still others that no such relationship exists. Evidence cited to refute or support these positions often confuses intensity and importance. Distinguishing between them focuses questions more sharply and indicates what sorts of new evidence should be sought. The more widely known debate over the prevalence of competition as an agent of community structure is a debate about the importance of competition, but evidence about the intensity of competition has often been used by both sides. We argue that intensity and importance need not be correlated, and so measurements of the intensity of competition are not directly relevant to this debate. This distinction also generates testable hypotheses and suggests directions for research. For example, we hypothesize that competition can be unimportant even if it is very intense: no such hypothesis is possible unless importance is distinguished from intensity. We discuss the application of these ideas to methods and theories used to study competition, ecological communities, and the evolution of competitive ability. We advocate a research approach that presumes multiple, interacting causes, including competition, affecting community structure, and we show how the distinction between intensity and importance helps to make this feasible.

Animals↗
PubMed2005

Competitive foods increase the intake of energy and decrease the intake of certain nutrients by adolescents consuming school lunch.

OBJECTIVE: To determine the influence of competitive foods on energy and nutrient intakes of adolescents participating in a school lunch program. DESIGN: Weighed plate waste data were used to assess the energy and nutrient consumption from school lunch menus items and competitive food items chosen and consumed by adolescents. The two test groups were students choosing competitive foods and those choosing no competitive foods. SUBJECTS/SETTING: Participants were sixth-grade students aged 11 to 13 years purchasing lunch in three public middle school cafeterias in Franklin County, KY (n=493 for no competitive foods and n=250 for competitive foods). The data were collected over 24 days in 2 school years. STATISTICAL ANALYSES PERFORMED: Significant differences between the competitive foods and no competitive foods groups were determined using frequency and analysis of variance procedures. RESULTS: The data indicate that sixth-grade adolescents consumed energy and other nutrients (except iron) at significantly lower than recommended levels; one third of students purchasing school lunch also purchased competitive food items; competitive foods purchasers reduced their school lunch servings, portion weight and/or item selection and increased school lunch item plate waste, resulting in lower intakes of energy (400 kcal vs 530 kcal for no competitive foods), calcium (300 mg vs 362 mg for no competitive foods), and vitamin A (77 retinol equivalents vs 113 retinol equivalents for no competitive foods) from the school lunch; and competitive foods supplied more than one third of total energy for the meal. CONCLUSIONS: These data will enable school cafeteria planners to develop menus that include more nutritious foods, snacks, and drinks instead of the currently available competitive foods.

Adolescent↗
PubMed2005

Effects of competition among fertility centers on pregnancy and high-order multiple gestation rates.

OBJECTIVE: To measure the effect of competition among fertility centers on pregnancy and high-order multiple (HOM) gestation rates after IVF. DESIGN: Retrospective cohort study. SETTING: Four hundred eight fertility clinics registered with the Society for Assisted Reproductive Technology as providing IVF services in 2000. Competition was defined as number of clinics in a geographically defined area. Demand for services was based on the population of reproductive-aged women. PATIENT(S): Three hundred eighty-one fertility clinics reporting clinical outcomes. INTERVENTION(S): Pregnancy rates, HOM gestation rates, population of reproductive-aged women, and number of competing clinics were calculated for each clinic from Society for Assisted Reproductive Technology and census data. MAIN OUTCOME MEASURE(S): The clinic HOM gestation rate (percentage of pregnancies that were HOM) and age-adjusted pregnancy rate. RESULT(S): The number of clinics in an area of competition ranged from 1 to 22. The HOM gestation rate per clinic ranged from 0% to 50%. As demand increased, competition increased. As competition increased, the number of HOM pregnancies per clinic decreased. In areas of low competition (1 to 2 clinics) the clinic HOM gestation rate was 8.43%, in areas of intermediate competition (3-7 clinics) 8.39%, and in areas of high competition (8-22 clinics) 8.24%. In areas with intermediate demand, high levels of competition resulted in fewer HOM pregnancies than intermediate competition (relative risk 0.56, 95% confidence interval 0.36-0.89) or low levels of competition (RR 0.57, 95% confidence interval 0.35-0.94). Age-adjusted pregnancy rates did not differ by level of competition. CONCLUSION(S): According to these data, the risk of HOM gestation decreases with increasing competition among clinics; however, pregnancy rates are unaffected.

Adult↗
PubMed2004

Spatial variation and density-dependent dispersal in competitive coexistence.

It is well known that dispersal from localities favourable to a species' growth and reproduction (sources) can prevent competitive exclusion in unfavourable localities (sinks). What is perhaps less well known is that too much emigration can undermine the viability of sources and cause regional competitive exclusion. Here, I investigate two biological mechanisms that reduce the cost of dispersal to source communities. The first involves increasing the spatial variation in the strength of competition such that sources can withstand high rates of emigration; the second involves reducing emigration from sources via density-dependent dispersal. I compare how different forms of spatial variation and modes of dispersal influence source viability, and hence source-sink coexistence, under dominance and pre-emptive competition. A key finding is that, while spatial variation substantially reduces dispersal costs under both types of competition, density-dependent dispersal does so only under dominance competition. For instance, when spatial variation in the strength of competition is high, coexistence is possible (regardless of the type of competition) even when sources experience high emigration rates; when spatial variation is low, coexistence is restricted even under low emigration rates. Under dominance competition, density-dependent dispersal has a strong effect on coexistence. For instance, when the emigration rate increases with density at an accelerating rate (Type III density-dependent dispersal), coexistence is possible even when spatial variation is quite low; when the emigration rate increases with density at a decelerating rate (Type II density-dependent dispersal), coexistence is restricted even when spatial variation is quite high. Under pre-emptive competition, density-dependent dispersal has only a marginal effect on coexistence. Thus, the diversity-reducing effects of high dispersal rates persist under pre-emptive competition even when dispersal is density dependent, but can be significantly mitigated under dominance competition if density-dependent dispersal is Type III rather than Type II. These results lead to testable predictions about source-sink coexistence under different regimes of competition, spatial variation and dispersal. They identify situations in which density-independent dispersal provides a reasonable approximation to species' dispersal patterns, and those under which consideration of density-dependent dispersal is crucial to predicting long-term coexistence.

Animals↗
PubMed2007

Two-species asymmetric competition: effects of age structure on intra- and interspecific interactions.

1. The patterns of density-dependent resource competition and the mechanisms leading to competitive exclusion in an experimental two-species insect age-structured interaction were investigated. 2. The modes of competition (scramble or contest) and strength of competition (under- to overcompensatory) operating within and between the stages of the two species was found to be influenced by total competitor density, the age structure of the competitor community and whether competition is between stages of single or two species. 3. The effect of imposed resource limitation on survival was found to be asymmetric between stages and species. Environments supporting both dominant and subordinate competitors were found to increase survival of subordinate competitors at lower total competitor densities. Competitive environments during development within individual stage cohorts (i.e. small or large larvae), differed from the competitive environment in lumped age classes (i.e. development from egg-->pupae). 4. Competition within mixed-age, stage or species cohorts, when compared with uniform-aged or species cohorts, altered the position of a competitive environment on the scramble-contest spectrum. In some cases the competitive environment switched from undercompensatory contest to overcompensatory scramble competition. 5. Such switching modes of competition suggest that the relative importance of the mechanisms regulating single-species population dynamics (i.e. resource competition) may change when organisms are embedded within a wider community.

Aging↗
PubMed2004

Competitive impacts and responses of an invasive weed: dependencies on nitrogen and phosphorus availability.

Changes in competitive interactions under conditions of enhanced resource availability could explain the invasion success of some problematic plant species. For one invader of North American grasslands, Centaurea diffusa (diffuse knapweed), we test three hypotheses: (1) under ambient (high resource) conditions, C. diffusa is better able to tolerate competition from the resident community (competitive response), (2) under ambient conditions, C. diffusa strong impacts the competitive environment (competitive effect), and (3) reductions in nitrogen and/or phosphorus availability diminish these advantages. In support of our first hypothesis, C. diffusa was the most tolerant to neighbor competition of the four focal species under current resource conditions. In opposition to our second hypothesis, however, neighborhoods that contained C. diffusa and those where C. diffusa had been selectively removed did not differ in their impact on the performance of target transplant individuals or on resource conditions. Reduction in resource availability influenced competitive tolerance but not competitive impact, in partial support of our last hypothesis. Reduction in soil nitrogen (via sucrose carbon addition) enhanced the degree of neighbor competition experienced by all species but did not change their relative rankings; C. diffusa remained the best competitor under low nitrogen conditions. Reduction of soil phosphorus (via gypsum addition) weakened the ability of C. diffusa to tolerate neighbor competition proportionately more than the other focal species. Consequently, under low phosphorus conditions, C. diffusa lost its competitive advantage and tolerated neighbor competition similarly to the other focal species. We conclude that C. diffusa invasion may be double-edged: C. diffusa is less limited by nitrogen than the other focal species and is better able to utilize phosphorus to its competitive advantage.

Centaurea↗
PubMed2005

Effects of payment incentives, hospital ownership and competition on hospitalization decisions for ambulatory surgical procedures.

OBJECTIVE: To test the hypotheses that (1) for-profit (FP) and not-for-profit (NFP) hospitals are less likely than public hospitals to admit cases reimbursed by prospective payment favoring ambulatory over inpatient care; (2) admission odds of public, FP and NFP hospitals will converge under increasing hospital competition. METHODS: Retrospective, population-based, cross-sectional study covering 29,699 cases of unilateral, femoral/inguinal hernia operation (major surgical procedure) and 60,626 cases of cataract surgery (local surgical procedure), from Taiwan's National Health Insurance database was used. Diagnosis-wise logistic regression analysis were done to examine associations between admission propensities of FP versus public and NFP hospitals (large teaching hospitals with > or = 250 beds versus district hospitals with < 250 beds) under high and low competition, adjusted for clinical complications, and patient as well as physician demographics. RESULTS: Large public teaching hospitals are significantly more likely than FP district hospitals to admit hernia patients (ORs = 1.9 and 2.6, respectively, under high and low competition), and cataract surgery patients (ORs = 5.0 and 5.4, respectively, under high and low competition). The corresponding odds ratios for public district hospitals (relative to FP district hospitals) are 1.2 and 3.9 for hernia and 4.9 and 2.7 for cataract surgery. Odds ratios show convergence of admission odds across hospital ownership under high competition relative to low competition for hernia (OR range for different hospital types under high competition, 1.0-1.9; and under low competition, 1.0-3.9). Cataract cases show high divergence of admission odds between public and FP/NFP hospitals regardless of competition level (OR range for different hospital types under high competition, 0.3-5.0; and under low competition, 0.3-5.4). CONCLUSION: Overall, our data support the study hypotheses. Differences in the relevance of inpatient care for hernia and cataract surgery may account for the lack of admission convergence of public hospitals and FPs under high competition among cataract surgery group.

Ambulatory Surgical Procedures↗
PubMed2002

A relational approach to measuring competition among hospitals.

OBJECTIVE: To present a new, relational approach to measuring competition in hospital markers and to compare this relational approach with alternative methods of measuring competition. DATA SOURCES: The California Office of Statewide Health Planning and Development patient discharge abstracts and financial disclosure files for 1991. STUDY DESIGN: Patient discharge abstracts for an entire year were used to derive patient flows, which were combined to calculate the extent of overlap in patient pools for each pair of hospitals. This produces a cross-sectional measure of market competition among hospitals. PRINCIPAL FINDINGS: The relational approach produces measures of competition between each and every pair of hospitals in the study sample, allowing us to examine a much more "local" as well as dyadic effect of competition. Preliminary analyses show the following: (1) Hospital markets are smaller than thought. (2) For-profit hospitals received considerably more competition from their neighbors than either nonprofit or government hospitals. (3) The size of a hospital does not matter in the amount of competition received, but the larger hospitals generated significantly more competition than smaller ones. Comparisons of this method to the other methods show considerable differences in identifying competitors, indicating that these methods are not as comparable as previously thought. CONCLUSION: The relational approach measures competition in a more detailed way and allows researchers to conduct more fine-grained analyses of market competition. This approach allows one to model market structure in a manner that goes far beyond the traditional categories of monopoly oligopoly, and perfect competition. It also opens up an entirely new range of analytic possibilities in examining the effect of competition on hospital perfomance, price of medical care, changes in the market, technology acquisition, and many other phenomena in the health care field.

Analysis of Variance↗